ALTERNATIVES TO ORGANIC ACID SURFACE MODIFICATION OF ZNO FOR EXCITONIC PHOTOVOLTAICS by Thomas M. Brenner A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Applied Physics). Golden, Colorado Date Signed: Thomas M. Brenner Signed: Prof. Reuben T. Collins Thesis Advisor Signed: Prof. Thomas E. Furtak Thesis Advisor Golden, Colorado Date Signed: Thomas E. Furtak Professor and Head Department of Physics ii ABSTRACT Surface modification of metal oxides with molecular monolayers is an effective strategy for tuning interface properties in excitonic devices employing metal oxides as charge accept- ing and transport layers. The most commonly used attachment chemistries are acid/base reactions employing organic acids. The use of acid/base chemistries has presented a problem for one of the most commonly used and promising metal oxides in excitonic devices, zinc ox- ide (ZnO). ZnO is easily etched by even weak organic acids, leading to non-ideal monolayers and the accumulation of surface complexes during etching, which is particularly problematic for ZnO-based dye sensitized solar cells (DSSCs). Two ways to address this issue have been explored. The first approach is to employ a triethoxysilane (TES)-based covalent attachment scheme instead of an acid/base reac- tion for attaching modifier molecules. We demonstrate that dipolar mixed monolayers of phenyltriethoxysilane-based molecules tune the work function of ZnO and the performance of bulk heterojunction photovoltaic devices containing modified ZnO layers. This indicates these modifiers are effective for tuning interfacial electronic structure. The second approach is to investigate Zn1-xMgxO (ZnMgO) alloys in order to produce a more etch resistant material with similar electronic properties to ZnO. These alloys, when exposed to the prototypical modifier benzoic acid (BA), demonstrate a steady-state, macro- scopic etch rate that decreases up to an order of magnitude (at 20% Mg) compared to ZnO. Infrared spectroscopic characterization of BA-modified ZnMgO indicates a monolayer of BA attaches to the ZnMgO surface nearly instantaneously and remains throughout etching. These results suggest that ZnMgO is a promising alternative material that may alleviate some of the problems with ZnO etching. However, for applications of this material as a substrate for dye sensitization, the initial etch rate, and not the steady-state rate, is really the quantity of interest. We investigated the initial etch rate of ZnMgO exposed to N3 dye (cis- iii bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylato)-ruthenium(II)). We find the initial etch rate of ZnMgO increases with Mg content, in contrast to the steady-state etch rates observed for BA-treated ZnMgO. We also find that the primary products of etching are Zn-carboxylate products. From these results we propose a mechanism for the observed etch resistance. iv TABLE OF CONTENTS ABSTRACT . iii LIST OF FIGURES . viii LIST OF TABLES . xii LIST OF SYMBOLS . xiii LIST OF ABBREVIATIONS . xvi ACKNOWLEDGMENTS . xviii CHAPTER 1 METAL OXIDE SEMICONDUCTORS IN EXCITONIC PHOTOVOLTAICS . 1 1.1 Metal Oxide Semiconductors . 1 1.2 Organic and Fullerene Semiconductors . 4 1.3 General Photovoltaic Device Physics . 10 1.4 Excitonic Solar Cell Device Physics . 17 1.5 Metal Oxide/Organic Interfaces . 21 1.6 Monolayer Modification of Metal Oxide Surfaces and Interfaces . 25 1.7 Acid Dissolution of Metal Oxide Semiconductors . 27 1.8 Thesis Organization . 29 CHAPTER 2 SAMPLE PREPARATION AND CHARACTERIZATION TECHNIQUES . 31 2.1 Production of Zn1-xMgxO Thin Films . 31 2.2 Triethoxysilane Modification of ZnO . 32 2.3 Carboxylic Acid Modification of Zn1-xMgxO ...................33 v 2.4 Fabrication of Bulk Heterojunction Solar Cells . 34 2.5 UV-Vis Absorption Spectroscopy . 35 2.6 Infrared Absorption Spectroscopy . 36 2.7 Kelvin Probe Surface Potential Measurements . 39 2.8 Tapping Mode Atomic Force Microscopy . 42 2.9 X-Ray Photoelectron Spectroscopy . 45 2.10 Contact Angle Goniometry . 48 2.11 Grazing Incidence X-Ray Diffraction . 49 2.12 Photoluminescence Spectroscopy . 51 CHAPTER 3 TUNING ZINC OXIDE/ORGANIC ENERGY LEVEL ALIGNMENT USING MIXED TRIETHOXYSILANE MONOLAYERS . 53 3.1 Introduction . 54 3.2 Experimental . 58 3.3 Results and Discussion . 61 3.4 Conclusions . 71 3.5 Acknowledgements . 71 CHAPTER 4 ETCH-RESISTANT ZN1-XMGXO ALLOYS: AN ALTERNATIVE TO ZNO FOR CARBOXYLIC ACID SURFACE MODIFICATION . 72 4.1 Introduction . 73 4.2 Experimental . 76 4.3 Results and Discussion . 78 4.4 Conclusions . 92 4.5 Acknowledgements . 92 vi CHAPTER 5 EXPLORING THE MECHANISM OF ZN1-XMGXO ETCH RESISTANCE THROUGH DYE SENSITIZATION . 94 5.1 Introduction . 96 5.2 Experimental Methodology . 99 5.3 Results and Discussion . 104 5.4 Conclusions . 116 5.5 Acknowledgements . 117 CHAPTER 6 CONCLUSIONS . 119 6.1 Project Conclusions . 119 6.2 Future Project Suggestions . 122 REFERENCES CITED . 125 APPENDIX A - FURTHER EXPLANATION OF METHODOLOGY . 140 A.1 Infrared Active and Inactive Modes: An Example . 140 A.2 Fourier Transform Infrared Spectrometer Design . 141 A.3 Simplified Schematic of the Kelvin Probe . 142 A.4 TM-AFM Feedback Loop Schematic . 144 APPENDIX B - SUPPORTING INFORMATION FOR CHAPTER 3 . 145 B.1 Calculation of Surface Proportion of 4CPTES and PTES from Infrared Spectrum . 145 B.2 Water Contact Angle Measurements . 148 B.3 Atomic Force Microscopy Measurements . 148 B.4 Dark J-V Curves . 150 B.5 Effect of Light Soaking . 150 APPENDIX C - PERMISSIONS . 152 vii LIST OF FIGURES Figure 1.1 Hexagonal wurtzite structure of ZnO . 3 Figure 1.2 Atomic force microscopy height images of Zn1-xMgxO films produced by a sol gel process . 5 Figure 1.3 UV-Vis spectra of the thin films of Zn1-xMgxO studied in this thesis . 6 Figure 1.4 Examples of common electro-active organic polymers and small molecules . 7 Figure 1.5 Illustration of Fermi level equilibration in junctions of electronic materials . 11 Figure 1.6 Example of a current density - voltage curve for a solar cell . 14 Figure 1.7 Equivalent circuit model of a solar cell . 17 Figure 1.8 The operational steps of excitonic solar cells . 18 Figure 1.9 Examples of excitonic photovoltaic device architectures . 19 Figure 1.10 Examples illustrating issues with metal oxide/organic semiconductor interfaces . 22 Figure 1.11 Electronic structure of a metal oxide and an organic in isolation and in contact . 23 Figure 2.1 UV-Vis absorbance spectra of materials used in this thesis . 37 Figure 2.2 Diagram of the working principle of the PM-IRRAS technique . 40 Figure 2.3 Basic working principle of Kelvin probe surface potential measurements . 41 Figure 2.4 Behavior of the tip in contact mode atomic force microscopy and tapping mode AFM . 43 Figure 2.5 Phase shift between AFM tip amplitude and driving force during TM-AFM . 44 Figure 2.6 TM-AFM height and phase images showing the relationship between topography, sample inhomogeneity, and phase . 45 viii Figure 2.7 X-ray photoelectron spectroscopy experimental setup . 47 Figure 2.8 Contact angle measurement setup and examples . 48 Figure 2.9 Experimental setup of grazing incidence X-ray diffraction . 50 Figure 2.10 Illustration of photoluminescence spectroscopy experiment setup and spectrum of N3 dye . 52 Figure 3.1 Energy level alignment at an ideal metal oxide/organic interface can be tuned by introducing a dipolar monolayer at the interface . 56 Figure 3.2 PM-IRRAS measurements of ZnO films treated with PTES and 4CPTES in different proportions . 63 Figure 3.3 Relative work function of treated ZnO as a function of 4CPTES and PTES mole fraction . 65 Figure 3.4 Representative light J-V measurements of IBHJ photovoltaic devices containing mixed monolayer modified ZnO . 67 Figure 3.5 Plot of open-circuit voltage of devices against relative work function of treated ZnO films . 70 Figure 4.1 Bandgaps of ZnMgO films as a function of Mg content . 79 Figure 4.2 Background-subtracted grazing incidence X-ray diffraction spectra of Zn1-xMgxO...................................80 Figure 4.3 PM-IRRAS infrared absorption spectra of benzoic acid-treated ZnMgO films showing the carboxyl stretch region . 83 − Figure 4.4 Peak fits to the dominant νasym(CO2 ) feature of the infrared spectra of ZnMgO films soaked in benzoic acid for 1 hr . 84 − Figure 4.5 Ratio of the integrated intensity of the νasym(CO2 ) modes observed on benzoic acid treated ZnMgO . 86 − -1 Figure 4.6 Variation of the νasym(CO2 ) modes at 1532 and 1569 cm in ZnO samples as a function of exposure time to benzoic acid . 88 -1 − Figure 4.7 Plot of integrated intensity of 1550 and 1571 cm νasym(CO2 ) modes and their sum as a function of bandgap . 89 ix Figure 4.8 Relationship between amount of unreacted acetate in ZnMgO films and amount of attached benzoic acid . 90 Figure 5.1 Molar absorptivity of N3 dye in ethanol solution . 101 Figure 5.2.