FABRICATION AND CHARACTERIZATION OF PHOTOELECTROCHEMICAL SOLAR CELLS USING CHEMICALLY SYNTHESIZED ZINC OXIDE THIN FILMS

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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

  1.       Nanomaterials                                                                                                
    1. General Introduction                                                       1
    1. Why Nanomaterials?                                                                          4
    1.      Zinc Oxide (ZnO)                                                          6
      1. Introduction to ZnO                                                 6
      1. Why ZnO Nanostructures?                                                   8
      1. Structural Properties of ZnO                                   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

  1. Introduction to DSSCs                                                                                   15
    1. Basic Principles of DSSCs                                              17
    1. Light absorption by the sensitizer (Dyes) monolayer on the mesoscopic semiconductor films     19
  1. Electron transfer processes in DSSCs                                       21
    1. Electron transfer dynamics                                        21
    1. Charge separation at semiconductor-dye interface          23
    1. Charge transport and back reaction                          25       
    1. Dye regeneration                                                                28
    1. Counter electrode                                                            29

1.5.      Sensitizers for Dye-Sensitized Solar Cells                                     30

  1. Introduction                                                                                30
    1. Inorganic dyes                                                                                32       
    1. Organic dyes                                                                                   35

1.6.      Literature survey on ZnO and PEC Cells                          38       

1.7.      Thesis Motivation                                                                        64       

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.      Chemical Bath Deposition (CBD)                                   69

2.2.2.      Successive Ionic Layer Adsorbtion and Reaction (SILAR)   74

2.2.3.      Electrodeposition                                                                                          77

2.2.4.      Doping in Semiconductors                                                  82

2.3.         Photoelectrochemical Solar Cells                                               83       

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       

2.4.1.      Thickness Measurement                                                          97

2.4.2.      X-ray Diffraction Technique                                                    99

2.4.3.      Scanning Electron Microscopy (SEM)                                    100

2.4.4.      Transmission Electron Microscopy (TEM)                          101

2.4.5.      Atomic Force Microscopy (AFM)                                       102

2.4.6.      Optical characterization                                                 103

2.4.7.      Fourier-Transforms Infrared Spectroscopy (FT-IR)        105

2.4.8.      FT-Raman Spectroscopy                                                  106

2.4.9.      Contact Angle Measurement                                                      106     

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     

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     

3.4.1.2. X-ray diffraction studies                                                           121     

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     

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.      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     

6.4. Challenges                                                                                                            195

7.0. APPENDIX                                                                                                        197

7.1. Appendix A                                                                                   197     

7.2. Appendix B                                                                                     209

REFERENCES

FABRICATION AND CHARACTERIZATION OF PHOTOELECTROCHEMICAL SOLAR CELLS USING CHEMICALLY SYNTHESIZED ZINC OXIDE THIN FILMS