ABSTRACT
Mesoporous
nanostructures of zinc oxide (ZnO) were successfully synthesized by chemical
bath deposition (CBD) technique and used to fabricate photoelectrochemical
(PEC) solar cells. The synthesis proceeded in an alkaline bath of an aqueous
solution of 0.1 M Zn(NO3)2.6H2O at a bath
temperature of 353 K and pH of 11.5 on microscope glass slide and stainless
steel slide substrates. The ZnO was doped with Al and Cu at varying
concentrations from 1-5 at. %. As-deposited films were annealed at 673 K for 2
h. The synthesized ZnO thin films had
thickness in the range of 2.03-10.43 µm. Crystal structure studies revealed
that all the ZnO thin films were polycrystalline with hexagonal wurtzite
structure and preferential growth in the 002 crystal plane. Crystallite sizes
in the range of 8-29 nm were obtained along the 002 crystal plane and the
crystallinity of the doped samples was strongly affected by the concentrations
of the dopants. Surface morphological studies indicated that the synthesized
ZnO thin films had nanoflakes, nanodendrites and nanorods morphologies which
confirmed the effects of surfactant, dopants and annealing on ZnO. Optical
studies revealed that all the films had low absorbance in the visible region of
the solar spectrum with transmittance ranging from 42-90 %. Energy band gaps of the undoped and doped ZnO
were found to decrease from 3.03 eV to 2.70 eV. All the measured optical
properties of the ZnO thin films showed strong dependence on the concentration
of the dopants. Surface wettability studies confirmed that all the synthesized
ZnO thin films were porous (hydrophilic), giving water contact angles in the
range of 0o to 71.3o. For the first time in literature,
this synthesized ZnO thin films were further sensitized with Indigofera arrecta
plant dye and also Rhodamine 6G to develop/fabricate dye-sensitized solar cells
(DSSCs). The fabricated PEC solar cells
produced short circuit current (ISC) of 12.34 μA/cm2 and
open circuit voltage (VOC) of 388 V for unsensitized undoped ZnO
electrode giving power conversion efficiencies (η) of 0.003 and fill factor
(FF) of 0.43. Unsensitized aluminum doped zinc oxide (AZO) electrode yielded ISC
in the range of 21 μA/cm2 to 29 μA/cm2, VOC in
the range of 333 mV to 641 mV, η in the range 0.0037 % to 0.01 % and FF in the
range of 0.34 to 0.43. While unsensitized copper doped zinc oxide (CZO)
electrodes produced ISC in the range of 16 μA/cm2 to 98 μA/cm2
and VOC in the rage of 774 mV to 796 mV, giving η in the range of
0.0009 % to 0.062 % and FF in the range of 0.06 to 0.63. Further upon
dye-sensitization of the ZnO electrodes with Rhodamine 6G and Indigofera
arrecta plant dye respectively, the PEC solar cells of undoped ZnO produced ISC
of 0.24 mA/cm2 and VOC of 360mV for Rhodamine 6G and ISC
of 0.29 mA/cm2 and VOC of 595 mV for Indigofera arrecta
plant dye. These produced η of 0.11 %
and FF of 0.45 for Rhodamine 6G and η of 0.16 % and FF of 0.44 for Indigofera
arrecta plant dye. AZO electrodes produced ISC in the ranges of 0.3
mA/cm2 to 0.4 mA/cm2 and VOC in the ranges 376
mV to 515 mV using Rhodamine 6G and ISC in the range of 0.84 mA/cm2
to 1.35 mA/cm2 and VOC in the range of 596 mV to 664 mV
for Indigofera arrecta plant dye. These yielded η in the range of 0.16 % to
0.22 % and FF in the range of 0.40 to 0.49 for Rhodamine 6G and η in the range
of 0.29 % to 0.51 % and FF in the range of 0.42 to 0.47 using Indigofera
arrecta plant dye. On the other hand, CZO electrodes produced ISC in
the range of 0.49 mA/cm2 to 0.97 mA/cm2 and VOC
in the range of 355 mV to 473 mV for Rhodamine 6G and ISC in the
range of 0.91 mA/cm2 to 6.8 mA/cm2 and VOC in
the range of 656 mV to 914 mV using Indigofera arrecta plant dye. These yielded η in the range of 0.18 % to
0.39 % and FF in the range of 0.39 to 0.46 using Rhodamine 6G and η in the
range of 0.40 % to 4.16 % and FF in the range of 0.42 to 0.54 for Indigofera
arrecta plant dye. The η of 4.16 % obtained in this work using Indigofera
arrecta plant dye is the highest ever obtained using natural dyes from plants
as found in the available literature.
Electrochemical impedance spectra of all the PEC solar cells shows
impedance variations that agrees with the ISC and VOC
results for all the cells as stated above.
ACKNOWLEDGEMENT.
When a man undertakes a journey, he has the good will of both God and men. When he returns, he brings with him great thankfulness for the grace and mercy of God and for the good wishes of men. My journey could not have been easily gotten this far if God had not granted it. I have indeed enjoyed special favours from God during the whole period of this work and so I glorify His name.
There are men who are akin to God in affairs of man. Their blessing, advice, assistance and challenging personalities are a source of strength, a sense of direction and dedication to duty and opens up a world of insatiable aspiration for us. Their lives provide us with the inspirational model in the process of struggling for self-actualization.
I am particularly grateful to my supervisors, Professor R.U. Osuji and Dr. F.I. Ezema for their relentless efforts, their great concern for the success of this research work, their tolerating patience and their commitment to academic excellence. They have gone extra miles from supervisor-student relationship to ensure excellence in the work. I owe a lot of appreciation to Professor C.D. Lokhande, coordinator of Thin film Laboratory, Department of Physics, Shivaji University, Klohapur, India and the entire students of the Department for their immense contributions to the success of this work. Professor C.D. Lokhande offered me his laboratory, materials and guidance for the experimental part of this work; above all, their overwhelming hospitality kept me comfortable during my stay in Kolhapur, India.
I am especially indebted to my wife, Mrs. Catherine Tyona and my children, Master Joshua Keghtor, Ms. Happiness Nguvan, Ms. Joy Mimi and Master Caleb Tavershima Tyona and also my Cousin, Ms. Msuur Nyiter. They have endured patiently all the hardships that besiege the family as a result of this work: my consistent absence and financial short fall amongst many others. My gratitude goes to the entire family of Late Evangelist Tyona Kange: Mr. Samuel Kange, Mr. Terkimbi Tyona just to mention few, for their persistent prayer, moral support and otherwise throughout the course of this work.
I would not forget the good relationship I enjoyed with the staff and postgraduate students of the Department of Physics and Astronomy, University of Nigeria, Nsukka. I really appreciate my friends who helped in the course of this work: Mr. James Ezema, Dr. S.B. Jambure, Dr. Ravindra Bulakhe and Dr. Nana Shinde just to mention few.
You have all made this journey a bearable one. God bless you all.
TYONA, MRUMUN DAVID OCTOBER, 2014.
TABLE OF CONTENTS
Title page i
Certification ii
Approval page iii
Dedication iv
Abstract v
Acknowledgement vii
Table of Contents viii
List of Figures xiii
List of Tables xvii
CHAPTER 1: Introduction and Theoretical Background                             1
-     Nanomaterials                                                                                               Â
- General Introduction                                                     1
- Why Nanomaterials?                                                                        4
- Â Â Â Â Zinc Oxide (ZnO)Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 6
- Introduction to ZnOÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 6
- Why ZnO Nanostructures?                                                  8
- Structural Properties of ZnOÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 9
1.3.     Photoelectrochemical (PEC) Solar Cells                                           1
1.3.1. Introduction 11
1.3.2.  Electrochemical Photovoltaic Cells without Dyes      14
1.4. Dye-Sensitized Solar Cells (DSSCs) 15
- Introduction to DSSCs                                                                                  15
- Basic Principles of DSSCs                                             17
- Light absorption by the sensitizer (Dyes) monolayer on the mesoscopic semiconductor films    19
- Electron transfer processes in DSSCs                                      21
- Electron transfer dynamics                                       21
- Charge separation at semiconductor-dye interface         23
- Charge transport and back reaction                         25      Â
- Dye regeneration                                                               28
- Counter electrode                                                           29
1.5.     Sensitizers for Dye-Sensitized Solar Cells                                    30
- Introduction                                                                               30
- Inorganic dyes                                                                               32      Â
- Organic dyes                                                                                  35
1.6.     Literature survey on ZnO and PEC Cells                         38      Â
1.7.     Thesis Motivation                                                                       64      Â
1.8.      Purpose of the study                                                                       65
CHAPTER 2: Theoretical Background of Chemical Deposition Methods, Photoelectrochemical Cells and Thin Film Characterization Techniques    66
2.1. Introduction 66
2.2.        Theoretical Background of Chemical Deposition Methods  68
2.2.1. Â Â Â Â Chemical Bath Deposition (CBD)Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 69
2.2.2. Â Â Â Â Successive Ionic Layer Adsorbtion and Reaction (SILAR)Â Â 74
2.2.3. Electrodeposition 77
2.2.4.     Doping in Semiconductors                                                 82
2.3.        Photoelectrochemical Solar Cells                                              83      Â
2.3.1.     Construction of Photoelectrochemical (PEC) Solar Cell      83
2.3.2.     Classification of Photoelectrochemical (PEC) Solar Cells       85
2.3.3.     Requirements of Photoelectrochemical (PEC) Cells         86
2.3.4.     Characterization of PEC Solar Cell                                   88
2.3.4.1.  Short circuit voltage measurements                         88
2.3.4.2.  Open circuit voltage decay                                               89
2.3.4.3.  Frequency response analysis techniques                  90
2.3.4.4.  Electrochemical impedance spectroscopy            90
2.3.4.5.  Power conversion efficiency of solar cell                96
2.3.4.6. Fill factor 96
2.3.5.     Advantages of Photoelectrochemical (PEC) Solar Cell      97
2.4.        Thin Film Characterization Techniques                       97      Â
2.4.1.     Thickness Measurement                                                         97
2.4.2.     X-ray Diffraction Technique                                                   99
2.4.3. Â Â Â Â Scanning Electron Microscopy (SEM)Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 100
2.4.4. Â Â Â Â Transmission Electron Microscopy (TEM)Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 101
2.4.5. Â Â Â Â Atomic Force Microscopy (AFM)Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 102
2.4.6.     Optical characterization                                                103
2.4.7. Â Â Â Â Fourier-Transforms Infrared Spectroscopy (FT-IR)Â Â Â Â Â Â Â 105
2.4.8.     FT-Raman Spectroscopy                                                 106
2.4.9.     Contact Angle Measurement                                                     106    Â
2.4.10.   Electrical Resistivity Measurement                                          107
CHAPTER 3: Un-Doped and Doped Zinc Oxide Thin Films: Synthesis and Characterization                                                               109
3.1.        Introduction                                                                       109    Â
3.2.        Experimental setup for the deposition of ZnO thin films by CBD method                                                                                         112
3.3.        Experimental details                                          112
3.3. 1.    Substrate cleaning                                                              112
3.3. 2.    Deposition of ZnO thin films                                                  113
3.3. 3.    Growth of ZnO nanodendrites and nanoflakes         115
3.3.4.     Deposition of Al-doped ZnO the film                             116
3.3.5.     Deposition of Cu-doped ZnO thin films                                    117
3.4.        Results and Discussions                                                118
3.4.1. Charcterization of Un-doped, Al-doped and Cu-doped ZnO Thin Films 118
3.4.1.1. Thickness measurement                                                                  118    Â
3.4.1.2. X-ray diffraction studies                                                          121    Â
3.4.1.3. Surface morphological studies                                           127
3.4.1.4. Surface wettability studies                                                     134
3.4.1.5. Optical studies                                                         136
CHAPTER 4: Photoelectrochemical (PEC) Solar Cells of Undoped Zinc Oxide, Aluminium-Doped and Copper-Doped Zinc Oxide Thin Film Photoelectrodes                                    145
4.1. Introduction 145
4.2. Experimental Setup for Measurement of PEC Cell Properties of undoped ZnO, Al-doped and Cu-doped ZnO Photoelectrodes                146    Â
4.3.        Results and Discussions                                                    148
4.3. 1.    Current-Voltage (I-V) measurement                                         148
4.3. 2.    Electrochemical impedance study                                    156
4.3. 3.    Power conversion efficiency (η) and fill factor (FF)  164
CHAPTER 5: Photoelectrochemical Solar Cells of Dye-Sensitized Zinc Oxide Thin Film Photoelectrodes                                                   166
5.1. Introduction 166
5.2. Experimental Details for the Preparation of Dye-Sensitized ZnO Thin Film Photoelectrodes                                                                          168
5.3. Experimental Setup for Measurement of PEC Cell Properties of dye-sensitized ZnO Thin Film Photoelectrodes                                         169
5.4.        Results and Discussions                                                    169
5.4.1. Â Â Â Â I-V Measurement of PEC Solar Cells of ZnO Oxide Photoelectrodes Sensitized with Rhodamine 6G and Indigofera Arrecta Plant Dye 169
5.4.1.1. Optical properties of rhodamine 6G and indigofera arrecta plant dye 169
5.4.1.2. Current-Voltage (I-V) measurement                                       170
5.4.2. Electrochemical impedance study                                                  176
5.4.3. Power conversion efficiency (η) and fill factor (FF)                        190
CHAPTER 6: Summary, Conclusion, Challenges and Recommendations 191
6.1. Summary 191
6.2. Conclusion 193
6.3. Recommendations for further research                            195    Â
6.4. Challenges                                                                                                           195
7.0. APPENDIX 197
7.1. Appendix AÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 197Â Â Â Â Â
7.2. Appendix BÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 209
REFERENCES