2, Department of Chemistry, Stanford University, Stanford, California, United States
3, Department of Applied Physics, Stanford University, Stanford, California, United States
4, Department of Materials Science and Engineering, Stanford University, Stanford, California, United States
5, Department of Chemical and Biological Engineering, University of Colorado Boulder, Boulder, Colorado, United States
In the pursuit of efficient tandem solar cells, high band-gap photovoltaic absorbers are required for top cells. Hybrid perovskites are promising candidates, as their band gaps can be widely tuned via compositional adjustments. Specifically, perovskites of the form FA1-xCsxPb(I1-yBry)3 have shown high device efficiencies as well as high band gaps suitable for tandem applications . In this family of perovskites, we have observed a tetragonal-cubic phase transition that is coincident with a change in the temperature coefficient of the band gap and also occurs within solar cell operating temperatures. It has been suggested  that the phase transition may impact light induced phase segregation, which has been identified as a major concern that lowers operating voltages in mixed cation perovskite solar cells . Thus, it is key to understand the phase behavior of this family of perovskites, as well as the impact of structural changes on electronic properties.
We explore here a wide compositional space of FA1-xCsxPb(I1-yBry)3 thin films with temperature-dependent synchrotron X-ray diffraction. At room temperature, many of these compositions exhibit tetragonal peaks that we have indexed to the P4/mbm space group, corresponding to concerted octahedral tilting about the c axis of the perovskite crystal structure. This is distinct from tetragonal MAPbI3, which has alternating tilt directions about the c axis corresponding to the I4/mcm space group. We compute the structure factors of a number of Bragg peaks that allow us to model the average octahedral tilt angle as a function of the temperature. Within the FA1-xCsxPb(I1-yBry)3 family, we show how varying the composition affects the phase transition temperature and we present a room temperature phase diagram. Additionally, we show that the temperature coefficient of the band gap presents a discontinuity at the phase transition temperature. This effect is related to band gap tuning observed in mixed Pb/Sn perovskites, which share the same tetragonal space group . This work establishes a phase diagram that may help interpret photostability and will enable better prediction of band gaps in this family of hybrid perovskites.
 K.A. Bush et al. Compositional Engineering for Efficient Wide Band Gap Perovskites with Improved Stability to Photo-induced Phase Segregation, ACS Energy Lett. 3, 428-435 (2018).
 A. J. Barker et al. Defect-Assisted Photoinduced Halide Segregation in Mixed-Halide Perovskite Thin Films, ACS Energy Lett. 2, 1416-1424 (2017).
 R. Prasanna et al. Band Gap Tuning via Lattice Contraction and Octahedral Tilting in Perovskite Materials for Photovoltaics, J. Am. Chem Soc. 139, 32, 11117-11124 (2017).