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Band Structure Engineering for Solar Energy Applications: Zno1-Xsex Films and Devices

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Band Structure Engineering for Solar Energy Applications: Zno1-Xsex Films and Devices Band structure engineering for solar energy applications: ZnO1-xSex films and devices By Marie Annette Mayer A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Engineering - Materials Sciences and Engineering in the Graduate Division of the University of California, Berkeley Committee in charge: Professor Eugene E. Haller, Chair Doctor Wladyslaw Walukiewicz Professor Tsu-Jae King Liu Spring 2012 Abstract Engineering ZnO for solar energy applications: Fabrication and characterization of ZnO1-xSex and related devices by Marie Annette Mayer Doctor of Philosophy in Engineering-Materials Sciences and Engineering University of California, Berkeley Professor Eugene E. Haller, Chair New technologies motivate the development of new semiconducting materials, for which structural, electrical and chemical properties are not well understood. In addition to new materials systems, there are huge opportunities for new applications, especially in solar energy conversion. In this dissertation I explore the role of band structure engineering of semiconducting oxides for solar energy. Due to the abundance and electrochemical stability of oxides, the appropriate modification could make them appealing for applications in both photovoltaics and photoelectrochemical hydrogen production. This dissertation describes the design, synthesis and evaluation of the alloy ZnO1-xSex for these purposes. I review several methods of band structure engineering including strain, quantum confinement and alloying. A detailed description of the band anticrossing (BAC) model for highly mismatched alloys is provided, including the derivation of the BAC model as well as recent work and potential applications. Thin film ZnOxSe1-x samples are grown by pulsed laser deposition (PLD). I describe in detail the effect of growth conditions (temperature, pressure and laser fluence) on the chemistry, structure and optoelectronic properties of ZnOxSe1-x. The films are grown using different combinations of PLD conditions and characterized with a variety of techniques. Phase pure films with low roughness and high crystallinity were obtained at temperatures below 450˚C, pressures less than 10-4 Torr and laser fluences on the order of 1.5 J/cm2. Electrical conduction was still observed despite heavy concentrations of grain boundaries. The band structure of ZnO1-xSex is then examined in detail. The bulk electron affinity of a ZnO thin film was measured to be 4.5 eV by pinning the Fermi level with native defects. This is explained in the framework of the amphoteric defect model. A shift in the ZnO1-xSex valence band edge with x is observed using synchrotron x-ray absorption and emission spectroscopy. Measurement of the optical absorption coefficient, α, shows a significant red shift as well as an increase in the low energy density of states with x. Fitting α revealed that the initial Se defect level is located 0.9 eV above the valence band edge and the coupling strength of the interaction is 1.2 eV. Optical reflection data are good agreement with the absorption onset at 2 eV. Taking the derivative of this data reveals experimental observation of the matrix-like band at higher energies. 1 ZnO1-xSex is explicitly evaluated for photoelectrochemical applications. An introduction to semiconductor electrochemistry is followed by flat band, photocurrent, and spectrally resolved photocurrent measurements. The flat band measurements are in excellent agreement with the measurements of the ZnO electron affinity using bulk methods, but show that the conduction band edge of ZnO1-xSex is too low for spontaneous water splitting. Measurements of the incident photon to current conversion efficiency (IPCE) indicated that photons with energies greater than 2 eV excite carriers that do conduct and induce chemical reactions. Tandem ZnO1-xSex/Si devices are made with a natural Ohmic contact between the p-Si and n-ZnO1-xSex. Electrochemical testing proves that the presence of the tandem photovoltaic provides an overpotential of ~0.5 V to electrons enabling the reduction of H+ in solution. Finally, the carrier scattering and recombination lifetimes in ZnO1-xSex are considered. Resistivity, Hall effect and Seebeck coefficient measurements are used to probe the scattering lifetime, while the recombination lifetime is investigated using photoluminescence spectroscopy. Electrochemical photocurrent measurements in light and dark are a function of the product of both lifetimes. Results indicate that significant scattering in the lateral direction does not prohibit the photoelectrochemical device from operating, but defects from high fluence growth are extremely detrimental to the recombination lifetime. A textured or otherwise irregular crystal that does not function well for a device designed for transport in one direction might be perfectly operational when the current flow is perpendicular. The final chapter provides perspective on the future of ZnO1-xSex in scientific research and obstacles to overcome before industrial applications are possible. Perspective on sustainable hydrogen production is given. The optimist can see a value for nearly all renewable energy technologies in a variety of value-driven applications. 2 Table of contents Table of contents ............................................................................................................................................ i List of figures ............................................................................................................................................... iv Acknowledgements ....................................................................................................................................... x Chapter 1: An Introduction ........................................................................................................................... 1 1.1 Semiconductors: Yesterday, today and tomorrow .......................................................................... 1 1.2 Semiconductors and the energy revolution ..................................................................................... 2 1.3 Semiconductors for photovoltaics ................................................................................................... 3 1.4 Semiconductors for photoelectrochemical applications .................................................................. 5 1.5 This Dissertation ............................................................................................................................. 6 Chapter 2: Band structure engineering .......................................................................................................... 7 2.1 Semiconductor bands and band gaps .............................................................................................. 7 2.2 Structural band gap engineering ..................................................................................................... 8 2.3 Chemical band gap engineering ...................................................................................................... 9 2.4 Highly mismatched alloys and band anticrossing ......................................................................... 10 Chapter 3: Synthesis of the highly mismatched oxide alloy ZnO1-xSex ...................................................... 14 3.1 Oxides for solar applications ......................................................................................................... 14 3.2 Pulsed laser deposition of ZnO1-xSex ............................................................................................ 14 3.3 Characterization techniques used in this chapter .......................................................................... 17 3.4 Effects of growth on the structural and chemical properties ......................................................... 18 3.5 Effects of growth on the electrical properties ............................................................................... 21 3.6 Effects of growth on optical properties ......................................................................................... 23 3.7 Vibrational modes in ZnO1-xSex .................................................................................................... 24 3.8 Summary ....................................................................................................................................... 27 Chapter 4: Electronic structure of ZnO1-xSex .............................................................................................. 29 4.1 Relevance and goals of understanding the electronic structure .................................................... 29 4.2 The electron affinity of ZnO found using the Fermi stabilization energy..................................... 30 4.2.1 The amphoteric defect model ................................................................................................ 30 4.2.2 Measuring electron affinity using stabilization of the Fermi level ....................................... 31 i 4.3 Band Edge measurements using synchrotron x-ray spectroscopies .............................................. 32 4.4 The optical absorption coefficient ................................................................................................. 35 4.5 Theoretical determination of the ZnO1-xSex dispersion relation .................................................... 36 4.6 Optical reflectivity .......................................................................................................................
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