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A STUDY OF ORGANIC SEMICONDUCTOR POLYMER MATERIAL AND DEVICE STRUCTURES FOR APPLICATION IN

OPTICAL DETECTORS

By SHEETAL LILADHAR BARAI

DEPARTMENT OF ELECTRICAL ENGINEERING INDIAN INSTITUTE OF TECHNOLOGY KANPUR

MAY, 2005

A STUDY OF ORGANIC SEMICONDUCTOR POLYMER MATERIAL AND DEVICE

STRUCTURES FOR APPLICATION IN OPTICAL DETECTORS

A Thesis submitted In partial fulfillment of the requirements

for the degree of

Master of Technology

By

SHEETAL LILADHAR BARAI

To The

DEPARTMENT OF ELECTRICAL ENGINEERING INDIAN INSTITUTE OF TECHNOLOGY KANPUR

MAY, 2005

CERTIFICATE

This is to certify the work contained in the thesis entitled “A Study of Organic

Semiconductor Polymer Material and Device Structures for Application in Optical

Detectors” by “Sheetal Liladhar Barai” has been carried out under our supervision and

this work had not been submitted elsewhere for a degree.

(Dr. Baquer Mazhari) (Dr. R. S. Anand)

Department of Electrical Engineering

Indian Institute of Technology

Kanpur

MAY, 2005

Abstract

In this work, polymer photo-detectors having good electrical and optical

characteristics have been demonstrated. It is shown that pure MEHPPV, an

electroluminescent material can be used as active material in organic photodetector.

Further, devices fabricated using blends of MEHPPV with PCBM as photoabsorbing

layer has an order of improved photoresponse with respect to the device having only

MEHPPV as active layer. The optimization of the processing conditions and change of

device structure has been done in order to get good quality devices. It is shown that the

use of aromatic solvent leads to best results. The photoresponse in the device with

polymer dissolved in 1-2 Dichlorobenzene is found to be better with maximum ratio of

photo current to dark current as 29.9 at -2.2 V, where as the leakage current in the device

with MEHPPV dissolved in Chlorobenzene is less. The thickness variation of the active

layer is incorporated and it is observed that photo-response is better in the device with

thinner active layer. The maximum ratio of the photocurrent to the dark current is in the

thinner device that is 29.51 at very low bias voltage of -0.8V.The leakage current is

reduced to -9 x 10-8 A/ cm2 as the active polymer layer thickness is increased. The device

using blend of MEHPPV: PCBM in 1:1 proportion shows a very high ratio of

photocurrent density to dark current density that is 2324.07 at a very low applied bias of -

0.6V. The physical demonstration of the photo-detector using MEHPPV: PCBM (1:4) as

photoabsorbing layer using an OP-AMP photodetector circuit has been made. The

response time of the detector at 680 Ω load was measured to be 450 ns and calculated

capacitance value is 0.566 nF.

Acknowledgements

I am indebted to my Supervisor, Dr. R. S. Anand, for his help and advice during

last one year. His generous support helped me throughout my work. His Advice was most

valuable to understand the obtained results and to determine next steps for the work

presented in this thesis.

A special thanks goes to my Co-Supervisor, Dr. Baquer Mazhari, who was the

first person to introduce me to the field of organic electronics. His enthusiasm motivated

me to learn more about this interesting and emerging field of electronics.

I am grateful to Dr. J. Narain and Dr. Asha Awasthi of Semiconductor Device

Laboratory for all their help and support.

I would also like to thank my colleagues Mr. Talari Manojaya, Mr. Ramesh and

Dr. Anjali Giri, for their encouragements and support during the work.

Further, I would like to thank all my fellow students for their co-operation and

suggestions for the accomplishment of the work.

And finally, I feel a deep sense of gratitude for my parents who taught me the

good things that really matter in life.

Table of Contents

PREFACE

ACKNOWLEDGEMENTS iv

LIST OF FIGURES vii

LIST OF TABLES xi

CHAPTER 1

INTRODUCTION 1

CHAPTER 2

BACKGROUND ON POLYMER PHOTODETECTOR 5

CHAPTER 3

FABRICATION OF PHOTODETECTOR 9

3.1 Patterning 10

3.1.1 Preparation of Mask 11

3.1.2 Cleaning of ITO coated substrate 11

3.1.3 Resist coating, Pre-bake and UV exposure 12

3.1.4 Developing the resist and post baking 13

3.1.5. ITO Etching 13

3.1.6 Stripping of resist 13

3.1.7 Cleaning 14

3.2 Ozone treatment 14

3.3 Coating of PEDOT-PSS layer & Vacuum Annealing 14

3.4 Spin Coating of Active layer & Solvent removal 15

3.5 Cathode Deposition 15

3.6 Encapsulation 16

CHAPTER 4

POLYMER PHOTODETECTOR: SINGLE LAYER 18

4.1 Introduction 18

4.2 Principle of operation 19

4.3 Experiments, Results and Discussion 21

4.3.1 Optimizing processing conditions of the device 23

4.3.2 Variation in Device structure 29

4.4 Summary 34

CHAPTER 5

POLYMER PHOTODETECTOR: DISPERSED (BULK)

HETEROJUNCTION

35

5.1 Introduction 35

5.2 Fabrication details 37

5.3 Principle of Operation 37

5.4 Experiments, Results and Discussion 39

5.4.1 An OP-AMP Photodetector Circuit 44

5.4.2 Rise Time Measurement 45

5.5 Summary 48

CHAPTER 6

CONCLUSION AND FUTURE WORK 50

BIBLIOGRAPHY 52

APPENDIX [A] 53

APPENDIX [B] 57

vii

LIST OF FIGURES

[1] Fig.2.1(a): shows how π bond is formed between two carbon atoms in a molecule

6

[2] Fig 2.2(b): shows when chain of carbon atoms comes together, π electron cloud is formed

6

[3] Fig.2.2: The organic polymer diode can be operated in various modes keeping the planer layered structure the same but by varying the biasing conditions. (a) LED mode, the forward bias is provided between the electrodes with electroluminescence as output. (b) Photodetector mode, the reverse bias is provided between the electrodes and the device is illuminated simultaneously to provide output voltage/current. (c) Photovoltaic mode, no bias is provided but the device is given irradiation to provide output voltage/current.

7

[4] Fig. 3.1: Basic Steps in Fabrication of Polymer Photo Detector. 9

[5] Fig. 3.2: Process of transferring mask features onto the ITO coated

substrate 10

[6] Fig.3.3: Design of mask transferred on photographic plate 11

[7] Fig.3.4: ITO coated Glass substrate before patterning 12

[8] Fig.3.5: Photo Resist coated ITO substrate. 12

[9] Fig.3.6: Hardened Photo resist after developing. 13

[10] Fig .3.7: Etched ITO substrate 13

[11] Fig.3.8: Resist stripped and patterned ITO substrate. 13

[12] Fig.3.9: Profile of ITO obtained from Alpha Step 500 Surface profiler 14

[13] Fig.3.10: PEDOT-PSS coated patterned ITO substrate. 15

[14] Fig.3.11: Vacuum annealed Polymer layer on patterned ITO substrates 15

[15] Fig.3.12: Cathode deposited on the substrates 16

[16] Fig.3.13: Glass Encapsulated ITO/PEDOT-PSS/Polymer Active Layer/Ca/Al Device

17

[17] Fig.3.14: Structure of Photodetector 17

[18] Fig.4.1: Chemical Structure (the picture shows one repeat unit of polymer) and energy level diagram of MEHPPV

18

[19] Fig. A: Structure of single layer polymer photodetector 19

viii

[20] Fig. 4.2: Energy level diagram. Upon irradiation an electron is promoted to LUMO leaving a hole behind in HOMO. Electrons are collected at Al electrode and holes at the ITO electrode. Φ: workfunction, χ: electron affinity, IP: ionization potential, Eg: optical band gap.χMEHPPV = 2.8 eV, IPMEHPPV = 5.2 V, ΦITO = 4.9eV, ΦCa = 2.9eV, ΦAl = 4.3eV,ΦPEDOT-PSS = 5.1 eV and Eg MEHPPV = 2.4 eV.

20

[21] Fig. 4.3: Absorption and Emission spectra of conjugated polymer MEHPPV,Courtesy: www.adsdyes.com. Absorption maximum= 490nm.

21

[22] Fig. 4.4: Reverse J-V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device. Chloroform as organic solvent.

22

[23] Fig 4.5(a): Reverse J-V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device. 1-2-Dichlorobenzene as organic solvent.

23

[24] Fig 4.5 (b): The ratio of photo current density to dark current density versus applied voltage

24

[25] Fig 4.6(a): Reverse J-V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device. Xylene as organic solvent.

24


25

[27] Fig 4.7(a): Reverse J-V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device. Chlorobenzene as organic solvent.

25


26

[29] Fig.4.8(a): Aromatic Conformation 28

[30] Fig.4.8(b): Non- aromatic conformation 28

[31] Fig.4.9(a):Reverse J-V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device with active layer thickness of 70 -80nm

30

[32] Fig.4.9(b):Forward J-L-V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device with active layer thickness of 70 -80nm under forward bias.

30

[33] Fig. 4.9(c): The ratio of Photo current density to dark current density plotted versus the applied voltage

31

ix

[34] Fig.4.10(a):Reverse J-V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device with active layer thickness of 150 -160nm

31

[35] Fig.4.10(b):Forward J-L-V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device with active layer thickness of 150 -160nm under forward bias

32

[36] Fig. 4.10(c): The ratio of Photo current density to dark current density plotted versus the applied voltage

32

[37] Fig 5.1: Chemical Structure and energy level diagram of PCBM (HOMO – LUMO = 2.4eV) 36

[38] Fig. 5.2: In blended device interface is distributed all over the device. Figure shows one such interface.

37

[39] Fig.5.3: Absorption spectra of PCBM and C60. Absorption maximum for PCBM = 284 nm & 341nm. Courtesy: www.adsdyes.com

38

[40] Fig.5.4(a): Reverse J-V characteristics of the device whose active layer has MEHPPV and PCBM are mixed in 1:1 proportion by weight in

Chlorobenzene.

39

[41] Fig.5.4(b) : The ratio of photo current density to dark current density versus applied voltage

40

[42] Fig.5.5(a): Reverse J-V characteristics of the device whose active layer has MEHPPV and PCBM are mixed in 1:2 proportion by weight in Chlorobenzene.

40


41

[44] Fig.5.6(a): Reverse J-V characteristics of the device whose active layer has MEHPPV and PCBM are mixed in 1:4 proportions by weight in Chlorobenzene.

41


42

[46] Fig. 5.7: An OP-AMP Photodetector Circuit. PD: Photodetector, R1= 5kΩ, R2= 10MΩ, Bias = 6V.

44

[47] Fig.5.8 (a): Light is incident on the polymer photodetector (on left) and the organic polymer LED’s are being driven through that using op-amp circuit.

45

[48] Fig. 5.8(b): Light is incident on the polymer photodetector (above right) and inorganic LED’s are being driven through that using op-amp circuit

45

x

[49] Fig. 5.9: Rise time measurement setup 46

[50] Fig.5.10 (a): R = 680 ohms, Fig.5.10 (b): R = 2.7k ohms, Fig.5.10 (c): R = 6.2k ohms, Fig.5.10 (d): R = 12k ohms.

46

[51] Fig. 5.11: Rise time Versus Resistance characteristics 47

[52] Fig : A.1 Typical band structure (a) Before ozone treatment ITO( ~4.7 eV) & HOMO of MEHPPV( ~5.2 eV) (b) After ozone treatment ITO (~4.9eV) & HOMO of MEHPPV (~5.2eV).

55

[53] Fig: A.2 (a) when PEDOT-PSS layer is not present, can lead to short (b) PEDOT-PSS layer is present, thus avoiding short

55

[54] Fig: A.3 Typical band structure showing alignment of workfuction with the addition of PEDOT –PSS layer.

56

[55] Fig. B.1:Typical system configuration to characterize PLED (PD is photodetector).

57

[56] Fig. B.2: Typical system configuration to characterize photodetector. 58

xi

xi

LIST OF TABLES

[1] Table 4.1: Comparison of the ratio of photo current density to dark current density of the various devices made using various solvent

27

[2] Table 4.2: Comparison of the devices with different active layer (MEHPPV) thickness

33

[3] Table 5.1: Comparison of the ratio of photo current density to dark current density of the various devices made using different proportions of the master solution of MEHPPV and PCBM.

42

[4] Table 5.2: Rise time values of the photodetector for different values of resistance 47

A Study of Organic Semiconductor Polymer Material and Device Structures for Application in Optical Detectors 1

Chapter 1

INTRODUCTION

In recent times there has been intense activity in the field of organic electronics.

The main driving force behind such an activity is the apparent capability of the organic

materials to affect the various devices of commercial interests. The activity that started

with the advent of polymer light emitting diodes (PLED’s) [1], now spans over all the

disciplines of semiconductor technology covering various devices for different

applications [2]. The organic approach finds advantage over inorganic due to its

mechanical flexibility, deposition over large substrate, ease of production and low cost.

Not only organic small molecules but also the conjugated organic polymers have been

studied extensively for the past several years due their potential applications in

optoelectronics devices such as PLED’s, photovoltaics, photodetectors, FET’s & displays.

Although Silicon technology has been the dominant technology in the above areas

for the past several decades, they suffer from several limitations such as the need for a

crystalline substrate which leads to inflexibility, the requirement for ultra-pure silicon

wafer as a starting material which leads to high processing costs, limited color sensitivity

in photovoltaic/photodetector applications etc. the organic molecules overcome all these

limitations and provide a simple, efficient and yet inexpensive alternative for all the

above applications.


The photodetectors/photovoltaic applications are especially the ones that are

severely hampered by the prevailing inorganic semiconductor technological limitations.

The present day photovoltaic devices are large and bulky and require a huge area for

setup. Also, their absorption range is also limited due to their band-gap dependencies. The

organic molecules, theoretically, can provide an option for co-evaporation/mixing of

molecules which can provide an absorption range much superior than the inorganic

materials. The capability to make an organic photo-device on the flexible substrates,

make them an attractive option from both mechanical and economical aspects.

Photodetectors are widely used in a variety of applications in fields like military,

bio-medical, space, traffic, consumer electronics etc. Few of the most common

applications of the photodetectors can be listed as: Optical scanners, Wireless LAN,

Remote control devices, Automatic lighting controls, Color sensor element for Digital

Camera, Flexible photodetectors for detection of optical field with non-planar wave-front,

extended large area photodiode arrays for industrial automation, security sensing, night

vision instruments etc.

The first few applications like Optical scanners, Wireless LAN, Remote control

devices, Automatic lighting controls, Color sensor element for Digital Camera, can be

achieved using silicon as well as polymer detectors but for the Flexible photodetectors,

extended large area photodiode arrays for industrial automation the applications, silicon is

found unsuitable. This is because the maximum area possible till date with the silicon as

substrate is 12” in diameter where as polymer thin films can very well be coated on

infinitely long flexible plastic sheets and glass substrates.


Also, organic polymer material provide us a number of other advantages as well

owing to their unique inherent properties in terms of their simple and inexpensive

processability, availability of different band gap materials which provide a broad

absorption range and modulation of absorption edge using different dyes. This means that

polymer photodetectors having response in entire visible and near IR region are possible

[3].

However, the area of organic electronics is still under intense study and requires a

greater maturity in terms of understanding of various physical mechanisms as well as

optimization of fabrication processes. In terms of photo detector applications, it is

required that the photo detector device has a very high photo-sensitivity (high quantum

efficiency), low leakage (dark) current, large dynamic range, fast response time and low

noise.

Out of all the above requirements, the dark current requires special attention because

the dark current is the photodetector leakage current, when reverse bias is applied and no

light is incident on photodetector. It also limits the photodetector dynamic range which is

another critical parameter of photodetector application. Dynamic range of the detector is

defined as ratio of the maximum signal level to the minimum signal level that can be

detected by the photodetector. If the minimum signal level, equivalent to the leakage/dark

current is high then the operating range of the device is reduced, which is an undesirable

situation. Hence the dark current needs to be minimized with a high priority in order to

achieve an efficient photodetector device with good dynamic range.


In this thesis, emphasis is given on the photodetector application of the organic

conjugated polymer materials. Not much data is available on the processing as well as the

mechanism of the process of photo-detection in the organic molecules. These organic

photodetector devices normally suffer from a high dark/leakage current and degradation

after illumination.

The focus of this thesis work is to optimize the dark current of the organic

photodetector. Initial work led to the dark current in the range of milli-amperes which

leads to almost no photo-response and these devices were prone to degradation after

illumination. After the optimization of the various steps of fabrication, dark current was

brought down to the levels of nano-amperes with a high photo-response and reasonably

good life times.

In the present thesis, ITO/PEDOT-PSS/MEHPPV/Ca/Al and ITO/PEDOT-PSS/

MEHPPV-PCBM/Ca/Al devices were fabricated and their current-voltage characteristics

are studied. Chapter 2 provides a basic background of photodetectors and the related

physical processes. In chapter 3, the fabrication steps of the above mentioned organic

devices are discussed in details. In chapter 4 & 5 two different structures of photodetector

are studied. The first structure (Chapter 4) uses only one type of polymer and other using

a blend of this polymer with Fullerene material (Chapter 5). The above chapters also

discuss the results and cause for the analysis drawn in case of the respective devices.

Chapter 6 gives the conclusion of the thesis work and the future aspects to the work done

in this thesis.

A Study of Organic Semiconductor Polymer Material and Device Structures for Application in Optical Detectors

5

Chapter 2 Background on Polymer Photodetectors

Polymers were initially studied as they have attractive mechanical and structural

properties. In mid 1970’s when conducting polymers were discovered, it led to a whole

new branch of materials having electrical properties that can range from insulating to

semiconducting to conducting [4]. These new semiconducting materials have electronic

and optical properties of inorganic semiconductor along with the mechanical flexibility of

a polymer. Though the electrical and optical properties are quite similar to those of

inorganic, the charge carrier generation and charge transport mechanisms in

semiconducting polymers are essentially different from their inorganic counterparts. A

brief background necessary for the same is discussed in this chapter.

Semiconducting polymers are conjugated polymers which refer to the alternating

single and double bonds between the carbon atoms on the polymer backbone. The carbon

atoms along with the polymer backbone are sp2 hybridized, which leaves the hybridized pz

orbital sticking up out of the plane of polymer. The electrons in the π-orbital form

delocalized electron cloud which is free to conduct as shown in Fig.1.1. Thus the

electrical conductance of the Semiconducting polymer comes in picture.


6

(a) (b)

Fig.2.1 (a) shows how π bond is formed between two carbon atoms in a molecule (b) shows when chain of carbon atoms comes together, π electron cloud is formed.

The molecular levels are grouped in bands. The band structure generally

associated with the inorganic semiconductor. In case of organic semiconductor there are

HOMO and LUMO levels. The band edge of valance band is referred to as “Highest

Occupied Molecular Orbital” (HOMO) and edge of conduction band is called the

“Lowest Unoccupied Molecular Orbital” (LUMO). The energy gap between HOMO and

LUMO levels in conjugated polymer is generally within range of visible photon. When

the incoming photon is absorbed electron is promoted to LUMO level, leaving behind

hole in HOMO layer. This electron and hole remains on the same polymer chain and are

bound to each other by their electrostatic force, commonly known as excitons.

Dissociation of these photo generated excitons requires an input of energy of

nearly hundreds of meV compared to only a few meV for crystalline semiconductor as

they are strongly bound and do not spontaneously dissociate into charge pairs. Therefore

the carrier generations do not necessarily result from the absorption of light. A strong

driving force such as an electric field should be present to break up the photogenerated

excitons. This electric field can be provided externally by applying a reverse bias voltage

or can be provided by doping different material with polymer such as carbon nano-

C C

π e- Cloud

π e- Cloud


7

particles or other polymers. The different material used with the polymers have large

difference in the electron affinities and which in turn helps to extract the electron from the

polymer chains and make it available for the conduction.

As discussed above, the photodetector principle can be summarized as follows: A

photodetector/photosensor is an electronic component that detects/senses light and

converts the optical signal to the electrical signal. It acts like a transducer. The operation

principle of the polymer photodetector can be considered as the combination of the three

processes [5] as:

1. Carrier generation by incident light

2. Carrier transport to respective electrode

3. Interaction of current with external circuit to provide output signal.

(a) LED mode (b) Photodetector (c) Photovoltaic mode (Photoconductive) mode

mode Fig.2.2: The organic polymer diode can be operated in various modes keeping the planer layered structure the same but by varying the biasing conditions. (a) LED mode, the forward bias is provided between the electrodes with electroluminescence as output. (b) Photodetector mode, the reverse bias is provided between the electrodes and the device is illuminated simultaneously to provide Photoconductive current. (c) Photovoltaic mode, no bias is provided but the device is given irradiation to provide output to drive current through the external circuit.

Al, Ca

Organic MaterialITO

Glass

Light

InputAl, Ca

Organic MaterialITO

Glass

Al, Ca

Organic MaterialITO

Glass

LightLight

Input

Al, Ca

Organic MaterialITO

Glass

Light

OutputAl, Ca

Organic MaterialITO

Glass

Al, Ca

Organic MaterialITO

Glass

LightLight

OutputAl, Ca

Organic MaterialITO

Glass

Al, Ca

Organic MaterialITO

Glass

LightLight

OutputAl, Ca

Organic MaterialITO

Glass

Al, Ca

Organic MaterialITO

Glass

LightLight

Output

Al, Ca

Organic MaterialITO

Glass

Al, Ca

Organic MaterialITO

Glass

LightLight

Output currentOutput current

BiasBiasAl, Ca

Organic MaterialITO

GlassITO

LightLight


8

The organic polymer diode structure can offer different functions simply by

varying the biasing conditions applied to the device as shown in Fig.2.2. Under the

forward bias the device emits light and thus acts as a simple Light emitting diode (LED).

When the diode is operated under the reverse bias and subjected to the illumination, then

it is said to be operating in the photoconductive mode of the photodetector (Fig. 2.2(b)).

In the third option an output voltage is obtained when the device is irradiated without any

external applied bias. This is termed as the photovoltaic mode of the detector shown in

Fig.2.2 (c). This operating mode is similar to that of a solar cell. But the need of large

area for the solar cell applications restricts the detector to be called solar cell.


Chapter 3

Fabrication of Photodetector

Polymer Photodetector in this work has been fabricated on commercially procured

Indium-Tin-Oxide (ITO) coated glass substrate. ITO, a transparent conducting oxide is

used as anode. The ITO film has been characterized for its sheet resistance, thickness and

roughness. The measured values are given below:

Sheet resistance: 20.4 to 20.9 Ω/

Thickness: ~145 -150 nm

Mean roughness: ~10.2 nm

The basic steps involved in Fabrication of the polymer photodetector are as shown

in figure 3.1.

Fig. 3.1 Basic Steps in Fabrication of Polymer Photo Detector

Cleaning of ITO coated substrate

Patterning of transparent Conducting Anode

Coating of active layer

Deposition of Cathode Metal

Encapsulation


3.1 Patterning

ITO coated glass substrate (0.1cm x 5 cm x 5 cm) are subjected to UV

Photolithography. The purpose of the lithography process is to transfer the mask feature

to the surface of the substrate. Fig. 2.2 shows overview of this typical transfer process.

Fig. 3.2 Process of transferring mask features onto the ITO coated substrate.

Coating of Positive Photo Resist (PPR) 2000 rpm (1 min)

Pre-bake in Oven (90-100°C) (30 min)

Baking of substrate at 95° to 100 °C for 20 min (to remove moisture)

UV Exposure through Mask (2 min)

Develop, Rinse & Dry

Post Bake in Oven (120-130°C) (30 min)

Etching

Removal of Resist and cleaning


3.1.1 Preparation of Mask

The mask is prepared on rubylith with the help of a co-ordinatograph. The mask

contains around 19 lines of dimension 0.1 cm x 5 cm. Then the pattern is transferred on

photographic plate. The typical picture of mask (particularly for Positive Photo resist) is

as shown in fig 3.3.

Fig. 3.3 Design of mask transferred on photographic plate.

3.1.2 Cleaning of ITO coated substrate

The ITO substrates are cleaned with RCA 5:1:1 solution (200ml DI + 40ml

H2O2 + 40ml NH4OH). The substrates are immersed in solution and heated for 30 min,

the temperature being 75° – 85°C.

DI De-ionized Water doesn’t have any metallic ion inside. It has ρ = 10MΩ/cm

to 18MΩ/cm. For our purpose 10 MΩ/ cm is sufficient.

H2O2 Functions to oxidize all organic contaminant (oxidizing agent)

NH4OH helps in removing heavy metals such as cadmium, cobalt, copper, iron,

nickel etc.

The substrates are subjected to ultrasonic cleaning in DI for 5 min, followed by drying.

The advantages of ultrasonic cleaning can be listed as follows:

(i) Ultrasonic cleans the surface and cavities without scratching, brushing or

scraping.


(ii) It takes very short time to clean. Even most complex geometries can be

thoroughly cleaned.

(iii) Ultrasonic cleaning is very simple and easy to handle. Concentration of

chemicals required is very less than in conventional cleaning.

The substrates are then baked at 95° to 100° C for 20 min. to remove the moisture

content.

3.1.3 Resist coating, Pre-bake and UV exposure

Fig.3.4 ITO coated Glass substrate before patterning

A positive Photo Resist (PPR) is flooded with the syringe onto the substrate,

followed by spinning at a speed of 2000 rpm for 1 min.

Fig.3.5 Photo Resist coated ITO substrate

A pre exposure bake of these photo resist coated ITO substrate at a temperature of

90-100°C is done for 30 min. The UV Exposure of substrates through the pattern mask is

carried out in 426 – 453 angstroms spectral region with the exposure time of 2 min.

ITO

Glass

PPR

ITO

Glass


3.1.4 Developing the resist and post baking

The exposed substrates are developed in developer (DI + KOH) followed by rinse in

DI water. The substrates are constantly agitated in these baths during this period. The

developed substrates are post baked at 120 – 130° C for 30 min.

Fig. 3.6 Hardened Photo resist after developing

3.1.5. ITO Etching

An ITO etchant consisting of 200 ml DI water, 60ml HCl and 15 ml HNO3 is used

for etching the ITO from the post baked substrate at a temperature between

55- 60°C for 6 -7 minutes.

Fig.3.7 Etched ITO substrate

3.1.6 Stripping of resist

After the etching, the resist is stripped using the acetone. The stripping of positive

resist becomes easier as compared to the stripping of negative photo-resist.

Fig. 3.8 Resist stripped and patterned ITO substrate

ITO

Glass

ITO

Glass

Hardened Resist

ITO

Glass

Hardened Resist


3.1.7 Cleaning

The resist stripped substrates are rinsed in DI water and then cleaned with RCA 5:1:1

solution (200ml DI + 40 ml H2O2 + 40 ml NH4OH typically) by heating it for 30 min.

Rinsing the substrates in DI water , using ultrasonic shaking for 5 min and then drying

completes the cleaning process. The profile of patterned ITO, obtained from profilometer,

is shown in fig.3.9,

0 1000 2000 3000 4000 5000

0

500

1000

1500

2000

Ang

stro

m

Length in Micro Meters

Fig. 3.9: Profile of ITO obtained from Alpha Step 500 Surface profiler

3.2 Ozone treatment

The substrates after the cleaning are subjected to the ozone treatment for 15 min.

Studies suggest that treating the ITO surface with oxygen plasma / ozone increases the

work function of ITO, which will also lowers the hole barrier at anode [6-7] . The role of

ozone treatment is discussed in Appendix [A].

3.3 Coating of PEDOT-PSS layer & Vacuum Annealing

The PEDOT- PSS (Poly (3, 4-EthyleneDioxyThiophene, Poly (Styrene

Sulfonate)) is spun onto the ozone treated substrates for a thickness of the layer ~ 40nm.

Subsequently the substrates are vacuum annealed, such that water content is removed.

The role of PEDOT is discussed in Appendix [A].


Fig. 3.10 PEDOT-PSS coated patterned ITO substrate.

3.4 Spin Coating of Active layer & Solvent removal

The active polymer layer is then spun onto vacuum annealed substrates at 1000 rpm

for 1 min. The substrates are then subjected to anneal in vacuum in order to ensure the

entire solvent removal.

Fig. 3.11 Vacuum annealed Polymer layer on patterned ITO substrates.

3.5 Cathode Deposition

Thermal evaporation is used for the deposition of the cathode layer. This evaporation

system consists of a diffusion pump backed by a rotary pump. The base pressure is of the

order of 4 x 10-6 to 3 x 10-6 mbar. The deposition is achieved by the application of current

through a filament of crucible. This then cause the filament or crucible to heat up and

allows the material to simply evaporate and is then deposited upon the polymer coated

ITO substrate.

ITOGlass

Annealed polymer layer Annealed PEDOT

ITO

Glass

PEDOT-PSS


The width of cathode deposited is restricted to 2mm, using metal mask. Therefore

the active area has been 1mm x 2mm. The cathode used is Calcium/ Aluminum. Firstly,

calcium is deposited and then aluminum is evaporated for deposition. The 2 lines of

cathode deposited by using metal mask is shown in Fig.2.11

(a) Top View

(b) Side View

Fig. 3.12 Cathode deposited on the substrates.

3.6 Encapsulation

Since these metals have tendency to be unstable in air hence they have to be

encapsulated. The cathode deposited substrate is sealed with a glass plate using U-V

epoxy resin, which is then treated with ultra-violet light. The device is now ready for test.

Typical system configuration for the characterization is discussed in Appendix [B].

ITOGlass

Cathode (Ca/Al) Annealed polymer layer Annealed PEDOT

ITO lines

Cathode deposited (Ca/Al)


Fig. 3.13 Glass Encapsulated ITO/PEDOT-PSS/Polymer Active Layer/Ca/Al Device.

Typical structure of Photo detector is shown in Fig 2.14. Each process in fabrication

has prominent effect on the device performance. Some of the modifications in fabrication

and their role on the device performance are studied in detailed in further chapters.

Fig. 3.14 Structure of Photodetector

+ -

ILLUMINATION

GLASS ITO PEDOT: PSS

POLYMER ACTIVE LAYER ALUMINUM

CALCIUM

Glass Encapsulation

Cathode deposited (Ca/Al)

ITO lines


18

Chapter 4

Polymer Photodetector: Single Layer

4.1 Introduction

Organic thin film diodes made of polymer MEHPPV (Poly (2-Methoxy-5-(2’-

Ethyl-Hexyloxy)-1, 4-PhenylVinylene) are generally used as polymer light emitting

diodes (PLED’s). Very little photoresponse has been observed when the device structure

is optimized as LED’s. To increase the photoresponse, certain materials like C60 and its

derivatives are usually added to pure electroluminescent material. This chapter gives an

idea that even pristine MEHPPV can be used to fabricate a photodetector by optimizing

the device structure and processing conditions.

The Single layer polymer photodetector is fabricated using only polymer

MEHPPV as active layer. The chemical structure of MEHPPV is shown in Fig. 4.1

Fig. 4.1: Chemical Structure (the picture shows one repeat unit of polymer) and energy level diagram of MEHPPV

Vacuum level

LUMO

HOMO

2.8eV

5.2eV

χ

IP


19

4.2 Principle of operation:

As discussed in chapter 2, the photodetector has a planar-layered structure, where

organic light absorbing layer is sandwiched between two different electrodes. One of the

electrode is semi-transparent, often ITO, but a thin metal layer can also be used. The other

electrode is Ca/ Al. The device structure of single layer photodetector is shown in Fig. A

given below.

Fig. A: Structure of single layer polymer photodetector

The working principle of photodetector is just the reverse of the operation of LED.

In LED’s an electron is injected from the cathode with the balanced introduction of hole

at anode. At some point in organic layer the electron and the hole meets, and gives light

upon recombination.

The photodetectors working phenomenon is just reverse. When light (photons) is

absorbed by an active layer, an exciton is formed. The exciton is basically an electron-

hole pair bound with an electrostatic force of attraction. These excitons must dissociate to

give free charge carriers i.e. a free electron and hole so that they reach respective

electrode and provide output current/voltage. In order to achieve the charge separation an

electric field is needed. The built-in electric field that arises due to difference in electrode

+ -

ILLUMINATION

GLASS ITO PEDOT: PSS

POLYMER ACTIVE LAYER ALUMINUM

CALCIUM


20

work functions is found to be insufficient to split the photogenerated excitons. Thus with

external application of electric field this exciton dissociation is achieved. Fig 4.2 explains

the charge transfer in single polymer photodetector when the light is incident on it.

The Fig. 4.2 shows the energy level diagram of the single layer polymer

photodetector. The device is illuminated from the ITO side. When the photon strikes the

active layer of the device, an electron is promoted from the HOMO layer to the LUMO

energy level. This leaves a hole in HOMO layer, thus forming a neutral exciton on the

polymer chain. With the external bias provided, the dissociation of the exciton is

achieved. The electron travels to the higher electron affinity electrodes (Calcium and then

Aluminum), and then towards the positive terminal of the battery. Similarly the hole

travels towards the negative terminal of the battery through the PEDOT –PSS layer and

ITO. Thus generation, separation and transport of the charge takes place in the detector.

Fig 4.2: Energy level diagram. Upon irradiation an electron is promoted to LUMO leaving a hole behind in HOMO. Electrons are collected at Al electrode and holes at the ITO electrode.

Φ: workfunction, χ: electron affinity, IP: ionization potential, Eg: optical band gap. χMEHPPV = 2.8 eV, IPMEHPPV = 5.2 V, ΦITO = 4.9eV, ΦCa = 2.9eV, ΦAl = 4.3eV, ΦPEDOT-PSS =

5.1 eV and Eg MEHPPV = 2.4 eV.

Vacuum LevelEnergy

ITO

Ca

AlHOMO

LUMO

IP

Φca

ΦAl

ΦITO

hν

χe -

h+

PEDOT

ΦPEDOT

Eg


21

The absorption and emission spectra of the conjugated polymer MEHPPV is given

in Fig. 4.3 below. The polymer mostly absorbs in the visible region of the spectrum. The

absorption peak is at 490 nm, where as its photo-luminescent maximum is at 585nm. The

difference between the two is because of Frank-Condon Shift.

Fig. 4.3: Absorption and Emission spectra of conjugated polymer MEHPPV, Courtesy: www.adsdyes.com. Absorption maximum= 490nm.

4.3 Experiments, Results and Discussion

The ultimate aim of this work is to fabricate a good quality polymer photodetector

with an improved dynamic range and lifetime. Hence the fabrication steps were under

continuous modification and the corresponding effects were thoroughly studied. The

analysis of the modifications led to an improvement in the overall performance of the

device.

The ITO/PEDOT/MEHPPV/Ca/Al devices were fabricated as discussed in

Chapter 3. Chloroform was used as an initial organic solvent for MEHPPV. The reverse

J-V characteristics obtained are shown in Fig. 4.4. There is a set of three measurements

taken on a device at a stretch. Firstly the device is kept under dark. The reverse voltage

bias is provided and is varied from 0 to -3.5V. The corresponding current values, which

Absorption Emission


22

represent the reverse leakage current of the device (also known as the Dark current), are

shown in Fig.4.4 with square symbols. In the second measurement, the device is

subjected to illumination with a broad light source. The device current under illumination,

which is the Photocurrent of the device, is represented by the circular symbols. Finally,

the device is again subjected to the reverse voltage under dark atmosphere. This is done in

order to determine any deterioration in the device because of exposure to light. This

current named as Dark current (2) is shown by the triangular symbols in Fig 4.4. The

graph is plotted for current density versus the applied reverse voltage, device area being 2

mm2.

-4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.01E-5

1E-4

1E-3

Reverse Voltage(V)

Cur

rent

Den

sity

(A/c

m2 )

Dark current density Photo current density Dark current(2) density

Chloroform

Fig 4.4: Reverse J-V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device. Chloroform as organic solvent.

The graph clearly shows that, the dark/leakage current density of the device is too

large ~ 1mA/cm2 and the photocurrent density is of the same order. The dark current (2)

density characteristic also follows the photo current density characteristic.


23

The optimization is undoubtedly required for the device especially to reduce the

leakage current. In this work optimization is achieved by two approaches: Firstly, by

varying the processing conditions for the device and secondly, by varying the device

structure.

4.3.1 Optimizing the Processing Conditions of the Device

The processing conditions of the device are varied by varying the organic solvent

in which the polymer MEHPPV is dissolved. The different organic solvents under test are

Chloroform, 1-2-Dichlorobenzene, Xylene and Chlorobenzene. The four different

solutions of MEHPPV using above four solvents were prepared keeping the material

concentration similar (8mg/cc) and devices were fabricated. The reverse J-V

measurements were taken on individual device as shown in Figures 4.4, 4.5, 4.6 and 4.7.

-4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0

1E-8

1E-7

1E-6

1E-5

1E-4

Reverse Voltage(V)

Cur

rent

Den

sity

(A/c

m2 )


1,2 Dichlorobenzene

Fig 4.5 (a) : Reverse J -V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device

1-2-Dichlorobenzene as organic solvent.


24

-8 -6 -4 -2 0

0

5

10

15

20

25

30

R

atio

(P/D

)

Voltage(V)

1-2- Dichlorobenzene

Maximum (P/D) =29.9 at -2.2V

Fig 4.5 (b): The ratio of photo current density to dark current density versus applied voltage

-4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0

1E-8

1E-7

1E-6

Reverse Voltage(V)

Cur

rent

Den

sity

(A/c

m2 )


Xylene

Fig 4.6(a) : Reverse J -V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device Xylene as organic solvent.


25

-8 -6 -4 -2 01.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

R

atio

(P/D

)

Voltage(V)

xylene

Maximum P/D = 2.79 at -7.9V


-4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0

1E-8

1E-7

1E-6

1E-5


Cur

rent

Den

sity

(A/c

m2 )

Reverse Voltage(V)

Chlorobenzene

Fig 4.7(a) : Reverse J-V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device. Chlorobenzene as organic solvent.


26

-8 -6 -4 -2 014.0

14.5

15.0

15.5

16.0

16.5

17.0

17.5

18.0

Rat

io (P

/D)

Voltage(V)

Chlorobenzene



The results shown in the above four graphs are tabulated under Table 4.1. It

compares the ratio of photo current density to dark current density of different devices

made using various solvents mentioned before.

It can be seen from the tabulated results that variation in the solvent in which

MEHPPV being dissolved does affect the photoresponse of the device.


27

Current

density (A/cm2)

Bias = -3.5V

1,2Dichlorobenzene Chloroform Xylene Chlorobenzene

Dark Current

density

-3.605 E-6 -1.05 E-3

-1.40 E-7

-9.05 E-8

Photo Current

density

-3.218 E-5 -0.98 E-3

-3.12 E-7

-1.51 E-6

Ratio = P / D 8.8 Nearly same 2.2

16.8

Table 4.1 Comparison of the ratio of photo current density to dark current density of the various devices made using various solvents

In the device where chloroform was used as the organic solvent for MEHPPV, no

response with the illumination is seen. The device also has higher leakage current density

as shown in Fig.4.4. With the chloroform as organic solvent, it is also seen that polymer

film after spin cast is not good. This is because the evaporation temperature of chloroform

is very low. By the time polymer is dropped on the substrate prior to the spinning,

chloroform found to be partially evaporated. Hence during the spinning of the polymer,

uniform film formation is not achieved.

In Xylene as organic solvent, small amount of the photoresponse is observed in

Fig. 4.6. In 1-2-Dichlorobenzene the photoresponse is indeed better, but device showed

degradation after subjected to the illumination as in Fig. 4.5. There is a remarkable

decrease in leakage current when Chlorobenzene was used as solvent. The photocurrent

density to the dark current density ratio in the device, with Chlorobenzene as organic

solvent, is nearly 1.909 times that of the device when1-2-Dichlorobenzene was used as


28

organic solvent. Thus, the overall performance of the device made using C6H5Cl

(Chlorobenzene) is found to be best [8, 9].

The Fig. 4.5(b), Fig. 4.6 (b) and Fig.4.7 (b) shows the ratio of photocurrent

density to dark current density plotted versus the applied voltage. It represents the optimal

value of the applied bias for the device with respect to get maximum ratio of P/D. The

choice of 1-2-Dichlorobenzene as an organic solvent thus can be further explored.

Discussion:

Basically conjugated polymer films are composed of many polymer chains and it

is becoming increasingly clear that the way in which a conjugated polymer films are cast

affects the interaction between polymer chains and thus the electrical and optical

properties [10].

The degree of interchain interaction in polymer strongly depends on the polymer

film morphology. This morphology of conjugated polymer in turn is controlled by the

way in which films are processed. The organic solvents are basically divided into two

categories Aromatic solvents and Non-aromatic solvents. Chloroform (CHCl3) comes

under non- aromatic group where as rest of the solvents falls under aromatic group. The

way in which polymer orients itself in the solvent does depend upon the type of solvent as

explained with the Fig. 4.8 (a), 4.8(b)

Fig. 4.8(a): Aromatic Conformation Fig. 4.8(b): Non- aromatic conformation


29

The polymer chains, dissolved in aromatic solvents (like Xylene Chlorobenzene,

1-2-Dichlorobenzene), have relatively open and straight conformation [11] with

maximum solute solvent interaction. The benzene rings of the conjugated polymer when

dissolved in the aromatic solvent aligns parallel to the surface, thus has planer

conformation as described in Fig. 4.8(a). It is expected that aromatic solvents can solvate

the π-conjugated segments better than the alkyl side chains [12]. This results in a

conformation which has better π-π stacking and subsequently better electrical conduction.

On the other hand, non-aromatic solvent (like chloroform) solvate the non-

conjugated segments i.e. alkyl side groups of the polymer. These alkyl side groups when

interacts among themselves, forms a tight coiled structure. This hinders the conjugation

length of the polymer backbone. Also the benzene ring structure of the polymers aligns

itself as perpendicular to the surface, with the non-conjugated group laying on the surface

as shown in Fig.4.8 (b). Thus the non aromatic solvents results in a polymer conformation

with a lower electrical conductivity

4.3.2 Variation in Device structure

The variation in the device structure is incorporated by varying the active layer

thickness. Now taking the best solvent i.e. Chlorobenzene (as discussed in section 4.3.1),

the active layer thickness can be varied by varying the spin speed or the by varying the

concentration of the polymer in the solvent. In this work, the concentration of the

polymers is varied. The concentration is reduced to 4mg of MEHPPV per cc of

Chlorobenzene. The reverse J-V and forward J-L-V characteristics of the devices are

shown in fig. 4.9(a), 4.9(b), 4.10(a) & 4.10(b) respectively for the thickness d = 70-80 nm

, d =150-160 nm.


30

-4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0

1E-8

1E-7

1E-6

1E-5

Reverse Voltage(V)

Cur

rent

Den

sity

(A/c

m2 )


Chlorobenzened = 70 - 80 nm

Fig. 4.9(a): Reverse J -V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device with active layer thickness of 70 -80nm

0 2 4 6 8-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

Cur

rent

Den

sity

(A/c

m2 )

Voltage(V)

d = 70 - 80 nm

Current Light Intensity

0

2

4

6

8

10

Light Intensity (a.u.)

Fig. 4.9(b): Forward J -L -V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device with active layer thickness of 70 -80nm under forward bias.


31

-8 -6 -4 -2 0

0

5

10

15

20

25

30

Rat

io (P

/D)

Voltage (V)


Chlrobenzene d = 70 -80 nm

Fig. 4.9(c): The ratio of Photo current density to dark current density plotted versus the applied voltage

-4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0

1E-8

1E-7

1E-6

1E-5Chlorobenzened = 150 -160 nm


Cur

rent

Den

sity

(A/c

m2 )

Reverse Voltage(V)

Fig. 4.10 (a): Reverse J-V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device with

active layer thickness of 150 -160nm


32

-2 0 2 4 6 8 10 12 14 16

0.00

0.01

0.02

0.03

0.04

0.05

d = 150-160nm

Cur

rent

Den

sity

(A/c

m2 )

Voltage(V)

Current Light Intensity

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Light Intensity(a.u.)

Fig. 4.10(b): Forward J-L -V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device with active layer thickness of 150 -160nm under forward bias

-8 -6 -4 -2 014.0

14.5

15.0

15.5

16.0

16.5

17.0

17.5

18.0

Rat

io (P

/D)

Voltage(V)

Chlorobenzened = 150-160nm


Fig. 4.10(c): The ratio of Photo current density to dark current density plotted versus the applied voltage

The above results are presented in Table 4.2. The characteristics in Fig 4.9(b) and

4.10(b) show that under forward bias the thinner device shows better performance than

the thicker one. The turn on voltage in thinner device is nearly 4V where as it is much


33

higher ~ 8 -10V in thicker device. Also the light output in the thinner device is much

better. In the device with thicker active layer, the light intensity was found to be poor

even at 16V.

Active Layer Thickness 150-160 nm 70-80 nm

Reverse Bias (at -3.5V) Forward Bias Reverse Bias (at -3.5V) Forward Bias Dark Current density = -9.05 E-8 A/ cm2

Photo Current density = -1.51 E-6 A/ cm2

Turn on Voltage= ~ 8-10V Light intensity = poor

Dark Current Density= -2.913 E -6 A/ cm2 Photo Current Density= -2.22 E -5 A/ cm2

Turn on Voltage= ~ 4V Light intensity = Very Good

Ratio P/D = 16.8

Ratio P/D = 7.6

Table 4.2 Comparison of the devices with different active layer (MEHPPV) thickness

Comparing the characteristics in Fig.4.9 (a) and 4.10(a) it can be stated that as the

device active layer thickness increases there is an improvement in the characteristics of

the device. The dark current was found to be less in case of thicker device than in thinner

device.

Discussion:

The dark current is found to be higher in devices having thinner active layer. This

may be because of the higher surface roughness in the commercially obtained substrates.

The roughness of the substrate has been found to be 10.2 nm. It is preferred to have low

value of roughness in order to have low leakage currents.

The photo-response in the thinner device was found to be higher as compared to

the thicker device. This may be due to the better charge collection of electrodes. In larger


34

active layer devices, the charges may be getting recombined before reaching the

electrodes.

The Fig. 4.9(c) and Fig. 4.10(c) represent the plot of ratio of photocurrent density

to the dark current density versus the applied voltage for the devices having active layer

thickness 70 -80 nm and 150 -160nm respectively. It is seen that if we bias the device at

-0.8V for the thinner device then the maximum ratio of P/D is obtained as 29.51 where as

in thicker device the bias voltage should be -3.1 V so as to get maximum ratio of 17.89.

It can be said that the device performance of the thinner device is good with respect to

irradiation than the thicker device except the leakage current values.

4.5 Summary:

The polymer MEHPPV is mostly used in the display like PLED applications.

When the device is optimized for the LED operation, very little photoresponse is

reported. The chapter shows that even pure MEHPPV can be used for the application in

photodetectors. This was achieved by changing the processing conditions and the active

layer thickness for the device. The processing conditions were varied by varying the

solvent in which the conjugated polymer MEHPPV is dissolved and the change in

polymer layer thickness changes the device structure.

The photoresponse of the detector using undoped MEHPPV polymer is still less

for the application in practical photodetectors. The response can be further improved with

the addition of the fullerene/polymer to the pure MEHPPV as discussed in next chapter.


35

Chapter 5

Polymer Photodetector: Dispersed (Bulk)

Heterojunction

5.1 Introduction

The analysis of the pure conjugated polymer single layer device points out that

charge separation process in these monocompound layers is rather weak. To improve the

charge generation and separation the idea is to use two materials with different electron

affinities and ionization potentials. This will favour the exciton dissociation: the electron

will be accepted by the material with the larger electron affinity and the hole by the

material with the lower ionization potential.

Among the known electron acceptor materials Fullerene/ C60 and its derivatives

are very popular. One reason for the popularity of above class of n-type nanoparticles is

the lack of wide range of n-type (electron transporting) conjugated polymers. In this

work, one derivative of C60 i.e. PCBM, [6, 6]-Phenyl-C61 Butyric acid Methyl ester is

used. The chemical structure and energy level diagram of PCBM are shown in Fig. 5.1.


36

Fig 5.1: Chemical Structure and energy level diagram of PCBM (HOMO – LUMO = 2.4eV)

In analogy to the classical p-n junction concept of electron donor material and

electron acceptor material forming bilayer structure [13] was quite of interest initially.

However for most of organic semiconductor the film thickness should be more than

100nm in order to absorb more light. It follows that thicker film layers increase light

absorption but only small fraction of the excitons will reach the interface and dissociate.

But the development of bulk heterojunction concept has opened up new research

directions. This bulk heterojunction concept is based on blends of the two organic

compounds, one with the donor properties and other with acceptor properties.

Device produced in a bilayer structure with a single interface between the electron

donor and acceptor had a low efficiency [14] because volume of the active layer where

efficient charge separation occurred was limited to a small fraction at the interface. An

early study shows that importance of close proximity of the polymer and fullerene for the

efficiency of the device. To overcome this problem, a device with a structure fabricated

from blend of MEHPPV and PCBM was incorporated [15].

Vacuum level

LUMO3.7eV

6.1eV

χIP

HOMO


37

5.2 Fabrication details:

The two master solutions of MEHPPV & PCBM (8mg/cc) were prepared. The

blend solutions with defined concentrations were then obtained by mixing the two master

solutions in proper molecular ratio. The MEHPPV: PCBM blend films were spin cast

from blend solution at room temperature. The device configuration is ITO/PEDOT-

PSS/MEHPPV-PCBM/Ca/Al.

5.3 Principle of Operation:

By blending the material, the interface is distributed throughout the device as

shown in Fig 5.2. The difference in electron affinities creates driving force at the interface

between two materials that is strong enough to spilt photogenerated exciton. Hence,

generally all photogenerated excitons are likely to find an interface and split before

recombining [16].

Fig. 5.2: In blended device interface is distributed all over the device.

Figure shows one such interface.


38

Fig.5.3: Absorption spectra of PCBM and C60. Absorption maximum for PCBM = 284 nm & 341nm. Courtesy: www.adsdyes.com In blended device, both electron accepting PCBM and a hole transporting

conjugated polymer are present throughout the bulk of the device, unlike the normal

bilayer device where the hole transport layer and electron transport layer are separately

defined. The absorption spectrum of PCBM is given in Fig. 5.3 and that of MEHPPV is

already given in Fig.4.11. Most of the absorption takes place in the conjugated polymer

layer. The excitons are generated with the absorption of the light. The dissociation of the

excitons into the electron and hole happens quickly in presence of the PCBM. Due to

higher electron affinity of PCBM than MEHPPV, electrons are attracted towards the

LUMO of PCBM, thus leaving a hole behind on MEHPPV polymer chains.

In the photodetector mode these charge carriers are drifted to the respective

electrode with the help of external electric field /voltage. Thus the charge transportation

becomes faster in the photodetector mode as compared to the photovoltaic mode where

no external bias is provided.

PCBM

C60


39

5.4 Experiments, Results and Discussion:

While using the blend of MEHPPV and PCBM as photo absorbing layer, the

basic aim remains same, to get good device with low dark current and to achieve the

maximum photoresponse out of the device. To implement this, concentration of the

master materials in blend is varied. The three compositions used in this work are

MEHPPV: PCBM ratio being 1:1, 1:2 & 1:4 by weight in organic solvent.

Chlorobenzene as common organic solvent is used. Active layer thickness was 100-

110nm and device area used as 2 mm2. The corresponding devices were fabricated and

the J-V characteristics were taken in dark and illuminated environment shown in fig. 5.4,

5.5 and 5.6.

-4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.01E-9

1E-8

1E-7

1E-6

1E-5

1E-4

Dark Current density Photo Current density Dark Current density(2)

Cur

rent

Den

sity

(A /c

m2 )

Reverse Voltage(V)

MEHPPV :PCBM (1:1)

Fig.5.4(a): Reverse J-V characteristics of the device whose active layer has MEHPPV and PCBM are mixed in 1:1 proportion by weight in Chlorobenzene.


40

-4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0

0

500

1000

1500

2000

2500

Rat

io (P

/D)

Voltage(V)

MEH:PCBM (1:1)


Fig.5.4(b) : The ratio of photo current density to dark current density versus applied voltage

-4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0

1E-9

1E-8

1E-7

1E-6

1E-5


Reverse Voltage(V)

Cur

rent

Den

sity

(A/c

m2 )

MEHPPV :PCBM (1:2)

Fig.5.5(a) : Reverse J-V characteristics of the device whose active layer has MEHPPV and PCBM are mixed in 1:2 proportion by weight in Chlorobenzene.


41

-4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0

0

20

40

60

80

100

Rat

io(P

/D)

Voltage(V)

MEH:PCBM (1:2)

Maximum (P/D) = 89.14 at -3.5V


-4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0

1E-8

1E-7

1E-6

1E-5

Revese Voltage(V)

Cur

rent

Den

sity

(A/c

m2 )

MEHPPV:PCBM(1:4)


Fig.5.6 (a) : Reverse J-V characteristics of the device whose active layer has MEHPPV and PCBM are mixed in 1:4 proportion by weight in Chlorobenzene.


42

-4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0-20

0

20

40

60

80

100

120

140

160

Rat

io(P

/D)

Voltage(V)

MEH:PCBM(1:4)

Maximum (P/D) = 156 at -3.2 V


For the ease of understanding the different results are being summarized in

following Table 5.1.

Current Density

(A / cm2) at -3.5V

MEHPPV: PCBM

(1:1)

MEHPPV: PCBM

(1:2)

MEHPPV: PCBM

(1:4)

Dark Current

Density -1.9036E-5 -2.004E-7 -2.495E-7

Photo Current

Density -8.103E-5 -1.7865E-5 -3.79965E-5

Ratio = P / D 4.25667 89.14671 152.29058

Table 5.1: Comparison of the ratio of photo current density to dark current density of the various devices made using different proportions of the master solution of MEHPPV and PCBM.


43

The tabulated result shows that as the concentration of the PCBM with respect

MEHPPV is increased from 1:1 to 1:2, the ratio of photocurrent density to dark current

density increases by almost 20 times at -3.5V. Further, improvement in the ratio is

achieved by increasing the PCBM proportion to 4 times. As seen from the table, the

absolute value of the photocurrent is decreased as the MEHPPV proportion is decreased.

This has been shown that the function of the MEHPPV is light absorption and formation

of exciton (bound electron-hole pairs), and that of PCBM is the charge separation. It

seems that at blend ratio of 1:4, the MEHPPV acts as the limiting factor in charge

generation, and hence we are observing decrease in photocurrent.

The above improvement in photocurrent to dark current ratio is mainly coming

from the decrease in dark current. As the PCBM concentration is increased (in 1:2 and 1:4

proportions) in the blend the dark current is decreased as compared to the 1:1 proportion.

It suggests that PCBM is better capable of covering the surface roughness in comparison

to MEHPPV. There may be other reasons, which are yet to be explored.

The Fig. 5.4(b), Fig. 5.5(b) and Fig 5.6(b) represent the plot of ratio of photo

current density to dark current density versus applied voltage. The graph gives the

optimal value of the bias voltage for the device to get maximum of ratio (P/D). The ratio

in the device with MEHPPV: PCBM (1:1) is very high as 2324.07 at bias voltage of -

0.6V which is very less. If the leakage current in the device is decreased further at higher

voltages the device with blend proportion of 1:1 can be further explored.


44

5.3.1 An OP-AMP Photodetector Circuit:

The device configuration where MEHPPV: PCBM blend ratio was kept 1:4 is

used in an op-amp photodetector circuit. The idea is to realize similar working of polymer

photodetector in place of inorganic photodetector for practical applications.

A photodetector produces current that is a linear function of the light intensity

incident on it. This current is converted to a voltage by an inverting op-amp in a current-

voltage converter mode. The output voltage depends on the input current. A simple light

sensing circuit consisting of a photodiode and an inverting op-amp is shown in Fig.5.7.

The anode terminal of the detector is connected to the negative terminal of a 6V supply.

To see the output, an organic LED or inorganic LED can be connected to the output pin

of the op-amp. Whenever light is incident on the detector, current is generated and then

converted into desired voltage through the op-amp in a current-voltage converter

configuration. This in turn helps to light up the LED connected to the output.

Fig. 5.7: An OP-AMP Photodetector Circuit. PD: Photodetector, R1= 5kΩ, R2= 10MΩ, Bias = 6V.

+

+12V7

4-12V

6

R1

R2

2

3

Bias

LEDPDLight +

+12V7

4-12V

6

R1

R2

2

3

Bias

LEDPDLight


45

The photographs of the practical demonstration are as shown in Fig.5.8 (a) & 5.8(b) Fig.5.8(a): Light is incident on the polymer photodetector (on left) and the organic polymer LED’s

are being driven through that using op-amp circuit. Fig. 5.8(b): Light is incident on the polymer photodetector (above right) and inorganic LED’s are being driven through that using op-amp circuit. 5.3.2 Rise Time Measurement: The rise time of the polymer photodetector is measured using the arrangement

shown in the Fig. 5.9. The photodetector was irradiated with the blue wavelength

(457nm). The blue light source is connected to the function generator generating a square

waveform of frequency 15 kHz having 50 % duty cycle. The photodetector is biased at -

6V. The output is taken across the resistance R. The value of R is varied from some ohms

to kilo ohms and corresponding rise time graphs are captured from the CRO. They are

shown in Fig. 5.10(a), 5.10(b), 5.10(c) and 5.10(d) respectively. The corresponding

values of R are also mentioned in these figures


46

Fig. 5.9: Rise time measurement setup

Fig.5.10 (a): R = 680 ohms Fig.5.10(b): R = 2.7k ohms

Fig.5.10 (c): R = 6.2k ohms Fig.5.10 (d): R = 12k ohms

Reference Pulse

Output Pulse

Reference Pulse

Output Pulse

Reference Pulse

Output Pulse

Reference Pulse

Output Pulse

To X channel of CRO

Function Generator

To Y channel of CRO

To Y channel of CRO

Bias

PD

Light Sourc

R


47

The rise time corresponding to each of the resistors used is being tabulated in the

Table 5.2 as shown.

Blue light wavelength used for the irradiation of photodetector = 457nm

Resistance Values Rise time values

680 Ω 450 ns

2.7 kΩ 1.915μs

6.2 kΩ 4.255 μs

12 kΩ 6.91 μs

Table 5.2: Rise time values of the photodetector for different values of resistor

The tabulated results are again expressed in graph as shown in Fig. 5.11. The rise

time data points are shown with the circular symbols. The linear fit for the data point is

obtained shown with the continuous line is the Fig. 5.11.

Fig. 5.11: Rise time Versus Resistance characteristics.

0 2000 4000 6000 8000 10000 12000

0.0

1.0x10-6

2.0x10-6

3.0x10-6

4.0x10-6

5.0x10-6

6.0x10-6

7.0x10-6

Data points Linear Fit

Ris

e tim

e (s

)

Resistance (ohms)


48

The equation (5.1) obtained from the graph for the linear fit is given as τ = 0.326 x 10-6 + 0.566 x 10-9 R (5.1) where τ is the total delay time of the device and R value of resistance connected with the

device. Thus comparing it with the equation (5.2) we get,

τ = τint + C R (5.2) Capacitance value C = 0.566 nF and intrinsic time delay τint = 0.32 μsec.

The capacitance value can also be calculated theoretically with the equation (5.3) as given

below

C = (A εo εR) / d (5.3)

Where A is the area of the device, εo is the free space permittivity, εR being relative

permittivity of the material and d is the active layer thickness. Inserting the values we get

C = (0.02 cm2 x 8.854 x 10-14 F/cm x 3) / 1000 x 10-8 cm

C = 0.53 x 10-9 F

C = 0.53 nF

The calculated value of the capacitance from the rise time measurement is 0.566nF and

the theoretical value of the capacitance is the 0.53 nF. Thus calculated value of the

capacitance shows excellent agreement with theoretical capacitance of the device.

5.4 Summary:

The fullerene derivative PCBM was added to the pure conjugated polymer

MEHPPV to get the better photoresponse. Generally the evaporation of the C60 material

over the MEHPPV is done to get the hetero-structured device. To get the blend of the

polymer and fullerene material, a derivative, PCBM was found suitable. The PCBM was

soluble in common organic solvent at room temperature. The concentration of the PCBM


49

in the blend of MEHPPV and PCBM found to affect the device characteristics of the

device with respect to light. With the higher concentration of PCBM in blend, the ratio of

photo current density to dark current density was improved.

The OP_AMP photodetector circuit shows the physical demonstration of the

polymer photodetector in the circuit. The inorganic as well as organic LED’s were driven

by the polymer photodetector. The transient response measurement was done on the

photodetector with different values of load resistors. The rise time measured at 650 Ω

load was 450 ns and the capacitance value calculated was 0.566 nF.


50

Chapter 6 Conclusion and Future Work

In this work photodetectors using polymer material having good electrical and

optical characteristic have been demonstrated. The pure conjugated polymer MEHPPV

and MEHPPV (polymer): PCBM (carbon nano particle) blend have been successfully

utilized as photo absorbing layer in the organic detector.

The pure conjugated polymer MEHPPV is known for its electroluminescence

effect. When the device conditions using electroluminescence layer are optimized for

LED operation, very little photoresponse is observed. The optimizations have been done

in this work, to get low dark current and better photoresponse. It is found that the

variation of the solvent leads to better solubility and film formation. Devices were

fabricated using different organic solvents like chloroform, 1-2-Dichlorobenzene, Xylene

and Chlorobenzene. Out of all these it was found that aromatic solvent, Chlorobenzene,

gives best results in terms of low leakage and film uniformity. Further the variation of the

active layer thickness was done to see the effect on the device characteristics. The device

leakage current was reduced when the active layer thickness was higher; this in turn

increased the ratio of the photocurrent density to dark current density.

Thus, it is shown that even pure conjugated polymer can be used as photodetector

with the optimization of the processing conditions. To further improve the photoresponse

of the device the studies were carried out using MEHPPV and PCBM as blend.


51

The fullerene /C60 derivative (PCBM) was added to the pure polymer MEHPPV.

The configuration of the device was ITO/PEDOT-PSS/MEHPPV-PCBM/Ca/Al. The

variation in the concentration of the PCBM in the blend of the MEHPPV and PCBM

leads to changes in the response of the device towards light. The devices were fabricated

using different ratios of the MEHPPV and PCBM like 1:1, 1:2 & 1:4. It is found that the

device with higher concentration of the PCBM with respect to the MEHPPV (MEHPPV:

PCBM = 1:4) was found to be the best device among all in terms of the ratio of

photocurrent density to the dark current density.

The transient response measurement of the device with MEHPPV: PCBM (1:4) as

photo-absorbing layer was made at different resistive loads. The rise time was measured

as 450 nano-seconds at 680Ω and the calculated capacitance value was 0.53nF.

Future Work:

1. The choice of polymers other than MEHPPV like MDMO-PPV, P3HT can be

employed. Also the electron acceptor material other than carbon nano particles

like CNPPV (other conjugated polymers) can be studied.

2. The operating region of the detector can further be extended to the near IR using

suitable dyes.

3. The photoresponse from the pure conjugated polymers MEHPPV needs better

understanding in terms of charge generation and charge separation phenomenon.

4. The same configuration of MEHPPV: PCBM (1:4) can be used under the

photovoltaic mode also. Further studies are needed in this direction.


52

BIBLIOGRAPHY

[1] C.W. Tang and S. A.Vanslyke, Appl. Phys. Lett., 1987, 51, 913.

[2] S. Forrest, P.Burrows & M.Thompson, featured article, IEEE Spectrum online,

(Aug 2000), 37.

[3] J.Wang, G.Yu, G. Srdanov & A.J. Heeger, Organic Electronics, 2000, 1, 33.

[4] T. Schimmel, M. Schwoerer, H. Naarmann, Synth. Met., 1990, 37(1-3), 1.

[5] S.M. Sze, "Physics of Semiconductor Devices, 2nd Edition," Wiley (1981).

[6] D. J. Milliron, et al., J. of Appl. Phys., 2000, 87(1), 572.

[7] J. S. Kim, et al., Journal of Applied Physics, 1998, 84(12), 6859.

[8] M.T.Rispens, Chem. Comm., 2003, 2116.

[9] W.L.Yu, J.Pei, Chem. Comm., 2000, 68.

[10] T. Q. Nguyen and B. J. Schwartz, J. Phys. Chem. B, 2001, 105, 5153.

[11] B. J. Schwartz, Annu. Rev. Phys. Chem., 2003, 54, 141.

[12] Y. Shi, J. Liu, Y. Yang, J. Appl. Phys., 2000, 87, 4254.

[13] C.J. Brabec et al., Adv. Funct. Mater., 2001, 11, 374.

[14] K. M. Coakley, M. D. McGehee, Chem. Mater. (Review), 2004, 16(23), 4533.

[15] G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science, 1995, 270, 1789.

[16] C. J. Brabec, Solar Energy Material and Solar Cells, 2004, 83, 273.

[17] C.C. Wu, C. I. Wu et al., Appl. Phys. Lett., 1997, 70(11), 1348.

[18] S.Y. Kim et al., J. Appl. Phys., 2004, 95, 2560.


53

Appendix [A]

Surface Treatment of Tin doped Indium

Oxide (ITO)

ITO finds itself suitable for electrode (anode) contact in most of the organic

devices due to its high conductivity, transparency to visible light and higher work

function (~ 4.7 eV). The device usually consists of a sandwich structure with organic thin

film deposited onto ITO (anode) coated glass substrate and covered by patterned top

metal electrode (cathode) contact.

The organic thin film is in direct contact with the ITO and work function of the

ITO is sensitive to the surface conditions hence device characteristics gets affected. To

minimize the problems, the properties of organic material/ ITO surface can be changed or

an intermediate stabilization layer with proper carrier transport characteristics between

ITO and active layer can be introduced.

The two treatments to modify the surface of ITO implemented in this work are

(a) treating the ITO surface by UV-Ozone.

(b) introduction of intermediate layer of PEDOT- PSS between ITO and

active layer.


54

(a) Role of UV-ozone treatment On ITO surface

Since the ITO films are initially patterned using photolithography process,

it is said that there are certain organic contamination on the surface along with

remnant carbon atoms which decreases the charge transfer across the interface.

Studies suggest modifying the ITO surface by various methods, such as

chemical treatment, oxygen plasma and UV-ozone treatment. Among such

surface treatments, UV-ozone treatment is one of the most effective techniques

which in turn increase the work function of the ITO surface [17, 18].

The ITO samples are placed in UV treatment chamber after the proper

cleaning. UV was irradiated using a low- pressure mercury lamp with the

wavelength of 254nm. Oxygen was flowed into the chamber. UV radiations are

absorbed by oxygen, changing it to ozone and nascent oxygen. It is thought that

carbon on ITO surface was oxidized into C-O or C=O during UV –Ozone

treatment followed by pumping out from the UV chamber by using a vacuum

pump.


55

(a) (b)

Fig : A.1 Typical band structure (a) Before ozone treatment ITO( ~4.7 eV) & HOMO of MEHPPV( ~5.2 eV) (b) After ozone treatment ITO (~4.9eV) & HOMO of MEHPPV (~5.2eV).

(b) Role of PEDOT-PSS layer

Another surface treatment includes introduction of PEDOT-PSS (Poly (3, 4-

ethylenedioxythiophene) poly (styrenesulfonate)) layer between ITO and

MEHPPV. The PEDOT-PSS layer performs dual functions in the device

(i) It solves the problem of the inconsistent ITO surface since it acts as a

smoothening interface between rough ITO and active MEHPPV layer

thus preventing shorts between +ve and –ve electrodes, especially if

active layer is thin.

(a) (b)

Fig: A.2 (a) when PEDOT-PSS layer is not present, can lead to short (b) PEDOT-PSS layer is present, thus avoiding short

HOMO Φh Ef

+

ITO MEHPPV

LUMO

Ef Φh

ITO MEHPPV

LUMO

HOMO


56

(ii) It has higher work function than (~5.1 eV) ITO which gives better

alignment of work function with the homo level of active layer for the

polymer.

Fig: A.3 Typical band structure showing alignment of workfuction with the addition of PEDOT –PSS layer.

A Study Of Organic Semiconductor Polymer Material and Device Structures For Application In Optical Detectors

57

Appendix [B]

Programming KEITHLEY SMU and

DMM

The Polymer Devices fabricated are characterized using computer interfaced

Keithley 236 High Voltage Source Measure Unit (SMU) and Keithley 196 Digital Multi-

Meter (DMM). While measuring the photo detector characteristics only SMU is used with

the broad light source with ~ 12 mW power output. Setup is shown as shown in Fig [B.2].

For characterization of polymer LED’s DMM and SMU are used, Fig [B.1] showing the

setup.

Fig. B.1:Typical system configuration to characterize PLED (PD is photodetector).

Keithley 236 SMU

PLED

PD

Keithley 196 DMM

Bias

Data Acquisition Computer

GPIB

A Study Of Organic Semiconductor Polymer Material and Device Structures For Application In Optical Detectors

58

Fig. B.2: Typical system configuration to characterize photodetector.

The Source measure unit and DMM are interfaced to computer using IEEE-488

Bus. These instruments are programmed using Lab VIEW 5.1 software. The experimental

data thus obtained is used for further analysis.

Keithley 236 SMU

Photodetector under Test

Data Acquisition Computer

Broad Light Source

GPIB

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