inkjet printing for commercial high-efficiency silicon solar cells

Inkjet Printing for

Commercial High-Efficiency

Silicon Solar Cells

by

Roland Yudadibrata Utama

ARC Centre of Excellence for Advanced Photovoltaics and Photonics

University of New South Wales

Sydney, Australia

A thesis submitted to the University of New South

Wales in fulfilment of the requirements for the degree of

Doctor of Philosophy

March 2009

PLEASE TYPE THE UNIVERSITY OF NEW SOUTH WALES

Thesis/Dissertation Sheet Surname or Family name: UTAMA

First name: ROLAND YUDADIBRATA

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Abbreviation for degree as given in the University calendar: PhD in Photovoltaic Engineering

School: School of Photovoltaic and Renewable Energy Engineering

Faculty: Engineering

Title: Inkjet Printing for Commercial High Efficiency Silicon Solar Cells

Abstract 350 words maximum: (PLEASE TYPE)

One way of reducing the cost of crystalline silicon solar cell fabrication is by increasing the conversion efficiency of the device. However, most high efficiency solar cell designs require more complex fabrication methods that also increase the fabrication cost. Photolithography is an example of such an indispensable but costly process. The most common use for photolithography in solar cell fabrication is for dielectric patterning. In this thesis, inkjet printing is proposed as an alternative method for dielectric patterning in solar cell fabrication. There are two inkjet printing methods developed in this thesis. The indirect inkjet patterning method involves the deposition of a suitable plasticiser droplet onto an intermediate resin coating layer on top of the dielectric surface. Diethylene glycol and novolac resin are used as the plasticiser and coating layer respectively. The plasticiser changes the permeability of the affected region of the resin such that it becomes permeable to liquid dielectric etchants. When the resin layer is removed, the printed pattern is transferred to the dielectric layer. The optimised process produces round openings with diameters as small as 30-35 μm and continuous line patterns with width as narrow as 40-50 μm. The direct inkjet patterning method involves the deposition of liquid phosphorus dopant sources onto both silicon and dielectric surfaces. Two types of phosphorus sources are used: phosphoric acid and specially-formulated dopant sources. Narrow lines as wide as 15-20 μm are produced after appropriate surface treatments on both silicon and dielectric surfaces. Using this method, a process that simultaneously pattern the dielectric layer and diffuse the silicon underneath is developed. Various high efficiency solar cell structures such as selective emitter, localised contacts, surface texturing and edge isolation are demonstrated using the indirect inkjet patterning method. Both inkjet patterning methods are then used in the fabrication of a selective emitter solar cell. Fill factors in the range of 0.79-0.80 are shown to be achievable with both patterning methods, thus indicating the high quality metal-silicon contacts formed by these inkjet techniques.

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Originality Statement

Inkjet Printing for Commercial High Efficiency Silicon Solar Cells iii

Originality Statement

I hereby declare that this submission is my own work and to the best of my knowledge it

contains no materials previously published or written by another person, or substantial

proportions of material which have been accepted for the award of any other degree or

diploma at UNSW or any other educational institution, except where due acknowledgment is

made in the thesis. Any contribution made to the research by others, with whom I have

worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that

the intellectual content of this thesis is the product of my own work, except to the extent that

assistance from others in the project’s design and conception or in style, presentation and

linguistic expression is acknowledged.

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Inkjet Printing for Commercial High Efficiency Silicon Solar Cells iv

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Abstract

Inkjet Printing for Commercial High Efficiency Silicon Solar Cells v

Abstract

One way of reducing the cost of crystalline silicon solar cell fabrication is by increasing the

conversion efficiency of the device. However, most high efficiency solar cell designs require

more complex fabrication methods that also increase the fabrication cost. Photolithography is

an example of such an indispensable but costly process. The most common use for

photolithography in solar cell fabrication is for dielectric patterning. In this thesis, inkjet

printing is proposed as an alternative method for dielectric patterning in solar cell fabrication.

There are two inkjet printing methods developed in this thesis. The indirect inkjet patterning

method involves the deposition of a suitable plasticiser droplet onto an intermediate resin

coating layer on top of the dielectric surface. Diethylene glycol and novolac resin are used as

the plasticiser and coating layer respectively. The plasticiser changes the permeability of the

affected region of the resin such that it becomes permeable to liquid dielectric etchants. When

the resin layer is removed, the printed pattern is transferred to the dielectric layer. The

optimised process produces round openings with diameters as small as 30-35 μm and

continuous line patterns with width as narrow as 40-50 μm.

The direct inkjet patterning method involves the deposition of liquid phosphorus dopant

sources onto both silicon and dielectric surfaces. Two types of phosphorus sources are used:

phosphoric acid and specially-formulated dopant sources. Narrow lines as wide as 15-20 μm

are produced after appropriate surface treatments on both silicon and dielectric surfaces.

Using this method, a process that simultaneously pattern the dielectric layer and diffuse the

silicon underneath is developed.

Various high efficiency solar cell structures such as selective emitter, localised contacts,

surface texturing and edge isolation are demonstrated using the indirect inkjet patterning

method. Both inkjet patterning methods are then used in the fabrication of a selective emitter

solar cell. Fill factors in the range of 0.79-0.80 are shown to be achievable with both

patterning methods, thus indicating the high quality metal-silicon contacts formed by these

inkjet techniques.

Inkjet Printing for Commercial High Efficiency Silicon Solar Cells vi


Acknowledgement

Inkjet Printing for Commercial High Efficiency Silicon Solar Cells vii

Acknowledgement

I would like to express my deepest gratitude to my supervisor Stuart Wenham for placing his

trust in me to undertake this new and exciting project. It has been a truly enriching experience

to work with one of the world’s most experienced photovoltaic specialist. His creativity,

innovation and vision in solar cells are second to none. And most importantly, his carefree

enthusiasm, kind generosity and endless optimism have been real inspirations to me.

I was extremely fortunate to have many great colleagues throughout my candidature. I would

like to especially thank Alison Lennon, whose expertise in inkjet and chemistry has made this

work possible and Budi Tjahjono, who has been the most generous with his time, idea and

advice. Special thanks must also go to Martha Lenio and Nicole Kuepper, two great friends

whose optimism and energy have always made the impossible possible. Many thanks to the

entire First Gen team: Anita Ho-Baillie, Allen Guo, Nino Borojevic, Stanley Wang, Adeline

Sugianto, Ly Mai, Ziv Hameiri, Anahita Karpour and my two thesis students, Andrew Barson

and Daniel Kong for all your contribution to this work. This is, after all, a team effort!

I must also thank the “behind-the-scene” teams: LDOT – Nicholas Shaw, Kian Fong Chin,

Alan Yee, Nancy Sharopeam, Lawrence Soria and Jules Yang – for keeping the lab

operational, School Office – Danny Chen, Trichelle Burns, Lisa Cahill, Shahla Zamani Javid

and Kimberly Edmunds – for solving many of my enrolment issues, and LG team – Jill

Lewis, Mark Silver, and Julie Kwan – for all the administrative help. Thank you also to Mark

Griffin and Tom Puzzer, two great engineers who helped me solve many equipment

problems, and also to the PL team – Yael Augarten, Thorsten Trupke and Robert Bardos.

I am grateful for the financial support from the Australian Government through the provision

of Australian Postgraduate Award and the University of New South Wales through the

Supplementary Engineering Award. I have been extremely privileged to have the opportunity

to work closely with Suntech Power Co. Ltd, whose generosity in providing access to

equipments, transportation and accommodation are very much appreciated. Thank you also to

Zhe Ding, Hongmin Huang and Charles Chen from Honeywell Electronic Materials (China)

for the provision of liquid dopant sources used in this work.

Acknowledgement

Inkjet Printing for Commercial High Efficiency Silicon Solar Cells viii

Finally, I would like to thank my family for being there all the way. To my mum and dad:

thank you for all the unconditional love and support you sent from home. You have worked

extremely hard to give me the greatest inheritance of all: knowledge. I dedicate this work to

both of you because only your proud smile in happiness makes all of this worthwhile. To my

brother: thank you for all the support and good luck with your study. Thank you also to my

aunt who has kindly provided care for me and treated me like her own son during my stay in

Sydney.

Table of Contents

Inkjet Printing for Commercial High Efficiency Silicon Solar Cells ix

Table of Contents

Originality Statement..............................................................................................................iii

Abstract ..................................................................................................................................... v

Acknowledgement ..................................................................................................................vii

Table of Contents .................................................................................................................... ix

Chapter 1 : Introduction.......................................................................................................... 1

1.1 Motivation .................................................................................................................. 1

1.2 Industrial Screen Printed Silicon Solar Cells ............................................................. 3

1.3 High Efficiency Silicon Solar Cells ........................................................................... 5

1.4 Traditional Patterning Techniques ........................................................................... 10

1.5 Thesis Objectives and Outline.................................................................................. 15

Chapter 2 : Review of Inkjet Printing Technologies........................................................... 17

2.1 Background .............................................................................................................. 17

2.2 The Inkjet Mechanism.............................................................................................. 19

2.2.1 Continuous Inkjet (CIJ)................................................................................ 20

2.2.2 Drop-on-demand Inkjet (DOD).................................................................... 21

2.3 The Inkjet System..................................................................................................... 25

2.3.1 Printhead....................................................................................................... 25

2.3.2 Fluid ............................................................................................................. 30

2.3.3 Substrate ....................................................................................................... 34

2.4 Inkjet Applications in Solar Cell Fabrication........................................................... 36

2.5 Summary .................................................................................................................. 38

Chapter 3 : Dielectric Patterning Using Inkjet ................................................................... 39

3.1 Introduction .............................................................................................................. 39

3.2 Fluid Selection and Formulation .............................................................................. 39

3.2.1 Fluid-Printhead Compatibility Evaluation ................................................... 40

3.2.2 Effects of Fluid on Substrate Surface........................................................... 42

3.2.3 Fluid Jetting Optimisation............................................................................ 49

Table of Contents

Inkjet Printing for Commercial High Efficiency Silicon Solar Cells x

3.3 Process Development and Characterisation ............................................................. 55

3.3.1 Formation of Hole and Line Patterns ........................................................... 59

3.3.2 Effects of Resin Thickness on Feature Sizes ............................................... 60

3.3.3 Effects of Drop Spacing on Feature Sizes.................................................... 63

3.4 Summary .................................................................................................................. 65

Chapter 4 : Device Fabrication Using Inkjet....................................................................... 67

4.1 Introduction .............................................................................................................. 67

4.2 Inkjet Printed High Efficiency Solar Cell Structures ............................................... 68

4.2.1 Selective Emitter .......................................................................................... 68

4.2.2 Localised Contacts ....................................................................................... 71

4.2.3 Surface Texturing and Sculpturing .............................................................. 74

4.2.4 Edge Isolation............................................................................................... 77

4.3 Simple Selective Emitter Solar Cell Using Inkjet .................................................... 80

4.3.1 Device Design .............................................................................................. 80

4.3.2 Device Fabrication ....................................................................................... 83

4.3.3 Device Characterisation ............................................................................... 85

4.4 Summary .................................................................................................................. 88

Chapter 5 : Inkjet Printing of Liquid Dopant Source ........................................................ 89

5.1 Introduction .............................................................................................................. 89

5.2 Direct Inkjet Patterning Concepts ............................................................................ 89

5.2.1 Localised Doping on Silicon Surfaces ......................................................... 89

5.2.2 Simultaneous Patterning and Doping on Dielectric Surfaces ...................... 93

5.3 Jettable Liquid Dopant Sources and Surface Treatment .......................................... 95

5.3.1 Phosphoric Acid-based Liquid Dopant Sources........................................... 95

5.3.2 Specially-formulated Liquid Dopant Sources .............................................. 99

5.3.3 Surface Preparations and Characterisation................................................. 102

5.4 Device Processing Using Direct Inkjet Printing .................................................... 106

5.4.1 Process Development ................................................................................. 106

5.4.2 Device Concept and Fabrication ................................................................ 114

5.4.3 Device Characterisation ............................................................................. 115

5.5 Summary ................................................................................................................ 119

Chapter 6 : Conclusion ........................................................................................................ 120

Table of Contents

Inkjet Printing for Commercial High Efficiency Silicon Solar Cells xi

6.1 Original Contributions............................................................................................ 121

6.2 Future Work ........................................................................................................... 122

References ............................................................................................................................. 125

Appendix A: List of Abbreviations..................................................................................... 138

Appendix B: List of Symbols............................................................................................... 140

Appendix C: List of Printhead Components ..................................................................... 142

Appendix D: Fluid-Printhead Compatibility Test ............................................................ 145

Appendix E: PC1D Modelling Parameters........................................................................ 146

Appendix F: General Approach of Liquid Dopant Formulation .................................... 148

Appendix G: Sheet Resistivity Correction Factor............................................................. 150

Appendix H: List of Author’s Publications ....................................................................... 152

Inkjet Printing for Commercial High Efficiency Silicon Solar Cells xii


Chapter 1: Introduction

Inkjet Printing for Commercial High Efficiency Silicon Solar Cells 1

Chapter 1 : Introduction

1.1 Motivation

For the past two hundred years, global economic growth has been fuelled almost exclusively

by conventional energy sources such as oil, coal and natural gas. Today, about 81% of the

global primary energy supply still comes from these three dominant fossil fuel sources (IEA

2008). Unfortunately, fossil fuels are finite energy sources that are rapidly depleting. The

constant increase in worldwide energy demand continues to outpace the discovery rate of new

reserves and the natural replenishment rate of these resources. Furthermore, greenhouse gas

emissions from the use of fossil fuel have been widely agreed as one of the major contributor

to man-made climate change. Considering these challenges, a more sustainable source of

energy is required as a long term solution to the world’s energy needs.

Electricity is one of the fastest growing end-use of primary energy. According to the

International Energy Agency, about 67% of global electricity generation in 2006 was still

derived from fossil fuel sources (IEA 2008). Almost two-thirds of this electricity was

generated from burning coal, which is one of the most greenhouse gas intensive fuels. In

order to satisfy the ever increasing demand of electricity in a sustainable manner, electricity

generation method needs to gradually shift from polluting fossil fuels to clean renewable fuel

sources.

One of the most attractive renewable electricity generation methods is solar photovoltaics

(PV) technology. The popularity of this technology has been bolstered by recent efforts of

many countries to set up a viable commercial market for solar generated electricity through a

range of government incentives. Consequently, there is currently a very strong demand for

solar cells, which are the building blocks of any solar power products.

Solar cells are devices that directly convert sunlight into electricity. The electricity generated

from solar cells is both clean and renewable because the generation process produces zero

waste and its energy source, sunlight, is virtually inexhaustible. Unlike other methods of

electricity generation, solar cell operation does not involve any moving parts, thus making it a



silent and an exceptionally durable system. In practice, many solar cells are usually combined

to form solar modules resulting in a mode of application that is modular and distributed.

Furthermore, solar modules can be well-integrated to the built environment thereby allowing

generation of electricity at the point of use with efficient utility of existing spaces.

Despite having many positive attributes, solar electricity is still more expensive than

conventional electricity in most parts of the world. Although sunlight is readily available for

immediate use at no cost, the solar cells used to convert it into electricity are relatively costly

to produce. One of the biggest cost factors in solar cell production is the raw material. Most

modern solar cells are solid-state devices which are based on the use of a variety of

semiconductor materials. Currently, silicon (Si) – in the form of thin wafers- is, by far, the

most popular semiconductor material used in commercial solar cell production. While Si is

the second most abundant element on earth’s crust (Lide 2008), they mostly occur naturally in

the form of quartz. Solar cell fabrication requires Si with extremely high purity as starting

material and unfortunately, the purification of Si is a very expensive process.

Two other important cost factors in solar cell production are cell conversion efficiency and

manufacturing technology. The economics of solar cell is determined by the amount of

electric power it can generate per unit production cost. The amount of electric power a solar

cell can generate from received sunlight depends on its conversion efficiency. Ideally, a solar

cell is designed to achieve the highest conversion efficiency possible. However, higher

efficiency designs are usually followed by an increase in fabrication complexity and

manufacturing cost. Therefore, improvements in manufacturing technology which can

produce higher efficiency solar cells without an increase in production cost are desirable.

From a technical point of view, further reductions in the cost of solar electricity can only be

achieved by taking into consideration these three key cost factors: raw material,

manufacturing technology and cell conversion efficiency. The present trend in solar cell

manufacturing is moving towards the use of thinner Si wafers and the development of higher

conversion efficiency solar cell designs. In order to realise both objectives in a large-scale

production environment, a suitable manufacturing technology that can produce high

efficiency structures and handle thin Si wafers without compromising production yield also

needs to be developed.



1.2 Industrial Screen Printed Silicon Solar Cells

Currently, the industrial production of solar cells is dominated by the screen-printed solar cell

technology. The success of this technology within the present solar industry stems from the

fact that it can achieve reasonably good conversion efficiency through simple manufacturing

processes that are applicable to both monocrystalline and the lower-cost multicrystalline Si

wafers. The screen-printed solar cell technology was first developed in the 1970s, but since

then has improved significantly in terms of its processing technology and final conversion

efficiency. The wide availability of relatively cheap, standardised manufacturing equipments

combined with mature understanding of the technology makes this technology highly suitable

for large-scale production of solar cells.

Almost all screen-printed solar cells in commercial manufacturing uses p-type Czochralski

(CZ) or multicrystalline Si wafers as substrate with general thickness between 210 - 250 μm.

After surface chemical cleaning and random pyramid texturing, the Si wafer surface is

thermally diffused by n-type impurities such as phosphorus in a high temperature furnace to

form the p-n junction. Following an edge isolation process to remove excess phosphorus

diffusion on the edges of the wafer, silicon nitride (SiNx) layer is deposited on the front

surface of the solar cell for both surface passivation and anti-reflection coating (ARC).

Finally, full-surface aluminium (Al) and silver (Ag) finger grid patterns are screen printed on

the rear and front surface of the solar cell respectively, followed by a quick metal co-firing in

a belt furnace to form a back surface field (BSF) and good ohmic contact between both metals

and the Si. A schematic of a typical screen printed solar cell design is shown in Figure 1-1.

Figure 1-1: Schematic of the screen-printed solar cell design (Green 1995).



Due to the simplicity of its design, the conversion efficiency of screen-printed solar cells is

fundamentally limited to around 17.0% for monocrystalline CZ Si wafers and 16.0% for

multicrystalline Si wafers. One of the main weaknesses of the screen-printed solar cell lies in

the front surface design. In order to let maximum amount of light into the solar cell, the metal

finger grid on the front surface must be designed to cover the smallest area possible. To retain

sufficient reliability and yield in the screen printing process, the minimum width of these

metal fingers are generally limited to around 120 μm. Therefore, it is necessary for the wide

metal fingers to be separated far enough, usually around 3 mm apart, so that light can enter

the solar cell without too much shading loss.

Such widely separated metal fingers also necessitate higher lateral conductance on the top

surface of the solar cell so that current can laterally flow with minimal resistive loss to the

metal fingers. Consequently, the emitter layer of screen-printed solar cells normally has to be

diffused fairly heavily to achieve a sheet resistance of around 40 - 50 Ω/. As a result of the

heavily diffused emitter, minority carrier lifetime within this layer is significantly reduced

resulting in low spectral response for the strongly-absorbed short-wavelength photons. These

constrains translate to reduced voltage, current and fill factor of the device which result in

limited maximum conversion efficiency achievable by the screen-printed solar cell design.

As the trend towards the use of thinner Si wafers in manufacturing continues, another

impending limitation is the screen printing process itself. Screen printing is a contact process

that subjects the Si wafer to significant mechanical pressure. Thinner Si wafers will be more

fragile and have less tolerance to such pressure, resulting in increased breakage rate. In

addition, the production cost of solar cells strongly correlates to the number of solar cells

produced rather than the size of each individual solar cell. Therefore, the problem with

reduced mechanical strength in thin Si wafers is further exacerbated by the tendency for

manufacturers to use larger area Si wafers.

It is clear that further cost reductions in solar cell production must come from cell designs

with conversion efficiency higher than screen-printed solar cells without incurring significant

cost increase. Many high efficiency solar cell concepts have been previously demonstrated in

the laboratory and several of them have been successfully adapted to large-scale production.



1.3 High Efficiency Silicon Solar Cells

The current record for the highest confirmed efficiency of crystalline Si solar cell is 25.0%

held by the University of New South Wales, Australia using the Passivated Emitter and Rear

Locally-diffused (PERL) cell technology (Green et al. 2009). A schematic of the PERL cell

design is shown in Figure 1-2.

Figure 1-2: Schematic of the Passivated Emitter Rear Locally-diffused (PERL)

solar cell design (Zhao et al. 1997).

The PERL cell design incorporates many high-efficiency solar cell structures. The front

surface of the solar cell is textured using inverted-pyramid structures and covered with

double-layer ARC resulting in extremely low front surface reflection. The front metal finger

grids are precisely defined by photolithography to be very thin as to minimise metal shading

loss. Both inverted-pyramid texturing and fine metal fingers significantly reduce the optical

losses which translate to higher current for the solar cell. The PERL cell also features highly-

defined selective emitter structure that minimise both contact resistance and contact area

recombination in heavily diffused regions underneath the metal contacts while the rest of the

top surface is lightly diffused to maintain excellent spectral response on the short-wavelength

photons. Combined with excellent oxide-based passivation on both surfaces of the solar cell,

chlorine-based high lifetime processing and the use of high quality float zone (FZ) Si wafer,

the voltage of the PERL cell is also appreciably increased. The higher voltage and current of

PERL solar cell design together with very high fill factor result in approximately an 8%

absolute improvement in efficiency compared to standard screen-printed solar cells.



Despite the high conversion efficiency, the materials and processes required to fabricate the

PERL cell is prohibitively expensive and complicated. PERL cell uses high-quality costly FZ

Si wafers as the substrate material. The fabrication of high efficiency structures such as

inverted-pyramid texturing, highly-defined selective emitter, high-quality SiO2 passivation,

rear locally diffused passivation and point contacting all require multiple long high

temperature and photolithographic-based patterning processes which are both low-throughput,

low-yield and expensive. Furthermore, the vacuum evaporation method of applying ARC and

metal contact in PERL cell fabrication is slow and unsuitable for large-scale production.

A low-cost adaptation of the PERL cell is the Buried Contact (BC) solar cell technology.

While many variants of BC cell design exist, the single-sided BC solar cell design is one that

has been proven suitable for large-scale production with conversion efficiencies of up to

18.3% demonstrated at BP Solar pilot line (Bruton et al. 2003). The single-sided BC cell

design retains many high efficiency features of the PERL cell, especially on the front surface

such as selective emitter structure, isolation of metal-semiconductor contact regions through

heavy diffusions, textured surface, good front surface passivation and the use of narrow but

deep conductive metal fingers. The required patterning processes are substantially simplified

by replacing photolithographic processes with laser scribing. A schematic of the single-sided

BC cell design is shown in Figure 1-3.

Figure 1-3: Schematic of the single-sided Buried Contact (BC) solar cell design

(Green 1995).



Further simplification of the BC technology is realised through Laser Doped (LD) solar cell

technology. The main advantage of LD technology is the simultaneous dielectric patterning

and contact region diffusion achieved through laser melting process. Such process

combination saves significant processing time and equipment requirements compared to the

separate process needed in the fabrication of BC solar cell. As a result, the LD technology is

much more attractive for large-scale manufacturing of high efficiency Si solar cell than the

BC technology. A number of solar cell manufacturers have implemented the LD technology

in pilot line production with demonstrated efficiencies of up to 17.5% (Tjahjono et al. 2007).

Besides PERL cell-based solar cell structures, there are other high efficiency Si solar cell

designs such as the backside point-contact solar cell fabricated at Stanford University with

highest confirmed efficiency of 22.7% (Sinton and Swanson 1990). This solar cell design was

originally developed for concentrator cell applications. A schematic of the backside point-

contact solar cell is shown in Figure 1-4.

Figure 1-4: Schematic of the backside point-contact solar cell design (McIntosh

et al. 2003).

Similar to the PERL cell, the backside point-contact solar cell also uses high-quality

expensive FZ Si wafer as the starting material. The use of FZ wafer is even more crucial for

this particular cell design since the collecting junction is located at the rear side of the solar

cell. This requires either higher lifetime material or thinner Si wafers in order for most

carriers generated on the front surface of the solar cell to travel to the rear to be collected by

the junction. The absence of metal grid on the textured and AR-coated front surface

significantly boosts the current of the backside point-contact solar cell. Well-defined thin

interdigitated rear metal interconnection combined with heavy diffusions underneath each



polarity of metal contact ensures minimal series resistance. Finally excellent passivation on

both surfaces, the use of front surface field to shield carriers from high front surface

recombination and isolation of high recombination metal-semiconductor contact regions with

heavy diffusions give sufficiently high voltage for this solar cell design. Despite utilising

considerably different design approach from the PERL cell structure, many of the

fundamental high efficiency features that require photolithographic patterning are similarly

present on both PERL and backside point-contact solar cell designs.

In recent years, SunPower Corporation has been commercialising the backside point-contact

solar cell design and putting the technology into large-scale production (Mulligan et al. 2004).

A number of process simplifications were implemented, particularly in the replacement of

photolithographic patterning with screen printing processes. This switch originally resulted in

substantial increase in feature sizes from 5 to 200 μm with a reported corresponding decrease

of 1.5% absolute in average efficiency. Through further optimisation of design parameters

such as diffusion width, metal finger pitch and reduction in cell thickness, SunPower has

reported that large-area backside point-contact solar cells with efficiency of around 22% using

FZ Si wafers should now be achievable in large-scale production (De Ceuster et al. 2007).

Another notable high efficiency Si solar cell design is the Heterojunction with Intrinsic Thin-

layer (HIT) cell developed by Sanyo with the latest highest confirmed cell efficiency of

22.3% (Taira et al. 2007). A schematic of the HIT solar cell structure is shown in Figure 1-5.

Figure 1-5: Schematic of the Heterojunction with Intrinsic Thin-layer (HIT)

solar cell design (Taguchi et al. 2000).



Unlike other previously discussed high efficiency solar cell designs, the HIT cell attains its

high efficiency using large-area n-type CZ Si wafer, which is of lower quality than FZ Si

wafers. The p-n junction in HIT cell is formed rather unconventionally by depositing a thin p-

type amorphous Si (a-Si) onto the n-type CZ Si substrate with another thin layer of n-type a-

Si deposited at the rear of the substrate. Excellent surface passivation and p-n junction quality

are achieved by inserting a thin layer of intrinsic a-Si layer in between both the p-type or n-

type a-Si and the n-type CZ Si wafer. All depositions of a-Si layers are done using Plasma

Enhanced Chemical Vapour Deposition (PECVD) at temperatures below 200 oC.

The unique design of HIT cell results in exceptionally high voltage due to superior surface

passivation properties. Surface lateral conductivity for current collection is achieved through

sputtering of transparent conducting oxide (TCO). Screen printing of silver paste is used to

form higher aspect ratio metal grids on both surfaces. Through the use of n-type Si wafer and

low temperature processing throughout its entire fabrication steps, high minority carrier

lifetime is retained resulting in significantly higher current of the device. The HIT solar cell

technology is commercialised by Sanyo and is currently in large-scale production.

A summary of typical performance parameters for the range of high efficiency Si solar cell

technologies described in this section in comparison with the dominant screen-printed solar

cell technology is presented in Table 1-1.

Cell Technology Voc

(mV)

Jsc

(mA/cm2)

FF

(%)

η

(%) Reference

Screen-printed 619 34.9 76.0 16.5 (Tjahjono et al. 2007)

PERL 696 42.0 83.6 24.4 (Zhao et al. 1998)

Buried contact 625 36.3 80.6 18.3 (Bruton et al. 2003)

Laser-doped 629 37.1 75.0 17.5 (Tjahjono et al. 2007)

Backside point-contact 680 40.8 79.7 22.1 (Mulligan et al. 2006)

HIT 725 38.9 79.1 22.3 (Taira et al. 2007)

Table 1-1: Comparison of typical performance parameters for different high

efficiency solar cell technologies and commercial screen-printed solar cell

technology.



1.4 Traditional Patterning Techniques

Patterning is a key processing technology that is commonly used in both commercial screen-

printed solar cells and high efficiency solar cell designs. The patterning process is usually

employed to selectively deposit functional materials such as dopant or metal on the substrate

surface. Similar to most semiconductor device fabrication, solar cell device performance

strongly correlates with precision in the patterning process. For this reason, photolithography

is the patterning method of choice in the fabrication of the highest efficiency solar cells. A

diagram depicting the photolithographic patterning process flow is shown in Figure 1-6.

Figure 1-6: Diagram of photolithographic patterning process flow (Sze 2002).



Photolithography is basically a pattern transfer process. As shown in Figure 1-6, the substrate

is first coated with a light-sensitive liquid known as photoresist and then dried (Step 1). Then,

the photoresist is exposed to ultraviolet (UV) light through a mask containing the desired

pattern (Step 2). Exposure to UV light changes the chemical properties of the photoresist.

Depending on the type of photoresist used, the exposed regions either become removable

(positive photoresist) or hardened (negative photoresist). After exposure, the substrate is then

immersed in a developer solution where either the exposed (for positive photoresist) or the

unexposed (for negative photoresist) areas are removed to reveal the underlying dielectric

layer (Step 3). An appropriate etchant is then used to etch the exposed dielectric layer (Step

4). Finally, the excess photoresist layer is removed from the surface (Step 5).

The main advantage of photolithography in solar cell fabrication is its high resolution

patterning capability. Very fine features can be patterned onto the substrate with excellent

precision and repeatability, therefore allowing localised processing that has been the key in

producing many high efficiency solar cell structures. While photolithography is capable of

patterning sub-micron feature sizes, the requirement for solar cell fabrication does not usually

extend below 1 µm. Moreover, the photolithographic ability to precisely align multiple

patterns is critical for the multiple processing sequences often required in fabricating high

efficiency solar cell devices.

Unfortunately, photolithography is an inherently complicated and expensive process. Each

patterning process involves multiple steps including coating, pre-baking, exposure,

developing, post-baking, etching and photoresist removal. The optical equipments used to

align and expose the photoresist-coated wafer is usually a major capital investment. Different

masks need to be created for each pattern resulting in high upfront cost particularly during

prototyping stage. Coating processes, especially spin coating, results in large wastage as only

a small portion of the deposited photoresist stays on the wafer. Furthermore, due to the

subtractive nature of photolithographic processes, virtually all of the deposited photoresist is

wasted upon removal. Photolithography is also a low-throughput process that requires clean-

room environment and it is not suitable for textured surfaces. While acceptable for use in

laboratory environment where high efficiency is the primary objective, photolithography is

not suitable for mass production of solar cells due to its very high fabrication cost per wafer.



Laser is becoming an increasingly important patterning tool in high efficiency solar cell

fabrication. Abbott (2006) and Grohe et al. (2007) provide comprehensive review on the

various ways laser patterning can be used to achieve high efficiency Si solar cell structures.

Some of these applications include laser cutting, scribing, drilling, alloying and doping which

can be applied to a range of relevant materials such as silicon, dielectric and metal layers.

Laser patterning usually involves the application of a narrow monochromatic energised beam

onto a specific region on the substrate that results in either surface ablation or surface melting.

A particularly attractive feature of laser patterning is its ability to form very small features on

the Si wafer surface. Although the achievable minimum feature size is still significantly larger

than photolithography, lasers can easily achieve feature sizes ranging from 10 - 40 μm, which

is sufficient for solar cell processing. These characteristics make lasers very useful for solar

cell processes such as surface sculpting and texturing, dielectric patterning, edge isolation,

selective doping and other localised heat treatments. Moreover, laser patterning is a simple,

low-cost, non-contact process with considerably higher throughput than photolithography.

One major drawback of laser processing for solar cells is the resulting crystallographic

damage on laser-processed regions, which can be detrimental to solar cell performance. Laser

ablation, commonly used in scribing and drilling to form structures such as buried contact or

emitter wrap-through (EWT) has been shown to cause lattice dislocations and deep crystal

defects (Chan 1993). In addition, the laser melting process, commonly used for alloying or

doping, causes significant crystallographic stress due to the rapid heating and cooling

experienced in the processed region (Sugianto et al. 2007).

Some of these laser-induced defects can also propagate further into the Si wafer when

subjected to subsequent long, high temperature processes (Wenham et al. 1997). While such

laser damage could be beneficial for lifetime improvement through gettering (Hayafuji et al.

1981), the resulting defects can cause unwanted shunting effects on some solar cell structures

as demonstrated by Guo et al. (2006) and Sugianto et al. (2007). Another potential

disadvantage of laser patterning is the cost of production-scale laser equipments. Despite

being less expensive than photolithographic equipments, the cost of large-scale laser systems

that are capable of sufficiently high throughput for mass production could still be substantial.



Screen printing is currently the lowest-cost, highest-throughput option for solar cell

patterning. It is used widely in solar cell manufacturing mainly for the patterning of front

surface metal contact grids and rear surface metal in commercial screen-printed solar cells.

This method is also used in SunPower’s commercial backside point-contact solar cell as a

replacement for photolithography in defining the contact regions (Mulligan et al. 2004). A

typical screen printing setup consists of a screen frame with stainless steel mesh and pre-

patterned emulsion, a squeegee system and a thixotropic paste. The operation is schematically

depicted in Figure 1-7.

Figure 1-7: Diagram of the operation of a typical screen printing system.

The main disadvantage of screen printing is its inability to produce very fine feature sizes.

The stainless steel mesh diameter and opening are critical parameters that determine the

feature size and the amount of materials that can be deposited onto the substrate. Generally,

smaller mesh diameter produces thinner line width. But it is also more fragile and costly to

produce. To keep cost to a minimum while maintaining quality and reliability, the minimum

screen-printable line width is usually limited to around 120 μm for the typical high volume

solar cell production environment. Finally, as mentioned before, screen printing is a contact

process which applies pressure on the fragile Si wafers, making it an increasingly undesirable

process for thinner Si wafers. Nevertheless, despite the shortcomings of screen printing, this

process is still the preferred patterning method in most manufacturing settings.

Besides photolithography, laser and screen printing, there are other less commonly used

patterning process such as mechanical scribing and masked patterning methods such as

plasma etching or thermal evaporation. Mechanical scribing is a contact process where a

diamond-tipped point or rotating blade is used to scribe the surface of the substrate. While it

is a high-throughput and low-cost method of patterning, it also tends to damage the Si surface.

Emulsion

Squeegee

FramePasteMesh

Substrate



Similarly, shadow mask-based patterning such as plasma etching or thermal evaporation is

cumbersome and low-throughput, thus not suitable for large-scale production.

To summarise, Table 1-2 compares five key process parameters of various patterning methods

commonly used in solar cell fabrication that are crucial determinants for the three cost factors

involved in large-scale production of solar cells.

Patterning

Method

Overall

Cost

Minimum

Feature Sizes

Si Crystal

Damage

Process

Throughput

Substrate

Contact Mode

Photolithography High < 1 μm None Low Proximity

Laser Medium ~10 μm Yes Medium Non-contact

Screen printing Low ~150 μm None High Contact

Mech. scribing Low ~35 μm Yes High Contact

Shadow mask Low ~4 μm None Low Proximity

Table 1-2: Comparison of various patterning methods in solar cell fabrication.

As evident from the processing requirements of both industrial screen-printed solar cells and

high efficiency solar cell designs, a patterning method that is suitable for large-scale

production is critical to minimise manufacturing costs. Due to the limitations of screen

printing, the trend towards thinner Si wafers and higher efficiency solar cells necessitate the

development of a suitable substitute patterning method. Ideally, the replacement method

should have the characteristics similar to screen printing as shown in Table 1-2, but capable of

producing much finer feature sizes using a non-contact mode of application.

In this thesis, inkjet printing is proposed as an alternative patterning technology that could

potentially satisfy the requirements set out above. Inkjet printing will be shown to be capable

of producing high efficiency solar cell structures using techniques that cost substantially less

than traditional photolithographic processes. Inkjet is also a non-contact and non-mechanical

process that is suitable for high throughput industrial environment. These attributes makes the

development of inkjet technology very attractive for high efficiency solar cell fabrication.



1.5 Thesis Objectives and Outline

The objective of this thesis is twofold: (1) to develop new patterning techniques using inkjet

printing technology and (2) to demonstrate the applicability of these inkjet patterning

techniques in producing various solar cell structures and devices. The structure of this thesis

is designed to chronologically address these objectives as follows:

Chapter 2 begins with a review of the state-of-the-art inkjet printing technology. Following

brief background information on historical inkjet technology development and its various

traditional applications, the chapter presents a detailed description on how different inkjet

mechanisms operate. An inkjet system is usually composed of three main components: (1)

printhead, (2) fluid, and (3) substrate. The relationship between these three constituents and

their importance in developing an inkjet process will be discussed. Finally, a summary of past

and present applications of inkjet printing in solar cell fabrication is given in order to provide

some perspectives on the relevance of the work presented in this thesis.

Chapter 3 details the development of a novel indirect inkjet patterning technique that can be

used to pattern dielectric layers as a direct replacement for photolithographic processes. It

begins by evaluating different fluids that could be used for the purpose of dielectric patterning

and examining their compatibility with the printhead and the substrate in an inkjet system. An

appropriate fluid is selected and optimised for the subsequent process development. The

entire patterning process flow is given with optimisations of various critical parameters in

order to achieve the smallest feature sizes possible.

The main purpose of the work described in Chapter 4 is to apply the indirect inkjet patterning

method developed in Chapter 3 to a working solar cell device. Firstly, the inkjet patterning

technique is shown to be a viable alternative for the fabrication of various high efficiency

solar cell structures that traditionally have been created using photolithography or other

methods, such as the formation of selective emitter structure, localised contacts, surface

sculpting and edge isolation. A simple selective emitter solar cell device is then fabricated as

a tool to demonstrate some of the advantages of using the indirect inkjet patterning method

compared to other methods such as laser patterning or photolithography.



Chapter 5 proposes two new concepts of using inkjet printing to selectively deposit liquid

dopant sources as a way to perform direct inkjet patterning on both Si and dielectric surfaces.

Due to the unavailability of suitable fluids, two types of solutions are formulated and

developed for inkjet printing use. In order to prepare the surfaces, several methods of surface

treatment are also developed and discussed. After the completion of the inkjet system

development, a novel direct inkjet patterning process is created using direct deposition of

liquid phosphorus dopant source. Finally, the inkjet patterning process is applied to a solar

cell device fabrication with the aim of identifying the limiting issues and concerns of the

process.

Finally, Chapter 6 summarises the important results of the study and analysis performed in

this thesis. Due to the novelty of the work presented in this thesis, there are numerous

opportunities to further develop the various inkjet patterning methods and innovative device

structures described here. Chapter 6 also proposes several suggestions on the possible

roadmap for future work on these topics.

Chapter 2: Review of Inkjet Printing Technologies


Chapter 2 : Review of Inkjet Printing Technologies

2.1 Background

In the past several decades, inkjet technology has revolutionised the printing industry by

providing a new low-cost and flexible method of printing. So far, inkjet technology has been

most successfully implemented in the field of graphic imaging. Recently, there has been an

increasing interest in the emerging area of inkjet material deposition. In this application,

functional fluids can be inkjetted onto various surfaces and substrates. Some examples of

functional fluids include metal inks, conductive polymers, surface coating, proteins and

nanoparticles. The versatility of inkjet is highlighted in a wide range of applications in areas

such as electronics, display, chemical, mechanical, optical and life sciences.

Inkjet can be broadly defined as a process where individual droplets of liquid are ejected

through an orifice in a controlled manner. From a scientific point of view, Rayleigh (1878)

was the first to describe the mechanisms of droplet formation due to the instability of fluid

jets. Rayleigh’s principle was used in several notable early technical developments of proto-

inkjet devices such as the analog jet writing devices of Schröter (1932) and Hansell (1933)

and the Mingograph by Elmqvist (1951), which was one of the first commercial inkjet

products available. These devices generally lack sufficient control of the jetting process.

Moreover, jetting frequency and resolution capability were relatively limited and design

complexities placed many restrictions on device manufacturability and practicality.

The advent of computers in the early 1960s spurred development of inkjet devices as a likely

candidate for generating physical output of computer data. Pond (2000, p. 9-42) provides a

comprehensive review of the historical development of modern inkjet technologies between

the 1960s and 1990s. A general observation of the development trends during these decades

indicates that many of the early research efforts were mostly directed towards improving

control of droplet generation, increasing printing speed and improving the reliability of inkjet

systems through design simplifications. These advancements helped establish inkjet as the

device of choice in many graphic imaging applications.



As inkjet device technologies mature in recent years, more effort has been directed towards

discovering novel applications of the technology where functional fluids are used instead of

traditional coloured inks. In many of these applications, inkjet technology usually has the

potential to either fabricate new structures that previously cannot be achieved through other

available methods or provide an alternative way of fabricating existing structures with lower

costs. A brief summary of these applications is presented in Table 2-1. The breadth of fluids

that have been deposited using inkjet technology underlines its huge potential for use in solar

cell fabrication.

Field Applications Fluids Examples

Electronics

Memory, RFID

tags, flex/printed

circuits

Metal nanoparticles,

solder, conductive

polymer, liquid silicon

Sirringhaus et al. 2000;

Subramanian et al. 2005;

Shimoda et al. 2006

Display OLED displays,

flexible displays

Nanoparticles,

conductive polymer,

light-emitting dye,

Hebner et al. 1998;

Shimoda et al. 2003;

Arias et al. 2007

Life science

Cell sorting, drug

delivery, tissue

engineering

Proteins (collagen),

DNA, antibodies,

bacteria

Roth et al. 2004;

Sanjana and Fuller 2004;

Henmi et al. 2008

Chemistry

pH sensors,

combinatorial

chemistry, coatings

Photopolymerisable

liquids, polymers,

solvents

Tekin et al. 2004;

Carter et al. 2005;

Morita et al. 2005

MEMS

Actuators, sensors,

cantilevers, micro-

fluidic systems

Metal nanoparticles,

colloids, thiols, solvents

Fuller et al. 2002;

Cooley et al. 2002;

Bietsch et al. 2004

Optics Microlenses,

optical array,

Solvent, epoxy resin,

silica, monomers,

prepolymers

Biehl et al. 1998;

Cox and Chen 2001;

Bonaccurso et al. 2005;

Table 2-1: Emerging inkjet applications in various fields, including the range of

typical fluids used in material deposition technologies.



The inkjet process possesses many attractive attributes. Firstly, it is a non-contact process,

which is a significant advantage compared to other contact processes. It can be applied to

almost any substrate regardless of its composition, morphology, thickness or other properties.

The deposited droplet sizes are very small, usually in the order of picolitres, with high

precision and reliability. For direct inkjet processes, materials are additively deposited on the

substrate only where required. It produces virtually no material waste and simplifies processes

by reducing the number of process steps. Due to its digital nature, fixed production costs

(such as screens and masks) are eliminated. As a result, the running cost of inkjet processes is

generally lower than other methods. This is particularly beneficial for processes that require

smaller sample runs such as prototyping. Finally, inkjet systems typically require lower

capital expenditure compared with other material deposition equipments.

2.2 The Inkjet Mechanism

By fundamental design and functionality, inkjet technology is usually classified into two

types: continuous inkjet (CIJ) and drop-on-demand (DOD) inkjet. Figure 2-1 depicts the

many variations of both CIJ and DOD mechanisms of depositing droplets.

Figure 2-1: Inkjet technology map adapted from Le (1998).

INKJET TECHNOLOGY

Continuous

Binary Deflection

Multiple Deflection

Hertz Microdot

Drop-on-demand

Thermal

Roof-shooter

Side-shooter

Piezo-electric

Squeeze mode

Bend mode

Push mode

Shear mode

Electro-static

Acoustic



2.2.1 Continuous Inkjet (CIJ)

As the name suggests, the operation of CIJ depends on the continuous generation of ink

stream from a pressurised fluid reservoir. When the fluid stream reaches a certain critical

length, surface tension forces break up the stream into individual droplets. Rayleigh (1878)

theoretically explained this phenomenon by calculating that such break-up occurs when the

stream length is longer than its circumference due to the instability of the jet stream. In order

to synchronise the droplets, a continuous acoustic waveform signal is introduced onto the

fluid stream by means of a transducer or a resonator. By changing the velocity of the stream

and frequency of the wave signal, the size, volume and distances between droplets can be

accurately controlled.

The ejected droplets then pass through a pair of electrodes where the droplets may be charged.

A high voltage deflection plate is then used to steer the charged droplets to the desired

position either on the substrate or onto a gutter where the unused ink is then recycled back

into the reservoir. Figure 2-2 shows the schematic of a typical continuous inkjet setup.

Figure 2-2: Design schematic of a typical continuous inkjet setup (Pond 2000).

In general, a CIJ setup can consists of either single or multiple jet streams, mostly depending

on the printing speed requirement for the particular process. CIJ setups can also be

differentiated by the deflection method used. Binary deflection identifies the droplets as only



either charged or uncharged which determines their placement on the substrate or the gutter

respectively. More control of droplet placement can be achieved through multiple deflection,

which identifies the droplets by their level of charging, thus allowing more flexibility on

droplet trajectory.

The main advantage of CIJ is its superior drop generation capability compared to other inkjet

setups resulting in very fast printing speed. Due to the high velocity of the generated droplets,

the inkjet head may be separated at relatively long distance from the substrate. Since the

waveform signal can be remotely delivered, a wide range of fluid can be used with CIJ as

long as proper material is used for the reservoir and fluid path. The nozzles in CIJ operation is

usually less prone to clogging because they are always in continuous use.

One of the major technical limitations of CIJ is the fact that the fluid used needs to be charged

to control its placement on the substrate. The ink handling system is another possible weak

point of the technology, particularly when highly sensitive fluid or extra cleanliness is

required. Due to its complexity, there is a real reliability concern with the long-term use of

CIJ as well as high costs associated with the system. More importantly for material deposition

purposes, CIJ might not provide sufficient drop precision that is required in many critical

applications.

2.2.2 Drop-on-demand Inkjet (DOD)

Most applications of concern to this thesis work would require the precision and versatility of

a DOD inkjet setup whereby individual droplets are only generated and deposited as required.

Using DOD, each droplet is individually formed and ejected through the same mechanism.

This is an important difference from CIJ setup where the applied pressure ejects the fluid

continuously while the individual droplets are controlled by a separate waveform signal. The

DOD concept greatly simplifies the design of the inkjet head because such integration of

droplet generation and ejection mechanism in DOD avoids the need for yet another separate

method of directing the trajectory of the droplets. In other words, the DOD technology

combines three functions present in CIJ design (droplet generation, droplet ejection and

droplet placement) into one step.



The simplified design of a DOD inkjet head has many advantages. Firstly, fluid use is greatly

conserved since the fluids are only deposited where required and there is a much lower

minimum fluid level required for DOD’s inkjet reservoir. This eliminates the need for a

separate ink recycling system and is particularly attractive for many material deposition

applications where the fluid itself might be expensive or difficult to make. A greater packing

density of multiple printheads is also possible since DOD inkjet heads are usually much

smaller than CIJ setups. Another important advantage is the increased deposition accuracy

which comes from more integrated control of droplet generation and placement with the

addition of a simpler binary electronics (i.e. “print” or “no print”). These factors results in

significantly lower costs and increased reliability for DOD inkjet compared to CIJ.

The two most common types of DOD inkjet head are the thermal inkjet (or “bubble-jet”) and

piezoelectric inkjet. The main differentiating feature between the two types is the way the

droplets are generated. In a thermal inkjet device, a heating element such as a resistor is built

onto the wall of the fluid chambers close to the nozzle orifices and is electrically connected to

a voltage source. As shown in Figure 2-3, there are two designs that are usually employed in

fabricating thermal inkjet printheads based on where the heating element is located: (1) the

roof shooter design, and (2) the edge shooter design. In a roof shooter design, the heating

element sits on top of the nozzle plate parallel to the nozzle orifice, whereas in an edge

shooter design, the heating element is on the side of the fluid chamber perpendicular to the

nozzle orifice.

Figure 2-3: Design schematic of a roof shooter design (left) and edge shooter

design (right) of a thermal inkjet device (Lee 2003).



During operation, an appropriate voltage pulse is delivered to the resistor which results in

heating. The heat then causes the fluid within immediate vicinity of the resistor to boil and

vaporise. This small volume of superheated fluid creates an expanding bubble (hence the term

“bubble-jet”) which pushes the entire fluid volume and ejects the fluid closest to the nozzle

orifice out. When the voltage pulse is switched off, the fluid inside the chamber then begins to

cool and the vapour condenses back into liquid form. This is a very important action since it

initiates: (1) fluid chamber refill, and (2) droplet formation. Once the fluid chamber is refilled

and the ejected droplet has separated from the nozzle, the device returns to its original state

and is ready to jet the next droplet. Surprisingly, this cycle can be reproduced in a reliable

manner for a very large number of times.

It is quite clear that the jetting speed in thermal inkjet devices is limited by the time required

to complete a droplet generation cycle. One thing to be aware of is the fact that this cycle

correlates strongly with the ejected droplet volume. In general, the smaller the drop volume,

the faster it is for the fluid chamber to refill since there is less heating required and less

volume to be replaced. For a 100 pL droplet, a typical cycle consists of 10 μs of bubble

formation and 90 μs of fluid chamber refill time resulting in a jetting frequency of about 10

kHz. This is significantly slower compared to continuous inkjet devices which can jet fluids

in the hundreds of kHz to MHz range. Furthermore, due to its much slower droplet velocity,

the separation distance between a thermal inkjet head and the substrate needs to be small (~1

mm) in order to achieve sufficient precision in droplet placement. Note that these limitations

also generally apply to piezoelectric inkjet devices since they possess similar jetting

characteristics with thermal inkjet devices.

For material deposition purposes, one of the major limitations of thermal inkjet device is the

need to heat and vaporise the fluid (albeit only a very small volume). In many instances, the

effect of heat to the fluid properties, composition and functionality can be quite detrimental

and undesirable. For example, many biological agents are very susceptible to heat, which

could render it unusable. Another issue worth considering is the suitability of the wide range

of solvents used to dissolve and carry many functional fluids because thermal inkjet relies on

bubble formation and vaporisation to eject droplets. For these reasons, piezoelectric inkjet

heads are usually the device of choice for material deposition applications.



In a piezoelectric inkjet device, a piezoelectric material is used instead of a heating element to

produce the force necessary to eject a droplet. Piezoelectric materials deform when a voltage

is applied onto them. In an inkjet head, this deformation can be used to displace volume in a

fluid chamber for droplet ejection. Unlike thermal inkjet, the process is purely mechanical and

therefore does not present a hazard to the fluid chemistry. By far, lead zirconate titanate (PZT)

ceramic is the most common piezoelectric material used in the fabrication of piezoelectric

inkjet head. Piezoelectric inkjet operation can be categorized into four types, based on the

PZT deformation mode. In squeeze mode (Figure 2-4(a)) the PZT crystal surrounds the fluid

chamber and “squeezes” it as voltage is applied. In shear mode (Figure 2-4(b)), the applied

voltage is perpendicular to the polarisation of the PZT material. On the other hand, in both

push mode (Figure 2-4(c)) and bend mode (Figure 2-4(d)) the applied voltage is parallel to the

polarisation of the PZT material.

Figure 2-4: Design schematic of a squeeze mode (figure a), shear mode (figure

b), push mode (figure c) and bend mode (figure d) of a piezoelectric inkjet

device - adapted from Brünahl (2003).

(a) squeeze mode (b) shear mode

(c) push mode (d) bend mode



Due to the use of piezoelectric material as the actuator, the typical piezoelectric inkjet device

design is more complicated and costly to fabricate than thermal inkjet device. It is also more

difficult to miniaturise piezoelectric material in order to produce the smallest droplet sizes and

greater resolution through higher packing density. Nevertheless, for many applications that

require the use of unconventional fluids, piezoelectric inkjet devices are the only choice

because it provides the widest range of fluid formulation possibilities as well as greater ability

to control the droplet formation process through voltage waveform shaping. For these reasons,

in the majority of this thesis work, only the DOD piezoelectric inkjet device is considered for

use.

2.3 The Inkjet System

To successfully develop a new inkjet process, it is important to consider the three key

components of an inkjet system: (1) printhead, (2) fluid, and (3) substrate (Pond 2000). There

is a strong interdependence between these three factors that determines the viability of an

inkjet process. As stated in the thesis objectives, one of the main aims of this thesis work is to

develop a patterning technique that could replace photolithography in high efficiency solar

cell fabrication. The essence of photolithography is pattern transfer, after which a material can

be selectively deposited onto a substrate. In the inkjet case, the central component to achieve

this is the fluid which acts as a medium of transfer. Therefore, for each specific process, a

suitable fluid must be selected or formulated to satisfy the process requirements. It needs to be

realised that the fluid selection or formulation process must also take into account the fluid-

printhead and fluid-substrate interactions. A review of each of this component as they are

relevant to this thesis work will be presented in the following sections.

2.3.1 Printhead

The main inkjet device used in this thesis work is the Dimatix Materials Printer (DMP) from

FUJIFILM Dimatix Inc. It is an integrated inkjet device consisting of cartridge-based

printheads, a substrate platen and a drop watcher system. The printhead used in the DMP is a

DOD piezoelectric inkjet device with two drop volumes available: 1 pL (DMC-11601) and 10

pL (DMC-11610). A photograph of the DMP machine together with the cartridge-based

printhead is available from Dimatix (2008).



The patent publication by Higginson et al. (2007) discloses the design and construction of the

DMP printer in details. An illustration of the DMP printer design adapted from that patent

document is shown in Figure 2-5.

Figure 2-5: Illustration of the Dimatix DMP machine (Higginson et al. 2007).

In the DMP machine, the substrate is held on the platen by a vacuum and the platen is

movable only in the x direction. The y-axis movement is served by the cartridge mounting

system. It is also possible to adjust the distance between the nozzle plate on the printhead and

the substrate in the z direction. A fiducial camera is attached near the cartridge for alignment

purposes. The positional repeatability of the stage is ±25 μm. The platen can also be heated to

a maximum of 60 oC. A drop watcher camera is integrated into the machine to allow jetting

characterisation through visual checks. The DMC cartridge module has a maximum fluid

capacity of 1.5 mL and is integrated with the printhead module, which can be heated up to 70 oC. The details of its design and construction have been disclosed in Bibl et al. (2006a).

Platen Drop

watcher

Fiducial camera

Cartridge



The DOD piezoelectric printhead used in the DMP machine operates in bend mode, where the

applied voltage bends the PZT crystal to push the fluid along the pumping chamber and ejects

it out of the nozzles. A schematic cross section of this printhead is shown in Figure 2-6.

Figure 2-6: Schematic cross section of the printhead used for the DMP

cartridge (adapted from Bibl et al. 2005).

For optimal printhead operation, the pumping chamber must first be filled with fluid. It is

critical that the space in the pumping chamber is devoid of any air bubbles because these

bubbles act as a damping agent that will absorb the actuating force of the piezoelectric

deformation. To degas the pumping chamber, the printhead usually needs to be primed before

jetting through a series of fluid purging cycles. During operation, the formation of air bubbles

can be avoided through the use of lung vacuum, which is a basically strong negative pressure

applied to the pumping chamber.

Filtering is another important issue that needs particular attention in miniature devices such as

DOD piezoelectric inkjet printheads. The fluid used must be filtered before being delivered

into the pumping chamber. The reason is because the small channels inside the printhead

construction and the small openings of the nozzle orifice can become easily clogged by large

particles and agglomeration of smaller particles. Printhead clogging due to particles are

usually very difficult to rectify and can be permanent. As a preventative measure, another

filter is also built into the printhead used with the DMC cartridge module.

Ink Flow

Nozzle

Filter Piezoelectric crystal

Pumping chamber

Actuator membrane



Besides fluid properties (which will be discussed in the next section), the other major factor in

determining the jetting quality is the driving pulse. Some of the parameters that can be

adjusted for optimisation include:

• Jetting voltage – refers to the amplitude of the piezo driving pulse.

• Jetting frequency – refers to the number of cycle of the piezo driving pulse.

• Jetting waveform – refers to the shape and width of the piezo driving pulse.

A typical driving pulse for the DMC printhead is shown in Figure 2-7. The jetting voltage

usually has the most impact in determining whether or not a droplet will eject at its operating

temperature since it affects the degree of the piezo crystal’s deformation. The jetting voltage

also determines the droplet volume and the velocity at which the droplet is ejected. The

quality of the jetted droplet is most affected by the jetting waveform. As shown in Figure 2-7,

the waveform can be divided into three segments. The slew rate and duration of each segment

can be adjusted to achieve the best jetting quality. The slew rate determines how fast the piezo

crystal bends and the duration determines how long it stays in that position. Segment A and

segment B are usually the most influential to jetting quality. In segment A, the piezo crystal

bends such that it creates a negative pressure in the pumping chamber, thus “sucking” the

fluid towards the piezo plate. In segment B, the piezo crystal is driven the opposite way,

creating a force on the fluid volume inside the pumping chamber to push fluid out. Segment C

then controllably reverses the bend to break off the ejected fluid into individual droplets.

Figure 2-7: Typical driving pulse to control jetting in a DOD piezo inkjet head.

Jetti

ng v

olta

ge

Pulse width

A B C



The jetting frequency then controls how fast the jetting cycle is repeated for a certain period

of time. The higher the jetting frequency, the faster printing speed it can achieve. However, an

increase in frequency usually results in poorer jetting quality, especially for fluids that possess

characteristics further away from the optimal requirements. Since piezo actuation is purely

mechanical, the jetting frequency for DOD piezo inkjet head mostly depends on the time it

takes for the fluid chamber to refill after an ejection event.

Due to the unavailability of the DMP machine with its DMC printheads at that time, some of

the early work for this thesis was performed using other DOD piezo inkjet printheads with

larger nominal droplet volumes from Spectra (former trade name of Dimatix). The two

Spectra printheads used for this thesis work are the Spectra Galaxy PH 256/30 AAA

(“Galaxy”) and the Spectra SX3 (“SX3”). A comparison table between the Galaxy, the SX3

and the DMC printheads are presented in Table 2-2.

Parameter Spectra Galaxy

PH 256/30 AAA Spectra SX3

DMC-11601/

DMC-11610

Number of addressable jets 256 128 16

Nozzle spacing 254 μm 508 μm 254 μm

Nozzle diameter 36 μm 27 μm 9 μm/21.5 μm

Calibrated drop size 28 pL 12 pL 1 pL/10 pL

Adjustment range for drop size 25-30 pL 10-12 pL N/A

Drop size variation 5% <2% 3.5%

Nominal drop velocity 8 m/s 8 m/s 8 m/s

Drop velocity variation 5% ±10% N/A

Operating temperature range Up to 90 oC Up to 70 oC Up to 70 oC

Fluid viscosity range 8-20 cP 10-14 cP 10-12 cP

Compatible jetting fluids Organic solvents, UV curables, aqueous, acidic, basic

Maximum operating frequency 20 kHz 10 kHz 80 kHz

Table 2-2: Product data of the Galaxy, SX3 and DMC printheads (available

from Dimatix 2008)



The Galaxy printhead was originally designed for industrial graphic printing applications,

whereas the SX3 printhead was specifically designed for high precision industrial material

deposition applications. Both printheads have significantly higher number of nozzles than the

DMC printheads and have a separate fluid handling and supply system consisting of remote

and built-in reservoirs connected by tubings. The nozzle plate of the Galaxy printhead is made

of stainless steel, while the nozzle plates of both the SX3 and the DMC printheads are

fabricated on Si substrates. The design and constructions of the Galaxy have been disclosed

by Moynihan et al. (2004), while the SX3 is MEMS-based printhead similar to those used in

the DMC design (Bibl et al. 2006b).

Both the Galaxy and the SX3 printheads are integrated into a motion precision stage provided

by iTi Corporation (2008). In this case, the substrate platen has a positional repeatability of ±2

μm and can be heated up to 75 oC. In contrast with the DMP system, both the x and y-axis

movement in this system is executed by the platen, while the printhead is stationary. Similar

to the DMC, the jetting temperature may be controlled up to 70 oC for both the Galaxy and

the SX3 printheads and the jetting voltage, frequency and waveform can be separately

adjusted.

2.3.2 Fluid

Before any fluid is loaded into a printhead, the first and foremost consideration must be

whether or not the fluid is compatible with the materials of the printhead construction. In

other words, it must be assured that the fluid does not chemically react with any part of the

printhead that comes in contact with the fluid. This includes the fluid supply and handling

system such as cartridges, reservoirs, tubings and filters. For large-scale industrial

applications, it is particularly important to understand the long term effects of the fluid

compatibility to the printhead because this could determine whether or not the process is

viable for such working environment.

In terms of fluid use, there is a relatively narrow range of fluid properties that can be used in

DOD piezoelectric inkjet device. This is because the mechanical force caused by the

piezoelectric crystal deformation on the fluid is usually quite weak. As a result, the typical

fluid must have sufficiently low viscosity to allow ejection. Viscosity refers to the resistance

of a fluid to flow. In this thesis work, all of the fluids used exhibit Newtonian properties,

http://www.iticorp.com/



meaning that the flow rate of the fluid is linearly proportional to the force applied to the fluid.

For such Newtonian fluids, the viscosity μ is simply described as:

γτμ&

= (2.1)

where τ is the shear stress, which is the force per unit area required to produce the fluid flow

and γ& is the shear rate, which is the differential velocity of the fluid flow dxdv . The fluid

viscosity can be easily measured using a viscometer.

As shown in Table 2-2, the ideal fluid viscosity range for the three printheads used in this

thesis work is between 8 and 20 cP. If the fluid viscosity is too high, the fluid may not be able

to be ejected through piezo actuation. On the other hand, if the fluid viscosity is too low, the

fluid may drip off the nozzle when idle and may result in poor jetting quality. Viscosity is

usually adjustable by altering the solvent or additive concentrations in the fluid. However,

caution is required in choosing the additives to ensure the functionality of the fluid is not

affected. Moreover, solvents with high boiling point are preferred to avoid excessive

evaporation that could cause nozzle clogging.

Although low viscosity is desirable for jetting purposes, a higher viscosity fluid is usually

preferable to minimise droplet spreading once the fluid lands on the substrate. This is

particularly important for inkjet applications in solar cell fabrication where small feature sizes

are generally more beneficial for the solar cell performance. Fortunately, in many cases,

slightly higher viscosity fluids can easily be modified for jetting through local printhead

heating and still retain its original viscosity as it is ejected and come into contact with the

substrate. It is clear from this discussion that viscosity is one of the most important

considerations in fluid formulation as it affects whether or not a fluid can be jetted.

Another important factor in the jetting viability of a fluid is the size of the particle content of

the fluid. In general, particle size of less than 1 μm in diameter is desirable for jetting using

DOD piezoelectric inkjet device. Particle size becomes more important as the diameter of the

nozzle decreases (for example in the 9 μm-diameter, 1 pL DMC-11601 printhead used in this



thesis) as it is easier for random group of particles to agglomerate and either clog the nozzles

or cause unreliable jetting. Therefore, it is critical that any particles required to be jetted must

be prepared to sufficiently small size and dispersed and suspended appropriately. It is also

necessary to filter the fluid before loading it into the printhead to remove any particle

contaminants from the fluid.

For most fluids, as long as both viscosity and particle size requirements are met, it should be

possible to jet them using a DOD piezoelectric inkjet device. In addition to these two

properties, surface tension is another important fluid characteristic that must be considered.

Surface tension of a fluid refers to the cohesive internal intermolecular forces within the fluid.

All fluids will naturally try to minimise its surface tension, usually by minimising its surface

area. In a fluid stream, the smallest surface area is usually attained through a spherical shape,

resulting in the characteristic individual round droplets formed when such stream breaks off.

In general the higher the surface tension, the more difficult it is for the fluid to separate from

itself and forms a droplet. However, once a droplet is formed and lands on the substrate, a

high surface tension is desirable since it holds itself well and prevents wetting. If the surface

tension is too low, the fluid might have difficulties in staying together as a droplet and might

separate and form long tails during flight. From jetting perspective, it is also undesirable to

have surface tension that is too low because it increases the possibility of the fluid flooding

the nozzle plate which inhibits good jetting.

For the DOD piezoelectric inkjet devices used in this thesis work, the ideal fluid surface

tension range is around 28-36 mN/m. Surfactants can be used to modify fluid surface tension.

Surface tension can be measured using a number of methods. In this thesis, all the measured

surface tension values were obtained using the Wilhelmy Plate method (Wilhelmy 1863).

More discussions on how surface tension interactions between various phases, particularly

those between the fluid and the substrate will be presented in the next section on substrates.

By ensuring that the fluid formulation results in viscosity, particle content and surface tension

values that are within the ideal range for jetting, a good jetting quality should be achievable

without the need for extensive optimisation of the jetting driving pulse. The aim of any inkjet

process optimisation is to achieve good jetting quality at the highest frequency possible for a



specific fluid formulation. This will ensure fast printing speed without sacrificing the

precision and reliability of the jetting process. A good jetting quality is usually characterised

by:

• No wetting of nozzle plate during ejection,

• Small jetting drop volume variation from droplet to droplet,

• Uniform droplet velocity from droplet to droplet,

• Absence of satellite droplets,

• Short droplet tail length that joins with the droplet head before it reaches the substrate,

• Straight droplet trajectory between the nozzle and the substrate,

• Long term jetting stability, including during on/off cycles.

Jetting quality characterisation is usually performed visually through the aid of a drop watcher

system. Such system typically would consist of a high speed camera pointed to the nozzle

plate with a backlight for clarity. Some examples of images from a drop watcher showing

various jetting quality issues are shown in Figure 2-8.

Figure 2-8: Drop watcher images of a good droplet formation (figure A),

formation of satellite droplets (figure B), random fluid spray (figure C) and

long tail formation due to low fluid surface tension (figure D) – (Lee 2003).

A B

C D



2.3.3 Substrate

The non-contact nature of inkjet deposition makes it a very versatile process that is applicable

to a wide range of substrate choice. In this thesis work, Si wafer is the only bulk substrate

type used, which is generally thin and rigid. More importantly for inkjet deposition is the

interaction between the deposited fluid and the surface of the substrate that will come in

contact with the fluid. Generally, the types of fluid-surface interactions that are commonly

observed involves: (1) a reaction, where the deposited fluid chemically or physically react

with the surface, or (2) a non-reaction, where the deposited fluid only wets the surface. In this

thesis work, both types of interactions are encountered.

In solar cell fabrication, it is desirable to have feature sizes as small as possible. This is why

photolithography, with its capability of forming very small and precise patterns on Si

substrates, is a crucial process in fabricating the highest efficiency Si solar cells. For inkjet

processes, the feature sizes are strongly determined by the spreading of the fluid droplets on

the surface of the substrate. From the fluid perspective, such spreading is controlled by both

the viscosity and surface tension. The primary contributor to spreading in terms of the solid

property is the surface energy of the surface, which is the equivalent of surface tension in a

solid. In contrast to the fluid surface tension, it is desirable for the surface energy to be as low

as possible to minimise spreading. Effectively, the least spreading can be achieved by

maximising the difference between the fluid surface tension and the surface energy.

When considering the fluid-substrate interaction, it is more useful to treat the surface tensions

as part of a system which consists of liquid, solid and vapour interfaces. This is because when

a droplet forms on a surface, three interface tensions forms on the solid-vapour interface, the

solid-liquid interface and the liquid-vapour interface. The concept of this relationship is

illustrated in Figure 2-9.

Figure 2-9: Schematic of droplet wetting on a solid surface creating a contact

angle as described by the Young equation (Holmberg 2002).



These three interfaces will always try to minimise its surface energy by changing its effective

surface area in a process more commonly known as wetting. In a wetting process, the

interface where the three surfaces intersect will form an angle known as the contact angle. For

a perfectly homogenous surface, the equilibrium condition at which a contact angle is formed

can be theoretically predicted by Young equation (Young 1805):

lv

slsve γ

γγθ

−=cos (2-2)

where θe is the contact angle and svγ , slγ , and lvγ are the interfacial tensions of the solid-

vapour, the solid-liquid and the liquid-vapour interfaces respectively.

When the contact angle θe is high (e.g. >90o), the surface is said to exhibit non-wetting

properties. In this case, the droplet shape is round and the contact area between the droplet

and the surface is typically very small. On the other hand, when the contact angle θe is low

(e.g. 0o<θe<90o), the surface is said to exhibit partial wetting and the contact area between the

fluid droplet and the substrate surface increases as the contact angle decreases. Finally, if the

contact angle θe is zero, then the surface wets the surface completely and forms a film layer on

the surface.

For a particular solid, a Zisman plot (Zisman 1964) can be constructed which shows the

relationship between the surface tension of a fluid and the resulting contact angle. From this

plot, it is possible to derive the critical surface tension of the solid, which quantifies the

surface tension value of a fluid that would completely wet the surface. Any fluid with surface

tension higher than the critical surface tension will form a droplet and have a contact angle

greater than 0o. It is clear that the critical surface tension value of the solid to be used is

critical in determining the best fluid formulation to achieve as small feature sizes as possible.

It is usually possible to alter the critical surface tension of a solid surface by applying certain

surface coatings or by chemically (surface fluorination) or physically (surface roughness)

treating the surface. In both reaction and non-reaction interactions, it is desirable to have as

low critical surface tension as possible in order to achieve the smallest initial contact area

possible between the deposited fluid and the surface of the substrate.



2.4 Inkjet Applications in Solar Cell Fabrication

Inkjet printing for Si solar cell processing has been previously investigated as a possible

alternative patterning method. One of the first demonstrated applications was to use inkjet

printing as a substitute for screen printing in applying the front surface metal fingers in

screen-printed Si solar cells. Teng and Vest (1988) developed a jettable Ag-containing

metallo-organic ink and demonstrated its use for front surface metallisation in solar cell. More

recently, similar work has also been pursued by other researchers such as Kaydanova et al.

(2005). Inkjet deposition of metal inks can potentially produce metal line widths as small as

20 µm, which results in significant reduction in shading losses compared to screen printing.

However, the low metal content in the deposited ink and the low aspect ratio of the printed

metal usually limit the conductivity of inkjet printed metal fingers.

To alleviate the conductivity problem, Mette et al. (2007) has recently proposed an alternative

metallisation method based on a combination of printed metal ink and subsequent thickening

of the metal line using light induced plating of copper. In this case, an aerosol jet system was

used to deposit the metal ink instead of an inkjet printer. Nevertheless, the same approach

could theoretically be done with an inkjet printer. Similar plating metallisation method of

nickel and copper has also been applied to other inkjet-structured devices such as those by

Biro et al. (2007) where inkjet is used to deposit masking materials to protect the dielectric

surface from patterning through chemical etching.

Another demonstrated application of inkjet printing for solar cell fabrication is in the

metallisation patterning of crystalline Si on glass (CSG) thin-film solar modules (Green et al.

2004). The inkjet printing process developed for this purpose involves the application of a

low-cost, electrically-insulating resin on the Si surface, followed by inkjet deposition of a

caustic etchant to locally remove the resin in areas where metal contacts are to be made

(Young and Lasswell 2007). Due to the monolithic p+-i-n+ structure, the exposed Si must then

be etched to gain access to the buried n+ layer. After a resin reflow process to avoid shunting,

the inkjet process is repeated to form contacts to the p+ layer. Finally, Al metal is sputtered

onto the patterned surface and isolated using laser. This inkjet metallisation process is

currently used in large-scale production of CSG modules (Basore 2006).



Besides inorganic solar cells, the inkjet process is also increasingly being utilised in the

fabrication of organic polymer-based solar cells. Such solar cells are usually thin-film devices

whereby suitable soluble polymer blends are applied onto a supporting substrate using

processes such as spin coating or screen printing. An alternative way to deposit such

functional liquid solutions is through inkjet printing. For example, Hoth et al. (2007) recently

demonstrated the fabrication of inkjet-printed organic solar cells with conversion efficiency in

the range of 3%. In this case, inkjet printing is used to coat the glass substrate with a

photoactive layer consisting of jettable polymer-solvent mixture. Konarka Technologies

(2008) have commercialised a flexible solar cell product based on roll-to-roll inkjet

manufacturing process using organic materials.

With the increasing interest in using inkjet as a viable alternative for surface patterning in

solar cell fabrication, there have been many efforts recently to formulate specialty materials

for inkjet deposition. For example, recent publications by Khaselev et al. (2008) and Köhler

et al. (2008) describe the development of a range of jettable materials that are useful for solar

cell fabrication such as improved silver inks, doping inks, diffusion barrier inks and dielectric

etchant inks. Other companies such as Rohm and Haas (2008a) and Filmtronics (2008) have

also developed similar jettable materials for use in solar applications. More novel materials

include Nanosolar’s (2008) CIGS ink that can be printed using roll-to-roll inkjet printer at

very high speed. Besides material development, much has also been done recently in large-

scale industrial inkjet equipment development specifically for solar cell production lines.

Some examples of this include inline inkjet systems from Schmid (2008) and PixDro (2008)

for Si-wafer based solar cells and from Litrex (2008) for thin-film solar cells.

The focus of this thesis work is to develop new inkjet printing processes for high efficiency Si

wafer-based solar cell fabrication, particularly for the purpose of surface dielectric patterning.

There are two approaches introduced in this thesis including a resist-based dielectric

patterning method and a dopant ink dielectric patterning method. Several advantages of the

resist-based method compared to the other available methods are: (1) faster processing time

per wafer, (2) the use of selective emitter structure, (3) smaller feature sizes, (4) high fill

factor contact formation, and (5) reversible process. The dopant ink method has the novelty of

simultaneously forming the heavy diffusion and contact dielectric patterning.



2.5 Summary

In this chapter, a thorough discussion of the various inkjet printing devices and technologies

currently available has been presented. It has been shown that inkjet devices can be used as

material deposition tools. Its successful applications have been demonstrated in a wide range

of research field. The versatility of inkjet provides a justifiable motivation to explore its

application in fabricating high efficiency Si solar cells.

The two main inkjet mechanisms in the form of continuous and drop-on-demand inkjet have

been reviewed. It has been concluded that the most suitable inkjet device type to use for this

thesis work is the drop-on-demand inkjet. Within the drop-on-demand inkjet technology, the

piezoelectric type is the most desirable for material deposition purposes. This decision is

mainly due to the ability for piezoelectric drop-on-demand inkjet devices to eject a wider

range of fluids and its capability for greater droplet ejection control.

In a typical inkjet system, there are three main components that are interdependent with one

another. They are the printhead, the fluid and the substrate. In a pattern transfer process using

inkjet printing, the fluid is the medium of transfer. As a result, it is important that in the fluid

formulation or choice, the fluid-printhead and fluid-substrate interactions must also be

considered. In this chapter, the significant parameters related to each of these three

components and the relevant background theory has been comprehensively discussed with the

aim of outlining some general guidelines on developing an inkjet process.

Finally, a current survey of the state-of-the-art of inkjet technology applications in solar cell

fabrication has been given. There has been many demonstration of inkjet printing use in

fabricating various solar cell structures with the aim of increasing the efficiency of the device,

simplifying the fabrication process and lowering the fabrication cost. A study of these prior

arts helps identify some novel applications that previously have been unexplored and define

the boundaries for the work performed during this thesis. By reading and understanding the

work presented in the following chapters, it is hoped that the reader will be able to put the

contributions of this thesis in context of the current progress in the inkjet solar cell research.

Chapter 3: Dielectric Patterning Using Inkjet


Chapter 3 : Dielectric Patterning Using Inkjet

3.1 Introduction

Inkjet printing has the potential to be a low-cost alternative to photolithographic patterning

methods. The aim is to develop an inkjet printing process that can produce feature sizes

comparable to those that are photolithographically-defined, while retaining the robustness of

the patterning process. This chapter details the development of a novel indirect inkjet

patterning technique which satisfies that aim. It begins by evaluating a range of different

fluids to identify the most appropriate fluid type for dielectric patterning purposes. Some of

the considerations include fluid-printhead compatibility, fluid’s functionality and fluid

jettability. The substrate, where the fluid is deposited, forms the surface area to be patterned

by the inkjet method. In order to achieve as small feature sizes as possible, the interaction

between the fluid and the substrate surface is then investigated. This also helps to narrow

down further the choice of suitable fluid for the process. Finally, an optimisation of the

various jetting controls such as jetting voltage, frequency and pulse length will be shown to

produce a high quality jetting of the selected fluid.

Using the identified fluid-substrate system, an inkjet patterning process flow is developed and

characterised. It will be shown that the inkjet patterning method is able to produce very small

openings in the form of both individual and continuous openings on various dielectric

surfaces relevant to typical crystalline Si solar cell substrates. In addition, an analysis of the

effects of a number of process variables such as platen temperature, resin thickness and drop

spacing on the size of the formed openings is presented.

3.2 Fluid Selection and Formulation

Two of the most widely used dielectric surface materials in industrial solar cell fabrication are

considered throughout this thesis: silicon dioxide (SiO2) and silicon nitride (SiNx). With the

aim of dielectric patterning in mind, the type of fluid that can be used in such a process can be

categorised into:



• Direct etching – using fluids that could directly etch the dielectric material.

• Indirect etching – using fluids that could not directly etch the dielectric material but

can be used to pattern an intermediary protective layer.

The former approach involves only the use of a chemical etchant for the specific dielectric

material. The latter approach is analogous to photolithographic process where a protective

layer (e.g. photoresist) is patterned to gain access to the dielectric layer where it could be

exposed to standard dielectric chemical etchants.

The choice of fluid for the simplest process would be to use direct chemical etchant of the

specific dielectric materials. For example, both SiO2 and SiNx could be effectively etched at a

sufficiently high etching rate by a common etchants such as hydrofluoric acid (HF). To a

much lesser extent, sodium hydroxide (NaOH) or potassium hydroxide (KOH) could also be

used to etch both dielectric materials, albeit at much lower etching rate. Another effective

etchant for SiNx is phosphoric acid (H3PO4), although such etching must be performed at an

elevated temperature.

Using the alternative indirect approach, a wide range of fluids can theoretically be evaluated

depending on the masking layer used. Young and Lasswell (2007) demonstrated this concept

by depositing droplets of NaOH using an inkjet system onto a novolac-resin based masking

layer. In this case, the NaOH etches the novolac resin and forms a series of patterned

openings on the masking layer. Kawase et al. (2001) also used a similar concept by using

inkjet to deposit solvent (ethanol) droplets onto polyvinyl phenol (PVP) film to form

openings.

3.2.1 Fluid-Printhead Compatibility Evaluation

The choice of suitable fluid to be used in the inkjet process firstly depends on the fluid’s

compatibility with the printhead used to deposit it. For each of the three printheads used in

this thesis work (see Table 2-2), there exists a Material Compatibility Kit (MCK) available

from the printhead supplier. The MCK contains sample materials, which makes up the

components of the printhead that come in contact with the fluid. Each component’s

compatibility with the fluid is tested by soaking the sample material in the fluid.



All the materials present in the fluid path for the Galaxy printhead, the SX3 printhead and the

DMP printheads and the function of each of these components are listed in Appendix C.

These materials also make up the supplied MCK samples to be tested for compatibility with

the fluid. Note that many of the same materials are used in more than one printhead.

The testing method is explained as follows: Before soaking, a baseline weight measurement

of each material was taken. Each material was soaked in the test fluids for seven days and

removed from the fluid solution to be checked daily. The containers used for soaking were

inert to the testing fluid and they were either made of glass or low density polyethylene

(LDPE). Although the actual jetting temperature may be higher, all the soaking tests were

performed at room temperature (25 oC). This was based on the assumption that fluids that are

incompatible at such low temperature were also unlikely to be compatible at higher

temperatures where reaction processes are usually stronger.

Basic evaluation methods involved visual checks and weight measurements of each of the

MCK materials. The short testing timeframe was not intended to simulate use of these fluids

in a large-scale continuous production environment, but rather only for evaluation of

laboratory proof-of-concept experiments. For a fluid to be suitable for laboratory use, it must

not chemically or physically affect any of the MCK materials during the seven-days soaking

period. When changes in the MCK material were observed, an evaluation of the mode of

failure was performed in order to identify the cause.

Many fluids (for both direct and indirect etching) were soak-tested for compatibility with the

printheads without success, especially the corrosive fluids. The compatibility test results of

four particular fluids (with their concentration in w/v) consisting of: (1) HF (49%), (2) NaOH

(50%), (3) acetone (100%) and (4) diethylene glycol (DEG-99%) are listed in Appendix D.

These four fluids represent a range of typical corrosive fluids (acidic HF and basic NaOH)

commonly used for direct etching of dielectric layers and organic solvents for indirect

etching.

The HF solution was found to be particularly damaging to the PZT, Kovar and nozzle plate

materials, while the NaOH solution strongly reacted with the rock trap filter and flex link.

These incompatibilities prevented these two fluids from being used in the Galaxy and SX3



printheads. In addition, HF and NaOH are well-known etchants to SiO2 and Si respectively,

which are the main materials used in the nozzle plate construction of the DMP printheads.

Acetone and DEG showed excellent short-term compatibility with all three printheads. The

demonstrated short-term compatibility of both fluids warranted an extended testing period to

evaluate their long-term compatibility. Therefore, an additional three-week soaking of the

components was performed on both acetone and DEG with weekly checking. It was found

that no apparent degradation occurred for the total one-month soaking for all of the tested

printhead components.

From these compatibility experiments, it was concluded that indirect etching methods using

fluids such as acetone and DEG were preferable to direct etching methods which require the

use of corrosive fluids in the printheads used in this thesis work. In addition, the use of highly

corrosive fluids such as HF and NaOH is not desirable due to the safety hazards that they

present, particularly in industrial environments where human operators are routinely exposed

to the chemicals.

3.2.2 Effects of Fluid on Substrate Surface

It is widely known in both semiconductor and photovoltaic manufacturing that certain organic

solutions can be used to remove photoresists. These solvents work by dissolving the resin’s

polymeric bonds under the generally accepted rule that polymers are likely to dissolve in

solvents with similar solubility parameters to its own. The most widely used description of the

solubility parameters was proposed by Hansen (2000). The Hansen solubility parameter is

based on the consideration of three molecular interactions in the material’s molecular

structure consisting of:

• Polar or permanent dipole interactions ( pδ ),

• Non-polar dispersive interactions due to atomic forces ( dδ ), and

• Hydrogen bonding interactions ( hδ ).

For both the polymer and solvent, the coordinate defined by each of these three interactions

can be plotted in a three-dimensional x-y-z graph. For a particular polymer, a spherical



volume of solubility can then be created with this point being the centre in addition of another

parameter, the interaction radius R. This solubility sphere determines which solvents can or

cannot dissolve the polymer. Solvents whose point lies inside the solubility sphere are likely

to dissolve, whereas those with points lying outside the sphere are not likely to dissolve the

polymer. These four parameters: pδ , dδ , hδ , and R define the Hansen Solubility Parameters

(HSP) of a particular polymer, which is a useful tool to help predict and choose a suitable

solvent for polymers.

Hansen (2000) formulated a mathematical expression to calculate Ra, which is the distance

between the central coordinate of the polymer solubility sphere and the coordinate of the

solvent’s HSP. In other words, Ra describes the relative position of the fluid’s HSP within the

resin’s solubility sphere. This is shown in Equation (3-1):

212

212

212

2 )()()(4)( hhppddRa δδδδδδ −+−+−= (3-1)

where 1dδ and 2dδ are the dispersive (non-polar) interactions of the respective polymer and

solvent, 1pδ and 2pδ are the polar interactions of the respective polymer and solvent and 1hδ

and 2hδ are the hydrogen bonding interactions of the respective polymer and solvent.

To determine whether or not a solvent is likely to dissolve a polymer, this distance Ra is then

compared with the interaction radius R of the polymer as:

RRaRED = (3-2)

where RED is the relative energy difference between the solvent’s and the polymer’s HSP, Ra

is the distance between the solvent’s and the polymer’s HSP and R is the interaction radius of

the polymer.

The relative energy difference RED is a ratio that indicates the similarity of two molecular

species, such as those between an organic polymer and an organic solvent. A RED ratio

equals to zero implies that the molecules are exactly the same and obviously will be soluble



with one another. For RED ratio of less than one but greater than zero, the two organic

molecules display sufficient similarities such that they are likely to be soluble. For RED ratio

approximately equals to one, it implies that the solvent’s HSP is on the boundary of the

polymer’s solubility sphere indicating that it could be partially soluble depending on other

factors. Finally, for RED ratio greater than one, the two molecules have sufficiently different

structure such that they are not soluble with one another.

A special attention must be paid towards the effect of molecular volume on the solubility of

two organic molecules. Smaller molecules are usually more soluble than larger molecules.

This is particularly important for molecules with an RED ratio of or close to one because

solubility could still occur for smaller molecules. Temperature also plays a major role in

determining a solvent’s ability to dissolve polymers. Higher temperatures usually results in an

increase in the solvent’s solubility rate as well as better absorption ability of the polymers for

the solvent (Hansen 2000).

In this thesis work, the chosen masking layer for use in indirect etching is the Microposit

FSC-M surface coating product from Rohm and Haas (2008b). It is a novolac-resin based

coating, which is a similar material used for photoresist without the addition of the

photoactive compounds. Novolac resins are phenol/cresol-formaldehyde polymer with low

molecular weight and phenol to formaldehyde ratio greater than 1. This synthetic resin was

invented by Baekeland (1909) and the chemistry/production of such resin has been described

extensively by Gardziella et al. (2000). It is a common, low-cost resin that has widespread

uses in microelectronics fabrication processes. In this case, the resin is dissolved in propylene

glycol monomethyl ether acetate (PGMEA) solvent at approximately 34% solid content.

The novolac resin has the following HSP correlations: dδ = 20.30, pδ = 15.40, and hδ = 5.30

with an interaction radius R of 15.10 (Hansen 2000). A plot of the novolac resin’s solubility

sphere is shown in Figure 3-1(a). A calculation of the RED values for both acetone and DEG

(listed in Table 3-1) using Equation (3-1) and Equation (3-2) yields 0.72 and 1.15

respectively. This result predicts that acetone is a solvent for the novolac resin whereas DEG

is somewhat not a solvent. Graphically, this is shown in Figure 3-1(b), which is a two-

dimensional pδ vs hδ projection of the three-dimensional Hansen solubility sphere

representation.



Figure 3-1: Hansen solubility sphere for novolac resin showing the relative

position of acetone (denoted by ) and DEG (denoted by ) to the sphere

(figure A) and the 2-dimensional δp vs δh projection of the Hansen solubility

sphere showing the relative position of acetone (denoted by ) and DEG

(denoted by ) to the sphere (figure B)

Acetone Diethylene glycol

Molar volume 74.00 94.90

dδ 15.50 16.60

pδ 10.40 12.00

hδ 7.00 20.70

RED value 0.72 1.15

Table 3-1: Hansen parameters of acetone and diethylene glycol (Hansen 2000).

It is useful to note that the exercise above could also be applied to a wide range of other

solvents in order to evaluate their solubility with the novolac resin. Similarly, the Hansen

parameters can also be used to conduct preliminary analysis of a myriad of polymer-solvent

combinations relatively quickly. This is particularly useful to determine the appropriate fluid

and surface type to choose for inkjet process development that involves polymer dissolution

without expending too much time on random approach.

A B



Acetone and DEG are examples of a dissolving and a non-dissolving fluid respectively. A

comprehensive review of the theory behind polymer dissolution has been presented by Miller-

Chou and Koenig (2003). When a droplet of solvent comes in contact with a polymer surface,

the solvent diffuses into the bulk of the polymer layer where it disentangles the bonds

between the polymer molecules. For a solvent such as acetone, this action results in the

dissolution of the polymer.

The dissolution of a polymer due to solvent drop has been demonstrated to work very well in

creating structural openings on polymeric surfaces. An excellent example of this application

is the formation of via-holes in all inkjet-printed organic thin film transistors by Kawase et al.

(2001). When a solvent drop is deposited using an inkjet device onto a soluble polymer

surface, a hole with a structure shown in Figure 3-2 is formed.

Figure 3-2: An AFM image of a solvent drop on polymer surface, showing a

characteristic hole shape with high ridges due to the "coffee-ring effect"

(Kawase et al. 2001).

One of the most interesting feature observable for the Atomic Force Microscope (AFM)

image of the hole formation through solvent drop is the presence of high ridges on the edge

surrounding the crater. This is explained by Kawase et al. (2001) as a result of the so-called

“coffee-ring effect” (Deegan et al. 1997). As the inkjet-deposited solvent droplet dissolves the

contacted polymer surface, the dissolved material flows out towards the edge of the droplet

through capillary flow. This is caused by the fact that evaporated fluids on the edge of the

droplet must be replaced by fluid from the centre of the droplet. This flow carries and



redeposits the bulk of the polymer material on the edge resulting in the characteristic

peripheral ridge seen in Figure 3-2.

A similar effect was observed when using acetone as a solvent for the novolac resin used in

this thesis work. While the inkjet deposition of solvent is a very elegant way of patterning

surfaces, the small volume of deposited inkjet droplets usually necessitates multiple

depositions at the same location in order for the opening to extend completely through the

polymer layer and to reach the underlying surface. Thicker polymer layers require higher

repetition of solvent deposition. This is a possible disadvantage for the processing throughput

in solar cell fabrication, particularly when applied to large-scale industrial environment.

An alternative approach is to inkjet-deposit a poorly-dissolving fluid such as DEG (as

predicted by its Hansen parameters). In the case of such fluid, the fluid-substrate interaction

involves a plasticising action whereby the fluid selectively weakens some bonds in the

polymer’s molecular structure to effect a change in the material properties. Plasticisers differ

from solvents in that they usually do not cause dissolution of the effected polymer. Therefore,

no holes are formed in the polymer layer. Instead, the plasticiser molecules enter the polymer

layer on contact and push the polymer molecules further apart, sometimes resulting in

swelling or even cracking (Sears and Darby 1982).

For the novolac resin used in this thesis work, DEG was found to be a plasticiser for the

polymer resin. Like most plasticisers, DEG has a higher molecular weight than solvents such

as acetone. DEG also has a RED ratio of more than one, indicating that it is a poor solvent.

When a droplet of DEG is inkjet-deposited onto the novolac resin surface, the DEG droplet

first wets the substrate and then penetrates into the resin to form a plasticised region in the

resin. The size of this plasticised region is strongly dependent on the wetting area. It was

observed that DEG penetration into the resin can be enhanced at higher temperatures. When

the DEG diffuses into the resin, swelling of the resin occurs as the individual resin molecules

are pushed apart by the plasticising DEG molecules. Such swelling is clearly visible from

Figure 3-3, which shows an AFM image of a plasticised region of a novolac resin that was

formed by the inkjet deposition of an 8 pL droplet of DEG.



Figure 3-3: An AFM image of an 8 pL plasticiser drop (DEG) on a novolac

resin layer showing the degree of polymer swelling on the surface.

The effect of the DEG on the novolac resin layer is markedly different from that of solvent.

Typically, a cured layer of novolac resin is used as a protective layer against the actions of

acidic etchants such as hydrofluoric acid (HF). When deposition of a solvent such as acetone,

is used to form a hole in the resin, the underlying dielectric layer is directly exposed to the

etching fluid. However, in the case of DEG plasticisation, the weaker bonds in the polymer

molecules allows aqueous etchants such as HF to permeate through the plasticised regions to

access the underlying dielectric without the removal of any resin or the direct exposure of any

dielectric layer. There are several advantages associated with the plasticiser approach such as

potentially smaller feature sizes and the possibility of reversing the permeability of the resin.

It is important to note that the solvent and plasticiser effects described above also apply to

organic fluids other than acetone and DEG. For example, another obvious choice of solvent

would be propylene glycol monomethyl ether acetate (PGMEA), which is the solvent used to

dissolve the novolac resin used in this thesis work. PGMEA and novolac resin has an RED

ratio of 0.95, which indicates that it is likely to dissolve the novolac resin. Other good

solvents for the novolac resin (i.e. those organic fluids with RED ratio less than one) such as

dimethyl sulfoxide (RED ratio of 0.41) and dimethyl formamide (RED ratio of 0.56) would be

expected to have a similar solvent effect too. On the other hand, there are alternative

plasticisers that could be used other than DEG in order to achieve the same plasticising action.

Some examples of such organic plasticisers for novolac resin are triethylene glycol (RED

ratio of 1.06) and propylene glycol (RED ratio of 1.34).



3.2.3 Fluid Jetting Optimisation

Table 3-2 shows the fluid properties that are relevant to fluid jetting for both acetone and

DEG. The last column shows the optimum fluid parameters range for the DOD piezoelectric

inkjet printheads used in this thesis work.

Acetone DEG Optimum jetting

Molecular weight 58.08 106.12 N/A

Particle content size (μm) none none < 1.00

Viscosity @ 25 oC (cP) 0.31 27.80 8.00 – 14.00

Surface tension @ 25 oC (mN/m) 22.30 44.80 28.00 – 36.00

Density (g/cm3) 0.79 1.12 > 1.00

Boiling point (oC) 55.10 244.50 > 100.00

Table 3-2: Fluid properties as relevant to jetting for acetone and DEG.

The viscosity of DEG is still higher than optimal. However, the viscosity of fluids can be

reduced to the optimal range by increasing the fluid temperature. This is not true for fluids

having a viscosity lower than the optimum value, such as acetone. In general, surface tension

also decreases with increasing temperature, in which case increasing the jetting temperature of

DEG would also reduce its high surface tension to a value that is closer to optimal. Table 3-2

also shows that the fluid density of DEG is within the optimal range. The DEG fluid also has

a high boiling point, which eliminates potential nozzle clogging and poor jetting caused by

fluid evaporation. Based on the discussion in Section 3.2.2 and the comparison of fluid

properties presented in Table 3-2, it is concluded that DEG is the most suitable fluid.

Despite the non-optimal fluid properties of fluids such as acetone, it is still prudent to realise

that it is possible to modify fluid viscosity and surface tension through the use of additives or

surfactants. Obviously, the used additive must achieve its objective without complications in

terms of printhead compatibility, jetting quality and most importantly, miscibility. In the case

of acetone, a good example of a suitable additive would be DEG, which would increase all of

its important jetting parameters depending on the mixing ratio of the two fluids.



In order to find the appropriate jetting temperature for DEG, a series of viscosity and surface

tension measurements were conducted across a range of temperature between 30 oC and 80 oC

with 10 oC intervals. The viscosity measurements were performed using a Brookfield DV-II+

Pro viscometer from Brookfield Engineering. The surface tension measurements were

performed using an Analite STM2141 surface tension meter from McVan Instruments. The

DEG solution used for the measurements and for the rest of this work was the Diethylene

Glycol ReagentPlus® 99% purchased from Sigma Aldrich. The results of these measurements

are presented in Figure 3-4.

Figure 3-4: Viscosity (solid squares) and surface tension (open squares) of DEG

as a function of temperature.

Both the viscosity and surface tension of DEG decreased as the temperature of the fluid

increased. This is mainly because increased temperature causes the fluid molecules to become

more energetic and thus more able to overcome their intermolecular forces. It is important to

recall that although low viscosity is desired for jetting, a high viscosity is preferable to

minimise droplet spreading as it touches the substrate surface. Therefore, in this case, the

DEG must be heated to a temperature such that the viscosity lies within the upper range of the

optimal jetting viscosity range. From the measurements, it was found that a jetting

temperature of 45 oC results in a viscosity of about 13.4 cP and surface tension of about 35.4

mN/m, which is theoretically optimal for jetting as well as wetting the polymer surface.



The optimisation of the DEG jetting parameters was conducted using the Dimatix Materials

Printer (DMP). The DMP printer can be operated with printhead cartridges which are capable

of depositing nominally 1 pL and 10 pL droplets. There are slight differences in the jetting

behaviour between the two printheads. For example, higher jetting voltages are usually

required for larger droplet sizes of fluid with the same viscosity and surface tension. In order

to achieve the smallest feature size possible, the 1 pL printhead was used for optimisation.

A 99% DEG solution was loaded into the Dimatix Materials Cartridge (DMC) using a

syringe. The fluid was filtered using a 0.2 μm syringe filter to minimise contamination. The

DMC was then attached to a 1 pL DMC printhead and was left with the nozzles facing down

in air for overnight in order to allow time for the fluid to flow and fill out the chambers inside

the printhead.

The printhead was heated to 45 oC to bring the DEG fluid properties to within the optimal

jetting parameters. Before jetting, the printhead was primed by continuous purging of the

fluid until the fluid was visibly expelled out of the nozzle orifices. Printhead priming is

important to ensure that the fluid level is held by negative pressure just above the nozzle

orifices and that any air bubbles that may have formed inside the pumping chamber while the

printhead was idle are removed.

There are three main jetting parameters that could be adjusted in order to characterise the

droplet velocity: jetting voltage, jetting frequency and jetting waveform. The droplet velocity

can be measured by dividing the distance travelled by the droplet by the time it takes to reach

that distance. In this experiment, all droplet velocity values were measured at a distance of 1

mm from the nozzle, which is also the distance between the nozzle and the substrate. This

gives the droplet velocity at the point of impact. While droplet velocity is quantifiable, the

characterisation of jetting quality is typically more qualitative. The main tool used to

characterise the jetting quality is through visual inspection using the drop watcher system

where jetting properties as described near the end of Section 2.3.2 are desirable.

Due to the optimal fluid properties of DEG at the jetting temperature, optimised fluid jetting

was readily achieved. It was found that the DEG solution could be jetted at a relatively wide

range of jetting parameters. The relationship between droplet velocity and the jetting voltage,



jetting frequency and jetting pulse length are shown in Figure 3-5. When the jetting voltage

was varied, both the frequency and the pulse length were kept constant at 5 kHz and 12.8 μs.

When the jetting frequency was varied, both the voltage and the pulse length were kept

constant at 15 V and 12.8 μs. Finally, when the jetting pulse length was varied, both the

voltage and the frequency were kept constant at 15 V and 5 kHz.

Figure 3-5: Droplet velocity dependence on varying jetting voltage (figure A),

jetting frequency (figure B) and jetting pulse length (figure C) for 99% DEG

solution.

Increasing the jetting voltage linearly increased the droplet velocity. This is due to more

bending of the piezoelectric crystal on the application of larger electric field, which causes

more force to be applied on the fluid. It was found that no jetting occurred for voltages below

9.5 V, while droplet quality began to deteriorate for voltages above 19 V (mainly through the

formation of satellite droplets). This is because for voltages below 9.5 V, the actuating force

was simply not strong enough to push fluid out of the nozzle. For voltages above 19 V, it is

likely that the initial part of the droplet was ejected out of the nozzle at a velocity greater than

its tail, resulting in sufficient velocity difference between the two parts of the droplet during

flight such that they separated before they reached the substrate. This is possible due to the

shape of the jetting waveform as shown in Figure 3-6. In segment B, maximum voltage is

applied to the piezoelectric crystal resulting in a maximum actuating force which ejects the

droplet. In segment C, this voltage is scaled back by 40% in order to break off the droplet.

Due to the linear relationship, the droplet velocity resulting from segment C is also about 40%

of segment B. The higher the jetting voltage, the bigger the difference is between the velocity

A C B



of the droplet head and the droplet tail. For a substrate distance of 1 mm, 19 V was found to

be the threshold jetting voltage for DEG using this particular waveform where this velocity

difference is such that it would lead to satellite droplet formation.

Figure 3-6: Jetting waveform used to jet DEG in the DMP.

It was found that for a particular voltage, a longer pulse length results in lower droplet

velocity. Note that while varying the total pulse length, the shape and proportion of each

segment of the waveform was maintained as the one shown in Figure 3-6. In general, a shorter

pulse length is desirable since it allows quick recovery of the fluid level inside the pumping

chamber during each jetting cycle, thus allowing higher frequency jetting. However, the pulse

length needs to be long enough so that enough actuation force is delivered to initiate droplet

formation and ejection. For the DEG fluid tested in this experiment, it was found that the

jetting quality began to deteriorate for pulse lengths below 10 μs.

For the jetting frequency, no apparent correlation with the droplet velocity was observed as

the drop velocity stayed virtually constant with increasing frequency. Such behaviour is

expected since a higher frequency only results in faster jetting rate, while each jetting cycle is

still actuated by the same voltage and pulse length. Therefore, jetting frequency variation does

not strongly affect droplet velocity but rather the jetting quality. Higher jetting frequency

typically results in less time for the fluid to reprime itself after each jetting cycle, which is

likely to negatively affect the droplet formation process.



Figure 3-7 shows several examples of jetting quality issues encountered during the jetting

optimisation of the DEG solution. In Figure 3-7(a), the droplet did not have a straight

trajectory due to excessively high jetting frequency. In Figure 3-7(b), satellite droplet was

formed due to high jetting voltage. Figure 3-7(c) shows an example of good jetting.

Figure 3-7: Drop watcher images of side-shooting droplet (figure A), satellite

droplet formation (figure B) and excellent jetting quality (figure C).

By combining the analysis presented above, it is concluded that the maximum droplet velocity

can be achieved by jetting at the highest voltage possible without sacrificing jetting quality at

the highest frequency possible while using the shortest pulse length possible. Depending on

the individual nozzles, a jetting voltage of 15-17 V was found to be stable for DEG solution

even for the highest frequency used in this work, which was 20 kHz. The selected pulse

length was 12.8 μs, which gave the required long-term stability in terms of jetting quality. The

resulting average droplet velocity obtained using these parameters were 9 m/s.

A B C

Nozzles



After optimisation, excellent jetting quality was observed for the DEG fluid solution jetted

using the parameters described above. Figure 3-8 shows collated time-lapse drop watcher

images of a DEG droplet jetted at 15 V with frequency of 20 kHz and pulse length of 12.8 μs.

The jetting temperature of the DEG was 45 oC.

Figure 3-8: Time-lapse drop watcher images of DEG droplet formation.

The jetted DEG shows all the characteristics of good jetting quality as described in Section

2.3.2. It can be seen that the head of the droplet was smoothly ejected out of the nozzle orifice

followed by an elongated tail that breaks off from the nozzle after 16 μs. The short tail

formation was beneficial as it only took another 15 μs for the tail to join up with the head of

the droplet. The entire droplet was formed within 31 μs of it leaving the nozzle, which is

equivalent to a distance of ~280 μm from the nozzle (assuming 9 m/s droplet velocity). As

evident from Figure 3-7(c), the droplet trajectory was very straight and no satellite droplets

were formed within the distance travelled. Thus, a standard 1 mm distance between the nozzle

and the substrate during printing would be suitable for this work.

3.3 Process Development and Characterisation

In the previous sections, the reasoning for the selection of DEG as the fluid of choice in this

thesis work was discussed by taking into account the various fluid-printhead and fluid-

substrate interactions. The jetting of DEG using an inkjet device was also optimised and

characterised in order to achieve the best jetting performance for the subsequent development

of an inkjet process. In this section, a new and patented inkjet process utilising the plasticising

action of DEG on novolac resin surface for the purpose of dielectric patterning is described.

6 8 10 12 14 16 19 21 23 25 27 29 31 33Time (μs)

Nozzle



Figure 3-9: Process flowchart of indirect inkjet dielectric patterning.

A novolac resin layer is applied on both the

top and bottom surfaces of the Si wafer either

by spin-coating or spray-coating. Ideally, the

coating layer needs to be low-cost, easy to

apply and plasticisable by a suitable fluid.

Droplets of DEG are selectively deposited by

inkjet onto the resin layer. DEG plasticises the

resin such that the affected region becomes

permeable to dielectric etchants such as HF.

The regions of the resin not affected by the

DEG maintain its resistance to such etchant.

The substrate is immersed in HF solution,

which allows the etchant to permeate through

the affected regions of the resin and etch the

underlying dielectric layer. The etching time

depends on the etching rate and the thickness

of the dielectric layer.

The resin layer is removed either by wet or

dry etching process or by solvent dissolution.

This exposes the localised Si surfaces on the

patterned dielectric layer.

A Si substrate may have an emitter diffusion

followed by dielectric layer formation such as

SiO2 or SiNx. The dielectric layer enshrouds

the entire surface of the Si wafer.



Figure 3-9 shows the flowchart of the entire inkjet patterning process. To demonstrate the

inkjet patterning process, three Si wafers with different types of surface finish were used as

substrates. The three surface finish used in this experiment were: (1) mirror polished surface,

(2) saw-damage-etched surface and (3) random upright pyramid textured surface. Due to its

high reflectivity, the mirror polished surface is preferred for initial optimisation work because

it provides a clear view of the patterned surface under an optical microscope. Both the saw-

damage-etched and the random upright pyramid textured surfaces were selected because they

are commonly-used surfaces in solar cell fabrication.

A thermal oxide was grown on both surfaces of all samples by standard furnace oxidation

process using the wet oxidation method. The thickness of the oxide layer grown for all

samples was between 300-400 nm. Both surfaces of the Si wafers were then coated with the

Microposit FSC-M novolac resin coating (from Rohm & Haas) by spin coating method. The

coating was spun at 5,000 RPM for 30 seconds and then baked in a conventional fan-forced

oven at 140 oC for 10 minutes. The resulting thickness of the resin layer as measured by

Dektak profile was ~2.6 µm thick.

After coating, the wafers were then placed on the platen of the DMP inkjet printer for

printing. The platen was heated to 60 oC to: (1) minimise the contact area between the DEG

droplet and the resin surface when they come in contact with each other and (2) enhance the

DEG diffusion into the resin layer. Since the DEG droplet is absorbed almost instantaneously

on the resin surface, heating the substrate ensures that the surface energy of the resin is kept to

a minimum. Once the DEG droplet begins to diffuse into the resin layer, elevated substrate

temperature helps increase the DEG permeation capability in the resin layer so that the DEG

permeates the entire depth of the resin.

Droplets of DEG were then inkjet-deposited onto the substrate using the jetting parameters as

described in Section 3.2.3. When the substrate was immersed in HF, the etchant would

permeate through the patterned regions of the resin and etch the underlying dielectric layer.

The dielectric etching was achieved without any removal of the overlying resist layer but

rather only through modification of the chemical properties of the resin layer by the

application of the appropriate plasticiser.



A swollen region was formed when the plasticising DEG was absorbed by the resin layer as

shown in Figure 3-3. After HF immersion, cracks were formed in this swollen region as

shown in Figure 3-10. One possible explanation for the observed cracking of the resin layer is

the presence of an additional volume in the resin layer caused by the permeation of HF into

the resin layer. The HF molecules is likely to push the resin molecules further apart and

causes further swelling of the polymer up to a point where the polymer surface begins to

crack. This explanation would also provide supporting evidence of the liquid permeation

effect during immersion since the additional volume could only come from the immersion

liquid molecules. The observation of surface cracks after the immersion etching process is

also a good indicator of whether or not the etchants have permeated through the resin layer.

Figure 3-10: Optical microscope image (figure A) and AFM image (figure B) of

the cracked region on the resin surface after HF immersion.

The length of etching time required to completely remove the dielectric layer and expose the

Si surface underneath strongly depended on the etching rate of the solution and the thickness

of the dielectric layer. Once etching was completed, the resin layer was removed by standard

photoresist removal technique. In this experiment, the resin layer was removed using a

Piranha etch solution consisting of a mixture of H2SO4 and H2O2 in a 3:1 ratio. After the resin

was removed, the dielectric layer was patterned with exposed Si surfaces according to the

inkjet printed DEG pattern. The surface was then ready for the next processing step.

10μm

10μm

A B



3.3.1 Formation of Hole and Line Patterns

The inkjet patterning method described above can be used to form any pattern on the

dielectric surface including individual hole patterns and continuous line patterns. Both types

of patterns are very useful in creating various high efficiency solar cell structures as will be

described in further detail in Chapter 4. The result of inkjet patterning using the technique

described above is shown in Figure 3-11. The three surface finishes used in this experiment

are polished, saw damage etched and random upright pyramid textured Si wafer surfaces.

Figure 3-11: Optical microscope images of hole and line patterns on mirror-

polished surface (figure A and D), saw damage etched surface (figure B and E)

and random upright pyramid textured surface (figure C and F).

The white-coloured surfaces shown in the optical microscope images in Figure 3-11 indicate

the exposed Si surfaces underneath the SiO2 dielectric layer. It can be seen that only regions

where DEG was deposited became permeable to HF, which etched the SiO2 all the way

through to the Si surface while the rest of the surface was protected by the resin, thus leaving

the SiO2 layer intact (green-coloured surfaces in the optical microscope images).

Although the SiO2 layer in this experiment was grown through oxidation process with a

thickness of approximately 300-400 nm, different SiO2 thickness could similarly be patterned

A B C

D E F

Scale 100μm

Scale 100μm



using the same inkjet technique. The same could also be said for SiO2 layers grown through

other methods such as dry oxidation or using spin-on glass. Other dielectric layers commonly

used in solar cell fabrication such as SiNx or TiO2 could also be used instead of SiO2 as long

as the appropriate dielectric etchant is used. In these cases, the only modification required for

the process would be the length of immersion etching required depending on the thickness

and density of the dielectric layer and the etching rate of the etchant solution.

Currently, the smallest feature sizes achievable using this inkjet patterning technique are 30-

35 μm diameter for individual holes and 40-50 μm line width for continuous lines. Figure

3-12 shows an AFM cross section image of an inkjet patterned line on a polished Si surface

similar to the one shown in Figure 3-11(d). In this case, the line width is about 44 μm and the

depth was measured to be about 360 nm, indicating that the SiO2 layer on the patterned region

has been completely removed.

Figure 3-12: AFM cross section image of an inkjet patterned line on a polished

Si surface similar to that shown in Figure 3-11(d).

3.3.2 Effects of Resin Thickness on Feature Sizes

One of the important process parameters that affect the feature sizes produced using this

inkjet patterning method is the resin thickness. Due to the indirect nature of the patterning

method, the way the resin is applied to the surface is crucial because the resin layer is the

actual patterned layer. This means that the inkjet deposited DEG would come in contact and

be functional to the resin layer instead of the underlying dielectric layer. Therefore, the

feature size is expected to have a strong correlation with the thickness of the resin layer.



The final thickness of the novolac resin depended primarily on the spin speed and to a lesser

extent, the baking/curing temperature. In general, faster spin speeds result in thinner resin

layers on the substrate surface because more solution is spun off the substrate during the spin-

coating process. Higher baking temperatures or a longer baking time would give thinner resin

layer due to more solvent being evaporated from the coating. It is also worthwhile to note that

other methods of applying the resin can be used, for example through spraying. In this case,

the thickness of the resin can be controlled by adjusting the spraying rate and time.

Figure 3-13 shows the relationship between varying resin thickness and the resulting hole

diameter formed on the dielectric layer for two different printheads. The data represented by

solid squares were obtained using 8 pL droplet volumes, while the data represented by the

solid triangles were obtained using 1 pL droplet volumes. For both sets of data, the substrate

was heated to 60 oC and the droplets were deposited using the jetting parameters as described

in Section 3.2.3. The inkjet patterning process was as described earlier in this chapter. The

pattern printed on the substrate was an array of individual holes separated about 500 μm apart.

Ten random hole diameters were measured on each of the four corners and the centre of the

pseudo-square 8 cm2 polished Si wafer and the average diameter was calculated.

Figure 3-13: Correlation between resin thickness and the resulting hole

diameter produced on the dielectric layer using inkjet deposition of one 8 pL

droplet (solid squares) and four 1 pL droplets (solid triangles) of DEG.



From the observed trend as shown in Figure 3-13, it is clear that the resulting hole diameter

decreased as the resin thickness increased for both the 8 pL and 1 pL droplets. A possible

explanation for this correlation is given by considering the DEG absorption mechanism in the

resin layer. Figure 3-14 shows the three possible scenarios of the absorption mechanism of

DEG in the resin layer.

Figure 3-14: DEG absorption mechanism in resin layer showing three possible

scenarios where the resin layer is too thick (figure A), optimal (figure B) or too

thin (figure C). The arrows show the directions in which the DEG diffuses.

When the DEG droplet comes in contact with the resin surface, it is almost instantaneously

absorbed into the resin bulk layer. The width of this region is mostly defined by the area

during the first instance of contact between the DEG droplet and the resin surface. After the

contact area has been defined, the DEG would then diffuse into the resin layer. If the resin

layer is too thick as shown in Figure 3-14(a), there is not enough volume of deposited DEG to

sufficiently plasticise the entire depth of the resin layer. Obviously, thinner resin layers

require less volume of DEG. The diameter of the openings formed using this method is

strongly dependent on the diameter of the droplet. Thus, it is preferable to deposit multiple

droplets of small volume rather than one droplet of larger volume since the nozzle diameter

used to deposit smaller volume droplets is smaller. For example, in this experiment, one 1 pL

droplet was found to be insufficient to plasticise the entire thickness of ~2 µm of the resin

layer. Thus, four layers of droplets had to be deposited in order to provide the required

plasticising volume when using 1 pL droplets. Nevertheless, the small feature size was still

maintained despite the multiple droplet deposition, indicating that the initial droplet-to-

surface contact area is the determining factor for etched hole diameter.

DEG Droplet

Resin layer

Substrate

A

B

C

Plasticised region



The trend observed in Figure 3-14 indicates that thinner resin layers results in larger etched

feature sizes. The likely situation in this case is depicted in Figure 3-14(c). Although the resin

layer is thinner, the resin still need to accommodate the same volume of deposited DEG

droplet. Since the DEG does not affect the underlying dielectric layer, the excess DEG must

occupy the lateral volume of the resin layer, which results in wider plasticised region. This

explains why the resulting hole diameter tends to be larger when thinner resin are used.

The optimal situation would be to find a resin thickness that is thin enough so that the

deposited DEG droplet can plasticise the entire depth of the layer without excessive lateral

expansion. This thickness was found to be approximately 2.6 μm for four droplets of 1 pL and

4.6 μm for one droplet of 8 pL. Any layer thicker than these values would not result in

opening formation on the dielectric layer due to the insufficient depth plasticised by the

deposited DEG as shown in Figure 3-14(a). It is also important to realise that these optimal

resin thickness values could change depending on the volume of DEG droplets deposited.

Therefore, the resin thickness would need to be re-optimised for different droplet volumes.

3.3.3 Effects of Drop Spacing on Feature Sizes

The formation of continuous line opening using this inkjet patterning method requires the

joining of individual holes. This means that the each droplet needs to be deposited in

overlapping fashion with its adjacent droplet. As a result of this overlapping, the lines formed

using this method is generally larger in width compared to the diameter of the individual

holes. In order to achieve the smallest line width possible, it is necessary to find the optimal

droplet spacing.

Using the DMP inkjet device, the drop spacing can be easily controlled through the operating

software. This was done by entering the required spacing and adjusting the printhead printing

angle. This printing angle is also known as the printhead sabre angle, which refers to the

specific angle required for the printhead to print at a resolution higher than its native

resolution. The use of sabre angle is particularly important to adjust printing resolution in the

vertical or cross process direction of the printhead.

As previously shown in Table 2-2, the DMC printhead used in this thesis work has a nozzle

spacing of 254 μm, which equates to a native resolution of about 39.4 drops/cm. Since the



DEG droplets deposited from a 1 pL printhead resulted in etched holes having a diameter of

approximately 30 μm, overlapping between droplets can only be achieved with a printing

resolution of 333.3 drops/cm or higher. Such higher resolution can be printed by rotating the

printhead and fixing it at an angle that corresponds to the particular required resolution.

In the horizontal (or process) direction, the jetting frequency is critical in determining the

printing speed at the specific printing resolution. At higher resolution, the drop spacing needs

to be smaller which requires the printhead to jet the liquid at higher frequency. If the

maximum jetting frequency is set to a low value (for instance due to poor jetting at higher

frequencies), higher resolution printing would require slowing down the printhead’s

horizontal movement. Thus, a trade-off exists between the drop spacing and the printing

speed for the particular jetting frequency selected in the specific process.

Figure 3-15 shows the correlation between the drop spacing and the resulting final patterned

line width on the dielectric layer. At drop spacing higher than 45 μm, continuous lines ceased

to be formed. The data then shows the increasing separation between individual drops.

Figure 3-15: Correlation between drop spacing and line width and the drop

separation in the case when the drop spacing become to large.

As evident from Figure 3-15, closer spacing between droplets results in wider resulting line

width in the patterned dielectric layer. This is due to the increasing amount of droplet overlap,

which increases the total volume of DEG deposited at a particular spot on the resin layer. If



the volume of deposited DEG exceeds the vertical depth absorption capacity of the resin

layer, the plasticiser tends to spread laterally as previously described in Figure 3-14(c) making

the adjacent regions also permeable to etchant solution. With increasing drop spacing, the

overlapping region is also reduced resulting in thinner line width.

However, as the drop spacing is further increased, there will be a point where the droplets

cease to overlap with one another. In this case, the droplets are separated from each other and

results in the standard individual holes with about 30-35 μm diameters. As expected, the

spacing between the droplets continues to increase linearly as the droplets are spaced further

apart.

From these results, it is concluded that assuming the inkjet deposition process is precise, the

optimal drop spacing would be around 40-45 μm. This drop spacing would result in the

narrowest line width. To be conservative however, slightly closer drop spacing was typically

used to offset the possibility of any inaccuracy in drop placement to ensure the printed line is

entirely continuous which is critical for solar cell design and fabrication.

3.4 Summary

In this chapter, a new and patented dielectric patterning technique using an indirect inkjet

printing method was described. The inkjet process development began by evaluating a range

of potential functional fluids in terms of their compatibility with the used inkjet device. The

corrosive nature of many acidic and basic solutions that could be used as direct dielectric

etchants rendered them unusable due to their damaging effects on components of the

printhead. The indirect inkjet patterning process was selected as the most suitable method

given the range of inkjet printheads available at the time. This method is quite similar to the

photolithographic patterning process. However, unlike photolithography, the inkjet process

makes use of low-cost chemicals and equipment to achieve patterning of dielectric layers.

A theory, which uses the HSP to identify solvents for polymers, was described. Using this

theory, DEG was identified as a suitable plasticiser for the novolac resin. It was found that the

inkjet deposition of DEG resulted in plasticised regions in the resin layer, which were

permeable to dielectric etchants, such as HF. These plasticised regions enabled the etching of



the underlying dielectric layers. Compared to solvents, some of the advantages of using

plasticisers are: (1) the volume of plasticiser required to form the permeable regions is less

than the volume of solvent required to form openings in the polymer, and (2) compounds that

can act as good solvents for the resin typically evaporate more easily than the higher

molecular weight compounds that act as plasticisers, thus making it less ideal for jetting. The

optimisation of the jetting parameters for DEG was described. It was found that the fluid

properties of DEG were very suitable for jetting, thus making it possible to achieve high

precision deposition by inkjet at relatively high frequency.

Finally, a robust process flow of the indirect inkjet patterning method was described.

Individual round openings with diameters as small as 30-35 μm and continuous line patterns

with width as narrow as 40-50 μm on 300-400 nm thick SiO2 layer were shown to be

routinely reproducible using this inkjet patterning method. In order to achieve the smallest

possible feature sizes, two important parameters: (1) resin thickness and (2) drop spacing

were optimised. The development steps involved in formulating and optimising this inkjet

patterning process are applicable to the development of other inkjet patterning processes. As

will be shown in the following chapter, this new indirect inkjet dielectric patterning process

can be very useful to solar cell fabrication.

Chapter 4: Device Fabrication Using Inkjet


Chapter 4 : Device Fabrication Using Inkjet

4.1 Introduction

Many higher efficiency Si solar cell designs have been proposed and developed over the

years. Some of the notable designs were discussed in Section 1.3. The final conversion

efficiency of a solar cell is determined by the total recombination, current generation and the

parasitic losses of the device. The relationship between these three device parameters is

summarised in Equation (4-1):

FFJV scoc ××=η (4-1)

where η is the solar cell conversion efficiency, Voc is the open-circuit voltage of the solar cell,

Jsc is the short circuit current of the solar cell and FF is the fill factor of the solar cell.

The open-circuit voltage (Voc) of the device is related to the total recombination in the device.

High voltages can be achieved by maintaining good minority carrier lifetime, excellent

surface passivation and minimum dark saturation current.

The short-circuit current (Jsc) of the device is related to the current generation in the device,

which in turn is determined by light absorption. Strategies to enhance light absorption include

the use of surface texturing, light trapping, anti-reflection coating and reducing front surface

shading.

The fill-factor (FF) is a measure of parasitic losses in the device. These losses can be in the

form of either shunt or series resistances, which typically manifests themselves within the

metal contact regions of the solar cell.

This chapter will show how the indirect inkjet patterning method developed in Chapter 3 can

be used to form a range of structures that maximises the Voc, Jsc and FF (and therefore the

conversion efficiency) of the solar cell. In addition, working solar cell devices that incorporate

several of these high efficiency structures are fabricated and analysed.



4.2 Inkjet Printed High Efficiency Solar Cell Structures

4.2.1 Selective Emitter

The benefits of selective emitter have been well known in solar cell fabrication. It is a higher

performance alternative to the homogenous emitter design typically used in screen-printed

solar cells. In selective emitter design, two regions with different emitter doping profiles are

formed on the surface of the Si wafer. The first region is the emitter, where the p-n junction is

located. This region is usually lightly diffused to maximise the short-wavelength response of

the device. The second region is the contact area, which provides the low resistance interface

between the metal and the Si. This is in contrast to a typical commercial screen-printed solar

cell where a one-step homogeneous emitter is employed.

In order to illustrate the electrical performance gain of selective emitter structure compared to

standard screen-printed solar cell design, a one-dimensional model was constructed using

PC1D (Clugston and Basore 1997). The modelling parameters are presented in Appendix E.

The simulation was performed for both screen-printed and selective emitter design with 120

μm metal finger line widths and a selective emitter design with 20 μm metal finger line

widths. The electrical parameters of the modelled solar cell devices are shown in Table 4-1.

Cell front surface design Voc (mV) Jsc (mA/cm2) FF (%) η (%)

Screen-printed (120 μm) 622.0 35.1 76.0 16.6

Selective emitter (120 μm) 626.1 35.5 77.4 17.2

Selective emitter (20 μm) 629.6 36.1 80.1 18.2

Table 4-1: PC1D-modelled electrical performance comparison between a

standard screen-printed design and a selective emitter design.

As also evident from the simulated electrical results of the solar cell devices in Table 4-1, the

performance gain of a selective emitter structure is maximised for narrower metal finger line

widths. The correlation between metal finger line widths and the corresponding simulated

device efficiency as a function of metal coverage is shown in Figure 4-1.



Figure 4-1: PC1D-modelled selective emitter cell efficiency as a function of

metal coverage percentage area for different metal finger line widths. The data

points represented by solid squares show the optimal metal coverage

percentage for each line width and its corresponding maximum cell efficiency.

There are several reasons for the observed increase in maximum device efficiency as the

metal finger line width is reduced. Firstly, narrower line widths allow the metal fingers to be

spaced closer to each other, which reduce the lateral series resistance in the emitter. Secondly,

closely-spaced fingers reduce the metal coverage area which improves light absorption

through less shading of the front surface. Finally, lower metal coverage area means increased

passivated surface area and less dark saturation current contribution from the metal-Si

interface regions. Therefore, to achieve the highest efficiency possible in a selective emitter

structure, it is preferable to use a patterning method that is capable of producing very fine

feature sizes.

The formation of selective emitter using indirect inkjet patterning method follows similar

processing steps as a photolithographically defined selective emitter. Typically, after a light

emitter is formed in the first diffusion step, a dielectric layer is applied on the front surface of

the solar cell. The dielectric layer serves as a diffusion mask for the subsequent heavy

diffusion process. Dielectric patterning steps as described in Figure 3-9 is then followed to

form the desired contact pattern on the Si surface. Finally, a second high temperature furnace



diffusion step is performed to form heavy diffusion regions on the patterned contact areas.

This processing method produces heavily doped contact lines of 45-50 μm widths with sheet

resistivities as low as 5 Ω/, thus allowing low metal coverage area and low contact

resistance.

One way to monitor the effects of the inkjet-formed selective emitter process on the device

quality is through its minority carrier lifetime which was measured using the flash photo-

conductance technique (Sinton and Cuevas 1996). A batch of ten standard commercial CZ, 1

Ω.cm, p-type Si wafer were alkaline-textured followed by a phosphorus thermal diffusion to

form a 100 Ω/ emitter and growth of a 350 nm thermal oxide. The front surface of the

wafers were then inkjet patterned and the wafers then underwent a second heavy phosphorus

thermal diffusion following the procedure described in the previous paragraph to create the

selective emitter structure. Average implied Voc of 664.1 mV and 662.7 mV were measured

after the first and second high temperature processes respectively, indicating that high bulk

lifetime and low surface recombination were maintained during the inkjet selective emitter

formation process.

Due to its similarity with the photolithographic patterning process, one of the main

advantages of the said inkjet patterning is the absence of crystallographic damages in the Si

like those commonly found in laser processing. Wenham et al. (1997) have previously studied

the far-reaching effects of laser-induced crystal defects in Si by performing electron-beam

induced current (EBIC) scan that shows subsequent high temperature process during the

heavy diffusion can cause the defects to propagate deep into the Si substrate. A similar

method using EBIC scan was used to demonstrate the inkjet patterning quality. EBIC can

determine the location of the junction in a freshly-cleaved surface by exposing the surface to

electron beams which generate current. However, only the current within the vicinity of the

space charge region in the junction is able to contribute due to very high recombination

elsewhere.

Figure 4-2(a) shows an EBIC scan superimposed with an SEM image of the heavily diffused

region in an inkjet patterned selective emitter structure. The light emitter was diffused with

phosphorus to about 100 Ω/ and the heavy emitter was diffused to about 5 Ω/. For

comparison, Figure 4-2(b) shows the EBIC scan of laser formed groove (Wenham et al. 1997)



Figure 4-2: EBIC images of inkjet selective emitter (figure A) and laser formed

groove - from Wenham et al. (1997) - (figure B). Both underwent heavy

diffusion process after patterning. The well-defined junction location in Figure

A indicates the absence of crystallographic damage in the patterned region.

From Figure 4-2, it is evident that the junction depth was clearly delineated within the reach

of the diffusion process, which indicated the absence of any defects that would have allow the

phosphorus to propagate deep into the substrate. This is in contrary to laser-processed regions

as shown by Wenham et al. (1997) whereby phosphorus was shown to diffuse as deep as 30

μm through the defected region. This comparison shows one of the advantages of forming

selective emitter structure using inkjet patterning.

4.2.2 Localised Contacts

Although most incoming photons in solar cells are absorbed within the first few microns of

entering the device, the rear surface of a solar cell still potentially represents a high

recombination centre that could drag down the device electrical performance. The rear surface

of the solar cell becomes important when it is the dominant recombination region in the

device. Improvements at the rear surface can be achieved through surface passivation together

with the use of a localised contacting scheme.

Some typical designs of well-passivated, localised rear contact structures in the fabrication of

high efficiency Si solar cell together with their typical open-circuit voltage values are shown

in Table 4-2. The dielectric layer used to passivate the rear surfaces in all of these solar cells

was SiO2. The contacting rear metal used was Al and dielectric patterning was done using

photolithography.

Inkjet-formed contact region

A BB 25 μm



Cell Technology Diagram of Rear Surface Design Voc (mV) Reference

PERC

(0.2 Ω.cm,

p-type, FZ)

696 (Blakers et al. 1989)

PERL

(2 Ω.cm,

p-type, FZ)

696 (Wang et al. 1990)

PERT

(1.5 Ω.cm,

p-type, MCZ)

707 (Zhao et al. 2001)

PERF

(1 Ω.cm,

p-type, FZ)

714 (Altermatt et al. 1996)

Table 4-2: Various low-area localised rear contacting schemes used in the

fabrication of high efficiency Si solar cell (the front surfaces are omitted from

the diagrams).

In the PERC structure, the rear comprised of SiO2-passivated p-type surface with

photolithographically defined localised metal directly contacting the substrate. In terms of

passivation quality, the PERL cell was mostly equivalent to the PERC passivation except for

the reduced dark saturation current contribution from the metal contact regions which in

PERL were passivated with heavy boron diffusion. A major improvement in Voc was observed

with the PERF structure through the incorporation of an n-type surface on the rear. This

creates a floating junction structure whereby the rear p-n junction is not contacted to the

external circuit. The high Voc achieved by the floating junction structure can be attributed to

the reduction of hole density in the vicinity of the rear surface in the n-type layer and due to

the superior dielectric passivation quality on an n-type surface compared to a p-type surface.

All of these localised rear contact schemes can be equivalently attained using the indirect

inkjet patterning method by printing and forming individual holes according to a

predetermined pattern throughout the rear surface.

p p++ p++

n+

pp++ p++

p+

p p++ p++

p



In most standard screen-printed solar cell process, the gaseous phosphorus used during

emitter formation usually also result in a rear n-type layer. This unwanted layer is usually

isolated from the front surface either by plasma etching, chemical etching or laser isolation

and then destroyed by screen-printed Al during the metal firing step. However, this n-type

layer could actually be beneficial by using it as a floating junction structure at the rear of the

solar cell as shown in Figure 4-3(a). Passivating an n-type surface using dielectric layer is

much more advantageous than a p-type surface resulting in a lower surface recombination

velocity. Furthermore, minority carriers are less likely to recombine in the bulk area near the

rear surface due to the low hole density within that vicinity.

Once the floating junction structure is set up, dielectric patterning is still required in order to

form localised rear contacting scheme. Indirect inkjet patterning was used to achieve this. In

this case, the n-type layer is the exposed layer on the rear surface while the metal needs to

make contact into the p-type substrate. After the rear dielectric passivating layer has been

patterned, the n-type Si surface is then isotropically etched until the buried p-type layer is

exposed. The isotropic etchant would create an overhanging dielectric structure as shown in

the Focused Ion Beam (FIB) image in Figure 4-3(b). The overhanging dielectric structure is

useful as a mask in a line-of-sight metal deposition to prevent metal from depositing into the

n-type walls. This provides an electrical isolation between the p-type and the n-type layers,

which avoids the shunting problem commonly encountered in PERF solar cells.

Figure 4-3: Floating junction-passivated localised rear contacting scheme (for

clarity, the rear surface is facing up in Figure A). Figure B shows a Focused Ion

Beam (FIB) image of the inkjet-fabricated overhanging dielectric structure.

p

Dielectric layer Aluminium

Silicon Overhanging dielectric

n+

A B

n+



The ability of the indirect inkjet patterning method to create both line and hole openings in

dielectric layer can also be beneficially used to form interdigitated rear contacting scheme.

Such rear contacting scheme alleviates the need for a metal grid on the front surface, thus

resulting in zero shading. The inkjet patterning can be applied multiple times in order to

create the alternating p-type and n-type contact regions on the rear surface. An innovative

application of this indirect inkjet patterning technique on a low-temperature simultaneous

contacting of n-type and p-type surfaces has been demonstrated by Kuepper et al. (2007).

4.2.3 Surface Texturing and Sculpturing

The surface of a bare Si wafer reflects about 30% of the incident light. In order for the Si

material to absorb more photons to generate carriers, more proportion of the incident light

needs to be coupled into the Si. Surface texturing is commonly used to reduce the surface

reflection of Si in solar cell fabrication.

The most common method of surface texturing used in large-scale production is the alkaline-

textured surface, which produces random upright pyramids on the Si surface with dimensions

between 3-8 μm. For high efficiency Si solar cells, other more effective surface texturing

using macrogroove and inverted pyramid structures have been used to further reduce

reflection from the surface of the Si wafer by increasing the number of bounces of incoming

light reflected from the Si surface. This improves the number of chances for the light being

absorbed by the Si as well as increasing the path length of the light inside the wafer due to

favourable incident angles. Comparison of the effectiveness of various textured surfaces for

solar cells has been reviewed by Campbell and Green (1987).

Despite the superior light-trapping properties of macrogroove and inverted pyramid

structures, the formation of such structures requires dielectric masking and photolithographic

patterning. The indirect inkjet patterning process offers an alternative patterning method to

photolithography. Individual closely-spaced holes and continuous lines can be formed on the

dielectric surface of the Si wafer followed by anisotropic alkaline etching to form inverted

pyramid and macrogroove structures respectively. In both cases, the dielectric layer acts as an

important etching mask, which defines the desired surface structures. The resulting inkjet

printed macrogroove and pyramid textured surfaces are shown in SEM images of Figure 4-4.



Figure 4-4: SEM images of indirect-inkjet-patterned macrogroove texturing

(figure A) and inverted pyramid texturing (figure B) on a polished Si surface

(Borojevic et al. 2008).

Compared to photolithographically defined structures, the main limitation of the inkjet printed

texturing is the minimum feature sizes. Another major limitation is the density of the printed

features, which determines how close the spacings between each feature. A typical array of

line patterns used to form macrogrooves such as in Figure 4-4(a) usually requires a minimum

spacing of ~20 μm between the lines. Similarly, the edge-to-edge spacing for hole patterns

like in Figure 4-4(b) is ~30 μm. This limit is imposed by several factors including the inkjet

deposition accuracy, substrate’s surface energy, resin adhesion and etching time.

Excessive space between the surface features causes flat surfaces on the Si wafer which have

poor reflection properties. Therefore, it is preferable to minimise the inter-feature space to

achieve the lowest possible surface reflection. In order to mitigate this issue, Borojevic et al.

(2008) have proposed a possible solution by using isotropic etches after inkjet patterning in

order to undercut the spaces in between features and reduce their effective spacing. This way,

the distance between two adjacent features is no longer limited by the resolution of the

dielectric patterning as a result of the inkjet printing step. The reflectance results of such

inkjet-formed macrogrooves and inverted pyramid structures have been shown to be

comparable to photolithographically-formed inverted pyramids used for the front surface

texturing in the world-record PERL solar cell design (Borojevic et al. 2008).

A B



The inkjet grooving method can also be similarly applied to form grooves in solar cell designs

such as the buried contact solar cell (BCSC). The standard grooves in BCSC have been

formed using laser ablation or mechanical scribing followed by alkaline etching to remove

slags or damage from the process. Alternatively, the indirect inkjet patterning method

combined with chemical etching can be used to form these grooves, which has the advantage

of causing no crystallographic damage to the material.

A particularly interesting application of surface sculpturing is when both sides of the Si wafer

surfaces are covered with macrogroove structures with the macrogrooves in one side of the

surface formed perpendicular to the other side. This so-called perpendicular slat solar cell

structure is shown schematically in Figure 4-5.

Figure 4-5: Perpendicular slat solar cell structure.

In order to form this structure, the inkjet printing method as described in Figure 3-9 is used to

create an array of closely-spaced lines on both the front and rear surfaces perpendicular to

each other followed by an alkaline anisotropic etching of the Si until a V-groove structure is

formed in each line. The location of the metal contacts can be predefined by setting an

appropriate spacing between the printed lines that would correspond with the metal line

width. Macrogrooves arranged in this perpendicular slat structure have been shown to

produce an excellent light trapping properties (Campbell and Green 1987). This is because

light that enters the perpendicular slat structure can continuously be total internally reflected

inside the material through the absence of parallel <111> planes on the front and the rear

surfaces.



The perpendicular slat structure is also especially useful for thin wafers because it provides a

reasonable mechanical strength through its corrugated-like structure. The current typical

inkjet-formed groove is about 90 μm wide and 65 μm deep, which implies that when the two

perpendicular groove arrays are designed to be back-to-back with each other such that the

peaks of the grooves from each side are in contact, a Si wafer of around 130 μm thick can be

used.

4.2.4 Edge Isolation

Most solar cell designs require an edge junction isolation step to remove any unwanted

electrical path between the p-type and the n-type polarities on the two wafer surfaces that

were inadvertently formed (usually during the diffusion process). Such electrical path

provides a low resistance path for the minority carriers to flow to the opposite polarity doped

region to recombine rather than flowing to the metal contacts to be extracted. This effect can

be modelled as a voltage drop across a resistor due to current flow and it electrically manifest

itself in the final device as a shunt resistance which lowers the fill factor of the solar cell.

The edge isolation needs to be performed in such a way that minimises its impact on the

carrier generation and collection in the active area of the solar cell. This usually means that

the isolation is performed as close as possible to the edge of the wafer before the intersection

between the p-type and n-type regions. There are three common edge isolation methods:

• Plasma etching

After diffusion process, Si wafers are stacked on top of each other and clamped from

both sides such that only the edges of the wafers are exposed. Dry etching in vacuum

is then performed on the exposed edges to remove the doped regions until the typically

higher resistivity substrate surface is exposed.

• Wet etching

After diffusion process, the side of the wafer not intended to be diffused are

chemically etched (usually by isotropic Si etch solution such as a mixture of HF and

HNO3) to remove the diffused layer. The diffused side can be protected by a masking

layer or maskless etching can be done in a single-side etching system.



• Laser isolation

Laser ablation around the edges is used to remove the diffused layer thus resulting in a

high resistance trench or gap between regions with opposite polarities. Laser isolation

is typically performed at the end of the cell fabrication. Due to short diffusion depth of

the dopants, the laser only needs to remove a couple of microns of the Si material.

Using the indirect inkjet patterning method, an alternative edge isolation process is introduced

by opening continuous lines on the dielectric layer along the edge perimeter of the Si wafer

combined with wet chemical etching of Si. This will result in a trench as shown in Figure 4-6,

which isolates the n-type emitter region on the front surface of the solar cell from the p-type

rear surface. In the fabrication of standard screen-printed solar cells, this edge isolation step

may be performed for example, after the application of SiNx on the front surface of the solar

cell and before the screen-printing process. In order to achieve excellent isolation, the alkaline

etch step must etch the Si to a depth more than the emitter diffusion depth such that the p-type

substrate is exposed. Furthermore, no conducting materials, such as metal, are allowed to be

deposited inside the trench, which might cause a shunting path.

Figure 4-6: Schematic of inkjet edge isolation in a standard screen-printed

solar cell showing electrical separation between the p-type and n-type regions.

Test structures were fabricated as follows. A CZ, p-type, 1 Ω.cm Si wafers with saw-damage-

etched planar surfaces were diffused with phosphorus using POCl3 furnace process to form an

emitter layer of ~100 Ω/ followed by the growth of passivating thermal oxide of ~350 nm

thick. The rear surface was then evaporated with Al of about 2 μm thick and sintered in a N2-

ambient furnace at 980 oC for 45 mins to form both BSF and the rear metal ohmic contact.

Ag

Al

p+

n+

Inkjet-formed trench



Isolation trenches were inkjet-patterned on the front surface of the wafer in the form of

rectangular-shaped perimeter about 0.1 mm from the edges of the wafer, except on one side

where a substantial distance is present between the inkjet-formed isolation trench and the

edge of the wafer. This is illustrated in Figure 4-7. The front surface was then deliberately

scratched within this area with a diamond-pen in a straight-line about 1 mm parallel to the

isolation trench. The scratch on the front surface deliberately damaged both the oxide

dielectric layer and the junction layer which induced shunt in the region.

Photoluminescence (PL) imaging (Trupke et al. 2006) was used to characterise the ability of

the inkjet-formed trench in isolating shunts in solar cells. The PL imaging technique has

previously been similarly employed in identifying edge isolation of shunts by Abbott et al.

(2006) and proved to be a quick and powerful method of spatial qualitative identification of

shunts in solar cell structures. The bright areas in a PL image shows regions with good

minority carrier lifetime, while dark areas shows regions with poor minority carrier lifetime.

PL images of the inkjet-isolated wafer were taken before and after the front surface were

scratched in order to show the effectiveness of the inkjet-formed isolation trenches in

shielding the inside of the rectangular region from the induced shunt. Both images were

measured under 0.1 suns illumination. These images are shown in Figure 4-7.

Figure 4-7: Photoluminescence images of test structure wafers clearly showing

a line on one side of the rectangular inkjet-formed isolation trench (figure A)

and its effectiveness in isolating the top two-third of the Si wafer from the

scratch-induced shunt on the bottom third of the wafer (figure B). Both images

were measured under 0.1 suns illumination.

A B

Location of scratch





It is clear from Figure 4-7 that without the inkjet-formed isolation trench, the low minority

carrier lifetime (dark) regions on the bottom third of the sample would have also occupied the

area above the scratch. The presence of the inkjet isolation trenches prevent carriers located

within the rectangular region from being sucked into the shunted region This clearly

demonstrates that inkjet-formed trenches can be effectively used to isolate shunted regions in

Si wafers. Note also that in the PL images, the area inside the rectangular region close to the

inkjet-formed isolation trenches is darker than the centre region. This artefact is due to the

unpassivated surfaces of the isolation trenches, which results in high surface recombination

velocity.

4.3 Simple Selective Emitter Solar Cell Using Inkjet

4.3.1 Device Design

In order to demonstrate the advantageous applications of the abovementioned inkjet-formed

high efficiency structures to solar cell electrical performance, complete devices incorporating

some of the simplest of these structures were designed, fabricated and characterised. In the

standard screen-printed solar cell design, the front surface of the device is the dominant

limiting factor to higher performance. To that end, the screen-printed front surface design was

replaced with the inkjet-formed selective emitter structure as described in Section 4.2.1 while

the rest of the solar cell design was retained. The device structure is shown in Figure 4-8.

Figure 4-8: Schematic of the selective emitter solar cell structure fabricated

using inkjet printing (not to scale).



A drawback of the inkjet patterning method is its relatively wider line width compared to

those formed by laser. The front surface contact design must be adapted to this finger width

limitation. To achieve a minimum electrical loss, the optimum balance between the lateral

emitter resistive losses and shading losses due to the metal coverage must be found. The

fractional power loss resulting from the lateral resistance losses is given by (Green 1998):

mp

mpstl V

JSP

×

××=

12

2ρ (4-2)

where Ptl is the fractional power loss due to lateral resistance, sρ is the emitter sheet

resistivity, S is the metal finger spacing, Jmp is the current of the solar cell at maximum power

point and Vmp is the voltage of the solar cell at maximum power point. Additionally, the

fractional power loss due to the finger and busbar shading losses is given by (Green 1998):

S

WP Fsf = (4-3)

B

WP B

sb = (4-4)

where Psf is the fractional power loss due to finger shading, Psb is the fractional power loss

due to busbar shading, WF is the finger width, WB is the busbar width, S is the metal finger

spacing and B is the metal finger length.

Therefore, the total combined fractional power losses due to lateral resistance and shading can

be calculated by adding each loss component as follows:

sbsftltot PPPP ++= (4-5)

In order to calculate the optimum finger spacing S, the following assumptions are used:

• An emitter sheet resistivity sρ of 100 Ω/, thus avoiding any degradation of the

spectral response, particularly in the short wavelength regions.



• Maximum power point current Jmp of 31.5 mA/cm2, which is the typical value for a

selective emitter device as shown in Figure 4-8.

• Maximum power point voltage Vmp of 515 mV, which is the typical value for a

selective emitter device as shown in Figure 4-8.

• Metal finger width WF of 50 µm, which is the typical continuous line width achieved

with indirect inkjet patterning method.

• Busbar width WB of 260 µm and metal finger length B of 26 mm (for a square 8 cm2

area device with front contact pattern as shown in Figure 4-9), which gives a typical

fractional busbar shading loss Psb of 1%.

Figure 4-9: Front contact pattern design for inkjet selective emitter solar cell.

Using Equations (4-2), (4-3), (4-4) and (4-5) and solving S to achieve a minimum value of

Ptot, it was found that the optimum finger spacing is 1.7 mm. This finger spacing results in

1.47% lateral resistance loss, 2.94% finger shading loss and 1% busbar shading loss, thus

giving a total fractional power loss of 5.41%. This is about 1.63% higher power loss

compared to a laser-formed fingers, which has an assumed width of about 25 µm.

Other than the metal covered areas, the rest of the front surface is passivated by thermally

grown SiO2. Due to the presence of the heavily doped contact regions, the emitter diffusion is

less critical compared to the screen-printed design. Therefore, the main consideration for the

emitter design is that it is light enough to achieve good spectral response for the short

wavelength photons without excessive lateral resistance loss. The rest of the device design

follows the structure similar to screen-printed solar cell, including rear Al BSF and the use of

standard p-type, 1 Ω.cm, Czochralski Si wafer with thickness of about 250 µm.

26 mm

1.7 mm

26 mm



4.3.2 Device Fabrication

An outline of the fabrication sequence of the inkjet selective emitter solar cell structure as

shown in Figure 4-8 is presented in Table 4-3. The process sequence is essentially the same as

the standard single-sided buried contact solar cell routinely fabricated at UNSW. The only 3

differences are: (1) the use of inkjet patterning instead of laser in forming the front contact

fingers, (2) the absence of alkaline etch after patterning as there is no Si damage to be

removed, and (3) a shorter Cu plating time because of the wider inkjet-formed finger width.

Steps Parameters

1. Wafer selection p-type boron-doped, 1 Ω.cm, CZ, 300 µm, as-lapped

2. Texturing Alkaline-textured (NaOH/IPA solution)

3. Phosphorus diffusion Solid source deposition at 850 oC for 16 mins – aim 100 Ω/

4. Thermal oxidation

800-1030 oC ramp up, 1030 oC/30 mins TCA oxidation, 980 oC/75 mins wet oxidation, 1030 oC TCA oxidation, 1000 oC

/15mins anneal, ramp down to 800 oC – final sheet resistance

of 100 Ω/

5. Spin coating

Microposit FSC-M Surface Coating (Rohm & Haas), spin

speed of 5,500 rpm/30 secs, followed by oven bake at 140 oC/10 mins – coat both sides

6. Inkjet patterning Front contact fingers 1.7 mm spacing, DEG 1 pL drops, 2

layers, platen at 60 oC

7. Pattern etching BOE (7:1) 8 mins, followed by H2SO4:H2O2 (3:1) 5 mins to

remove surface coating

8. Front contact diffusion Solid source diffusion at 940 oC for 45 mins – aim 5 Ω/

9. Rear contact formation Thermal evaporation of Al (2 µm), alloy at 980 oC/15 hrs

10. Deglazing BOE (15:1) 40 secs

11. Ni plating Ni electroless plating 80-85 oC/5 mins, sinter at 350 oC/10

mins (N2 ambient), 2nd Ni electroless plating 80-85 oC/1 min

12. Cu plating Cu electroless plating 42 oC/3 hrs

13. Edge isolation Laser scribe from rear and cleave

Table 4-3: Fabrication sequence of inkjet selective emitter solar cell.



While the majority of the processing sequences are relatively standard, several problems were

encountered during the processing of inkjet selective emitter solar cells, particularly in the

metallisation step due to the use of inkjet patterning. Figure 4-10 depicts some of the

metallisation problems observed during the development of the device.

Figure 4-10: Optical microscope image of discontinuous metal finger (figure A),

overplated surface (figure B) and excellent plated metal finger (figure C).

Figure 4-10(a) shows an optical microscope image of a discontinuous inkjet patterned finger

line where the plated metal failed to connect. This could result in high series resistance in the

final device due to poor conductivity along the metal fingers. In some cases where the inkjet

patterned lines are not completely continuous, the plated metal would still be able to bridge

the gap because the metal growth is usually isotropic. However, the thickness of the plated

metal is usually only around 15-20 µm, which means that any gaps greater than this would

result in discontinuous metal finger. In order to alleviate this problem, the drop spacing of the

inkjet printed droplets was slightly reduced to 35 µm. This reduction slightly sacrificed

feature size for greater line formation reliability.

Figure 4-10(b) shows an optical microscope image of oxide-protected Si surface overplated

by Cu during the plating process. It was found that the overplating occurred due to the

inability of the spin-coated resin layer to completely protect the peak of the surface pyramids

during the inkjet patterning process. Originally, the typical resulting pyramid heights from the

surface texturing were about 3-5 µm whereas the thickness of the resin as a result of the spin-

coating parameter used was only around 2.5 µm. To avoid overplating, the texturing solution

was modified in order to produce smaller pyramids of about 1-2 µm heights. With these slight

modifications, excellent plated metal fingers could be formed as shown in Figure 4-10(c).

A B C

100μm 100μm 100μm



4.3.3 Device Characterisation

Figure 4-11 shows the current-voltage electrical characteristics of the best-performing initial

device fabricated according to the inkjet selective emitter solar cell structure. The light J-V

curve shown in Figure 4-11(a) was measured against a calibrated reference under STC

condition at 1-sun illumination. The dark J-V curve shown in Figure 4-11(b) was measured

without illumination (in the dark). Additionally, Figure 4-11(b) also plots the calculated local

ideality factor as a function of voltage, which was derived from the differential of the dark J-V

curve. The average electrical performance parameter of a batch of ten solar cells that was

fabricated according to the process sequence of Table 4-3 is also shown.

Figure 4-11: Best electrical performance of an initial inkjet selective emitter

solar cell device showing the measured current-voltage curves against a

calibrated reference under STC conditions at 1-sun illumination (figure A) and

in the dark (filled squares, figure B), together with the calculated local ideality

factor curve as a function of voltage (open squares, figure B). The table shows

the average electrical results of a batch consisting of ten fabricated solar cells.

One of the main highlights of the result shown above is the achievement of an average fill

factor of 80%, which indicates that the inkjet-formed contacts displays excellent contact

formation quality which minimises parasitic losses, particularly due to possible shunts. Due to

its similarity with photolithographically-formed contacts and the absence of structural damage

A B



during its formation, this result is originally expected. The high diode quality of the device is

also clearly shown from its measured electrical response in the dark where the device shows

no signs of parasitic shunts as highlighted by the excellent m = 1 and m = 2 local ideality

factor behaviours in the moderate and low voltage regions respectively.

Despite the excellent fill factors, the final conversion efficiency of the device was rather

limited by the lower than expected Voc and Jsc. According to PC1D modelling using

parameters as listed in Appendix E but with a slight change in the optical coating parameters

in order to account for the use of SiO2 (t = 110 nm, n = 1.46) instead of SiNx, the expected

electrical performance of the device are as follows: Voc of 620.0 mV, Jsc of 35.0 mA/cm2 and

FF of 0.80 thus resulting in a final efficiency of 17.3%. Except of the FF value, both the Voc

and the Jsc measured device values were much lower than the model predicted which

warranted further investigations.

A typical measured spectral response of the fabricated inkjet selective emitter device is shown

in Figure 4-12, showing the measured IQE, EQE and reflectance values of the device. Figure

4-12 also shows the fitted IQE and EQE values as modelled using PC1D. The PC1D

parameters are listed in Appendix E.

Figure 4-12: Spectral response of the inkjet selective emitter solar cell device

showing the measured IQE (open triangles), EQE (open squares) and

reflectance (open circles) values. The black solid lines represent the fitted IQE

and EQE values for the device as modelled using PC1D.



The measured spectral response revealed IQE of slightly less than unity in the short

wavelength range between 300-600 nm. This indicates an increased recombination rate on the

top surface of the device. Two likely causes of the observed increased recombination rate are

heavier emitter diffusion and poor passivation on the top surface. Further investigation of the

emitter diffusion process for the device showed that the sheet resistivity of the emitter turned

out to be around 60-65 Ω/, which is heavier than the desired 100 Ω/. This was simply due

to the non-optimised process parameters used in the fabrication process. The front surface

SRV value was also varied to fit the measured IQE better, which resulted in a slightly

increased fitted SRV value of 2500 cm/s.

The major performance degradation however, was likely due to a significant drop in bulk

lifetime during the metallisation process. After the front contact diffusion process (step 8 in

Table 4-3), the average implied Voc was around 660 mV. After the metallisation process, the

average final Voc was only around 605 mV, which indicates a significant 55 mV drop. The

voltage drop is unlikely to be caused by a junction recombination mechanism mainly because

of the device’s high measured fill factor value and the absence of any non-ideal local ideality

factor behaviour in the measured dark J-V curve as shown in Figure 4-11(b).

The most likely process in the metallisation sequence that could cause such a dramatic

reduction in bulk lifetime is the long high temperature Al sintering (980 oC for 15 hours). The

lifetime degradation was probably caused by the presence of damaging contaminants within

the furnace tube during processing due to the equipment’s poor condition. When such

contaminants are diffused into the Si bulk, it usually increases the Shockley-Read-Hall

recombination in the semiconductor material. Other processes in the metallisation sequence

only involve relatively low temperatures, which are unlikely to significantly degrade the bulk

lifetime of the material. The value extracted from the IQE modelling in PC1D indicated a

post-processing bulk lifetime of only around 5 μs.

Regardless of the equipment and process shortcomings, the ability of inkjet-patterned contact

openings to reach very high fill factors strongly indicates the applicability of the inkjet

patterning method towards higher efficiencies in solar cell fabrication. With further

optimisations, it is highly likely that conversion efficiencies above 18% can be reached by

inkjet-processed selective emitter solar cell devices.



4.4 Summary

In this chapter, a range of high efficiency solar cell structures have been fabricated using

indirect inkjet printing method. These structures have previously been demonstrated by using

traditional patterning methods (such as photolithography) to produce significant increases in

solar cell device efficiency. One of the most important high efficiency structures is the

selective emitter structure. The best implementation of selective emitter structure is through

photolithographic patterning. More recently laser patterning has been used to produce similar

structures. This chapter proposed inkjet printing as an alternative patterning method. The

many advantages of using inkjet printing have been discussed. One of the major

disadvantages however, is the comparatively larger feature sizes produced by inkjet

patterning.

While selective emitter structures are usually implemented on the front surface of the solar

cell, major performance improvements can be achieved through the use of localised

contacting scheme through passivating dielectric layer at the rear of the solar cell. A

combination of these types of structures has resulted in the world-record efficiencies

demonstrated by the PERL solar cell. A range of localised contacting schemes that can be

produced using inkjet printing have been discussed. A particularly innovative way of

implementing localised contacts and floating junction passivation using inkjet patterning has

also been proposed. Such structure was previously demonstrated to produce very good

passivation quality on the rear surface of the solar cell.

Other inkjet-formed solar cell structures have also been demonstrated, including: (1) surface

texturing through the formation of inverted pyramids on the front surface of the device,

similar to the light-trapping scheme used by the world-record holding PERL cell, and (2) an

alternative edge isolation technique through a combination of inkjet patterning of dielectric

layer and chemical etching.

Finally, a complete working device that utilises the inkjet-patterned selective emitter structure

was designed and fabricated to provide an example of the use of inkjet printing as a

processing technique in solar cell fabrication. Fill factors as high as 0.800 have been routinely

achieved in these devices, with efficiencies above 18% feasible with further process

optimisations.

Chapter 5: Inkjet Printing of Liquid Dopant Source


Chapter 5 : Inkjet Printing of Liquid Dopant Source

5.1 Introduction

One of the most attractive features of inkjet printing is the ability to selectively deposit

materials on the substrate without the use of any mask. In semiconductor fabrication, this is

commonly termed additive processing as opposed to the traditional subtractive method

employed in photolithography. In this case, the fluid used is usually functional, meaning that

the fluid carries an active payload that will eventually stay on the substrate and become part

of the final device.

This chapter presents the evaluation of the viability of using inkjet-printable liquid dopant as a

method of localised doping as well as potentially simultaneous dielectric patterning on Si

wafer substrates. For this initial work, the focus was on using phosphorus-containing liquid

sources that are specially formulated for inkjet printing. Several innovative ways of direct

patterning Si and dielectric layers through the use of inkjet-printed liquid phosphorus sources

will be described, followed by discussions on the fluid formulation and characterisation. Due

to the hydrophilic nature of many dielectric surfaces, appropriate surface treatments are

required. By combining these developments, the direct inkjet patterning method was used to

fabricate a solar cell device in order to demonstrate the applicability of the technique.

5.2 Direct Inkjet Patterning Concepts

5.2.1 Localised Doping on Silicon Surfaces

For standard p-type screen-printed solar cells, the two most common ways of performing

diffusion is by furnace diffusion using either solid, liquid or gaseous sources and by room-

temperature deposition of phosphorus-containing source, such as H3PO4, followed with belt

furnace diffusion. Typically, the H3PO4 is delivered using spray deposition equipment.

Theoretically, inkjet printing can also be used to deposit H3PO4 or other liquid phosphorus

sources onto the entire Si wafer surface. However, it is easy to see that the inkjet printing

deposition method would be disadvantageous compared to the spray deposition method,

mainly due to its much slower speed.



A more suitable application of liquid dopant sources by inkjet printing is to perform localised

deposition of dopant sources on a Si wafer substrate, followed by subsequent high

temperature treatment in a furnace. This approach fully utilises the strengths of the inkjet

deposition method. Selective inkjet deposition of liquid dopant sources with appropriate

polarities would allow the maskless direct patterning of multiple distinct diffusion regions on

Si surfaces. The significant reduction in process steps by using inkjet patterning instead of

photolithography is depicted in the process flowchart comparison shown in Figure 5-1.

Figure 5-1: Process steps flowchart comparison between localised diffusion

using traditional photolithography and inkjet printing of liquid dopant.

The illustration in Figure 5-1 shows that the number of processing steps can be potentially

reduced from seven to just two steps by using inkjet liquid dopant deposition to achieve local

diffusion on Si wafer surfaces. In addition, there are significant cost savings such as:

• Materials savings with the inkjet method (only liquid dopant required) compared to

costly photoresist and developer solution and others such as acid dielectric etchants

needed for photolithography.

• Less material wastage in the inkjet method as the liquid dopant is only deposited

where needed. This is in contrast to the use of photoresist, which is completely

removed afterwards.

• Only one high temperature step (furnace drive-in) is required with the inkjet method

compared to two steps (oxide growth and furnace diffusion) in photolithography.

Oxide growth

Photo-resist

UV expose & develop

Oxide removal

Photoresist removal

Furnace diffusion

Inkjet print

Oxide removal

Furnace drive-in



The application of inkjet liquid dopant as shown in Figure 5-1 does not retain any dielectric

layer at the end of the localised diffusion process. While this structure might not be suitable

for solar cell structures that require pre-patterned dielectric layers, there are some other

structures where this inkjet patterning method could be useful. An example of a solar cell

design that would benefit from this particular application of inkjet printed liquid dopant is the

semiconductor finger solar cell structure (Wenham et al. 2005).

The semiconductor finger technology implements a selective emitter structure while still

employing screen-printed metallisation. This is achieved through the formation of closely-

spaced, thin and heavily doped transparent “fingers” on a lightly doped front surface. The

metal fingers are then screen-printed perpendicular to these transparent fingers such that metal

contacts are made only where the two fingers intersect. The transparent fingers help transfer

collected carriers to the metal fingers. This allows the metal fingers to be spaced further apart

resulting in reduced shading without increasing the lateral emitter resistive losses.

Instead of the traditional method of using a laser to form shallow grooves followed by furnace

heavy diffusion to form the semiconductor fingers, liquid dopant sources could be inkjet

printed on bare textured Si surfaces to define the heavily-doped semiconductor fingers on the

wafer. The following lists a possible process sequence for fabricating a semiconductor finger

solar cell structure using inkjet printing of liquid dopant sources on standard commercial CZ

wafers:

1. Surface texturing using alkaline etching.

2. Inkjet printing of liquid dopant sources to define semiconductor fingers on the front

surface, followed by suitable drying of the printed dopant sources.

3. High temperature drive-in of printed dopant sources, combined with light emitter

diffusion on the front surface.

4. Diffusion glass removal.

5. Edge junction isolation.

6. PECVD SiNx deposition on the front surface.

7. Screen printing of rear and front metal pastes.

8. Metal co-firing.



Besides forming localised doped surfaces on bare Si surfaces, inkjet printing can also be used

as a precision method of depositing liquid dopants on pre-fabricated localised openings such

as holes or grooves. A comparison between the traditional method of achieving such localised

diffusion using furnace diffusion and the alternative approach using inkjet deposition of liquid

dopant followed by high temperature drive-in step is shown in Figure 5-2. In both cases, the

dielectric opening is assumed to be fabricated beforehand using an arbitrary method.

Figure 5-2: Process steps flowchart comparison between diffusion through

localised dielectric opening using conventional furnace diffusion and inkjet

printing of liquid dopant.

One of the advantages of locally inkjet depositing the liquid dopant sources instead of the

conventional homogenous furnace diffusion process is the ability to constrain dopant

diffusion only to areas where the dopant sources are present. This is particularly beneficial

when diffusion of dopants are undesirable in other parts of the Si wafer, for instance on the

rear, sides or other regions on the front surface.

Localised deposition of dopant sources using inkjet is also advantageous when used to

fabricate solar cell structures where two different diffusion polarities are required either on

single or both surfaces of the Si wafer. An example of such structure is the interdigitated

contacting scheme whereby an inkjet device can be used to deposit both p-type and n-type

dopant sources, followed by only a single high temperature step to drive both dopants sources

into the Si.

Oxide growth

Dielectric patterning

Furnace diffusion

+

Furnace drive-in

Inkjet print



5.2.2 Simultaneous Patterning and Doping on Dielectric Surfaces

In some solar cell structures, it is still preferable to be able to pattern dielectric layers and

perform localised diffusion on the subsequently exposed Si surfaces. Generally, at least two

processing steps are required to achieve this: one process for patterning and another for

diffusion. The concept of inkjet liquid dopant deposition as presented in Section 5.2.1 is only

targeted towards the direct patterning of Si surfaces, but not dielectric surfaces. It is desirable

to develop a method of combining both dielectric patterning and Si diffusion.

A novel concept for using inkjet-printed liquid dopant sources as a dielectric patterning tool

as well as a selective diffusion method is presented in Figure 5-3. Note that in this particular

technique, phosphorus is the only preferred liquid dopant source due to its known ability to

alter the etching rate of certain dielectric materials.

Figure 5-3: Process steps flowchart of simultaneous dielectric patterning and

selective diffusion through inkjet printing of liquid dopant source.

This patterning method relies on the presence of etching selectivity between intrinsic and

phosphorus-doped dielectric layer. The starting surface maybe a dielectric layer commonly

used in solar cell fabrication such as SiO2 or SiNx. Inkjet printing is then used to deposit

liquid phosphorus sources on top of the dielectric surface, followed by high temperature

drive-in of the printed phosphorus droplets. The drive-in step must allow the phosphorus

source to diffuse through the entire thickness of the dielectric layer, and preferably also

diffuse the underlying Si surface to create selective doping in these regions. The wafer may

then be immersed in dielectric etchant solution, such as dilute HF, which etches the

phosphorus-diffused regions of the dielectric significantly faster than the unaffected regions.

The etching selectivity generally depends on the amount of phosphorus doping present within

the dielectric layer thickness with higher concentration resulting in faster etching.

Apply dielectric

Inkjet print

High temp drive-in

Wet etch



As mentioned before, the high temperature drive-in step not only results in changing the

etching selectivity of the dielectric layer but also has the potential to diffuse the Si layer

underneath to create a localised doped Si region. Using this method, only one high

temperature step is required to form both the dielectric opening and the heavy diffusion. The

final concentration of phosphorus dopant in the Si depends on the amount of dopant atoms

available at the dielectric-silicon interface. Obviously, the thicker the dielectric layer, the

more difficult it is for the dopant atoms to reach the dielectric-silicon interface.

The above-mentioned inkjet patterning method can be immediately applied to simple

selective emitter structures whereby metals are directly deposited onto the heavily diffused Si

regions that are exposed. An example of a suitable self-aligning metallisation method to

achieve this purpose is through chemical plating where the metal preferably deposits onto the

Si. In this case, the rest of the unpatterned surfaces are protected from any metal deposition by

the retained dielectric layer. The deposited metal then makes intimate contact onto the heavily

doped Si surface, which result in a good ohmic contact.

A possible process sequence for fabricating a selective emitter solar cell structure using the

inkjet printing of liquid dopant sources method as described above is as follows:

1. Surface texturing using alkaline etching.

2. Light emitter diffusion on the front surface.

3. Diffusion glass removal.

4. Edge junction isolation.

5. PECVD SiNx deposition on the front surface.

6. Inkjet printing of liquid dopant sources on the SiNx surface.

7. Furnace drive-in of printed liquid dopant sources.

8. Selective wet chemical etching to form openings on the patterned SiNx layer

9. Screen printing of rear Al metal, followed by firing.

10. Ni/Cu/Ag chemical plating.

Two extra equipments that are needed are an inkjet device and a chemical plating system. The

relative suitability of this type of concept to existing production lines makes it an attractive

proposition for retrofitting in pursuit of higher efficiency device structures.



5.3 Jettable Liquid Dopant Sources and Surface Treatment

In order to conduct a preliminary proof-of-concept work for these innovative approaches of

using inkjet printed liquid dopant sources in solar cell fabrication, it is necessary to first

develop an appropriate inkjet process for this application. The approach taken for such

development is essentially similar to the one described at length in Section 2.3. Once again,

the central role of inkjet process development is the fluid formulation which determines the

viability of the printhead used to eject it. Another particularly important consideration in this

development process is the specific treatments that may need to be applied to the surfaces as

both surfaces used in this work (Si and dielectric surfaces) exhibit markedly different

behaviours when in contact with the inkjet printed liquid.

For convenience, boron and phosphorus are the designated dopants of choice in this work for

the p-type and n-type dopants respectively due to their widespread use in solar cell

applications. It is important to mention that for the localised doping on Si concept discussed

in Section 5.2.1, both boron and phosphorus dopant sources can be used. On the other hand,

for the dielectric patterning concept described in Section 5.2.2, only phosphorus dopant

sources are suitable for this method due to the observed increase in etching rate of

phosphorus-doped dielectric layers. As a result, the preliminary development work presented

in this section focuses only on liquid phosphorus dopant sources. Nevertheless, it is noted that

mostly similar limitations, requirements and performance are similarly applicable to liquid

boron dopant sources.

5.3.1 Phosphoric Acid-based Liquid Dopant Sources

The most obvious choice of phosphorus-containing liquid that is readily available as a source

for inkjet-printable dopant diffusion is phosphoric acid (H3PO4). It is a low-cost solution that

is produced commercially at a significant quantity and is widely used in semiconductor

process, particularly as the preferred etchant of SiNx layers. The solution is acidic (pH ~1.5),

so corrosion in the printhead material requires particular attention.

In order to investigate the compatibility between H3PO4 and the materials used in the

printhead construction, a compatibility test was conducted by immersing the material

compatibility kit (MCK) provided by the printhead manufacturer in the solution to be jetted.



The highest possible liquid concentration of H3PO4 is 85%, above which the solution becomes

solid. Therefore, the tests were conducted at room temperature using this concentration. In

this trial, only the Dimatix DMC-11601 printhead was investigated for compatibility due to its

suitability for preliminary work. A list containing the DMC-11601 printhead components is

presented in Appendix C.

After seven consecutive days of immersion in H3PO4 (85%) solution, these five components

were removed from the solution, rinsed under deionised (DI) water and then dried using N2

gun. Preliminary visual checks by naked eye showed no observable physical changes to the

materials. Closer examination through the use of an optical microscope on the surfaces of the

components further confirmed the positive results.

Table 5-1 lists the fluid properties of H3PO4 (85%) that are relevant to jetting quality. The

right column shows the optimum jetting parameters of the DMC-11601 printhead.

H3PO4 (85%) Optimum jetting

Molecular weight 98.00 N/A

Particle content size (μm) none < 1.00

Viscosity @ 25 oC (cP) 47.00 8.00 – 14.00

Surface tension @ 25 oC (mN/m) 80.70 28.00 – 36.00

Density (g/cm3) 1.69 > 1.00

Boiling point (oC) 158.00 > 100.00

Table 5-1: Fluid properties as relevant to jetting for H3PO4 (85%).

Both the viscosity and surface tension of H3PO4 (85%) are too high for optimum jetting. To

rectify these parameters, the most straight-forward way is to heat up the fluid. To reduce the

viscosity of H3PO4 (85%) to the upper range of the optimum viscosity value (14.0 cP), the

fluid needs to be heated up to around 60 oC, which gives a viscosity of around 13.4 cP.

However, in the case of H3PO4 (85%), fluid heating is undesirable due to: (1) possible

compatibility issues due to increased reactivity between the fluid and the printhead, and (2)

operational safety reasons of using acidic solutions at elevated temperature.



An alternative way of modifying the viscosity and surface tension of a fluid is by diluting the

solution. In the case of H3PO4, the solvent used is H2O, which has viscosity and surface

tension values of 0.89 cP and 71.97 mN/m (at 25 oC) respectively. The low viscosity of H2O

should dramatically reduce the viscosity to within the optimum value. However, the high

surface tension of H2O limits the achievable minimum diluted surface tension to ~72 mN/m.

Both the change of viscosity and surface tension, as well as density of the diluted H3PO4 as a

function of various concentrations is shown in Figure 5-4. The H3PO4 was diluted by adding

H2O to the 85% concentrated solution. The viscosity values were measured using Brookfield

DV-II+ Pro viscometer (Brookfield Engineering). The surface tension values were measured

using an Analite STM2141 surface tension meter (McVan Instruments). Both measurements

of viscosity and surface tension were performed at a fluid temperature of about 25 oC. The

density values were obtained from Corbridge (2000).

Figure 5-4: Viscosity (open squares), surface tension (open circles), and density

(open triangles) of diluted H3PO4 solution as a function of its concentration.

The closest optimal viscosity was obtained at about 50% H3PO4 concentration. The measured

surface tension and density at this concentration were 75.73 mN/m and 1.33 g/cm3

respectively. Despite the high surface tension value of the H3PO4 solution, it was found that

the fluid was still jettable with good jetting quality as long as the jetting parameters

(particularly the jetting waveform) are appropriately optimised.



Approximately 1.5 mL of H3PO4 (50%) solution was loaded into a 1 pL DMC-11601

printhead using a syringe injection through a 0.2 μm pore size filter. The printhead was left

idle for at least 24 hours with the nozzles facing down to allow the fluid to fill the pumping

chambers inside the printhead and to ensure that the fluid is properly degassed. Afterwards,

the printhead was installed onto the DMP system and then primed for use. The fluid was

maintained at room temperature during jetting. The jetting quality was characterised visually

through the drop watcher system by adjusting the jetting voltage and waveform.

The best jetting parameters were found to be a jetting voltage of 16 V, jetting pulse length of

8.768 μs and jetting frequency of 5 kHz. The jetting performance quickly deteriorates as the

jetting frequency is increased, particularly due to the high surface tension of the fluid which

inhibits priming of the jets at the beginning of each jetting cycle. Figure 5-5 shows the jetting

waveform used to jet the H3PO4 (50%) solution using the DMP device.

Figure 5-5: Jetting waveform used to jet H3PO4 (50%) in the DMP.

There are two additional considerations with regard to the H3PO4 (50%)-based inkjet-

printable liquid dopant sources described above. The first one is related to the use of a lower

concentration of H3PO4 solution. It is obvious that to achieve the heaviest diffusion possible,

higher concentration of phosphorus source is preferable. Nevertheless, calculations of the

available amount of phosphorus atoms in a 1 pL droplet of H3PO4 (50%) reveals that the

concentration is in the order of 1021 atoms/cm3, which is higher than the solid solubility value

of phosphorus in Si at 800-1000 oC temperature range.



The second consideration relates to the beneficial effect of the high surface tension value of

the H3PO4 (50%) solution in this direct inkjet patterning application. Unlike the indirect inkjet

patterning method, in which the droplet almost immediately diffuses into the “absorbent”

resin layer, the deposited droplets of dopant sources remain as liquid on the Si or dielectric

surfaces. To achieve the finest feature sizes on the surface, higher fluid surface tension is

preferable to prevent spreading of the deposited droplet. The surface energy of the substrate

also plays an equally important role in determining the degree of spreading. This aspect of the

deposition method will be discussed in more details in Section 5.3.3.

5.3.2 Specially-formulated Liquid Dopant Sources

One of the key advantages of specially-formulated solutions is the greater control on the

overall composition of the fluid, which optimises the fluid properties to obtain desirable

results such as fine feature size and heavy diffusion. Another advantage is the ability to

synthesise safer formulations that are preferably less acidic and corrosive, which is beneficial

both from printhead compatibility and operational safety points of view.

At the beginning of this thesis work, there were no known commercially-available products

for inkjet-printable liquid dopant sources. The closest available product for this purpose was

liquid spin-on-dopants (SOD). Most of the trialled SOD products were difficult to jet due to

their lower-than-optimal viscosities. The most serious drawback, however, was the low

boiling point of the solution due to the use of alcohols as the main solvent. It was found that

consistent jetting was virtually impossible to achieve as the rate of solvent evaporation (at

room temperature) far exceeded the jetting frequency, thus resulting in depletion of solutions

in liquid phase at the nozzle orifice. Over time, this evaporation effect left an accumulation of

dried residues, particularly around the orifice perimeter which further inhibited good jetting.

In order to create a suitable inkjet-printable liquid phosphorus dopant source, Honeywell

Electronic Materials (HEM) was engaged in a collaborative development. Honeywell was

responsible for the chemical formulation of the liquid phosphorus dopant source, while the

author’s work was mainly focused on the testing, characterisation and application of the

resulting product. A general approach used in the formulation process is presented in

Appendix F.



Several prototype solutions were tested and the formulation, which in this thesis is termed as

Solution A, was found to be the best performing solution in terms of jetting quality. A fluid-

printhead compatibility test was also performed for the DMC-11601 printhead with Solution

A resulting in no observable effects after seven consecutive days of MCK immersion. The

fluid properties of Solution A are presented in Table 5-2.

Solution A Optimum jetting

Molecular weight N/A N/A

Particle content size (μm) none < 1.00

Viscosity @ 25 oC (cP) 6.00 – 7.00 8.00 – 14.00

Surface tension @ 25 oC (mN/m) 51.00 – 53.00 28.00 – 36.00

Density (g/cm3) 1.10 – 1.20 > 1.00

Boiling point (oC) multiple > 100.00

Table 5-2: Fluid properties as relevant to jetting for specially-formulated liquid

phosphorus dopant source used in this thesis.

One major difference between H3PO4 (50%)-based liquid phosphorus dopant source and

Solution A was the ability to maintain fine feature sizes during the required post-printing

processing, particularly at elevated temperatures (these issues will be discussed further in

Section 5.4.1). In this respect, Solution A performed significantly better than H3PO4 (50%).

While the viscosity and surface tension of both solutions do not differ by much, the key

difference between the two solutions lies in Solution A’s use of a variety of undisclosed

additives and multiple solvents. Therefore, unlike aqueous H3PO4 which has a single boiling

point, the various solvents in Solution A evaporate at different temperatures thus enabling

greater control of the phosphorus source’s dehydration process.

Due to the similarities in fluid properties, the parameters required to jet Solution A were

found to be similar to those used to jet H3PO4 (50%). A similar jetting voltage (in the range of

15 to 17 V) was used, while the firing frequency was kept low at 5 kHz for this preliminary

work. A slight modification to the jetting waveform was used as shown in Figure 5-6. The

resulting jetting quality was excellent, with consistent droplet generation and straightness.



Figure 5-6: Jetting waveform used to jet Solution A in the DMP.

The slightly less acidic nature of Solution A compared to H3PO4 (50%) did not result in

significant printhead lifetime extension. The printhead lifetime was characterised by the

ability of at least one nozzle to jet the fluid with the same jetting quality as when the fluid was

first loaded into the printhead. For the DMC-11601 printhead, it was observed that the jetting

quality deteriorated after a period of about 3-4 weeks. The most common failure mode was

found to be either non-jetting or poor droplet flight trajectory. A likely cause of both problems

is the accumulation of dried fluid residues around the perimeter of the nozzle orifice. This

build-up of residue obstructs the fluid flow as it is pushed out of the nozzle. In the worst case

scenario, the accumulated residue completely blocked the nozzle and prevented any fluid

jetting.

The attempt to formulate a less acidic fluid resulted in a solution that contained a lower

concentration of phosphorus. In the case of Solution A, the actual phosphorus concentration

after dissolution in the solvent is only 5-7% (w/v), while the phosphorus solid content ranges

from around 30-40% (w/v). Assuming that the density of Solution A is around 1.1-1.2 g/cm3

as given in Table 5-2, a 1 pL droplet still contains a phosphorus atom concentration in the

order of 1021 atoms/cm3, which is a figure that is at least similar or higher to the solid

solubility of phosphorus in Si in the process temperature range between 800-1000 oC.



5.3.3 Surface Preparations and Characterisation

Two types of surfaces are considered in the development of direct inkjet patterning concepts

through the printing of liquid dopant sources. The bare, HF-treated Si surface used in the first

application as described in Section 5.2.1 is a hydrophobic surface, while the dielectric surface

(primarily SiNx and SiO2) used in the second application as described in Section 5.2.2 is a

hydrophilic surface. Obviously, in order to achieve the finest feature sizes possible, a

hydrophobic surface is desirable as the spherical droplet shape is retained when the droplet

contacts the surface, thus creating a small contact area between the surface and droplet.

As discussed in Section 2.3.3, the solid surface energy determines the wetting behaviour of a

particular surface. Contact angle measurements using a goniometer can be used to

characterise the surface energy of a solid surface. To better predict the wettability of a surface,

the critical surface tension of a solid surface can be measured and calculated. This parameter

represents the minimum surface tension threshold of a liquid, below which complete wetting

will occur.

Gould and Irene (1988) found that the critical surface tension of an HF-treated Si surface was

~27 mN/m. This means that any liquid with a surface tension above 27 mN/m will not wet the

surface. Both the H3PO4 (50%) and Solution A used in this work have very high surface

tension values of 80.7 mN/m and 51.0 mN/m respectively. It is important to note that this low

critical surface tension value only applies for HF-treated Si surface as Si by itself has been

identified by Jaccodine (1963) as a very high energy solid with measured surface free energy

value of 2130 mN/m for (100) surfaces. This highlights the importance of having hydrogen

and fluorine species passivating the Si surface in order to achieve the low surface energy

value.

In order to measure the resulting contact angle, goniometer measurements were performed on

pipette-formed droplets of H3PO4 (50%) and Solution A onto three different Si surface

finishes: random upright pyramid textured, saw-damage-etched and mirror-polished surfaces.

The Si wafer surfaces were not diffused. Any surface oxide was removed from these surfaces

by immersion in BHF (7:1) solution before any measurements were made. The measurements

were taken at room temperature using a Rame-Hart Model 200-F1 goniometer. Table 5-3 lists

the contact angle measurement results for the two fluids.



HF-treated Si surface H3PO4 (50%) Solution A

Random pyramid textured 104.6 62.8

Saw damage etched 58.9 43.2

Mirror-polished 74.8 63.2

Table 5-3: Measured contact angles (in degrees) of pipette-dropped H3PO4

(50%) and Solution A on three different HF-treated Si surface finishes.

Droplets of H3PO4 (50%) form higher contact angles compared to Solution A due to their

higher surface tension value. With contact angles higher than 90o for textured surfaces, these

droplets of H3PO4 (50%) results in very narrow printed lines on the HF-treated Si surface. The

higher contact angle exhibited by the textured surface can be attributed to the rougher surface

structure due to the presence of the microscopic pyramids. Although rough surface usually

makes it difficult for the fluid droplet to pin down a contact line that minimises its energy, the

dimension of the pyramids (~1 µm) in this case is at least an order of magnitude smaller

compared to the droplet diameter (~15-20 µm). As a result, the roughness is hardly ‘visible’

to the droplet and may minimise spreading by allowing the droplet to quickly define its

contact line along the valleys of the pyramids.

Surface roughness becomes less advantageous when the surface structure is comparable to the

size of the droplet. This situation is clearly demonstrated by the lowest contact angle shown

by the saw-damage-etched surface. In this case, the droplet attempts to minimise its energy by

spreading along the surface until it meets a trough on the rough surface. The flat case is

shown by the mirror-polished surface which shows a marked improvement in its measured

contact angle compared to the saw-damage-etched surface.

The formation of narrow inkjet-printed lines is more complicated when dielectric surfaces

such as SiO2 or SiNx are considered. Williams and Goodman (1974) have investigated the

wetting behaviour of H2O on SiO2 surfaces with varying oxide thickness and found that these

surfaces are completely hydrophilic (i.e. contact angle of almost 0o) for oxide thicknesses

greater than 30 Å. Goniometer contact angle measurements performed on both SiO2 and SiNx

surfaces using pipette-dropped H3PO4 (50%) and Solution A further verified the complete-

wetting behaviour of dielectric surfaces, thus implying high surface energies.



A common way to modify surface properties is through surface functionalisation. Specific

functional groups can be added onto the surface through a number of methods. Plasma

processing is one surface functionalisation method that is regularly used to introduce certain

atoms or molecules onto a surface. Another popular method is through immersion or spin-

coating of specific solutions that attach layers of functional groups onto a surface.

The functionalisation of SiO2 surfaces to achieve hydrophobicity has been previously

demonstrated. For instance, Lim et al. (2005) showed that the application of hexamethyl-

disilazane (HMDS) or octadecyltrichlorosilane (OTS) onto SiO2 surfaces by spin-coating or

immersion resulted in an increase of the measured contact angle of H2O droplets with values

of 53.76o and 78.98o respectively.

Similarly for SiNx, functionalisation of the surface through the use of attached organic

monolayers by reacting SiNx-coated Si wafer surfaces with solutions of 1-alkenes and 1-

alkynes have been demonstrated by Arafat et al. (2007) to produce high measured contact

angles in excess of 100o. Despite this, SiNx is known to be a difficult surface to functionalise

due to its intrinsic electronegative nitrogen groups on the surface. This makes it easier for

hydrogen atoms in a polar molecule to interact in a strong dipole-dipole interaction in the

form of hydrogen bonding.

One important issue that needs to be considered when functionalising surfaces is the fact that

the inkjet-deposited liquid phosphorus dopant source must be able to penetrate these layers to

act on the underlying SiNx layer. Another consideration is when the deposited fluid has

penetrated the functionalised layer and then comes into contact with the underlying SiNx

(which is hydrophilic). Since the fluid is not reactive to the SiNx underneath but reactive to

the adjacent functionalised layers, the fluid droplet is likely to have a tendency to spread

laterally along the surface.

In this work, an alternative and simple method of creating hydrophobic SiO2 and SiNx

surfaces was used. The method involved heating the SiO2 or SiNx-coated Si wafers in a

conventional fan-forced oven at 250 oC in ambient air for at least 1 hour. This heat treatment

appears to change the surface from being hydrophilic to hydrophobic as indicated in the

goniometer images shown in Figure 5-7. The fluid used was H3PO4 (50%) deposited by a



syringe onto both an as-deposited SiNx surface (Figure 5-7(a)) and a heat-treated SiNx surface

(Figure 5-7(b)). Both surfaces were textured with random upright pyramids. Similar contact

angle behaviours were observed when using Solution A and also when textured SiO2 surfaces

were used instead of SiNx.

Figure 5-7: Goniometer images showing a drop of H3PO4 (50%) on as-deposited

SiNx surface (figure A) and heat-treated SiNx surface (figure B) clearly showing

the complete spreading and high contact angle formation respectively.

Unlike active surface functionalisation where functional groups are introduced onto the

surface, heat treatment does not add any extra layer on the sample surface. A likely reason for

the increased hydrophobicity due to the heat treatment can be traced to the original makeup of

the dielectric surface. The surface of SiO2 contains a mixture of silanol (Si-OH) and siloxane

(Si-O-Si) groups. The siloxane groups are naturally hydrophobic. However, adjacent silanol

groups adsorb H2O readily through strong hydrogen bonding, thus resulting in hydrophilic

surfaces. Bolis et al. (1991) suggested that surface dehydration (i.e. hydrophobicity) in SiO2

occurs when only siloxane bridges and isolated silanols are present after heat treatment. Heat

treatments above 250 oC evaporate off many of the silanol groups and eliminate the adsorbed

H2O molecules from the surface, thus resulting in largely hydrophobic surface.

Table 5-4 lists the contact angles measured on textured SiNx and SiO2-coated Si wafers as-

deposited and after heat treatment. The contact angles were measured using Rame-Hart Model

200-F1 goniometer at room temperature. The Si wafer surfaces were not diffused and were

cleaned with standard RCA1/RCA2 sequence, followed by a quick HF dip before the

A B



dielectric surfaces were deposited or grown. The SiNx was deposited using an inline PECVD

machine with a thickness of 75 nm and a refractive index of ~2. The SiO2 was thermally

grown in a high temperature furnace under dry O2 ambient with a thickness of ~350 nm. The

heat treatments were performed in a conventional fan-forced oven at a temperature of 250 oC

for 1 hour in air ambient.

Measured contact angles Dielectric surfaces

H3PO4 (50%) Solution A

As-deposited textured SiNx 0 o 0 o

Heat-treated textured SiNx 72.9 o 65.9 o

As-grown textured SiO2 0 o 0 o

Heat-treated textured SiO2 112.6 o 102.3 o

Table 5-4: Measured contact angles (in degrees) of pipette-dropped H3PO4

(50%) and Solution A on random upright pyramid textured SiNx and SiO2

surfaces before and after heat-treatment.

The measured contact angle for H3PO4 (50%) is higher for both types of dielectric layers

compared to Solution A due to its higher surface tension value. The higher contact angles

observed for SiO2 surfaces compared to SiNx surfaces are possibly due to the less stable Si-N-

Si bridges in SiNx surfaces compared to the Si-O-Si bridges in SiO2 surfaces causing

increased reactivity with H2O molecules through adsorption (Fubini et al. 1989).

5.4 Device Processing Using Direct Inkjet Printing

5.4.1 Process Development

For the direct inkjet patterning process described in Section 5.2.1, in which the liquid dopant

sources are inkjet printed directly onto HF-treated Si surfaces, the general process is relatively

simple. A standard commercial alkaline-textured Si wafer is used as a substrate. After

standard wafer cleaning, the Si wafer was immersed in HF solution to remove all remaining

native oxide and more importantly, to create a hydrogen-passivated surface that was

hydrophobic in nature. At this stage, two process routes are possible. The first process route is



where the liquid dopant sources are inkjet-printed directly onto the un-diffused Si surface.

The second process route is where a standard impurity diffusion process (phosphorus is used

in the case of p-type substrate) is performed on the surface, followed by diffusion glass

removal and then inkjet printing of liquid dopant sources. The general process flowchart for

the direct inkjet patterning of phosphorus liquid dopant sources on Si surface is shown in

Figure 5-8.

Figure 5-8: Process flowchart of direct inkjet patterning of phosphorus liquid

dopant sources on Si surface.

Further investigations of both the diffused and un-diffused surfaces of Si revealed significant

differences in the wetting behaviour of inkjet-printed liquid dopant sources. Both the HF-

treated un-diffused and diffused Si surfaces were found to have a very high degree of

hydrophobicity immediately after the HF-treatment as evidenced by the high contact angle

measurements obtained using both types of phosphorus liquid dopant sources. However, it

was observed that the diffused Si surface lost its hydrophobicity considerably quicker than the

un-diffused Si surface. This phenomenon can be attributed to the fact that native oxide grows

much faster in n+ (100) Si surfaces than in n (100) Si surfaces at room temperature in air

ambient as has been previously shown by Morita et al. (1990).

The inkjet printing step selectively deposited the liquid phosphorus dopant sources on the

surface of the solar cell. The substrate was heated to ~60 oC during printing in order to

enhance the evaporation of some of the solvent from the deposited liquid dopant source upon

contact with the substrate surface. Using the 1 pL DMC-11601 printhead to inkjet-print both

the H3PO4 (50%) and Solution A onto textured diffused and un-diffused Si surfaces, line

widths of 15-20 µm were routinely achieved as shown in Figure 5-9(a) and Figure 5-9(b).

Optimisation of droplet spacing had to be performed to achieve the most continuous line

possible during printing. If the drop spacing was too small, bulges along the printed line

Diffused surface

Un-diffused surface

HF treatment

Inkjet printing Drying High temp

drive-in



began to appear. Similar bulges were observed by Duineveld (2003). If the drop spacing was

too large, the printed droplets failed to connect with each other, and thus formed separate

individual droplets instead of a continuous line.

Figure 5-9: Optical microscope images of inkjet-printed H3PO4 (50%) (figure

A) and Solution A (figure B) on textured, un-diffused HF-treated Si surface.

Since the deposited volume is small, the heated substrate substantially boiled off the jetted

solvent content. However, the presence of moisture from the ambient air usually re-hydrated

the deposited droplets almost instantaneously. Further drying of the inkjet printed droplets of

dopant sources was required before the high temperature drive-in step. The drying process

was performed either in a stand-alone dryer oven separately from the drive-in process or in an

integrated multiple zone furnace system where the Si wafer undergoes lower temperature

drying before the high temperature drive-in.

It was observed that the deposited droplets of liquid dopant sources on the substrate surface

underwent substantial (unwanted) spreading when exposed to elevated temperatures. The

degree of spreading varied, but spreading from the original 20 µm line width to over 300 µm

line widths was commonly observed when the printed substrate was heated to around 200 oC.

There are a number of factors that causes the droplet spreading at elevated temperatures.

Firstly, both the viscosity and surface tension of the liquid dopant sources decreases with

increasing fluid temperatures. As the viscosity decreases, it becomes easier for the fluid to

flow across the surface. Similarly, as the surface tension decreases, there are less

A B

100μm

100μm



intermolecular forces within the liquid molecules to hold it together and it is eventually

overcome by the fluid-solid molecular forces with the substrate surface. The spreading

tendency is further exacerbated by the formation of native oxide as the temperature is raised

in air ambient. The presence of native oxide considerably increases the surface energy of the

substrate, which causes complete wetting on the surface.

Essentially, the drying step is a dehydration process. For instance, in the case of H3PO4

(50%), the drying process converts liquid H3PO4 into solid phosphorus pentoxide (P2O5).

Similarly, for Solution A, the acid-based liquid compound is also converted into P2O5 through

dehydration. Water is the main solvent in the case of H3PO4, which has a boiling point of 100 oC. H3PO4 itself has a boiling point of 158 oC. Above this temperature, polyphosphoric acid is

formed. Further removal of water results in a range of dehydrated forms of H3PO4, consisting

of pyrophosphoric acid (H4P2O7) and metaphosphoric acid (HPO3). When the acid is fully

dehydrated, the resulting compound is solid P2O5. This solid compound itself sublimes at

temperatures above 300 oC (Corbridge 2000).

In order to achieve dehydration at elevated temperatures without droplet spreading, a special

vacuum drying method was developed. The technique involved establishing a vacuum

environment before the temperature was raised for drying. The low pressure environment is

believed to remove much of the moisture out of the ambient air, which prevents native oxide

growth on the surface. Once the drying process was completed, it was presumed that only

solid P2O5 remained on the surface. Although P2O5 is a strong dessicant, it was observed that

after vacuum dehydration, further high furnace temperature treatment in atmospheric

conditions did not cause any spreading.

The vacuum drying technique is equally applicable to both HF-treated Si surfaces and heat-

treated dielectric surfaces such as SiO2 and SiNx. The visual appearance before and after the

vacuum drying did not appear to change significantly except that the wet liquid look of the

inkjet-printed lines before drying turned into a more glassy solid look after drying. This is

illustrated in the optical microscope images shown in Figure 5-10, which shows the

appearance of inkjet-printed Solution A lines on heat-treated textured SiNx surface before and

after the vacuum drying.



Figure 5-10: Optical microscope images of Solution A inkjet-printed on heat-

treated SiNx surface before (figure A) and after (figure B) vacuum drying.

After the inkjet-printed phosphorus dopant sources were dried, high temperature treatment

was used to drive the phosphorus atoms into the substrate. The process is essentially a

standard solid state diffusion process that is commonly used in semiconductor fabrication and

has been reviewed extensively on many textbooks such as by Sze (2002).

One major difference is the limited amount of phosphorus atoms available from inkjet-printed

droplets of liquid dopant sources. Due to the sublimation of P2O5 at temperatures above 300 oC and the absence of continuous source of phosphorus dopants in the ambient, it is inevitable

that the diffusion process is severely limited by the availability of the printed dopants.

Evidently, it is much more difficult to diffuse impurities onto a narrow 20 µm line from a

similarly-sized dopant source compared to diffusion on entire surface such as those performed

on an inline diffusion system with H3PO4 as sprayed precursor liquid. Furthermore, there are

no accurate measurement technique to determine the actual doping concentration and profile

of an active area as small as those produced by this inkjet printing technique (~15-20 µm).

To characterise the diffusion that was achieved using inkjet printing of liquid phosphorus

dopant sources, four-point-probe measurements were used to measure the sheet resistivity of

the diffused layer. However, it is difficult to use a four-point-probe to measure resistivities for

narrow regions such as those produced by inkjet printing. Therefore, a solid large area pad

(~10 mm 10 mm) was inkjet-printed on the surface to enable probing using the available

four-point-probe instrument. Obviously, the measurements made using this technique are not

A B

100μm

100μm



an accurate representation of the actual doping that occurs from the 20 µm printed lines.

However, measurements from the large area pads were useful for initial investigations into

whether the liquid dopant sources, were able to diffuse into the substrate surface.

The actual diffusion of inkjet-printed phosphorus liquid dopant sources was estimated by

extrapolating measured sheet resistivities resulting from printed pads of decreasing size.

Three types of surfaces on Si substrates were considered: (1) HF-treated diffused textured Si

wafer, (2) heat-treated diffused textured Si wafer with SiNx, and (3) heat-treated diffused

textured Si wafer with SiO2.

Inkjet printing of Solution A was done on each surface in the shape of individual solid pads of

varying area with a layout as shown in Figure 5-11. During the inkjet printing step, the

substrate was heated to 60 oC. Afterwards, the wafers were all vacuum-dried in a vacuum

chamber at 200 oC, followed by belt furnace drive-in at 980 oC for 3 hours in air ambient. The

sheet resistivity of each of the pads was then measured using a four-point-probe.

Figure 5-11: Layout of inkjet-printed pads of varying sizes.

Due to the small area of the measurement pads, the four-point-probe measurements had to be

adjusted by an area-related correction factor. A method to calculate the appropriate correction

factor was given by Smits (1958) and is presented in Appendix G.

The resulting sheet resistivities are presented in Figure 5-12 as a function of the various pad

widths for all three different surfaces (Si surface, SiNx surface and SiO2 surface).

15x10mm

15x8mm

15x6mm

15x4mm

15x2mm

Pad areas:

• 15 x 10mm • 15 x 8mm • 15 x 6mm • 15 x 4mm • 15 x 2mm • 15 x 1mm • 15 x 0.5mm



Figure 5-12: Measured sheet resistivities of diffused inkjet-printed pads as a

function of pad area for Si surface (filled circles), SiNx surface (filled squares)

and SiO2 surface (filled triangles). Trend lines are added as guides to the eye.

The measurement result on the largest-size pad on Si surface indicates that Solution A was

able to diffuse the Si very heavily, in the region of ~3 Ω/. This heavy diffusion confirms that

the formulation does not fundamentally limit the diffusion capability of printable dopant

sources. The diffusions performed through dielectric coatings resulted in much higher sheet

resistivities due to the presence of dielectric barrier. Nevertheless, the reasonably heavy

diffusions of around 25-30 Ω/ is sufficient for contacting purposes.



With decreasing pad sizes, the sheet resistivities appear to decrease. This trend is particularly

evident in the results obtained for diffusions involving Si surfaces where the phosphorus

dopants are able to directly diffuse into the Si. The trend is less clear on the dielectric

surfaces, presumably due to the presence of the dielectric barriers, which makes the diffusion

less uniform due to various non-uniformities on the dielectric layer such as thickness or

hardness (density). This result must be viewed with caution because even the smallest

measurable pad width is 1 mm wide or 50 times wider than the width of the inkjet-printed

liquid phosphorus dopant source line. At such a microscopic scale, various effects such as

vapour mass transfer or P2O5 sublimation may become very influential and might drastically

change the amount of phosphorus atoms available for the diffusion.

The concern regarding insufficient phosphorus on single pass, single line printing of a liquid

phosphorus dopant source was confirmed when a test structure was fabricated as follows. A

batch of Si wafers was coated with PECVD SiNx on one surface with the standard 75 nm

thickness and refractive index of ~2. The wafers were then heat-treated to create hydrophobic

SiNx surface and then inkjet printed with a series of parallel lines of Solution A spaced 1 mm

apart. Only a single pass for each line was printed. The wafers were vacuum-dried and the

resulting line width was ~20-25 µm. The wafers then underwent high temperature drive-in in

a belt furnace at a temperature of 980 oC for a total of 3 hours in air ambient. The wafers were

then immersed in HF (1:10) solution for a total of 30 minutes with regular visual and optical

microscope checks every 5 minutes to see whether or not selective etching has occurred.

Selective etching was observed for the first 5 minutes and then appeared to have stopped

thereafter. This result suggests that some penetration through the SiNx did occur, but not for

the entire thickness of the dielectric layer. Some of the possible implications are: (1)

insufficient available dopant source after drying, or (2) significant loss of dopant sources

during the high temperature treatment, or (3) a longer drive-in time was required. The process

temperature for drying is less than the sublimation temperature of P2O5 (>300 oC) and thus

unlikely to cause significant loss of phosphorus atoms. However, it is quite likely that the

high drive-in temperature could cause either sublimation of P2O5 or vapour mass transfer of

phosphorus atoms from the inkjet-printed region to the ambient. The third hypothesis was

found to be unlikely as longer high temperature treatments as long as a total of 6 hours at 980 oC did not show any improvement in the etching selectivity of the SiNx layer.



To alleviate this problem, multiple pass printing was trialled instead of single pass printing.

This resulted in wider lines, although presumably also resulted in an increased phosphorus

source volume on the surface. Using similar SiNx-coated wafers as substrate and the standard

direct inkjet patterning process, successively higher number of passes were printed onto a test

wafer. It was found that the printing of just two adjacent lines was sufficient to produce the

desired etching selectivity of the SiNx. Double line printing means that two connected parallel

lines were printed instead of one single line, thus doubling the line width from 20-25 µm to

40-50 µm. This method enabled the direct inkjet patterning of the SiNx layer.

5.4.2 Device Concept and Fabrication

A simple selective emitter solar cell structure similar to the one used to demonstrate the

applicability of the indirect inkjet patterning method in Chapter 3 was also used to

demonstrate the usefulness of the direct inkjet patterning method. A schematic of the device

structure was shown in Figure 4-8.

Instead of using indirect inkjet patterning method to form the front metal grid patterning, the

direct inkjet patterning of SiNx layer and simultaneous diffusion underneath the metal contact

regions were used to create the front metal contacts. If SiO2 was used as opposed to SiNx, the

method would have been similarly applicable. Also, in this case, Solution A was used as the

liquid phosphorus dopant source, although H3PO4 (50%) could also have been used.

An outline of the fabrication sequence for this direct-inkjet-patterned selective emitter solar

cell structure is presented in Table 5-5. The main differences between this fabrication

sequence and the one for indirect inkjet patterning are: (1) SiNx is used as a dielectric layer

following the industry standard, (2) the use of direct inkjet patterning instead of indirect inkjet

patterning, (3) no second heavy diffusion was required due to the intended simultaneous

heavy diffusion achieved during the direct inkjet patterning process. The rest of the

processing sequence remains the same.



Steps Parameters

1. Wafer selection p-type boron-doped, 1 Ω.cm, CZ, 300 µm, as-lapped

2. Texturing Alkaline-textured (NaOH/IPA solution)

3. Phosphorus diffusion Solid source deposition at 850 oC for 40 mins – aim 100 Ω/

4. Diffusion glass removal HF (1:10) etch

5. PECVD SiNx deposition Remote-PECVD, thickness: 75 nm, refractive index: ~2

6. Heat treatment Conventional fan-forced oven, 250 oC for 1 hour, air ambient

7. Inkjet patterning Front contact fingers 1.7 mm spacing, Solution A 1 pL drops,

50 µm wide (2 parallel connected passes), platen at 60 oC

8. Vacuum drying Pressure ~0.1-0.3 mbar, temperature: 200-300 oC, time: ~10-

15 minutes

9. Drive-in Belt furnace, 980 oC for a total of 3 hours

10. Etching HF(1:10) for 3-5 mins

11. Rear contact formation Thermal evaporation of Al (2 µm), alloy at 980 oC/15 hrs

12. Deglazing HF (10:1) 50 secs

13. Ni plating Ni electroless plating 80-85 oC/5 mins, sinter at 350 oC/5 mins

(N2 ambient), 2nd Ni electroless plating 80-85 oC/1 min

14. Cu plating Cu electroless plating 42 oC/3 hrs

15. Edge isolation Laser scribe from rear and cleave

Table 5-5: Fabrication sequence of direct-inkjet-patterned selective emitter

solar cell.

5.4.3 Device Characterisation

Figure 5-13 shows the current-voltage electrical characteristics of the best-performing initial

device fabricated according to the inkjet selective emitter solar cell structure. The light J-V

curve shown in Figure 5-13(a) was measured against a calibrated reference under STC

condition at 1-sun illumination. The dark J-V curve shown in Figure 5-13(b) was measured

without illumination (in the dark). Additionally, Figure 5-13(b) also plots the calculated local

ideality factor as a function of voltage, which was derived from the differential of the dark J-V

curve.



Figure 5-13: Best electrical performance of an initial direct-inkjet-patterned

selective emitter solar cell device showing the measured current-voltage curves

against a calibrated reference under STC conditions at 1-sun illumination

(figure A) and in the dark (filled squares, figure B), together with the calculated

local ideality factor curve as a function of voltage (open squares, figure B).

Although the device fabricated here is only a test structure with no optimisation, some

explanations on the poor electrical parameters are useful to determine the factors that could

improve the device performance in subsequent work. There are several possible reasons for

the measured low Jsc value. The non-optimised front surface design in this device causes

higher-than-usual shading losses. This is further exacerbated by the presence of overplated

copper on the front surface due to inability of SiNx coating to fully mask the peaks of the

pyramids from the plating process.

The low Voc value can be attributed to several reasons. Firstly, the poor quality of the heavily

contaminated belt furnace at UNSW that was used for the high-temperature drive-in severely

degraded the bulk lifetime of the device. The same belt furnace was also used by Edwards

(2008) to fire screen-printed dopant pastes at high temperature with similar degradation in

bulk lifetime. The second effect that reduces the Voc can probably be attributed to high

junction recombination, particularly in the contact regions of the device. The impact of this

recombination loss will be explored further in the discussion on fill factor losses.

A B



The fill factor loss is the main investigation point in this case, especially in evaluating the

impact of direct inkjet patterning as a method of forming metal contacts on solar cell devices.

The dark J-V measurement result shown in Figure 5-13(b) clearly indicates that the m > 1

recombination dominates in this device in the medium to high voltage range. This

characteristic ‘hump’ implies that there is likely to be a detrimental shunt recombination

effect that limits the fill factor of the device (McIntosh 2001). Subsequent Suns-Voc

measurement of this sample gave a pseudo fill factor (pFF) of 0.601. The fact that the

measured pFF value is not much higher than the measured FF of 0.590 under illumination

provides further evidence that the dominant loss is due to a parasitic shunt effect rather than a

series resistance effect.

A likely cause for the observed shunt is the non-uniformity of the phosphorus diffusion in the

direct-inkjet-patterned regions such that there is a mixture of p-type and n-type regions

contacting the deposited metal. When these localised p-type regions make a contact with the

metal, they create localised Schottky barriers and also localised shunts with the adjacent n-

type regions, thus contributing to the m > 1 recombination as observed in the dark J-V curve.

This theory was verified by introducing additional dopants through the following method.

Another batch of samples was fabricated following the same procedure as described in Table

5-5. An additional inkjet deposition of liquid phosphorus dopant source was dispensed only

into the exposed Si after the etching step (Step 10), followed by an extra high temperature

drive-in step in the same belt furnace at 980 oC for 1 hour. The purpose of this extra selective

diffusion step is to ensure that enough phosphorus was diffused into the Si underneath the

metal contacts. A comparison of the measured light J-V electrical parameters of the devices

with and without additional phosphorus diffusion is shown in Table 5-6.

Voc (mV) Jsc (mA/cm2) FF η

No additional diffusion 498.0 27.50 0.590 8.11

With additional diffusion 549.1 27.96 0.736 11.30

Table 5-6: Comparison of electrical parameters of direct-inkjet-patterned

selective emitter solar cell with and without additional phosphorus diffusion

measured against a calibrated reference under STC condition at 1-sun

illumination.



There is a significant improvement in the measured fill factor after the extra diffusion process

on the patterned contact regions from 0.590 to 0.736. This result provides further evidence to

the hypothesis of there being insufficient phosphorus atoms diffused in the Si because it

clearly shows that additional phosphorus diffusion eliminates the localised Schottky barriers

caused by localised metal-p-type region contacts. The Voc is also noticeably increased by

about 50 mV due to the elimination of these parasitic ‘high junction recombination’ regions.

Suns-Voc measurement was performed on the sample with additional phosphorus diffusion

showing a high pFF of 0.794, thus indicating that most of the fill factor losses can now be

attributed to the parasitic series resistance losses due to the non-optimised emitter design.

Finally, a comparison between the m-V curve of the samples before and after extra

phosphorus diffusion as shown in Figure 5-14 clearly shows the elimination of the increased

local ideality factor previously observed in the medium to high voltage range.

Figure 5-14: Comparison of m-V curve between samples before (filled squares)

and after (open squares) extra phosphorus diffusion.

The results presented above indicate that although direct inkjet patterning of SiNx using the

concept discussed in Section 5.2.2 proves to be challenging, it still appears to be possible to

achieve high fill factor contact formation using the method. The problems concerning the

poor electrical performance of the initial fabricated device have been identified, thus making

improvements possible and demonstrating that there is a feasible application of the newly

developed direct inkjet patterning concept to solar cell fabrication.



5.5 Summary

In this chapter, two new concepts of directly patterning Si and dielectric surfaces by utilising

inkjet-printable liquid dopant sources have been proposed. The first concept simply involves

the direct deposition of liquid dopant sources onto bare Si surfaces to form selectively

diffused regions. The second concept involves the direct deposition of phosphorus liquid

dopant sources onto dielectric surfaces such as SiO2 or SiNx, whereby the phosphorus dopants

are used to diffuse through both the dielectric layer and the underlying Si. The diffused

phosphorus modifies the etchability of the dielectric layer creating high etching selectivity,

which can be advantageously-used for dielectric patterning purposes. Simultaneously, the

phosphorus dopants also diffuse into the underlying Si, thus creating a highly-doped region

where the metal contact will eventually be deposited.

Two types of liquid phosphorus dopant source formulations were described and their potential

to act as printable dopants compared. The first type of solution was based on off-the-shelf

standard highly concentrated phosphoric acid. The second solution was jointly developed with

an external chemical manufacturer with the aim of reducing the acidity of the original

phosphoric acid-based solution as well as improving the fine line width capability and

jettability of the solution. Various fluid properties, printhead compatibility and jetting

optimisation were performed and discussed in order to illustrate the development path of an

inkjet-printable solution as well as the ink-jetting capability of the formulated solutions. To

complete the inkjet system development procedure, the various surface types used in this

work were reviewed. Several issues regarding their hydrophobicity were identified and

solved. A method of creating hydrophobic SiO2 and SiNx surfaces was developed and was

shown to be capable of producing fine lines as narrow as 15-20 µm using inkjet printing of

liquid phosphorus dopant sources.

Finally, the newly developed direct inkjet patterning concept was applied to the fabrication of

a solar cell device. The inkjet process was used to fabricate a simple selective emitter solar

cell structure to demonstrate the feasibility of the patterning technique. The limited amount of

diffused phosphorus that resulted from using the inkjet doping process proved to be the

biggest challenge in achieving high fill factor contact formation in the device. Nevertheless,

the limiting cell performance factors have been identified and discussed, which is useful for

future work.

Chapter 6: Conclusion


Chapter 6 : Conclusion

The primary aim of this thesis was to conceptualise, develop and demonstrate the applicability

of using inkjet printing as a viable alternative patterning method. In the past,

photolithography, laser patterning and screen-printing have been the most commonly used

patterning method in solar cell fabrication. As a result, many higher efficiency solar cell

designs have been developed using these tools. However, the drive to reduce fabrication costs

requires an alternative patterning approach that combines low-cost, high-throughput, high-

resolution and contactless characteristics. In this thesis, inkjet printing was proposed as a

possible solution that satisfies all of these requirements.

Although inkjet printing have been previously used as a tool to deposit a number of functional

fluids (such as metal inks) in solar cell fabrication, the use of such method to directly pattern

and form localised openings on dielectric layers have never been explored before. The lack of

systematic way of developing a suitable inkjet system, particularly for solar cell fabrication

purposes, provided the motivation for the synthesis of such method. It was found that the

inkjet system can be broken down to three main components consisting of printhead, fluid and

substrate.

The development of two novel and distinct inkjet patterning methods represents the key

innovation resulting from this thesis work. The first method involved the deposition of

plasticiser droplets onto an intermediate resin layer that coats the dielectric layer on the

substrate surface, thus creating regions that are permeable to aqueous dielectric etchants such

as HF. The underlying dielectric layer can then be selectively etched and patterned. The final

removal of the resin layer exposed the patterned dielectric layer.

The second method involved the direct deposition of functional fluids, specifically in the form

of liquid dopant sources onto both Si and dielectric surfaces. In the case of Si surfaces, the

inkjet-printed liquid dopant sources can be selectively deposited to form localised diffusion

layers. In the case of dielectric surfaces, the inkjet-printed liquid dopant sources can be used

to simultaneously form openings on the dielectric layer and selective diffusion in the

underlying Si surface.



Using the indirect method, individual round openings with diameters as small as 30-35 μm

and continuous line patterns with width as narrow as 40-50 μm could be formed on dielectric

surfaces. Using the direct method, lines as narrow as 15-20 μm were formed on both Si and

dielectric surfaces. Additionally, a vacuum-drying method was developed to prevent droplet

spreading at elevated temperatures and a simple baking method was developed to make

normally-hydrophilic dielectric surfaces become hydrophobic.

As a result of these developments, solar cell devices with fill factors above 0.800 have been

fabricated using the indirect method, thus showing the excellent quality of the patterning

process. Furthermore, working solar cell devices have been fabricated using the direct

method, with the performance-limiting factors identified.

6.1 Original Contributions

The following are the original contributions arising from this thesis work:

• The development of a novel dielectric patterning method by inkjet-depositing droplets

of plasticiser onto a resin layer thus creating a permeable layer that would allow

aqueous etchants to etch the underlying dielectric layer.

• The identification of diethylene glycol and novolac resin as a suitable plasticiser-resin

combination for the above-mentioned dielectric patterning method.

• The demonstration of patterns produced using the above-mentioned dielectric

patterning method producing round openings with diameters as small as 30-35 μm and

continuous line patterns with width as narrow as 40-50 μm.

• The demonstration of various high efficiency solar cell structures, including selective

emitter structure, localised contacting scheme, surface texturing and edge isolation

produced the above-mentioned dielectric patterning method.

• The fabrication of a complete solar cell with front surface contacts patterned using the

above-mentioned dielectric patterning with fill factor as high as 0.80.



• The development of a novel selective diffusion method by inkjet-depositing droplets

of liquid phosphorus dopant sources onto a silicon surface followed by a high

temperature treatment.

• The development of a novel simultaneous dielectric patterning and selective diffusion

method by inkjet-depositing droplets of liquid phosphorus dopant sources onto a

dielectric layer followed by a high temperature treatment and selective chemical

etching of the dielectric layer.

• The demonstration of the use of vacuum drying to produce a narrow line width of ~15-

20 μm of the inkjet-printed liquid phosphorus dopant sources on both silicon and

dielectric surfaces.

• The demonstration of the use of a heat treatment to transform the originally

hydrophilic dielectric surfaces into hydrophobic surfaces.

• The fabrication of a complete solar cell with front surface contacts patterned and

selectively diffused using the simultaneous inkjet-printed patterning/diffusion process

of liquid phosphorus dopant sources with fill factor as high as 0.79.

6.2 Future Work

Due to the novelty of the work, there are numerous opportunities to further develop the inkjet

patterning methods described here. Some suggestions for further works are given below:

1. Indirect inkjet patterning method

• In this work, only the novolac-DEG combination of resin-plasticiser was

evaluated as the initial proof-of-concept. Theoretically, any resin-plasticiser is

a potential candidate to be used in the indirect inkjet patterning method. It is

likely that there are other resin-plasticiser combinations capable of producing

smaller feature sizes than the novolac-DEG combination. It would be useful to

further evaluate a wide range of plasticisers and resin formulations.



• One of the main advantages of using plasticiser instead of solvent in the

indirect inkjet printing method is that plasticiser droplets do not form openings

on the resin layer. The retention of the resin layer offers the possibility of

reversing the permeability of the patterned resin layer. The reversibility would

allow multiple patterning to be performed on the same resin layer, thus

reducing wastage and the number of process steps involved.

2. Direct inkjet patterning method

• One of the major problems encountered in the direct inkjet patterning process

is the tendency for the droplets to form bulges along the printed line, which

results in discontinuous lines. This raises serious difficulties in forming

continuous diffusion regions that are essential to conduct currents generated by

the solar cell. More work is required to further understand the surface

behaviour and to find possible surface treatments that avoid this problem.

• Another major problem was the low level of attainable diffusion of dopants

from the inkjet-deposited liquid. The various physical and chemical events that

occur during the high temperature drive-in process needs to be understood in

order to determine the best way to perform such localised diffusions.

Furthermore, it may be possible to formulate a fluid containing chemical

binder such that loss of phosphorus atoms is minimised during processing.

3. Inkjet-processed high efficiency solar cell devices

• Both the indirect and direct inkjet patterning method described in this thesis

can be used as a patterning tool to create many different solar cell structures

that are capable of achieving higher efficiencies. Some of these concepts have

been proposed and discussed in Chapter 4. The implementation of these

structures onto a real solar cell design and the fabrication of such devices

would be invaluable in evaluating the benefits of using these inkjet patterning

methods on the device’s electrical performance.

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Appendix A: List of Abbreviations



AFM atomic force microscope

AR anti-reflection

ARC anti-reflection coating

BC buried contact

BOE buffered oxide etch

BSF back surface field

CIJ continuous inkjet

CSG crystalline silicon on glass

CZ czochralski

DI de-ionised

DEG diethylene glycol

DMC dimatix materials cartridge

DMP dimatix materials printer

DNA deoxyribonucleic acid

DOD drop-on-demand

EBIC electron-beam induced current

EPDM ethylene propylene diene M-class

EWT emitter wrap-through

FIB focused ion beam

FZ float zone

HIT heterojunction with intrinsic thin-layer

HMDS hexamethyldisilazane

HSP hansen solubility parameters

IQE internal quantum efficiency

KOH potassium hydroxide

LD laser-doped

LDPE low density polyethylene

MCK material compatibility kit

OLED organic light emitting diode

OTS octadecyltrichlorosilane



PECVD plasma-enhanced chemical vapour deposition

PERC passivated emitter and rear contacts

PERF passivated emitter and rear floating-junction

PERL passivated emitter and rear locally-diffused

PERT passivated emitter and rear totally-diffused

PGMEA propylene glycol monomethyl ether acetate

PL photoluminescence

PV photovoltaics

PVP polyvinyl phenol

PZT lead zirconate titanate

RFID radio frequency identification

RPM rotation per minute

SOD spin-on-dopant

SRH shockley-read-hall

SRV surface recombination velocity

STC standard testing condition

TCO transparent conducting oxide

UV ultraviolet

UNSW University of New South Wales

Appendix B: List of Symbols



a-Si amorphous silicon

A area of the device

Ag silver

Al aluminium

Au gold

cm centimetre

cm2 square centimetre

cP centipoise

Cu copper

γ& shear rate

lvγ liquid-vapour interfacial tension

slγ solid-liquid interfacial tension

svγ solid-vapour interfacial tension

dδ dispersive (non-polar) interaction in HSP

hδ hydrogen bonding interaction in HSP

pδ polar interaction in HSP

FF fill factor

H2O2 hydrogen peroxide

H2SO4 sulphuric acid

H3PO4 phosphoric acid

H4P2O7 pyrophosphoric acid

HF hydrofluoric acid

HNO3 nitric acid

HPO3 metaphosphoric acid

Hz hertz

Jsc short-circuit current

kHz kilohertz

μ viscosity



μm micrometre

μs microseconds

m local ideality factor

mbar millibar

MHz megahertz

mL mililitre

mm millimetre

mV millivolt

η conversion efficiency

n refractive index

NaOH sodium hydroxide

Ni nickel

nm nanometre

Ω ohms

θe contact angle

Ω/ ohm per square (unit for sheet resistivity)

pFF pseudo fill factor

pL picolitres

P2O5 phosphorus pentoxide

PO4 phosphate

R interaction radius in HSP

Ra distance between Hansen parameter in Hansen space

RED relative energy difference in HSP

Rs series resistance

Rsh shunt resistance

Si silicon

SiNx silicon nitride

SiO2 silicon dioxide

t thickness

TiO2 titanium dioxide

τ shear stress

V volt

Voc open-circuit voltage

Appendix C: List of Printhead Components



The following list describes the function of each component of the printhead used in this

thesis work. These components are present in the fluid path of the printhead construction and

therefore come in contact with the fluid. The material of each component is part of the

Material Compatibility Kit (MCK) used to test the compatibility between the printhead and

the fluid. The name of the printhead in which these components are present is also indicated

next to the component’s name.

1. Apical prebond samples (Galaxy, SX3)

This sample is a completely epoxy-bonded component consisting of the ceramic PZT

material (component 4), flex link material (component 6), and Kovar material

(component 8). This assembly forms part of the wall of the pumping chamber where

the PZT actuates the jetting action.

2. Apical rock trap (Galaxy, SX3)

Rock trap is a filter inside the printhead assembly made from polyimide films such as

Kapton or Upilex. This is the last filter in the printhead assembly before the fluid

reaches the nozzle orifice.

3. Carbon pieces (Galaxy, SX3)

Carbon is the material used as the bulk of the printhead assembly body. It provides the

printhead assembly’s mechanical strength. It is also the material used for the printhead

manifolds.

4. Ceramic PZT material (Galaxy, SX3, DMC)

PZT is the ceramic material used as the piezoelectric element in most DOD piezo

inkjet devices, including the Galaxy, SX3 and DMC printheads used in this work. In

the design, the PZT element is chemically isolated from the fluid path by the flex print

component. Nevertheless, it is still desirable that the fluid is compatible with the PZT

material.



5. Epoxy-coated glass slides (Galaxy, SX3, DMC)

Epoxy is the material used as a sealant to bond various components of the printhead

with each other. In this MCK, in order to test the epoxy compatibility, a glass slide

coated with epoxy material was provided for testing.

6. Flex link/plate material (Galaxy, SX3)

The flex link/plate is a Kapton or Upilex material that includes a series of electrical

contacts which connects to the pumping chambers. The attached electrical contacts are

made of nickel-copper-gold (Ni/Cu/Au) for the Galaxy printhead and copper (Cu) for

the SX3 printhead.

7. Flex print (Galaxy, SX3)

The flex print is the component of the printhead that provides chemical isolation

between the piezoelectric element and the fluid as well as electrical isolation between

the jet body and the fluid and between the piezoelectric element and its electrodes. The

flex print is made from polyimide film and contains the integrated circuit for the

printhead’s driver electronics.

8. Kovar material (Galaxy)

Kovar is an iron-nickel-cobalt alloy that is used as the material for both the stiffening

and cavity plates in the Galaxy printhead construction. The stiffening plate is a thin

metal plate used to stiffen the jetting assembly. The cavity plate is a thin metal plate

into which the pumping chamber has been chemically milled.

9. Parylene-coated glass slides (Galaxy)

Parylene is a polyxylylene polymer that can provide a smooth, continuous and

conformal coating. It is used to coat the walls of the pumping chamber to eliminate

nucleation sites for air bubbles within the fluid volume as described by Moynihan

(1990).



10. Peroxide-treated EPDM and Kalrez rubber for o-ring seal (DMC, SX3)

Ethylene propylene diene M-class (EPDM) and Kalrez rubber are used for the o-ring

seal between the cartridge and the printhead in the DMC to prevent fluid leaking out

of this connection interface and between the adapter bar and jetting assembly in SX3.

11. Polypropylene cartridge material (DMC)

Polypropylene is the material used for the cartridge bag and interface material, which

is used as a fluid reservoir in the DMC design.

12. Silicon wafer pieces and silicon dioxide (SX3, DMC)

Silicon is the material used as the printhead assembly material in both SX3 and DMC

printhead constructions. The entire fluid path and nozzle orifices are micromachined

onto a silicon wafer piece. Due to the micromachining process, some silicon dioxide

layers are present within the printhead construction.

13. Stainless steel (Galaxy, SX3)

Stainless steel is the material used as the nozzle plate material in the Galaxy printhead,

where the orifices are fabricated onto. It is also used as the material for the cavity plate

and adapter bar for the SX3 printhead.

Appendix D: Fluid-Printhead Compatibility Test


Appendix D: Fluid-Printhead Compatibility Test

HF NaOH Acetone DEG

Apical prebond

samples

Corroded and

discolouration

Metal peeled off,

solution affected

No observable

effect

No observable

effect

Apical rock trap No observable

effect

Etched into debris

solution affected

No observable

effect

No observable

effect

Carbon pieces No observable

effect

No observable

effect

No observable

effect

No observable

effect

Ceramic PZT

material

Mostly etched

away into debris

No observable

effect

No observable

effect

No observable

effect

Epoxy-coated

glass slides

Possibly very

minor reactions

No observable

effect

No observable

effect

No observable

effect

Flex link/plate

material

No observable

effect

Metal peeled off,

solution affected

No observable

effect

No observable

effect

Flex print Not tested Not tested No observable

effect

No observable

effect

Kovar material Corrosion visible No observable

effect

No observable

effect

No observable

effect

Parylene-coated

slides

Possibly very

minor reactions

No observable

effect

No observable

effect

No observable

effect

O-ring seal

rubber

No observable

effect

No observable

effect

No observable

effect

No observable

effect

Polypropylene

material

No observable

effect

No observable

effect

No observable

effect

No observable

effect

Si wafer pieces

and SiO2

Si: no effect

SiO2: etched

Si: very slight

weight difference

No observable

effect

No observable

effect

Stainless steel Completely

etched away

No observable

effect

No observable

effect

No observable

effect

Appendix E: PC1D Modelling Parameters



1) PC1D parameters for simulation in Table 4-1:

Parameter Screen Printed

(120 μm)

Selective Emitter

(120 μm)

Selective Emitter

(20 μm)

Cell thickness 220 μm 220 μm 220 μm

Bulk doping 1 Ω.cm 1 Ω.cm 1 Ω.cm

Emitter diffusion 45 Ω/ 100 Ω/ 100 Ω/

Bulk lifetime 30 μs 30 μs 30 μs

Front SRV 1 103 cm/s 1 103 cm/s 1 103 cm/s

Rear SRV 1 107 cm/s 1 107 cm/s 1 107 cm/s

Front optical coating 75 nm SiNx (n=2) 75 nm SiNx (n=2) 75 nm SiNx (n=2)

Front surface texture Random pyramid Random pyramid Random pyramid

Notes:

• The cell thickness is the typical thickness for commercial Si substrate.

• The bulk doping is the typical substrate resistivity for commercial Si substrate.

• The emitter diffusions are the typical emitter sheet resistivity used for standard commercial

screen-printed solar cells (45 Ω/) and the minimum emitter sheet resistivity required to

achieve 100% short wavelength IQE response (100 Ω/).

• The bulk lifetime is the typical minority carrier lifetime for CZ, 1 Ω.cm, p-type Si substrate.

• The rear of the device consists of full area Al BSF region.

• The front SRV is the typical value for hydrogenated SiNx passivated n-type surfaces with the

respective sheet resistivities. The rear SRV is the typical value for full area Al BSF rear.

• Series resistance for the front contacts of the devices was calculated based on fraction metal

coverage (finger width and distances between fingers).

• Broadband reflectance based on fraction of metal coverage was added to the front optical

coating parameter.

• Dark saturation current components were calculated based on fraction of metal coverage and

SRV (1 103 cm/s for non-metallised regions and 1 107 cm/s for metallised regions).



2) PC1D parameters for simulation in Figure 4-12:

Parameter Value

Cell thickness 220 μm

Bulk doping 1 Ω.cm

Emitter diffusion 65 Ω/

Bulk lifetime 5 μs

Front SRV 2 103 cm/s

Rear SRV 1 107 cm/s

Front optical coating 110 nm SiO2

Front surface texture Random pyramid

Notes:

• The cell thickness is the typical thickness for commercial Si substrate.

• The bulk doping is the typical substrate resistivity for commercial Si substrate.

• The bulk lifetime is the typical minority carrier lifetime for CZ, 1 Ω.cm, p-type Si substrate.

• The rear of the device consists of full area Al BSF region.

• The rear SRV is the typical value for full area Al BSF rear.

• Series resistance for the front contacts of the devices was calculated based on fraction metal

coverage (finger width and distances between fingers).

• Experimental reflectance data was used for the reflectance values in the model.

• Dark saturation current components were calculated based on fraction of metal coverage and

SRV.

• See text in Section 4.3.3 for more explanations of the fitting parameters.

Appendix F: General Approach of Liquid Dopant Formulation


Appendix F: General Approach of Liquid Dopant

Formulation

H3PO4 (85%) represents the highest possible concentration for a liquid-phase phosphorus-

containing solution. Increasing the H3PO4 concentration above this value, results in the H3PO4

dehydrating to phosphorus pentoxide (P2O5). H3PO4 is also an acidic solution that potentially

poses a compatibility issue with the printhead, especially at highly concentrated levels. But

the main objective of reformulation was to create a solution that can produce finer features

when inkjet printed onto a substrate and heavy phosphorus diffusions with an appropriate

diffusion profile when thermally treated. Table F-1 lists the general objectives that were

considered during the formulation process.

Parameter General requirements

Acidity Reduce acidity by preferably increasing pH level to neutral for

better fluid-printhead compatibility and longer printhead life.

Purity The phosphorus compound must not contain any trace of

metal or organic to ensure high quality junction formation.

Particle size Preferably no particles or very fine particles less than 1 μm in

diameter.

Viscosity and surface

tension

Viscosity and surface tension values that are similar to the

optimum values for the printhead, preferably in the upper

range to reduce droplet spreading.

Solubility Phosphorus-containing compound must be completely soluble

in the solvent used.

Table F-1: General requirements on the formulation of inkjet-printable liquid

phosphorus dopant sources.

For phosphate (PO4)-based compounds, there are two general types of liquid phosphorus

dopant sources: inorganic acid-based and polymer-based solutions. The acid-based solution is

formulated through the reaction of elemental molecules with PO4, whereas the polymer-based

Appendix F: General Approach of Liquid Dopant Formulation


solution utilises polymer molecules instead. In order to achieve the highest possible

phosphorus solid content in the final solution, acid-based formulations are preferable than

polymer-based formulations due to the use of lower molecular weight compounds.

In the acid-based formulation, it is clear that H3PO4 would contain the highest phosphorus

solid content possible due to the use of hydrogen atoms, which are the lightest element in the

periodic table. However, in order to reduce the acidity of the solution, the hydrogen needs to

be replaced with another atom or molecule, which will inevitably be larger than hydrogen.

Therefore, any modification away from the structure of H3PO4 would always result in lower

phosphorus solid content.

The method of finding the right molecular substitute to hydrogen to balance the charge of the

PO43- ion is largely an experimental exercise. However, there is a general tendency that pH is

increased with decreasing phosphorus concentration. By examining the available elements

across the periodic table, and keeping in mind the general requirements as set out in Table F-

1, it is possible to narrow down a small number of potentially suitable compounds. It is, of

course, desirable to select a compound that contains as high phosphorus solid content as

possible whilst retaining a liquid formation.

Once a suitable compound has been selected, it is very important to evaluate the solubility of

the compound in a high boiling point solvent while still satisfying the viscosity and surface

tension requirements. Finding a single solvent that would suit all of these requirements is

evidently difficult. As a result, a combination of solvents is usually employed. The solvent

with the highest concentration in the solution is the designated dissolving solvent, which

should provide most, if not all, of the solubility. Sometimes smaller amounts of a second

dissolving solvent are added to improve the solubility of the phosphorus compound. Finally,

extra chemicals may be added in order to adjust the viscosity and surface tension values of the

final solution to within the desired optimal range.

Appendix G: Sheet Resistivity Correction Factor



Smits (1958) provided a method for calculating the correction factors of a thin-rectangular

slice such as that shown in Figure G-1.

Figure G-1: Sheet resistance measurement using four-point-probe on a thin,

rectangular slice.

In this case, the sheet resistivity is given by:

IVG=ρ , where ⎟

⎠⎞

⎜⎝⎛⋅⋅=⎟

⎠⎞

⎜⎝⎛⋅⋅=

ba

sbRt

ba

sbRtG ll ,5324.4,

)2ln(π (G-1)

where ρ is the sheet resistivity, G is the correction factor, V is the voltage measured by the

probes, I is the current flowing through the diffused layer, t is the thickness of the diffused

layer, a is the length of the pad, b is the width of the pad and s is the spacing of the probes.

Equation (G-1) assumes that the thickness t of the diffused layer is much smaller than the

probe spacing s. This is valid for the case considered here whereby t is about 1 µm deep and s

is 1 mm giving a t/s ratio of 0.001 << 1. According to the created pattern, a is 15 mm whereas

b varies from 10 to 0.5 mm.

Therefore the following values for ⎟⎠⎞

⎜⎝⎛

ba

sbRl , and G are obtained:



b (mm) b/s a/b R1 G

10 10 1.5 0.935 4.24

8 8 1.875 0.905 4.10

6 6 2.5 0.840 3.81

4 4 3.75 0.710 3.22

2 2 7.5 0.425 1.93

1 1 15 0.220 1.00

0.5 0.5 30 b/s too small N/A

Table G-1: Sheet resistivity correction factors.

Using measured voltages V for a fixed current I on the different-sized diffused pads and

multiplying the V/I with the correction factors G as given in Table G-1 according to the pad

width (size), the actual measured sheet resistivities can be calculated.


Appendix H: List of Author’s Publications

Utama R. Y., Lennon A., Wenham S. R., and Ho-Baillie A. W. Y. 2008, “Methods of

Forming Openings in Selected Materials”, Intl. PCT Publication No. WO/2008/092186.

Lennon A. J., Utama R. Y., Lenio M. A. T., Ho-Baillie A. W. Y., Kuepper N. B., and

Wenham S. R. 2008, “Forming openings to semiconductor layers of silicon solar cells by

inkjet printing” Solar Energy Materials and Solar Cells, Vol. 92, No. 11, pp. 1410-1415.

Utama R., Lennon A., Lenio M., Borojevic N., Ho-Baillie A., Karpour A., and Wenham S.

R. 2008, “Inkjet printing for high efficiency selective emitter silicon solar cell”, 23rd

European Photovoltaic Solar Energy Conference and Exhibition (Valencia).

Borojevic N., Ho-Baillie A., Utama R., Lennon A., Lenio M., Karpour A., and Wenham S.

2008, “Inkjet texturing for high efficiency commercial silicon solar cells”, 23rd European

Photovoltaic Solar Energy Conference and Exhibition (Valencia).

Lennon A., Utama R., Ho-Baillie A., and Wenham S. 2008, “Inkjet method for direct

patterned etching of silicon dioxide” DF 2008 International Conference on Digital

Fabrication Technologies (Pittsburgh).

Utama R., Lennon A., Ho-Baillie A., Lenio M., Borojevic N., and Wenham S. 2007, “Inkjet

printing for high efficiency silicon solar cell structures”, 17th International Photovoltaic

Science and Engineering Conference (Fukuoka).

Kuepper N., Utama R., Guo A., Wells M., Ho A. W. Y., and Wenham S. R. 2007,

“Photovoltaic technology for developing countries” 22nd European Photovoltaic Solar Energy

Conference and Exhibition (Milan).

Lennon A., Utama R., Ho-Baillie A., Lenio M., Kuepper N., and Wenham S. 2007, “Inkjet

method for forming openings to buried semiconductor layers of silicon solar cells”, DF 2007

International Conference on Digital Fabrication Technologies (Anchorage).

inkjet printing for commercial high-efficiency silicon solar cells

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