The sun represents the most abundant potential source of sustainable energy on earth. Solar cells for producing electricity require materials that absorb the sun's energy and convert its photons to electrons, a process called photovoltaics. Recently, materials based on inorganic-organic halide perovskite materials have achieved promising solar energy power conversion efficiency approaching that of silicon solar cells, and can be made from earth-abundant elements using lower-cost, solution based fabrication methods. However, these perovskite materials contain lead, which is toxic, and they also degrade in the presence of moisture, which prevents their commercial use. To address these limitations, this project will develop new solar perovskite-like solar materials based on the element bismuth, which is both abundant and has low toxicity. This unexplored class of materials was identified through theoretical and computational techniques, and the research will use these theoretical insights to make and test the performance of this potentially exciting new class of materials for photovoltaic applications. The experimental findings, in combination with theoretical analysis, will feed back to the materials design criteria to help identify promising new materials for continued study. The educational activities associated with this project include laboratory internships offered in coordination with minority-serving organizations, hands-on learning modules, and the continued development of a semester-long solar photovoltaics course.
Bismuth (Bi) compounds with perovskite-like band structures, including ternary bismuth halides and bismuth chalcohalides, are promising absorption materials for solar photovoltaic applications that go beyond conventional perovskite materials in three ways. First, their electronic structure and strong spin-orbit coupling can enable tolerance to intrinsic defects in a way that is similar to methylammonium lead iodide perovskite materials, but do not contain lead. Second, the bismuth cation has a large Born effective charge to provide high dielectric constants and screening of charged defects. And third, lead-free methylammonium bismuth iodide materials can be phase stable in the presence of water vapor due to the preferential formation of protective oxide layers. Given these potential advantages over lead-based organic metal halide perovskite materials, the overall goal of this research is to gain a fundamental understanding of the photovoltaic performance of bismuth-containing compounds as perovskite-like solar PV materials through theoretical and experimental investigation. Towards this end, the research has three objectives. The first objective is to investigate the importance of symmetry for favorable transport properties by investigating the alkali metal metathiobismuthites, one of the few classes of Bi-based materials that are not layered. The second objective is to develop models to determine the recombination processes and recombination rate in materials from time-resolved photoluminescence measurements. The third objective is to develop strategies for growing the new Bi-compounds with higher purity and correlate impurity content with minority carrier lifetime. Density functional theory will be used to determine the electronic structure, dielectric constant, and charge carrier effective masses. Experimental studies will establish the materials design criteria for efficient solar absorption, focusing on determining the role of intra-granular structural defects, molecular cations, and crystal symmetry on transport diffusion length, optical properties, and overall device performance. The diffusion length of the thin films will be obtained through time-resolved photoluminescence measurements as well as single photon and two-photon spectroscopy for depth-resolved lifetime measurements that decouple the effects of surface recombination.