ENGINEERING OF BINARY METAL OXIDE NANOSTRUCTURES FOR HIGHLY EFFICIENT AND STABLE EXCITONIC SOLAR CELLS NAVEEN KUMAR ELUMALAI (B.Eng., Anna University) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF MECHANICAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2013 DECLARATION I hereby declare that the thesis is my original work and it has been written by me in its entirety. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously. ____________________________ Naveen Kumar Elumalai 12 August 2013 ACKNOWLEDGEMENT First and foremost, I would like to express my heartfelt gratitude to my supervisor Prof. Seeram Ramakrishna for providing me this valuable opportunity to perform the research under his supervision. I would like to thank him for the immense faith and tremendous encouragement he provided during the course of my research. Without his constant support, guidance and patience this thesis would have never been possible. I would also like to express my sincere gratitude to Dr. Chellappan Vijila for her excellent cosupervision and guidance throughout this entire project. A special thanks to her for allowing me to carry out most of my research work in her laboratory. The valuable suggestions and motivation she has provided me during this period is truly prominent and it is imperative to acknowledge the time and energy she has put into this project. I would also like to express my heartfelt gratitude to Prof. Rajan Jose for his unparalleled guidance and immense support throughout this project. He is one of the cornerstones of this project and a source of constant motivation, enabling me to move forward consistently with positive energy during this period of research. My sincere gratitude to the Department of Mechanical Engineering for offering me the prestigious NUS research scholarship throughout the entire course of my PhD study. And a very special thanks to the Institute of Materials Research and Engineering (IMRE) for offering me the Post Graduate Student Attachment during this period, enabling to carry out most of my PhD research in their facility. i I would like to thank all members of the Prof. Seeram’s lab for their assistance in the completion of my PhD thesis. A special thanks to Ms. Archana Sathyaseelan for her valuable support and assistance during the course of my PhD research. My heartfelt gratitude to Dr. J. Venugopal for providing me valuable advice and suggestions. I also thank Dr. Sreekumaran Nair, Dr. Velmurugan Thavasi and Dr. Sundarrajan for their guidance and suggestions. Special thanks to lab coordinator Ms. Wang Charlene for the support provided during my PhD tenure. I would also like to thank Ms. Teo Lay Tin Sharen and Ms. Thong Siew Fah from Mechanical department for their help on administrative matters. My gratitude also goes to lab members at IMRE - Kam Zim Ming, Tan Mein Jin, Siew Lay, and Goh Weipeng. A special thanks to Dr. Zhang Jie. Hearty thanks to all my friends at NUS - Dr. Rajeshwari, Hemant, Anand, Bhavadharini, Dr. Suresh and Wong Kim Hai. A special thanks to my friend R.Saravanan whose support in various capacities helped me to complete this thesis. My heartfelt gratitude to all my friends at Yew Tee who assisted me in completing this thesis. I am sincerely grateful to Ms. Jhansy Thomson, Dr. L. Karthikeyan and Dr.P.Chinnadurai for their support and good wishes. Finally, I am out of words to express my love and gratitude to my beloved mother, Vijaya and my father, Elumalai who have given me this wonderful life and love to cherish. I would also like to express my hearty gratitude to my beloved fiancée, Indu for her constant support, patience and motivation she provided me during the entire course of this PhD. Dedicating this thesis to God’s Lotus feet. ii TABLE OF CONTENTS ACKNOWLEDGEMENT i TABLE OF CONTENTS iii SUMMARY viii LIST OF TABLES x LIST OF FIGURES xi LIST OF ABBREVIATIONS xviii LIST OF PUBLICATIONS xx Introduction 1.1 Solar energy – the ultimate renewable resource 1.2 Solar cell classification 1.3 Organic solar cells 1.3.1 Organic semiconducting materials 1.4 Device structure of OSCs 1.4.1 Bilayer organic solar cells 1.4.2 Bulk heterojunction solar cells 1.5 Working principle of OSCs 11 1.5.1 Exciton generation 11 1.5.2 Exciton diffusion and dissociation 13 1.5.3 Charge carrier transport and recombination 15 1.5.4 Charge collection at electrodes 19 1.6 Normal and Inverted OSCs 21 1.7 Role of metal oxide nanostructures in OSCs 22 1.8 Stability of the OSCs 24 1.9 Basics of dye sensitized solar cells (DSCs) 26 1.9.1 Key components of a dye-sensitized solar cell 27 1.9.2 Photoelectrode 27 iii 1.9.3 Sensitizers for dye-sensitized solar cells 29 1.9.4 Electrolytes for dye-sensitized solar cells 30 1.9.5 Counter electrode 31 1.9.6 Working mechanism of DSCs 31 1.9.7 Photochemical processes and recombination in DSCs 33 1.10 Role of metal oxide nanostructures in DSCs 37 1.11 Scope and structure of the thesis 38 Materials and methods 43 2.1 Material synthesis and characterization techniques 44 2.1.1 Electrospinning technique 44 2.1.2 Scanning Electron Microscope (SEM) 45 2.1.3 Transmission Electron Microscope (TEM) 46 2.1.4 X-Ray Diffraction (XRD) 47 2.1.5 X-ray Photoelectron Spectroscopy (XPS) 48 2.1.6 Brunauer–Emmett–Teller (BET) measurement 49 2.1.7 UV-Vis Spectroscopy 49 2.2 Fabrication of OSCs 50 2.2.1 Substrate design 51 2.2.2 Spin-coating 51 2.2.3 Thermal vacuum deposition 52 2.2.4 Preparation of photoactive blend layer 53 2.2.5 Fabrication of inverted OSCs 53 2.3 Fabrication of DSCs 54 2.3.1 Substrate preparation 54 2.3.2 Deposition of metal oxide nanostructures (photoanodes) 54 2.3.3 Sensitization of the metal oxide nanostructure electrodes 55 2.3.4 Deposition of the electrolyte 55 2.3.5 Counter electrode 55 2.4 Device characterization 55 2.4.1 I-V characterization 55 2.4.2 Incident Photon-to-Current Conversion Efficiency (IPCE) 60 iv 2.4.3 Temperature and photon flux dependence measurement 61 2.4.4 CELIV and Photo-CELIV measurements 61 2.4.5 Transient Photovoltage measurements 64 2.4.6 Electrochemical Impedance Spectroscopy (EIS) 64 Band structure engineered interfacial layers for highly efficient and stable organic solar cells 66 3.1 Introduction 66 3.2 Functions of MoO3 as hole transport layer 66 3.3 Fabrication of inverted device with Ca and MoO3 68 3.4 Photovoltaic performance and IPCE of the device (ITO/Ca/P3HT: PCBM/ MoO3/Ag) 69 3.4.1 Dark stability of the device with Ca and MoO3 73 3.4.2 Photo-stability of the device with Ca and MoO3 74 3.5 Function of ZnO as electron transport layer 75 3.6 Preparation of the solution processed ZnO interlayer 76 3.6.1 ZnO film morphology 77 3.6.2 XRD characteristics of the ZnO interlayer 78 3.7 Fabrication of inverted device with ZnO and MoO3 79 3.8 Photovoltaic performance and IPCE of the device (ITO/ZnO/P3HT: PCBM/MoO3/Ag) 80 3.8.1 Dark stability of the device with ZnO and MoO3 83 3.8.2 Photo-stability of the device with ZnO and MoO3 83 3.9 Conclusions 84 Charge transport in the IOSCs employing the modified interfacial layers 86 4.1 Introduction 86 4.2 Photon flux and temperature dependent current-voltage characteristics of the device (ITO/Ca/P3HT: PCBM/ MoO3/Ag) 86 4.2.1 Evaluation of trap depth 91 4.2.2 Effect of trap depth on the open circuit voltage 94 4.2.3 Determination of charge mobility and carrier concentration 97 v 4.2.4 Origin of enhanced dark stability in device (ITO/Ca/P3HT: PCBM/MoO3/Ag) 98 4.3 Photon flux and temperature dependent current-voltage characteristics of the device (ITO/ZnO/P3HT: PCBM/MoO3/Ag) 100 4.3.1 Evaluation of trap depth 101 4.3.2 Effect of trap depth on the open circuit voltage 104 4.3.3 Evaluation of charge mobility from CELIV transients 106 4.3.4 Origin of enhanced photo-stability in device (ITO/ZnO/P3HT: 108 PCBM/MoO3/Ag) 4.4 Conclusions 109 ZnO nanowire plantations in the electron transport layer for high efficiency and stable IOSCs 111 5.1 Introduction 111 5.2 Synthesis of the ZnO nanostructures 112 5.2.1 Preparation of ZnO sol-gel thin films on ITO 112 5.2.2 Preparation of ZnO nanowire plantations 113 5.3 Morphology of the ZnO nanostructures 114 5.4 Crystal structure of the ZnO particles and wires 116 5.5 Device fabrication and characterization 117 5.5.1 Effect of ZnO morphology and surface states on photovoltaic parameters 118 5.5.2 Evaluation of charge carrier lifetime by transient photovoltage technique 122 5.5.3 Determination of carrier recombination from delay dependent Photo-CELIV 124 5.6 Conclusions 128 Engineering of Tin Oxide Nanostructures for efficient Dye-Sensitized Solar Cells 130 6.1 Introduction 130 6.1.1 Synthesis of SnO2 nanostructures by electrospinning 131 6.1.2 Morphological characterization of fibers and flowers 132 6.1.3 Structural characterization of the flowers and fibers 133 vi 6.1.4 Evaluation of flat band potential and electron density – MottSchottky Analysis 134 6.1.5 Cyclic voltammetry studies 136 6.1.6 Difference in electronic bands of SnO2 flowers and fibers 137 6.1.7 Proposed growth model of nanoflowers 139 6.1.8 Fabrication of solar cells and evaluation of photovoltaic properties 142 6.2 Estimation of trap density – open circuit voltage decay measurements 144 6.3 Charge transport through the SnO2 nanoflowers and nanofibers 146 6.3.1 Effect of trap states on carrier transport – EIS analysis 148 6.3.2 Effect of trap states on carrier lifetime – OCVD analysis 150 6.3.3 Evaluation of diffusion coefficient and mobility by photocurrent transient measurements 152 6.3.4 Origin of high JSC in the flower based device 156 6.3.5 Origin of high VOC in the flower based device 159 6.4 Conclusions 160 Future Outlook and Recommendations 162 Bibliography 164 vii SUMMARY Excitonic solar cells (ESCs) such as Dye Sensitized Solar Cells (DSCs) and Organic Solar Cells (OSCs) are promising candidates of third generation photovoltaics owing to their higher performance efficiency, ease of fabrication, and low cost. Immense research has been carried out in these areas for the last two decades, focusing on improving the device performance and stability in order to make it economically viable. Nanostructured binary metal oxide semiconductors (n-MOS) form an inevitable part in ESCs serving as an interfacial buffer layer in OSCs or an electron transporting layer (photoelectrode) in DSCs. One of the unresolved problems in OSCs despite large investments in this technology, is to unite high efficiency and operational stability. In general selective charge collection at the respective electrodes in OSCs is achieved by using hole- and electron-transporting buffer layers at the collecting electrode – photoactive layer interface. In this thesis, Molybdenum Oxide (MoO3) and Zinc Oxide (ZnO) is used as hole and electron transporting interfacial layers respectively. 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Zhang, "Photoelectrochemical properties of TiO2 electrodes sensitized by porphyrin derivatives with different numbers of carboxyl groups," Journal of Electroanalytical Chemistry, vol. 537, pp. 31-38, 2002. 182 183 [...]... organic solar cells 20 Figure 1.12 Schematic representation of (A) normal and (B) inverted device structure of OSCs 22 Figure 1.13 Schematic view of the energy levels of metal oxides and orbital energies of some of the organic components used in OSCs.[52] 23 Figure 1.14 Band energies of conduction band (CB) and valence band (VB) of different metal oxides.[75] 28 Figure 1.15 Schematic of the... is the basis of ESCs Examples of this type of ESCs include organic solar cells (OSCs), dye-sensitized solar cells (DSCs), and quantum dot solar cells Conjugated polymers and/ or organic materials such as PCBM, P3HT etc are the materials of choice in OSCs In the DSCs, a wide band gap metal oxide semiconductor, such as TiO2, is anchored to a dye In the third generation quantum dot solar cells, quantum... Energy-level diagram for an excitonic solar cell Although an inferior energy technology when compared to other renewable energy technology such as wind and hydro-electricity, photovoltaics–the science and technology of solar cells have steadily progressed Figure 1.1 shows the state -of the-art performance of various types of solar cells published by NREL One may note that the p–n junction solar cells made from... stage of performing in their theoretical conversion 5 efficiency On the other hand, performance of “emerging solar cells in the second and third generation is relatively inferior Compared to the first generation solar cells, these emerging sol Although an inferior technology ar cells offer ease of fabrication, flexibility, lower cost, and higher performance efficiency1 Although p–n junction solar. .. fuels.[7] In fact, solar photovoltaic energy is presently the most expensive of all renewable sources 1.2 Solar cell classification Solar cells are classified into different schemes based either on the historical evolution or on their principles of operation The class of solar cells based on a p–n junction is the first of its evolution; and therefore, are typically called first generation solar cells Semiconductors,... Figure 1.4 Representatives of conjugated polymers and fullerene derivative used in organic solar cells 8 Figure 1.5 Bilayer organic solar cell and its energy level alignment 9 Figure 1.6 Schematic representation of the bulk heterojunction solar cells (left) and blend morphology with interpenetration network of the donor and acceptor (right) 10 Figure 1.7 Operation of the OSC device at the molecular... Si Silicon SnO2 Tin Oxide SrTiO3 Strontium Titanate TiO2 Titanium di -Oxide V2O5 Vanadium Pentoxide Voc Open Circuit Voltage WO3 Tungsten Oxide ZnO Zinc Oxide xix LIST OF PUBLICATIONS 1 High performance dye-sensitized solar cells, with record open circuit voltage using tin oxide nanoflowers, developed by electro spinning Naveen K Elumalai, R Jose, P S Archana, C Vijila, M M Yusoff and Seeram Ramakrishna... electron donor D and acceptor A 12 Figure 1.8 Comparison between solar spectrum and the photoresponse of an organic solar cell.[22] 13 Figure 1.9 Representation of the (A) exciton, (B) geminate pair or bound electron-hole pair, and (C) free electrons and holes in the donor and acceptor layers of OSCs 15 Figure 1.10 Elementary charge transport processes and recombination in organic solar cells. [43]... upper limit of 34% PCE for p–n junction solar cells for unconcentrated light uner standard AM 1.5 The second generation solar cells are based on the charge separation at an interface either between two conjugated polymers or a fluorophore molecule conjugated with a metal oxide semiconductor The National Renewable Energy Laboratory (NREL) at Colorado, US categorize them as “emerging solar cells (Figure... Table 6.1 Comparison of normalized transition time and electron mobility of the SnO2 with popular metal oxide semiconductors 154 x LIST OF FIGURES Figure 1.1 Best Research-Cell efficiencies, NREL 3 Figure 1.2 Energy-level diagram for an excitonic solar cell 5 Figure 1.3 Schematic representation of the bonding–antibonding interactions between the HOMO and LUMO levels of an organic semiconductor . ENGINEERING OF BINARY METAL OXIDE NANOSTRUCTURES FOR HIGHLY EFFICIENT AND STABLE EXCITONIC SOLAR CELLS NAVEEN KUMAR ELUMALAI (B.Eng.,. energies of some of the organic components used in OSCs.[52] 23 Figure 1.14 Band energies of conduction band (CB) and valence band (VB) of different metal oxides.[75] 28 Figure 1.15 Schematic of. organic solar cells. 20 Figure 1.12 Schematic representation of (A) normal and (B) inverted device structure of OSCs. 22 Figure 1.13 Schematic view of the energy levels of metal oxides and orbital