Rachel Beal1 2 Hans Steinrueck2 Nanna Zhou Hangström4 Michael Toney2 Ana Nogueira3

1, Stanford University, Stanford, California, United States
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.