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, present organic-inorganic perovskite photovoltaic devices degrade in the presence of moisture in air, and the organic component is one reason for this lack of stability. The goal of this project is develop new class of perovskite solar cell materials that are made entirely of inorganic, nanometer sized crystals. The innovative aspect of this approach is that by fusing together different types of nanocrystals which contain the elements cesium and chloride, the stability of the perovskite in moisture will be much improved. As part of the educational activities associated with this project, solar photovoltaic concepts will be integrated into hands-on demos at Philly Materials Day in Philadelphia, PA, as part of an effort to reinforce state education standards in STEM teaching.
The overall goal of this research is to employ chemically-induced sintering of colloidal nanocrystals to produce doped, polycrystalline thin films of cesium metal halide perovskites for use in solar photovoltaic devices. Cesium lead iodide and cesium tin iodide perovskite phase materials share many attributes with the hybrid organic/inorganic methylammonium lead iodide perovskite materials for solar photovoltaics. However, relative to organic metal halide perovskite materials, the all-inorganic cesium perovskites are both less volatile and less chemically reactive, but are still not stable in the presence of atmospheric moisture. It is hypothesized that doping with chloride will stabilize the cesium lead iodide in the perovskite form. To carry out the chloride doping, a hetergenous-phase reaction is used to sinter solid films of mixed cesium lead chloride and cesium lead iodide nanocrystals. By this approach, non-equilibrium, mixed nanocrystal thin films can be synthesized. This nanocrystal-based approach also makes it possible to follow the progress and products of the synthesis reactions with in-situ, temperature-dependent absorption spectroscopy to detect both mixing and phase-separation at microscopic levels, allowing for elucidation of kinetically vs. thermodynamically controlling processes. The analytical approaches proposed here present an opportunity to mechanistically understand the intrinsic miscibility of two dissimilar halide perovskite phases to promote the overall phase stability of mixed crystal perovskite thin films.
