university of the punjab, lahore,...

University of the Punjab, Lahore, Pakistan. Department of Physics

Ph.D. Thesis

Academic year 2009-2012

Nisar Ali

Synthesis and characterization of metal based Chalcogenide thin films

for solar cell application

Supervisor: Prof. Dr. Mohammad Azhar Iqbal

January 2013

All rights reserved. No part of this publication may be reproduced without the written permission

of the copyright holder.

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DECLARATION

I, Nisar Ali, Ph.D. scholar in the subject of Physics, enrolled in 2009 in the Department of Physics,

University of the Punjab, Lahore, hereby declare that the work submitted in this dissertation

entitled “Synthesis and characterization of metal based Chalcogenide thin films for solar cell

application” is the result of my own investigation, except where otherwise stated. It has not

already been accepted for any degree, and is also not being concurrently submitted for any other

degree. If found otherwise, I will be responsible for the consequences.

NISAR ALI

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CERTIFICATE

This is to certify that the work presented in this dissertation entitled “Synthesis and

characterization of metal based Chalcogenide thin films for solar cell application” has been

performed in the, Material characterization lab. Department of Physics, University of the

Punjab, Lahore, Pakistan, NS & CD National Centre for Physics Islamabad, Pakistan and

joint collaboration with Department of Engineering and Applied Science, Cranfield University,

UK by Nisar Ali, Ph.D. student (Physics) in partial fulfillment of the requirement of the degree of

Doctor of Philosophy in Physics. It is hereby recommended for submission for the award of Ph.D.

degree.

Supervisor Prof. Dr. Muhammad Azhar Iqbal

Department of Physics,

University of the Punjab, Lahore, ______________________

Pakistan.

Chairman Prof. Dr. Shaukat Ali

Department of Physics,

University of the Punjab, Lahore, ______________________

Pakistan.

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DEDICATION

This thesis is dedicated to my parents and the rest of my family members who have supported me

all the way since the beginning of my studies. Also, this thesis is dedicated to my wife and three

sons Mohammad Faheem Khan, Mohammad Haris Khan, Mohammad Muddasir Khan who

missed me a lot during this study. Finally, this thesis is dedicated to all those who believe in the

richness of learning.

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Title

Synthesis and characterization of metal based Chalcogenide

thin films for solar cell application

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ABSTRACT OF DISSERTATION

To reduce the cost of expensive solar cells, new, cheap, nontoxic and more abundant new materials

have been proposed as suitable to use as absorber/window layers in solar cells. One such new

material as absorber layer is tin antimony sulphide. We have attempted two methods for the growth

of these films. A two stage process involving combinatorial sputtering of metallic targets tin and

antimony followed by sulphurization by heating libraries in the presence of elemental sulphur in

vacuum thermal evaporator. The sulphurized films were annealed for 1 hour in sealed quartz

ampoule containing argon gas at low pressure ( 1 atm) at 425°C, 450°C, 475°C, 500°C and 525°C

in tube furnace

The 2nd method of combinatorial tin antimony sulphide thin films deposition is vacuum thermal

evaporator. Thin films of Sn-Sb-S were synthesized on soda lime glass substrate from SnS and

Sb2S3 binary compounds. SnS and Sb2S3 were evaporated in vacuum chamber at 10-4 torr without

substrate heating simultaneously. The films were annealed in argon atmosphere at 85°C, 105°C,

150°C, 275°C and 325°C inside glass ampoules. The elemental composition of the films was

characterized by EDX and the XRD analysis was done for crystallographic phase’s confirmation.

The XRD pattern of combinatorial tin antimony sulphide thin films shows that the as deposited

films are amorphous while the low annealed temperature thin films are poly crystalline.

The optical properties and thickness of the films were measured by ellipsometry techniques.

Electrical properties were calculated from photoconductivity and hot point probe measurement.

The photoconductivity of the library was calculated by photoconductivity spectrometer while hot

point probe was used for the type of conductivity. It was found that Sb2Sn5S9 is a good candidate

for photovoltaic application with a band gap of 1.15-2.5eV, absorption coefficient above 105cm-1,

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transmittance above 700nm and whose conductivity changing from n-type to p-type at high

annealing temperature (325°C).

The effect of air annealing of tin antimony sulphide was also studied in the current study. Tin

antimony sulphide (SnSb2S4) thin films were deposited on glass substrate and annealed thermally

at 150°C, 200°C and 300°C. The 300°C annealed films have good photoconductivity response and

low transmittance. The band gap calculated by ellipsometry technique was found in the range of

2.65eV-1.45eV. The absorption coefficient of the films is ~105cm-1 while the refractive index and

other optical properties of the library presented have good results.

The influence of temperature dependence on Cadmium Sulfide deposited on corning 7059 glass

substrate (150⁰C to 300⁰C) was also studied in this study. Transmittance, absorbance, band gap

and reflectance were obtained by UV spectroscopy. The transmittance for 300nm to 1100nm

thickness was grater then 80%. The resistivity and mobility was calculated by Vander Pauw

method which were 10-80 Ωcm and 2-60 cm2V-1S-1 respectively. The thermoelectric properties of

the film were measured by hot and cold probe method which shows the n-type nature of the film.

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ACKNOWLEDGEMENTS

First and foremost, I thank to Allah, the creator and sustainer of all, who gave me the courage and

power to accomplish this research work. Great attitude is due to the Prophet (S A W) whose holy

teaching inspired me to accomplish this research work in time.

There are a number of people without whom this thesis might not have been written, and to whom

I am greatly indebted.

To my Parents, who continues to learn, grow and develop and who has been a source of

encouragement and inspiration to me throughout my life, a very special thank you for providing a

‘writing space’ and for nurturing me through the months of writing. And also for the myriad of

ways in which, throughout my life, you have actively supported me in my determination to find

and realize my potential, and to make this contribution to our world.

Special appreciation goes to my supervisors, Prof. Dr. Muhammad Azhar Iqbal and Dr. Sayed

Tajammul Hussain, for his supervision and constant support. His invaluable help of constructive

comments and suggestions throughout the experimental and thesis works have contributed to the

success of this research.

I would like to express my appreciation to the ex-chairman Department of Physics Prof.

Dr.Shaukat Ali, chairman Department of Physics Prof. Dr. Muhammad Azhar Iqbal, Ph.D

coordinator Prof. Dr. Mahmood-ul-Hasan for their support and help towards my postgraduate

affairs. I would also acknowledge Prof. Dr. Rashid Ali, Prof. Dr. Munazza Zulfiqar, Prof. Dr.

Fazal Mahmood and the rest of teaching faculty in the Department of Physics University of the

Punjab, Lahore, whose support was unlimited during my stay in the department. My

acknowledgement also goes to all the technicians and office staffs for their co-operations.

To my dear wife and sons Mohammad Faheem khan, Mohammad Haris Khan, who missed

me a lot and remains willing to engage with the struggle, and ensuing discomfort, while I was

accomplishing this research work.

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Special Thanks to Mr. Hamid Mahmood and M. Tufil for being so supportive in my official and

technical assistance during this study.

This work has been supported and financed by Higher Education commission (HEC) of

Pakistan. I would like to express my gratitude to HEC Pakistan for providing me financial support

under HEC indigenous scholarship scheme and International research Support Initiative Program

(IRSIP).

It is also dedicated to Dr. David Lane, Dr. Scilla Roncallo, Mike swallow and the rest of the

staff member in the Department of Engineering and Applied Science, Cranfield University, UK -

who knowingly and unknowingly- led me to an understanding of some of the more subtle

challenges to our ability to thrive.

Loving thanks to my friends / learning partners/ relatives, Mansoor Ahmad, Musarrat Jabeen,

Nisar Ahmad, Gohar Ali, Usman Ali, Barkat Ali, Kamran Khan, Safdar Ali, Salman Khan, Hazrat

Rehman, jamil-ur-Rehman, Rafaqat Hussain, who played such important roles along the journey

toward completion of this thesis, as we mutually engaged in making sense of the various challenges

we faced and in providing encouragement to me at those times when it seemed impossible to

continue.

NISAR ALI

Table of contents

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Contents

Chapter 1 .......................................................................................................................... 1

Chapter 1 ___________________________________________________________ 1

Introduction _________________________________________________________ 1

1.1 Outline of thesis ___________________________________________________ 1

1.2 Introduction ______________________________________________________ 2

1.3 Solar cell parameters: ______________________________________________ 4

1.4 Losses in solar cells ________________________________________________ 4

1.4.1 Loss of low energy and excess energy photons ............................................... 4

1.4.2 Voltage loss ...................................................................................................... 5

1.4.3 Fill factor loss ................................................................................................... 5

1.4.4 Loss by reflection ............................................................................................. 5

1.4.5 Loss due to incomplete absorption ................................................................... 5

1.4.6 Loss due to metal coverage .............................................................................. 5

1.4.7 Recombination losses ....................................................................................... 6

1.5 The band gap and efficiency losses ____________________________________ 6

1.6 Band Gap calculation ______________________________________________ 7

1.7 Solar spectrum ____________________________________________________ 8

1.8 Air mass coefficient AMX _________________________________________ 10

1.9 Solar absorptance and absorption coefficient ___________________________ 11

1.10 Quantum efficiency (QE) _________________________________________ 12

Chapter 2 __________________________________________________________ 13

Literature review ____________________________________________________ 13

2.1 Photovoltaic principles ____________________________________________ 13

Table of contents

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2.2 Equivalent circuit for thin film solar cell ______________________________ 14

2.3 Combinatorial approach to material synthesis __________________________ 17

2.4 Background – Challenges to photovoltaics _____________________________ 18

2.5 Material for solar cell _____________________________________________ 20

2.6 History of thin film solar cell _______________________________________ 24

2.7 Thin film (sulfosalt) tin antimony sulphide (SnSbS) solar cell ______________ 28

2.8 Objective of research ______________________________________________ 32

Chapter 3 ........................................................................................................................ 34

Deposition Techniques _______________________________________________ 34

3.1 The glass substrate ________________________________________________ 35

3.2 Vacuum thermal evaporation technique _______________________________ 36

3.3 DC Magnetron Sputtering __________________________________________ 38

3.4 X-ray diffraction (XRD) ___________________________________________ 41

3.5 Energy dispersive X-ray spectroscopy (EDXS) _________________________ 44

3.6 Scanning Electron Microscopy (SEM) ________________________________ 47

3.7 X-ray fluorescence spectroscopy (XRFS) ______________________________ 49

3.8 Variable Angle Spectroscopic Ellipsometry Techniques (VASE) ___________ 51

3.8.1 Modeling of ellipsometric data _____________________________________ 55

3.9 Photoconductivity measurement _____________________________________ 57

3.10 Hot-point probe measurement ______________________________________ 58

Chapter 4 ........................................................................................................................ 60

Experimental Methods ________________________________________________ 60

Introduction ________________________________________________________ 60

4.1 Calibration of Antimony and Tin targets _______________________________ 60

Table of contents

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4.2 Calibration of Molybdenum target ___________________________________ 63

4.3 Calibration of Tin sulphide (SnS) and antimony sulphide (Sb2S3) targets _____ 64

4.4 Combinatorial thin films deposition __________________________________ 65

4.4.1 DC magnetron sputtering ............................................................................... 65

4.4.2 Vacuum thermal evaporation technique ......................................................... 68

4.5 Annealing of sputtered SnSbS thin films ______________________________ 69

4.6 Annealing of evaporated SnSbS thin films _____________________________ 71

4.7 Synthesis of tin sulphide (SnS) powder ________________________________ 72

4.8 Antimony sulphide (Sb2S3) powder ___________________________________ 73

4.9 Preparation of pallets ______________________________________________ 73

4.10 Sample preparation ______________________________________________ 73

4.10.1 Preparation of SnSb thin films ..................................................................... 73

4.10.2 Preparation of molybdenum thin films ........................................................ 74

4.10.3 Preparation of SnSbS thin films ................................................................... 74

Chapter 5 ........................................................................................................................ 75

Thermal evaporation of Cadmium sulphide thin films _______________________ 75

5.1 Cadmium sulphide thin film window layer _____________________________ 75

5.2 Experimental ____________________________________________________ 76

5.3 Result and discussion _____________________________________________ 76

5.4 Conclusion ______________________________________________________ 85

Chapter 6 ........................................................................................................................ 86

Properties of tin antimony sulphide thin films deposited by sputter coater _______ 86

6.1 Introduction to combinatorial approach for material synthesis ______________ 86

6.2 Results and discussion _____________________________________________ 87

6.2.1 Energy Dispersive X-ray spectroscopy .......................................................... 87

6.2.2 Combinatorial thin film X-ray diffraction ...................................................... 89

Table of contents

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6.2.3 Contour plot ................................................................................................... 93

6.3 Conclusion ______________________________________________________ 97

Chapter 7 ........................................................................................................................ 98

Combinatorial synthesis of Sb2Sn5S9 thin films by thermal evaporation _________ 98

7.1 Metal chalcogenide thin films _______________________________________ 98

7.2 Experimental ____________________________________________________ 98

7.3 Results and discussion ____________________________________________ 100

7.3 Conclusion _____________________________________________________ 113

Chapter 8 ...................................................................................................................... 114

Effect of air annealing on the properties of SnSbS thin films _________________ 114

8.1 Air annealing ___________________________________________________ 114

8.2 Experimental ___________________________________________________ 114

8.3 Results and analysis ______________________________________________ 115

8.3 Conclusion _____________________________________________________ 123

Chapter 9 ...................................................................................................................... 124

Conclusions _______________________________________________________ 124

9.1 Conclusions ____________________________________________________ 124

9.2 CdS thin films as window layer _____________________________________ 124

9.3 Sputtering of SnSb metallic thin films and its sulphurization ______________ 125

9.4 Preparation and analysis of argon annealed SnSbS thin films by thermal evaporation

technique _____________________________________________________ 125

9.5 Preparation and analysis of air annealed SnSb2S4 thin films by thermal evaporation

technique _____________________________________________________ 127

Future Work _______________________________________________________ 128

List of Publications _________________________________________________ 129

Table of contents

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Journals __________________________________________________________ 129

Conferences and workshops __________________________________________ 130

References ________________________________________________________ 131

List of figures

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LIST OF TABLES Page

Table-2.1 Global energy scenario 23

Table-2.2 World total energy consumption 24

Table-2.3 Development and installation of photovoltaic electricity in various

Countries 24

Table-2.4 US patents on CIS solar cell material 26

Table-2.5 year wise efficiencies of best solar cell 29

Table-2.6 Visible spectrum range 31

Table-2.7 Energy conversion efficiencies achieved in the laboratory for

different photovoltaic thin-film technologies 32

Table-3.1 Elements profile present in glass substrate 35

Table-3.2 EDS parameters 44

Table-4.1 Antimony (Sb) target calibration, Density=6.62 g/cc, Z- ratio=0.

Purity=99.999% 61

Table-4.2 Tin (Sn) target calibration, Density=7.2g/cc, Z-ratio=0.724,

Purity=99.998% to 99.999% 61

Table-4.3 Molybdenum (Mo) target calibration, Density=10.28g/cc,

Purity=99.998% to 99.999% 63

Table-4.4 SnS and Sb2S3 target calibration 64

Table-4.5 Power and evaporation rate for Tin and Antimony Targets 68

Table-4.6 Sulphurization conditions for Sn Sb and SnSb 68

Table-5.1 Films parameters deposited by thermal evaporation 78

Table-5.2 Resistivity Vs. thickness 80

Table-6.1 Elemental composition (Atomic percentage) 87

Table-7.1 Elemental composition (atomic percentage) 100

List of figures

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LIST OF FIGURES

Page

Figure-1.1 Band gap calculation for SnSbS thin films (our work) 8

Figure-1.2 The standard AM1.5 global solar spectrum 9

Figure-1.3 Air mass coefficient calculation 10

Figure-2.1 diffusion length for p-type absorbing layer 14

Figure-2.2 Equivalent circuit for thin film solar cell 15

Figure-2.3 Schematic of superstrate and substrate solar cell configuration 16

Figure-2.4 Equivalent Circuit of (a) Ideal Solar Cell (b) Practical Solar cell 17

Figure-2.5 Comparison of quantum efficiency of (a) crystalline silicon (b) CZTS

(c) CdTe (d) α-Si (e) SnSbS solar cells (our work) 22

Figure-2.6 Actual and planned PV production capacities of different solar cell

technologies 27

Figure-2.7 Solar cell efficiencies (NREL) 30

Figure-3.1 Vacuum thermal evaporation (a) Operation (b) Camera Photograph 36

Figure-3.2 Magnetron sputtering schematic 39

Figure-3.3 XRD illustrates diffraction for different orientations resulted from

the plains in different grains 42

Figure-3.4 Thin Film Analysis by X-Ray Scattering 42

Figure-3.5 Philips PW1820 diffractometer camera photograph 43

Figure-3.6 Components of EDS system, The charge pulse from sample is converted

in the free amplifier to an electric signal 46

Figure-3.7 Photograph of Energy Dispersive X-ray spectrometer 46

List of figures

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Figure-3.8 Schematic of a SEM 48

Figure-3.9 Electronic process in XRF 49

Figure-3.10 XRF Experimental set up (Rutherford Lab, Cranfield University UK) 50

Figur-3.11a. Schematic setup of an ellipsometry experiment 52

Figure-3.11b. Incident linearly polarized light of arbitrary azimuth θ is reflected

from the surface S as elliptically polarized 52

Figure-3.12 Camera photograph of Ellipsometer 53

Figure-3.13a Modeling of experimental curve 56

Figure-3.13b Modeling of Sb5Sn2S9 thin film annealed at 300°C ellipsometry

data 56

Figure-3.14 Photoconductivity measurement set up 57

Figure-3.15a Hot-point probe measurement set up for thin films 59

Figure-3.15b Photograph of Hot-point probe setup 59

Figure-4.1 Antimony (Sb) target calibration 62

Figure-4.2 Tin (Sn) target calibration 62

Figure-4.3 Molybdenum target calibration 62

Figure-4.4 Targets and substrate settings for combinatorial growth of SnSb

metallic thin film 66

Figure-4.5 Vacuum Thermal Evaporation schematic 69

Figure-4.6 Quartz ampoule containing sputtered SnSbS libraries 70

Figure-4.7 Glass ampoule containing SnSbS thin film libraries 71

Figure-4.8 Glass ampoule containing SnS ingots 72

Figure-5.1 X-ray diffraction pattern for CdS films (top film 1, down film 2) 77

Figure-5.2 Effect of substrate temperature on mobility 78

Figure-5.3 Effect of substrate temperature on resistivity 79

List of figures

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Figure-5.4 Effect of evaporation rate on the electronic properties 79

Figure-5.5 Temperature Vs Thermal EMF for CdS/Glass thin film 81

Figure-5.6(a) Transmittance Vs wavelength for substrate 82

Figure-5.6(b) Transmittance Vs wavelength for film 82

Figure-5.6(c) Transmittance Vs wavelength for film 82

Figure-5.6(d) Absorbance Vs wavelength for film 82

Figure-5.6(e) Absorbance Vs wavelength for film 83

Figure-5.7 Reflectance Vs wavelength for CdS thin films 83

Figure-5.8 Comparison of transmittance Vs Glass Substrate and film 84

Figure-5.9 Band gap calculation of cadmium sulfide thin film 85

Figure-6.1 EDS study of the SnSb combinatorial films 87

Figure-6.2 EDS study of the SnSbS combinatorial films (a) 425°C annealed

(b) 450°C annealed (c) 475°C annealed (d) 500°C annealed (e) 525°C

annealed 88

Figure-6.3 EDS plots of selected points 89

Figure-6.4 XRD set up for combinatorial analysis 90

Figure-6.5 (a) Combinatorial SnSbS thin film XRD patterns annealed at 425°C

for 1 hour. (b) Combinatorial SnSbS thin film XRD patterns annealed

at 450°C for 1 hour (c) Combinatorial SnSbS thin film XRD patterns

annealed at 475°C for 1 hour (d) Combinatorial SnSbS thin film

XRD patterns annealed at 500°C for 1 hour (e) Combinatorial

SnSbS thin film XRD patterns annealed at 525°C for 1 hour 91

Figure-6.6 (a) Contour combinatorial SnSbS thin film XRD patterns annealed

at 425°C for 1 hour. (b) Contour combinatorial SnSbS thin film

XRD patterns annealed at 450°C for 1 hour. (c) Contour

combinatorial SnSbS thin film XRD patterns annealed at

475°C for 1 hour. (d) Contour combinatorial SnSbS thin film

XRD patterns annealed at 500°C for 1 hour.

(e) Contour combinatorial SnSbS thin film XRD patterns annealed

at 525°C for 1 hour 92

List of figures

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Figure-6.7 Photoconductivity response of SnSbS thin films (a) 425°C annealed

(b) 450°C annealed (c) 475°C annealed (d) 500°C annealed (e) 525°C

annealed 94

Figure-6.8 SEM images of SnSbS library of selected points at different annealing

temperature 96

Figure-7.1 Schematic for combinatorial SnSbS evaporator 99

Figure-7.2a SnS ingot EDX 99

Figure-7.2b SnS ingot XRD 99

Figure-7.3 EDX study of one of the library 100

Figure-7.4a XRD pattern of SnSbS combinatorial thin films 102

Figure-7.4b Sn0.7Sb0.3S1.45 XRD pattern 103

Figure-7.4c Contour graph of XRD annealed at different temperature 103

Figure-7.5 Photoconductivity measurement of Sb2Sn5S9 combinatorial thin

films library annealed at different temperature 105

Figure-7.6 Optical transmittance spectra of the Sb2Sn5S9 libraries 107

Figure-7.7 The dependence of absorption coefficient of Sb2Sn5S9 thin films

Annealed at different temperature with the wavelength of the incident

Photon 108

Figure-7.8 Variation of refractive index of Sb2Sn5S9 thin films of different elemental

composition with wavelength at different annealing temperature 109

Figure-7.9 Variation of band gap of Sb2Sn5S9 thin films of different elemental

composition with wavelength at different annealing temperature 111

Figure-7.10 Hot point probe measurements of selected points annealed at

different temperature 112

Figure-7.11 Hot point probe of n-type silicon used for calibration 113

Figure-8.1 Elemental composition of as it is Sn0.25Sb0.5S0.5 115

Figure-8.2 XRD of tin antimony sulphide thin films air annealed.

(A) 150°C annealed (B) 300°C annealed. 116

Figure-8.3 Photoconductivity response of the as it is and annealed films 118

List of figures

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Figure-8.4 Filament lamp output data 118

Figure-8.5 Transmittance Vs. wavelength 119

Figure-8.6 Refractive index n versus wavelength 120

Figure-8.7 Absorption coefficient of Sn-Sb-S 121

Figure-8.8 Dependence of extinction coefficient on the wavelength and

annealing conditions 121

Figure-8.9 Energy band gap calculation of SnSbS thin films (a) as it is

(b) 150°C air annealed (c) 200°C air annealed (d) 300°C air annealed 122

Figure-8.10 Refraction Vs. wavelength 123

List of symbols and abbreviations

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LIST OF SYMBOLS AND ABBREVIATIONS

Å Angstrom

α Absorption coefficient

AM Air mass

a-Si amorphous Silicon

a-Si:H amorphous silicon hydrogenated

c-Si Crystalline Silicon

CBD Chemical Bath Deposition

CdS Cadmium sulphide

CdTe Cadmium telluride

CIS Copper indium selenide

CIGS Copper indium gallium diselenide

CZTS Copper Zinc Tin Sulphide

DC Direct current

EDS Energy Dispersive Spectroscopy

Eg Band gap energy

eV Electron volt

FWHM Full Width Half Maximum


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FIR Far Infra Red

h Plank constant

Hz Hertz

IR Infra Red

J Joule

Kw Kilo watt

MCA Multichannel analyzer

MW Megawatt

n Refractive index

NREL National Renewable Energy Laboratory

nm Nano meter

PECVD Plasma enhanced chemical vapor deposition

PLD pulsed Laser Deposition

PV Photovoltaic

QE Quantum Efficiency

Sb2S3 Antimony Sulphide

SEM Scanning Electron Microscopy

SnSbS Tin antimony sulphide

SnSbSe Tin antimony selenide

SnS Tin Sulphide

Si Silicon

TCO Transparent Conducting Oxide

Tw Terawatt


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UV Ultra Violet

VASE Variable angle spectroscopic ellipsometry

XRD X-Ray Diffractometer

XRF X-Ray Fluorescence

ZnS Zinc Sulphide

µm Micrometer

Chapter 1 Introduction

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Chapter 1

Chapter 1

Introduction

1.1 Outline of thesis

The chapter 1 is an introduction to thin film solar cells. This explains the basics of solar

cells along with the physics related to solar cells and some idea about the absorbing layers

used in solar cells.

The literature review is discussed in chapter 2.

Chapter 3 includes the techniques used in the deposition of thin films along with the

analytical techniques used for the characterization of tin antimony sulphide thin films of

the current research. In chapter 4 experimental setup was discussed which includes the

calibration of machines and the deposition of combinatorial SnSbS thin films.

Chapters, 5, 6, 7 and 8 are the results and discussion part of the experimental work. The

properties of cadmium sulphide thin films were discussed in chapter no 5. In chapter 6,

the physical properties of tin antimony sulphide thin films deposited by sputter coater

were analyzed. The effect of argon annealing and air annealing are studied in chapter 7

and chapter 8 respectively. Finally, conclusion and future plans for research and

investigation in the current material were discussed in chapter 9.


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1.2 Introduction

The major problems of 21st century are energy, health and environment. These three

problems are interrelated with each other to a great extent. In other words the processing,

distribution and harvesting of various energy sources have major environmental and

health implications (Galarraga, Markandya et al. 2011). These implications can be

minimized by using a clean source of energy and electric energy obtained directly from

the sun via photovoltaic effect is one of the prime contributors in getting clean energy

which leads to “new energy” future.

The demand of energy is constantly growing up. In 2008, 474×1018J of energy was

consumed globally with 80% to 90% contribution of fossil fuel. This number decreases

1.1% in 2009 due to financial crisis. And the demand of energy in 2035 will be 7.247×1018

J and 30TW is estimated for 2050 (Zweibel 2005). In order to achieve these goals we have

to explore alternative sources of energy as the existing reservoirs will not be enough in

future. We have to develop renewable sources of energy whose raw materials should be

cheap and abundant. Solar energy, wind energy, biomass energy, geothermal energy and

hydro energy are the major sources of renewable energy. Among these energy sources,

solar energy has enough potential to meet the major fraction of the world energy needs

(Umweltveranderungen 2004). Photovoltaic industry provides a viable option of such

energy production. In this connection conventional solar cells were developed using

different semiconductor materials. In conventional solar cells silicon and other

semiconductor materials were used as absorbing material. Such materials were difficult

to purify and also large quantity of materials was needed.

This conversion of sunlight into electricity can be done with solar cells, also called

photovoltaics. French physicist Henri Becquerel first discovered photovoltaic effect in

1839. However the 1st solar cell was built by Charles Fritts in 1883 by coating the

semiconductor selenium with an extremely thin layer of gold to form the junctions. The

electron-hole pairs are generated from the suitable incident photons inside the absorbing

layer of the solar cell which is separated by the built-in potential of the p-n junction in the

material to make a current flow (Ngô and Natowitz 2009). On the average 100W more


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energy per 30 minute is used to make our life more comfortable and electrical energy is

the premium form of energy as it is used in nearly every ongoing phenomenon in progress

globally (Crane, Kinderman et al. 2010).

The use of electrical energy has much importance in our life. Renewable and non-

renewable energy resources can be transformed into electrical energy by different

methods. Since there are many unpleasant side effects related with the non-renewable

energy sources, therefore in 21st century the researcher and scientists tuned their research

toward a renewable energy which has no unpleasant side effects and photovoltaic industry

causes revaluation in the world energy market. The fuel for solar energy is abundant and

frees (Würfel 2008). The electricity produced by photovoltaic technology is very clean

and is maintenance free.

The photovoltaic devices are very docile. They include no mechanical parts and therefore

produce no sound. The priority option for research in converting solar radiation into

electrical energy emerges basically from early 1970’s Middle East oil crisis which

stimulate the researchers to think about alternate energy source (Shwadran 1977).

Research in Photovoltaic is not only the subject of interest of scientists and engineers but

also of public interest.

If 19th century is declared as the century of coal and the 20th century as of oil then

definitely 21st century will be the age of solar energy. Solar energy is set to play an

everlasting role in generating the form and affecting the appearance. PV systems are the

key element in production of electricity directly from solar radiation and are going to

spread widely as their advantage become apparent and coat fall. Silicon is not an ideal

material for solar cell because of its lower absorption power of solar radiation. Theoretical

efficiency of silicon solar cell is 30% while in solar cars silicon solar cell with efficiency

of the order of 25% is used (Goetzberger and Hoffmann 2005). For efficient absorbance,

a thick layer of silicon is required. The reason behind is its indirect band gap. So therefore

a thick material is needed for high absorption. In order to reduce the material (in quantity

and cost as well) the research for more suitable material was started which should be used


Solar cell Page 4

as thin films in solar cells. Such material should be characterized by a direct band gap

and give very strong light absorption.

The renewable energy resources are fossil fuels, renewable resources and nuclear

resources (Demirbas 2000). These are resources of energy which can be used for energy

production time and again often called alternative source of energy (Rathore and Panwar

2007), e.g. wind, solar, biomass, geothermal etc. It is of highly concerned that solar

energy is one of the most abundant forms of energy available in direct and indirect form.

The earth intercepts 1.8×1014 KW power out of the total power of sun 3.8×1023KW which

has a vast scope of utilization in heating and cooking.

1.3 Solar cell parameters:

The solar cell parameters, measured from current-voltage (I-V) characteristics is of

essential importance for the quality control and evaluation of the solar cells. Several

authors proposed methods to devise ways for extracting these parameters which describe

the non-linear electrical nature of solar cells. These parameters are (Chegaar,

Ouennoughi et al. 2003) Series resistance, Saturation current, Ideality factor, Shunt

resistance and photocurrent.

1.4 Losses in solar cells

The efficiency of solar cell is associated directly with the conversion of photo energy into

electrical energy. The losses in solar cells may be optical or electrical losses. Optical

losses are caused by reflection, shadowing and non-absorbed radiation. While the cause

of electrical loss may be ohmic or recombination losses. Semiconductor material, base-

emitter contact material metal junction are the cause of ohmic losses while recombination

losses occur in emitter region material, surface base region material and surface space

charge region. The photon energy losses occur due to the following factors (Solanki

2011).


Solar cell Page 5

1.4.1 Loss of low energy and excess energy photons

Photons with energy incompatible with the band gap will be absorbed giving rise to

phonons or passes straight away without generating electron hole pairs, the ingredient of

solar cell. For single junction solar cell, the transmission loss is about 23% while the

thermalization loss is 33%.

1.4.2 Voltage loss

Band gap voltage loss is referred as voltage loss inside the solar cell. This voltage loss

(gE

q) is due to unavoidable intrinsic Auger recombination. The ratio of solar cell voltage

Voc and voltage loss oc

q

V

E / qmust be in the range of 0.65 to 0.72.

1.4.3 Fill factor loss

The fill factor of a best solar cell is found to be 0.89. The fill factor loss is because of

parasitic resistance of the cell. Parasitic resistance arises due to series and shunt resistance

of the cell.

1.4.4 Loss by reflection

Some incident photons are reflected from the top layer of the solar cell. Surface texturing

and anti-reflecting coating will minimize the reflecting problem.

1.4.5 Loss due to incomplete absorption

The photons which are not compatible with the band gap are absorbed in solar cell

elsewhere. It may be absorbed due to limited thickness of solar cell. This loss can be

minimized by using appropriate light trapping schemes.

1.4.6 Loss due to metal coverage

The front metal contact shadows some light (nearly 10%) and serve as metal contact

coverage loss. This problem can be overcome by using suitable TCO.


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1.4.7 Recombination losses

Due to defect in crystals or even at interfaces, some electron-hole pairs recombine and

give rise to a phonon and are lost. Such loss due to recombination centers may be

minimized by using defect free polycrystalline layers.

1.5 The band gap and efficiency losses

The efficiency of solar cell is strongly dependent on the band gap of semiconductor

material. The band gap of semiconductor material can be tailored by doping concentration

and particle size control. In direct band gap semiconductors light absorption is much

efficient as no third particle is needed for the excitation of electron from valance band to

conduction. Si and Ge is indirect band gap for which electron cannot be excited directly

to the conduction band. In such semiconductors two particles (photon and phonon) are

involved in generation and recombination process (Twidell and Weir 2006). In the direct

band gap semiconductors (e.g. Cu2ZnSnS4, SnSbS, CdTe, CdS, etc) absorption of photon

occurs without lattice interaction and therefore have sharp absorption band transition with

large value of extinction coefficient (ν > Eg/h). For indirect semiconductor materials the

above effect is reversed. For semiconductors e.g. Si [Eg=1.1ev] band gap absorption

occurs at a frequency ν > Eg/h i.e.

15-19(1.1eV)(1.6×10 j / eV)

= 0.27 10 hz-346.634×10

1.1

And 8

15

3 10 / sec 1.1 m

0.27 10

mµ

1.2

As photon flux in solar spectrum is ~ 3×1021photon/m2.sec, the absorption of solar

radiation by semiconductors greatly increase exciton generation apart from thermal

generation.

The photovoltaic cells are limited in efficiency by many losses in which some are

avoidable and the rest are intrinsic to the system. Using proper dopant qualitatively and

quantitatively, the properties of semiconductors can be enhanced. In solar cell


Solar cell Page 7

configuration top surface contact obstruction provide ~ 3% loss during current transfer

from solar cell to the circuit, reduce the performance of the cell as well (Solanki 2011;

Rau, Abou-Ras et al. 2011). It is observed that 40% of solar radiation is reflected from

semiconductor. This reflection can be reduced to 3% dramatically by thin film surfaces.

1.6 Band Gap calculation

The forbidden energy gap in semiconductor materials between valance band and

conduction band is fundamentally important in studying the properties of solid. Most

properties of the material such as intrinsic conductivity, optical transitions, depend on it.

Any change in the band gap may lead to significant variation in the chemistry and physics

of the materials.

The basic transition involved at the fundamental edge of crystalline semiconductor

materials are direct and indirect transitions. Both types of transitions are due to the

interaction of an electromagnetic radiation with electron in a valance band. This electron

is raised to the conduction band across the forbidden gap. However indirect transitions

also involve simultaneous interactions with lattice and impurity centers. Thus the wave

vector of electron can change the optical transition of the semiconductor (Rangarajan

and Rangarajan 2004).

For direct transition

αhυ ≈ A(hυ-Eg)n 1.8

Here α is the absorption coefficient of the material, h is a plank constant and n is a constant

equal to 1/2 for direct band gap and 2 for indirect band gap semiconductors (Orlianges,

Champeaux et al. 2011). The value of band gap can be calculated by extrapolating the

linear portion of (αhν)2 vs. hν to the horizontal axis (Fig 1.1).

For more than one layer of thin film, (amorphous and crystalline) constant optical band

gap is assumed.


Solar cell Page 8

Fig-1.1 Band gap calculation for SnSbS thin films (our work)

Theoretical models are also available for the calculation of electronic band structure. Dv-

Xα method is much successful for studying transition metal compounds. Dv-Xα model is

a linear combination of atomic and molecular orbital. This model is useful in explaining

the electronic states and energy level distribution in short computing time. This model is

also helpful for complexes and large clusters (Kim and Jiang et al. 2000).

1.7 Solar spectrum

The solar spectral irradiance consists of 200nm–2400nm laying from ultraviolet region

to far infrared (fig 1.6). The objective of our research is to comprehend the whole solar

spectrum for solar cells. The sun releases a huge capacity of energy enough for human

needs. The sun supply energy to our planet at the rate of 1.37KW/m2 with output power

of 3.86×1020MW each second. Though the output power of the sun is larger than the

above quoted values, the rest of light energy and spectral distribution that arrives the

Earth’s surface is significantly altered by atmospheric absorption and scattering (Peippo

1992).


Solar cell Page 9

The characteristic fingerprint to the solar radiation spectrum is shown in figure 1.2. The

graph in the figure relates the spectral irradiance with wavelength. The total yearly solar

irradiance is also a function of territories and is different for different region of the globe

(Thuillier, Herse et al. 2003).

Figure-1.2 Standard AM1.5 global solar spectrum

The current research in thin film solar cells is based on the engineering of band gap of

different layers in such a way that a wide spectrum of photon energies is utilized for

photovoltaics. The higher band gap of window layer allows these photons to reach the

absorbing layer and the small band gap of the absorbing layer will capture all the photons

for the production of excitons.


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1.8 Air mass coefficient AMX

The air mass coefficient is a typical method for the characterization of the performance

of solar cell under standard conditions referred to as AM (air mass). For the

characterization of solar panels a universal air mass coefficient AM 1.5 is used. Air mass

intensity (AM) is used to denote the ratio of the optical path length L to a normal path

length Lo at sea level on a clear day (figure 1.3). AM for a path length L through the

atmosphere, incident at angle θ relative to the normal to the Earth's surface is (Würfel

2008).

o

L 1AMx = =

L cosθ 1.3

Where Lo is the zenith path length (i.e. normal to the Earth's surface) at sea level and θ

is the zenith angle in degrees. AM 1.5 is the air mass coefficient at a zenith angle of

48.2°. Thus the air mass number is different at different time of the day and different

seasons of the year.

Figure-1.3 Air mass coefficient calculation


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1.9 Solar absorptance and absorption coefficient

The ability of the material to convert the incoming solar radiation into heat is solar

absorptance. Solar absorptance is sensitive to the micro structure of the material as well

as surface morphology. Metal particles which are compatible to the wavelength of solar

radiation distributed in transparent thin films greatly enhance the solar absorbance of the

films. Al-Cu-Fe is nearly an ideal absorber with high absorbance value at short

wavelength and poor absorbance at higher wavelength (above 2nm). Pores in the material

interior structure scatter light and transform a transparent medium into highly absorbing

medium in thin films. The lower density regions between grains which serve as void

network and are equivalent to pores which increase the absorbance as well. These low

density regions inside thin films scatter the incoming photons and increase its path length

inside thin films which enhance its probability of absorbance. The annealing effect of the

films adjusts the void network of the films which control the transmittance or absorbance

of thin films (Chang, Hsieh et al. 2012, Kameya and Hanamura 2011).

Thin films deposited by evaporated techniques have absorption coefficient 10 times larger

than the bulk. However the films deposited by energetic ion beams eliminate some of the

low density regions and therefore have same properties as that of bulk (Machlin 2005).

If I(o) is the intensity of the incident ray traversing a distance x inside an absorbing

medium and I(x) is the intensity of the traversed ray (Hecht 1988). Then

I(x) = I(o)e−αx 1.4

Where α is the absorption coefficient of the material. From the imaginary part of the

refractive index

α= 4πk/λ 1.5

Here λ is the wavelength of the incident radiation and k is the extension coefficient of the

material can be calculated from ellipsometry.


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1.10 Quantum efficiency (QE)

Quantum efficiency is an important parameter in characterizing solar cell materials. It is

the ratio of free exciton generated by photons to the number of available photons.

Collected electron hole pairs J / qQE( )

photon flux

1.6

Current out

Photon flux ( )

1.7

Experimentally monochromatic beam of light with flux density ϕ (cm-2s-1) of different

wavelength range (300nm-1200nm in our case) is scanned for QE response. The QE data

can estimate

(a) Thickness of the layer (Davies, Sites et al. 2007)

(b) Minority carrier diffusion length (Liu and Sites 1994)

(c) Form of the collected function by changing chopped frequency and light

wavelength (Nagle and University 2007).

The optimum value of the QE is 1 (100%) which is an ideal value. There are always

losses due to different layers and interfaces in solar cell. These layers and their

interfaces absorb and scatter light before reaching the absorber layer and thus

quantum efficiency is reduced.

The optical losses such as reflection affect the QE and if reflection losses are

considered, the measured QE will be external quantum efficiency (EQE). In most

cases, the light left over the transmitted and reflected light is the measure of QE

known as internal quantum efficiency (IQE) (Lu 2008).

Chapter 2 Literature review

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Chapter 2

Literature review

2.1 Photovoltaic principles

Photoelectric effect is the basic for photovoltaic effect which was observed by Plank and

Albert Einstein in the beginning of 20th century. The absorption of light in semiconductor

takes place via excitation of electron from valance band to conduction band. When an

electron is excited to the conduction band, it leaves behind a free state called hole. The

electrons in conduction band spend some time until it recombines with the hole in the

valance band. Appropriate combinations of materials separate these electrons spatially

from the holes and electric contact direct them into power circuit.

In order to excite an electron to the conduction band, a photon energy hν must be greater

than or equal to the band gap energy Eg. i.e.

hν ≥ Eg 2.1

In case of hν < Eg, photon will be reflected or transmitted by the material.

The probability of absorbing a photon energy hν by a material is defined by the absorption

coefficient α(hν). It is the property of the material and is independent of the geometry of

the material. It is proportional to the density of filled states in valance band and empty

states in conduction band.

For a distance from the surface L = 1/ α(hν), the intensity is attenuated by a factor of e and

1/α is known as penetration depth of photon denoted by Lγ. Direct band gap

semiconductors like GaAs, SnSbS, CdTe, ZnTe etc have large absorption coefficient and

small penetration depth. A couple of micrometer thick layer of such direct band gap

materials will absorb all the absorbable part of the spectra with photon energy hν > Eg.

The schematic of a free exciton generation is shown below. A photon of energy hν is

absorbed after traversing a distance Lγ in the material and is converted to exciton.


Solar cell Page 14

Figure-2.1 diffusion length for p-type absorbing layer

2.2 Equivalent circuit for thin film solar cell

Solar cells are fabricated in superstrate and substrate configurations. In substrate

configuration the substrate is opaque. Flexible opaque polymers and metal foils are best

substrates in this configuration. The front side of this cell is encapsulating by a transpiring

capping layer which serve as a front contact. The light enters through capping layer into

the solar cell.

In the superstrate configuration, different layers are grown on glass substrate and the light

enters through the glass substrate and reaches the cell. Both configurations are given in

the figure 2.3.

Superstrate Solar cell with typical glass/TCO/CdS/CZTS/metal along with band diagram

is also depicted in the figure 2.2 and 2.3. Spectral photons striking the solar cell will be

absorbed, reflected or transmitted. The absorbed photons by the absorbing layer will be

transferred to electron hole pair. The built- in potential, which is the difference in Fermi

level between p-type and n-type materials, will separate electrons and holes for electric

power. The built-in potential depends on the acceptor and donor impurity concentration

as well as the temperature.


Solar cell Page 15

A Dbi 2

i

N NKTV = ln ( )

q n 2.2

The equation for current in pn-junction diode under dark condition is given by

eV

mKToI = I (e -1) 2.3

Where Io is the reverse saturation current and m is the diode quality factor.

Figure-2.2 Equivalent circuit for thin film solar cell


Solar cell Page 16

Figure-2.3 Schematic of superstrate and substrate solar cell configuration

Under illumination, the photons are absorbed and create exciton within the diffusion

length of the depletion region. The built-in potential sweeps these excitons and generation

current is produced. With generation current equation 1.3 become

eV

mKTL oI = I - I (e -1) 2.4

When the effect of contact resistances (shunt and series) and ohmic losses are taking into

account. Then the equation for solar cell (fig-2.2) can be written as

se(V+IR )

smKTL o

P

V + IRI = I - I [e -1] - 2.5

R

Rs and Rp are the extrinsic behavior of solar cell which can be adjusted for efficient solar

cells. Small parallel resistance will reduce open circuit voltage Voc and fill factor FF,

while a large series resistance will reduce fill factor as well as short circuit current Isc.

When the cell is short circuit with zero external voltage, the junction is still biased with a

voltage produced by the passage of Isc through shunt resistance Rp i.e. IscRp. Practically

IscRp must not be greater than 25mV (Lorenzo 1994).


Solar cell Page 17

Figure-2.4 Equivalent Circuit of (a) Ideal Solar Cell (b) Practical Solar cell

2.3 Combinatorial approach to material synthesis

Many deposition techniques are consistently being used for thin films such as thermal

evaporation, electron beam evaporation, chemical bath deposition, and many more

chemical deposition techniques (Wan, Bai et al. 2010). Among many metallic

combinatorial deposition techniques for thin films, Sputtering is found to be a sound

technique in term of quality of films and controlling different parameters (Wasa,

Kitabatake et al. 2004). Combinatorial deposition refers to techniques to fabricate, test,

and store the resulting data for a material library containing tens, hundreds or even

thousands different materials or compounds.

The term combinatorial or more specifically “combinatorial chemistry” was employed

for the 1st time by Merrifield while studying peptides (Merrifield 1963; Yanase, Ohtaki

et al. 2002). Since then this technique has been used for the characterization of organic

as well as inorganic materials. The concept behind the combinatorial synthesis is to study

a variation of a specific effect or property across a chemical composition. Combinatorial

study is assumed to be an efficient way for exploring the optimal value of best elemental

composition, which can have high performance in a sequence of material synthesis and

characterization. The study of combinatorial deposition will explore nearly all the

elemental composition in a single slide. This method is expected as a novel technique

for research in thin films. Plenty of work has been devoted in the past for the analysis of


Solar cell Page 18

a particular compound with different elemental composition but is time consuming job

due to conventional approach. The combinatorial deposition allows us to analyze a

particular compound even in a single stage (Bob, Anzelmo et al. 2001). We make special

arrangement in the sputter coater (dc magnetron sputtering, Kurt J Lesker) as well as in

thermal coater for the deposition of SnSb metallic combinatorial thin films. These

arrangements will be discuss later on.

2.4 Background – Challenges to photovoltaics

The demand of energy has been increasing day by day and it has been predicted that

28TW of energy will be required in 2050. The conventional sources of energies available

are fossil fuel and nuclear energy. These energy sources are depleting at a constant rate

and also pollute the environment (Prakash and Garg 2000). Sun supplies a tremendous

amount of energy powering atmosphere, oceans and every cycle of life. The annual

energy consumption on earth (4.6×1020J) is supplied by sun in one hour which is

approximately 104 times larger than its use. This inferred the worth of available solar

energy on earth available at about 1KW per square meter. This area is quiet dilute and

require sufficiently large area of energy conversion (Sukhatme and Nayak 2008).

Therefore the requirement of an efficient solar cell will be crucial for such energy

conversion in order to meet the world energy consumption.

The attempts for the fabrication of solar cell are categorized in three generations.

1st generation: crystalline silicon solar cells

2nd generation: thin film polycrystalline solar cells

3rd generation: organic and dye sensitized solar cells

The 2nd generation solar cells which are based on high potential thin films technology is

of promising interest due to its fabrication techniques on low cost substrates and cheap

material usage. The market share of amorphous silicon (a-Si) solar cell was 5% and CdTe,

Cu(ln,G)(Se,S)2 solar cells has 7% in 2010 (Chapin, Fuller et al. 1954; Gordon, Carnel


Solar cell Page 19

et al. 2007). For the operation of 100W bulb, the solar power falling on 1m2 area must at

least be utilized. Different solar cells are used for the conversion of this light energy into

electrical energy. Among these solar cells the most widely commercialized technology is

that of single crystal silicon solar cells with 26.5% efficiency. It is quite expensive to

achieve single crystal silicon for solar cell fabrication. Amorphous silicon solar cell

technology was introduced to overcome the crystallinity problem. Amorphous silicon has

the ability of low deposition temperature and polycrystalline nature. This technology has

also a drawback of degradation while exposing to light which reduces its existing

efficiency by 10% to 20%. Presently poly crystalline silicon based solar cells are the

leading commercial photovoltaic products in the market but cannot compete with the

conventional sources of energy. It is doubted that crystalline silicon technology can meet

the necessary cost reductions.

Due to these issues arising for silicon solar cell technology, thin film solar cell has the

potential to be cheaper in large scale production. The recent technologies of thin film

solar cells have lead the competition of electricity prices with conventional energy

sources (Keshner and Arya 2004). Therefore new materials that are nontoxic, stable,

abundant and cheaper than single crystal silicon and also that are compatible with large

scale production, are required in solar cell technology. The thin film technology and

manufacturing processes are the leading contender regarding lowering the production

cost of solar cell technology. In thin film technology 1 to 2µm thin layer of semiconductor

material is deposited on a low cost substrate such as plastic, glass or metal.

Thin film compound solar cells replaces silicon technology for its low cost, better

efficiency and longer life time. These polycrystalline thin film solar cells are efficient to

create electric field (built-in electric field) between the interfaces of the hetro-junction.

This built-in electric field will separate the electrons and holes generated in the absorbing

layer. A typical polycrystalline thin film has a window layer thickness in the range 80nm-

120nm and the absorbing layer thickness 1-2μm. The window layer must have large band

gap such that it allow most of the light through interface to the absorbing layer. The

absorbing layer must have smaller band gap so that all the photons must trap in this layer

for the generation of electron-hole pair (Afzaal, and Brien 2005). The optimum value of


Solar cell Page 20

the band gap for absorbing layers in solar cells is between 1.0 eV to 1.8 eV for ensuring

sufficient absorption of the solar irradiations (Wakkad, Shokr et al. 2008). The materials

used for capturing these irradiations within the above quoted band gap are Si, CdTe,

CIGS, CZTS, Sb2S3 etc. Due to low band gap of these materials, they have high

absorption coefficient for visible light (105cm-1). Thus 1μm to 2μm thick absorbing layer

is sufficient for complete absorption of solar spectrum except Si, for which 300μm thick

absorbing layer will be used for complete absorption (Rafat and Habib 1998). Material

cost and stability problems associated with these materials leads for engineering new

materials in the field of photovoltaics.

2.5 Material for solar cell

The 1st material used as thin film was amorphous silicon which has different properties

than crystalline silicon. However it took some time to understand the basic properties of

the material. Amorphous silicon (a-Si) thin film technology put forward a remarkable

opportunity to reduce the cost of materials for the solar cells (Chang, Hsieh et al. 2011;

Samanta, Das et al. 1997) because a-Si absorbs light more powerfully than crystalline

silicon. The thickness of a-Si solar cell can be up to 300 times less than that of

conventional cells which significantly reduces the cost of the material. A significant step

in this development was the introduction of the triple junction modules in 1997 that

provides comparatively high efficiency and stability (8.0–8.5%) (Smil V 2006).

Beyond a-Si certain other compound III-IV, IV-V-VI, II-IV and I-III-VI2 can also be used

as an absorbing layer in solar cells (Hermann, Westfall et al. 1998; Han, Spanheimer et

al. 2011). Studying empirically few promising materials have been resulted. Foremost

among these compounds are copper indium sulfide (CIS) and Cadmium Telluride (CdTe)

by the 1960.

Thin film solar cells are of great importance due to its reduction of cost of material. With

this consideration the technology shifted to polycrystalline thin film hetrojunction solar

cells. The advantage of such solar cell over conventional pn-junctions solar cells includes;

enhanced short-wavelength spectral response because most photons are absorbed inside

the depletion region of the 2nd semiconductor. The surface recombination of high energy


Solar cell Page 21

photon generated electron hole pair is greatly decreased; lower series resistance because

the 1st semiconductor will be heavily doped without causing the life time of minority

carrier short; high radiation tolerance.

The current thin film solar cell technologies are based on chalcogenide based material

which should overcome the deficiencies relating toxicity and material cost. Our focus in

this work is on SnSbS thin films for solar cells application grown by different techniques

combinatorially.

Tin antimony sulphide is a new material and has a potential to be a prominent material as

an absorbing layer for solar cell. In the following figure 2.5 the comparison of quantum

efficiencies of different solar cells at different conditions is presented. Careful analysis

shows that the higher wavelength cutoff edge of the solar spectrum for all the materials

lie below 900nm. While for SnSbS this edge shifts to larger value (>1000nm) which

extended the spectrum range into NIR region.


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Fig-2.5 Comparison of quantum efficiency of (a) crystalline silicon (Chang, Hsieh et al. 2011). (b)

CZTS (Platzer-Björkman, Scragg et al. 2012). (c) CdTe (Krishnakumar, Han et al. 2011). (d) α-Si

(Dagkaldiran, Gordijn et al. 2009). (e) SnSbS solar cells (our work)

Electrical energy is the backbone of all developmental efforts carrying out both in

developing and non-developing nations. The production of energy from fossil fuel

involves the environmental impacts due to the consumption of fuel leading to the

pollutants emission (Okoro and Madueme et al. 2006; Stoppato 2008). The recent bang

in photovoltaic modules demands has created a shortage in silicon supply, leads an

opportunity for thin-film solar cell modules to enter the energy market replacing silicon.

Thin films have revolutionized the cost structure of the solar cell modules as above 50%

cost in silicon technology is that of module manufacturing (Panwar, Kaushik et al. 2001).


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This huge variation in price outstrips the demand of silicon supply, providing an

opportunity for a number of thin film technologies. Categorically, they are

Silicon; in crystalline, amorphous, nano-crystalline and polycrystalline phases

Polycrystalline chalcogenide compounds

Organo-metalic dyes and polymers

Silicon is the most widely used solar cell technology for production of electricity and is

known as clean source of energy. The enhancement in the efficiency of solar cells, the

quantity of material and system design, will reduce the requirement of energy and CO2

emission (Demirbas 2007). There are so many energy resources, we are consistently

utilizing. Such scenario of renewable energy has been represented in table 2.1. This table

speculates the energy scenario till 2040 in which 13310 million ton oil equivalent energy

is speculated. Similarly table 2.2 shows world total energy consumption (Million Tons of

Oil Equivalent) and Table 2.3 represents development and installation of solar

photovoltaic electricity in various countries.

Table-2.1 Global renewable energy scenario

Year 2001 2010 2012 2013 2040

Total consumption (million

tons oil equivalent)

10038 10549 11425 12352 13310

Biomass 1080 1313 1791 2483 3271

Large hydro 22.7 266 309 341 358

Geothermal 43.2 86 186 333 493

Small hydro 9.5 19 49 106 189

Wind 4.7 44 266 542 688

Solar thermal 4.1 15 66 244 480

Photovoltaic 0.1 24 221 784

Solar thermal electricity 0.1 0.4 16 68

Marine (tidal/wave/ocean) 0.05 0.1 0.4 20

Total RES 1,365.5 1,745.5 2,964.4 4289 6351

Renewable energy source

contribution (%) 13.6 16.6 23.6 34.7 47.7


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Table-2.2 World total energy consumption (Million Tons of Oil Equivalent) (Solangi, islam et al. 2011)

1971 2002 2010 2030 2002–2030 (%)

Coal 617 502 516 526 0.2

Oil 1893 3041 3610 5005 1.8

Gas 604 1150 1336 1758 1.5

Electricity 377 1139 1436 2263 2.5

Heat 68 237 254 294 0.8

Biomass and waste 641 999 1101 1290 0.9

Other renewable 13 41 6.2

Total 4200 7075 8267 11, 176 1.6

Table-2.3 Development and installation of solar photovoltaic electricity in various countries.

Year USA (MW) Europe (MW) Japan (MW) Worldwide (MW)

2000 140 150 250 1000

2010 3000 3000 5000 14,000

2020 15,000 15,00 30,000 70,000

2030 25,000 30,000 72,000 140,000

2.6 History of thin film solar cell

Silicon solar cells are the initial solar cells in the photovoltaic industry. They are operated

as a-Si, a-Si:H, c-Si and thin film silicon solar cells. a-Si are deposited on glass (<600°C)

commercialized in 1970,s (aberle 2009). In 1980s, a-Si:H solar cell deposited by plasma

enhanced chemical vapor deposition (PECVD) at ~200°C appear to be the base line in

thin film silicon technology (Kuwano and Tsuda et al. 1980).

This technology was able to produce low cost electricity and having high optical

absorption coefficient with low deposition temperature. The only drawback of a-Si:H was

its low PV module efficiency (6%) (Lechner and Schade 2002), this low efficiency may

have many factors. Among them Staebler–Wronski effect (Staebler and Wronski 1977)

i.e. the light-induced degradation of the initial module efficiency to the stabilized module

efficiency. Research is continuing for finding ways for reduction of the above effect and


Solar cell Page 25

only improvement was 9.5% (Green, Emery et al; Meier, Spitznagel et al. 2004) in 2003

by University of Neuchatel which is the highest one.

Due to the stable efficiency of a-Si:H (6%), researchers at the University of Neuchatel

succeeded in the early 1990s to fabricate the 1st hydrogenated microcrystalline silicon

(μc-Si:H) cells at 200°C with reasonable efficiency of 4.6% (Meier Flückiger et al. 1994).

Importantly, the efficiency of the solar cell was stable under light soaking conditions,

giving hope to significantly higher stable module efficiency than that for a-Si:H modules.

The drawback in the μc-Si:H single-junction solar cells is its low deposition rate (<

40nm/min) and technical difficulties which seems not to be commercially viable

presently.

Sanyo Electric during the early 1990s worked on polycrystalline silicon at ~600°C of a

thick (~5 μm) PECVD-deposited a-Si film gives 9.2% efficiency on a metal substrate

(Matsuyama, Terada et al. 1996).

Pacific Solar Pty Ltd, in the late 1990,s a spin-off company of the University New South

Wales (UNSW) in Australia, successfully transferred the PECVD-based SPC approach

to borosilicate glass. Pacific Solar achieved a major breakthrough in the following years

in light trapping (novel glass texture (Ji and Shi 2002) and cell metallization and back

and front contact (Basore 2002; Proc 2006). The greatest efficiency obtained so far with

this “Crystalline Silicon on Glass (CSG)” technology is 10.4% in 2007 (Keevers, Young

et al. 2007).

The reported efficiencies of CdTe thin film solar cells by the end of 1970,s were about

8% (Mitchell, Fahrenbruch et al. 1977; Fahrenbruch, Fahrenbruch et al. 1980). The

efficiency was improved in 1980,s to about 13% (Nakazawa, Takamizawa et al. 1987;

Nakayama, Matsumoto et al. 1980). At present 16% efficiency solar cell is available on

glass substrate (Britt and Ferekides 1993).

In mid-1990’s, (Mathew, Thompson et al. 2003) the interest on flexible and light weight

substrate, molybdenum substrate, stainless steel and their post deposition techniques


Solar cell Page 26

improve different properties of the CdTe solar cells (McClure, Singh et al. 1998; Seth,

Lush et al. 1999).

In 1974 the research on copper indium selenides bears fruits and the 1st single crystal CIS

solar cell device with 12.5% efficiency was produced (Jager-Waldau 2011). This

efficiency was almost double in those days for other thin film solar cell devices

(Kazmerski, Ayyagari et al. 1976; Kazmerski 1977). In the early 1980,s, the 1st

polycrystalline CIS solar cell with 9.4% efficiency (Mickelsen and Chen 1982) was

discovered and the researcher turned their attention toward polycrystalline technology.

The band gap of Cu(In,Ga)(S,Se)2 have various band gap range, ranging from 1.04eV to

1.2eV. Boeing group invented the bi-layer process in 1980s by the process of co-

evaporation of Cu, Se and In on molybdenum-coated glass substrate and recorded 14.6%

efficiency. In 2010, the Centre for Solar and Hydrogen Research in Stuttgart, Germany,

reported the world record efficiency of 20.3% for Cu(In,Ga)Se2 solar cell. The following

table 2.4 represents an overview of early US patents on CIS materials for solar cells.

Table-2.4 US patents on CIS solar cell material

Company Period of

granting

Topics Reference

The Boeing Company, Seattle,

WA

1982–1993 Window layer, multi-junction, module

design, deposition

Boeing Company US patent

Atlantic Richfield Company

(Arco Solar), Los Angeles, CA

1984–1991 Co-deposition by magnetron sputtering,

window layer, multi junction cells

Richfield Company US patent

Matsushita Electric Industrial

Co., Ltd., Kadoma, Japan

1990–1998 Precursor deposition, deposition

equipment, window layer, co

Matsushita Electric US patent

Siemens Solar and other

Siemens companies

1992–1997 Sequential deposition; sodium control,

module design, window layers, TCO,

multi junction

Siemens company US patent

Yazaki Corporation, Tokyo,

Japan

1996–2001 Deposition system, electro-deposition Tokyo, Japan US patent

Other solar companies 2002-2012 New materials for solar cells SUPERGEN,

UK

In the year 2009, 166MW of CIGS-Se modules was manufactured all over the world

representing 1.5% of the total annual production. The expansion in the other technology

is much higher (3.4 GW) but 166MW is still realistic. This is shown in figure 2.6.


Solar cell Page 27

Figure-2.6 Actual and planned PV production capacities of different solar cell technologies

Ito and Nakazawa at Shinshu University fabricated CZTS thin film by sputtering

technique, and calculated the optical band gap energy of 1.36-1.62 eV and use them as a

solar cell absorber layer in 1988 (Ito and Nakazawa 1988) with photovoltaic effect of

165 mV. In 1996, Katagiri and Jimbo fabricated the CZTS thin film by an electron beam

deposition followed by the sulphurization with conversion efficiency of 0.66% .In 1997,

Friedlmeier et al. at the Stuttgart University reported a conversion efficiency of 2.3% for

CZTS solar cell deposited by simultaneous deposition technique (Friedlmeier, et al.

1997). In 2007, Moriya et al. at Nagaoka University of Technology reported a conversion

efficiency of 1.74% and open circuit voltage of 546 mV for CZTS solar cell deposited by

pulsed laser deposition (PLD) technique (Moriya, et al. 2007). In the following year

Shimada et al. at Shinshu University (Shimada 2007) prepared CZTS thin films by

sulphurization in the Ar+H2S atmosphere and the reported solar cell conversion efficiency

of 4.02% and 2.69% respectively. Similarly in 2007, Momose et al. at Nagano National

College of Technology fabricated the CZTS thin film solar cell with sputtering technique

with conversion efficiency of 1.36% (Momose 2007). In 2008 solar cell with overall

properties of 1.0% were obtained (Mathew, Thompson et al. 2003).


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2.7 Thin film (sulfosalt) tin antimony sulphide (SnSbS) solar cell

Sulfosalt materials in the form of thin films have been the subject of substantial research

due to its technological importance. Their applications have been utilized in the field of

thin film photovoltaic, thermoelectric energy conversion and phase change memory

devices (Gassoumi and Kanzari 2011). Dittrich et al. studied different materials for

photovoltaics and suggested Sulfosalts to be a new absorber for solar cell applications

(Dittrich and Vaughan et al. 1996). Presently Sb2S3, SnS, CZTS sulfosalts have been

used as an absorbing layer for solar cells (Tanuševski and Poelman 2003; Maghraoui et

al; Li, Chawla et al. 2012). Sulphide materials have potential as semiconductor materials

whose properties matches’ best in solar cell applications. Low band gap (≈ 1.2eV) is

required for trapping maximum number of photons for free exciton conversion and

sulphur is a useful contingent in reducing energy gaps. The nontoxic, abundant and cheap

metal based sulphide thin films can be the desired materials for this technology. SnSbS

thin films are also a potentially stable sulfosalt which has not been studied in detail yet.

Sn–Sb–Se ternary compounds have already been reported as a potential candidate for

thermoelectrical transition and some other technological purposes (Wakkad, Shokr et al.

2008; Kumar and Bindra et al. 2006). Therefore the need for alternative materials to CIS

and CIGS have been attempted to reduce the cost of solar cell by using tin antimony

sulphide thin films. The new ternary compound, sulfosalt tin antimony sulphide thin films

appear to be potential candidates but not yet widely studied. Sulfosalts are complex

sulfides with generalized formula of AmBnXp, where A represents metallic elements like

tin; B stands for trivalent, semi- metallic elements like antimony and bismuth; while X

may be either sulfur or selenides (Kryukovaa, Heuerb et al. 2005). SnSbS is a group IV-

V-VI ternary compound semiconductor material and has a potential to be cheap, abundant

and having no negative impact on the environment. Thin film polycrystalline SnSbS-

based solar cells will be one of the market leading solar cells due to an optimal band gap

of 1.2 eV which cover maximum part of solar spectrum with high optical absorption

coefficient and low cost synthesis techniques (Gassoumi and Kanzari 2011). The reported

deposition techniques for Sn-Sb-S are thermal vacuum evaporation while in the present

study we used magnetron sputtering and thermal vacuum evaporation (Gassoumi and


Solar cell Page 29

Kanzari 2011). Very little work has been reported on the current material in thin film

form for solar cell applications. In the following table (2.5) the comparison of different

solar cell and its efficiency is shown (Rommel and Ken 2006).

Table-2.5 year wise efficiencies of best thin solar cell

Years Reference Efficiency

1970 Bonnet et al 6%

1982 Tyen at el 10%

1991 T.L. Chu at el 13.4%

1993 Ohyama, H at el 15.8%

1997 Ferekides et al 16%

2001 NREL Researchers 16.5%

2011 Radboud Versity ~26%



Attempts have been made to convert conventional solar cells to thin film photovoltaic

devices in order to overcome the difficulties facing conventional solar cells. Research on

thin film solar cells was carried out and it was found that absorbing materials which have

direct and small band gap will cover the whole solar spectrum. The window material

should have larger band gap such that all the light should reach the absorbing material.

The efficiency chart up to 2013 is presented in figure 2.5 which summarize the efficiency

of all solar cells under study. The chart is published by NREL. The maximum efficiency

obtained ever is 44.4% for multi junction solar cell.


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Figure-2.7 Solar cell efficiencies (NREL)

The band gap engineering of the window layer is also an active part of the present

research. The band gap of as deposited CdS thin film is 2.42ev which will cover only

some part of the visible spectrum. i.e.

-7

g

hcλ = =5.14×10 nm

E

= 514nm


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With this band gap we are able only to transmit light of wave length 514nm (green color)

and the aim is to cover the blue shift as well. Different wave length range is shown in

table 2.6.

The variation of optical band gap with doping as well as post deposition annealing in

different atmosphere was studied in order to increase the blue response of the solar cell.

For the transmittance of whole spectrum we have to tune the band gap of CdS up to 3.5ev

[will cover the whole visible spectrum]. This may be achieved by proper doping

concentration and post deposition annealing (Sharma, Jain et al. 2003). The annealing of

CdS thin films shows crystal growth and reduces the defects which lead to the

improvement of electrical and optical properties (Wan, Bai et al. 2010).

Table-2.6 Visible spectrum range

Color Wavelength

violet 380–450 nm

blue 450–475 nm

cyan 476–495 nm

green 495–570 nm

yellow 570–590 nm

orange 590–620 nm

red 620–750 nm

The variation of band gap is also doable with lattice parameters and the lattice parameters

have opposite dependence on band gap. It is possible that both hexagonal and cubic

structures are present together and at annealing temperature of 250-300oC, cubic structure

dominate which decrease the band gap (Ortuño-López, Sotelo-Lerma et al. 2004). Since

the band gap of thin films depend on the particle size and reducing the particle size below

10nm, quantum size effect start appearing and the band gap increases. It is reported by D.


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Saikia at el that for a particle size of 3.58nm, band gap of 3ev is obtained (Saikia. D at el.

2010).

After many years research in thin films for solar cells, the technology is now transferred

from laboratory scale to industrial production. Besides the silicon technology, CdTe,

CIGS, ZnTe etc technologies are also competitive technologies (Vaseashta, Dimova-

Malinovska et al. 2005). The Energy conversion efficiencies achieved in the laboratory

for different photovoltaic thin-film technologies is presented in table 2.7. It shows that

the module efficiency is at least 50% lower than the cell efficiency.

Table-2.7 Energy conversion efficiencies achieved in the laboratory for different photovoltaic thin-film

technologies

No Technology Cell efficiency % Module efficiency

%

1 Thin silicon (Si) high

temperature process

12 8

2 Thin silicon (Si) high

temperature process

10 -

3 Thin amorphous Si 13 8

4 Chalcopyrites (CIGS) 19 12

5 Cadmium Telluride 16.5 10

2.8 Objective of research

The objective of the study is to explore new materials for solar cells which should be

abundant as well as cheap and nontoxic. The 2nd important factor in this work is the use

of combinatorial approach for the synthesis of sulfosalt materials with two different

deposition techniques, sputtering and thermal evaporation. The combinatorial method has

not yet been used for thin films deposition in thermal evaporation technique for solar cell

research. This will be a novel approach which will allows us to explore nearly all the

elemental composition in one sample. To prove that sulfosalt are promising layer in solar

cells, Sn-Sb-S thin film will be deposited on clean soda lime glass. Sn-Sb-S has an

optimized band gap which can be used for solar energy production and the shift in the

band gap and variation in other properties with different elemental composition will be

shown.


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It is also aiming to optimize the deposition technique for Sn-Sb-S thin films and the

correct elemental composition which have optimized properties for solar cells. To achieve

these aims the following objectives should be met:

Use of sputter coater for elemental SnSb combinatorial thin films followed by its

sulphurization in vacuum thermal coater.

Demonstrate that single step two source deposition techniques (thermal

evaporation) can be used to deposit Sn-Sb-S thin films.

Study all the properties of the layer at different points in different samples with

variable elemental composition.

Optimize the best elemental composition and annealing temperature for Sn-Sb-S

layer.

Study the air annealing effect on the Sn-Sb-S thin films both in argon and air.

Cadmium sulphide thin film as a window layer deposition via thermal evaporation

technique.

Chapter 3 Deposition and analytical techniques

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Chapter 3

Deposition Techniques

Three techniques of Physical Vapor Deposition (PVD) are in use for thin film deposition

technology, evaporation, ion plating and sputtering (Bunshah 1994). Current study used,

vacuum thermal evaporation and magnetron sputtering technique for the deposition of

combinatorial tin antimony sulphide thin films.

For characterization, X-ray diffraction (XRD) and Energy Dispersive X-ray Spectroscopy

(EDS) was used for structural and compositional analysis of thin films. Optical properties

of the films was obtained from Variable Angle Spectroscopic Ellipsometry (VASE).

Band gap, transmittance, absorption coefficient, refractive index and extinction

coefficient was accomplished from VASE. The photoconductivity response of the library

is calculated by photoconductivity spectrometer.


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3.1 The glass substrate

Soda lime standard microscopic glass substrate 1.2mm thick was used in this work for

layer development. XRF study (table-3.1) is the analysis of clean glass substrate used in

the current study which shows the presence of different elements.

Table 3.1 Elements profile present in glass substrate

SiO2 70.4702(±0.2774) (wt %) 5280.872(±18.464)(cps)

TiO2 0.0000(±0.0242) (wt %) 0.217(±0.230)(cps)

Al2O3 3.7855(±0.0548) (wt %) 201.544(±2.894)(cps)

Fe2O3 0.0108(±0.0029) (wt %) 3.538(±0.950)(cps)

MnO 0.0027(±0.0046) (wt %) 0.512(±0.862)(cps)

MgO 0.8080(±0.0747) (wt %) 17.343(±1.611)(cps)

CaO 13.3572(±0.1867) (wt %) 238.441(±3.368)(cps)

Na2O 10.4955(±1.0439) (wt %) 17.777(±1.696)(cps)

K2O 0.2457(±0.0296) (wt %) 2.790(±0.384)(cps)

P2O5 0.6491(±0.0125) (wt %) 124.136(±2.195)(cps)

S1O3 0.0000(±7.0847) (wt %) 0.154(±0.263)(cps)

PbO 0.0118(±0.0006) (wt %) 33.946(±1.803)(cps)

Sb2O5 0.0215(±0.0026) (wt %) 8.254(±1.097)(cps)

CuO 0.0105(±0.0011) (wt %) 12.222(±1.298)(cps)

CoO 0.0358(±0.0142) (wt %) 2.821(±0.970)(cps)

ZnO 0.0068(±0.0007) (wt %) 12.998(±1.302)(cps)

BaO 0.0888(±0.0031) (wt %) 43.702(±1.858)(cps)


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3.2 Vacuum thermal evaporation technique

When a material is heated in high vacuum above its melting point, it will evaporate or

sublimate and condense on a substrate attached at a proper position. This is done by

Thermal evaporation technique (fig-3.1).

Figure-3.1 Vacuum thermal evaporation (a) Operation (b) Camera Photograph


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Vacuum thermal evaporation is one of the oldest and versatile thermal deposition

technique used for the deposition of most materials. Faraday’s in the late 1880s made an

attempt with metal wires to explode in vacuum which leads to thermal evaporation. This

process was reviewed for so many years until 1940-1950, a commercialize way of

deposition named as vacuum thermal evaporation technique was discovered (Maissel and

Glang 1970; Wasa, Kitabatake et al. 2004). Evaporation takes place in vacuum where

the mean free path of atoms in the evaporant material is much larger than source to

substrate distance (Elshabini-Raid and Barlow 1998). Early work on evaporation theory

is referred to Hertz, Knudsen and Langnuir. They worked on ideal behavior of

evaporation by using the concept of thermodynamics, kinetic theory and solid state

theory. From kinetic theory

c

dN P

A dt 2 mKT

3.1

Where Ni, is the number of molecules/atoms striking the substrate per unit area Ai. Here

m is the molecular weight, k is Boltzmann constant and T is the temperature in Kelvin.

In 1882, Hertz measure the rate of evaporation of mercury and found that the rate of

mercury in high vacuum is proportional to the difference between the vapor pressure P*

at mercury surface and hydrostatic pressure, P (Bunshah 1994).

Thus the rate of evaporation at equilibrium vapor pressure is

2

c

dN P Pcm / sec

A dt 2 mKT

3.2

Hertz measured that the maximum possible evaporation rate at p=0 is only about 1/10th

of the maximum theoretical rate. Knudsen in 1915 (Bunshah 1994) postulated that all the

molecules/atoms impinged are not deposited on the substrate. Some fraction (1-αv) was

reflected back on the gas phase and contributes to the evaporant pressure. Due to this

correction equation 2.2 is modified known as Hertz-Knudsen equation given by


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V

c

(P P)dN

A dt 2 mKT

3.3

αv (0 < α < 1), the evaporation coefficient has maximum value for clean evaporant surface.

3.3 DC Magnetron Sputtering

Sputtered depositions are important for elemental depositions of thin films due to its

controlling parameters ability and more specifically in controlled combinatorial growth.

Therefore we will use magnetron sputtering technique in the current study for the

deposition of thin elemental films (tin antimony) as well as molybdenum contacts for

electrical characterization (shown in figure 3.2). Sputtering deposition is usually

practiced by the removal of surface atoms of target through energetic charged particles.

The energetic particles must have enough energy to overcome the surface binding energy

Usurf that they may detach the surface atoms from the target. The sputtering of a surface

is expressed in sputtering yield and is a mean number of sputtered target atoms per

incident ion (Seshan 2001, Bishop 2011, Depla and Mahieu 2008)

Number of sputtered atomsSputtering yield

Number of Incident Ions

The calibration of the targets is an important step before experiment which can be done

with the help of quartz crystal and balance techniques. Details calibrations of tin,

antimony and molybdenum metallic targets have been discussed in detail in chapter 5.

It has been established theoretically, experimentally as well as by computer simulations

that the energy spectrum of sputtered atoms emerging from a collision cascade is the

replication of Thompson formula (Thompson 1968), Such cascade is represented in terms

of differential sputtering yield Y(E) of atoms ejected with energy E for normal incidence

of incoming ions.


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Figure-3.2 Magnetron sputtering schematic


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Y (E) dE

S

o

2 2

S

U E1

E dE

E (1 U / E)

3.4

Here E and E0 are the energies of the sputtered and incident ions, Us is the surface binding

energy, γ ≈ 4M1M2/(M1+M2)2, M1, M2 are the masses of incoming ions and target atoms.

For γoE >> Us approximation, Equation 3.4 approaches to

Y (E) dE 2

S

E dE

(E U )

3.5

This theoretical formula was tested experimentally and with ACAT code and found the

reproduced result. The same formula was also derived by Falcone et al (Thompson 1968).

For qualitative energy distribution, equation 3.5 can be generalized and will have the

following form.

Y (E) dEk

S

E dE

(E U )

3.6

Where k for elastic collision depends on the interaction potential.

For light ions (>1KeV), the Thompson formula deviate (Bay, Berres et al. 1982). Falcone

(Kenmotsu, Yamanura et al. 2004) prove that the primary recoils are the sputtered atoms

which undergoes elastic collisions

Y (Eo, E) dE o

5/2

S S

EE dE ln

(E U ) E U

3.7

The flux of sputtered particles composes of atoms, molecules and clusters. The

understanding of clusters emission in sputtering is not fully explained yet. Both the

computer simulations and experiments have specified the importance of the cluster

formation (Depla and Mahieu 2008, Shin, Kim et al. 2009).


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3.4 X-ray diffraction (XRD)

The crystalline properties of the material can be studied with XRD. The X-rays diffract

from the consecutive layers of the ordered atoms with geometrical variation within the

material and construct peaks on the diffractogram. Braggs explain the phenomena of

diffraction and gave a mathematical explanation about the diffraction (Scherer 1918).

According to Braggs law

2dSinθ = nλ 3.8

Here “d” is the interplaner spacing, θ is half of the diffraction angle between the diffracted

beam and the transmitted beam and λ is the wavelength of the impinging X-rays.

There are three different types of interactions involved between x-rays and matter in the

relevant energy range. The photoionization, in which electrons are liberated from their

bound atomic state which is an inelastic scattering process. The Compton scattering

which is also an inelastic scattering, the energy is transferred to an electron and do not

radiate. The third type is elastic scattering (Thomson scattering) in which the electron

oscillate with the frequency of impinging x-rays and become source of dipole radiation.

The structural investigation by x-ray diffraction is calculated by Thomson scattering.

The copper target for x-rays production was used for this work where the intense

characteristic x-ray lines kα1 and kα2 causes the reflection produced by the sample to be

doublet with less intense higher peak value due to kα2 and more intense lower angle peak

value kα1.

The thin films to be examined with XRD are polycrystalline and therefore any preferred

orientation within thin film will affect our data. Only the planes of atoms parallel to the

surface of the material will satisfy Bragg’s law. The reflection for the different

orientations within the material is due to the result of different grains inside material, see

figure 3.3. This is a vital concern when we are looking forward at the difference in the

properties of each crystallographic direction.


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The Scherer formula 3.9 which was published in 1918 can be used for the measurement

of crystalline size.

kλB(2θ) =

Lcosθ 3.9

B is the peak width which is inversely proportional to crystalline size (L). The Scherer

constant K is assumed to be 0.9 (Gassoumi and Kanzari 2009).

Figure-3.3 Diagram illustrates diffraction for the different orientations will result from the plains in

different grains. This diagram applies to the sample with rotational symmetry around the axis normal to

the sample surface.

Figure-3.4 Thin Film Analysis by X-Ray Scattering


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Philips PW1820 diffractometer (figure-3.5) with Cu-Kα radiation (λ=1.54Ǻ, 40

KV, 20 mA) at a scanning speed of 2° per 20 minutes was used in the current study for

the structural analysis of combinatorial thin films. 14 different points 2mm apart are

selected for experimental purpose in each combinatorial library.

Figure-3.5 Philips PW1820 diffractometer camera photograph


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3.5 Energy dispersive X-ray spectroscopy (EDXS)

The energy dispersive X-ray spectroscopy (Leo Electron Microscopy) will be used for

the elemental analysis of combinatorial thin films in the current study. EDS has the

capability of line scan in a short time interval in intermediate vacuum. EDXS is a crucial

way for elemental analysis with accuracy up to 10th of a percent for standard. It quantifies

all elements heavier than Boron with relative abundance.

The energy of the emitted X-rays is the characteristics of the material which give the

qualitative as well as quantitative determination of the elements present. Figure-3.5 shows

the photograph of EDXS system used in the current study. The following EDXS

parameters (Table-3.2) were used for the elemental study.

Table-3.2 EDS parameters

No Parameter Range

1 Beam current 200µA

2 I probe 5.0nA

3 Fill target 2.44 A

4 EHT 20 kV

5 Magnification 27-100

6 Working distance 24 mm

7 Contrast 45%

8 CPS(X-rays) 4000-5000

9 Dead time % 4-5%

10 Amplitude 3.2µsec


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The Lithium-drifted Silicon crystal Si (Li) and Germanium crystal (Ge) semiconductors

are used as detectors in the EDS measurement system. Lithium is commonly used as it

compensates the impurity effect in silicon by creating an intrinsic zone inside the crystal

for effective X-ray detection. The energy required to generate one electron hole pair is of

the order of 3.8 eV for Si (Li) and ~2.9 eV for Ge detectors. The energy of the incident

X-ray photon is several KeV which generate thousands of electron hole pairs inside

detector (Friedbacher and Bubert 2011). The electric field (0.5-1.0 KeV) is used to

separate the electron hole pairs yielding an electric pulse. These pulses are amplified with

preamplifier and analyzed for pulse height and then feed to the multi-channel analyzer

(MCA) or computer for creating spectrum via software (figure-3.6). Liquid nitrogen is

constantly circulating in the system for keeping the detectors cool.


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Figure-3.6 Components of EDS system, the charge pulse from sample is converted in the free amplifier to

an electric signal

Figure-3.7 Photograph of Energy Dispersive X-ray spectrometer and scanning electron microscopy


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3.6 Scanning Electron Microscopy (SEM)

Scanning electron microcopy (SEM) is a surface analytical technique used for the

analysis of materials in macro and submicron ranges along with the capability of

generating three-dimensional images for analysis. The elemental analysis of the material

or contaminant can be done on SEM when it is used in conjunction with EDS.

The electron beam scan the surface of the specimen inside vacuum just like a television

camera and produce a rasterized digital image with imaging capability down to 10

angstroms. When the wavelength of the speedy electrons approaches 1/100,000 of the

white light, they are focused on a specimen and are absorbed or scattered for an electronic

image procession.

The three dimensional image of a microscopic areas do not provide sufficient information

about the material. Often it is necessary to identify and quantify different element

associated with the specimen. The energy dispersive X-ray spectrometer (EDS) in

conjunction with SEM for the elemental analysis which utilizes the X-rays emitted from

specimen when bombarded with high energy electrons. The energy of the emitted X-rays

is the characteristics of the material which give the qualitative as well as quantitative

determination of the elements present (Wisconsin 1999).

De Broglie materialization equation (equation-3.3) is the basis for electron microscopy.

h h 1.22(nm)

mv 2qmV V 3.10

For high resolution, smaller electron wavelength is required which are supplied by

accelerating voltage. The electrons in SEM chamber after acceleration passes through

lenses and aperture deposits on the sample. The dissipated energy of electrons upon

interaction with the sample yields many signals for analysis (Schroder 2006). They are

Back scattered electrons, Secondary electrons, Auger electrons, Elastic and inelastic

scattered electrons, X-rays etc. SEM uses secondary electrons for image formation

(figure-3.8).


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Figure-3.8 Schematic of a SEM


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3.7 X-ray fluorescence spectroscopy (XRFS)

XRF also known as X-ray secondary emission spectroscopy is a quantitative and

qualitative analytical techniques used for elemental and chemical analysis of materials in

solid as well as liquid form. It is a nondestructive, multi-elemental, cost effective and fast

technique widely used for a broad range of concentration from 100% to few parts per

million (ppm). It will measure all the elements heavier than fluorine. EDXS has been used

for elemental analysis in the current study; however XRFS was also used for some of the

selective samples. Figure-3.9 shows the electronic process in X-ray fluorescence

spectroscopy. Primary X-ray photon create an inner shell vacancy (e.g. K-shell) which is

filled by the electronic transition (e.g. L-shell) liberate characteristic secondary X-rays

with energy (Nikawa, Inoue et al. 1999).

EXRF= EK (Z) - EL2, 3 (Z) 3.11

The energy of the emitted X-rays identifies the impurity level and their peak gives

density. Energy dispersive or a wavelength dispersive spectrometer detects the secondary

X-rays emitted from the sample. Low power excitation source is used for qualitative and

quantitative detection of elements (Z≈11). For the analysis of elements down to Z≈4,

vacuum as the analysis environment is needed.

Figure-3.9 Electronic processes in XRF


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XRF is a suitable technique for conductors as well as insulators due to neutral nature of

X-rays and having good resolving power. It can analyze area as small as 10-4cm2 to 10-

6cm2 (Schroder 2006). The X-rays in XRF spectroscopy penetrate inside the sample

material which is governed by the absorption coefficient of the material. For good XRF

spectroscopy results the penetration depth of the incident X-Ray Should be 50%. This

makes this technique as a non-surface-sensitive technique. The intensity of the secondary

X-rays also measure the thickness of the films (up to 10nm) by the reference of standard

film of known thickness (Drude and Ichten 1889).

Figure-3.10 XRF Experimental set up (Rutherford Lab, Cranfield University UK)


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3.8 Variable Angle Spectroscopic Ellipsometry Techniques (VASE)

Variable angle spectroscopic ellipsometry technique was used in this study for measuring

the optical properties of combinatorial tin antimony sulphide thin film libraries. It is a

non-destructive and contactless powerful optical technique used for measuring several

different optical properties. VASE (J.A. Woollam M2000 ellipsometer, scan range

300nm-1800nm) can yield information about layers ranging from even single atomic

layer to few microns thick layer. Most importantly we can ascertain film thickness,

refractive indices, surface roughness, interfacial mixing, composition crystallinity,

absorption coefficient, band gap, transmittance and uniformity of different materials

(Azzam and Bashara 1987). Ellipsometry is based on the measurement of the state of

polarization of a polarized wave when reflected from the sample surface (Cigal 2002).

This change is associated with the optical, chemical and optical properties of the sample.

In the graph it is shown that the polarized light is incident on the surface of a sample with

different polarization from source controlled by polarizer. It takes changes in polarization

of the wave that reflected of the surface in both the magnitude of polarization of plane. It

is observed (figure-3.11b) that the incident light beam which is arbitrary azimuth and is

linearly polarized while the reflected beam is elliptically polarized.


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Figure-3.11a Schematic setup of an ellipsometry experiment

Fi

Figure-3.11b Incident linearly polarized light of arbitrary azimuth θ is reflected from the surface S as

elliptically polarized

a

b


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Figure-3.12 Camera photograph of Ellipsometer


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The light being detected is expressed in term of coefficient Ψ (amplitude) and ∆ (phase

difference between coefficients). The associated ellipsometric parameters for a sample

are

N= cos (2Ψ)

C= sin (2Ψ) cos (∆)

S= sin (2Ψ) sin (∆)

For non-polarizing films

N2+C2+S2=1

And the SE data can be converted into ρ representation

p i

s

r C iS tan e

r 1 N

3.12

This allows us in calculating the complex reflectance ratio ρ of a surface (Jellison,

Modine et al. 1998). P and S are the plan of incidence of electric field perpendicular

to each other, rs and rp are their reflection coefficients. The Phase shift ∆ is the

difference between coefficients (Synowicki 1998).

Phase shift ∆ = δp - δs 3.13

Varity of different type of materials can be measured using the VASE technique such as

transparent materials like a glass slide, absorbing materials i.e. a dye or thin film on glass

and also opaque substance such as silicon oxides on silicon, different metals or metal

layers on different substrates, transparent conducting oxide like ITO can also be measured

using this techniques.

For transparent materials like glass slide, we apply a piece of scotch tape on the back of

substrate. Because while when measuring transparent substrates, there can be back side

reflection that will have to take account during modeling.


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3.8.1 Modeling of ellipsometric data

The ellipsometric data have been done at room temperature and a model was proposed

for extracting different parameters from the raw SE data. The modeled data of 300°C

annealed Sb2Sn5S9 thin film is shown in figure 3.13a.

Cauchy model was used for measuring the thickness and refractive index of the film by

defining the transmission wavelength range of the film. The substrate constants were not

allowed to vary during fitting the data.

Lorentz oscillator model was used for measuring the absorptivity and band gap

calculation. The absorption in the material was due to free carriers and its information

was obtained from the free carrier conductivity of the material by fitting at longer

wavelength for absorption (Schroder 2006). The following model parameters were used

for modeling the ellipsometric data.

1st layer B-Spline matrix (for absorbing layer)

Starting material GaAs-2

Back reflection 20.00

1st reflection 100.00

Module intensity data ON

Weight% 500

Transmission data % weight 100.0

Slight variation was made in these parameters for fitting the curve on experimental curve.

The following figures-3.13a, b are the experimental results of our study. The mean square

error (MSE) for this result is 14 with thickness of 1.264µm.


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Figure-3.13(a) Modeling of experimental curve

Figure-3.13(b) Modeling of Sb5Sn2S9 thin film annealed at 300°C ellipsometry data


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3.9 Photoconductivity measurement

The photoconductivity of the thin film library was calculated by shining a monochromatic

light of variable wavelength range on the film from 240W tungsten halogen bulb and the

response was recorded via computer controlled program. The entrance slit Acton

Research model Sp-150 monochromator is used to control the incident light. The exit slit

dimensions and monochromatic grating (1200g/mm) is selected to achieve a

monochromatic beam ≈2 nm of spectral range of interest. A chopper is used to chop the

monochromatic beam of light at a frequency of 27 Hz. The beam frequency was also

changed to 75 Hz and 110 Hz and the change in the function was observed. The

monochromatic beam chopped at 27 Hz collimated by a 100 mm focal length lens and

then focused on the thin film with the help of second lens. Molybdenum contacts are

deposited via sputter coater on thin films for collecting output data and feed to computer

via a circuit shown in figure. SR570 current-to-voltage preamplifier detects the ac current

produced by the monochromatic beam. The ac current signal is converted into an ac

voltage signal with the help of preamplifier. The SR570 is used for dc voltage biasing

and filtering dc response from the film. SR810 model lock-in amplifier detects the voltage

signal from the preamplifier and feed to the computer.

Figure-3.14 photoconductivity measurement set up


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3.10 Hot-point probe measurement

Hot point probe measurement is a commonly used technique for the determination of

bipolar conductivity of semiconductors. The “thermal see back effect” create potential

difference between hot and cold contacts and the carrier type is identified by the

deflection of central-zero meter. In the experimental setup (fig 3.15a), the soldering iron

is used to heat up (200°C) one of the probe for creating potential difference across the hot

and cold probe via see back effect. The free carrier diffuses more rapidly near the hot end

leading current away from the hot contact and vice versa. The direction of electric current

(toward or away the hot contact) identify the type of conductivity (Plummer, Deal et al.

2000). In this study the instrument was calibrated for n-type silicon wafer and then the

conductivity type of thin films were measured.

The majority charge carriers in n-type samples are electrons. The difference in thermal

energy between hot and cold probes will diffuse these electrons from hot probe towards

cold probe. If both probes are shorted by a wire, a detectable current will tend to flow

whose direction will correspond to the conductivity type. A positively charged immobile

donor atom is left behind with the diffusion of electron from hot probe in n-type samples.

As a result hot probe become positive with respect to cold probe. Similarly the direction

of current would be reversed for p-type materials. So the short circuit current or open

circuit voltage tells us about material conductivity type (Wei, Zhang et al. 2008).

The majority carrier current is given by

n n n

dTJ qnµ p

dx 3.14

For minority current carriers

p p p

dTJ qpµ p

dx 3.15

Here pp > 0 and pn < 0 are the thermo electric powers.


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This voltage measured by the hot point probe is known as see back voltage or thermal

EMF.

Figure-3.15a Hot-point probe measurement set up for thin films

Figure-3.15b Photograph of Hot-point probe setup

Chapter 4 Experimental Methods

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Chapter 4

Experimental Methods

Introduction

Semiconductor industry is the backbone of our society and thin films have revolutionized

this industry in the last few years. In this chapter, the deposition technique used for tin

antimony combinatorial metallic thin films deposited through sputtering for solar cells

followed by its sulphurization for SbSnS libraries via vacuum thermal evaporator, along

with the post deposition annealing processes will be employed. Thin films of SbSnS with

thermal evaporation techniques will also be discussed in the current chapter.

4.1 Calibration of Antimony and Tin targets

Sputtering coater was used in the current study for the deposition of SnSb metallic thin

films. Before any depositions, the target holder of the machine was replaced by an aligned

target layers for the growth of combinatorial libraries of metallic SbSn thin films. The

targets were calibrated with the help of quartz crystal and balance techniques. The target

to substrate distance was fixed to 10cm and the quartz crystal was inserted in the chamber

to calibrate the target i.e. sputtering rates of both targets at different powers (temperature)

and pressures and time rates for thickness. The calibration via quartz crystal is based on

the principle that when the sputtered ions/atoms fall on the quartz crystal, its vibration

rate changes with time. Normally the quartz crystal vibrates at 105 – 106 hertz. This

variation in vibration is measured by a computer controlled program which is connected

to the quartz crystal (Jespersen and Fitz-Randolph 1999). The following data is obtained

after calibrating the Antimony and Tin targets. These calibrations are summarized in

tables 4.1, 4.2 and plotted in figures 4.1, 4.2 and 4.3.


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Table-4.1 Antimony (Sb) target calibration, Density=6.62 g/cc, Z- ratio=0. Purity=99.999%

No Power (watt) Pressure

(torr)

Voltage (V) Evaporation rate

(Å/sec)

1 25 5×10-3 373 4.20

2 50 5×10-3 428 9.62

3 75 5×10-3 457 14.50

4 100 5×10-3 480 19.80

Table-4.2 Tin (Sn) target calibration, Density=7.2g/cc, Z-ratio=0.724, Purity=99.998% to 99.999%

No Power

(watt)

Pressure

(torr)

Voltage (V) Evaporation rate

(Å/sec)

1 10 5×10-3 359 0.97

2 20 5×10-3 449 2.11

3 30 5×10-3 507 3.28

4 40 5×10-3 529 4.66

5 50 5×10-3 547 6.16


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20 30 40 50 60 70 80 90 100 110

2

4

6

8

10

12

14

16

18

20

22

y = 0.2067x - 0.89

Evap

orat

ion

Å/Se

v

Power (W)

B

Figure-4.1 Antimony (Sb) target calibration

10 20 30 40 50

1

2

3

4

5

6

7

y = 0.1293x - 0.443

Evap

orat

ion

rate

Å/S

ec

Power (W)

B

Figure-4.2 Tin (Sn) target calibration

0 50 100 150 200 250

0

1

2

3

4

5

6

7

8

Evap

orat

ion

rate

Å/S

ec

Power (W)

Figure-4.3 Molybdenum target calibration


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4.2 Calibration of Molybdenum target

Molybdenum contacts were used for the electrical characterization of thin films. The

molybdenum target was calibrated with quartz crystal and balance techniques and was

found that the maximum rate was 7Å/sec at 250W. The whole data is summarized in

table 4.3.

Table-4.3 Molybdenum (Mo) target calibration, Density=10.28g/cc, Purity=99.998% to 99.999%

No Power (watt) Pressure (torr) Voltage

(V)

Evaporation rate

(Å/sec)

1 25 5×10-3 375 0.8

2 50 5×10-3 403 1.38

3 75 5×10-3 425 2.14

4 100 5×10-3 431 2.85

5 125 5×10-3 439 3.57

6 150 5×10-3 443 4.25

7 175 5×10-3 448 4.97

8 200 5×10-3 453 5.67

9 250 5×10-3 465 7.01


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4.3 Calibration of Tin sulphide (SnS) and antimony sulphide (Sb2S3) targets

Combinatorial SnSbS thin films were also deposited through vacuum thermal coater by

two source technique. In order to obtain same sputtering rate from both the targets, it is

important to calibrate SnS and Sb2S3 targets to get the same sputtering rate for the

deposition. It was found that SnS and Sb2S3 targets have same sputtering rate (3.25Å/sec)

at 40A and 20A current respectively.

Table-4.4 SnS and Sb2S3 target calibration

SnS Sb2S3

Current (A) Rate (Å/sec) Current (A) Rate (Å/sec)

36 1.5 16 1.31

40 3.27 20 3.21

42 6.10 24 5.28

44 10.00 28 7.00

46 16.39 32 10.89

48 21.50 36 13.7

50 26.13 40 22.5


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4.4 Combinatorial thin films deposition

The combinatorial deposition of electronic and functional materials is a new wave for the

formation of thin films combinatorial libraries which dramatically increase the rate of

discovery of new materials. With small substrate of 1cm2 area, one can synthesize

thousands of different compositions of desired physical properties. In other words a large

number of super lattice samples can be deposit on a single substrate with combinatorial

synthesis. This concept is extremely feasible in the discovery of new materials with much

improved physical properties. This concept was first developed in a pharmaceutical

industry in 1980s for the discovery of new drugs (Xiang et al. 1995).

Two deposition techniques were used in the current study for the combinatorial

deposition of thin films.

1. DC magnetron sputtering

2. Vacuum thermal evaporation technique

4.4.1 DC magnetron sputtering

The combinatorial deposition of SnSb metallic thin film is easy to control via dc

magnetron sputtering technique. The SnSb combinatorial thin film libraries were

deposited through sputtering techniques in the Rutherford laboratories Cranfield

University United Kingdom. For combinatorial depositions, both the targets (Sn and Sb)

were adjusted at 45 degree normally such that the maximum sputtered ions from both the

target approaches the center of the substrate. The distance from target to the center of

substrate was fixed to 10cm, with this arrangement (fig-4.4) we got a combinatorial SnSb

compound thin film which is Sn rich at one side and Sb rich at the other side while having

1:1 elemental composition (Sn:Sb) at the central point.


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Figure-4.4 Targets and substrate settings for combinatorial growth of SnSb metallic thin film

4.4.1.1 Targets power in dc magnetron sputtering for combinatorial growth

Tin and antimony targets have different sputtering rates for same power. For

combinatorial growth same evaporation rate is required for both the targets. Therefore

from the power vs. sputtering rate, we conclude that both Sn and Sb targets will have

same evaporation rate 5Å/sec at 43w and 28w respectively. Although both targets have

different densities and atomic weights. Using the same sputtering rate we will get a

combinatorial thin film with 1:1 elemental ratio at the central point and different ratio

elsewhere in the same thickness. With the above sputtering rate the following calculation

shows that 500nm thick film will be deposited in approximately 9.0 minutes.


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From the slopes of both graphs 4-1 and 4-2

RSb = 0.2067PSb – 0.89 4-1

RSn = 0.1293PSn – 0.443 4-2

We also need to use a factor to calculate atomic number ratios. Let that number be

#i #i = kiRi 4-3

Where ki = Densityi / Atomic weighti 4-4

K for antimony is KSb = 0.06138 g/cm3/amu

And KSn = 0.05489 g/cm3/amu

For combinatorial growth we will have 1:1 elemental composition at the middle point.

Therefore we will concentrate on that ratio. So using equation 4-3 for both the targets and

for 1: 1 i.e.

#Sb=#Sn

KSbRSb =KSnRSn 4-5

From graph 4-1 we see that for certain low evaporation rate (5Å/sec) the Sb target power

is 28 watt. Using this power in equation 4-1

RSb = 4.8976Å/sec 4-6

Now using equation 4-5 for the rate of Sn target

RSn = 4.3797 Å/sec 4-7

And the correct power for Tin target we use equation 4-2

PSn = 37.2989 w 4-8

This calculation leads us to the correct power for both the targets.


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Table-4.5 Power and evaporation rate for Tin and Antimony Targets

Element Power Rate

Tin 37 W 4.3797 Å/sec

Antimony 28 W 4.8976 Å/sec

4.4.2 Vacuum thermal evaporation technique

The second technique used for the deposition of combinatorial SnSbS thin films as well

as the sulphurization of metallic SnSb thin films is vacuum thermal evaporation. It is easy

to sulphurize the elemental film in vacuum chamber by evaporating sulphur powder.

Table 4-6 summarize the sulphurization condition of SnSb metallic films.

The combinatorial SnSbS thin film libraries growth is a difficult task in term of elemental

control. Because the elemental variation in the deposited film varies due to difference in

evaporation rate and the adjustment of baffle position. However at certain suitable

position of the baffles and similar evaporation rates, combinatorial deposition was

achieved. The schematic 4.5 shows that how the baffles were placed for combinatorial

depositions.

Table-4.6 Sulphurization conditions for Sn Sb and SnSb

Evaporation

Rate

Thickness Frequency Life time

(Crystal)

Pressure Density Z-factor

110Å/sec 1µm 5687097Hz 68.7% 5×10-4mbar 2.09g/cc 2.29

Besides sputtered thin films, seven tin antimony sulphide combinatorial thin film libraries

were also deposited via vacuum thermal evaporator for studying their structural, electrical

and optical properties.


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Figure-4.5 Vacuum Thermal Evaporation

4.5 Annealing of sputtered SnSbS thin films

Five SnSbS combinatorial thin films of same elemental compositions were used in this

study. All the films were annealed in the presence of argon gas under same condition of

pressure in sealed quartz ampoule at 425°C, 450°C, 475°C, 500°C and 525°C for 1 hour

each in tube furnace. Temperature gradient of the tube furnace was 1/2°C per second.

Cooling of the sample was done by putting the ampoules containing films at the edge of

the furnace until cooled to room temperature. Figure 4.6 is the representation of five

samples in quartz ampoules after annealing.


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Figure-4.6 Quartz ampoule containing sputtered SnSbS libraries


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4.6 Annealing of evaporated SnSbS thin films

The seven SnSbS combinatorial thin films deposited via vacuum thermal evaporator were

selected for the study. One of the film is treated as as-deposited while the rest of six

combinatorial thin films were annealed at 85°C, 105°C, 150°C, 175°C, 250°C and 325°C

in sealed glass ampoules containing argon gas at low pressure in tube furnace.

Temperature gradient of the tube furnace was same as for sputtered films i.e. 1/2°C per

second. Cooling of the sample was done by putting the glass ampoules containing the

combinatorial film libraries at the edge of the furnace overnight while the furnace was

turned off. Figure 4.7 is the representation of six samples in glass ampoules after

annealing.

Figure-4.7 Glass ampoule containing SnSbS thin film libraries


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4.7 Synthesis of tin sulphide (SnS) powder

Tin sulphide powder was synthesized from tin powder (99.99%) and sulphur (99.998%)

powder by using the following ratio (Massalski and Okamoto 1990).

Sn = 0.7873g

S = 0.2127g

Both the powders were grounded in a pestle and mortar to mix well. The mixed tin

sulphide powder was annealed in sealed quartz ampoule containing argon gas at low

pressure for 24 hours at 600°C in a tube furnace. After cooling the ampoule by putting at

the edge of the tube furnace overnight, the tin sulphide powder was grounded again such

that a fine powder was obtained. EDS study and XRD study of the synthesized SnS

powder was carried out for the confirmation of SnS phase.

Figure-4.8 Glass ampoule containing SnS ingots


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4.8 Antimony sulphide (Sb2S3) powder

Antimony sulphide (Sb2S3) powder with 99.99% purity was used from sigma Aldrich

(Kurt J. Lesker) as a second evaporation source. Two source methods have been

employed for combinatorial deposition; therefore Sb2S3 and SnS powder have been used.

4.9 Preparation of pallets

Powder evaporation in the vacuum chamber may produce micro particles in the chamber

before thermal evaporation starts. These micro particles embedded in the thin film will

affect the surface morphology as well as the optical properties (Hornyak, Tibbals et al.

2009). In order to avoid this effect, the material for evaporation was used in pallet form.

Hydraulic press was used for this purpose to press the powder in the form of pallet

(10mm).

4.10 Sample preparation

Sputter coater and vacuum thermal coater were employed in the current research. In the

1st part of the research the metallic film SnSb was deposited via sputter coater and

sulphurized through vacuum coater. In the 2nd part, SnSbS combinatorial thin films were

deposited through vacuum thermal coater. The molybdenum contacts were deposited for

electrical characterization.

4.10.1 Preparation of SnSb thin films

The SnSb metallic combinatorial thin films were deposited through sputtering techniques

in the Rutherford laboratories Cranfield University United Kingdom. The sputter coater

was calibrated for the combinatorial deposition in such a way that both the targets are

pointed to the same central point of the substrate. The Argon gas was used as a sputter

gas at a chamber pressure of 4.5×10-3 mbar. Power used for Sn target was 43w while that

for Sb was 37w. The choice of power is selected for the same rate of evaporation from

both the targets. The deposition time was 9 minutes as already calculated in section


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4.4.1.1, which will give us 500nm thickness. Clean soda lime glass substrate was used

for the deposition and glow discharge was run before coating which is helpful for

removing any moister, grease or some other impurity on substrate.

4.10.2 Preparation of molybdenum thin films

Molybdenum contact was used for electrical characterization in our absorbing layers for

both deposition methods. From the calibration data, the selected sputtering power for

molybdenum was 250w with corresponding evaporation rate of 7Å/sec. The sputtering

time was 19 minutes which is sufficient for the deposition of 800nm.

4.10.3 Preparation of SnSbS thin films

The SnSbS combinatorial thin films were deposited through vacuum thermal coater in

the Rutherford laboratories Cranfield University UK. The thermal coater was calibrated

for the combinatorial deposition by using baffle in front of the crucible inside chamber.

The pressure of the chamber was maintained at ~5×10-4 mbar. It was calculated in section

4.3 that SnS and Sb2S3 targets have same evaporation rate (~3.25Å/sec) at 40A and 20A

current respectively. This will give us 1.4µm thick SnSbS thin films in about 20 minutes.

Quartz crystal monitor is used as thickness monitor.

Chapter 5 Thermal evaporation of Cadmium sulphide thin films

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Chapter 5

Thermal evaporation of Cadmium sulphide thin films

5.1 Cadmium sulphide thin film window layer

The window layer play an important role in solar cell and the scientists have focused on

window layer in order to increase the blue response and overcome the recombination and

short circuit problem in solar cell. CdS has the ability to be doped with different impurities

so as to achieve the desired properties and to make them multifunctional (Kafafi, Lane et al.

2004; Sun and Sariciftci 2005; Liang Feng et al. 2005). Due to the antiseptic nature of

transitional metals, major research is carried out on it, in order to get an optimum condition

of dopant for an explicit application. A uniform and compact window layer can tune the

efficiency of solar cell beyond the optimum value. The cadmium sulfide film has been

deposited by different methods in order to obtain a best possible method for deposition. The

main deposition techniques are chemical bath deposition, chemical vapor deposition,

physical vapor deposition, close space deposition, thermal evaporation, Pulse Laser

deposition etc (Mahdavi, Irajizad et al. 2008; Townsend et al. 2002). Among above one of

the best methods is thermal evaporation because one can get a controlled and reproducible

thin film. Solar cell at present supplies power for satellite as well as domestic purposes and

also for small scale terrestrial application. In this work thin films were deposited on glass

substrate and mainly the temperature effect was addressed (Boer 1992; Iacomi, Purca et al.

2007; Manickathai et al. 2008; Souri and Shomalian 2009; Varghese, Lype et al. 2002;

Chandramohan, Kanjilal et al. 2009).


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5.2 Experimental

Pallets of CdS powder were used of 4cm3 volume in the preceding experiment. Before

deposition, corning 7059 glass substrates were rinsed with isopropyl alcohol (IPA) and then

dried for 30 minutes at 250⁰C in a furnace. High vacuum coating (A550V) heat resistive

system along with the control panel (IC 6000 controller) was used for deposition. Control

panel was used to control source temperature, evaporation rate, substrate temperature,

deposition time, etc. Substrate temperature was measured with the help of quartz crystal

and the source temperature was measured with thermocouple. After deposition, the films

were annealed in the presence of argon gas for 45 minutes at 420⁰C.

The optical properties of the film were measured by double beam spectrometer (specard-

200). This measure the absorbance, transmittance and reflectance of the film.

The nature of the film was measured with the help of hot and cold probe method. The four

probe techniques determine the electrical properties i.e. resistivity and mobility of the film.

The band gap was measured from the data of absorbance verses wave length.

5.3 Result and discussion

The cadmium sulfide films studied are polycrystalline with high preferred orientation along

[002]. Some other phases were also found whose contribution is shown in XRD with phases.

They are shown in the XRD pattern below (fig 5.1). The pattern confirms the hexagonal

crystalline phase. The average particle size for (112) calculated by Scherrer’s formula was

~11.868nm (N. Ali, Z. Ali et al. 2012).


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Figure-5.1 X-ray diffraction pattern for CdS films (top film 1, down film 2)

The electric properties of the films were measured by Vander Pauw method. The resistivity

varies from 10Ωcm to 80Ωcm while the corresponding mobilities associated are in the range

from 2cm2V-1S-1 to 60 cm2V-1S-1. It was found that the resistivity and mobility of the layers

are mainly controlled by the substrate temperature and evaporation rate. This effect is shown

in a set of plots 5.2, 5.3, 5.4. It is inferred from these graphs that resistivity of the film

increases with an increase in substrate temperature and evaporation rate. The variation in Cd

to S ratio will increase because both have different evaporation rate and also because of

different substrate temperature which in turn increase the resistivity of the films.


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Table-5.1 Films parameters deposited by thermal evaporation

Film

No.

Film thickness

(nm)

Substrate

temperature (C⁰)

Source

temperature(C⁰)

Vacuum

pressure (torr)

1 500 150 800-900 2×10-6

2 500 200 800-900 2×10-6

3 500 250 800-900 2×10-6

4 500 300 800-900 2×10-6

Figure-5.2 Effect of substrate temperature on mobility


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Figure-5.3 Effect of substrate temperature on resistivity

Figure-5.4 Effect of evaporation rate on the electronic properties of as deposited films

As deposited As deposited


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Table-5.2 Temperature of hot probe verses thermal EMF for CdS/glass configuration

No

Temperature of hot probe

Thermal EMF (mv) °C K

1 5 278 0.25

2 10 283 0.56

3 18 291 0.78

4 32 305 1.47

5 34 307 1.61

6 42 315 2.01

7 58 331 2.62

The thermo electric properties were analyzed by hot and cold probe method. It was

calculated that due to deficiency of Cd in the film, the film is n-type. The thermo electric

power for n-type semiconductor is

n

Ka =

Nee[r - ln( )]

n

Or ne

a Kn = N exp[-(r - )]

e

Where n= electron density

R = scattering parameter = 1 for amorphous semiconductors

eN = effective density of states in conduction band

T = 300K

eN = 2(2πKTme/h2)3/2

For CdS effective mass of electron= me = 0.21m


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By using these values, N= 2.40×1024 m-3

And from graph # 5-5. an = 5.4×10-5V/K

Which gives the electron density “n” = 8.65×1017cm-3

Figure-5.5 Temperature Vs Thermal EMF for CdS/Glass thin film

The spectrometer (specard200, UV-VIS 300nm-1100nm) analysis measured the

transmittance, absorbance and reflectance of the film. Transmittance was obtain from the

plots between transmittance percentage and wavelength λ. 1st the transmittance of substrate

was calculated and the result is shown in graph 5.6(a). The transmittance has least value at

300nm and increases as the wavelength increases. At 500nm its transmittance is maximum.

No further change was observed in the transmittance above 500nm. The transmittance at

500nm is 95% and no further change is observed above 500nm (Abu-safe, Hossain et al.

2004).

The absorbance was recorded by plotting absorbance against wavelength of light. The

absorbance was very high and decreases from 80% to 5% in the wavelength range of 330nm-

400nm as in graph 5.6(d) and 5.6(e). For the wavelength greater than 400nm, absorbance


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was almost constant and was approximately equal to 5%. Because of this high transmittance

CdS thin films are uses as window layer in solar cells. In fig 5.6(c, d) the absorbance and

transmittance for the films are compared.

Fig-5.6(a) Transmittance Vs wavelength for substrate Fig-5.6(b) Transmittance Vs wavelength for film

Figure-5.6(c) Transmittance Vs wavelength for film Figure-5.6(d) Absorbance Vs wavelength for film

Film 1

Substrate Film 1

Film 2


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Figure-5.6(e) Absorbance Vs. wavelength for film

Figure-5.7 shows the plot for percent reflectance verses wavelength from which we can

estimate the reflectance of light in different spectrum.

As A+T+R=1

R=1-(A+T)

The percent reflectance at different wavelength was different. In the range 300nm-400nm

the reflectance was less than 5% and was less than 2.5% for visible and infrared region. At

wavelength of 200nm the reflectance is 25%.

Reflectance Vs wavelength Reflectance Vs wavelength

Figure-5.7 Reflectance Vs wavelength for CdS thin films

Film 1 Film 2

Film 2


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Figure-5.8 Comparison of transmittance Vs Glass Substrate and film

The transmittance from CdS thin is compared with glass substrate in figure 5.8. it is inferred

from it that the film has good transmittance in the visible and NIR region.

The band gap of the film can be calculated by plotting (αhυ) 2 verses hυ shown in figure 5.9.

The absorption coefficient α2 is calculated and it is related to the band gap Eg and photon

energy hυ according to the relation (Orlianges, Champeaux et al. 2011).

αhυ= A(hυ-Eg)n

Where n=1/2 for direct allowed transition. The absorption coefficient α is defined by the

Beer-Lambert’s law as: α = 2.303×A/t where A is the absorbance and t is the thickness of

the film absorbance. The extrapolation of the straight line to (αhυ)2=0 gives the value of the

energy band gap of prepared film etc (Mahdavi, Irajizad et al. 2008). The reported band gap

for CdS thin films annealed at 250°C is 2.35eV (Mariappan, Ponnuswamy et al. 2012). In

our case the band gap was 2.46ev.


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Film No.1 Film No.2

Figure-5.9 Band gap calculation of cadmium sulfide thin film

5.4 Conclusion

Thin film of cadmium sulphide was deposited via thermal evaporation techniques aiming to

study the effect of substrate temperature on the optoelectronic properties of the thin films. It

was found that the substrate temperature plays a vital role in the characteristics of deposited

films. We studied few characteristics of cadmium sulphide thin film deposited at different

temperature (150°C-300°C) on corning 7059 glass substrate. The properties are grain size

dependent as the XRD shows variation in grain size. The band gap measured for the films

was 2.42ev for 0.5μm thick film and usually such thick films of cadmium sulfide have very

low pin hole density. So with this optimum thickness it can be used as a window layer of

CdS/CdTe solar cells.

Chapter 6 Properties of Sn-Sb-S thin films deposited by sputter coater

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Chapter 6

Properties of tin antimony sulphide thin films deposited by

sputter coater

6.1 Introduction to combinatorial approach for material synthesis

The term combinatorial or more specifically “combinatorial chemistry” was employed for

the 1st time by Merrifield while studying peptides (Merrifield 1963; Yanase, Ohtaki et al.

2002). Since then this technique has been used for the characterization of organic as well as

inorganic materials. The concept behind the combinatorial synthesis is to study a variation

of a specific effect or property across a chemical composition. Combinatorial study is

assumed to be an efficient way for exploring the optimal value of best elemental

composition, which can have high performance in a sequence of material synthesis and

characterization. The study of combinatorial deposition will explore nearly all the elemental

composition in a single slide. This method is expected as a novel technique for research in

thin films. We make special arrangement in the sputter coater (dc magnetron sputtering, Kurt

J Lesker) for the deposition of SnSb metallic combinatorial thin films. Plenty of work has

been devoted in the past for the analysis of a particular compound with different elemental

composition but is time consuming job due to conventional approach. The combinatorial

deposition allows us to analyze a particular compound even in a single stage (Bob, Anzelmo

et al. 2001). XRD techniques is one of the suitable way of rapid screening due to its high

penetrating power, non destructiveness and rapid data collection. Two dimensional X-ray

diffraction have been used for the structural information of the combinatorial libraries with

high accuracy (Roncallo, Karimi et al 2010). The combinatorial SnSb thin films deposition

together with XRD apparatus is a time reducing agent for exploring the crystalline phases in

different samples. All the libraries were illuminated with extended X-ray source with a larger

detector area for data collection (Roncallo, Karimi et al 2010). SnSbS is an attempt to

explore new material for solar cells as this material has depth in term of optical and electrical


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properties. The motto behind new materials is to fabricate a device with excellent photon-

to-current conversion (Gassoumi, M. Kanzari 2011).

6.2 Results and discussion

6.2.1 Energy Dispersive X-ray spectroscopy

The EDS study [fig-6.1] of the deposited films confirms the combinatorial deposition of

synthesis. X-axis represents position of points where as point 1 starts from Sn rich and ends

up on Sb rich point, number 14. Where Y-axis represent atomic percentage. It shows the

varied composition of Sn and Sb at different points.

Table-6.1 Elemental composition (Atomic percentage)

Figure-6.1 EDS study of the SnSb combinatorial films


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Figure-6.2 EDS study of the SnSbS combinatorial films (a) 425°C annealed (b) 450°C annealed (c) 475°C

annealed (d) 500°C annealed (e) 525°C annealed


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Figure-6.3 EDS plots of selected points

6.2.2 Combinatorial thin film X-ray diffraction

Fig 6.4 shows schematic of the combinatorial thin films X-ray diffraction developed by

Keith Roger at Cranfield University. Thin films in a sample holder were affixed in order in

front of X-rays for exposure. The apparatus was set in a way that a continuous scan of the

thin combinatorial films was possible 2mm apart. The sample holder was retained fix in the

apparatus while the X-ray gun scans the sample. There are 70 points, 14 in each library.

Figure-6.5(a) shows SnSbS XRD pattern for 14 different points in a single library annealed

at 425°C treated as a started material. The elemental composition can be seen in figure 6.2

for all libraries. Point 1 is Sn rich while point 14 is Sb rich side of each library.


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Figure-6.4 XRD set up for combinatorial analysis

We see the existence of SnS phases from point 1 to point 7 in 425°C annealed samples

because of Sn richness. However, strong phases of Sn2Sb2S5 and SnSb2S4 exist in this

library. The SnS phases also exist at 450°C and 475°C for 1st seven points while at high

annealing temperature it spills up to all 14 points in the library. There is another phase of

Sb2Sn5S9 as well consistently present at each point in each library at all annealing

temperature. This phase becomes very strong with annealing temperature and this is shown

in figure 6.5e. XRD of combinatorial libraries shows that no alloys are formed as no peak

shift is observed for Sn and Sb. This is generally an indication of solid state solution is

formed (Roncallo, Karimi et al 2010).


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Figure-6.5 (a) Combinatorial SnSbS thin film XRD patterns annealed at 425°C for 1 hour. (b) Combinatorial

SnSbS thin film XRD patterns annealed at 450°C for 1 hour. (c) ombinatorial SnSbS thin film XRD patterns

annealed at 475°C for 1 hour. (d) Combinatorial SnSbS thin film XRD patterns annealed at 500°C for 1 hour.

(e) Combinatorial SnSbS thin film XRD patterns annealed at 525°C for 1 hour.


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Figure-6.6 (a) Contour combinatorial SnSbS thin film XRD patterns annealed at 425°C for 1 hour. (b) Contour

combinatorial SnSbS thin film XRD patterns annealed at 450°C for 1 hour. (c) Contour combinatorial SnSbS

thin film XRD patterns annealed at 475°C for 1 hour. (d) Contour combinatorial SnSbS thin film XRD patterns

annealed at 500°C for 1 hour. (e) Contour SnSbS thin film XRD patterns annealed at 525°C for 1 hour.


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6.2.3 Contour plot

The contour graph represents the existence of different peaks and their variations with

temperature. There are 14 divisions on y-axis with which peak height were compared. Angle

2θ was plotted on x-axis.

Figure 6.6a shows the existence of Sn2Sb2S5, Sb2Sn5S9 and SnSb2S4 phases along with SnS

phase. It was found that SnS peak is as high as 7th point (on y-axis). This is because points

1-7 are Sn rich in library. Figure 6.6b shows 450°C annealed library. We see that Sn2Sb2S5

is shifted toward Sb rich side on the film. Change in SnS is still observed while its peak

height decreases. However Sb2Sn5S9 and SnSb2S4 are still appear to be strong phases.

Figure-6.6c (475°C) shows strengthening in Sn2Sb2S5 and SnSb2S4 while Sb2Sn5S9 at the Sb

rich end becomes weak. The SnS peak in 475°C annealed sample still exist with higher

strength. Figure 6d (500°C) is a complicated graph and may refer to the transition state of

the compound. It shows that sulphur gas has started reaction with Sn and Sb in the whole

library and appears to be in transition state. The SnS peak strength further increases in the

library. Figure-6.6e (525°C) shows that Sn2Sb2S5 and Sb2Sn5S9 overlap and strengthen in the

library. While the other phase of SnSbS has shifted to a more stable phase Sb2Sn5S9. It also

shows that sulfur fully react with Sn and Sb for the final phases. The SnS peak is constantly

appearing in the higher annealing libraries.


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Figure-6.7 Photoconductivity response of SnSbS thin films (a) 425°C annealed (b) 450°C annealed (c) 475°C annealed

(d) 500°C annealed (e) 525°C annealed

Figure-6.7 shows the photoconductivity plots of the tin antimony sulphide libraries. It shows

that SnSbS is photoactive in the visible as well as infrared region. The 425°C annealed

library contains SnS and Sb2S3 phases and the library might be photoactive because of these

phases. At a high annealing temperature, these phases disappear and the photoconductivity

increases due to the presence of different tin antimony sulphide phases. The

photoconductivity response span over a wide range of spectrum starting from visible to near

infrared. All points in 500°C annealed library are not photoconductive due to its transition

phase confirmed by XRD. The 525°C annealed library looks better in photoconductivity.


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Figure-6.8 represents the SEM images of selected points of Sn-Sb-S thin film sputtered

library at different annealing temperature. The 425°C annealed sample shows that the

sulphur is present in the boiled condition. This is confirmed from the appeared morphology

shown in SEM images (Figure 6.8). As the annealing temperature is increased, we see that

the sulphur is adsorbed and react with Sn and Sb in making the required phases.

The combinatorial library of SnSb metallic thin films confirmed by the energy dispersive X-

ray spectroscopy which was sulphurized and annealed in closed inert environment for SnSbS

combinatorial thin films. XRD and photoconductivity measurement confirmed the reaction

of sulphur with metallic SnSb combinatorial thin films at different temperatures. The

relationship between XRD and photoconductivity followed by the contour XRD graph for

SnSbS thin films at different annealing temperature validates our experimental findings for

photoconductivity. The presence of SnS and Sb2S3 phases reduces the photoconductivity

response of the tri metallic compound SnSbS as SnS and Sb2S3 are not very good photoactive

materials (N. Ali, Z. Ali et al. 2012). The increased photoconductivity was noted when the

samples were annealed at 525°C can be attributed to the complete reaction of sulphur with

metallic film to form SnSbS rather than SnS and Sb2S3 phases


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Figure-6.8 SEM images of SnSbS library of selected points at different annealing temperature (a)

425°C (b) 425°C (c) 450°C (d) 450°C (e) 475°C (f) 475°C (g) 500°C (h) 525°C annealed


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6.3 Conclusion

This study reveals the role of Sn-Sb-S thin film as a low cost material deposited by sputter

coater for solar cells. It has been concluded from the study that combinatorial approach is a

fastest way of analysis in any research area especially material research. Hundreds of

different samples can be analyzed in a singly slide in no time. The metallic thin films of

SnSb were sputtered from Sn and Sb tilted targets on glass substrates. The films were then

sulphurized by thermal evaporation techniques. The films were annealed at 425°C, 450°C,

475°C, 500°C and 525°C inside sealed quartz ampoules containing argon gas. The elemental

composition study was carried out with EDS which confirms the combinatorial growth of

the films. We study XRD and photoconductivity combinatorially of tin antimony sulphide

sputtered thin films in detail. The contour graphs provide enough information about phase’s

formation and we conclude finally that SnSb2S4, Sn2Sb2S5 and Sb2Sn5S9 phases exists for all

the libraries. The XRD shows that the peaks of different compounds have no shift at different

annealing temperature suggest that no solid state solution is form. The combinatorial

photoconductivity study leads us to pick a highly photo active point in the whole library.

We see that point 9 in library annealed at 525°C has maximum photoconductivity response.

Point 9 elemental compositions are Sn-25%, Sb-40%, S-62%.

Chapter 7 Combinatorial synthesis of Sb2Sn5S9 thin films by thermal evaporation

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Chapter 7

Combinatorial synthesis of Sb2Sn5S9 thin films by thermal

evaporation

7.1 Metal chalcogenide thin films

Attempts are being carried out at various laboratories for the development of new solar cell

materials in order to improve the solar cell efficiencies. These new materials should have (i) an

appropriate energy band gap that matches the solar spectrum to absorb maximum solar

radiation, (ii) be able to deposit the material with an acceptable efficiency using an inexpensive

deposition technique, (iii) high abundance of the elements (AbuShama, Johnston et al. 2004).

Metal chalcogenide (sulphide’s, telluride’s, selenide’s) are very important materials for solar

cell applications. Among various chalcogenide materials, sulfosalt ore minerals possess very

promising semiconductor properties but are not yet widely studied (Reddy and Reddy 2005;

Gassoumi and Kanzari 2009). Tin antimony sulphide (Sn2Sb5S9) with a direct energy band gap

of about 1.4eV and a high optical absorption coefficient is one of the promising materials that

satisfies the above criteria. In addition, both p-type conductivity as well as n-type conductivity

has been reported by a group of researchers (Hirshman, Hering et al. 2006), opening up a range

of new types of device structures.

7.2 Experimental

Tin antimony sulphide thin films were prepared on clean glass substrate by thermal vacuum

evaporation using a combinatorial approach and two evaporation sources. Sb2S3 was purchased

from sigma Aldrich (Kurt J. Lesker) with 99.99% purity and SnS was synthesized from tin and

sulphur powder by the following ratio (Massalaski and Okamoto 1990).

Sn = 0.7873g

S = 0.2127g

Both the powders were mixed, ground together in a pestle and mortar and annealed in a quartz

ampoule under an Argon gas atmosphere for 24hours at 600°C. After annealing, the powder


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was ground together again and hydraulic press was used to make separate pallets of SnS and

Sb2S3 for thermal evaporation. Figure 7.1 shows the combinatorial evaporation.

Figure-7.1 Schematic for combinatorial SnSbS evaporator

XRD and EDX of synthesized tin sulphide powder confirm the structural and elemental

composition of the ingots [Fig 7.2 (a, b)]. These powders were evaporated from Al2O3 crucibles

together in a vacuum chamber for combinatorial thin films on glass substrates. The pressure of

the chamber was 2x10-4 mbar with no substrate heating. The deposited combinatorial thin films

were annealed at 85°C, 105°C, 150°C, 275°C and 325°C in sealed glass ampoules containing

argon gas.

Figure-7.2a SnS ingot EDX Figure-7.2b SnS ingot XRD


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Energy dispersive X-ray spectroscopy (EDX) with beam current of 200µA and probe current

of 5.0nA was used for elemental composition of the films. D-8 Discover diffractometer with

Cu-Kα radiation (λ=1.54Ǻ) was used for the structural analysis of the films. J. A. Woollam

variable angle ellipsometry (VASE) was used for the measurement of optical properties of the

films. Band gap, thickness, refractive index and transmittance were measured by the

ellipsometry techniques.

For the electrical properties of the films spectro-photometer and hot point probe techniques

were used. For the photoconductivity measurement molybdenum contacts were deposited 1

mm apart with dc-sputtering machine. Light of variable wavelength from 350nm to 1100nm is

allowed to fall in the absence of light on the film and the response of the photoconductivity is

measured with the help of attached PC controlled with software. It is to be noted that all

measurements are made from tin rich side.

7.3 Results and discussion

The elemental composition of the films was calculated from Energy Dispersive X-ray

Spectroscopy (EDX) as shown in figure 7.3. The EDX measurement was done from tin rich

side where points 6 of the films represent the stoichiometric amount of tin and antimony

metals in our combinatorial films. The contents of the sulphur is constant (≈ 54%) throughout

the sample.

Table-7.1 Elemental composition (atomic percentage)

Figure-7.3 EDX study of one of the library


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The XRD pattern of combinatorial tin antimony sulphide thin films [fig 7.4a] shows that the as

deposited films are in amorphous state and its polycrystalline nature enhanced with the increase

in annealing temperature. When the annealing temperature is increases above 105°C, the

stability in structure start appearing and with further annealing till 325°C, the stability in

Sb2Sn5S9 phase is consistent.

Fig 7.4b is the XRD of Sn-Sb-S thin film with Sn0.7Sb0.3S1.45 stoichiometry. Sb2Sn5S9 phase

was the dominant phase appeared in the films. The crystalline size of the low annealed films is

calculated by the Scherrer’s equation (N. Ali, M. A. Iqbal et al. 2012).

D =0.9λ/βcosθ (1)

Where β is the value of the full width at half maximum (FWHM). The average grain size of the

layers was calculated from diffraction line was of the order of 50nm.


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Figure-7.4(a) XRD pattern of SnSbS combinatorial thin films


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Figure-7.4(b) Sn0.7Sb0.3S1.45 XRD pattern

Figure-7.4(c) Contour graph of XRD annealed at different temperature


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Figure 7.4(c) is the contour graph of XRD annealed at different temperature. The heated

libraries show a reduction in the number of peaks with increase in annealing temperature.

Figure 7.5 is the photoconductivity response of Sb2Sn5S9 library annealed at different

temperature. The photoconductivity in the material due to the generation of free electrons and

holes with the absorption of photons was measured by photoconductivity spectrometer. The

generation of photocurrent is governed by the generation of free excitons from the absorption

of incident photons, the transport of these free excitons through it under the influence of electric

field and their recombination. The actual photocurrent is resulted by subtracting the dark

photocurrent arises due to thermal equilibrium density of the excitons inside material.

The photo-conductivities of 85°C, 105°C and 150°C annealed samples have excellent response

of the photoconductivity in the visible and near infrared regions. The as deposited library

photoconductivity response is very low. Few photoconductivity peaks in the as deposited

library is attributed to the SnS and Sb2S3 binary compounds, used as target materials. It is found

that the tin rich end of the films has good photoconductivity response but as the tin

concentration decreases, the photoconductivity of the layer decreases. Negative

photoconductivity is also noted in few samples.

The negative photo-conductivities have been found in 105°C and above annealed samples. This

is attributed to different phenomena’s during light incident when carrying out measurement.

Romanychev et al and Grigorovich et al referred the negative photoconductivity to the

reduction in the carrier mobility due to the change in the light scattering by impurities and

transition to higher density trapping states (Yunxia Zhang, Guanghai Li et al. 2004; Femrndez,

A. M. and Merino 2000). Zhuze et al and Dobrego et al. attributed the negative

photoconductivity to the noping mechanism. In noping mechanism the conduction is due to

electron jumping from localized/traps to nearby Free State (Arkhipov, V.I., Iovu et al. 1981;

Adriaenssens, G. J., Kuamhih, N at al. 1996). The negative photoconductivity is due to the

variation in carrier concentration because of the presence of high concentration of traps. These

traps serve as recombination centers and reduce number of minority carriers which tend to

decrease the full photocurrent.


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Figure-7.5 Photoconductivity measurement of Sb2Sn5S9 combinatorial thin films library annealed at different

temperature


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Figure 7.6 is the optical transmittance spectra of the as deposited and annealed combinatorial

libraries. It shows that the transmittance of the as deposited library is slightly high comparing

to polycrystalline libraries due to amorphous structure. The transmittance of the films decreases

with annealing temperature due to its transition into polycrystalline structure and remain nearly

constant during the polycrystalline phase. However when the annealing temperature is

increased further the material poly crystallinity remain stable with the reduced number of

impurities. This effect increases the transmittance again slightly and the conductivity peaks

become smooth. It was noted that the transmittance of the antimony rich side decreases due to

the opaqueness of the antimony. The variation in transmittance with phase transition is due to

the presence of low density regions between grains which scatter light and increase absorbance

(Plummer, Deal et al. 2000). The transmittance starts above visible region of spectrum (700nm)

for amorphous libraries and increases to 800nm in the lower edge for some points in

polycrystalline libraries.

The absorption coefficient of the library is calculated by plotting α (cm-1) vs. λ (nm). Where α

is the absorption coefficient and is

4 k

Where k is the extinction coefficient of the material obtained from ellipsometry.

The absorption coefficient of Sb2Sn5S9 thin films is in the energy range 800nm–1150nm. The

absorption coefficient of the library is very high and lies in the range 8×105-2×106cm-1.


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Figure-7.6 Optical transmittance spectra of the Sb2Sn5S9 libraries


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Figure-7.7 Dependence of absorption coefficient of Sb2Sn5S9 thin films annealed at different temperature with

the wavelength of the incident photon


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Figure-7.8 Variation of refractive index of Sb2Sn5S9 thin films of different elemental composition with

wavelength at different annealing temperature

The refractive index “n” of the films has been determined from ellipsometry. The wavy pattern

of n is highly dependent on transmittance of light incident over thin films. It is observed that

the refractive index increases with the antimony concentration. The refractive index at 85°C

annealed combinatorial thin film library lies in the range 2.7-4.0, which increase slightly with


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wave length. The film annealed at 105°C shows a haphazard behavior due to its transition state

of crystallinity. At 150°C the libraries shifted into crystalline phase whose refractive index lies

in the range 2.2-3.6 and slightly decrease with wave length. At higher annealing temperature,

the refractive index increases and lies in the range 1.2-5.0 (Prabahar, Balasubramanian et al.

2010). The refractive index is wavelength dependent which obeys the Cauchy dispersion

relation in the visible spectrum (Rank et al. 1954).

n = n0 + a/λ2+b/λ4 7.1

For higher wavelength the refractive index increases in correspondence with the following

dispersion relation (Hertzberger and Salzberg 1962).

n = A+BL+CL2+Dλ2+Eλ4 7.2

λ is measured in µm and L=1/(λ2-λ02) and the coefficients for IR region are

A= 3.999, B= 3.3917, C= 0.1634, D= -0.000006, E= 0.000000053

For the band gap calculation, the absorption coefficient α can be related with the band gap

energy Eg by the equation (Rabhi, Kanzari et al. 2008)

αhν =A (hν –Eg) n 7.3

Where h is Plank constant, A is constant and n is equal to ½ for a direct band gap and 2 for

indirect band gap semiconductors. The band gap was calculated by extrapolating the straight

section of the (αhν) 2 vs. hν curve to the horizontal energy axis (Gassoumi and Kanzari 2009).

The energy gaps were estimated from extrapolating the curves which correspond to the valance-

conduction band transition is in the range 1.6-3.2eV for different elemental composition and

annealing temperature. The band gap for crystalline phase is in the range of 1.9-2.55eV while

that of amorphous phase is in the range of 1.6-2.65eV. The variation in band gap is due to

different elemental composition as well as annealing temperature which varies the grain size

and hence the band gap.

For the conductivity type, hot point probe technique was used. The known n-type silicon

conductivity was measured initially for calibration (figure-7.11). Then the conductivity type of

Sb2Sn5S9 was calculated (Plummer, Deal et al. 2000). It is observed from the experimental data

that Sb2Sn5S9 has n-type conductivity as in figure-7.10.


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Figure-7.9 Variation of band gap of Sb2Sn5S9 thin films of different elemental composition with wavelength at

different annealing temperature.


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It was also found that the conductivity type changes with high annealing temperature and at

325°C annealed library the conductivity type changes to p-type. The bipolar conductivity in

semiconductors is a classical problem. It is believed that the compensation of native defects

(vacancies and interstitials) is responsible for the bipolar conductivity which is supported by

the theoretical models and fitting of electrical data (Fitzpatrick, Neumark et al. 1983). The

chalcogenide based compounds get converted from a high resistivity region to n-type polarity

and vice versa due to annealing. It is expected that a deviation from stoichiometry will result

in native donor creation leading to bipolar conductivity.

Figure-7.10 Hot point probe measurements of selected points annealed at different temperature


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Figure-7.11 Hot point probe of n-type silicon used for calibration

7.3 Conclusion

This study reveals the fact that Sb2Sn5S9 thin film is a low cost material deposited by vacuum

thermal evaporation technique for solar cells. Combinatorial thin films of Sb2Sn5S9 were

deposited by two source method from SnS and Sb2S3 precursors. The films were annealed at

85°C, 105°C, 150°C, 275°C and 325°C inside sealed glass ampoules containing argon gas. The

elemental composition study was carried out with EDS which confirm the combinatorial

growth of the films. XRD shows that the film has Sb2Sn5S9 phase. The photoconductivity

measurement confirms that the material is highly photo conductive in the visible and NIR

region. Ellipsometry measurement was done for optical properties of the library. The

transmittance of the films is low in the crystalline phase due to low density regions between

grains. The refractive index of the films is found to be 2.2-4.0. The absorption coefficient is in

the energy range 850nm-1100nm have high value of ~105cm-1. The band gap of the films is

found to be in the energy range 1.6eV-3.2eV which is the optimum value for absorbance layer

in solar cell materials. The conductivity type of Sb2Sn5S9 thin film is n-type but changes to p-

type at 325°C annealing.

Chapter 8 Effect of air annealing on the properties of SnSbS thin films

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Chapter 8

Effect of air annealing on the properties of SnSbS thin films

8.1 Air annealing

The studies of low cost, abundant and nontoxic materials have been emphasized for cost

competitive solar cells (Engelken, McCloud et al. 1987). SnSbS semiconductor could be

of particular interest for photovoltaic applications because of its optical energy gap of 1.4

eV comparable to that of silicon (Ristov, Sinadinovski et al. 1989). The metal

chalcogenide thin films are of significant interest because of its continuous use in target

materials for television cameras, microwave, switching and various optoelectronic

devices (Maghraoui-Meherzi, Ben Nasr et al. 2010). H. Zhao et al show that that the

effect of oxygen annealing increase the density and hence improves the rest of optical and

electrical properties. The air annealing increases the process of nucleation and hence

reduces the grain size which leads to the homogeneity and increasing density (Zhao,

Farah et al. 2009). Oxygen is a paramagnetic material and its presence inside the films

leads to increase the conductivity by effecting the carrier concentration of the material.

8.2 Experimental

Sn–Sb–S was deposited by thermal evaporator using the two sources method. The starting

materials were tin sulphide (SnS) and antimony sulphide (Sb2S3). SnS was synthesized

from tin and sulphur powders. Sn and S powders were mixed in crucible and annealed in

argon gas at low pressure for 24 h at 600°C. XRD and EDX of the powder confirmed the

SnS formation. Sb2S3 was purchased from Sigma Aldrich (Kurt J. Lesker) with 99.99%

purity. The powders were pressed into 10 mm diameter pellets employing 6 ton hydraulic

pressure for 1.5 min. Thin films were deposited on glass substrate by evaporating SnS

and Sb2S3 pallets simultaneously in vacuum chamber at 2×104 mbar pressure. The

evaporation rate of Sb2S3 at 40 A filament current was 22.5Å/s and SnS was 21.5 Å /s at

48 A filament current. It takes 5 min to deposit about 1.5mm thick film. The deposited

films were air annealed at 150°C, 200°C and 300°C in a tube furnace. The films were

characterized with XRD (D-8 Discoverer with Cu Kα radiation), energy dispersive X-ray


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spectroscopy (EDX) at a beam current of 200mA and probe current of 5.0 nA, and J.A.

Woollam variable angle ellipsometry (VASE) to obtain the optical absorption and

transmission data of the films. The conductivity type of the films was verified by the hot

point probe method.

8.3 Results and analysis

The elemental composition of the as deposited and annealed thin films was confirmed by

energy dispersive X-ray spectroscopy (EDX). Figure 8.1 shows the EDX spectra for the

annealed and as it is thin films.

Figure-8.1 Elemental composition of as it is Sn0.25Sb0.5S0.5

The XRD of tin antimony sulphide air annealed thin films is shown in figure 8.2. It is

noted that the main peak corresponds to SnSb2S4 phase in air annealed samples. The

particle size calculated from Scherrer’s formula was ~85nm [N. Ali et al 2012]

The XRD patterns of the as-deposited and annealed tin antimony sulphide thin films are

shown in Fig. 8.2. The as-deposited films were amorphous and the crystallinity develops

150°C annealed

200°C annealed

300°C annealed

As it as


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at annealing temperatures around 150°C. In all the annealed films, the peak with

maximum intensity corresponds to the SnSb2S4 phase. Using the Scherrer’s formula, the

grain size of the films annealed at 150°C was around ~23 nm. Incorporation of oxygen

in the films is evident at high annealing temperatures; the peaks appearing can be indexed

to the (110) and (101) reflections of SnO2, consistent with the standard JCPDS no. 41-

1445. Incorporation of oxygen and annealing increases the grain size to 27 and 34 nm for

the films annealed at 200°C and 300°C, respectively.

Figure-8.2. XRD of tin antimony sulphide thin films air annealed. (A) 150°C annealed (B) 300°C

annealed.

The photoconductivity results (Fig. 8.3) of the films show that all films have good

photoconductivity response at high annealing temperature. The as-deposited and 150°C

annealed films have very low photoconductivity response and goes into negative at

certain points. The presence of high concentration of traps lead to variation in carrier

concentration and hence negative photoconductivity observed for the samples. These

traps serve as recombination centers and reduce the number of minority carriers which


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tend to decrease the full photocurrent. When the annealing temperature increases, the

photo activeness of the material start appearing and was found to be in the range of 700

nm–1100 nm for the 200°C annealed films. The change in the spectral region is observed

(500 nm–1000 nm) with further annealing (300°C) and the humps at 600 nm and near

800 nm might be an indication of dual band appearance at high annealing temperature (N.

Ali, Z. Ali et al. 2012). The surface defects at the grain boundaries are positively charged

sulphur vacancies in tin antimony sulphide thin films. When thermal annealing is carried

out, these vacancies are passivated by the oxygen atoms which decrease the charge on the

grain boundaries. This reduces the band bending as well as the recombination probability

for photo-generated electron. This also results in reducing the transmittance and refractive

index while increases the absorption coefficient. Fig. 2B shows the filament output data

from which it is clear that oxygen annealing increase the photoconductivity of the tin

antimony sulphide thin films, the humps appearing in the photoconductivity data is a

characteristic of material not lamp. Two effects are responsible for this enhancement in

photoconductivity, the improvement in crystalline quality with annealing temperature and

the increase in carrier concentration due to oxidation of tin and tin antimony. In a previous

study on the oxygen annealing of CdTe thin films, the paramagnetic nature of oxygen

was responsible for the increase in carrier concentration (Zhao, Farah et al. 2009).


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Figure-8.3 Photoconductivity response of the as it is and annealed films

Figure-8.4 Filament lamp output data

Fig. 8.5 represents the transmittance curve for the samples which shows that the

transmittance decreases with the annealing temperature. The as-deposited and 150°C

annealed films have nearly same transmittance while the transmittance decreases with

annealing temperature. The air annealing increases the traps inside the material leading

to the decrease in the transmittance. For 300°C annealed sample, there is no transmittance

below 850 nm. The maxima and minima of the transmitted spectrum at same wavelength

is an indication of the optical homogeneity of all the films. It also shows that the

transmittance slightly decreases at longer wavelength (Maghraoui-Meherzi, Ben Nasr et

al. 2010).

Figure 8.6 represent the refractive index variation with annealing temperature as well as

with the wavelength. The refractive index of as it as and low temperature annealed have

high refractive index. As the annealing temperature increases, the refractive index

decreases with an average value of 1.9 for 300°C.


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Figure-8.5 Transmittance Vs. wavelength

Figure-8.6 Refractive index n versus wavelength


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The absorption coefficient of all the films is plotted in figure 8.7. Absorption coefficient

of the thermally annealed films is 4×105 cm-1. While that of as deposited is much high

~106cm-1. The variation in optical properties is due to the increase in the density of the

films while annealed in air at atmospheric pressure.

The band gap can be calculated by plotted the absorption coefficient α with the band gap

energy Eg by the equation (Rabhi, Kanzari et al. 2008).

αhν =A (hν –Eg) n 8.1

Where h is Plank constant, A is a constant and n is equal to ½ for a direct band gap and 2

for indirect band gap semiconductors. The band gap was calculated by extrapolating the

straight section of the (αhν) 2 vs. hν curve to the horizontal energy axis (Gassoumi and

Kanzari 2009; N. Ali, Z. Ali et al. 2012). The energy gaps were estimated by extrapolating

the curves which correspond to the valance band-conduction transition.

As shown in Fig. 8.9, the band gap decreases from 2.65 eV for the as-deposited films to

1.45 eV for the films annealed at 300°C. This decrease in band gap can be associated with

increasing grain size of the films upon annealing as confirmed from the XRD

measurements. We therefore suggest that the variation in band gap and other optical and

electrical properties is because of the annealing effect in oxygen atmosphere. At 200°C

annealing in air propose that the effect of oxygen enhance the optical and electrical

properties of thin film. Further increase in the annealing temperature will saturate all the

properties.


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Figure-8.7 Absorption coefficient of Sn-Sb-S

Figure-8.8 the dependence of extinction coefficient on the wavelength and annealing conditions


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Figure-8.9 Energy band gap calculation of SnSbS thin films (a) as it is (b) 150°C air annealed (c) 200°C

air annealed (d) 300°C air annealed

Figure 8.8 gives us the dependence of extinction coefficient on the wavelength and

annealing temperature. Monotonic change is observed in the extinction coefficient with

the incident photon wavelength. It is found that the extinction coefficient decrease with

increase in wave length except the 300°C annealed film for which the extinction

coefficient slightly increases (Prabahar, Balasubramanian et al. 2010). The refraction of

the films is shown in figure 8.10. It is observed that the refraction decreases with the

increase in annealing temperature.


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Figure-8.10 Refraction Vs. wavelength

8.3 Conclusion

Tin antimony sulphide thin films deposited by the thermal evaporation technique has a

band gap of 2.65 eV which was found to be 1.45 eV when thermally annealed at 300°C

in air. This decrease in band gap is because of the increase in grain size upon annealing

and the paramagnetic behavior of oxygen which not only increases the conductivity of

the films but also amends the optical properties of the material. The photoconductivity

response of the as deposited and 150°C annealed films is very low while the

photoconductive response increase with annealing temperature and shifts toward a lower

band edge with high annealing.

Chapter 9 Conclusions

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Chapter 9

Conclusions

9.1 Conclusions

Thin films for photovoltaic application was the main objective of current work. Window

layers and absorbing layers were studied after depositing via thermal and sputtering

techniques for photovoltaics. CdS thin film deposited via thermal evaporator at different

substrate temperature was studied as window layer while SnSbS thin film was studied as

an absorber layer depositing via thermal and sputtering techniques.

9.2 CdS thin films as window layer

Using as window layers in solar cells, the influence of temperature dependence

on Cadmium Sulfide thin film deposited on corning 7059 glass substrate was studied. The

substrate temperature was kept from 150⁰C to 300⁰C with a step of 50°C. Transmittance,

absorbance, band gap and reflectance were calculated by using UV-visible-NIR

spectroscopy. It was found that the transmittance is in visible and near IR region is greater

than 80%. The absorbance of the films is extremely low and very smooth curve showing

absorbance below 6% was observed. The percent reflectance for wavelength range

300nm-400nm was 5% and less than 2.5% for visible and IR regions. The reflectance was

found to be 25% for 200nm. It was found that the resistivity of the films decreases with

increase in thickness and for 0.6µm thin film the resistivity was found to be 80Ωcm. We

also found that the resistivity of CdS thin films increases with an increase in substrate

temperature. The band gap calculated for CdS was 2.42eV. The thermoelectric properties

of the film were measured by hot and cold probe method which shows the n-type nature

of the film.


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9.3 Sputtering of SnSb metallic thin films and its sulphurization

A two stage process involving combinatorial sputtering of metallic targets, tin

and antimony for SnSb metallic combinatorial thin films, followed by its sulphurization

by heating libraries in the presence of elemental sulphur in vacuum thermal evaporator

was synthesized and characterized. The sulphurized films were annealed for 1 hour at

425°C, 450°C, 475°C, 500°C, 525°C in tube furnace in sealed quartz ampoule containing

argon gas at low pressure. The results show the presence of SnSb2S4, Sn2Sb2S5 and

Sb2Sn5S9 phases. The photoconductivity shows that the material is photoactive.

The elemental composition was studied with EDS which confirms the combinatorial

growth of the films. The contour graphs provide enough information about phase’s

formation and we conclude finally that SnSb2S4, Sn2Sb2S5 and Sb2Sn5S9 phases exists for

all the libraries at different annealing temperature. The XRD shows that the peaks of

different compounds have no shift at different annealing temperature suggest that no solid

state solution is form. The combinatorial photoconductivity study leads us to pick a highly

photo active point in the whole library. We see that point 9 with Sn-25%, Sb-40%, and

S-62% in a library annealed at 525°C has maximum photoconductivity response. The

optical properties were not easy to calculate through ellipsometry techniques due to the

roughness of the films surface and the presence of tin sulphide and antimony sulphide

phases. In order to avoid these problems for making the films ready for optical

measurement, proper etchant should be used and therefore I switched out my experiment

by using thermal evaporation technique for the deposition of thin films in a single step.

9.4 Preparation and analysis of argon annealed SnSbS thin films by thermal

evaporation technique

A new and novel approach of combinatorial material synthesis, developed for the first

time for the preparation of SnSbS thin films was employed. Special arrangement was

designed and implemented inside the vacuum chamber already loaded with two sources

via baffles for combinatorial deposition simultaneously evaporated from SnS and Sb2S3

pallets. This deposition technique allowed a large number of depositions over a wide


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range of experimental conditions. Thin film libraries with variable elemental composition

were deposited after optimization of the best possible conditions of vacuum thermal

coater.

The films were annealed at 85°C, 105°C, 150°C, 275°C and 325°C inside sealed glass

ampoules containing argon gas. EDS confirms the combinatorial growth of the library

while XRD shows that the film has Sb2Sn5S9 phase. XRD study reveals that Sn-Sb-S goes

into polycrystalline state with increase in the annealing temperature and 150°C annealed

film have greater crystallinity. The photoconductivity measurement confirms that the

material is highly photo conductive in the visible and NIR region. Ellipsometry

measurement was done for optical properties of the library. The transmittance of all the

films is well above the visible range. While the refractive index calculated lies in between

2.2-4.0 for tin antimony sulphide thin films.

The absorption coefficient for this new absorber tin antimony sulphide thin film has high

value of ~1.5×106cm-1 and is wave length as well as thickness and annealing temperature

dependent. The variation with annealing temperature may be due to thermal disorder and

structural disorder (stained bonds) produced in thin films. The increase in the absorption

coefficient for thin films is due to the presence of states in the energy gap, creation of

defect levels as well as the metallic precursor production. The highest value of the

absorption coefficient in our library was found for Sb18.94Sn26.44S54.61 in the 275°C

annealed sample. Arvind Shah state for amorphous silicon that absorption coefficient of

a- Si is 10 time higher for crystalline silicon due to photo generation of exciton with in

solar cells (Shah 2009).

The band gap of the films is found to be in the energy range 1.5eV-2.5eV which is in

agreement with the band gap values used as absorbing layers in solar cell materials. The

best value of the band gap (1.5eV) is Sb31.15Sn14.34S54.16 in 325°C annealed. From the

overall library it is found that the tin rich side in each library has low band gap and can

be used for the solar cell application.


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The photoconductivity calculated for tin antimony sulphide thin films shows that most of

the tin rich points are photoactive. It was noted that the 150°C annealed library has good

photoconductivity and nearly all the points are photoactive. 150°C annealed thin film

libraries are polycrystalline in phase. There photoconductivity lies from visible to near

infrared region of the solar radiations.

The presences of high concentration of traps lead to variation in carrier concentration and

hence negative photoconductivity observed for the libraries. These traps serve as

recombination centers and reduce number of minority carriers, which tend to decrease the

full photocurrent.

Sn-Sb-S has bipolar conductivity at different annealing temperature. The compensation

of native defects (vacancies and interstitials) is responsible for the bipolar conductivity

in Sn-Sb-S thin films. The chalcogenide based compounds convert from a high resistivity

region to n-type polarity and vice versa due to annealing. It is expected that a deviation

from stoichiometry will result in native donor creation leading to bipolar conductivity

(Fitzpatrick, Neumark et al. 1983).

9.5 Preparation and analysis of air annealed SnSb2S4 thin films by thermal

evaporation technique

In the last section of the thesis, the role of thermal annealing was discussed on tin

antimony sulphide thin films deposited by thermal evaporation technique. The Sn-Sb-S

thin films were annealed at 150°C, 200°C and 300°C in oxygen atmosphere. As oxygen

is a paramagnetic material and its passivation in the thin film via annealing will change

its electrical as well as optical properties. It was noted that the band gap of as deposited

Sn-Sb-S (2.37eV) changes to 1.43ev when thermally annealed at 300°C in air. The

transmittance of the as deposited and 150°C air annealed films starts above 600nm while

the 200°C and 300°C air annealed have low transmittance starting from 850nm.

The photoconductivity response noted for as deposited and 100°C annealed films was

very low while an increase in red shift in photoconductivity is observed for 200°C

annealed film. Further increase in the annealing temperature, increase in the blue shift is


Solar cell Page 128

observed. The absorption coefficient also increases with an increase in wavelength till

NIR region for the films annealed at 200°C and 300°C. While the absorption coefficient

is 1.1×106cm-1 and 4×105cm-1 for as deposited and 150°C annealed film respectively. For

these films (as deposited and 150°C), there is no absorption above the visible range.

The refractive index of the films decreases with increase in annealing temperature. The

refractive index of the as deposited, 150°C and 200°C is in the range of 2.5-4.0, while the

refractive index of the 300°C air annealed is found to be 1.2 and increase with the increase

in wavelength. The variation in optical properties is due to the increase in the density of

the films while annealed in air at atmospheric pressure.

The surface defects at the grain boundaries are positively charged sulphur vacancies in

tin antimony sulphide thin films. When thermal annealing is carried out, these vacancies

are passivated by the oxygen atoms which decrease the charge on the grain boundaries.

This reduces the band bending as well as the recombination probability for photo-

generated electron. This is also resulted in reducing transmittance and refractive index

while increases the absorption coefficient (McEvoy, Markvart et al. 2011).

Future Work

Tin antimony sulphide thin films are n-type at low annealing temperature while have p-type

conductivity at high annealing temperature. Copper dopant has been reported for CdS p-type

conductivity. Cu should be doped for attaining p-type conductivity. The possibility of engineering

the band gap of the SSS film could also be investigated by the addition of doping elements such

as Copper.

List of Publications

Solar cell Page 129


Journals

1. Ali, N. et al. , Effect of air annealing on the band gap and optical properties of

SnSbS thin films for solar cell application Material letters, 100 (2013) 148-151

2. Nisar Ali, et al, Physical Properties of the Absorber Layer Sn2Sb2S5 thin Films

for Photovoltaics, Journal of current Nanoscience, volume 9 page 149-152

(2013).

3. Nisar Ali et al. Structural and optoelectronic properties of antimony tin

sulphide thin films deposited by thermal evaporation techniques, Optik -

International Journal for Light and Electron Optics, Volume 124, Issue

21, November 2013, Pages 4746–4749

4. Nisar Ali et al. Doping effect of Sn on the properties of antimony sulphide

thin films for solar cells. Current Nanoscience Vol 9(4) 2013, 532-535.

5. Ahmad Saeed and Nisar Ali, Photovoltaic effect in the metal based sulfosalt

thin film deposited by physical vapor deposition technique, Chalcogenide

letters, Vol. 10, No. 4, April, p. 143-150.

6. Nisar Ali et al, Sputtering of SnSb thin films and its sulphurization for

solar cell applications, Journal of thermal energy and power engineering

Vol. 2(3) 86-88 (2013)

7. N. Ali, Z. Ali, R. Akram, S. Aslam, M. J. M. N. Chaudhry, M. A. Iqbal, N.

Ahmad, Study of sb28. 47Sn11. 22S60. 32 compound as thin film for photovoltaic

applications. Chalcogenide Letters. 9(8)2012,329.

8. Nisar Ali, S. T. Hussain, M. A. Iqbal, N. Ahmad, Yaqoob Khan, David Lane.,

Combinatorial study of SnSbS thin films by X-Ray Diffraction and

Photoconductivity, Journal of Global Energy Issues, Volume 1, page 1-8

(2013).

9. Ali, N. et al. 2012, Metal based chalcogenide thin films for photovoltaics,

Chalcogenide letters, Vol. 9, No. 11, November, p. 435 – 440.

10. Ali, N. et al. 2013, Optoelectronic properties of the evaporated antimony tin

sulphide thin films for solar cell applications, Renewable Energy Engg.55

(2013)13129-13132.


Solar cell Page 130

11. N. Ali, M.A. Iqbal et al. Optoelectronic Properties of Cadmium Sulfide Thin

Films Deposited by Thermal Evaporation Technique, Key Engineering

Materials Vols. 510-511 (2012) pp 177-185.

12. Ali, N. et al. 2012, Combinatorial synthesis of Sb2Sn5S9 by thermal evaporation

techniques for solar cell applications (with editor) Solar energy materials

13. Ali, N. et al. 2012, Photovoltaic activity in combinatorially deposited

chalcogenide based metallic thin films (with editor) MAEJO Journal

14. Ali, N. et al. 2012, Annealing effect on thermally deposited Tin Antimony

Sulfide (SnSb2S4) thin films for solar cell applications (with editor) Lithuanian

Journal of Physics

15. Ali, N. et al. 2012, Sputtered deposition of Sn-Sb-S thin films for solar cells

application (Manuscript)

16. Ali, N. et al. 2012, monitoring the changes in properties of Sn2Sb2S5 thin films

with Sb variations for solar cell applications. (Manuscript)

17. Ali, N. et al. 2012, Photo-spectrometry to estimate track densities in irradiated

and etched CR-39 foils (Accepted) Pakistan Journal of Engineering and

Applied Sciences

Conferences and workshops

18. Properties of the new absorber antimony tin sulphide thin films for solar cell

applications (Symposium on Hydrogen and Fuel Cells, 9-11 July, 2012)

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Solar cell Page 131

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