nanostructured electrodes and their use in organic solar cells · 2017-09-15 · renewable energy...

Nanostructured Electrodes

and Their Use in Organic Solar Cells

Master Thesis Reza Abolhassani

Mads Clausen Institute University of Southern Denmark

Supervisors:

Associate Professor Morten Madsen Associate Professor Jost Adam

November 2015

Master Thesis- Reza Abolhassani MCI, University of Southern Denmark

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Acknowledgements

I would like to express my sincere gratitude to my supervisors, Morten Madsen and Jost

Adam, for giving me the chance to work on this project, and for their continuous support and

guidance throughout my work. I have learned so much from them and it is my honor to be their

student.

I would also like to thank PhD students: Mina Mirsafaei, André Luis Fernandez Cauduro,

Arkadiusz Jaroslaw Goszczak, and Mehrad Ahmadpour. They were not only good mentors that

gave me their valuable experiences and provided me information, but also good friends that

motivated me to finish the project as good as possible for me.

Last but not least, I wish to thank my family for their borderless love, supports, and

encouragement. They will be in my heart forever.


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Abstract

Energy demands have increased dramatically in last decades and predicted to increase

by three times in 2050. Fossil fuels are the main reasons for environmental degradation, acid

rain, ozone depletion, forest destruction, and global climate change. Therefore, renewable

sources of energy have attracted attention from industries and scientists. One of these

renewable energy sources is solar power. Solar cells convert sun light directly to the electricity.

However organic solar cells power conversion efficiency is rather low in comparison with

conventional solar cells, it is an interesting topic for researchers due to their flexibility and

lightweight, low cost materials and manufacturing, and simple fabrication process.

In this research, light-trapping structures in OSC electrodes, and their effect on light

absorption profile is studied. The project divided into two main parts: modeling and

experiments.

Modeling is a good tool to study devices optical properties which experimentally are

time and cost consuming. OSCs were simulated using finite element method by COMSOL

Multiphysics 5. Different device configurations were studied and the effects of pitch and height

dimensions on light absorption profile were investigated.

In the experimental part, wrinkled PDMS substrates used as stamps for imprinting and

transferring structures onto the electrodes. Then optical properties of fabricated devices were

characterized and light absorption enhancement in respect to planar device was studied.

Finally, three different configurations were characterized for OSC parameters under 1 sun

illumination by solar simulator.


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Table of Contents

Acknowledgements ....................................................................................................................................... 0

Abstract ......................................................................................................................................................... 2

List of Figures ................................................................................................................................................ 5

List of Tables ................................................................................................................................................. 9

List of Acronyms .......................................................................................................................................... 10

1. Introduction ............................................................................................................................................ 11

1.1. Project Background ..................................................................................................................... 11

1.2. Project Objectives ....................................................................................................................... 12

1.3. Outline......................................................................................................................................... 13

2. Theory ..................................................................................................................................................... 15

2.1. Introduction ................................................................................................................................ 15

2.2. Light Absorption in Thin-Films .................................................................................................... 16

2.3. Photovoltaic Effect ...................................................................................................................... 19

2.4. Basics of Solar Cell Operation ..................................................................................................... 20

2.5. Organic Solar Cells....................................................................................................................... 22

2.5.1. Organic Semiconductors ..................................................................................................... 22

2.5.2. Principles of Organic Solar Cells Operation .............................................................................. 23

2.5.3. Organic Solar Cells Structure............................................................................................... 24

2.5.4. Organic Solar Cell Characterization ........................................................................................... 27

2.5.5. Prospects and Challenges ......................................................................................................... 30

3. Simulation ............................................................................................................................................... 31

3.1. Introduction ..................................................................................................................................... 31

3.2. Simulation Methods ......................................................................................................................... 31

3.3. Comsol Multiphysics ........................................................................................................................ 33

3.3.1. Wave Optics Module; Theories and Principles ......................................................................... 33

3.4. Simulation Setup .............................................................................................................................. 35

4. Experimental ........................................................................................................................................... 38

4.1. Introduction ..................................................................................................................................... 38

4.2. Stamp Fabrication for Nano-imprinting ........................................................................................... 38


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4.3. Fabrication Methods and Equipment .............................................................................................. 43

4.3.1. Sample Preparation................................................................................................................... 43

4.3.2. Nano-Imprinting ........................................................................................................................ 43

4.3.3. Metal Deposition ...................................................................................................................... 46

4.3.4. Organic Material Deposition ..................................................................................................... 48

4.4. Measurements and Characterization .............................................................................................. 50

4.4.1. Surface Characterization; Atomic Force Microscope ................................................................ 50

4.4.2. Absorption Measurement; Optical Microscope ....................................................................... 51

5. Results and Discussions........................................................................................................................... 52

5.1. PDMS Stamps ................................................................................................................................... 52

5.2. Simulation Results ............................................................................................................................ 56

5.3. Light Absorption in Organic Solar Cells ............................................................................................ 62

5.4. Organic Solar Cells Parameters ........................................................................................................ 67

6. Conclusion ............................................................................................................................................... 71

7. Outlook.................................................................................................................................................... 73

8. Bibliography ............................................................................................................................................ 75

9. Appendices .............................................................................................................................................. 81

9.1. Appendix 1- Project Timeline ........................................................................................................... 81

9.2. Appendix 2- Efficiency Chart ............................................................................................................ 82

9.3. Appendix 3- PDMS Stamps Process ................................................................................................. 83


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List of Figures

FIGURE 1: METALS EMIT ELECTRONS WHEN THEY ARE IRRADIATED BY LIGHT. THIS PHENOMENON IS CALLED PHOTOELECTRIC

EFFECT WHICH SHOWS WAVE-PARTICLE DUALITY NATURE OF LIGHT. ................................................................. 16

FIGURE 2: WHEN LIGHT PASSES FROM A MEDIUM TO ANOTHER MEDIUM WITH HIGHER REFRACTIVE INDEX, PART OF IT

REFLECTED AT THE INTERFACE AND PART OF IT TRANSMITTED TO THE SECOND MATERIAL. ..................................... 17

FIGURE 3: LIGHT ABSORPTION IN SEMICONDUCTORS: WHEN A PHOTON WITH HIGHER ENERGY THAN BANDGAP FALLS INTO

SEMICONDUCTOR (A), AN ELECTRON MOVES TO CONDUCTION BAND AND LEAVES A HOLE IN VALENCE BAND (B). ..... 18

FIGURE 4: PHOTOVOLTAICS EFFECT: ELECTRONS (RED CIRCLES) DIFFUSE TO THE P-TYPE AND HOLES (BLUE CIRCLES) DIFFUSE

TO THE N-TYPE MATERIAL. AFTER A WHILE, FIXED IONS BUILD DEPLETION REGION AND STOPS CARRIERS FROM

DIFFUSION UNTIL EQUILIBRIUM HAS CHANGED (E.G. BY ABSORBED PHOTONS) .................................................... 19

FIGURE 5: CROSS-SECTIONAL VIEW OF A TYPICAL P-N JUNCTION SOLAR CELL. CHARGES ARE GENERATED BY INCIDENT LIGHT

AND COLLECTED BY CONTACTS. .................................................................................................................. 20

FIGURE 6: ELECTRONIC BANDS IN MATERIALS: AN ELECTRON NEEDS TO OVERCOME EG (BAND GAP ENERGY) TO GO FROM

VALENCE BAND TO CONDUCTION BAND. (A) IS INSULATOR, (B) IS SEMICONDUCTOR, AND (C) IS CONDUCTOR. .......... 21

FIGURE 7: SHORT CIRCUIT FLOW OF ELECTRONS AND HOLES AT A P-N JUNCTION: (A) ELECTRON-HOLE PAIR IS CREATED BY

PHOTON ABSORPTION; (B) ELECTRON FLOWS TO THE CIRCUIT AND HOLE CROSSES THE JUNCTION; (C) ELECTRON PASSES

THROUGH THE EXTERNAL LOAD AND RECOMBINES WITH A HOLE AND COMPLETES THE CIRCUIT. ............................. 21

FIGURE 8: DIAGRAM OF HOMO AND LUMO IN GROUND STATE: ELECTRONS IN HOMO ARE ILLUSTRATED BY RED CIRCLES.

............................................................................................................................................................ 22

FIGURE 9: FOUR STEPS OF AN ORGANIC SOLAR CELL OPERATION: (A) PHOTON IS ABSORBED AND EXCITON IS GENERATED; (B)

EXCITON DIFFUSED TO DONOR/ACCEPTOR INTERFACE; (C) EXCITON DISSOCIATED AND CHARGES CARRIER IS

GENERATED; AND (D) CARRIER TRANSPORTED AND THEN EXTRACTED AT THE ELECTRODES. ................................... 23

FIGURE 10: STRUCTURE OF A BILAYER ORGANIC SOLAR CELL.................................................................................... 24

FIGURE 11: ACTIVE LAYER IN BULK HETEROJUNCTION ORGANIC SOLAR CELLS: 1) EXCITON IS GENERATED UPON LIGHT

ABSORPTION; 2) EXCITON RECOMBINE OR DIFFUSE AT DONOR/ACCEPTOR INTERFACE AND SEPARATED TO ELECTRON

AND HOLE, THEN THEY TRAVEL TO THEIR CORRESPONDING ELECTRODES; 3) ELECTRON AND HOLE MIGHT RECOMBINE

WITH TRAPPED CHARGES. .......................................................................................................................... 25

FIGURE 12: MOLECULAR STRUCTURES OF SOME ORGANIC ELECTRON DONOR AND ACCEPTOR MATERIALS [35] .................. 26

FIGURE 13: AIR MASS (AM) REPRESENTS THE LIGHT POWER DROP AS IT TRAVELS THROUGH THE ATMOSPHERE AND HITS THE

EARTH. ................................................................................................................................................... 27


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FIGURE 14: EQUIVALENT ELECTRICAL CIRCUIT FOR A SOLAR CELL .............................................................................. 28

FIGURE 15: I-V CURVE OF AN IDEAL SOLAR CELL. RED LINE AND BLUE LINE REPRESENT THE I-V CURVE IN PRESENCE OF LIGHT

ILLUMINATION AND WITHOUT LIGHT ILLUMINATION RESPECTIVELY. .................................................................. 28

FIGURE 16: KEY PARAMETERS OF A SOLAR CELL; FILL FACTOR IS DEFINED AS AREA (A) DIVIDED BY AREA (B). SHORT CIRCUIT

CURRENT AND OPEN CIRCUIT VOLTAGE, MAXIMUM POWER, AND CURRENT AND VOLTAGE AT THE POINT OF MAXIMUM

POWER ARE SHOWN AS IMP AND VMP RESPECTIVELY. [50] ................................................................................... 29

FIGURE 17: INCIDENT WAVE IN A MULTILAYER DEVICE (LEFT); ELECTROMAGNETIC WAVE INTERACTIONS AT LAYERS

INTERFACE (RIGHT). ................................................................................................................................. 32

FIGURE 18: SKETCH OF THE PLANAR DEVICE THAT IS USED AS REFERENCE. TOP LAYER IS AIR WHICH PROPAGATING LIGHT

ENTERS THE DEVICE. ................................................................................................................................. 35

FIGURE 19: OPTICAL PROPERTIES OF MATERIALS WHICH USED IN COMSOL AFTER [62] ................................................ 36

FIGURE 20: ELASTIC PDMS SUBSTRATE .............................................................................................................. 39

FIGURE 21: WRINKLES FORMATION PROCESS ON PDMS: (A) CLEAN NON-DEFECTED PIECE OF PDMS SUBSTRATE IS CHOSEN;

(B) PDMS SUBSTRATE IS STRETCHED; (C) STRETCHED PDMS IS EXPOSED UPON OXYGEN-PLASMA; (D) A THIN HARD

LAYER IS FORMED ON TOP OF PDMS; (E) BY RELEASING THE STRETCH, PERIODIC WRINKLED PATTERNS ARE FORMED ON

THE TOP LAYER DUE TO IN-PLANE COMPRESSIVE STRAIN. ................................................................................. 41

FIGURE 22: AFM IMAGES (5×5 µM) OF PDMS 28 WHICH IS FABRICATED AND USED IN THIS PROJECT. .......................... 41

FIGURE 23: ILLUSTRATION OF (LEFT) SIDE VIEW AND (RIGHT) TOP VIEW OF THE CLAMP WHICH USED IN THIS PROJECT TO

STRETCH PDMS SUBSTRATES. .................................................................................................................... 42

FIGURE 24: ILLUSTRATION OF IMPRINTING PROCESS: (A) DROPLETS OF PHOTORESIST IS APPLIED ON THE GLASS SUBSTRATE;

(B) SPINNING PROCESS LEAVES A DETERMINED THICKNESS OF PHOTORESIST FILM; (C) PDMS STAMP WITH A COLUMN

WEIGHT OF 500 GR IS LAID ON THE SUBSTRATE; (D) THE FORCE DUE TO COLUMN WEIGHT TRANSFERS STRUCTURES

ONTO PHOTORESIST; (E) SOLVENT IS EVAPORATED AND PHOTORESIST IS HARDENED; (F) PDMS STAMP AND SUBSTRATE

ARE DETACHED AND STRUCTURES ARE REMAINED ON HARDENED PHOTORESIST. ................................................. 45

FIGURE 25: ILLUSTRATION OF AN E-BEAM DEPOSITION SYSTEM: HIGH ENERGY ELECTRON BEAM IS FOCUSED ON THE

MATERIAL IN A CRUCIBLE; ATOMS EVAPORATED FROM THE CRUCIBLE TRAVEL THROUGH THE VACUUM CHAMBER,

REACH THE SUBSTRATE AND FORM A THIN FILM ON IT. ................................................................................... 47

FIGURE 26: ILLUSTRATION OF A DC-SPUTTER SYSTEM; VOLTAGE DIFFERENCE BETWEEN ANODE AND CATHODE CREATES

PLASMA. ENERGETIC IONS ARE ACCELERATED AND STRIKE THE TARGET AND SPUTTER ATOMS FROM IT. THESE ATOMS

STRIKE WAFER AND CONDENSE ON IT AND FILM FORMS ON THE SUBSTRATE SURFACE. ......................................... 48


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FIGURE 27: ILLUSTRATION OF A PLANAR DEVICE: (A) IS A TOP VIEW OF A FABRICATED SUBSTRATE (EACH SUBSTRATE

CONSISTS OF SIX CELLS; (B) CROSS-SECTIONAL VIEW OF THE SELECTED AREA OF (A). ............................................ 49

FIGURE 28: ATOMIC FORCE MICROSCOPY PRINCIPLE: ATOMIC FORCE BETWEEN TIP AND SURFACE CAUSE DEFLECTIONS IN THE

CANTILEVER. DEFLECTIONS TRANSFER TO THE DETECTOR BY REFLECTED LASER BEAM AND IMAGE IS FORMED. .......... 50

FIGURE 29: (LEFT) AFM IMAGE (5×5 µM) OF PDMS 28 AND (RIGHT) HEIGHT PROFILE OF WRINKLED PATTERN FOR THE

SELECTED LINE 1. ..................................................................................................................................... 52

FIGURE 30: AVERAGE PITCH (LEFT) AND AVERAGE HEIGHT (RIGHT) FOR 10% STRETCH PDMS STAMPS AS A FUNCTION OF

PLASMA EXPOSURE TIME ........................................................................................................................... 54





FIGURE 33: SKETCH OF THE STRUCTURE AFTER [56] TO EVALUATE COMSOL MODEL VALIDITY. .................................... 56

FIGURE 34: LIGHT ABSORPTION AS A FUNCTION OF LIGHT WAVELENGTH FOR THE STRUCTURE SHOWN IN FIGURE 33. GREEN

LINE IS THE REFERENCE [56] WHICH USED TM METHOD. RED LINE IS FDTD SIMULATION RESULT AND BLUE LINE IS

COMSOL SIMULATION RESULT WHICH IS USED IN CURRENT RESEARCH. ............................................................ 57

FIGURE 35: (A) PLANAR STRUCTURE AS REFERENCE; AND (B) STRUCTURED ELECTRODE DEVICE. PITCH AND HEIGHT IN (B)

VARY TO ENHANCE LIGHT ABSORPTION IN DEVICES. ........................................................................................ 58

FIGURE 36: LIGHT ABSORPTION IN PEDOT AND ACTIVE LAYER FOR PLANAR DEVICE WHICH IS SHOWN IN FIGURE 3 (A) AS

REFERENCE IN TE MODE. ........................................................................................................................... 58

FIGURE 37: TOTAL LIGHT ABSORPTION FOR PLANAR DEVICE WHICH IS SHOWN IN FIGURE 3 (A) AS REFERENCE IN TE MODE.

............................................................................................................................................................ 59

FIGURE 38: TOTAL LIGHT ABSORPTION IN DEVICES WITH LIGHT-TRAPPING STRUCTURED ELECTRODES OF DIFFERENT HEIGHT

WITH THE SAME PITCH OF 500 [NM] IN COMPARED WITH PLANAR STRUCTURE AS REFERENCE IN TE MODE. ............ 59

FIGURE 39: TOTAL LIGHT ABSORPTION IN DEVICES WITH LIGHT-TRAPPING STRUCTURED ELECTRODES OF DIFFERENT HEIGHT

WITH THE SAME PITCH OF 500 [NM] IN COMPARED WITH PLANAR STRUCTURE AS REFERENCE IN TM MODE. ........... 60

FIGURE 40: LIGHT ABSORPTION IN ACTIVE LAYER FOR DEVICES WITH LIGHT-TRAPPING STRUCTURED ELECTRODES OF

DIFFERENT HEIGHT WITH THE SAME PITCH OF 500 [NM] IN COMPARED WITH PLANAR STRUCTURE AS REFERENCE IN TE

MODE. ................................................................................................................................................... 61

FIGURE 41: TOTAL LIGHT ABSORPTION IN DEVICES WITH LIGHT-TRAPPING STRUCTURED ELECTRODES OF DIFFERENT PITCH

WITH THE SAME HEIGHT OF 50 [NM] IN COMPARED WITH PLANAR STRUCTURE AS REFERENCE. ............................. 61


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FIGURE 42: SIMULATED LIGHT ABSORPTION FOR F0 (BLUE LINE), F1 (DARK YELLOW LINE), F11 (GREEN LINE), AND F15 (RED

LINE). .................................................................................................................................................... 63

FIGURE 43: MEASURED REFLECTION SPECTRUM IN RESPECT TO F0 FOR F1 (DARK YELLOW LINE), F11 (GREEN LINE), AND

F15 (RED LINE). ...................................................................................................................................... 64

FIGURE 44: SIMULATED LIGHT ABSORPTION FOR F0 (BLUE LINE), F11 (DARK YELLOW LINE), F17 (GREEN LINE), AND F13

(RED LINE). ............................................................................................................................................. 65

FIGURE 45: MEASURED REFLECTION SPECTRUM IN RESPECT TO F0 FOR F11 (DARK YELLOW LINE), F17 (GREEN LINE), AND

F13 (RED LINE). ...................................................................................................................................... 65

FIGURE 46: OBSERVED DEFECTS IN ONE OF THE FABRICATED DEVICE AFTER ORGANIC MATERIAL DEPOSITION UNDER THE

OPTICAL MICROSCOPE WITH 5× OBJECTIVE (LEFT) AND 10× OBJECTIVE (RIGHT) .................................................. 67

FIGURE 47: OSCS PARAMETERS FOR DEVICES LISTED IN TABLE 8. ............................................................................ 70


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List of Tables

TABLE 1: SPIN-COATING PARAMETERS TO OBTAIN 1.4 µM THICKNESS OF PHOTORESIST. .............................................. 43

TABLE 2: AVERAGE PITCH AND HEIGHT OF PDMS STAMPS AND THEIR FABRICATION PARAMETERS. ................................ 53

TABLE 3: WRINKLES AVERAGE PITCH AND HEIGHT WITH CORRESPONDING STANDARD DEVIATION FOR THE STRETCH OF 10%

............................................................................................................................................................ 53

TABLE 4: WRINKLES AVERAGE PITCH AND HEIGHT WITH CORRESPONDING STANDARD DEVIATION FOR THE STRETCH OF 20%

............................................................................................................................................................ 54

TABLE 5: TABLE 3: WRINKLES AVERAGE PITCH AND HEIGHT WITH CORRESPONDING STANDARD DEVIATION FOR THE STRETCH

OF 20% ................................................................................................................................................. 55

TABLE 6: SAMPLES WHICH ARE USED TO INVESTIGATE LIGHT ABSORPTION ENHANCEMENT IN THIS RESEARCH; AND THEIR

CONFIGURATION. ..................................................................................................................................... 62

TABLE 7: SAMPLES WHICH WERE USED TO CHARACTERIZED UNDER SOLAR SIMULATOR AND THEIR CONFIGURATION. ........ 69

TABLE 8: DEVICES WHICH CHARACTERIZED UNDER SOLAR SIMULATED AND THEIR PARAMETERS. .................................... 69


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List of Acronyms

AFM Atomic Force Microscopy AM Air Mass BCP bathocuproine BHJ Bulk Heterojunction CVD Chemical Vapor Deposition DBP Dibenzo{[f,f' ]-4,4',7,7'-tetraphenyl}diindeno[1,2,3-cd :1',2',3'-lm ]perylene DC Direct Current FDTDM Finite-Difference Time-Domain Method FEM Finite Element Method FF Fill Factor HOMO Highest Occupied Molecular Orbital JSC Short Circuit Current LUMO Lowest Unoccupied Molecular Orbital MDMO-PPV Poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] MEH-PPV Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] MEMS Microelectromechanical systems OLED Organic Light-Emitting Diode OSC Organic Solar Cell P3HT Poly(3-hexylthiophene-2,5-diyl) PCBM Phenyl-C61-butyric acid methyl ester PCE Power Conversion Efficiency PDMS Polydimethylsiloxane PEDOT Poly(3,4-ethylenedioxythiophene) PPV Polyphenylene Vinylene PSS Poly(styrenesulfonate) PTs Polythiophenes PVD Physical Vapor Deposition TE Transverse Electric TM Transverse Magnetic TMM Transfer Matrix Method VOC Open Circuit Voltage


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1. Introduction

1.1. Project Background

The source of almost all energies on the earth, from warmth and food to wind and fossil

fuels, is solar energy. When people generate electricity by using different forms of energies,

they actually convert solar energy to electricity indirectly.

176 years have passed since Alexandre-Edmond Becquerel discovered photovoltaic

effect in his father’s laboratory [1]. In 1883, Charles Fritts made a solar cell using selenium with

less than 1% efficiency [2] [3]. Albert Einstein opened a new vision in light-matter interaction by

explaining the photoelectric effect in a published paper in 1905 [4] and won the noble prize for

that in 1921. The first silicon based solar cell was patented by Russel Ohl in 1941 [5]. Thereafter,

many investments and researches have been developed to enhance the efficiency by varying

fabrication methods, solar cell architectures, and materials; and the highest photovoltaic

research cell efficiencies have been reported as 45.7% for a four-junction inverted

metamorphic (4J IMM) cell [6].

Tang et al. demonstrated the first OLED in 1986 [7] and it showed semiconductors

potential for a new generation of solar cells: Organic solar cells. This idea became popular

quickly due to unique properties of organic semiconductors: Flexibility, thinness, simple

fabrication methods, and low-cost processing [8] [9] [10]. Although vast researches and

developments have been performed on organic solar cells, their rather low efficiency is the

main drawback in comparison with inorganic solar cells. Charge carrier mobility in organic

semiconductors is low. In addition, although the absorption coefficient is high in organic

semiconductors, excitons diffusion length is small. Those properties cause restriction to active

layer thickness [11].

This master thesis focuses on the light absorption of organic solar cells and investigates

the effects of nano-structured features on electrodes to enhance maximum light absorption;

using Comsol Multiphysics as simulation software and compare results with fabricated solar


12

cells in University of Southern Denmark. Procedure had been discussed regularly with two

supervisors: Morten Madsen (associate professor at MCI, SDU), Jost Adam (associate professor,

MCI, SDU), and Mina Mirsafaei (PHD student, MCI, SDU), André Luis Fernandez Cauduro (PHD

student, MCI, SDU), and Arkadiusz Jaroslaw Goszczak (PHD student, MCI, SDU) to study results,

find out experimental and simulation problems, and plan next steps.

1.2. Project Objectives

This project began with simulation of organic solar cells using Comsol Multiphysics 5 to

evaluate light absorption in different layers. In order to have a reliable model, some published

researches were simulated; then outcomes were compared and confirmed by their results. The

next step was fabricating planar organic solar cells as a reference and characterizing light

absorption through optical spectroscopy techniques, and to check its results to the simulation.

These organic solar cells consist of Titanium and Aluminum as bottom electrode on a glass

substrate, blend of P3HT:PCBM as active layer, and PEDOT:PSS as top electrode.

One approach to increase power conversion efficiency in OSCs is to increase light

absorption in the active layer. Light-trapping in semiconductor devices was patented in 1968

[12]. Increasing effective absorption makes it possible to decrease active layer thickness; results

to reduce losses. This particularly has to be considered in organic photovoltaics which active

layer thickness has strong effects on internal quantum efficiency [13]. Soft embossed grating

fabrication [14], buried nano-electrodes [15], and surface plasmon [16] are techniques to enhance

light absorption in active layer.

The main goal of this project is to understand nano-structured electrode effects on light

absorption in active layer which affects directly JSC, and thereby in power conversion

efficiencies. It was done by varying nano-structures pitch, height, and width in simulation, and

then nano-structures were developed by Nanoimprint lithography, and characterized by AFM

measurements. In addition, optical spectroscopy was used to characterize their light

absorption. Furthermore, the cells were characterized through standard I-V measurements

under a solar simulator.


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Finally, derived models were compared to experimental findings to understand how

nano-structured electrodes affect light absorption and power conversion efficiency in organic

solar cells.

1.3. Outline

Chapter 1 is a brief historical revision of invention and development of solar cells and

main objectives of this project.

Chapter 2 provides theories behind the solar cells. It starts with nature of light and light

absorption in thin films; and how solar cell structure gets advantage of this feature. Then solar

cell operation with emphasis on photovoltaic effect is explained and its main parameters are

described and formulated. Finally, it focuses on OSCs with some details about organic

semiconductors and electron donor and acceptor materials. OSCs structure and operation

relying on active layer is elucidated and its prospects and challenges are listed in the last part.

Chapter 3 includes a short introduction to simulation methods. Then an overview on

Comsol Multiphysics and its abilities are given; alongside the theory and principle backgrounds

for optics module which is used in this project. In the last part, detailed simulation setup for this

project is explained.

Chapter 4 is dedicated to experimental aspects and measurements. In the first part of

this chapter, all the fabrication theories, methods, and devices which are used in this project

are explained briefly. It includes metal deposition, nano imprint lithography, and organic

material deposition. In the second part, measurements methods and theories about optical

microscopy and solar simulator are described.

Chapter 5 shows results and discussions about the effects of nano-structured features

on electrodes in OSCs. All the results from simulations and measurements for various

dimensions of nano-structured solar cells are compared with the reference (planar cell) and

each other and the optimum geometry is enhanced.

Chapter 6 summarizes and concludes main results.


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Chapter 7 makes suggestions for future works and developments.


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2. Theory

2.1. Introduction

All technological developments and industrialization have been based on energy. This

has resulted in increasing energy consumption in past decades and the global energy demand is

expected to increase by three times in the middle of 21st century. [17]

Fossil fuels, renewable resources and nuclear resources are three main energy resource

categories [18]. Nuclear energy causes health and environmental problems such as Chernobyl

incident in 1986 and Japan’s earthquake incident in 2011. Furthermore, fossil fuels are the main

reason for environmental degradation, acid rain, ozone depletion, forest destruction and global

climate change (greenhouse effect) [19]. Industries use almost 40% of worldwide energy and

emit almost 37% of global greenhouse gases [20]. The global temperature has increased by 0.4-

0.8 °C in the last century; and arctic sea ice thinned by 40% and decreased by 10-15% since the

1950s [21]. These drawbacks, in addition to shortage of coil, oil, and natural gas, has increased

human interests to look for alternative sustainable kind of energy: renewable energy.

One common definition of sustainable development is: “development that meets the

needs of the present without compromising the ability of future generations to meet their own

needs” [22]. Today, renewable energy sources supply 14% of the total world energy demand [23].

One of these renewable resources is the sun. Sun emits energy at a rate of 3.8×1023 kW, and

almost 1.8×1014 kW is intercepted by the earth [24]. This reveals why 26% of global research

effort in the last 30 years in the field of renewable energy has been dedicated to solar energy

[18].

This chapter starts with light-matter interaction, specifically light absorption in thin

films. Then basic principles of solar cell operation and characterization are shown. Finally,

organic solar cell structure and operation are discussed in detail, including its prospects and

challenges.


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2.2. Light Absorption in Thin-Films

Light nature had been a debate between scientists in the sixteenth and seventeenth

century. Although Isaac Newton considered light as a stream of particles travelling in straight

lines, Christian Huygens had a different hypothesis in which light is wave, spreading out from a

source and propagating in a medium in all directions; Augustin Fresnel developed Fresnel’s

equation to analyze the reflection and refraction the light as a transverse wave, and James Clerk

Maxwell introduced a set of partial differential equations which describes light propagation as

an electromagnetic wave. [18, 19]

Wave theory of light had some difficulties where the light interacts with matter. Max

Planck observed atoms emitted light in discrete energy quantities when they intercepted by

light. This energy 𝑬 is proportional to the radiation frequency, 𝒗:

𝐸 = ℎ𝑣 (2-1)

where 𝒉 is Planck’s constant. In 1905, Albert Einstein explained photoelectric effect: the

emission of electrons from a metal surface when irradiated with light. These observations led to

wave-particle duality nature of light, which on, light behaves as wave in propagation and

interference and diffraction; and behaves like particle when exchanging energy with matter, as

in photoelectric effect [20]. (Figure 1)

Figure 1: Metals emit electrons when they are irradiated by light. This phenomenon is called photoelectric effect which shows wave-particle duality nature of light.


17

When a beam of light impinges a transparent material, part of it is scattered backward

and part of it is transmitted to the medium with bending at the interface. These phenomena

are called reflection and refraction respectively which is shown in Figure 2.

Figure 2: When light passes from a medium to another medium with higher refractive index, part of it reflected at the interface and part of it transmitted to the second material.

Regarding to wave continuity at boundaries, laws of reflection and reflection are

described as below:

Law of reflection: 𝜃𝑖 = 𝜃𝑟 (2-2)

Law of refraction: 𝑛1 sin 𝜃𝑖 = 𝑛2 sin 𝜃𝑡 (2-3)

where 𝜽𝒊 is incident angle, 𝜽𝒓 is reflection angle, 𝜽𝒕 is transmitted angle, 𝒏𝟏 is real part of

refractive index in the medium 1, and 𝒏𝟐 is real part of refractive index in medium 2. Refractive

index is the ratio of speed of light in vacuum 𝒄 to speed of light in medium 𝒗.

𝑛 =𝑐

𝑣 (2-4)

Materials are not perfect insulators, i.e. they have non-zero electric conductivity 𝝈.

Relation between electric field, current density, and conductivity is formulated by Ohm as

below:

𝑱 = 𝜎𝑬 (2-5)


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This physically means part of the light is absorbed while passes through a medium. This

optical property is taken account into the imaginary part of refractive index, called extinction

coefficient (𝒌) and it represents absorption:

𝜂 = 𝑛 + 𝑖𝑘 (2-6)

Note that in most of the materials either 𝒏 and 𝒌 are frequency dependence. Equations

solutions have been in many references. [21, 22]

When light impinges upon a semiconductor material, photons with energy (𝑬𝒑𝒉) more

than bandgap energy interact with electrons, break bonds and generate electron-hole pairs.

The bandgap (𝑬𝒈) is the energy gap between the valence band (𝑬𝒗) and the conduction band

(𝑬𝒄), and it is defined as the minimum energy required to excite an electron from valence band

to conduction band. The higher the energy of photon, the more electrons are excited to the

conduction band in the case of semiconductors. Absorption coefficient depends on wavelength

of light (𝜆) and extinction coefficient (𝑘) [23]. This phenomenon is sketched in Figure 3:

Figure 3: Light absorption in semiconductors: When a photon with higher energy than bandgap falls into semiconductor (a), an electron moves to conduction band and leaves a hole in valence band (b).


19

2.3. Photovoltaic Effect

Edmond Becquerel discovered photovoltaic effect in 1839 when he observed that two

silver-coated platinum electrodes immersed in acid generated electric power when illuminated

with sunlight [1]. However, developments of quantum mechanics and semiconductor

technologies have played a key role to understand photovoltaics effect and improve

photovoltaic devices [31].

Voltage and current must be generated in order to generate power in photovoltaic

devices. It is done by p-n junction formation. Simply, n-type and p-type materials are doped

semiconductors with high electron and hole concentration respectively. When they join,

electrons diffuse to the p-type part at the junction and similarly holes diffuse to the n-type part;

leave exposed charges on dopant atom sites behind. As result, an electric field is built in the

depletion region around the junction and it stops the flow and reaches equilibrium. Absorbed

photons can generate electron-hole pairs. If these generated pairs are close enough to the

junction within the diffusion length, the electric field separates and transfers them to the

corresponding regions and changes the equilibrium. An external load provides a closed loop

(circuit) for electrons; voltage and current are generated [28] [32]. This phenomenon is illustrated

in Figure 4.

Figure 4: Photovoltaics Effect: electrons (red circles) diffuse to the p-type and holes (blue circles) diffuse to the n-type material. After a while, fixed ions build depletion region and stops carriers from diffusion until equilibrium has changed (e.g.

by absorbed photons)


20

2.4. Basics of Solar Cell Operation

Solar cells are devices that directly convert incident sunlight into electricity using

photovoltaic effect. This needs in the material which light is absorbed; electron-hole pairs are

generated. Then electron excites to a higher energy state and then moves to an external circuit.

This electron then dissipates its energy and returns to the solar cell. Almost all the materials

satisfy these requirements, but practically all photovoltaic devices use p-n junction

semiconductor materials [33]. A cross-section of a typical solar cell is shown in Figure 5:

Figure 5: Cross-sectional view of a typical p-n junction solar cell. Charges are generated by incident light and collected by contacts.

Anti-reflection coating is a transparent or semi-transparent material which lets the light

imposes itself to the cell. The base consists of materials which generate charges when light is

absorbed by them. Charges are collected by contacts and conducted to the external load.

Semiconductor is a material which has conductivity in the range between an insulator

and a metal. Unique properties of semiconductors lie in their bandgap. They have bandgaps in

the range of 1-4 eV, whilst insulators have bandgap greater than 5 eV [34]. Energy band gaps in

different materials are illustrated in Figure 6:


21

Figure 6: Electronic bands in materials: an electron needs to overcome Eg (band gap energy) to go from valence band to conduction band. (a) is insulator, (b) is semiconductor, and (c) is conductor.

A conventional solar cell simply is two sandwiched semiconductors, one is n-type doped

and another is p-type, between two electrodes. Pure silicon doped by elements in column V of

periodic table results in n-type semiconductor which in electrons have more concentration than

holes. Similarly, p-type silicon is an intrinsic silicon doped by an element from column III of

periodic table and has hole concentration more than electron.

When light is absorbed by semiconductor material, electron-hole pairs are generated if

the incident photon has higher energy than the bandgap. An electric field exists at the p-n

junction may separate charge carriers before recombination. If base and emitter connected to

each other, electrons flow through the external circuit and fill the holes on the other side of the

base. This process is shown in Figure 7:

Figure 7: Short circuit flow of electrons and holes at a p-n junction: (a) electron-hole pair is created by photon absorption; (b) electron flows to the circuit and hole crosses the junction; (c) electron passes through the external load and recombines with

a hole and completes the circuit.


22

2.5. Organic Solar Cells

Flexibility and lightweight, low cost materials and manufacturing, and simple fabrication

process of organic solar cells have attracted researchers’ attention to organic photovoltaics

since the first OLED was introduced in 1986 [35]. Organic solar cells use organic semiconductors

to absorb the light and generate electricity.

2.5.1. Organic Semiconductors

Organic semiconductors are carbon-based materials which show semiconductor

properties. Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular

Orbital (LUMO) are representatives of the valence band and conduction band in inorganic

semiconductors respectively; HOMO is filled with electrons and LUMO is electron-free [35].

Diagram of the HOMO and LUMO is illustrated in Figure 8.

Figure 8: Diagram of HOMO and LUMO in ground state: Electrons in HOMO are illustrated by red circles.

Organic semiconductors have low mobility in comparison with inorganic ones, e. g.

electron mobility in commonly used P3HT:PCBM blend in organic semiconductors is in order of

10−7 − 10−8 [𝑚2

𝑉𝑠] while silicon has a much higher electron mobility of 0.1 [

𝑚2

𝑉𝑠]. [36][37]


23

2.5.2. Principles of Organic Solar Cells Operation

In organic semiconductors, an electron is excited from the HOMO to the LUMO upon

the absorption of a photon; and an exciton is generated. Then the bound electron-hole pair has

to be separated to generate free charges. This is done by aligning band levels of two different

organic materials [7], i. e. donor and acceptor materials. When an exciton is generated in a

donor material, in a correctly aligned band level materials, electron and hole diffused and

dissociated; electron transfers to acceptor material. In this step, electron-hole pair forms a

charge carrier and this carrier has to transfer to electrodes; extracted from active layer and

generate electricity. These four steps are illustrated in Figure 9.

Figure 9: Four steps of an organic solar cell operation: (a) photon is absorbed and exciton is generated; (b) exciton diffused to donor/acceptor interface; (c) exciton dissociated and charges carrier is generated; and (d) carrier transported and then

extracted at the electrodes.


24

2.5.3. Organic Solar Cells Structure

The simplest structure of organic solar cells with around 1% efficiency is the bilayer solar

cell which was first demonstrated in 1986 by Tang [38]. This architecture consists of a

donor/acceptor interface as active layer sandwiched between a transparent anode and a metal

cathode with different work functions. Two inter-layers, electron collection layer and hole

collection layer, are used to modify electrodes work function. Structure of a bilayer organic

solar cell is sketched in Figure 10:

Figure 10: Structure of a bilayer organic solar cell

Upon the absorption of light, active layer one electrons are moved from HOMO to

LUMO and excitons are formed. These excitons diffuse and reach at the planar donor/acceptor

interface and separated; electrons and holes travel in the acceptor and donor respectively.

2.5.3.1. Active Layer Developments

Exciton diffusion lengths in organic semiconductors are a few tens of nanometers which

are very short compared to the inorganic counterpart [39]. It means that excitons generated at a

distance more than this length from donor/acceptor interface, will recombine before reaching

the interface and only excitons generated within this distance will reach the electrodes and

thus contribute to the photocurrent generation. This is a significant drawback of bilayer active

layer and it limits the thickness of donor and acceptor layers. On the other hand, this limitation

reduces light absorption in active layers.


25

Bulk Heterojunction Solar Cells, which are an important improvement in organic solar

cells operation, were introduced in 1995 [40]. Operation principle of BHJ OSCs is the same as

bilayer. The only difference is that the active layer is a blend mixture of donor and acceptor

materials. This increases the interfacial area between donor and acceptor and thus, excitons

reach the interface in their life-time; almost all generated excitons are dissociated [41].

Excitons are generated upon light absorption. They diffuse to the active layer and

dissociate to a polaron pair, and then separated electrons and holes transfer to the electrodes.

Schematic of bulk heterojunction organic solar cell active layer is sketched in Figure 11.

Figure 11: Active layer in bulk heterojunction organic solar cells: 1) Exciton is generated upon light absorption; 2) Exciton recombine or diffuse at donor/acceptor interface and separated to electron and hole, then they travel to their corresponding

electrodes; 3) Electron and hole might recombine with trapped charges.


26

2.5.3.2. Electron Donor and Electron Acceptor Materials

As mentioned above, an organic solar cell consists of an active layer sandwiched

between two electrodes. This active layer typically is a blend mix of conjugated polymer donor

and fullerene derivative acceptor. Donor and acceptor material properties have a strong effect

on power conversion efficiency of organic solar cells. Absorption band, molecular energy levels

(HOMO and LUMO), charge carrier mobility, chemical stability, and solubility in different solvers

are main features that should be considered in molecular design of organic photovoltaic

materials [8].

Typically, conjugated polymers as electron donors and fullerene derivatives as electron

acceptors are key materials in organic photovoltaic devices. Various materials as donor and

acceptor have been tested and developed in the past decades. Poly (phenylene vinylene) (PPV),

Polythiophenes (PTs), MEH-PPV and MDMO-PPV are some kinds of organic materials which are

used as electron donor in organic photovoltaic devices [37] [42] [43]. For electron acceptor

materials, fullerene C60 and its derivatives exhibit good electron mobility [44]. To improve its

solubility, Phenyl-C61-butyric acid methyl ester (PC60BM) is used in organic photovoltaic

devices. Molecular structures of these materials are shown in Figure 12.

Figure 12: Molecular structures of some organic electron donor and acceptor materials [35]


27

The most commonly used donor material is Poly (3-hexylthiophene) (P3HT). It has a

bandgap of ~ 1.9 eV; lower than most organic materials of ~ 2 eV which increases absorption

efficiency. In addition, PCBM energy bandgap matches well with P3HT, and they match with

commonly used electrodes work function (such as ITO and Al) as well. [11]

P3HT:PCBM blend is the most commonly used active layer in bulk heterojunction

organic solar cells [45]. It exhibits proper absorption range and charge mobility; and by thermal

annealing, ratio optimization, and using additives, power conversion efficiency has reached

over 5% [46]. The absorption band of this blend is from 380 to 670 nm, and photons with energy

between 2 and 3.3 eV can be absorbed by the active layer [35].

The weight ratio between P3HT and PCBM has a strong effect on PCE in BHJ solar cells.

The optimum P3HT:PCBM ratio of 1:0.8 to 1:1 has been reported. Furthermore, controlling

other factors, such as deposition parameters, annealing temperature and time, and active layer

thickness, varies the PCE in organic solar cells. [47]

2.5.4. Organic Solar Cell Characterization

Organic solar cells are characterized under 1000 W/m2 Air Mass 1.5 solar spectrum [48].

Air mass (AM) represents the light power drop as it travels through the atmosphere and hits the

earth and it is defined as:

𝐴𝑀 =1

cos 𝜃 (2-7)

where 𝜃 is the angle from normal incidence as shown in Figure 13. Regarding to equation (2-7),

AM 1.5 occurs when 𝜃 = 48.2.

Figure 13: Air mass (AM) represents the light power drop as it travels through the atmosphere and hits the earth.


28

Organic solar cells generally are modeled by a current source parallel with a diode [49].

Dark current is a small amount of current flow in the absence of light. When a solar cell is

exposed by light, photovoltaic effect generates a current. In addition, there are two modeled

resistances regarding to materials which are used in solar cell and current leakage through the

cell [49]. Equivalent electrical circuit for a solar cell is illustrated in Figure 14.

Figure 14: Equivalent electrical circuit for a solar cell

I-V curve of this ideal solar cell is shown in Figure 15 with no light, and under light

illumination:

Figure 15: I-V curve of an ideal solar cell. Red line and blue line represent the I-V curve in presence of light illumination and without light illumination respectively.


29

Short circuit current, open circuit voltage and fill factor are key parameters which are

used to characterize solar cells:

Short circuit current (JSC) is defined as the current without external voltage. Small bandgap, high

absorption coefficient, and high charge mobility can improve JSC [35]. Open circuit voltage (VOC) is

defined as voltage when the current is 0 and it mainly depends on the band alignment of the

electron-donor composites. Fill factor (FF) is defined as below:

𝐹𝐹 =𝐽𝑚𝑝𝑝𝑉𝑚𝑝𝑝

𝐽𝑆𝐶𝑉𝑂𝐶 (2-8)

where Jmpp and Vmpp are current and voltage at maximum output power respectively. Power

conversion efficiency (PCE) determines the solar cell efficiency and it is calculated by:

𝑃𝐶𝐸 =𝑉𝑂𝐶𝐽𝑆𝐶𝐹𝐹

𝑃𝑖𝑛 (2-9)

where Pin is the input power. These parameters are shown in Figure 16 in respect to I-V curve:

Figure 16: Key parameters of a solar cell; Fill factor is defined as area (A) divided by area (B). Short circuit current and open circuit voltage, maximum power, and current and voltage at the point of maximum power are shown as Imp and Vmp

respectively. [50]


30

2.5.5. Prospects and Challenges

OSCs have lower PCE than conventional solar cells due to organic semiconductor

materials properties, but their flexibility and lightweight, low cost materials and manufacturing,

and simple fabrication process such as roll-to-roll method, have attracted scientists and

researchers’ attention in organic solar cells.

Low electron and hole mobility in organic materials in comparison with silicon is one of

their major disadvantages [36, 51, 52]. In addition, although the absorption coefficient in organic

materials is rather high, short exciton diffusion length makes it essential to keep active layer in

order to a few hundred nanometers, which causes decreasing in light absorption. These

drawbacks and some other losses such as junction and contact losses, and recombination

losses, have to be overcome to increase organic solar cell efficiency.


31

3. Simulation

3.1. Introduction

Efforts and experiments have been devoted on organic solar cells to improve their

optical and electrical properties, and thus their efficiency, in recent years. Modeling and

simulation are very effective and valuable tools to study and investigate devices which might be

expensive and time consuming to fabricate and characterization. By achieving a reliable model,

one can study how changing architecture in solar cells would affect their properties.

In this project, a model for light absorption in planar organic solar cell by Comsol

Multiphysics 5 was enhanced and its results were compared with FDTDM model as well as

references. Then by implementation of structured electrodes in geometry, desired

architectures were examined.

In this chapter, most common simulation methods are explained briefly. Comsol

Multiphysics is used in this project to simulate optical properties and light absorption in organic

solar cells. Theories and principles behind optics module in Comsol are detailed, and how

Comsol solves equations is described; and setups which are used in this thesis are explained at

the end.

3.2. Simulation Methods

Optical modeling is used to find the magnitude of electrical field in a solar cell as a

function of position and frequency. By finding the electrical field magnitude in every position,

one can calculate light intensity profile in the device, and then total absorbed power as a

function of frequency.

Three different methods are described in this section: Transfer Matrix method (TMM),

Finite-Difference Time-Domain method (FDTDM), and Finite Element Method (FEM).


32

Transfer Matrix

TMM is an analytical method to calculate electromagnetic waves distribution in one-

dimensional structures [53]. This method has been used to simulate light absorption in

multilayered planar solar cell structures and has shown reliable results [54] [55] [56].

Incident light in a planar multilayered solar cell is absorbed in different layers and

interacts in boundaries, results in reflection and transmission. Distribution of electromagnetic

waves is dependent on optical properties and parameters of materials and their thicknesses, as

well as incident wave intensity and frequency as illustrated in Figure 17: [57]

Figure 17: Incident wave in a multilayer device (left); Electromagnetic wave interactions at layers interface (right).

Two matrices are used to model wave propagation in the device: layer matrix which

models how each layer affects the propagating wave, and interface matrix which models the

reflection and transmission of the wave at interfaces. Numerical solutions for this method with

more details are in reference [58].

Finite-Difference Time-Domain

Finite-difference time-domain method (FDTDM) is a time domain method to model

computational electrodynamics using partial differential Maxwell’s equations [59]. In terms of

optical simulation, absorption and transmission are determined in the desired frequency range


33

by calculating E-field and H-field at any point. Major advantage of this method is the possibility

to study a wide range of frequencies with a single simulation run.

Finite Element Method

Finite element method is a numerical technique to calculate boundary problems for

partial differential equations. In this method, the entire domain is divided into small parts by

mesh generation, and equations are solved approximately for each element.

3.3. Comsol Multiphysics

Comsol Multiphysics is simulation software which uses finite element analysis to solve

and simulate different physics and engineering problems. The company was founded in 1986 in

Sweden and the first version of software was released in 1998. The product has expanded to a

wide range of modules such as structural mechanics, high and low frequency electromagnetics,

fluid flow, heat transfer, chemical reactions, MEMS, acoustics, and many other applications [60].

3.3.1. Wave Optics Module; Theories and Principles

Wave optics module is used in this thesis to calculate and simulate light absorption in

organic solar cells. To understand how Comsol solves Maxwell’s equations in domains and

boundaries, it is essential to have an overview on theories and principles behind this module.

The wave optics module solves electromagnetic waves problems at optical frequencies

[61]. Since electromagnetic waves behavior in frequency domain is favorable, The

Electromagnetic Waves, Frequency Domain User Interface branch is chosen in this project. This

branch solves time-harmonic wave equations.

Differential equation form of Maxwell’s equations which are used in finite element

method can be written as:

∇ × 𝑯 = 𝑱 +𝜕𝑫

𝜕𝑡 (3-1)

∇ × 𝑬 = −𝜕𝑩

𝜕𝑡 (3-2)


34

∇. 𝑫 = 𝜌 (3-3)

∇. 𝑩 = 0 (3-4)

where 𝑬 is electric field, 𝑫 is electric flux density, 𝑯 is magnetic field, 𝑩 is magnetic flux

density, 𝑱 is current density, and 𝜌 is electric charge density. Electric flux density (𝑫) and

magnetic flux density (𝑩) are related to the electric field (𝑬) and magnetic field (𝑯) respectively

as:

𝐃 = ε𝐄 (3-5)

𝐁 = 𝜇𝐇 (3-6)

where ε and 𝜇 are permittivity and permeability of the medium respectively. The electric and

magnetic energies are calculated as:

𝑊𝑒 = ∫ (∫ 𝑬. 𝑑𝑫𝐷

0)

𝑣𝑑𝑉 = ∫ (∫ 𝑬.

𝜕𝑫

𝜕𝑡𝑑𝑡

𝑇

0)

𝑣𝑑𝑉 (3-7)

𝑊𝑚 = ∫ (∫ 𝑯. 𝑑𝑩𝐵

0)

𝑣𝑑𝑉 = ∫ (∫ 𝑯.

𝜕𝑯

𝜕𝑡𝑑𝑡

𝑇

0)

𝑣𝑑𝑉 (3-8)

The time derivatives of (3-7) and (3-8) are the electric and magnetic power:

𝑃𝑒 = ∫ 𝑬.𝜕𝑫

𝜕𝑡𝑣𝑑𝑉 (3-9)

𝑃𝑚 = ∫ 𝑯.𝜕𝑩

𝜕𝑡𝑣𝑑𝑉 (3-10)

Boundary conditions at the interfaces between materials have to be satisfied to solve

the problem. These boundary conditions can be written as:

𝒏𝟐 × (𝑬1 − 𝑬2) = 0 (3-11)

𝒏𝟐. (𝑫1 − 𝑫2) = 𝜌𝑠 (3-12)

𝒏𝟐 × (𝑯1 − 𝑯2) = 𝑱𝑠 (3-13)

𝒏𝟐. (𝑩1 − 𝑩2) = 0 (3-14)


35

where 𝜌𝑠 is surface charge density and 𝑱𝑠 is surface current density. To understand Maxwell’s

equations solution in details, reference [25] is recommended.

3.4. Simulation Setup

As mentioned above, Wave Optics Module, Frequency Domain User Interface branch is

used in this project to simulate light absorption in organic solar cells which solves time-

harmonic electric field wave equation. Plane transverse electric wave which is used to simulate

light is given as:

∇ × (∇ × 𝑬) − 𝑘02𝜖𝑟𝑬 = 0 (3-15)

where 𝑘 is wave vector.

Since light-trapping nano-structures in this thesis have two dimensions, 2D space

dimension is chosen. Planar device which is used as reference consists of a sticking Titanium

layer of 3 nm, 80 nm Aluminum with a thin layer of Titanium (8 nm) as bottom electrode, 150

nm of P3HT:PCBM as active layer, and 200 nm PEDOT as top electrode (Figure 18).

Figure 18: Sketch of the planar device that is used as reference. Top layer is air which propagating light enters the device.

Complex refractive index is the parameter which is necessary to investigate and

simulate light propagation in different materials, and to determine reflection and transmission

at interfaces. These optical properties, as a function of wavelength, are extracted from [56] and

converted to table using a Matlab code written by Jost Adam (associate professor, MCI, SDU),

and then implemented in Comsol using interpolation function. Propagation constants for


36

materials at any wavelength are defined as variables. Index of refraction and extinction

coefficient for materials used in Comsol in visible light range are plotted in Figure 19.

Figure 19: Optical properties of materials which used in Comsol after [62]

Wave equation (3-15) is solved for all domains where

𝜖𝑟 = (𝑛 − 𝑖𝑘)2 (3-16)

is calculated by real part and imaginary part of refractive index shown in Figure 19.

Top boundary is defined as port 1, which called excitation port and electric wave enters

the device from. Port 2 is the bottom boundary which is defined as listener port. These ports

will be used later in S-parameters to calculate light absorption in the device.


37

Pitch distance is determined by the light-trapping structure pitch dimensions. The device

consists of an array of these structures stacked side by side. Boundaries at both sides of device

are chosen as Floquet periodicity; simply means “what goes out from one side, comes in from

the other side”.

Mapped mesh node is chosen to create structured quadrilateral mesh on boundaries.

Due to the Comsol condition that each boundary must be bounded by at least four boundary

segments, minimum element size is defined as 1 nm. In addition, maximum element size is

chosen 6 nm to obtain more accurate results. Finally, parametric sweep is defined in the

wavelength range of 395 nm to 705 nm with steps of 10 nm.

Default result of the simulation is electric field intensity in whole the domain. As

mentioned above, Comsol uses port 1 and 2 to calculate S-parameters. These parameters are

pre-defined in Comsol in terms of power flow as:

𝑆11 = √𝑃𝑜𝑤𝑒𝑟 𝑟𝑒𝑓𝑙𝑒𝑐𝑡𝑒𝑑 𝑓𝑟𝑜𝑚 𝑝𝑜𝑟𝑡 1

𝑃𝑜𝑤𝑒𝑟 𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑜𝑛 𝑝𝑜𝑟𝑡 1 (3-17)

𝑆21 = √𝑃𝑜𝑤𝑒𝑟 𝑑𝑒𝑙𝑖𝑣𝑒𝑟𝑒𝑑 𝑡𝑜 𝑝𝑜𝑟𝑡 2

𝑃𝑜𝑤𝑒𝑟 𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑜𝑛 𝑝𝑜𝑟𝑡 1 (3-18)

Wave transmission and absorption are calculated from (3-17) and (3-18) and plotted as

a function of wavelength later.

Results for planar device as reference and devices with light-trapping nano-structures of

different pitch and height will be discussed in Result and Discussion Part.


38

4. Experimental

4.1. Introduction

The main goal of this research is to investigate light-trapping nano-structured electrodes

effects on light absorption in organic solar cells active layer, and thereby their power

conversion efficiency. This was done by developing nano-structured electrodes by a

lithography-free pathway using wrinkled Polydimethylsiloxane (PDMS) stamps, which were

developed in plasma cleaner chamber under controlled conditions. These conditions are time,

stretch, and plasma generator power. By varying the conditions, sinusoidal structures with

different pitches and heights are obtained and characterized by Atomic force microscope

(AFM). Then structures were transferred to BK7 glass substrates, and electrodes were

deposited on them. A blend of Poly(3-hexylthiophene-2,5-diyl) (P3HT) and Phenyl-C61-butyric

acid methyl ester (PCBM) were spin-coated on prepared electrodes as active materials,

following by poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT) as top electrode.

Optical properties of nano-structures were investigated through optical microscopy; I-V curve

measurements under 1 sun illumination were characterized by solar simulator.

In this chapter, PDMS stamps fabrication method and theories are explained briefly.

Then fabrication methods and equipment consisting of sample preparation, nano-imprinting,

metal deposition, and organic material spin-coating are discussed. Finally, atomic force

microscopy, optical microscopy, and solar simulator which were used for characterization and

measurements in this project will be described.

4.2. Stamp Fabrication for Nano-imprinting

As mentioned before, wrinkled PDMS was used as stamp for a nano-imprinting in this

project. Polydimethylsiloxane (PDMS) is a chemical-stabled silicon-based organic polymer which

is used mostly in microfluidic devices. By mixing liquid PDMS with a cross-linking agent

following by heating, elastomeric PDMS is obtained which can be used as stamp for soft

lithography (Figure 20).


39

Figure 20: Elastic PDMS substrate

Plasma exposure onto PDMS substrate was introduced to the current project researcher

by André Luis Fernandez Cauduro (PHD student, MCI, SDU). To fabricate PDMS substrate for

later oxygen-plasma treatment use, a silicon wafer of 100 mm radius is chosen. The wafer is

cleaned by acetone and isopropanol, and heated on hotplate for 10 minutes at 150 °C, and the

wafer is transferred to a container immediately. One droplet of Trichlorosilane 97% is applied

into the container, closes the lid, and is left for one hour. Trichlorosilane 97% is used as anti-

sticking layer on silicon wafer; it evaporates inside the container and sits on silicon wafer

surface, making a thin film on top.

13.5 gr of silicone elastomer is mixed with 1.5 gr curing agent properly and is placed in

vacuum chamber for one hour to gas out. When all the bubbles disappear, the mixture is

poured gently on the silicon wafer, which was prepared before with anti-sticking layer,

restricted by a ring and let in the vacuum chamber for 30 minutes to gas out again. Last step is

to bake the wafer with PDMS mixture on top on the hotplate for two hours at 75°C.

Cured PDMS is obtained now in size of silicon wafer. This PDMS is cut in 20×24 mm

substrates which are used as stamps in this project. For cleaning process, PDMS substrates are

rinsed in Toluene for one hour, following by one hour baking on hotplate at 100°C.


40

In this thesis, wrinkled PDMS with sinusoidal grating structures are used as stamps to

transfer light-trapping structures to the glass substrate. Wrinkles on PDMS were observed first

time in 1982 by aluminum evaporation onto PDMS [62], but metal evaporation on PDMS to

obtain structures was done in 1998 [63]. For this purpose, a layer of titanium following by gold

was deposited onto the PDMS which was expanded thermally. The sample then brought back

to the room temperature and PDMS shrank, caused wrinkles on metal layer. This wrinkled

phenomena is created due to the fact that bottom layer, that is elastic, shrinks and exposes

compressive strains to the top layer, which is a hard film. In response to this strain, hard layer

deforms perpendicular to the stress direction and periodic patterned wrinkles are formed as a

result of in-plane compressed thin film [64].

Another way to create hard film on top of the elastic PDMS substrate is to oxidation of

PDMS using oxygen plasma exposure [65]. Using a clamp to stretch PDMS in specific lengths,

following by oxidization, and then releasing the exposed PDMS, forms periodic wrinkled

patterns perpendicular to the stretch direction [66] [67]. Process of wrinkles formation on PDMS

upon oxygen-plasma exposure is illustrated in Figure 21. Figure 22 is AFM images from wrinkled

PDMS used in this project.

Under well-controlled conditions such as controlled stretch and applied dose (plasma

power multiplied by oxygen-plasma exposure time), wrinkled patterns are regular and

predictable [64]. Cracks are observed during relaxation process are observed. These cracks are

parallel to the stretched direction (perpendicular to wrinkles) and can be decreased by

releasing stretched exposed PDMS as quick as possible [68]. Since analytical description of

wrinkled pattern formation only is described by nonlinear theories [64] and due to complexity, it

is out of this master thesis topic scope and is not mentioned in this report.


41

Figure 21: Wrinkles formation process on PDMS: (a) clean non-defected piece of PDMS substrate is chosen; (b) PDMS substrate is stretched; (c) stretched PDMS is exposed upon oxygen-plasma; (d) a thin hard layer is formed on top of PDMS;

(e) by releasing the stretch, periodic wrinkled patterns are formed on the top layer due to in-plane compressive strain.

Figure 22: AFM images (5×5 µm) of PDMS 28 which is fabricated and used in this project.

Well-ordered periodic wrinkled patterned structures with pitch distance in the visible

light wavelength order can be used as soft nano-imprinting lithography stamps to fabricate

light-trapping electrodes with diffraction gratings [65] [69]. The main advantages of this method


42

are simple fabrication methods and easy lithography process. Parameters and materials which

have to be considered, as well as lithography process, are detailed later in this chapter.

PDMS substrates which were fabricated and cut before are rinsed by distilled water and

dried by Nitrogen blow just before oxygen-plasma treatment. They are stretched by a clamp

with a fixed arm in one side and a mobile arm in the other side mounted on a base with ruler to

recognize the expansion length. Designed clamp is illustrated in Figure 23. First, PDMS substrate

is tightened at both sides and stretched to obtain desired expansion. Then mobile arm is

tightened at the base to keep the expansion during the process. Oxygen-plasma treatment is

done in PDC-002 Harrick Plasma Cleaner. This process recipe is provided in appendix 3 in

details.

Figure 23: Illustration of (left) side view and (right) top view of the clamp which used in this project to stretch PDMS substrates.

As mentioned, pitch distance and height of wrinkled patterns vary by stretch and plasma

dose. All the stamps were fabricated in same pressure of 150 mTorr with maximum plasma

power of 29.7 W. Structures dimensions as function of stretch and dose are shown in result and

discussion part.


43

4.3. Fabrication Methods and Equipment

4.3.1. Sample Preparation

BK-7 glass wafer with 700 µm thickness was chosen as substrate in this project. This

glass wafer has a very smooth surface which is essential to form a uniform layer of AZ 5214E

photoresist onto it later.

The glass wafers were diced by DISCO DAD-2H/5 dicing machine into 15 mm × 20 mm

rectangles which were used as solar cell substrates. Cell substrates were transferred to the

Cleanroom, rinsed in acetone in sonic bath for 10 minutes; following by 10 minutes rinsed in

isopropanol and blown dried. Since next step is photoresist spin-coating, substrates were

transferred to HMDS oven with 140 °C to evaporate remained residuals from substrates and

increase adhesion.

4.3.2. Nano-Imprinting

Nano-imprinting has enhanced and optimized by Arkadiusz Jaroslaw Goszczak (PHD

student, MCI, SDU), and results from this research were discussed with him to improve

parameters and reach more optimization. Nano-imprinting is done to transfer light-trapping

structures from PDMS stamps to the substrates. Electrodes will be deposited on these

structures later by metal deposition methods. AZ 5214E image reversal photoresist was spin-

coated on the substrates. Film thickness depends on photoresist viscosity and spin speed.

Regarding to the program was used to spin-coat photoresist on substrate (Table 1); a film

thickness of 1.4 µm is obtained.

Step Time (s) Rotation (rpm/s) Ramping (rpm/s2)

1 2 0 100

2 5 500 5000

3 30 4000 10000

Table 1: Spin-coating parameters to obtain 1.4 µm thickness of photoresist.


44

Nano-imprinting was done immediately after photoresist spin-coating. PDMS stamp

which was laid on a column weight of 500 gr, stayed on top of the substrate on the hot plate at

140°C for five minutes; solvent evaporated, photoresist hardened and PDMS stamp structures

were transferred and remained onto photoresist. As PDMS surface is extremely hydrophobic,

no photoresist remained on it. Surface characterization confirmed that all the structures

transferred completely from stamp to photoresist in respect to dimensions and defects. Nano-

imprinting process is illustrated in Figure 24.

Some details have to be considered to have a proper uniform structured photoresist. It

is recommended to check stamp-substrate contact visually before placing them on the hotplate

to be sure whole the stamp is in touch with substrate and there is no bubble between them. As

it was observed, stamp and substrate stick together after five minutes on hotplate. They should

separate accurately to avoid any friction between them that may cause to destroy structures in

some spots on the photoresist.


45

Figure 24: Illustration of imprinting process: (a) droplets of photoresist is applied on the glass substrate; (b) spinning process leaves a determined thickness of photoresist film; (c) PDMS stamp with a column weight of 500 gr is laid on the substrate; (d) the force due to column weight transfers structures onto photoresist; (e) solvent is evaporated and photoresist is hardened;

(f) PDMS stamp and substrate are detached and structures are remained on hardened photoresist.


46

4.3.3. Metal Deposition

Deposition of different layers of dielectrics, metals, insulators, and semiconductors

above substrates plays a key role in manufacturing and research in micro- and nanotechnology.

Issues such as film quality and uniformity have to be considered to choose the best method for

thin film deposition. Quality is referred to contamination and defect which determines

mechanical and electrical properties of materials. In addition, uniform thickness is essential

across the substrate, as well as filling spaces within topographical structures.

There are two main categories in thin film deposition: Chemical Vapor Deposition (CVD)

and Physical Vapor Deposition (PVD). In CVD, gases are introduced into the chamber and

desired film is formed due to chemical reactions which take place on the substrate surface. PVD

mainly uses physical process to deposit thin film on the substrate. In this method, knocked off

atoms from the target travel through the vacuum and condense on the surface; form the thin

film.

Aluminum and Titanium are used as bottom electrode in this research. A Titanium layer

of 3 nm following by 80 nm Aluminum were electron beam evaporated on a glass substrate

with structured AZ 5214E on top (for reference device, they were deposited on bare glass

substrate), and 8 nm of Titanium was DC-sputtered on top of the Aluminum. Metal deposition

was done by Cryofox Explorer 600LT in the Cleanroom in University of Southern Denmark.

Electron beam evaporation, which will be referred as e-beam in this project, is a PVD

method to form a thin layer on the substrate. In this method, high energy electron beam is

focused on the material in a crucible and melts it. Atoms evaporated from the molten material

travel through the vacuum chamber, arrive the substrate, condense on it and form a thin film

on it. To have a uniform thickness of material on the substrate, high vacuum condition (10-5

Torr) is essential for whole the process. Deposition rate for Titanium and Aluminum are chosen

0.5 Å/s and 3 Å/s respectively. Deposition rate is regulated by beam intensity. A quartz

microbalance is monitored film thickness during the deposition, and process is stopped when

desired thickness is reached. A typical e-beam deposition system is illustrated in Figure 25.


47

Figure 25: Illustration of an e-beam deposition system: high energy electron beam is focused on the material in a crucible; atoms evaporated from the crucible travel through the vacuum chamber, reach the substrate and form a thin film on it.

8 nm of Titanium was deposited on Aluminum with the rate of 0.5 Å/s to form TiOx

which works well as hole blocker using dc-sputtering. DC-sputter is a PVD method to deposit a

thin film on the substrate. In this method, plasma is created by applying DC voltage across two

electrodes. The electrode with negative voltage served as the source of material and named

target. Other electrode, which substrates sit upon, is grounded and becomes the anode.

The positive ions in plasma are accelerated to the target and strike it. As a result, atoms

knocked off from the target and travel through the plasma strike the substrate surface,

condense on it, and form a thin film. This is illustrated in Figure 26.


48

Figure 26: Illustration of a DC-sputter system; voltage difference between anode and cathode creates plasma. Energetic ions are accelerated and strike the target and sputter atoms from it. These atoms strike wafer and condense on it and film forms

on the substrate surface.

Solar cell substrates which were diced before in 15×20 mm rectangles and then

imprinted as explained above were placed on a shadow mask consisting of six electrodes for

each device. Then they were fixed by a clamp onto wafer holder in Cryofox explorer 600LT and

electrodes were deposited on substrates.

4.3.4. Organic Material Deposition

Bulk Heterojunction organic solar cells use a blend of donor/acceptor material as active

layer. In present research, poly(3-hexylthiophene-2,5-diyl) (P3HT) as electron donor mixed with

Phenyl-C60-butyric acid methyl ester (PCBM) as electron acceptor with 1:1 ratio is the active

layer. This solution can be diluted in chlorobenzene and comes in liquid form; easily spin-coated

on the cell substrate.

Organic material deposition was held in nitrogen-filled glove box. As mentioned above,

solar cell substrates are 15×20 mm rectangles with six electrodes deposited on them. A blue

tape was used to cover a small portion of electrodes before organic materials deposition. This

was done to have access on electrodes later for characterization. 50 ml of P3HT:PCBM (1:1) is

applied on the substrate and spin-coater is programmed to spin 4000 rpm for 45 seconds, and


49

baked on hotplate for five minutes at 140°C. It results in approximately 150 nm active layer

thickness on electrodes.

A mixture of high work function CPP PEDOT and PSS in a 1:1 ratio is served as anode. As

same as active layer, a blue tape is used to cover electrodes and overlaps small part of active

layer. 50 ml of PEDOT:PSS is applied on the substrate and spun 4000 rpm for 45 seconds, and

baked on hotplate for 150 seconds at 140°C. It results in approximately 200 nm PEDOT:PSS on

active layer.

A layer of silver paste was applied on PEDOT:PSS layer and electrodes to improve

electrical contact for I-V curve characterization in solar simulator. Side view and top view of

fabricated planar solar cell which is used as reference is illustrated in Figure 27.

Figure 27: Illustration of a planar device: (a) is a top view of a fabricated substrate (each substrate consists of six cells); (b) cross-sectional view of the selected area of (a).


50

4.4. Measurements and Characterization

4.4.1. Surface Characterization; Atomic Force Microscope

Atomic force microscopy is designed to investigate surface topographical profile such as

surface roughness and structure height in the scale of nanometer. The idea was first

demonstrated in 1986 [71]. In this method, the force between probe and surface is measured by

a cantilever lateral and vertical deflection. These deflections are due to forces between surface

and the tip of cantilever. Deflections are transferred to a detector by a laser beam which is

focused on top of the cantilever. Any deflection in cantilever causes a slight change in reflected

laser beam; detector tracks these changes and forms an image. This is illustrated in Figure 28.

Figure 28: Atomic force microscopy principle: Atomic force between tip and surface cause deflections in the cantilever. Deflections transfer to the detector by reflected laser beam and image is formed.

Fabricated PDMS stamps in this research were characterized by Veeco Dimension 3100

Atomic Force Microscope. Three square areas of 5×5 µm were chosen in left, middle, and right

side of each 20×24 mm stamp and 20 height and pitch measurements were done in each area.

Finally, average pitch and height and their standard deviations were calculated.


51

4.4.2. Absorption Measurement; Optical Microscope

Main goal of this thesis is to investigate light-trapping structured electrodes and their

effects on light absorption in organic solar cells. Nano-structures in the visible light wavelengths

order diffracts and reflects back the light several times before it leaves the surface. This

increases light absorption in the active layer.

Reflectivity spectra were measured by Nikon Eclipse ME600D microscope in laser lab in

University of Southern Denmark. A white light source and MAYA 2000 Pro, Oceans Optics fiber

coupled spectrometer were used to transfer and save the spectra to the computer.

Measurements were done using 50× objective. This lens is responsible to condense incident

light onto the surface and then to collect reflected light.

Reflection measurement is a comparison between reflected light from a device with

non-structured electrodes and devices with light-trapping structured electrodes with different

dimensions (in pitch and height). Therefore, devices with planar electrode were used as

reference and cells with structured electrodes were compared with references. Spectra were

normalized in order to have a proper reference to compare with.


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5. Results and Discussions

5.1. PDMS Stamps

As mentioned before, wrinkled PDMS were used as stamps for nano-imprinting to

fabricate light-trapping structures on organic solar cells electrodes. Stretch, plasma dose, and

chamber pressure are parameters which influence wrinkles dimensions. Chamber pressure was

held constant for all fabricated stamps on 150 mTorr, and plasma power of 29.7 W was selected

in this research. Hence, variables which had effects on wrinkled sinusoidal patterns dimensions

(pitch and height) are plasma exposure time and stretch in respect to original length of PDMS

substrates.

PDMS stamps were stretched to 10%, 20%, and 30% expansion regarding to their

original length and exposed by oxygen-plasma from 5 minutes to 30 minutes with 5 minutes

steps. Then they were characterized by AFM in three areas of 5×5 µm in middle, right, and left

side; 60 spots of pitches and heights were measured (Figure 29) and average pitch and height

with corresponding standard deviations were calculated and summarized in Table 2.

Figure 29: (left) AFM image (5×5 µm) of PDMS 28 and (right) height profile of wrinkled pattern for the selected line 1.


53

PDMS Stamp

Stretch (%) Plasma

Exposure time (min)

Pitch (nm) Height (nm)

Average Standard Deviation


27 10 10 410.64 60.13 38.75 2.83

28 10 15 414.18 19.52 41.4 5.59

29 10 5 308.865 22.12 23.85 2.19

31 10 25 605.34 19.68 100.3 3.17

37 20 25 685.875 19.46 163.6 4.94

43 20 5 158.415 21.53 16.8 4.96

45 20 20 426.57 22.58 118.9 5.43

46 30 5 292.05 19.41 62 5.08

47 30 15 402.675 20.75 138.9 11.14

48 30 20 372.585 13.96 117.5 4.98

53 10 20 404.445 28.1 77.4 5.92

55 20 15 397.365 23.85 101.2 5.57

58 30 25 439.845 20.26 143.9 5.25

59 10 30 584.985 23.94 81.8 2.59

60 20 30 579.675 25.2 170 4.89

61 30 30 567.285 19.16 195.4 5.03

62 20 10 323.91 23.06 80.7 7.21

63 30 10 391.17 16.49 112.4 4.7

Table 2: Average pitch and height of PDMS stamps and their fabrication parameters.

In order to evaluate how parameters affect wrinkles dimensions, average pitch and

height are drawn as a function of plasma exposure time for each stretch separately; i.e. 10%,

20%, and 30%.

Average pitch and height for wrinkled PDMS for the stretch of 10% is shown in Table 3.

Then they are drawn in Figure 30 as a function of exposure time:

PDMS Stamp Plasma

Exposure Time (min)




29 5 308.865 22.12 23.85 2.19

27 10 410.64 60.13 38.75 2.83

28 15 414.18 19.52 41.4 5.59

53 20 404.445 28.1 77.4 5.92

31 25 605.34 19.68 100.3 3.17

59 30 584.985 23.94 81.8 2.59

Table 3: Wrinkles average pitch and height with corresponding standard deviation for the stretch of 10%


54

Figure 30: Average Pitch (left) and average height (right) for 10% stretch PDMS stamps as a function of plasma exposure time

Table 4 provides average pitches and heights for 20% stretch of PDMS substrates;

plotted in Figure 31.

PDMS Stamp Plasma

Exposure Time (min)




43 5 158.415 21.53 16.8 4.96

62 10 323.91 23.06 80.7 7.21

55 15 397.365 23.85 101.2 5.57

45 20 426.57 22.58 118.9 5.43

37 25 685.875 19.46 163.6 4.94

60 30 579.675 25.2 170 4.89

Table 4: Wrinkles average pitch and height with corresponding standard deviation for the stretch of 20%



55

Finally, results of 30% stretch of PDMS substrates are shown in Table 5 and plotted in

Figure 32 as a function of plasma exposure time.

PDMS Stamp Plasma

Exposure Time (min)




46 5 292.05 19.41 62 5.08

63 10 391.17 16.49 112.4 4.7

47 15 402.675 20.75 138.9 11.14

48 20 372.585 13.96 117.5 4.98

58 25 439.845 20.26 143.9 5.25

61 30 567.285 19.16 195.4 5.03

Table 5: Table 3: Wrinkles average pitch and height with corresponding standard deviation for the stretch of 20%


As can be seen, pitch and height follow the same trend for a constant stretch.

Furthermore, another trend which can be observed is that pitch and height increase linearly by

time (Plasma dose). Since wrinkles dimension depends on the hard film thickness formed on

top of the elastic PDMS [65], it is predicted that these dimensions show a linear behavior by

increasing the time.

Clamp which was used to stretch PDMS substrates and its screws were made by plastic.

It caused cracks and deformation in it after several times uses and affected its strength to keep

the constant stretch in a whole process (especially for long plasma exposure time, i.e. more

than 20 minutes). PDMS substrates were fixed onto the clamp using four screws on corners. It

means that substrates were tightened in their four corners stronger than sides, and stretch was


56

not equally distributed in whole the substrates surface. Clamp was marked by researcher to

measure the substrate expansion length, and then substrates were stretched and fixed on the

marked clamp. This caused observational error in experiments.

5.2. Simulation Results

As it was discussed in chapter 3, COMSOL Multiphysics 5.0 is used to simulate light

absorption in organic solar cells in this research. There have been many published researches to

simulate light behavior in solar cells with different methods. F. Burkhard and his team used

Transfer Matrix method in their research for a planar structure, and their model shows a good

convergence with experiments [56]. Their structure consists of 110 nm of ITO as top electrode,

35 nm of PEDOT as hole transport layer, active layer of 220 nm which is the blend of

P3HT:PCBM, 7 nm Calcium, and 200 nm Aluminum; as sketched in Figure 33.

Figure 33: Sketch of the structure after [56] to evaluate COMSOL model validity.

This research was chosen as a reference to evaluate COMSOL model reliability.

Comparing COMSOL simulation results and FDTDM results which was done by Mina Mirsafaei

(PHD student, MCI, University of Southern Denmark) for the same structure as [56] confirmed

the COMSOL model validity. Light absorption as a function of light wavelength is plotted in

Figure 34.


57

However results are slightly different, FDTD method and COMSOL show a very similar

behavior in both trend and values. This is because in these two simulations, researchers applied

exactly same setups and parameters which were not available for TM method in details.

Figure 34: Light absorption as a function of light wavelength for the structure shown in Figure 33. Green line is the reference [56] which used TM method. Red line is FDTD simulation result and blue line is COMSOL simulation result which is used in

current research.

Organic solar cells which are simulated in this project consist of 200 nm PEDOT as top

electrode, 150 nm blend of P3HT:PCBM as active layer, and sinusoidal light-trapping structured

Titanium and Aluminum of 8 nm and 80 nm respectively as bottom electrode. To avoid

unnecessary complexity in simulation setup, Titanium layer is excluded and Aluminum thickness

is assumed 100 nm. This causes a very small difference in optical properties of the structure

which is negligible. Geometry of planar structure which is used as reference in COMSOL, and

devices with structured electrodes are illustrated in Figure 35.


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Figure 35: (a) planar structure as reference; and (b) Structured electrode device. Pitch and height in (b) vary to enhance light absorption in devices.

Pitch and height in Figure 35 are light-trapping structure dimensions; and vary in

simulation to enhance light absorption in the device.

Listener port (port 2) in COMSOL setup is defined at the interface between P3HT:PCBM

and Titanium. This setup shows the amount of light which is absorbed in PEDOT and active

layer. Light absorption for planar structure in TE mode is shown in Figure 36:

Figure 36: Light absorption in PEDOT and active layer for planar device which is shown in figure 3 (a) as reference in TE mode.


59

In Figure 36, light absorption peak is witnessed at the wavelength of 500 [nm] which is

important for P3HT based organic solar cells [72]. Total light absorption in TE mode is plotted in

Figure 37:

Figure 37: Total light absorption for planar device which is shown in figure 3 (a) as reference in TE mode.

To examine the effect of nano-structured electrodes height in light absorption in OSCs,

pitch distance was kept constant at 500 [nm] and structures height of 40 [nm],

60 [nm], 80 [nm], 100 [nm], and 120 [nm] are simulated and plotted in Figure 38:

Figure 38: Total light absorption in devices with light-trapping structured electrodes of different height with the same pitch of 500 [nm] in compared with planar structure as reference in TE mode.


60

As can be seen in Figure 38, nano-structures change the total light absorption profile.

These changes can be explained in four wavelength ranges. Planar device has a peak in the

wavelength of 350 [nm]. This peak shifts to shorter wavelength of around 300 [nm]. Between

wavelengths of 370 [nm] and 450 [nm], slight gain for total light absorption is observed by

increasing nano-structures height. From 450 [nm] to 600 [nm] wavelength, total light

absorption has a very similar behavior in both trends and values for different structures with a

small shift in peak points to the shorter wavelengths. Dramatic elevation in light absorption is

observed in the wavelength range of 600 [nm] to 700 [nm] by increasing the nanostructures

height. Total light absorption profile is very similar in TM mode except a peak which appears in

the wavelength of around 630 [nm] (Figure 39).

Figure 39: Total light absorption in devices with light-trapping structured electrodes of different height with the same pitch of 500 [nm] in compared with planar structure as reference in TM mode.

Figure 40 shows enhancement in light absorption of different heights in active layer in

TE mode. Light absorption increases by increasing heights of structures.


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Figure 40: Light absorption in active layer for devices with light-trapping structured electrodes of different height with the same pitch of 500 [nm] in compared with planar structure as reference in TE mode.

In Figure 41, nano-structured electrode pitch effect on total light absorption in organic

solar cells is studied. Pitch distances of 300 [nm], 400 [nm], 500 [nm], 600 [nm], and 700 [nm]

for the constant height of 100 [nm] were simulated; light absorption were plotted, and

compared with reference device:

Figure 41: Total light absorption in devices with light-trapping structured electrodes of different pitch with the same height of 50 [nm] in compared with planar structure as reference.


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Light absorption profiles which are shown in Figure 41 for structured electrodes are very

similar from 300 [nm] to 600 [nm] light wavelengths. Peak shifts are corresponded to structures

height which was observed before in Figure 38. Light absorption enhancement takes place in

the wavelength range of 5700 [nm] to 700 [nm]. Light-trapping structured electrode with 500

[nm] and 700 [nm] pitches show the highest enhancement in light absorption in this range.

5.3. Light Absorption in Organic Solar Cells

As mentioned in experimental chapter, light absorption in organic solar cells was

measured by reflection measurement in respect to a planar device as a reference through the

optical microscope. In the planar sample which is labeled as F0, no imprinting took place and

electrodes were deposited directly on the glass substrate, following by spin-coating the active

materials. Then reflection spectrum of this sample was chosen as the reference. It means that

reference reflection spectrum turns to 100% for all the wavelengths. Absorption profile for

other samples is the graph that shows light absorption enhancement for different wavelengths

in respect to the planar sample in percentage. Table 6 is the list of samples and their

configuration which are used to investigate optical properties in this research:

Sample Dimensions (nm)

Pitch Height

F0 - -

F1 426 119

F11 404 78

F13 584 82

F15 397 101

F17 323 80

F19 372 117

F24 354 70

Table 6: Samples which are used to investigate light absorption enhancement in this research; and their configuration.

Each sample consists of six cells. Reflection measured on a non-defected spot (defects in

samples are explained later in this report) on each cell in the substrate, and the reflection

spectrum as a function of wavelength extracted in a .txt file; their average calculated and used

as the final reflection spectrum for the sample.


63

To evaluate how different pitches and heights influence optical properties in cells,

reflection spectra is studied into two main categories: cells with same pitch and different

heights, and cells with same height and different pitches. For each category, reflection spectra

are plotted in one graph and compared with corresponding simulation results.

As can be seen in Table 6, F1, F11, and F15 have pitch in order of 400 nm with heights of

119 nm, 78 nm, and 101 nm respectively. Simulated light absorption profile for these cells is

plotted in Figure 42. Figure 43 shows the reflection measurements in respect to planar device in

percentage.

Figure 42: Simulated light absorption for F0 (blue line), F1 (dark yellow line), F11 (green line), and F15 (red line).


64

Figure 43: Measured reflection spectrum in respect to F0 for F1 (dark yellow line), F11 (green line), and F15 (red line).

As it is observed in Figure 43, reflection spectra for samples follow the same trend, but

different in values. This can be seen in Figure 42 too. There is a light absorption enhancement

for all three structured electrode solar cells in the wavelength range between 420 nm and 470

nm. Reflection at around 490 nm wavelength is very close to the planar device (≈100%).

Dramatic optical properties enhancement is obtained in the wavelengths longer than 550 nm,

and F1 with higher height shows the most light absorption than other two samples which is also

observed in simulation results. F19 and F24 have the same pitch of around 360 nm with 117 nm

and 70 nm heights respectively. Their simulated light absorption and measured reflection

spectra showed similar trend and effect.

F11, F13, and F17 are the samples with same height of around 80 nm with different

pitches of 404 nm, 584 nm, and 323 nm respectively. They are selected to study the effects of

different pitch distances on organic solar cells optical properties. Simulated light absorption

profile for these cells is plotted in Figure 44. Figure 45 shows the reflection measurements in

respect to planar device in percentage.


65

Figure 44: Simulated light absorption for F0 (blue line), F11 (dark yellow line), F17 (green line), and F13 (red line).

Figure 45: Measured reflection spectrum in respect to F0 for F11 (dark yellow line), F17 (green line), and F13 (red line).

Regarding to simulation results in Figure 44, enhancement in light absorption takes

place in wavelengths longer than around 520 nm. Devices with structured electrodes follow the

same trend; F11 and F13 have very similar enhancements in values, more than light absorption

in F17; as can be seen in measurements in Figure 45.


66

Simulation results in comparison with measured ones studied qualitatively in this

research. Although experiments showed very similar trends with simulated results, there are

various parameters that make it not possible to match the measured results with simulations,

quantitatively.

Light absorption measurement method which is used in this project is different from the

one which is calculated in simulation. In this method, normal incident light hit a small spot of

the cell by an objective, and reflected light from the surface is collected by that objective. Then,

reflected light intensity is plotted in respect to the reflected light from planar device in

percentage. So, total light absorption in device is not measured by this method. In addition,

other light sources such as sun light in the lab may shine on the sample and reflected into the

objective, which interferes in reflected light collected by objective, and may increase the

reflected light intensity. Furthermore, a part of light diffracted out of the range of objective and

not collected by it.

Optical properties of the materials which are used in simulation were extracted from

other publications. To obtain accurate results, one has to measure the optical properties of the

used materials in experiments, and implements their measurements in simulation.

Organic material thickness is another issue that has to be considered. 200 nm of

PEDOT:PSS thickness kept constant in all simulations. P3HT:PCBM which was used as active

layer, has a thickness of 150 nm in planar structure. This thickness used for simulations with

different heights from the origin of the sinusoidal structure. In practice, these thicknesses must

be measured device fabrication, and measurements be applied in simulation in order to have

precise simulations.

Last but not least, researchers witnessed some defects in the substrates which will be

explained later in this report.


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5.4. Organic Solar Cells Parameters

As it was mentioned before, some defects were observed in fabricated OSCs. These

defects are bubbling effects and cracks in electrodes after active layer deposition as shown in

Figure 46:

Figure 46: Observed defects in one of the fabricated device after organic material deposition under the optical microscope with 5× objective (left) and 10× objective (right)

Different approaches have tried and discussed with the team researcher was worked

with to investigate defects reasons. At the first step, many samples were imprinted and

metalized, and then baked on the hotplate for 5 min at 140°C to simulate thermal treatment of

active material deposition. Cracking was observed in some of them, while the rest remained

defect-less. Thus, these defects are the consequence of thermal treatment. It was interesting

that the samples, which rainbow effect could be easily seen, did not exhibit any defects.

Checking the dimensions of the defect-less samples showed that sinusoidal structured

electrodes with the height higher than around 80 nm survives after thermal treatment. This

might because of remained solvents in photoresist. Imprinting method consists of 5 min baking

glass substrate with 1.4 µm spin-coated photoresist, and PDMS stamp on top at 140°C. It is

probably not enough to evaporate all the solvents from photoresist, so that an amount of

solvent is still remained in the photoresist, and evaporates after metal deposition through the


68

hardened photoresist and metalized parts, causes cracks in electrodes. By increasing height,

surface energy changes and makes it easier for solvents to release from the surface.

Glass substrate cleaning process is crucial to remove all particles and residuals and to

improve photoresist adhesion onto the glass. Using a resist with the capability of obtaining

thinner layer in order to nanometer may solve the cracking problem due to less remained

solvent after post-bake.

Due to cracking issue which is pointed in previous paragraph, PDMS stamps with heights

more than 80 n were chosen to do imprinting and making devices. Surprisingly, cracking issue

was observed again in many of devices after active material spin-coating and thermal

treatment. As thermal treatment had done before on the same dimension devices and all of

them showed defect-less features electrodes, P3HT:PCBM blend was suspicious as the only

factor which varied in the process. Chlorobenzene is the common solvent and is used for this

blend. It might penetrate into the photoresist at some points and evaporates during baking

process, causes cracking defects in electrodes.

To avoid cracking defect due to P3HT:PCBM solution and thermal treatment, small

molecules evaporation was tried. In this method, 10 nm of bathocuproine (BCP) evaporated on

top of the electrodes as electron transport layer, following by evaporation 30 nm of C70 and 10

nm of DBP as electron acceptor and electron donor respectively. Finally, PEDOT:PSS was spin-

coated as top electrode and left in vacuum condition to release its solvents. Devices which were

fabricated by small molecules showed very low JSC and FF, and did not involve in further

characterization and investigation. Their low JSC and FF is because they are not optimized with

Aluminum/Titanium and PEDOT:PSS as electrodes.

The cracking issue, and approaches to avoid them, needs an effort to be classified

scientifically. Nevertheless, three devices did not show cracking defects after whole process and

they were used to characterize under solar simulator to evaluate OSCs parameters i.e. short

circuit current (Jsc), open circuit voltage (Voc), fill factor (FF), and power conversion efficiency

(PCE). Devices whose parameters are characterized are listed in Table 7 with their

configuration:


69

Sample Dimensions (nm)

Pitch Height

F0 - -

F1 426 119

F19 372 117

F21 328 80

Table 7: Samples which were used to characterized under solar simulator and their configuration.

J-V characterization was done using Abet 3000 solar simulator under one sun

illumination in laser lab., MCI, University of Southern Denmark. OSCs parameters are detailed in

Table 8 and plotted in Figure 47:

Samples

Dimension (nm)

VOC (V) JSC (mA/cm2) FF (%) PCE (%)

Pitch Height Average Standard Deviation




F0 - - 0.43 0.03 3.12 0.33 24.5 1.41 0.35 0.07

F19 372 117 0.43 0.01 3.96 0.7 25 0 0.42 0.08

F21 328 80 0.41 0.01 4.38 0.22 24.66 0.57 0.44 0.02

F1 426 119 0.48 0.04 7.17 1.96 25 0 0.75 0.16

Table 8: Devices which characterized under solar simulated and their parameters.

As can be seen, open circuit voltage and fill factor stayed almost constant for all devices,

but short circuit current which is directly related to the light absorption varies for different

configurations. Structured devices show enhancement in JSC. Sample F1 with the pitch of 426

nm and the height of 119 nm shows highest JSC than other samples. It is straight forward that

with constant VOC and FF, PCE increases by increasing short circuit current.


70

Figure 47: OSCs parameters for devices listed in Table 8.

OSCs parameters in this experiment are lower than typical devices which are made by

polymers. Main reason is related to the top electrode which is PEDOT:PSS in current research.

PEDOT:PSS mostly uses as hole transport layer in OSCs. This blend is not highly conductive and

results in high sheet resistant in top electrode; results in increase in recombination, and thus JSC

and PCE decrease. In addition, TiOx which is used as hole blocker on top of bottom electrode,

increases sheet resistant from around 1.5 Ω to around 15 Ω; results in S-shape J-V curve,

decreases FF and PCE.

In the other hand, cracking defect which can be obviously seen in macroscopic scale in

some devices may occur in microscopic scale too. In this situation, even the samples that do not

show any defect on their electrodes might suffer from these defects and it affects their

functionality.


71

6. Conclusion

In this thesis, sinusoidal light-trapping structures on organic solar cell electrodes, and

their effects on light absorption were studied in two phases: modeling and experimental.

In modeling section, COMSOL Multiphysics 5 software was used which solves

electromagnetic wave equations by finite elements method. Enhanced model was compared

with other methods such as FDTDM and TMM, and verified. Then it was configured for

structures and materials which were used in experimental part of current project. Simulation

results showed enhancements in light absorption profile for structured electrodes in OSCs. This

enhancement takes place mostly in the wavelength longer than 550 nm. For constant pitch

distance, light absorption in visible light wavelength range increases by increasing structures

height. In addition, for constant height, structures with 500 nm and 700 nm pitch distances

show the most light absorption enhancement in visible light wavelength range.

In experimental part, Wrinkled PDMS substrates were used as stamps for nano-

imprinting. PDMS substrates were stretched by a specific length in respect to their original

length in percentage, treated under plasma exposure to form a thin hard film on top of them,

and then released. Structures dimensions depend on stretch and plasma dose. To investigate

their effects on dimensions, PDMS substrates were stretched by 10%, 20%, and 30% in respect

to their original length, and plasma treatment was done from 5 min to 30 min by 5 min steps;

and then characterized by AFM. Achieved dimensions (pitch and height) show a linear behavior

by increasing time.

In fabrication part, AZ 5214E was spin-coated on the BK7 substrates and PDMS stamp

lied on top of the substrate with a column weight of 500 gr, left 5 min on hotplate at 140°C.

Imprinted substrates characterization showed that structures were transferred onto the

substrates with same dimensions as stamps. Then metal deposited on them, following by

P3HT:PCBM and PEDOT:PSS.


72

Reflection spectra of fabricated cells were measured in respect to the planar device (as

the reference) and they showed enhancement in light absorption profile for structured

electrodes. This enhancement matched qualitatively with simulation results i.e. for structures

with same pitch distances, reflection spectrum decreases (and thus light absorption inceases)

by increasing height. J-V curves were measured under 1 sun illumination, and enhancement in

JSC, and thereby PCE, was observed.


73

7. Outlook

Modeling in this project was used to evaluate optical properties of devices. Another

crucial factor which is considered in solar cells is their electrical properties. By implementation

electrical properties of the materials which are used in OSCs and their corresponding equations,

one can simulate their electrical properties and even extract J-V curve and other OSCs

parameters such as JSC, VOC, FF, and PCE. This needs very hard effort on theory of both materials

and light properties.

Optical properties of the materials which are used in simulation in current research

were extracted from other publications. These materials might have been provided from

different supplier than the one who provides materials for SDU. A defined project to measure

and characterize optical (and electrical) properties of the materials that are used in this

academy could help future researchers to obtain more precise results. A light absorption

measurement method with the capability of measuring total light absorption in whole cell,

instead of a small spot of it, is vital to obtain accurate results that are comparable quantitatively

with simulation results.

Active layer kept constant in simulation. In practice, regarding that nano-structures are

different on the electrodes, active layer thickness varies. Measuring the thickness of all the

layers for each cell, and then configure the simulation with them, will give even more precise

results.

PDMS substrates were stretched by a clamp that was made by plastic. It deformed after

few times using and made the stretch less accurate. The base of clamp was marked manually to

measure stretching length. In addition, stretching was measured by eyes which made it even

less precise. A precise clamp and stretching process is required to obtain accurate results.

Cracking defect that was observed in some of the samples after thermal treatment and

active layer deposition shows changes in microscopic level. To avoid this defect, more studies

on the photoresist are necessary. Use a photoresist with the ability to obtain thinner layer on


74

top of the glass substrate may disappear this defect. Other cures to make photoresists more

chemically stabled, such as UV exposure, may prevent cracking or bubbling effects in

photoresist, and thereby in electrodes.

Finally, parameters which were optimized for planar devices may not be optimized for

structured devices due to changes in their thicknesses. More work on their optimization is

recommended by researcher.


75

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81

9. Appendices

9.1. Appendix 1- Project Timeline


82

9.2. Appendix 2- Efficiency Chart


83

9.3. Appendix 3- PDMS Stamps Process

Cleaning the plasma chamber

1) Open and immediately close the air capsule valve

2) Turn on the vacuum pump

3) Turn the plasma cleaner power on

4) Open the vacuum pump valve

5) When the base pressure is about 70 mTorr, open the protection valve on the chamber

and let air into the chamber by turning the chamber valve to the left

6) Repeat step 4

7) Turn the selector on the chamber all the way to the right (29.4 w) and leave it for 20

minutes

8) Cut the sample and tight it in the sample holder and stretch it to the clamps

Put the sample in the chamber

9) Turn the selector on the chamber all the way to the left “off”

10) Close the chamber valve (it should point down)

11) Close the vacuum pump valve

12) Open the chamber and put the sample in it

13) Close the chamber and open the vacuum pump valve

14) Repeat steps 1 to 5

15) Turn the selector on the chamber all the way to the right (29.4 w) and leave it for 1-5

minutes

Take out the sample and turn off the machine

16) Turn the selector on the chamber all the way to the left “off”

17) Close the chamber valve (it should point down)

18) Close the vacuum pump valve

19) Close the protection valve on the chamber and capsule


84

20) Vent the chamber slowly (valve should point to the right)

21) Take out the sample

22) Close the chamber and pump it down for 20 seconds

23) Turn off the vacuum pump

24) Turn off both chamber and controller

nanostructured electrodes and their use in organic solar cells · 2017-09-15 · renewable energy...

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