Bismuth Rudorffites: Promising New Materials for the Top Cell in Solution Processed Tandem PV

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Non-Technical Description: An effective way to decrease the overall cost of electricity from solar energy is to increase the power conversion efficiency of solar cells, particularly if it can be accomplished with minimal increase in manufacturing cost. This goal can be achieved by adding a low-cost high-bandgap solar cell on top of a silicon solar cell. To date, the most promising materials for this top-cell are hybrid perovskites. These new materials have extraordinarily high performance, but they contain lead, which is toxic and raises environmental concerns. This project focuses on developing new low-toxicity bismuth-based high bandgap semiconductors that can potentially be used instead. The team is exploring a recently discovered class of bismuth materials called rudorffites and is building an understanding of the critical relationships between material structure, properties, and processing for these non-toxic semiconductors. This basic research could have significant societal impact by enabling the development of low-cost tandem solar cells from non-toxic elements. Furthermore, the project provides training of a graduate student, a postdoctoral scholar, and several undergraduates, as well as engage the public though the Pacific Science Center in Seattle and the Clean Energy Institute's Clean Energy Ambassadors program.Technical Description: Bismuth-based semiconductors are interesting non-toxic optoelectronic materials since the partial oxidation of Bi3+ with the 6s2 lone pair is expected to lead to similar defect tolerance as the lead-based hybrid perovskites. The recently discovered bismuth rudorffites are of particular interest since they exhibit high bandgaps suitable for tandem solar cells. Solar cells from bismuth rudorffites have a few-percentage efficiency, but they have the potential for much higher efficiency. In this project, investigators are conducting a body of fundamental research to explore the potential of solution grown bismuth rudorffites for optoelectronic applications, to understand the fundamental processes responsible for potential performance limitations and to develop strategies to overcome them. Preliminary research shows that nanoscale layer morphology and doping/alloying have a tremendous effect on the optoelectronic quality of these materials. The project utilizes combinatorial spray coating methods to synthesize a large number of alloy/doping compositions, absolute intensity photoluminescence to assess quasi-Fermi level splitting, and photoconductivity-based methods to assess the carrier diffusion length. The goal is to connect structure and processing with the most important material performance metrics for photovoltaic materials.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.