enhancement of photoelectrochemical characteristics of cds
TRANSCRIPT
An-Najah National University
Faculty of Graduate Studies
Enhancement of Photoelectrochemical Characteristics of CdS Thin Film Electrodes Prepared by Chemical Bath Deposition:
Effect of Annealing and Rate of Cooling
By
Rania Muhammad Abd-Allateef Ismail
Supervisors
Professor Hikmat S. Hilal, Ph. D.
Dr. Amer Hamouz, Ph. D.
Submitted in Partial Fulfillment of the Requirement for the Degree of Master in Clean Energy and Conservation Strategy Engineering, Faculty of Graduate Studies, at An-Najah National University, Nablus, Palestine.
2008
i
Enhancement of Photoelectrochemical Characteristics of CdS Thin Film Electrodes Prepared by Chemical Bath Deposition:
Effect of Annealing and Rate of Cooling
By
Rania Muhammad Abd-Allateef Ismail
Supervisors
Professor Hikmat S. Hilal, Ph. D.
Dr. Amer Hamouz, Ph. D.
This thesis was defended successfully on 15.6.2008, and approved by:
Committee Members Signature
1. Prof. Hikmat Hilal .
2. Dr. Ayman Al Haj Daoud .
3. Dr. Iyad Saadeddin .
ii
Biographical Sketch
Rania M. A. Ismail was born on February 27, 1979 in Tulkarm,
Palestine. In 1997 she graduated from Kafr Zebad Secondary School
(Kafr Zebad, Qalqelia-Palestine). In 2001 she graduated from An-Najah
National University (Nablus-Palestine) with the degree of Bachelor of
Science in Electrical Engineering. In 2005 she enrolled in the Graduate
School at An-Najah National University, and she has been engaged in
research under the supervision of Professor Hikmat S. Hilal and Dr.
Amer Hamouz.
iii
Dedication
To my family, who offered me unconditional love and support
To those who are looking forward to enrich their knowledge .
iv
Acknowledgments
The author wishes to express her sincere appreciation to her
supervisors Professor Hikmat S. Hilal and Dr. Amer Hamouz for their
guidance, help and encouragement throughout research work and writing
up.
Thanks are extended to Mrs. Elisabeth Dufour-Gergam, Institut
d Etudes Françaises (IEF) laboratories, Université Paris XI, France, for
help with SEM measurements. Donation of free FTO/glass samples, by Dr.
Guy Campet of ICMCB, University of Bordeaux, France, is acknowledged.
Many thanks are due to the teaching and technical staff in the
laboratories in the departments of Chemistry at An-Najah N. University.
Research equipment donated by French-Palestinian University Cooperation
program is acknowledged.
The author would like to acknowledge the assistance of each of the
following: Dr. Imad Ibrik, the director of Energy Research Center,
Professor Marwan Mahmoud, from Electrical Engineering Department, Dr.
Abderaheem Abusafa and Dr. Hassan Arafat, from Chemical Engineering
Department for continued assistance.
I wish to express my appreciation to my colleagues at the University.
Last, but in no way least, the patience and encouragement given to me by
my family are truly appreciated.
Rania M. Ismail
Nablus - An-Najah N. University
May-2008 A.C.
1429 H.
v
:
.
Declaration
The work provided in this thesis, unless otherwise referenced, is the
researcher's own work, and has not been submitted elsewhere for any other
degree or qualification.
: Student's name:
: Signature:
: Date:
vi
List of Contents
Title
Page
Committee Decision
. .i Biographical Sketch . . ii Dedication .. .. . ... .iii Acknowledgements . . . ... . iv Declaration . v List of Contents .. .. .. vi List of Tables . .ix List of Figures . .. . . ... x Abstract (in English) .. ... . xiv
Chapter1: Introduction ............................................................................... 1 1.1 Objectives .................................................................................................................. 1
1.2 Hypothesis ................................................................................................................. 3
1.3 Previous Studies ........................................................................................................ 4
Chapter2: Fundamentals & Historical Background ............................... 6 2.1 Potential of Solar Energy ........................................................................................... 6
2.2 Solar Energy Conversion Technologies Overview.................................................... 7
2.3 Photoelectrochemical Cells vs. Photovoltaic Cells ................................................. 10
2.4 Fundamentals of PEC Cells ..................................................................................... 14
2.4.1 How Dark Current Occurs in PEC Cells? .............................................................. 17
2.4.2 How Photocurrent Occurs in PEC Cells? .............................................................. 18
2.4.3 Perturbations of the Ideal Model of PEC Systems ................................................ 20
a) Corrosion ................................................................................................................. 20
b) Surface States .......................................................................................................... 21
c) Doping Instability .................................................................................................... 24
2.4.4 Stability Enhancement of SC in PEC Systems ...................................................... 24
a) Protective Layer ....................................................................................................... 24
b) Chemical and Photochemical Etching ..................................................................... 25
c) Redox Couple and Solvent Variation ...................................................................... 25
d) Annealing of Semiconductors ................................................................................. 26
2.5 Semiconductor Materials for PEC Cells .................................................................. 26
2.6 CdS in PEC Systems? .............................................................................................. 27
2.6.1 Historical Background ........................................................................................... 27
2.6.2 Fundamental Properties of Bulk CdS .................................................................... 29
2.7 SC Thin Film Deposition Techniques ..................................................................... 31
vii 2.7.1 Chemical Bath Deposition (CBD) ......................................................................... 33
2.7.2 Review of CBD-CdS Growth Mechanism ............................................................ 33
2.7.3 Characterization of CBD-CdS Thin Films ............................................................ 34
2.8 What is This Work All About? ................................................................................ 37
Chapter3: Experimental Work ................................................................ 38 3.1 Materials .................................................................................................................. 38
3.1.1 Chemicals and Solvents ......................................................................................... 38
3.1.2 Preparation of CdS Films ...................................................................................... 38
3.1.3 Etching Process...................................................................................................... 40
3.1.4 Annealing Process ................................................................................................. 40
3.1.5 Cooling Process ..................................................................................................... 41
a) Slow Cooling Process .............................................................................................. 41
b) Quenching (Fast Cooling) ....................................................................................... 41
3.2 Equipment ............................................................................................................... 42
3.2.1 Measuring Devices ................................................................................................ 42
3.2.2 PEC Cell Description ............................................................................................. 42
3.2.3 Light Source .......................................................................................................... 43
3.3 Measurements .......................................................................................................... 43
3.3.1 Current Density-Potential Plots ............................................................................. 43
3.3.2 Stability Testing ..................................................................................................... 44
3.3.3 Scanning Electron Microscopy (SEM) Results ..................................................... 44
Chapter 4: Results ..................................................................................... 46 4.1 General Remarks ..................................................................................................... 46
4.2 Effect of Experimental Preparation Conditions on CdS Cell Performance ............. 47
4.2.1 Mono-Deposition Experiments .............................................................................. 47
4.2.2 Multi-Deposition Experiments .............................................................................. 48
4.3 Effect of CdS Film Modification on Absorption Spectrum ..................................... 49
4.4 Effect of CdS Film Modification on Dark J-V Plots ............................................... 52
4.4.1 Effect of Annealing ............................................................................................... 52
4.4.2 Effect of Rate of Cooling ....................................................................................... 53
4.5 Effect of CdS Film Modification on Photo J-V Plots .............................................. 59
4.5.1 Effect of Annealing ............................................................................................... 59
4.5.2 Effect of Cooling Rate ........................................................................................... 59
4.5.3 Effect of Surface Etching ...................................................................................... 65
4.5.4 Effect of Redox Couple ......................................................................................... 65
4.6 Effect of CdS Film Modification on Electrode Stability ......................................... 71
4.6.1 Effect of Annealing ............................................................................................... 71
4.6.2 Effect of Cooling Rate ........................................................................................... 71
4.6.3 Effect of Surface Etching ...................................................................................... 72
4.6.4 Effect of Redox Couple ......................................................................................... 72
4.7 Cell Efficiency Studies ............................................................................................ 76
viii 4.8 SEM Result for CdS Thin Film Electrode ............................................................... 77
4.8.1 Effect of Number of Depositions on SEM Results ................................................ 77
4.8.2 Effect of Annealing on SEM Results ..................................................................... 82
4.8.3 Effect of Cooling Rate on SEM Results ................................................................ 82
Chapter 5: Discussion ............................................................................... 86 5.1 Introduction ............................................................................................................. 86
5.2 Effect of Experimental Conditions on CdS Film Characteristics ............................ 87
5.3 Effect of Annealing on CdS Film Characteristics ................................................... 88
5.3.1 Effect of Annealing on Dark J-V Plots .................................................................. 90
5.3.2 Effect of Annealing on Photo J-V Plots ................................................................. 92
5.3.3 Effect of Annealing on Cell Efficiency ................................................................. 93
5.3.4 Effect of Annealing on Film Stability ................................................................... 94
5.4 Effect of Cooling Rate on CdS Film Characteristics ............................................... 95
5.4.1 Effect of Cooling Rate on Dark J-V Plots ............................................................. 96
5.4.2 Effect of Cooling Rate on Photo J-V Plots ............................................................ 97
5.4.3 Effect of Cooling Rate on Cell Efficiency ............................................................. 98
5.4.4 Effect of Cooling Rate on Film Stability ............................................................... 98
5.5 Effect of Surface Etching on CdS Film Characteristics .......................................... 99
5.6 Effect of Redox Couples on CdS Film Characteristics ......................................... 100
5.6.1 Effect of Redox Couples on the Photo J-V Plots ................................................. 100
5.6.2 Effect of Redox Couples on Electrode Stability .................................................. 102
Conclusions .............................................................................................. 103 Suggestions for Further Work ............................................................... 104 References................................................................................................. 105 Appendix 1 ............................................................................................... 114
ix
List of Tables
Table No.
Title
Page
2.1 PEC cells with CdS electrode 29
2.2 Electrical Properties of CdS 31
2.3 Survey and Classification of Thin-Film Deposition Technologies 32
4.1 Composition of chemical bath, temperature and deposition time.
48
4.2 Composition of chemical bath, repeated depositions 49
4.3 Values of , Voc, Jsc
and for FF of CdS electrodes. 75
x
List of Figures
Figure No.
Title
Page
2.1 Solar energy conversion paths and technologies 9
2.2 Energy level diagram for a p-n junction, showing band bending and the creation of an electron-hole pair upon absorption of a photon with wavelength equal or shorter than threshold wavelength
10
2.3
Energy level diagram for SC/electrolyte interface showing band bending and the creation of an electron-hole pair in n-type SC upon absorption of a photon with wavelength equal or shorter than threshold wavelength. Ec,s and Ev,s mark the surface energies of the conduction and valence edges respectively
11
2.4 Progress in solar cell efficiencies 13
2.5
(a) n-type SC/electrolyte interface (b) n-type semiconductor (c) p-type semiconductor, the two semiconductors are of the same kind. E = electron potential; EC = lower conduction band edge; EV = upper valence band edge; EF = Fermi level; ERed/Ox
= standard redox potentials.
14
2.6
n-type SC/electrolyte junction in the dark (a) before
equilibrium (b) at equilibrium. E = electron potential; EC = lower conduction band edge; EV = upper valence band edge; EF
= Fermi level; ERed/Ox = standard redox potentials. 15
2.7
Photovoltage formation under illumination. E = electron potential; EC = lower conduction band edge; EV = upper valence band edge; EF = Fermi level; ERed/Ox = standard redox potentials.
15
2.8 Operational principles for the two types of PEC cells: a) photoelectrolytic energy storage cell and b) regenerative photovoltaic cell
16
2.9 Dark current for n-type SC 17
xi
2.10 Dark and photo current voltammogram for n-type SC. Jan = anodic current density; Jcat = cathodic current density; Voc = open circuit potential; ( ) dark current; (---) photo current
19
2.11 Photocurrent generation at n-type SC. V>Vfb; a depletion layer is formed. Photogenerated holes move to the surface and oxidize solution resultants
20
2.12 Isoenergetic charge transfer of hole from n-type SC to occupied state of electrolyte redox couple. Redox energy levels have a distribution due to solvent-ion interactions
22
2.13
Mediation of charge transfer by surface state at n-type SC a) Majority carrier injection through the barrier produced by band bending. b) Photo-generated minority carrier injection via surface state
23
2.14 a) cubic (zincblende) structure of CdS. b) hexagonal (wurtzite) structure of CdS.
30
2.15 X-ray spectrum of a typical CBD-CdS sample 35
2.16 Typical photoluminescence spectrum of CdS obtained by CBD. The photoluminescence spectrum was obtained at a temperature of 10 K
36
2.17 AFM pictures of CdS prepared by CBD 36
2.18 Transmission spectrum of CdS prepared by CBD 37
3.1 Three electrode photoelectrochemical cell (PEC). 39
3.2 The annealing system 41
3.3 Experimental arrangement for solution growth of CdS film 43
4.1 a) Ideal dark J-V plot b) ideal photo J-V plot for semiconductor
47
4.2
Absorption spectra of deposited CdS electrodes from 0.12M CdCl2, 0.2M NH4Cl, 2.0M NH3 and 0.6M thiourea chemical bath at 80 C for 30 min. (a) untreated CdS electrode prepared from one deposition, (b) untreated CdS electrode prepared from three depositions, and (c) annealed at 250 C CdS electrode prepared from three depositions.
50
xii
4.3
Photo J-V plots for deposited CdS electrodes from 0.12M CdCl2, 0.2M NH4Cl, 2.0M NH3 and 0.6M thiourea chemical bath at 80 C for 30 min. a) one deposition, b) two depositions, c) three depositions and d) four depositions.
51
4.4
Dark J-V plots for CdS electrodes a) untreated, b) quenched from 150 C, and c) slowly cooled from 150 C.
54
4.5 Dark J-V plots for a) untreated CdS electrode, and quenched CdS electrodes from b) 250 C, and c) 350 C .
55
4.6 Dark J-V plots for a) untreated CdS electrode, and slowly cooled CdS electrodes from b) 250 C, and c) 350 C.
56
4.7 Dark J-V plots for a) slowly cooled and b) quenched CdS electrodes from 250 C.
57
4.8 Dark J-V plots for a) slowly cooled and b) quenched CdS electrodes from 350 C.
58
4.9 Photo J-V plots for a) unheated CdS electrode. Slowly cooled CdS electrodes from b) 150 C, c) 250 C, and d) 350 C.
59
4.10 Photo J-V plots for a) unheated CdS electrode. Quenched CdS electrodes from b) 150 C, c) 250 C, and d) 350 C.
61
4.11 Photo J-V plots of a) Quenched and b) slowly cooled CdS electrodes from 150 C.
62
4.12 Photo J-V plots of a) Quenched and b) slowly cooled CdS electrodes from 250 C.
63
4.13 Photo J-V plots of a) Quenched and b) slowly cooled CdS electrodes from 350 C.
64
4.14 Photo J-V plots of slowly cooled CdS electrodes from 250 C a) unetched and b) etched in HCl (10% v/v).
67
4.15 Photo J-V plots of untreated CdS electrodes in a) KOH/Fe(CN)6
3-/4- and b) S2-/Sx2 aqueous solution.
68
4.16 Photo J-V plots for slowly cooled from 250 C CdS electrode in a) KOH/Fe(CN)6
3-/4- aqueous solution and b) S2-/Sx2 aqueous
solution. 69
4.17 Photo J-V plots for a) untreated CdS electrode and b) slowly cooled from 250 C CdS electrode. Both J-V measurements were conducted in S2-/Sx
2 aqueous solution.
70
4.18 Short circuit current vs. time for CdS thin film electrodes (a) untreated, (b) slowly cooled from 250 C, and (c) quenched from 250 C in KOH/Fe(CN)6
3-/4- aqueous solution 73
xiii
4.19
Short circuit current density vs. time measured for CdS thin film electrodes: (a) untreated unetched, (b) untreated etched in HCl (10% v/v), (c) unetched slowly cooled from 250 C, and (d) slowly cooled from 250 C etched in HCl (10% v/v). All measurements were conducted in KOH/Fe(CN)6
3-/4- aqueous solution.
74
4.20
Short circuit current vs. time for CdS thin film electrodes (a) untreated in KOH/Fe(CN)6
3-/4- aqueous solution, (b) untreated in S2-/Sx
2 aqueous solution, (c) slowly cooled from 250 C in KOH/Fe(CN)6
3-/4- aqueous solution, and (d) slowly cooled from 250 C in S2-/Sx
2 aqueous solution
75
4.21 SEM results for untreated CdS electrodes prepared from one deposition, scale of 5µm.
78
4.22 SEM results for untreated CdS electrodes prepared from two depositions, scale of 5µm.
79
4.23 SEM results for untreated CdS electrodes prepared from three depositions, scale of 5µm.
80
4.24 SEM results for untreated CdS electrodes prepared from four depositions, scale of 5µm.
81
4.25 SEM results for quenched CdS electrodes from 300 C, scale of 5µm.
84
4.26 SEM results for slowly cooled CdS electrodes from 300 C, scale of 5µm.
85
5.1 Photo J-V curves of photocells with a CdS electrode and two different redox electrolytes
101
xiv
Enhancement of Photoelectrochemical Characteristics of CdS Thin Film Electrodes Prepared by Chemical Bath Deposition:
Effect of Annealing and Rate of Cooling
By Rania M. A. Ismail
Supervisors Professor Hikmat S. Hilal, Ph. D.
and Dr. Amer Hamouz, Ph. D.
Abstract
Polycrystalline CdS thin films were prepared by chemical bath
deposition technique (CBD) on fluorine-doped tin oxide (FTO) coated
glass substrates. Enhancement of deposited CdS thin film characteristics at
solid/liquid interface in photoelectronchemical (PEC) systems was
investigated. Deposited CdS thin films were exposed to different treatment
methods and different experimental conditions. The films were heated to
desired temperatures (150 C, 250 C and 350 C) under air. Cooling of
heated films to room temperature was done using two different methods
(slow cooling and quenching). Etching of film surface was conducted using
dilute HCl solution. Different redox couples were also used in the PEC
measurements. The effect of such treatment on electrode PEC
characteristics, such as: open-circuit voltage (Voc), short-circuit current
density (J sc), dark current density-potential (J-V) plots, photo J-V plots,
conversion efficiency ( ), fill factor (FF), Surface Morphology and
stability, was studied.
The characteristics of CdS thin films in PEC systems were enhanced
by using different experimental conditions, controlling preheating
xv
temperatures and cooling rates. Improving the stability of the prepared CdS
electrode by etching and using suitable redox couple was also achieved.
The dark and photocurrent densities vs. potential plots were
improved by annealing. Film stability was also improved by annealing.
Cell efficiency, fill factor, short-circuit current densities Jsc and SEM
results were enhanced for the annealed CdS films. The best annealing
temperature for CdS films was found to be 250 C at which the photo J-V
plots and cell efficiency were improved significantly.
Slowly cooled electrodes from temperatures above 150 C, gave
better dark and photo current density vs. potential plots with higher
efficiency than their quenched counterparts. SEM measurements were
consistent, and showed better surfaces for slowly cooled CdS thin film
electrodes. Maximum values of efficiencies were obtained by slow cooling
of preheated CdS film electrodes from 250 C. Cell efficiency values
exhibited sharp decrease for CdS film electrodes cooled from temperature
350 C compared to that of electrodes cooled from 250 C.
The effect of etching on pre-heated/pre-cooled CdS electrode
stabilization was also studied. Etching of CdS film surfaces improved the
output stability of CdS electrode in the PEC system to a great extent. The
Jsc values of CdS films increases with etching. While etching enhanced the
Jsc vs. time plots; it showed no significant effect on the photo J-V plots.
Two different redox couples were used in the PEC measurement.
KOH/Fe(CN)63-/4- and polysulphide redox couple systems were
xvi
investigated. It was found that PEC cells with KOH/Fe(CN)63-/4- system
gave better photo J-V plots and higher cell efficiency than cells with
polysulphide system. On the other hand, CdS films in polysulphide system
were more stable against photodegration than in KOH/Fe(CN)63-/4- system.
1
Chapter One
Introduction
1.1 Objectives
The main goal of this work is to enhance the characteristics of
polycrystalline cadmium sulfide (CdS) thin film electrodes, at the
solid/liquid interface in photoelectronchemical (PEC) systems. CdS thin
films have been grown using chemical bath deposition technique (CBD) on
fluorine-doped tin oxide (FTO) coated glass substrates. Different methods
have been employed to enhance CdS films, including: (1) studying the
effect of different preparation conditions, (2) investigating the effect of
annealing and cooling rates of CdS thin film electrodes, (3) controlling the
stability of CdS thin film electrodes by surface etching and using different
redox couples.
Enhancements of prepared polycrystalline CdS thin film character-
istics have been investigated by measuring performance parameters
including; open-circuit voltage (Voc), short-circuit current (Isc), dark current
density-potential (J-V) plots, photo J-V plots, conversion efficiency ( ) and
fill factor (FF).
Why polycrystalline thin films?
The current high production cost of conventional solar cells is a
major barrier to large-scale deployment of solar energy. The price of the
2
active material and the manufacturing methods are the main elements
determining the total price of solar energy conversion technologies.
Researchers are required to explore new processes for producing low-cost
methods and material suitable for solar energy conversion. In recent years
there has been considerable interest in the application of semiconductor
(SC) electrodes to PEC solar cells for solar energy conversion [1]. Single
crystal SC electrodes are known to give the highest efficiency for such
purposes. But due to the prohibitive cost of single crystal SC materials, a
necessity to develop less expensive, but stable PEC cells based on thin film
materials, has emerged. Thin film SC materials have a major advantage
over single crystalline, since most of them have direct band-gap, resulting
in higher optical absorption. This allows typical thin film solar cells to use
very thin layers of active materials (~1 m) that can thus be of lower
quality [2].
Why Chemical Bath Deposition Technique?
Thin films are deposited using various techniques such as: vacuum
evaporation, sputtering, spraying, electrodeposition, molecular-beam
epitaxy, laser-induced, electroless plating and CBD [3]. CBD technique is
not the most efficient polycrystalline thin film deposition technique.
However, it appears to be relatively simple, inexpensive and convenient for
large area deposition of polycrystalline thin films. CBD technique has been
intentionally employed in this work, so as to see if our modification
3
techniques are competent or not. If it is found to be satisfactory, it may then
be employed for SC electrodes deposited by other techniques.
1.2 Hypothesis
Direct band gap SC with high absorption coefficients are favorite
candidates for solar cells. CdS belonging to the II IV group is one
important SC used in photovoltaic and PEC solar cells. The basic problem
with CdS deposition is uniformity over large surface area, stability and
reproducibility. However, because of inherent defects present in the
polycrystalline materials, a considerable portion of the photo-generated
charge carriers is lost due to recombination. Considerable efforts are
currently being devoted towards removing these grain boundary effects by
subjecting these films to surface treatments.
In earlier works [4-6], the effect of annealing and cooling rates of
single crystalline n-Si and n-GaAs wafers has been investigated. It has been
found that, n-Si and n-GaAs electrode efficiency can be easily enhanced by
controlling preheating temperatures and rates of cooling. Annealing of
single n-Si and n-GaAs surfaces improves the surface quality by
minimizing defects and improving crystal characteristics. Also, slow
cooling of these SC s surfaces improves crystal properties. If the heated
crystal is slowly cooled, the crystal will be able to gain its original order. If
quenched, the crystal will retain its imperfection. Imperfection will be
exhibited in many ways, such as the ill-defined band-edge structure, and
the distorted crystal surface. To our knowledge, such technique has not
4
been investigated as a tool to enhance the characteristics of CdS thin films.
Therefore, we intend to formulate a convenient method to enhance the
surface structure of CdS thin films for the purpose of enhancing their PEC
properties, for the first time.
In this work, we expect to enhance the characteristics of CdS thin
films prepared by CBD method in PEC systems by controlling film
thickness, preheating temperatures and rates of cooling. Improving the
stability of the prepared CdS electrode by etching and using suitable redox
couple is also expected.
1.3 Previous Studies
The effect of annealing on crystallinity of different semiconductor
types has been conducted earlier. Effect of rate of cooling preheated single
crystalline SC materials, on their characteristics, has also been conducted.
Annealing of semiconductor surfaces plays an important role in
crystallization process of the SC crystals. It enhances the crystals
homogeneity, quality, performance and reliability. Annealing also reduces
defects, and lowers surface roughness. This has been reported in literature
[7-16].
Cooling rate of heated SC electrodes also has an obvious effect on
the quality of the SC crystals through improving their parameters.
Examples of such parameters are: composition uniformity, growth film
thickness, and luminescence properties. The dislocation density and
5
concentration of structural defects also depend on cooling rate of SC
crystals. This has been reported in literature for monocrystallline SC
materials [17-22].
Moreover, etching of CdS thin film enhances the film surface and
improves the surface roughness of the electrode. Consequently, etching
removes recombination centers at the surface and at grain boundaries,
reduces surface impurity and changes surface morphology. This has been
reported in literature [23-26].
Detailed citations of earlier work regarding the effect of heating,
cooling rate and etching on crystal quality of SC are presented in the
discussion sections (Chapter 5, sections 3-5).
6
Chapter Two
Fundamentals & Historical Background
Global warming, environmental pollution, and impending shortage
of fossil fuel sources are factors that force modern society towards an
increase in the utilization of renewable sources of energy. One of the most
abundant resources on the surface of the earth is sunlight. Sunlight reaches
the earth in a quantity that is sufficient to supply the total global energy
consumption [27].
The use of solar energy has many advantages such as [28]:
Solar energy is free and abundant.
Solar energy has low environmental impact.
Solar energy does not require large centralized supplies or expensive
distribution networks.
Solar energy has high public acceptance as a natural form of energy.
2.1 Potential of Solar Energy
Solar radiation represents the largest energy flow entering the terrestrial
ecosystem. After reflection and absorption in the atmosphere, some
(100,000TW [29]) hit the surface of Earth, and undergoes conversion to all
forms of energy used by humans, with the exception of nuclear, geothermal,
and tidal energy. This resource is enormous and corresponds to almost 6,000
fold the current global consumption of primary energy (13.7TW [30]). Thus,
solar energy has the potential of becoming a major component of a
7
sustainable energy portfolio with constrained greenhouse gas emissions.
However, the slow deployment of solar technologies is due to:
The current high production cost of conventional solar cells compared
with other fossil fuel based and renewable technologies.
The intermittent nature of the energy input, and hence the requirement
for energy storage systems to match the energy supply with the
electricity demand and to decrease the capital cost.
If we want solar energy to significantly contribute to the world s
energy supply, more effort has to be put into improving efficiencies while
reducing the manufacturing costs.
2.2 Solar Energy Conversion Technologies Overview
A wide variety of solar technologies have the potential to become a
large component of the future energy portfolio. Passive technologies are
used for indoor lighting and heating of buildings and water for domestic
use. Also, various active technologies are used to convert solar energy into
various energy carriers for further utilization. Two distinct types of energy
conversion systems have emerged over the years, Figure (2.1) [31]:
(1) Solar Thermal Conversion Technologies convert the energy of
direct light into thermal energy using concentrator devices. These systems
reach temperatures of several hundred degrees with high associated
energy. Electricity can then be produced using various strategies including
thermal engines (e.g. Sterling engines) and alternators, direct electron
extraction from thermionic devices, Seebeck effect in thermoelectric
8
generators, conversion of IR light radiated by hot bodies through
thermophotovoltaic devices, and conversion of the kinetic energy of
ionized gases through magneto-hydrodynamic converters [27, 31].
(2) Solar Photonic Processes, in which, the photons of sunlight are
used as the driving force in the conversion process. The use of photons of
the sunlight is possible in several ways [27]:
Photochemical processes, in which, sunlight is absorbed in isolated
molecules, being reactants or catalysts, in solution. The molecule absorbs a
photon and reaches its excited state. The energy of excitation is then
transferred to electrons that are necessary to drive a chemical reaction [27].
Photobiological processes, which are based on absorption of photons
by a leaf chloroplast or algae. Photosynthetic organisms use photons of the
sunlight for an energy storing reaction. Energy storage is based on the
reduction of carbon dioxide to form carbohydrates. It is possible to modify
conditions in these systems such that the photosynthetic process is coupled
to a hydrogen-generating enzyme [27].
Semiconductor systems, in which, the sunlight is absorbed in a
semiconductor material. The absorption of a photon results in the transfer
of an electron from its valence band to its conduction band. This electron
can be used to drive a chemical reaction. The semiconductor can be in the
form of a small particle suspended in a liquid or in the form of a film
deposited on a support, built into a macroscopic unit like a photovoltaic cell
or an electrochemical cell [27].
9
Figure (2.1): Solar energy conversion paths and technologies [31].
10
2.3 Photoelectrochemical Cells vs. Photovoltaic Cells
Both photovoltaic and photoelectrochemical cells are based on using
semiconductor materials in solar energy conversion. These semiconductor
materials need to be photosensitive in a certain part of the solar spectrum to
absorb photons with wavelengths that are equal or shorter than their
threshold wavelengths and this will lead to excitation of an electron from
the upper valence band to the lowest conduction band and creates excess
electrons in the conduction band and excess holes in the valence band [32].
If this semiconductor contains a p-n junction (e.g. a p-type in one
part and n-type in the other, Figure (2.2)), as in photovoltaic cell, electrons
will move downhill to n-type through the solid junction and holes move
upward towards p-type side. These photogenerated electron-hole ( -h+)
pairs are separated with some built-in electric field within the cell resulting
in photovoltage/photocurrent [33].
Figure (2.2): Energy level diagram for a p-n junction, showing band bending and the creation of an electron-hole pair upon absorption of a photon with wavelength equal or shorter than threshold wavelength (reproduced from ref. [34]).
11
Alternatively, when n-type SC electrode is brought in contact with
an electrolytic solution, Figure (2.3), the ( -h+) pairs will be generated
within the depletion layer and the generated electric field will drive
photogenerated holes toward the interface and the electrons will be driven
toward the interior (bulk) of the electrode. By joining a SC electrode with a
suitable counter electrode a PEC cell is constructed [35].
Figure (2.3): Energy level diagram for SC/electrolyte interface showing band bending and the creation of an electron-hole pair in n-type SC upon absorption of a photon with wavelength equal or shorter than threshold wavelength. Ec,s and Ev,s mark the surface energies of the conduction and valence edges respectively (reproduced from ref. [36]).
The only difference between photoelectrochemical and photovoltaic
(PV) devices is that in photoelectrochemistry, a semiconductor electrolyte
junction is used as the active layer instead of the solid-state junctions in
photovoltaic. In both cases, a space charge region is formed where contact
formation compensates for the electrochemical potential differences of
electrons on both sides of the contact.
Potentially, there are several advantages for the use of PEC cells for
solar energy conversion over conventional PV devices [35]:
12
(a) PEC devices can store energy in the form of conventional fuel and
can convert light to electrical energy as well;
(b) PEC devices can be fabricated and modified with considerable ease.
Contrary to PV devices, no solid-solid junctions are demanded in
PEC. The band bending characteristics of the SC can be
conveniently varied by suitable choice of electrolyte and cell
variables;
(c) PEC cells do not have problems associated with different thermal
expansions of solid-solid junction; and
(d) Unlike PV devices, no antireflection coatings are required in PEC
cells.
However, absorption of light by the electrolyte solutions, reflection
losses from the cell and the stability of the electrode limit the energy
conversion efficiencies in PEC devices in many cases and are shortcomings
of PEC systems. Figure (2.4) shows progress in cell efficiencies of
photovoltaic and PEC systems [37].
13
Figure (2.4): progress in solar cell efficiencies (copied from ref.[37]).
14
2.4 Fundamentals of PEC Cells
As we mentioned earlier, PEC systems are based on the formation of
a SC/electrolyte junction when a SC is immersed in a suitable electrolyte
solution. For the SC, the Fermi level (EF) is determined by chemical
potential of majority carriers (electrons in n-type SC and holes in p-type
SC). For electrolyte solution, the redox couples determine the initial
chemical potential [38], Figure (2.5).
Figure (2.5): (a) electrolyte solution (b) n-type semiconductor (c) p-type semiconductor, the two semiconductors are of the same kind. EC = lower conduction band edge; EV = upper valence band edge; EF = Fermi level; ERed/Ox = standard redox potentials.
In n-type SC/electrolyte junction, before equilibrium the EF will
remain above Eredox,. Electrons will therefore flow down to Eredox,
Figure (2.6a). As a result, EF will be lowered. The electron flow will
continue until the equilibrium is established, Figure (2.6b). At equilibrium,
EF and Eredox will match up. This produces a positive depletion region or
space charge layer (SCL) in the SC because of chemical reduction process
A+2 + A+ which will produce an electric field in the depletion layer. As
(c)
(b)
(a)
15
a result, conduction and valence band edges are bent, and a potential barrier
is established against further electron transfer into the electrolyte [39].
Figure (2.6): n-type SC/electrolyte junction in the dark (a) before equilibrium (b) at equilibrium. EC = lower conduction band edge; EV = upper valence band edge; EF = Fermi level; ERed/Ox = standard redox potentials.
Under illumination, light of energy(h ) graeter than SC band gap
(Eg) is absorbed within the SCL of the electrode. The resulting ( -h+) pair
is split apart. The minority carriers (h+) will go to the interface. The
majority carriers ( ) will move into the bulk of the SC electrode,
Figure (2.7), [40].
Figure (2.7): Photovoltage formation under illumination. E = electron potential; EC = lower conduction band edge; EV = upper valence band edge; EF = Fermi level; ERed/Ox = standard redox potentials.
(b)
(a)
16
Using these principles two types of photoelectrochemical cells (PEC
cells) can be built. (1) Regenerative PEC cell used to convert light energy
into electrical energy, and (2) Photoelectrolytic PEC cell used to store
chemical energy, Figure (2.8) [41].
When a redox couple (Ox/Red) is present in the electrolyte
solution, the holes in the valence band oxidize species (Red to Ox) with
a redox potential more negative than (i.e., above) the band edge of the
valence band at the interface. In contrast, electrons in the conduction
band of the semiconductor reach the metal counter electrode surface and
may reduce Ox to Red of another redox couple (Ox /Red ) whose
redox potentials are located below the conduction band (i.e., the redox
potential EOx/Red is positive of (Ox /Red )), thus the net result of the PEC
process: Red + Ox
Ox + Red is thermodynamically uphill reaction
leading to the storage of chemical energy, Fig. (2.8a) [42].
Figure (2.8): Operational principles for the two types of PEC cells: a) photoelectrolytic energy storage cell and b) regenerative photovoltaic cell (copied from ref. [43]).
17
In contrast, when a common redox couple is reaching at both the
photoanode and the metal cathode, no net chemical energy storage
occurs, but electrical energy can be withdrawn in the external circuit,
Figure (2.8b) [42-43].
2.4.1 How Dark Current Occurs in PEC Cells?
The dark current results from electrons transfer from the Conduction
Band to the electrolyte redox couple in n-type SC (cathodic current flow).
At equilibrium, the band bending creates an energy barrier for the
electrons to overcome before they transfer. For any given SC and
electrolyte, there exists a unique potential for which the potential drop
between the surface and the bulk of the electrode is zero (no SCL). This
potential is called the flat band potential (Vfb), since the band edges do not
bend. Thus, for dark current to occur, the n-type SC must be negatively
biased to provide the electrons with enough energy to overcome the barrier,
equals E1, Figure (2.9).
Figure (2.9): Dark current for n-type SC (copied from ref. [43])
18
2.4.2 How Photocurrent Occurs in PEC Cells?
When light absorption generates a population of excited holes and
electrons, the majority carrier concentration changes relatively little and the
minority carrier concentration is greatly enhanced. Hence photo-effects are
greatest when minority carriers dominate the electrode response. This
occurs when the electrode is biased to form a depletion layer and the photo-
generated minority carriers migrate towards the electrode/electrolyte
interface [44].
The SC exhibits a threshold response to photon energy dictated by
the band gap energy. The photo-effects switch on as the wavelength of
incident light ( ) becomes shorter than the threshold wavelength ( g); with
above g the electrode is relatively insensitive to light. A useful relation
between the threshold wavelength g and the band gap energy Eg [45] is:
g=1240/Eg (1)
where g has units of nanometers, and Eg units of electron-volts (eV).
The dependence of the photocurrent or photopotential on excitation
wavelength provides information about the band gap energy Eg and nature
of optical transition (direct or indirect). Recombination is another important
phenomenon of photo-effects in the PEC. Recombination can occur
directly with the electron descending from the conduction band edge to the
hole at the valence band edge, or indirectly via intermediate energy levels
(bulk or surface states). Recombination reduces the magnitude of
19
photoeffect, and consequently lowers the power and the efficiency of PEC
cell. When n-type SC electrode is biased sufficiently positive of Vfb the
dark currents are very low, due to the blocking effect of the depletion layer.
Upon irradiation of SC through the electrolyte with light ( < g), large
anodic photocurrents appear, Figure (2.10).
Figure (2.10): Dark and photo current voltammogram for n-type SC. Jan = anodic current density; Jcat = cathodic current density; Voc = open circuit potential; ( ) dark current; (---) photo current (reproduced from ref. [44]).
These photocurrents arise from the flux holes (minority carrier)
arriving at the surface. The generated electron-hole pairs in the depletion
layer are separated by the electric field in the depletion layer, with the
electron moving towards the bulk of the crystal and the hole migrating
towards the surface, Figure (2.11). Electron-hole pairs generated beyond
the depletion layer also diffuse into the electric field and become separated.
The shape of the photocurrent voltammogram depends on the energy
distribution of the incident photons, the absorption coefficient of the SC,
the diffusion distance of the excited holes and electrons, and the
recombination rates.
20
Figure (2.11): Photocurrent generation at n-type SC. V>Vfb; a depletion layer is formed. Photogenerated holes move to the surface and oxidize solution resultants (copied from ref. [44]).
As the applied potential approaches Vfb the space charge layer
thickness decreases. Recombination rates increase because the holes and
electrons are no longer being separated by the electric field. The
photocurrents drop sharply and merge with the dark current near Vfb , the
electrode is no longer blocking, and the dark current increases dramatically,
with small cathodic photocurrent sometimes observed.
2.4.3 Perturbations of the Ideal Model of PEC Systems
SC electrode behavior is not ideal as discussed in the preceding
section. A number of phenomena occur that add a complication to the ideal
model.
a) Corrosion
Rapid change in the surface composition, results when the SC crystal
becomes in contact with the aqueous electrolytes. The photogenerated holes
E
21
and electrons are generally characterized by strong oxidation and reduction
potentials, respectively. Instead of being injected into the electrolyte to
drive redox reactions they cause SC surface corrosion [46]. Generally an
oxide layer forms and causes surface decomposition. The oxides include
pH sensitivity of Vfb electrode surface. When current flows across the solid-
liquid interface, the surface can undergo further alteration. The most severe
change result when holes oxidizing equivalents arrive at the surface. The n-
type semiconductors are particularly susceptible to photo-corrosion. Both
soluble and insoluble products may be formed. The insoluble products
build up on the surface, and may block current flow [47-48].
b) Surface States
In the usual model of charge transfer across SC/electrolyte interface,
it is assumed that the occurrence of this process is isoenergetic. That is, as
shown in Figure (2.12), electrons can be exchanged between the SC and
electrolyte due to either of two reasons. The first is when the energy levels
of the valence band overlap with the occupied energy levels of the
electrolyte redox couple (anodic current flow by hole injection). The other
reason is when the conduction band overlaps with the empty energy levels
of the electrolyte redox couple (cathodic current flow by electron injection)
[49]. However, experimental results show that the inverse is generally true.
These results are generally explained by involving the existence of surface
states.
22
Figure (2.12): Isoenergetic charge transfer of hole from n-type SC to occupied state of electrolyte redox couple. Redox energy levels have a distribution due to solvent-ion interactions (copied from ref. [49]).
The surface state, or intermediate trapped charges, exists within the
forbidden Eg of the SC [50]. These states result from different sources,
including structural defects, metallic impurities, or bond breaking
processing [51]. Surface states are most effective when they are located
energetically between the band edges, at the surface. They mediate charge
transfer of majority and minority carriers, control surface charge and band
bending, and catalyse recombination of photo-generated carriers. The
existence of surface states lowers the resulting photocurrent density.
Researchers always try to eliminate or minimize surface state formation.
Surface state may be classified as intrinsic or extrinsic [47]. Intrinsic
states arise from the abrupt termination of the crystal lattice at the electrode
surface. The dangling bonds in the crystal will dangle long in an
electrolyte. The new chemical entity formed by reaction with a solution
species may then behave as a surface state. Extrinsic states are created
23
when a new phase is formed at the interface e.g. a surface oxide, or when
the electoactive species (of the redox couple) adsorb to the electrode
surface. They are, in principle, removable when the surface is renewed.
Consider an n-type SC in contact with an electrolyte containing an ox
species. The standard potential for the reduction of Ox is more positive
than Vfb, Figure (2.13a).
Figure (2.13): Mediation of charge transfer by surface state at n-type SC a) Majority carrier injection through the barrier produced by band bending. b) Photo-generated minority carrier injection via surface state (reproduced from ref. [47]).
Surface states can provide an alternative path; the electrons tunnel
through the depletion layer to the surface states and then transfer to the Ox
species. The holes in the valence band are captured by surface states in the
band gap, which then oxidize the Red molecules Figure (2.13b). The
surface states overlap more effectively with the higher Red energy levels.
The arrival of holes at the surface may create surface states.
24
c) Doping Instability
When an electrode is biased in the depletion layer, very strong
electric fields are present in the depletion layer. The field can act on
charged donor and acceptor doping species, causing them to migrate slowly
in the crystal lattice. After several hours or days of use, the doping density
becomes inhomogeneous near the electrode surface.
2.4.4 Stability Enhancement of SC in PEC Systems
The perturbations (corrosion, surface states, and doping instability)
cause sever effects on SC stability in PEC systems. SC stability can be
enhanced by different methods:
a) Protective Layer
The unstable SC is prevented from direct contact with the electrolyte
and the photo-generated carrier can tunnel through the thin protective
layers. Different techniques of protective layer are reported such as:
(i) Thin layer of metallic coating on the surface stabilizes the
electrode. Consequently the device will be transformed into a buried SC-
metal Schottky barrier [52]. The strategy is to shield SC coming into direct
contact with the electrolyte. However, it is obvious that the hole transfer or
tunneling through the metallic layer is not effective as for the case of direct
contact, especially if Schottky barrier exists.
(ii) Coating semiconductors with a wide band gap material (like TiO2,
ITO, or SnO2) which is relatively more stable [53-54]. This strategy is also
not effective for the same reasons in (i).
25
(iii) Electrode stabilization was achieved using electro-active species
attached to the electrode surface. The photo-generated holes first oxidize
the molecules attached to the surface which subsequently oxidize the
reduced species of the electrolyte [55].
(iv) Encouraging results of stabilization have been obtained by growing
thin polymer films on the SC electrode [6, 56].
b) Chemical and Photochemical Etching
Chemical and photochemical etching of many SC electrodes has
been found to reactivate them. It is found that different etchants change
differently the stability and efficiency of some SC electrodes [57-58]. Short
time etching yields better results [59]. Removal of surface impurity and
change in surface morphology are apparently responsible for stabilization.
c) Redox Couple and Solvent Variation
The variation in the nature of redox couple and solvent can profitably
be used in stabilizing SC electrode. Examples of stabilization are described
below:
(i) A fast redox couple is added to the electrolyte which can provide an
alternative path to minority carrier capture. Rapid carrier transfer
effectively competes with corrosion [60].
(ii) Non-aqueous electrolytes containing the appropriate redox couple
sometimes creates less damage to SC electrodes than aqueous electrolytes.
The anodic decomposition potentials have been observed to shift to a more
positive value in non-aqueous solutions [61].
26
(iii) Addition of NaCl and LiCl salts in the aqueous electrolyte seems to
stabilize CdS and CdSe electrodes [61]. This increases the overall kinetics
of hole collection from the CdS and CdSe electrodes.
(iv) Some simple techniques like stirring or relative ratio of redox
species can be used to suppress corrosion.
(v) Various reducing agent species in the electrolyte would give different
stabilities [62].
d) Annealing of Semiconductors
Annealing of semiconductors was found to increase the efficiency of
the PEC cells. This is due to the fact that there is an increase in the grain
size and removal of frozen-in defects in the SC. Consequently, there is an
increase in minority carrier diffusion to electrolyte solution in PEC solar
cell. Surface enhancement in SC can be obtained by annealing of SC [63].
2.5 Semiconductor Materials for PEC Cells
One of the most challenging aspects of the solar cell research is that
of finding an absorber material with needed properties at acceptable cost.
The following basic requirements should be fulfilled [64]:
a) The band gap of the material should be such that maximum part of solar
spectrum is used. Most of the visible part of solar energy reaching the
earth s surfaces is centered in the range 1.0 - 3.0 eV (1240 - 413nm).
b) The efficiency of the cell, which depends on absorption coefficient,
band gap, diffusion length, conductivity, recombination, surface state, etc,
should be high.
27
c) The electrode must be stable against corrosion when placed in the
specific redox electrolyte. Low band gap semiconductors are generally
easily corroded.
Therefore, the choice of semiconductor material for photoelectron-
chemical cells is not an easy task. On the other hand, the form in which the
photoelectrode is used is one of the major factors affecting the efficiency of
the photoelectrochemical cell. Photoelectrodes could be made in the form
of a single crystal, thin film, thick film or pellet. Single crystal electrodes
are known to give the highest efficiency for solar energy conversion; the
electrodes made of pellets are known to produce the lowest yield for a
given material. The most popular semiconductors used in solar energy
conversion can be catalogued as [65]:
1- Elements (e.g. Si and Ge)
2- III-V Compounds (e.g. GaAs, GaP and InP)
3- II-VI Compounds (e.g. CdS, CdSe and CdTe)
4- Transition Metal Di-chalcogenides (e.g. MoSe2 and ZrS2)
5- Ternary compounds (e.g. CuInS2 and CdIn2Se4)
6- Oxides Semiconductors (e.g. TiO2, WO3 and ZnO)
7- Zinc phosphides (e.g. ZnP2)
2.6 CdS in PEC Systems?
2.6.1 Historical Background
In recent years, there has been considerable interest in the use of
direct band gap II-VI binary and ternary compound semiconductors in solar
28
cell application. The most famous semiconductors that belong to this group
are Cadmium Chalcogenides (CdS, CdSe & CdTe) [65]. They comprise one
of the most important classes of semiconductors in photoelectrochemical
research. Of particular interest are CdS photoanode based photoelectro-
chemical (PEC) cells because of its suitable band gap of ~2.42eV, high
index of refraction, high absorption coefficients, long lifetimes, important
optical properties, excellent stability, ease of fabrication and numerous
device applications [16, 66]. It is typically sulfur deficient, possessing the
sulfur vacancies with a high electron affinity. This causes CdS to acquire
electrons easily, resulting in the material of n-type in nature [16].
Historically, the single crystalline CdS photoelectrochemical solar
cell was first reported by Gerischer (1975) [69]. The redox couple was
Fe(CN)63 /4
with a doped tin oxide electrode. The efficiency was about
1.5%. The most stable cell is obtained with an S2 /S22
redox couple [69].
A PEC cell with CdS in iodine electrolyte has also been investigated and
found to be quite unstable. Also, some studies for non-aqueous electrolyte
have been carried out [69]. A number of studied PEC systems with CdS
electrodes are listed in Table (2.1).
Most of the studies reported in literatures are on single crystals. The
change in surface crystallinity has been noted as a reason for unstability
and low efficiency. However, systems of commercial interest would use
polycrystalline thin films because of cost consideration. Unfortunately,
because of inherent defects present in the polycrystalline materials, a
29
considerable portion of the photo generated charge carriers is lost due to
recombination.
Table (2.1): PEC cells with CdS electrode [69].
SC electrode
Mono crystal or
Film
Counter Electrode
Electrolyte Redox Couple
Efficiency
( ) Stability
CdS (Eg=2.4eV)
M SnO2 KCl+K4 Fe(CN)6+ K3
Fe(CN)6 Fe(CN)6
3 /4
1.5-5.5 Unstable
M C KOH+Na2S+S S2 / S22
1.0 -
M Pt KCl+CH3OH - 5.0 Unstable
M Pt KCl+hydroquinone - - -
M Pt NaOH+Na2S+S S2 / S22
6.8 -
M Pt HaOH+Na2S+S S2 / S22
1.7 -
M Pt NaOH+Na2Se - 3.4 -
M Pt NaOH+Na2Te - - -
M - NaOH+Na2(SCH2COO)
+S S2 / S2
2
1.0 -
M - NaI+I2+CH3CN I / I2 9.5 -
M C NaOH+Na2S+S S2 / S22
1.3 Stable
M Pt NaOH+Na2S+S S2 / S22
0.9 Stable
M C Na2S+ Na2S4 S2 / S22
- Stable
M SnO2 glass
or Au NaOH+Na2S+S S2 / S2
2
0.02 -
M Pt NaOH+Na2S+S S2 / S22
0.28 Stable
M Pt NaOH+Na2S+S S2 / S22
0.5 Stable
F Pt NaOH+Na2S+S S2 / S22
- -
- NaOH+Na2S+S S2 / S22
2.5 -
F Pt NaOH+Na2S+S S2 / S22
0.4 Stable CdS:Li (doped)
F Pt NaOH+Na2S+S S2 / S22
2-4 Stable
F Pt NaOH+Na2S+S S2 / S22
0.9 Stable
2.6.2 Fundamental Properties of Bulk CdS
CdS exists in two crystalline forms: hexagonal (wurtzite) phase,
Figure (2.14a) and cubic (zincblende) phase, Figure (2.14b), and it is
possible to grow CdS films in both these phases depending on the
deposition conditions and techniques [67,70].
Experiments on optical reflection and transmission give the
following expression for the value of CdS band gap as a function of
temperature:
(2)
30
where T is measured in K. For T=300K, i.e., at nearly room
temperature, this turns out to be 2.42 eV. The density of intrinsic carriers is
very low and the conductivity is controlled by the presence of natural
defects and impurities. Table (2.2) shows some important electrical
properties of bulk CdS [70].
The resistivity of CdS is generally very high; it is more often
thought of as a semi-insulator rather than semiconductor. Between 100 and
300 K, the electronic mobility follows the behavior predicated for polar
scattering; while for temperatures ranging from 25K down to absolute zero,
piezoelectric scattering is dominant. Substituting chlorine, bromine or
iodine for the sulfur, and aluminium, gallium or indium for cadmium,
creates donor levels at 0.03eV from the conduction band. Copper and silver
give accepter levels of 0.6 eV and 1.0 eV respectively from the valence
band. Sodium, potassium and lithium are acceptor impurities as well [70].
Figure (2.14): a) cubic (zincblende) structure of CdS. b) hexagonal (wurtzite) structure of CdS [67].
(a) (b)
31
Table (2.2): Electrical Properties of CdS [70].
Parameters CdS (Wurtzite) Relative density 4.92 Molecular weight 144.46
Lattice parameters a = 4.136 , c = 6.713
Direct band gap 2.42eV (300K) Effective mass of electrons mn*/mc 0.153 - 0.171
Effective mass of holes mp*/mc 0.7 light holes 5 heavy holes
Thermal conductivity 0.20 W K-1
cm-1
( c axis)
Dielectric constant c = 8.64 c = 8.28
Reflective index 2.3 ( =2µm) 2.26( =14µm)
Electron mobility ~ 400 cm2
V-1 s-1
Hole mobility 15 cm2
V-1 s-1
2.7 SC Thin Film Deposition Techniques
There are many dozens of deposition technologies for material
formation [71-75]. Since the concern here is with SC thin-film deposition
methods for forming layers in the thickness range of a few nanometers to
about ten micrometers, the task of classifying the technologies is made
simpler by limiting the number of technologies to be considered.
Basically, SC thin-film deposition technologies are either purely
physical (such as evaporative methods) or purely chemical (such as gas-and
liquid-phase chemical processes). A considerable number of processes that
are based on glow discharges and reactive sputtering combine both
physical and chemical processes. Such overlapping processes can be
categorized as physical-chemical methods. A classification scheme is
presented in Table (2.3) [75].
32
Table (2.3): Survey and Classification of Thin-Film Deposition Technologies [75].
EVAPORATIVE METHODS
Vacuum Evaporation
Conventional vacuum evaporation
Electron-beam evaporation
Molecular-beam epitaxy (MBE)
Reactive evaporation
GLOW-DISCHARGE PROCESSES
Sputtering
Diode sputtering
Reactive sputtering
Bias sputtering (ion plating)
Magnetron sputtering
Ion beam deposition
Ion beam sputter deposition
Reactive ion plating
Cluster beam deposition
Plasma Processes
Plasma-enhanced CVD
Plasma oxidation
Plasma anodization
Plasma polymerization
Plasma nitridation
Plasma reduction
Microwave ECR plasma CVD
Cathodic arc deposition
GAS-PHASE CHEMICAL PROCESSES
Chemical Vapor Deposition (CVD)
CVD epitaxy
Atmospheric-pressure CVD (APCVD)
Low-pressure CVD (LPCVD)
Metalorganic CVD (MOCVD)
Photo-enhanced CVD (PHCVD)
Laser-induced CVD (PCVD)
Electron-enhanced CVD
Thermal Forming Processes
Thermal oxidation
Thermal nitridation
Thermal polymerization
Ion implantation
LIQUID-PHASE CHEMICAL TECHNIQUES
Electro Processes
Electroplating
Electroless plating
Electrolytic anodization
Chemical reduction plating
Chemical displacement plating
Electrophoretic deposition
Mechanical Techniques
Spary pyrolysis
Spray-on techniques
Spin-on techniques
Chemical Bath Deposition (CBD)
Liquid phase epitaxy
33
2.7.1 Chemical Bath Deposition (CBD)
CBD is a process in which a film of the SC is plated spontaneously
onto a substrate dipped in a bath of appropriate composition. In order to
prepare CdS films by CBD method, a chemical bath containing a cadmium
salt (e.g., CdCl2, CdSO4 or Cd(CH3COO)) as a Cd source, thiourea
( as a sulfur source and ammonia water (NH4OH), with
ammonium salt (e.g., NH4Cl or NH4CH,COO) as a buffer in suitable
proportions is used. The bath temperature is controlled and the substrate
(glass, Ti, ITO, FTO, etc) is immersed in it with continuous stirring
[1, 67, 76-78].
2.7.2 Review of CBD-CdS Growth Mechanism
CBD-CdS thin film growth processes have been discussed by
different authors [1, 67, 77-79]. They have shown that CdS precipitation
takes place by homogeneous as well as heterogeneous reactions. In former
one, Cd2+ ion combines with S2
ion and is precipitated as CdS particles
which are very undesirable in chemical bath deposition technique. These
CdS particles settle on the substrate surface or settle to the bottom of the
beaker and can be used as pure CdS in powder or pallet form. In fact,
heterogeneous reaction is most desirable for CBD technique where CdS
film formation mechanism may be assumed to be due to the adsorption
reaction of Cd2+ from Cd(NH3)42 + and S2- from the hydrolysis of thiourea
onto the glass substrate. This may be shown in the form of chemical
reactions as in Equations ((4)-(8)) [76].
34
(4)
(5)
(6)
(7)
and
(8)
2.7.3 Characterization of CBD-CdS Thin Films
The structural, optical and electrical properties of the prepared CdS
film prepared by CBD are influenced by many parameters such as the
composition of the chemical bath, deposition temperature and pH of the
solution, [55, 80-82].
Ximello-Quiebras et. al. [80] prepared CdS thin films using the same
technique and deposition parameters as we used. They used several
techniques for the structural, morphological, optical and electrical
characterization of the films, as follows:
1. The thicknesses and roughness of the film were determined in a Sloan
Dektak II A profilometer.
2. The optical transmission spectra of the samples were recorded on a
Shimadzu 3101 PC UV-VIS spectrophotometer with air as reference.
3. The morphology of the surface was analyzed with a Park Scientific
Instruments AFM machine with AFM-STM head and ULTRALEVERS
of 0.6 µm points.
35
4. The diffraction patterns were recorded using the radiation of copper (K
radiation, with = 1:54 in a D-500 Siemens equipment.
5. The photoluminescence spectra were recorded using an Ar+ laser as the
exciting beam ( = 457:9 nm, 2.71 eV), and with a 1403-SPEX double
monochromator. The detector was an RCA-C31034 photomultiplier
tube.
The difractogram of an as-deposited CBD-CdS prepared by Ximello-
Quiebras et. al. [80] is shown in Figure (2.15). The X-Ray Diffraction
(XRD) pattern shows only one line that corresponds to the reflection (0 0 2)
of the Greenockita (hexagonal), showing in general that the preferential
orientation of the film is along the (0 0 2) direction. However, we can
observe also that the films have some amorphous component.
Figure (2.15): X-ray spectrum of a typical CBD-CdS sample [80].
36
The photoluminescence spectrum shown in Figure (2.16) shows a
defect structure characteristic of the films obtained by CBD [80], the band
at 1.7 corresponds to sulfur vacancies. The thicknesses of the film ranged
from 500 to 1100 and roughness
ranged from 15 to 50 .
Figure (2.16): Typical photoluminescence spectrum of CdS obtained by CBD. The photoluminescence spectrum was obtained at a temperature of 10 K [80].
The Atomic Force Morphology (AFM) pictures of an as-deposited
CBD-CdS prepared by Ximello-Quiebras et. al. [80] is presented in Figure
(2.17) and can be seen that small grains are grouped to form big clusters
like a cauliflower. The corresponding grain sizes range from tenth of a
micron to 0.5 µm. The pin holes of similar size are also visible in the
Figure.
Figure (2.17): AFM pictures of CdS prepared by CBD [80].
37
Figure (2.18) shows the transmission spectrum of a CBD-CdS
sample prepared by Ximello-Quiebras et. al. [80]. A sharp absorption edge,
at about 450nm (dashed line) corresponding to a band gap of approximately
2.47 eV, can be observed. The transmittance in the high-energy region
extends up to 300 nm, and this is an evidence of disorder effects or
presence of amorphous components in the film.
Figure (2.18): Transmission spectrum of CdS prepared by CBD [80].
2.8 What is This Work All About?
As mentioned earlier, different thin film deposition methods and
different SC enhancement techniques have been reported in literature. In
this work, we study the effect of annealing and cooling rate of
polycrystalline SC thin films, deposited by using CBD method, on their
characteristics in PEC systems. Polycrystalline CdS thin films will be
prepared by CBD method under different experimental conditions to find
the best electrode. The prepared samples will be treated by heating to
different temperatures and by cooling at different rates. The effect of such
treatment on different parameters such as: open-circuit voltages short-
circuit currents, dark J-V plots, photo J-V plots, stability, and SEM
measurements will be investigated in this work.
38
Chapter Three
Experimental Work
3.1 Materials
3.1.1 Chemicals and Solvents
LiClO4, KOH, K3Fe(CN)6, K4Fe(CN)6, CdCl2, thiourea (CS(NH2)2),
NH4Cl and NH3were purchased from Aldrich, and HCl from Frutarom. All
organic solvents (methanol, dichloromethane, DMF) were obtained from
Riedel-DeHa n in a pure form. Highly conductive FTO/glass samples were
kindly donated by Dr. Guy Campet of ICMCB, University of Bordeaux,
France.
3.1.2 Preparation of CdS Films
CdS films have been grown using CBD technology on glass plates
covered by Fluorine-doped tin oxide (FTO) thin film. The choice of
deposition parameters has been guided by kinetic studies carried out by
other authors [80-82]. The experimental arrangement for CdS film
preparation is shown in Figure (3.1). It consists of a chemical bath
containing cadmium chloride, ammonium chloride, ammonium hydroxide
and thiourea in suitable proportions. The bath has been maintained at 80 C
in a constant-temperature oil bath and under constant stirring during the
deposition. This was prepared by adding into the beaker containing 25ml of
stirred deionized water at 80 C, 2.5ml of 0.12M CdCl2, 10ml of 0.2M
NH4C, and 15 ml of 2.0M NH3. Two cleaned FTO/glass substrates, held by
39
substrate holders, were then partially immersed in the solution (about 1×1
cm2 of the substrates were kept out of the solution). The system was firmly
closed using a rubber seal, otherwise ammonia may evaporate out and pH
of bulk solution cannot be controlled. Moreover Cd(OH)2 may deposit
along with CdS. Finally 2.5 ml of 0.6M thiourea was added into the beaker.
The total volume of the bulk solution was kept as 55 ml at pH level 10 to
11.
Figure (3.1): Experimental setup for solution growth of CdS film: 1- beaker (60ml); 2- substrates; 3- chemical bath of CdCl2, thiourea, NH4Cl and NH4OH; 4- oil input for the constant-temperature bath; 5- oil output for the constant-temperature bath; 6- rubber seal; 7- substrate holder; 8- magnetic bar; 9-magnetic stirrer.
The substrate dimensions were 1.0×5.0 cm2. In order to obtain good
adherence and uniformity for the films, it is very important to use clean
substrates in the CBD system. The substrate cleaning steps were: (i)
40
washing with Liquinox soap, (ii) washing with distilled water, (iii) washing
with methanol, (iv) washing again with distilled water, (v) soaking in dilute
HCl (10% v/v), (vi) washing with distilled water, (vi) washing with
methanol, (vii) washing with deionized water, (viii) finally, the substrates
were dried in nitrogen atmosphere and inserted into the chemical bath. The
experiments were carried out in a hood. The deposition time was 30 min.
During the deposition the color of the solution changed from pale yellow to
yellow within 4 or 5 min and finally to bright orange. After each
deposition, the coated substrate was cleaned in distilled water and dried in
nitrogen atmosphere and preserved surface contamination.
3.1.3 Etching Process
The prepared electrodes of CdS use etched were treated by dilute
HCl (10% v/v) solution. The electrode was immersed in the dilute HCl
solution for about 5 seconds and then rinsed with distilled water. The above
procedure was repeated for two or three times to obtain a shiny film
surface. The electrode was then rinsed with distilled water and methanol,
and dried with nitrogen.
3.1.4 Annealing Process
Annealing was conducted using a thermostated horizontal tube
furnace. The prepared CdS film substrates to be annealed were inserted in
the middle of a long Pyrex cylinder. The heat was raised to the desired
temperature (150 C, 250 C or 350 C), under air and was left for 30 min
41
before cooling. The system was left to cool, in the desired method
(quenching or slow cooling), Figure (3.2).
Figure (3.2): The annealing system
3.1.5 Cooling Process
a) Slow Cooling Process
After the film substrates were annealed to the desired temperature,
the temperature was decreased in steps of 50 C, and the annealing system
was left for 20 min intervals at each cooling step. When the cooling time
interval between various temperature cooling steps rises above 30 min, the
furnace was shut down, and left to cool to room temperature. The time
intervals of slow cooling process were varied from between 4-8 hour,
depending on the annealing temperature.
b) Quenching (Fast Cooling)
The annealing system (Pyrex cylinder and film substrate) was left to
cool under continuous flow of air from the desired temperature to room
temperature within 2 min, from the desired temperature. This was achieved
by opening the furnace immediately after switching off the heater, and
allowing the system to rapidly cool in air.
Air
Air
CdS Film electrode
42
3.2 Equipment
3.2.1 Measuring Devices
The measurements of current-voltage data were performed using a
computer controlled Princeton Applied Research (PAR) Model 263A
potentiostat. Light intensity was measured with Lutron-LX 102 light meter.
The light meter, which measures the illuminance in lux units, was
calibrated against a Kipp & Zonen CM11 pyranometer. The pyranometer
measures irradiance in watt.m-2 [55]. The calibration was conducted by
exposing both light meter and pyranometer sensors to sunlight
simultaneously. Therefore, unless otherwise stated, the illuminance used in
our experiments was measured in lux and calibrated into irradiance in
watt.cm-2 units.
3.2.2 PEC Cell Description
The prepared CdS film electrode was incorporated as a working
electrode into a three-electrode one-compartment photoelectrochemical
cell, with a platinum counter electrode and a reference saturated calomel
electrode (SCE) Figure (3.3).
Two types of redox couples were used in this study. The first was:
KOH/Fe(CN)63 /4 (1.0M KOH, 0.05M K3Fe(CN)6, and 0.05M K4Fe(CN)6)
as a redox couple in distilled water (pH 6.5), with 0.1M LiClO4 was used
as supporting electrolyte [55]. The second was: NaOH/S2 /Sx2
as a redox
couple (0.1M Na2S, 0.1M NaOH and 0.1M S) [83]. The solution was
stirred at the beginning, and was stopped as PEC experiment started.
43
High purity nitrogen gas was bubbled through the solution for at
least 5 min before each experiment, and was kept to bubble over the
solution during the experiment to minimize contamination with air.
Figure (3.3): Three electrode photoelectrochemical cell (PEC).
3.2.3 Light Source
Illumination was carried out using a 50 watt Xenon lamp equipped
with a housing and a concentrating lens. The lamp has a high stability and
an intense coverage of wide spectral range, from about 450 to 800 nm,
without any preference [84]. The Xenon lamp was placed at a defined
distance from the working electrode. The illumination intensity on the
electrode was 0.035 W.cm-2.
3.3 Measurements
3.3.1 Current Density-Potential Plots
Current-density-voltage (J-V) plots were measured using the PC
controlled potentiostat described above at room temperature under nitrogen
44
atmosphere. The dark experiments were performed by completely
excluding the light, by covering the system with a black thick cloth. On the
other hand, a 50 W Xenon lamp was used in the photocurrent
measurements.
3.3.2 Stability Testing
Experiments were conducted using the same electrochemical cell.
The values of short circuit current (Isc) were measured over a range of time,
while keeping the electrode under steady illumination (0.035Wcm 2) and
under a 0.00 V bias vs. SCE. Values of short circuit current density (Jsc)
then calculated by dividing the measured short circuit current by the
effective electrode area. Effect of using different redox couples (S2 /Sx2
with different concentrations and KOH/Fe(CN)63 /4 ) on the electrode
stability were studied. The short circuit current Isc was measured with time
throughout the experiment under room temperature.
3.3.3 Scanning Electron Microscopy (SEM) Results
SEM is useful to directly study the surface of solid objects. By
scanning with an electron beam, that is generated and focused by the
operation of the microscope, an image is formed in much the same way as a
TV. SEM allows better surface study than optical microscope. For this
reason SEM produces an image that is a good representation of the three-
dimensional sample.
SEM uses electrons instead of light to form an image. A beam of
electrons is produced at the top of the microscope by heating of a metallic
45
filament. The electron beam follows a vertical path through the column of
the microscope. It makes its way through electromagnetic lenses which
focus and direct the beam down towards the sample. Once it hits the
sample, other electrons (backscattered or secondary) are ejected from the
sample. Detectors collect the secondary or backscattered electrons, and
convert them into a signal that is sent to a viewing screen to produce an
image.
SEM measurements, obtained here, were conducted in Institut
d Etudes Françaises (IEF) laboratories of the Université Paris XI, France.
46
Chapter Four
Results
4.1 General Remarks
In this work, CdS semiconductor thin films have been prepared using
chemical bath deposition technique. Deposited CdS thin films were
exposed to different treatment methods and different experimental
conditions. The influence of different experimental conditions on the PEC
cell performance as, changing deposition time, bath concentration and
repeated depositions have been investigated. The prepared films have been
treated by heating and cooling. The films are heated to a desired
temperature (150 C, 250 C or 350 C) under air. Cooling of heated films to
room temperature has been done using two different methods (slow cooling
and quenching). Two different redox couples have been used in here for the
PEC measurements. The effect of treatment on the films characteristics has
been measured by monitoring different parameters, namely:
1. Values of short-circuit currents and open-circuit voltages.
2. Dark J-V plots: A better dark J-V plot is the one that approaches
ideal dark J-V plot.
3. Photo J-V plots: A better photo J-V plot is the one that approaches
ideal photo J-V plot with higher measured power.
4. Value of short-circuit current and stability testing.
5. Efficiency enhancement.
6. Surface morphology by SEM measurements.
47
The ideal dark J-V plot, Figure (4.1a), can be defined as : a smooth
plot in which the current density remains zero for a given potential, and
suddenly it drops negative at the onset potential (Vonset). An ideal photo J-V
plot, Figure (4.1b), can be defined as: a smooth plot in which the fill factor
approaches 100%.
Figure (4.1): a) Ideal dark J-V plot b) ideal photo J-V plot for semiconductor
4.2 Effect of Experimental Preparation Conditions on CdS Cell Performance
In this work, we investigated the influence of different experimental
conditions on the PEC cell performance as, changing deposition time, bath
concentration and repeated depositions.
4.2.1 Mono-Deposition Experiments
Effects of bath concentrations and deposition time on CdS cell
efficiency have been studied. Table (4.1) shows the composition of the
chemical bath for a set of four representative samples with measured short-
circuit currents and open-circuit voltages in electrolyte solution with
KOH/Fe(CN)63-/4- redox couple.
Jsc
Potential (V) Voc
Vonset
Potential (V)
(b) (a)
48 Table (4.1): Composition of chemical bath, temperature and deposition time.
Exp.
#
CdCl2
(M) Thiourea
(M) Temperature
(C)
Time (min)
Jsc
(mA) Voc (V)
1 0.12 0.6 80 30 0.18 -0.42
2 0.12 0.6 80 15 0.05 -0.22
4 1 1 80 30 0.09 -0.28
5 1 1 80 15 0.11 -0.3
Note: in all cases after the mixing of CdCl2 (2.5ml), 10ml of NH4Cl (0.2M)
and 15ml of NH3 (2.0M) were added and finally 2.5 ml of thiourea (0.6M)
was added into the beaker.
From Table (4.1), it can be shown that the film prepared using
chemical bath of (0.12M CdCl2, 0.2M NH4Cl, 2.0M NH3 and 0.6M
thiourea) at 80 C for 30 min has the best Jsc and Voc. Therefore, unless
otherwise stated, this bath has been used throughout this work.
4.2.2 Multi-Deposition Experiments
Multiple deposition preparations of CdS films have been conducted,
using repeated depositions. In each deposition step, the conditions giving
best results for mono-deposition, Table (4.1), have been employed.
Table (4.2) shows the Jsc and Voc values for a set of CdS films
prepared by repeated depositions with KOH/Fe(CN)63-/4- as a redox couple.
Figure (4.3) shows the photo J-V plots for the resulting samples. A mong
the different samples, the CdS film electrode after three depositions has
highest Jsc value and most negative Voc value. Therefore, unless otherwise
stated, electrodes of three depositions have been used throughout this work.
49
Table (4.2): Composition of chemical bath, repeated depositions
Exp. #
CdCl2
(M) Thiourea
(M) Temperature
(C) Time (min)
# of
Deposition Times
Jsc
(mA)
Voc
(V)
1 0.12 0.6 80 30 One 0.147
-0.12
2 0.12 0.6 80 30 Two 0.171
-0.25
3 0.12 0.6 80 30 Three 0.182
-0.42
4 0.12 0.6 80 30 Four 0.175
-0.28
4.3 Effect of CdS Film Modification on Absorption Spectrum
The influence of repeated depositions and annealing on the
absorption spectra of prepared films were investigated. Figure (4.2) shows
the electronic spectra of: (a) untreated CdS electrode prepared from one
deposition, (b) untreated CdS electrode prepared from three depositions,
and (c) annealed at 250 C CdS electrode prepared from three depositions.
While the absorption edge for sample prepared from one deposition is not
well defined, (Fig.(4.2a), a sharp absorption edge, at about 525nm
corresponding to a band gap of approximately 2.36 eV, can be observed for
sample prepared from three depositions (Fig. (4.2b).
Annealing at 250 C, of triply deposited samples, did not affect the
band gap value, as observed in figures (4.2b-c). This consistent with earlier
literature, where annealing below 300 C did not affect band gap value [10].
50
Figure (4.2): Absorption spectra of deposited CdS electrodes from 0.12M CdCl2, 0.2M NH4Cl, 2.0M NH3 and 0.6M thiourea chemical bath at 80 C for 30 min. (a) untreated CdS electrode prepared from one deposition, (b) untreated CdS electrode prepared from three depositions, and (c) annealed at 250 C CdS electrode prepared from three depositions.
51
Figure (4.3): Photo J-V plots for deposited CdS electrodes from 0.12M CdCl2, 0.2M NH4Cl, 2.0M NH3 and 0.6M thiourea chemical bath at 80 C for 30 min. a) one deposition, b) two depositions, c) three depositions and d) four depositions. All measurements were conducted in aqueous KOH/K4Fe(CN)6/K3Fe(CN)6 at 25 C.
52
4.4 Effect of CdS Film Modification on Dark J-V Plots
4.4.1 Effect of Annealing
The CdS thin film electrodes prepared by three-time deposition were
pre-heated, to different temperatures, and then cooled (slowly or quenched)
to room temperature. Heating and cooling procedures were conducted
under air. The dark J-V plots of the heated electrodes have been affected by
both annealing temperature and cooling rate.
Figure (4.4) shows the dark J-V plots of quenched and slowly cooled
CdS electrodes from 150 C. Low temperature (150 C) annealing showed
no significant enhancement on the dark J-V plots. No significant difference
can be noticed between the dark J-V plots of the untreated electrodes and
the dark J-V plots of electrodes slowly cooled from temperature 150 C.
Moreover, low values of dark current densities have been observed for the
quenched CdS film electrodes from 150 C.
The dark J-V plots of quenched and slowly cooled CdS film
electrodes from 250 C and 350 C are shown in Figures (4.5-6) respectively.
It can be observed that the dark J-V plots of CdS film electrodes have been
enhanced by annealing at 250 C and 350 C. Better dark J-V plots of
quenched and slowly cooled CdS electrodes are obtained. While showing
comparable Vonset values to untreated counterparts, the annealed electrodes
gave higher dark-current densities at same applied bias.
53
4.4.2 Effect of Rate of Cooling
Slowly cooled CdS electrodes from 150 C have no significant
difference compared to untreated counterparts. On the other hand, they
showed higher dark current densities than quenched counterparts, (Fig.4.4).
However, as shown in figures (4.7-8), slowly cooled CdS electrodes
from 250 C and 350 C showed higher dark current densities than the
quenched counterparts.
From Figures (4.4-8), it can be concluded that:
Enhancement of dark J-V plots of CdS electrodes by annealing depends
on the treatment temperature and on rate of cooling.
Slow cooling of CdS electrode, heated at 150 C, 250 C and 350 C,
shows better dark J-V plots than quenching.
Slow cooling from 250 C shows the best dark J-V plots.
54
Figure (4.4): Dark J-V plots for CdS electrodes a) untreated, b) quenched from 150 C, and c) slowly cooled from 150 C. All measurements were conducted in aqueous KOH/K4Fe(CN)6/K3Fe(CN)6 at 25 C.
a
b
c
55
.
Figure (4.5): Dark J-V plots for a) untreated CdS electrode, and quenched CdS electrodes from b) 250 C, and c) 350 C. All measurements were conducted in aqueous KOH/K4Fe(CN)6/K3Fe(CN)6 at 25 C.
56
Figure (4.6): Dark J-V plots for a) untreated CdS electrode, and slowly cooled CdS electrodes from b) 250 C, and c) 350 C. All measurements were conducted in aqueous KOH/K4Fe(CN)6/K3Fe(CN)6 at 25 C.
57
Figure (4.7): Dark J-V plots for a) quenched, and b) slowly cooled CdS electrodes from 250 C. All measurements were conducted in aqueous KOH/K4Fe(CN)6/K3Fe(CN)6 at 25 C.
58
Figure (4.8): Dark J-V plots for a) quenched, and b) slowly cooled CdS electrodes from 350 C. All measurements were conducted in aqueous KOH/K4Fe(CN)6/K3Fe(CN)6 at 25 C.
59
4.5 Effect of CdS Film Modification on Photo J-V Plots
4.5.1 Effect of Annealing
Generally speaking, preheated multi-deposited CdS film samples
showed better photo J-V plots than untreated samples. As explained earlier,
better photo J-V plots mean higher Jsc value and more negative Voc.
Photo J-V plots, (Figure 4.9), observed for CdS thin film electrodes
quenched from 250 C and 350 C are better than
J-V plots for the untreated
counterpart. Photo J-V plots, measured for samples quenched from 150 C,
are not significantly different from untreated counterparts. CdS samples,
slowly cooled from any preheating temperature, have better photo J-V
plots, (Figure 4.10), than the untreated counterparts.
4.5.2 Effect of Cooling Rate
CdS thin film electrodes, slowly cooled from (150 C, 250 C and
350 C) have better photo J-V plots than their quenched counterparts.
Figures (4.11-13) show the photo J-V plots for slowly cooled electrodes
and for quenched electrodes from 150 C, 250 C and 350 C respectively.
The best photo J-V plots are for slowly cooled samples from
temperature 250 C, (Fig. 4.10) where the short circuit current density has a
value is four-fold that for the untreated samples. The short-circuit current of
the photo J-V plots, measured for samples quenched or slowly cooled from
temperatures 350 C, is found to sharply decrease compared with that of
samples quenched or slowly cooled from temperatures 250 C, Figures
(4.9-10).
60
Figure (4.9): Photo J-V plots for a) unheated CdS electrode, and quenched CdS electrodes from b) 150 C, c) 250 C, and d) 350 C. All measurements were conducted in aqueous KOH/K4Fe(CN)6/K3Fe(CN)6 at 25 C.
61
Figure (4.10): Photo J-V plots for a) unheated CdS electrode, and slowly cooled CdS electrodes from b) 150 C, c) 250 C, and d) 350 C. All measurements were conducted in aqueous KOH/K4Fe(CN)6/K3Fe(CN)6 at 25 C.
62
Figure (4.11): Photo J-V plots of a) Quenched and b) Slowly cooled CdS electrodes from 150 C. All measurements were conducted in aqueous KOH/K4Fe(CN)6/K3Fe(CN)6 at 25 C.
63
Figure (4.12): Photo J-V plots of a) Quenched and b) Slowly cooled CdS electrodes from 250 C. All measurements were conducted in aqueous KOH/K4Fe(CN)6/K3Fe(CN)6 at 25 C.
64
Figure (4.13): Photo J-V plots of a) Quenched and b) Slowly cooled CdS electrodes from 350 C. All measurements were conducted in aqueous KOH/K4Fe(CN)6/K3Fe(CN)6 at 25 C.
65
4.5.3 Effect of Surface Etching
The effect of etching CdS film electrode, on PEC characteristics has
been studied. Multideposited CdS electrodes, slowly cooled from 250 C,
have been etched by dilute HCl (10% v/v) solution. Etching of CdS
electrode surface did not enhance the photo J-V plots. Figure (4.14) shows
the photo J-V plots of etched electrodes with respect to their unetched
counterparts. Despite this, etching showed effects on electrode stability, as
will be shown later.
4.5.4 Effect of Redox Couple
All experiments, discussed so far, employed aqueous
KOH/Fe(CN)63-/4- as a redox couple in the aqueous electrolyte of the PEC
system. The effect of using different redox couples has been studied here.
Polysulphide electrolyte, (S2-/Sx2- redox couple) with different
concentrations of NaOH, Na2S and S, has been used.
In case of electrolyte consisting of 1M of NaOH, 1M of Na2S and
1M of S, the CdS thin film surface has been degraded rapidly and the FTO
layer became in direct contact with the electrolyte solution leading to a
short circuit error. With redox couple concentrations 0.1M of NaOH, 0.1M
of Na2S and 0.1M of S, the photo J-V plot was more stable. Therefore,
redox couple concentrations 0.1M of NaOH, 0.1M of Na2S and 0.1M of S
have been used.
66
Figure (4.15) shows the photo J-V plots of untreated CdS electrodes
in two different redox systems. Photo J-V plots of CdS electrode slowly
cooled from 250 C in two different systems are shown in Figure (4.16).
The PEC system with KOH/Fe(CN)63-/4- redox system gives a better photo
J-V plot than S2-/Sx2- redox system, but the latter system is more stable, as
will be shown later.
As expected, pre-annealed CdS electrodes used in S2-/Sx2- redox
system give better photo J-V plots than their untreated counterparts. Figure
(4.17) shows the photo J-V plots of untreated CdS electrode and slowly
cooled CdS electrode from temperature 250 C using S2-/Sx2 redox system.
In this aspect, the results parallel those observed for the KOH/Fe(CN)63-/4-
redox system, (Figure 4.10).
67
Figure (4.14): Photo J-V plots of slowly cooled CdS electrodes from 250 C a) unetched and b) etched in HCl (10% v/v). All measurements were conducted in aqueous KOH/K4Fe(CN)6/K3Fe(CN)6 at 25 C.
68
Figure (4.15): Photo J-V plots of untreated CdS electrodes in a) aqueous KOH/Fe(CN)63-/4- redox system, and b)
aqueous S2-/Sx2 redox system.
69
Figure (4.16): Photo J-V plots for slowly cooled from 250 C CdS electrode in a) aqueous KOH/Fe(CN)63-/4-
redox system and b) aqueous S2-/Sx2 redox system.
70
Figure (4.17): Photo J-V plots for a) untreated CdS electrode and b) slowly cooled from 250 C CdS electrode.
Both J-V measurements were conducted in aqueous S2-/Sx2 redox system.
71
4.6 Effect of CdS Film Modification on Electrode Stability
The stability of CdS thin films under PEC conditions has been
studied. The value of Jsc vs. time has been measured; using 0.0V applied
potential (vs. SCE). The effects of different parameters, such as annealing,
rate of cooling, etching and redox couples, on Jsc vs. time plots, have been
studied.
4.6.1 Effect of Annealing
Effect of annealing on film stability has been studied using aqueous
(KOH/Fe(CN)63-/4-) systems under illumination. Plots of Jsc vs. time have
been measured for untreated electrodes and for pre-heated/pre-cooled ones
from 250 C using slow and rapid cooling. Figure (4.18) summarizes these
findings. In each case, the Jsc vs. time plots start with very small Jsc values,
which increase with time, leveling at almost a steady value for more than
240 min. This indicates the relative stability of CdS thin films under PEC
conditions. As expected, the preheated CdS electrodes show higher Jsc
values than untreated counterparts. Quenched CdS electrode from
temperature 250 C has a value of Jsc five-fold than their untreated
counterpart. Slowly cooled CdS electrode from temperature 250 C shows a
rather higher value of Jsc, seven-fold, than their untreated counterpart.
4.6.2 Effect of Cooling Rate
Figures (4.18) indicate that the Jsc values have been improved by
annealing at 250 C. Consistent with earlier results, slowly cooled
electrodes exhibited better enhancing effect on Jsc values than quenched
72
counterparts. Slowly cooled CdS electrode from temperature 250 C shows
a higher value of Jsc , 1.5-fold, than the quenched counterpart.
4.6.3 Effect of Surface Etching
CdS thin films have been etched by dilute HCl (10% v/v) solution.
The effect of etching on stabilizing the CdS electrode surface has been
studied while using aqueous (KOH/Fe(CN)63-/4-) systems under
illumination. Figure (4.19) shows the effect of etching on untreated and
pre-heated/pre-cooled CdS electrode stabilization. The Jsc values of CdS
films increase with etching, reaching 1.5-fold the unetched counterparts
throughout the 240 min exposure period.
It is worthwhile to note that, while etching affected the Jsc vs. time
plots, it showed no significant effect on the photo J-V plots as discussed
earlier.
4.6.4 Effect of Redox Couple
The effect of using different redox couples on the surface stability of
different CdS films was studied. Values of Jsc vs. time (at applied potential
0.0 V vs. SCE) were measured using polysufide and (KOH/Fe(CN)63-/4-)
systems. Untreated CdS electrodes showed better Jsc vs. time plots, when
studied using low concentration polysufide (NaOH 0.1M/Na2S 0.1M/S
0.1M) redox couple than when using (KOH/Fe(CN)63-/4-) counterparts.
Moreover, the preheated CdS electrodes showed higher Jsc values (two-
fold) using polysulfide redox couple than when using (KOH/Fe(CN)63-/4-)
counterparts, (Fig. 4.20).
73
Figure (4.18): Short circuit current density vs. time measured for CdS thin film electrodes: (a) untreated, (b) slowly cooled from 250 C, and (c) quenched from 250 C. All measurements were conducted in KOH/Fe(CN)6
3-/4- aqueous solution.
74
Figure (4.19): Short circuit current density vs. time measured for CdS thin film electrodes: (a) untreated unetched, (b) untreated etched in HCl (10% v/v), (c) unetched slowly cooled from 250 C, and (d) slowly cooled from 250 C etched in HCl (10% v/v). All measurements were conducted in KOH/Fe(CN)6
3-/4- aqueous solution.
75
Figure (4.20): Short circuit current vs. time for CdS thin film electrodes (a) untreated in KOH/Fe(CN)63-/4-
aqueous solution, (b) untreated in S2-/Sx2 aqueous solution, (c) slowly cooled from 250 C in KOH/Fe(CN)6
3-/4-
aqueous solution, and (d) slowly cooled from 250 C in S2-/Sx2 aqueous solution.
76
4.7 Cell Efficiency Studies
The light-to-electricity conversion efficiency of CdS electrodes has
been enhanced by modification. Values of electrode percentage conversion
efficiencies and fill factor values are calculated for different electrodes.
Table (4.3) summarizes data on electrode conversion efficiency ( ), Voc, Jsc
and fill factor (FF). Table (4.3) shows that slow cooling of CdS electrodes
from temperatures 150 C, 250 C and 350 C gives higher efficiency than
quenching. Among different systems, maximum values of efficiencies have
been obtained by slow cooling of preheated CdS samples from temperature
250 C, Table (4.3, Entry 5).
Table (4.3): Values of , Voc, Jsc and for FF of CdS electrodes.* Entry number
Annealing Temperature ( C)
Cooling Rate Voc Jsc
(%)** FF (%)***
1 Room Temp. - -0.42 0.18 0.21 21
2 150
Quenched -0.44 0.19 0.23 35
3 Slow -0.46 0.22 0.28 30
4 250
Quenched -0.51 0.4 0.6 41
5 Slow -0.58 0.79 1.3 42
6 350
Quenched -0.56 0.34 0.54 33
7 Slow -0.49 0.42 0.55 41 * All measurements were conducted in aqueous KOH/K4Fe(CN)6/K3Fe(CN)6 at 25 C.
** (%)= [(maximum observed power density)/(reach-in power density)] ×100%. ***FF = [(maximum observed power density)/Jsc×Voc] ×100%.
77
4.8 SEM Result for CdS Thin Film Electrode
The study of CdS electrode surface quality was conducted in a
parallel manner to electrode PEC characteristics study. Scanning Electron
Microscopy was used to study the electrode surface quality. While
investigating effects of different preparations and treatment on PEC
characteristics, as shown earlier, we also investigated effects of such
treatment on electrode surface quality (SEM micrographs) as shown here.
4.8.1 Effect of Number of Depositions on SEM Results
SEM results for CdS thin film samples with different numbers of
depositions have been obtained. Figures (4.21-24) show the SEM results of
untreated CdS films prepared from one, two, three and four depositions
respectively. It is clear that film prepared from three and four depositions
have more uniform film surfaces and larger grain sizes than those prepared
from one and two depositions. .
The SEM measured for CdS film prepared with four depositions
shows better surface than that with three depositions, but the higher
thicknesses of these samples may result in photo J-V plots with lower
efficiency, as shown earlier.
SEM results, observed so far, are consistent with photo J-V plots
shown earlier, (Fig. 4.3). In each case, CdS thin film electrodes prepared
with three depositions showed better J-V plots and SEM results than that
with one, two or four depositions.
78
Figure (4.21): SEM results for untreated CdS electrode prepared from one deposition, scale of 5µm.
79
Figure (4.22): SEM results for untreated CdS electrode prepared from two depositions, scale of 5µm.
80
Figure (4.23): SEM results for untreated CdS electrode prepared from three depositions, scale of 5µm.
81
Figure (4.24): SEM results for untreated CdS electrode prepared from four depositions, scale of 5µm.
82
4.8.2 Effect of Annealing on SEM Results
CdS thin film that were heated and then cooled to room temperature,
show higher quality surfaces than untreated counterparts. This was evident
in both quenching and slow cooling processes.
The SEM surface quality results for the untreated CdS thin film
electrode, shown in Figures (4.21-24) are rough, with small grain size and
many islands and aggregates.
SEM indicates that CdS film surfaces, quenched from 300 C, have
rough surface, but to a lesser extent and larger grain size than untreated
ones, (Fig. 4.25). The grain size was enhanced by treatment. Moreover,
slow cooling of heated samples reduces the surface roughness and
increases the grain size, compared to untreated and quenched ones, (Fig.
4.26).
4.8.3 Effect of Cooling Rate on SEM Results
SEM results for samples, quenched and slowly cooled, from 300 C
are shown in figures (4.25-26). The slowly cooled samples show better
surface quality than the quenched counterparts. The slowly cooled samples
show a crosslinked surface with no islands, aggregates, or cracks.
Therefore, SEM results indicate that slowly cooled surfaces give better CdS
film surfaces. Figure (4.25) shows that the CdS surface quenched from
300 C involves special dark holes which may be associated with peeling
out of CdS from FTO surface due to shrinkage. On the other hand, slowly
cooled counterparts (Fig. 4.26) did not show any similar dark holes.
83
SEM results, observed so far, are consistent with J-V plots shown
earlier. In each case, the CdS thin film electrodes were enhanced by
heating. Furthermore, the slowly cooled samples showed better J-V plots
and SEM results than their quenched counterparts.
84
Figure (4.25): SEM results for quenched CdS electrodes from 300 C, scale of 5µm.
85
Figure (4.26): SEM results for slowly cooled CdS electrodes from 300 C, scale of 5µm.
86
Chapter Five
Discussion
5.1 Introduction
The rate of cooling of heated metal crystal affects the crystal
properties. For example, it is reported that quenching (rapid cooling) of
heated aluminum rods prevents its contraction to its original length [85].
The metastable spheres, in the heated crystal, will not return to original
stable positions with quenching due to kinetic restriction. Alternatively,
slow cooling will kinetically allow the return of metastable spheres to
original positions. Therefore, it is assumed that slow cooling improves
crystallinity, by allowing removal of imperfections.
Annealing of single SC wafers was reported to enhance their
character-istics, such as crystal structure, surface and doping distribution.
Slow cooling has also been reported to enhance SC characteristics [63, 86-
91]. Slowly cooled crystals may retain their original order. On the other
hand, quenched crystals will retain imperfection. Such imperfections will
be exhibited in many ways, such as ill-defined band-edge structures,
distorted crystal surfaces and increased surface states. This will
consequently affect the dark- and photo-current/potential plots.
Moreover, chemical and photochemical etching of many SC
electrodes reactivates them. It is found that different etchants change
differently the stability and efficiency of some SC electrodes [57-59].
87
Removal of surface impurity and change in surface morphology are
apparently responsible for stabilization.
We hereby utilized such ideas to improve the PEC characteristics of
CdS thin film semiconductor. To our knowledge, such technique has not
been investigated as a tool to enhance the characteristics of CdS thin films.
5.2 Effect of Experimental Conditions on CdS Film Characteristics
In this work, CdS cell efficiency has been affected by different
parameters such as, changing deposition time, bath concentration and
multiple depositions. We found that CdS films prepared from a bath of low
concentrations (0.12M CdCl2 and 0.6M thiourea) have a shiny surface and
a better photo J-V plot than films prepared from a bath of high
concentrations (1M CdCl2 and 1M thiourea). This is presumably due to
kinetic reasons. With lower bath concentrations, the deposition rate is
expected to be lower and consequently allows better crystals with less
imperfection. With multiple depositions, the best film performance has
been obtained from films with three depositions.
The question that may surface is: why do we need to do multi-
deposition, knowing that a single deposition from heavily concentrated
bath may yield same total thickness? The answer is straight forward. As
discussed above, high concentrations yield low quality crystals. Therefore,
to get reasonable film thicknesses without scarifying crystal quality, it is
necessary to follow multi-deposition techniques using lower bath
concentrations. Literature supports such discussions [67, 80-81]. Photo-
88
luminescence, optical, electrical and photoelectrochemical characteristics
of deposited films are affected by bath concentrations. Structure, crystal
type and defect density are known to vary with bath concentrations. Optical
absorption coefficient of the films decreases by increasing the thickness.
This effect can be explained by proposing that the thicker film has bigger
crystallites resulting in more inter-granular space so the optical density of
the film decreases. The energy band gap value was found to decrease with
increasing film thickness.
5.3 Effect of Annealing on CdS Film Characteristics
CdS thin films grown by CBD technique are generally polycry-
stalline, composed of metastable cubic and stable hexagonal phases. The
untreated CdS films are expected to be poorly crystallized and quite
inhomogeneous with small crystalline size. Thermal annealing leads to
improvement in the crystalline quality of the films by increased grain size
and removal of random strain, which can lead to changes in resistivity.
Interesting part of thermal annealing is that it may lead to phase transition
and thereby may change the band gap. This has been reported in literature
[7-16].
Maliki et. al. [7] showed that the main influence of annealing CdS
thin films grown by CBD is the film crystallization and the escape of some
oxygen species (SO2, H2O) which induces a red shift effect on the band
gap. Lozada-Morales et. al. [8] showed that the structure of CdS thin films
grown by CBD can by transformed from cubic sphalerite metastable
89
structure to hexagonal wurtzite stable structure by annealing in the range of
100-500 C in Ar+S2 gas atmosphere. The critical point of the phase
transformation is at temperature 300 C. Zinoviev et. al. [9] demonstrated
that CBD-CdS film resistivity changes with annealing temperature. A
limited temperature region yielding films with very low resistivity is found
at temperature interval of 240 260 C under H2 atmosphere. Resistivity
values exhibit sharp increase in the region 260 300 C as a result of the
film-stoichiometry change on account of Cd evaporation. Cetin rgü et. al.
[10] reported that annealing of CBD-CdS thin films in air atmosphere has a
significant effect on the electrical and optical properties of the films. They
found that the absorption edge is shifted towards longer wavelength, i.e.,
decrease in the gap values. They found also that the resistivity of these
films decreases to a value of ~105 .cm for samples annealed in air at
250 C. Mishra et. al. [11] found that CdS thin films prepared by CBD on
GaAs undergo structural phase transitions from the metastable cubic phase
to the stable hexagonal phase, when, annealed at 500°C. This was
accompanied by significant improvement in crystalline quality of the film.
Metin et. al. [12] reported that the crystallite sizes of CBD CdS thin films
are increased by annealing. However, the energy gap of the film is found to
decrease by annealing. They also found that the best annealing temperature
for CBD-CdS films is 300°C from the optical properties. On the other
hand, annealing above 300°C is seen to degrade the optical properties of
the film. Murthy et. al. [13] reported that annealing and etching CdS thin
films prepared by CBD help improve the I-V curves and the conversion
90
efficiency because of increased grain size and decreased film resistance.
Goto et. al. [14] found that annealing CdS thin films on O2 atmosphere can
reduce concentration of defects, and thus, is useful to prepare good quality
CdS films. Guillén et. al. [15] found that 200°C and 400°C air-annealing
treatments of CBD-CdS thin films modify electrical and optical properties
by decreasing dark and light resistivity and making absorption edge shifts
towards lower photon energies. Finally, Jia et. al. [16] investigated the
influence of different annealing temperatures on the PEC cell performance
in order to optimize the annealing condition. They found that the higher
PEC cell performance is obtained when the CdS film electrode is annealed
at 350 C in air atmosphere for 30 min. But the output was found to sharply
decrease when the electrode is annealed at 450°C in air atmosphere for 30
min. From these reports it is obvious that annealing CdS thin films
enhances the film crystal structure and lowers the film resistance.
Moreover, there is an optimal range (250°C-300°C) at which annealing
yields best film characteristics.
In this work, we intended to enhance efficiency of CdS thin films in
light-to-electricity conversion using economic and feasible techniques.
Annealing is one technique to improve crystal structure, as documented
above, and consequently cell efficiency, as discussed earlier.
5.3.1 Effect of Annealing on Dark J-V Plots
In this study, we have measured J-V plots, in the dark, for annealed
and untreated CdS film. Low values of J-V plots have been observed for
91
the untreated CdS films. This can be partly explained due to the high
resistivity which is attributed to dislocations and imperfections of the films.
It has been also found that annealing CBD-CdS thin films improves the
SEM characteristics of the electrode. Moreover, it has been observed that
the degree of improvements depends on the annealing temperature and rate
of cooling. Generally, annealing of CdS thin films improves their dark J V
plots. The effect of annealing at lower temperatures (150 C) is not
significantly pronounced, Figure (4.4). On the other hand, for samples
annealed at 250 C and 350 C the effect is more pronounced. Figures (4.5-6)
indicate that the dark J-V plots were improved by annealing at 250 C and
350 C. Quenched and slowly cooled samples (at temperatures 250 C and
350 C) give better dark J-V plots than their untreated counterparts.
The observed annealing effect on the dark J-V plots of CdS films
can be explained as follows: enhancement in dark J V plots is presumably
due to increased grain size and enhanced film structure by annealing.
Consequently the carrier mobility is enhanced across the crystallite.
Moreover, annealing presumably I ncreases sintering between crystallites.
Thus the grain boundary resistive effect will be lowered. Therefore, intra-
and inter-particle carrier mobility will be enhanced. Furthermore the heat
treatment increases the adherence of the films to their substrates. Thus
annealed films will have lower overall resistance compared to that of
untreated films. This is consistent with earlier reports [7-16]. Another
observation is that annealing leads to phase transition from the metastable
cubic phase of CdS to the more stable hexagonal phase with much
92
improved crystalline quality. Reorganization of the film by annealing
always favors of better quality films with more stable phases. We also
assume that the untreated crystallites will contain relatively high disorders
in the forms of either dislocations or point defects during random
deposition during growth. Annealing would thus eliminate such disorders
yielding more ordered crystallites. Furthermore, annealing may help
remove foreign impurities (such as oxygen atoms) from the thin films.
These effects together are thought to be responsible for the enhancement of
the characteristics of CdS thin film electrode. This in turn improves dark
currents.
Convincing evidence to this discussion comes from our SEM results.
Figures (4.23, 4.25-26) show that heated CdS films exhibit lower surface
roughness, higher cross linking and larger grain size than untreated
counterparts. Moreover, annealing increases percolation between
crystallites. This result supports our claim that heating SC thin film
improves dark J-V plots by lowering surface state densities, which are
commonly associated with higher surface imperfections.
5.3.2 Effect of Annealing on Photo J-V Plots
Photo J V plots exhibit similar behavior like dark J-V plots, as
shown in Figures (4.9-10). Generally, annealing CdS thin films improves
the photo J V plots. Values of short circuit photo-current density have been
enhanced by annealing the electrode. But the degree of enhancements
depends on the annealing temperature and cooling rate. The effect of
93
annealing on photo J V plots at lower temperatures (150 C) is not
significantly pronounced. On the other hand, for samples annealed at 250 C
and 350 C the effect is more pronounced especially for slowly cooled
samples. Figures (4.9-10) indicate that the photo J-V plots have been
improved by annealing at 250 C and 350 C. Quenched and slowly cooled
samples (at 250 C and 350 C) give better photo J-V plots than their
untreated counterparts. The best photo J-V plots were observed when the
CdS film electrode is annealed at 250 C where the short circuit current
density values of slowly cooled samples from 250 C increases by four-fold
than their untreated counterparts, Figure (4.10). But photo J-V plot
enhancement decreases for the electrode annealed at 350 C,
Figures (4.9-10). The optimal 250 C annealing temperature is consistent
with literature findings discussed above [7-16].
For higher annealing temperatures, the film enhancement has been
found to be lower, Figures (4.9-10). This result can be explained due to
oxidation of smaller particles, which might have accompanied
microcrystallite depositions. Higher temperature may also induce excessive
crystal imperfections in the film. Excessive Cd evaporation from the film
may also occur at elevated temperatures, as evidenced by experimented
measurements [10]. All these factors collectively increase film resistivity
values.
5.3.3 Effect of Annealing on Cell Efficiency
In previous sections, the effect of CdS thin film preheating on quality
of dark and photo J V plots have been demonstrated. In here, preheating
affected the CdS thin film conversion efficiency in a manner that parallels
94
the effect on J V plots. In general, preheating enhanced electrode
efficiency. Annealing of CdS thin from 150 C, 250 C or 350 C shows
better efficiency than untreated counterparts. Table (4.3) indicates that
electrodes after preheating give better conversion efficiency and fill factor
than untreated counterparts. Slow cooled sample from 250 C has the best
conversion efficiency (1.3%) of more than six-fold their untreated
counterpart. Moreover the fill factor has been improved from 21% for
untreated samples to 42% for slowly cooled samples from 250 C.
Enhancements of conversion efficiency and fill factor are due to the short
circuit current density and open circuit voltage enhancements discussed
earlier for the modified electrode. The conversion efficiency of quenched
electrodes from 250 C also shows some enhancement in cell efficiency, but
to a lower extent (three-fold), Table (4.3).
CdS film preheating improves the crystal in many ways. Film quality
and performance will presumably be improved. Film resistance and crystal
defects are lowered by treatment. Such improvements in crystal quality
would yield enhancement in electrode J V plots and cell efficiency, as
discussed earlier.
5.3.4 Effect of Annealing on Film Stability
The effect of annealing on stabilizing CdS thin film, under PEC
conditions, have been studied here while using the unfavorable aqueous
LiClO4/Fe(CN)63-/4- system. Plots of Jsc vs. time were measured for
untreated electrodes and pre-heated/pre-cooled ones from different
95
temperatures using slow and rapid cooling. Figure (4.18) indicates that the
Jsc values have been improved by annealing at 250 C. In general, preheated
electrodes have better enhancing effect on Jsc values than untreated
counterparts. Consistent with earlier discussions, the annealing of CdS thin
films enhances the film surface and improves the stability of the electrodes.
This is due to the fact that there is an increase in the grain size and removal
of frozen-in defects in the deposited films. Consequently, there is an
increase in minority carrier diffusion to electrolyte solution in PEC solar
cell, which prevents electrode photocorrosion normally associated with
hole accumulation in the SCL.
5.4 Effect of Cooling Rate on CdS Film Characteristics
Cooling rate of heated SC has an obvious effect on the quality of the
SC structure through improving their parameters. Examples of such
parameters are: composition uniformity, growth film thickness, and
luminescence properties. The dislocation density and concentration of
structural defects also depend on cooling rate of SC crystal.
Christmann et al. [17] showed that heat treatment of CdS platelets
leads to changes in the native defect concentration and distribution. Such
changes depend on both temperature of treatment and cooling rate. Kita et
al. [18] showed that a uniform film growth of Si from Cu-Si solution, with
constant temperature gradient distribution, at the cooling rate of 0.1 C/s and
0.05 C/s (low cooling rate), was possible. Nishijima et al. [19] prepared
InGaAs crystal with uniform composition in the growth direction by
96
adjusting the cooling rate of growth zone to be 0.2 C/h. Kulakov et al. [20]
reported that with higher cooling rate, higher surface phase shifts were
observed in Si wafers. Lee et al. [21] showed that the dark current could be
greatly reduced through rapid thermal annealing of porous silicon SC.
Miyazazi et al. [22] found that efficient doping of wide-gap SCs can be
introduced by controlled cooling of a sample after doping. They also found
that the formation efficiency of large and loosely bound impurity
complexes is very sensitive to the cooling rate and they would successfully
form only when the cooling rate is carefully tuned. Therefore, our findings
are consistent with literature reports.
5.4.1 Effect of Cooling Rate on Dark J-V Plots
Literature reports [17-22] show that slow cooling, of heated SC
samples, improved their crystallinity by affecting their imperfections,
including dislocation density and structural defect concentration. Slow
cooling also improves composition uniformity and luminescence
properties. In this work, it has been found that slowly cooled CdS film
electrodes, from temperatures, 250 C or 350 C have better dark J-V plots
than their quenched counterparts. Figures (4.7-8) show that the slowly
cooled samples from 250 C and 350 C respectively give better dark J-V
plots than their quenched counterparts. Therefore the improvement of the
dark J-V plots, of slowly cooled samples, can be explained. The
consistency between these findings with the literature reports is obvious.
97
The dark J-V plots of untreated electrode, Figures (4.5-6), were not
smooth with high Vonset compared to other heated electrodes. This
presumably is due to higher defect density in the untreated electrode.
Annealing the sample provides atoms with enough energy to overcome the
energy barrier and to move from stable positions to metastable new
positions. Therefore heating itself increases the structural defect density in
the crystal. On cooling, metastable atoms try to move back to their stable
positions. Slow cooling of the CdS film electrodes allows enough time for
most metastable atoms of annealed electrodes to return to equilibrium
positions at the storage temperature [85]. This in turn will minimize
structural defects, and dislocations created during growth and annealing.
On the other hand, quenched electrodes will partly keep the atoms in their
new positions after annealing. Quenching will deprive the metastable atoms
from any pathways to return to their stable positions. The defect density in
quenched samples will therefore be greater than their slowly cooled
counterparts. Thus, slow cooling reduces the surface state density [4]. Such
effect will improve CdS film crystallinity, and therefore, enhance dark
current density.
5.4.2 Effect of Cooling Rate on Photo J-V Plots
Photo J-V plots, Figures (4.11-12), show that the slowly cooled CdS
film electrodes from temperatures 150 C, 250 C or 350 C exhibited better
photo J-V plots than their quenched counterparts. These results support the
discussions and results presented for dark J-V plot enhancement shown
earlier.
98
These discussions have direct supporting evidence from SEM. Figure
(4.26) indicates that CdS film, slowly cooled from 300 C, exhibits lower
surface roughness and higher sintering between crystallites than the
quenched counterparts. As stated earlier, the lower roughness and higher
crystallites sintering is an indicator of lower dislocations and lower point
defect densities.
5.4.3 Effect of Cooling Rate on Cell Efficiency
Consistent with earlier results, cooling rates of the preheated CdS
thin films also affect the CdS electrode conversion efficiency. In general,
preheating enhances electrode efficiency. However slow cooling of CdS
thin films from temperatures 150 C, 250 C or 350 C show higher
efficiency and higher fill factor values than quenching, as shown in
Table (4.3). Sample slowly cooled from 250 C has the best conversion
efficiency (1.3%) with more than two-fold its quenched counterpart (0.6%),
Table (4.3). Thus slow cooling of heated CdS thin films improves their
crystallinity by affecting their dislocation density and structural defect
concentration. Slow cooling also improves composition uniformity and
luminescence properties [18]. This in turn enhances CdS thin film
efficiency.
5.4.4 Effect of Cooling Rate on Film Stability
Figures (4.18) indicate that the Jsc values have been improved by
annealing at 250 C. In general, slowly cooled electrodes have better
enhancing effect on Jsc values than quenched counterparts. Consistent with
99
earlier discussions, slow cooling of CdS thin films enhances the film
surface and improves the stability of the electrodes. This is due to the fact
that slow cooling improves CdS film crystallinity and lowers dislocation
density and surface roughness of the film. The enhanced Jsc values for
prolonged times is a good indication of crystal quality and/or lower surface
state density.
5.5 Effect of Surface Etching on CdS Film Characteristics
Commonly speaking, etching of SC is performed for a number of
purposes: to remove damaged surfaces layers, to study the defect structure
of the SC or to shape or polish a surface. For PEC uses, the first reason is
the most important one.
Etching of CdS thin film enhances the film surface and improves the
surface roughness of the electrode. Consequently, etching removes
recombination centers (surface states) at the surface reduces surface
impurity and changes surface morphology [24].
In this work, the effect of etching on stabilizing the CdS electrode
surface was studied under PEC conditions using aqueous
(KOH/Fe(CN)63-/4-) systems. CdS thin films were etched by dilute HCl
(10% v/v) solution. Before etching, a lot of heaps are randomly distributed
on the surface of the films as could be envisioned with naked eye. After
etching, the under layer morphology appears more homogeneous. The Jsc
100
values of CdS films increases with etching, reaching four-fold more than
the unetched counterparts, Figure (4.19).
Etching of CdS thin film enhances the film surface and improves the
surface roughness of the electrode. Consequently, etching removes
recombination centers at the surface, reduces surface impurity and changes
surface morphology. These factors together are apparently responsible for
improving the stability of the CdS thin film surfaces under PEC conditions.
5.6 Effect of Redox Couples on CdS Film Characteristics
Literature report that decomposition processes in prototypical CdS
photoelectrochemical cell can be efficiently suppressed by addition of an
appropriate polychalcogenide redox couple to the electrolyte [92-94].
However, conversion efficiencies remain low [95]. Moreover, although
increased light-to-electricity conversion rates can be obtained by using a
redox couple such as Fe(CN)63 /4 , the cell lifetime is greatly diminished
[96-98]. In this work, we have studied the effect of using two different
redox systems, in CdS photoelectrochemical cells, namely
KOH/Fe(CN)63/4- and polysulfide systems.
5.6.1 Effect of Redox Couples on the Photo J-V Plots
Figure (4.15), shows the photo J-V plots of untreated CdS electrode
using KOH/Fe(CN)63/4- and polysuredox electrolytes. The photo J-V plots
of slowly cooled CdS electrodes from 250 C using KOH/Fe(CN)63-/4- and
polysulfide redox electrolytes are shown in Figure (4.16). The PEC system
101
with KOH/Fe(CN)63-/4- redox couple has a better photo J-V plot than that
with polysulfide redox couple, but the latter system gave higher stability.
Historically, CdS photoelectrochemical solar cells have been
investigated using different redox couples (Fe(CN)63-/4-, S2-/Sx
2-, I-/I2, etc.)
[69]. The first cell studied in this field was a single crystalline CdS
electrode in contact with the redox system Fe(CN)63-/4- [69,70]. The power
characteristics of such cells vary with the quality of the SC. However, the
fill factor of such cell depends not only on the properties of the SC and the
cell geometry but also on the redox couple used [70]. Figure (5.1) gives
two curves with different redox couple systems and a single crystalline CdS
electrode as the energy converter [70].
Figure (5.1): Photo J-V curves of photocells with a CdS electrode and two different redox electrolytes. Illumination by Xenon lamp with 4mW.cm-2 (copied from ref. [70]).
These results support the results presented for photo J-V plots shown
in Figures (4.15-17). It is clearly shown that the photo J-V plots of CdS thin
films depends on the redox couple system used. In polysulfide system, the
open voltage is somewhat lower than the Fe(CN)63-/4- because |Vredox Vfb| is
smaller. The reason is that energy-rich intermediates like S2- and S22- have
4mW/cm-2
[Fe(CN)6]3-/4-
max=9.4%
S2O32-/S4O6
2-
max=1.6
102
to be formed in the single steps of this redox reaction, which causes large
over voltages in anodic and cathodic directions. It is mainly the large
polarization of the counter electrode which causes the high voltage losses
in this system.
5.6.2 Effect of Redox Couples on Electrode Stability
Generally, SC electrodes in contact with the redox couples S2-/S22-,
Se2-/Se22-, and Te2-/Te2
2- are particularly stable. This has been demonstrated
by different workers [69- 70]. They have proposed to use semiconductors
in their saturated solution as photoelectrodes, like CdS in sulfide
electrolytes, to prevent decomposition. They found that, saturation of
electrolyte up to the solubility product of the SC can slow down the
deterioration to such an extent that extended use becomes possible.
In this work, we have found that that PEC system with S2-/Sx2- is
more stable than PEC system with KOH/Fe(CN)63-/4- redox electrolyte,
(Fig.4.20), although, the latter system has better photo J-V plots, (Fig.4.15).
This can be explained as follows: in the case of CdS, it turned out
that the crystal slowly decomposes even in a saturated solution containing
the redox system S2-/S22- [99]. The result of decomposition in this case is an
oversaturation of CdS due to an increased Cd2+ concentration at the
interface, and consequently microcrystals of CdS are formed as a very thin
layer on the top of the original film. This thin layer seems to slow down
further decomposition after some time, but its presence decreases the
photocurrent quantum yield.
103
Conclusions
- Thin film CBD-CdS efficiency in PEC systems was affected by
different deposition parameters such as, changing deposition time, bath
concentration and multiple depositions.
- CdS films prepared from low bath concentrations with multi-deposition
showed higher PEC cell performance than films prepared from high
bath concentrations.
- Annealing of CdS film electrodes showed enhancement in PEC cell
efficiency in both KOH/Fe(CN)63-/4- and polysulfide aqueous systems.
- Slowly cooled CdS electrodes annealed from temperatures above 150 C
showed better PEC characteristics than quenched counterparts.
- Maximum values of efficiencies were obtained by slow cooling of
preheated CdS film electrodes from 250 C.
- While etching of CdS film surfaces enhanced electrode stability in PEC
systems, it showed no significant effect on the photo J-V plots.
- Redox couples of KOH/Fe(CN)63-/4- system gave better photo J-V plots
and higher cell efficiency than polysulphide system.
- CdS films in polysulphide system were more stable against
photodegration than in KOH/Fe(CN)63-/4- system.
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Suggestions for Further Work
For future work, we recommend doing the following:
1) Prepare CdS thin films using other techniques than CBD such as
electrodeposition and spraying.
2) Use the heating and cooling technique to improve other thin films such
as CdSe thin films.
3) Modify the heat-treated CdS thin films with coating material such as
polymers and electroactive species.
4) Study the effect of different cooling rates.
5) Apply different PEC cells using different experimental conditions to
enhance electrode stability.
6) Further investigate effect of heating on FTO film conductivity.
7) Use additional techniques to study effects of different treatments on
CdS surfaces, such as X-ray Diffraction Pattern (XRD), Atomic Force
Microscopy (AFM), Photoluminance (PL) and capacitance studies.
8) Make resistivity hall measurements to measure carrier concentration
and mobility.
9) Measure film thickness using optical and/or mechanical techniques.
105
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114
Appendix 1
Results obtained from this study were accepted as oral presentations before the following conferences and workshops:
1. The Third Palestenian Physics Conference, Al-Quds University, Abu-Dees, Palestine, April 21-23, 2008.
2. 1st International Workshop on Renewable Energies and their Applications, Amar Telidji University of Laghouat, Laghouat, Algeria, May 10-12, 2008.
115
116
117
118
119
120
121
122
123
(CdS)
:
. .
.
.
2008
(CdS)
:
. .
.
(CdS Thin Films)
(FTO).
.
)(
)150 250 350 (
.
-
.
(dark current density)
(photo current density)
(short
circuit current)
(open circuit voltage)
(conversion efficiency)
(scanning electron microscopy).
.
.
.
.
250
350 .
150
.
.
250
.
.
-
(II)
(III)
-
- .
.
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