fabrication of cadmium sulfide and cadmium telluride...

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





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

http://en.wikipedia.org/wiki/%C3%85

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.

http://www.photometrics.net/fesem.html

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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fabrication of cadmium sulfide and cadmium telluride...

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