Pathways to improved stability in organic-inorganic halide perovskite photovoltaic materials
Keywords: Materials Science and Engineering.

The development of reliable, cost-effective, carbon-free energy resources is paramount in mounting a response to climate change that ensures global welfare and economic stability. While a complete decarbonized energy portfolio will include a range of renewable energy technologies, there is no question that solar solar energy should be used to meet an appreciable portion of energy demand. While solar energy has already achieved grid parity, or has a lower levelized cost than electricity from coal-fired power plants, in many U.S. states and other countries around the world, further reductions in cost are still needed. Organic-inorganic metal halide perovskite (OIHP) photovoltaics offer a promising route to reducing the dollars per watt cost of solar energy because they can be deposited via scalable, low cost methods and still achieve high performance. OIHP processing methods such as slot-die coating could be easily integrated into the existing silicon PV infrastructure to make perovskite-on-silicon tandems that outperform current modules for a minimal increase in cost. This thesis will explore several factors limiting the stability and performance of OIHPs that currently prevent their widespread deployment in the PV market. First, it will show that the thermal stability of OIHP materials can be improved through chemical substitution of cesium for the inorganic methylammonium cation. Cesium substitution also improves stability to a more pernicious degradation mechanism: phase segregation of the halide anions under illumination. Tuning chemical composition of OIHPs through halide substitution also tunes the band gap across the ideal range for perovskite-on-silicon tandem solar cells, but segregation of the different halide ions results in the formation of low band gap domains that act as recombination centers and limit the voltage of devices. Fully-inorganic perovskite semiconductors do not achieve the same high photoconversion efficiencies as OIHPs, so next, the impact of incremental cesium substitution in formamidinium lead halide perovskite materials, which have comparable thermal stability, is explored. Structural characterization is coupled with time-dependent photoluminescence to study a range of alloyed formamidinium cesium perovskites. Because cesium and formamidinium have different ionic radii, structural changes are observed as the composition of the alloys is varied, but these structural changes are not perfectly correlated with the observed changes in stability to phase segregation. While several conflicting mechanisms for phase segregation have been proposed in the literature, the experimental data presented here cannot isolate which, if any, are correct. The final chapter will highlight an alternative approach to both understanding and mitigating photo-induced phase segregation. Halide diffusion is a vacancy-mediated process, so phase segregation is necessarily tied to defect chemistry, but very few empirical studies have been done. Here, a method for probing the concentration of halide vacancies using \emph{in situ} X-ray diffraction is demonstrated. Preliminary results are presented and are in agreement with vacancy concentrations on the order of tenths of a percent predicted by first principles calculations. Altogether, this work maps out several pathways to improved stability for high-efficiency OIHP materials so that they can realize their potential in the next generation of renewable energy technologies