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FABRICATION OF CADMIUM SULFIDE AND CADMIUM TELLURIDE
SOLAR CELLS AND THEIR CHARACTERISATIONS
ABDULKAREM ALBALUSHI
A dissertation submitted in fulfilment of the
requirements for the award of the degree of
Master of Science (Physics)
Faculty of Science
Universiti Teknologi Malaysia
JUNE 2014
v
To beloved mother, father and wife
vi
ACKNOWLEDGEMENT
First of all, my heart and soul say thanks to God “ALLAH” (SWT), the Lord
Almighty, for His generousity to giving me the health, strength and ability to
complete this thesis.
I would like to express my sincerest appreciation to my project supervisor
Prof. Dr. Samsudi Sakarani for his advice, guidance, encouragement, spent a lot of
time to assist and consult me in this venture.
Furthermore, this thesis would not have been possible without the very
pleasant and creative working atmosphere that at the Vacuum Laboratory, Physics
Department Faculty Science, Universiti Technologi Malaysia, Skudai.
I shall forever be grateful to my parent, my siblings, their families and my wife
for their belief in me even when I did not and for their unending support, spiritually and
emotionally. To them I am highly indebted and words alone cannot describe my
gratitude.
vii
ABSTRACT
The thin film CdTe based solar cells have the potential for high efficiency
and have been investigated for 40 years due to their variations in flexibility in
manufacturing technology, rapid deposition, and an excellent match to the solar
spectrum. Cadmium Telluride (CdTe) is a promising thin film photovoltaic (PV)
material due to its near ideal bandgap of 1.5 eV and its high optical absorption
coefficient. The typical CdTe thin film solar cell was a substrate configured with a
window layer (CdS). The gold (Au) was used as an absorber (CdTe) and a back
contact. Both semiconductor films, CdS and CdTe, were deposited by high vacuum
evaporation at room temperature, and deposited on glass substrates with fluorine
doped tin oxide (FTO) front contact. The back contact was made to enhance carriers
flow from CdTe layer to the electrode. The structure and morphology of the prepared
cells were determined by X-ray diffraction (XRD), Energy Dispersive X-ray (EDX)
top views and Field Emission Scanning Electron Microscopy (FESEM) cross-
section. The XRD of the prepared films showed a polycrystalline of cadium sulphide
and cadium telluride structure with peaks at 2 = 26.54o and 23.71
o, respectively. It
was found that the distance between the created particles (d) is 3.34 Å for CdS, and
3.6 Å for CdTe. The FESEM image showed clearly the layers of fabricated cell and
the junction between N-type and P-type was a visible from cross-section. The
efficiency of cells was investigated by using three different thicknesses of CdS and
CdTe. Efficiency about 7.98 %, Ics = 19.1 mA and Voc = 0.76 V were achieved by
deposited thin layer of CdS and thick layer of CdTe with a gold as a back contact.
viii
ABSTRAK
Sel-sel solar yang diperbuat daripada tapak filem tipis CdTe mempunyai
potensi kecekapan yang tinggi dan telah dikaji selama lebih 40 tahun kerana
kelebihannya dalam teknologi pembuatan, pemendapan pantas, dan sifatnya yang
sesuai sebagai pasangan kepada spektrum suria. Foto-voltan filem tipis Cadmium
Telluride (CdTe) dikenalpasti sebagai bahan yang berpotensi tinggi kerana keunikan
nilai jurangnya iaitu ~1.5 eV selain pekali penyerapan optiknya yang tinggi. Pada
kebiasaannya, CdTe filem nipis sel solar adalah sejenis substrat yang
dikonfigurasikan dengan lapisan tetingkap (CdS). Emas (Au) digunakan sebagai
penyerap (CdTe) dan penghubung belakang. Proses pemendapan kedua-dua filem
semikonduktor, CdTe dan Cd telah dilakukan menggunakan pemendapan vakum
tinggi yang dibiarkan pada suhu bilik. Kepingan kaca yang mempunyai penghubung
depan oleh fluorin yang didopkan timah oksida (FTO) digunakan sebagai substrat.
Penghubung belakang dibuat untuk meningkatkan pengaliran cas-cas pembawa
daripada lapisan CdTe ke elektrod. Struktur dan morfologi sel-sel dikaji dengan x-
ray pembelauan (XRD), Sebaran Tenaga sinatan-x (EDX) dan keratan rentas Field
Emission Scanning Electron Microscopy (FESEM). XRD filem tersebut
menunjukkan proses polihabluran antara struktur kadmium sulfida dan kadmium
Telluride dengan puncak masing-masing di 2 = 26.54 dan 23.71. Kajian XRD
menunjukkan jarak antara zarah ialah (d) 3.34 untuk Cd dan 3.6 untuk CdTe.
Imej keratan rentas FESEM menunjukkan dengan jelas lapisan-lapisan sel dan
simpangan antara jenis-n dan jenis-p. Kecekapan sel-sel Cd dan CdTe dikaji dengan
menggunakan tiga ketebalan yang berbeza. Menurut kajian yang dilakukan, sel-sel
tersebut menunjukkan nilai kecekapan sekitar 7.98 %, dengan ICS = 19.1 mA dan Voc
= 0.76 V. Nilai ini diperolehi daripada proses pemendapan antara lapisan nipis Cds
dan lapisan tebal CdTe dengan emas sebagai penghubung belakang.
ix
TABLE OF CONTENTS
CHAPTER TITEL PAGE
DECLARATION ii
DEDICATION iii
ACKNOWLEDGEMENT vi
ABSTRACT vii
ABSTRAK viii
TABLE OF CONTENTS ix
LIST OF TABLES xii
LIST OF FIGURES xiii
LIST OF ABBREVIATION/SYMBOLS xv
1 INTRODUCTION 1
1.1 Background 1
1.2 Problem Statement 3
1.3 Research Objectives 5
1.4 Scope of the research 5
1.5 Significant of the study 6
2 LITERATURE REVIEW 7
2.1 Introduction 7
x
2.2 Basics of Solar Cells 8
2.2.1. Photovoltaic 8
2.2.2. Solar Cells 9
2.2.3 CdTe Solar Cells 12
2.2.4 Window Layer (CdS) 14
2.2.5 Thin CdS 14
2.2.6 Deposition Methods 15
2.2.7 Transparent conducting film 16
2.3 Solar cell theory 16
2.3.1 PN Junctions 17
2.3.2 Metal-Semiconductor Contacts 20
2.3.2.1 Schottky Barrier 21
2.3.2.2 Ohmic metal semiconductor contact 25
2.4 Semiconductor Heterojunctions 26
2.5 Solar Cell Parameter 28
2.5.1 Short Circuit Current, Isc 29
2.5.2 Open Circuit Voltage, Voc 29
2.5.3 Fill Factor, FF 30
2.5.4 Efficiency of solar cell 30
3 RESEARCH METHODOLOGY 32
3.1 Introduction 32
3.2 Glass Substrate 33
3.3 Fluorine Doped Tin Oxide (FTO) 33
3.4 Solar Cell Structure 34
3.5 Substrate Preparation and Set up 35
3.5.1 Glass Substrate Preparaion 36
3.5.2 Fabrication of CdS layer 36
3.5.3 The Cadmium Telluride (CdTe) Layer 37
3.5.4 Back Contact 38
3.5.4.1 The gold and silverlayers 39
3.6 Experimental procedures 40
3.7 Equipment 41
3.7.1 High vacuum evaporation 41
xi
3.7.1.1 Operating procedure for HVE 42
3.7.2 X-ray diffraction 44
3.7.3 Field Emission Scanning Electron Microscope (FESEM) 45
3.7.4 IV measuements 46
4 RESULTS AND DISCUSSION 48
4.1 Introduction 48
4.2 XRD Analysis 48
4.3 Field Emission Scanning Electron Microscope (FESEM) 50
4.4 Solar cell efficiency (Light on) 52
4.5 IV measurement in dark condition 58
5 CONCLUSION AND FUTURE WORK 61
5.1 Introduction 61
5.2 Conclusion 61
5.3 Future work 62
REFERENCES 63
LIST OF APPENDICES 66
xii
LIST OF TABLES
TABLE NO TITLE PAGE
4.1 The percentage of weight and atomic of Au, S, Cd and Te 51
4.2 Solar cells efficiency and evaluation under t different power 52
4.3 Comparison between the current resulte and USF group (1991) 53
xiii
LIST OF FIGURES
FIGURE NO. TITLE PAGE
1.1 Market share for PV technologies 2
1.2 Flowchart of the researarch work 4
2.1 Typical structure of a solar cell 10
2.2 Simplified band structure of a direct band gap material at T>0K 11
2.3 Annual growth of different thin film PV technologies from 2008 13
to 2013
2.4 PN junction energy band diagram at equilibrium 18
2.5 PN junction under forward bias 19
2.6 PN junction under reverse bias 20
2.7 Energy band diagrams of (a) metal and (b) P-type semiconductor 21
2.8 Metal-semiconductor Schottky barrier band diagram 23
at equilibrium
2.9 Schottky barrier under (a) forward bias and (b) reverse bias 24
2.10 Band–diagram of an ohmic metal-semiconductor contact 25
2.11 Energy band diagrams of an P-N heterojunction before contact 26
2.12 Energy band diagram of an P-N heterojunction in thermal 27
equilibrium
2.13 IV curve of the solar cell in the Dark and under Illumination 28
3.1 Solar cell structure based on CdTe /CdS mixture 35
3.2 Theoretical efficiency for a given band gap 37
3.3 The fabricated solar cells 39
xiv
3.4 Current experimental works 40
3.5 Thermal evaporation E306-Model at Thin Film laboratory 43
in Univerciti Teknologi Malaysia
3.6 The X-Ray Diffractometer performed in the current research 44
3.7 FESEM analysis facility based on SU8020 Hitachi Company Japan 46
3.8 A special current circuit for I-V measurements 47
4.1 The XRD patterns of: a) CdS and b) CdTe films on glass substrates 49
4.2 FESEM crossection images of the CdS and Au surface deposited 50
on top of CdTe substrates at 27 oC
4.3 EDX spectrum of CdS, CdTe and Au thin films 51
4.4 IV characteristic of our solar cell processed under light on , VOC = 55
0.76 V, Isc= 19.1 mA, FF = 0.55 and efficiency = 7.98 %
4.5 IV characteristic of our solar cell processed under light on, VOC = 55
0.63 V, Isc= 17.8 mA, FF = 0.53 and efficiency = 5.94 %
4.6 IV characteristic of our solar cell processed under light on, VOC = 56
0.58 V, Isc= 16.6 mA, FF = 0.45and efficiency = 4.33%
4.7 IV characteristic of our solar cell processed under light on, VOC 56
= 0.47 V, Isc= 9.7 mA, FF = 0.40, efficiency = 3.6 %
4.8 IV characteristic of our solar cell under dark condition, CdS 57
thickness is 105 nm and CdTe thickness is 3.5 μm
4.9 IV characteristic of our solar cell under dark condition, CdS 58
thickness is 120 nm and CdTe thickness is 2.2 μm
4.10 IV characteristic of our solar cell under dark condition, CdS 58
thickness is 150 nm and CdTe thickness is 1.2 μm
xv
LIST OF ABBREVIATION/ SYMBOLS
A Area of solar cell
A The diode quality factor
Å The angstrom
Ag Silver
Au Gold
oC Celsius
CdCl2 Cadmium chloride
CdS Cadmium sulphide
CdTe Cadmium telluride
CSS Close Space Sublimation
Cu Cupper
d Distance between particles
Eo Vacuum energy
Ec Conduction band energy
EDX Energy Dispersive X-ray
EF Fermi energy level
Eg Energy bandgap
Egn Energy bandgap (electron)
Egp Energy bandgap (hole)
Ev Valence band energy
eV Electron volte
xvi
FF Fill Factor
FESEM Field Emission Scanning Electron Microscopy
FTO Flourine doped Ten Oxide
HF Hydroflouride acid
HVE High Vacuum Evaporation
Im Maximum current
IV Current and Voltage
In2O3 Indium(III) Oxide
Igen Generated current
Isat Saturation current
Isc Short circuit current
Jsc Current dencity
k Poltzmann's constant
K Kelvin
KV Kilovolts
mA Milliampere
MOCVD Meta Organic Chemical Vapor Deposition
mW Milliwatt
n Electron
n Order diffraction
η Efficiency
NA Doping concentration in the holes
ND Doping concentration in the electrons
nm Nanometer
p Hole
Pm Maximum power
q Charge
Si Silicon
Sq Square
T Temperature
TCF Transparent Conducting Films
TCO Transparent Conducting Oxide
UV Ultra violet
µm Micrometer
xvii
V Voltage
VA Voltage applied
Vm Maximum voltage
Voc Open circuit voltage
VTD Vapor Transport Deposition
Χ The electron affinity energy
χn Electron affinity
χp Hole affinity
ZnO Zinc Oxide
ZnO2 Zinc peroxide
Wavelength of X-ray
Diffraction angle
Ohm
B Schottky barrier
m The metal work function
n Fermi level N-type
p Fermi level P- type
s The semiconductor work function
1
CHAPTER 1
INTRODUCTION
1.1 Background
About 66% of the world electrical power provide by fossil fuels, and world's
total energy demands about 95%. As worldwide requirement of energy is more than
supply, due to this reason the total cost of suppling electricity becomes expensive.
Due to use these fossil such as, coal, gas and oil the global warming and climate
have been changed. Also using of fossil fuels to produce electricity and
transportation vehicles products carbon dioxide, sulphur and nitrogen oxide leads to
acide rain. Hence, it is necessary to look for clean, efficient and sustainable form
energy source. The solar energy is one of the most important significant renewable
source of energy, which is abundant. The sun is continual source of light and heat.
About 5,000,000 tons of energy per second emits from the sun in gama ray. All these
rays move to the ground's surface, some of them is absorbed and some of them re-
emitted towards universe. The total energy radiate from the sun toward the earth
about 98% within 0.25μm to 3μm wavelength (Bapanapalli, 2005).
CdTe is a nearly perfect absorber material for solar cell because the bandgap
closely matches the peak of the solar spectrum, relatively high absorption coefficient
and good electronic properties in the polycrystalline phase. Fabricating high-
efficiency CdS/CdTe solar cells with an ultra-thin absorber layer is a challenging yet
highly desirable step in improving CdTe technology. Most of today‟s CdTe solar
2
cells utilize an absorber layer which is about 2.5 μm to 8 μm thick. Thinning this
layer down typically results in poorer cell performance due to shunting, incomplete
photon absorption, fully depleted CdTe layer or interference between the main and
the back contact junction when the CdTe layer thickness approaches a certain limit.
While some of these losses are fundamental, others can be minimized by careful
optimization of the fabrication steps (Rogach, 2000).
Figure 1.1 shows the global market share of the different PV technologies for
the years 2008 and 2009.The crystalline silicon accounts for 85% of the PV market.
The biggest change this past decade has been the emergence of CdTe thin-film
technology from 8% in 2008 to 13% in 2009. First Solar has brought CdTe to mass
production and in 2009 it became the world‟s first PV manufacturer to exceed 1
GW/year production rate capturing 13% of the global market (Palekis, 2011).
Figure 1.1: Market share for PV technologies (Palekis, 2011)
3
1.2 Problem Statement
Cadmium telluride (CdTe) has been recognized as a very promising material
for thin-film solar cells. Furthermore, it is a II–VI compound semiconductor with a
direct optical bandgap 1.5 eV that is nearly optimally matched to the solar spectrum
for photovoltaic (PV) energy conversion. It has a high absorption coefficient, which
means that 99% of photons with energy greater than the bandgap (Eg) that can be
absorbed within 2 cm2 of CdTe film. In contrast, cadmium sulfide (CdS) with its
large bandgap and chemical stability, is an N-type semiconductor used as a window
layer in many types of solar cells in conjunction with absorbers CdTe. The best cell
efficiencies attained so far are 16.5% for CdTe and CdS. Thin films of CdS have
been studied extensively over the past three decades (Wu et al., 2004). Based on the
fact that CdS has been the most widely used and most successful N-type window
layer, thin film solar cells of CdS with CdTe has proposed as a new solar cell.
Various techniques such as chemical vapor deposition, magnetron sputtering, and
chemical bath deposition have been used to make uniform and transparent CdS films
to produce high efficiency solar cells (Kumazawa et al., 1997; Moutinho et al., 2003;
Aksu et al., 2011; Bhandari et al., 2013).
In the present study, different materials have been proposed to produce a high
efficiency thin film solar cell layer by layer based on CdTe and CdS heterojunction
by using high vacuum thermal evaporation. The gold (Au) will be used as a back
contact due to its work function greater than P-type CdTe. Inaddition, silver (Ag)
will be used as electrode to measure the current and voltage that created by the solar
cell.
4
Figure1.2: Flowchart of the research work
Solar Cells Thin Film Fabrication
Cadmium Sulfide
(CdS)
Cadmium Telluride
(CdTe)
Solar Cells
High Vacuum Evaporation
The cell
efficiencies attained
so far are 16.5%
Transparent
Conducting Film
(FTO)
Back Contact
Gold and Silver
Heterojunction
between N-type
and P-type
5
1.3 Research Objectives
This study embarks on the following goals:
i) To fabricate CdS/CdTe solar cells using high vacuum evaporation .
ii) To characterise the structure of CdS/CdTe solar cells.
iii) To determine the performance of electrical characteristics based
on current – voltage measurement.
1.4 Scope of the research
The current study was splited into three main scopes to achieve the stated
objectives. First, the proposed solar cell samples will prepared by using the basic of
N-type and P-type (CdS/CdTe). As a supplementary step, gold and silver will
perform as contact back materials by using high vacuum evaporation. Second, X-ray
diffraction (XRD) and Field Emission Scanning Electron Microscopy (FESEM) will
use to identify the structure and amorphouse phase of the prepared solar cell. Finally,
a significant measurement for the current and voltage (IV) will calculate taking the
consideration of dark and light effect.
6
1.5 Significant of the study
The current study will be done to clarification of making CdS/CdTe solar
cells by using high vacuum evaporation and using gold (Au) as contact layer with
silver (Ag) for electrode. Furthermore, understanding solar cell working based in PN
junction and determines electrical properties of solar cells based on current and
voltage measurement in light and dark condition.
63
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