2, Materials Sciences Division, Stanford Sychotron Radiation Lightsource, Stanford, California, United States
4, Stockholm University, Stockholm, , Sweden
3, Institute of Chemistry, University of Campinas, Campinas, , Brazil
Organic-inorganic perovskite materials offer a promising route to reducing the dollars-per-watt cost of solar energy due to their ease of deposition and favorable optoelectronic properties. A wide range of perovskite compositions can be solution processed with relative ease where changing the stoichiometry of the material allows for the preparation of materials with bandgaps tailor-made for specific tandem and single-junction applications. Early work showed that varying the Br:I ratio in (CH3NH3)Pb(BrxI1-x)3 tunes the bandgap between 2.3 and 1.6 eV, but photo-induced phase segregation leads to the formation and I-enriched regions in materials with x ≥ 0.2 that trap carriers and pin the voltage of photovoltaic devices. Materials with a combination of formamidinium (FA) and cesium of the A-site in the general ABX3 stoichiometric formyla have demonstrated improved stability to this phenomenon, but the fundamental and mechanistic underpinnings of photo-induced phase segregation are not well understood. We have studied the structural origins of photo-induced phase segregation by coupling synchrotron X-ray diffraction with photoluminescence experiments. We examine materials with a range of FA:Cs and Br:I ratios and show that optical stability is observed at the phase boundary between Br-poor cubic and Br-rich tetragonal compositions, with material compositions farther from the boundary demonstrating a greater extent of segregation. Ours is the first study to examine materials in the FA:Cs phase space with bandgaps relevant for high-efficiency device applications. By mapping out the phase boundary, we provide a roadmap for compositional selection for photostable devices.