Utilizing Photoelectron Spectroscopy to Influence the Design of Earth-abundant Solution-Processed Chalcogenide Thin-film Photovoltaics
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Solution processing offers many key advantages to the manufacturing of photovoltaic cells. This includes lower costs, higher throughput and lower temperature conditions resulting in shorter energy payback times and better scalability. Solar cells developed using these techniques then offer greater potential to fill the growing demand for low cost and sustainable energy production. Presented in this thesis is the characterization of each primary interface in solution-deposited Cu2BaSnSxSe4-x (CBTSSe) solar cells using photoelectron spectroscopy techniques. This material is set to improve upon high efficiency predecessor Cu2ZnSnSxSe4-x (CZTSSe) materials by suppressing inherent antisite defect formation through dissimilar ionic-sizes and coordination mismatch. From the electron affinity (EA) values determined by ultraviolet and inverse photoelectron spectroscopies a large conduction band offset of -0.6 eV was measured at the buffer/absorber (CdS/CBTSSe) interface, meaning the conduction band edge of CdS is significantly lower than that of CBTSSe. A cliff-like band profile of this magnitude can promote charge carrier recombination at this interface, lowering the open circuit voltage of the photovoltaic cell and therefore reducing its power conversion efficiency. It is then suggested, based on these findings, that lower electron affinity electron transport materials need to be developed for future optimization of these devices.