Diana LaFollette1,Juanita Hidalgo1,Carlo Andrea Riccardo Perini1,Juan Pablo Correa Baena1
Georgia Institute of Technology1
Diana LaFollette1,Juanita Hidalgo1,Carlo Andrea Riccardo Perini1,Juan Pablo Correa Baena1
Georgia Institute of Technology1
One of the ever-present issues with perovskite solar cells (PSCs) is the lack of long-term stability, particularly because popular material methylammonium (MA) lead iodide undergoes a phase transition at 57C and decomposes into PbI<sub>2 </sub>even in an inert environment. Past work exploring the formamidinium (FA) and cesium (Cs) compositional space demonstrated that these compositions have increased thermal stability while approaching the impressive efficiency of MA-based PSCs. However, it has still proven to be extremely difficult to stabilize the photoactive phases of these mixed-cation mixed-halide perovskites, due to the favorability of the photoinactive phases at room temperature.<br/>A thorough investigation of 14 compositions with varying Cs-FA and I-Br ratios was carried out using in-situ XRD and in-situ GIWAXS in combination with X-ray fluorescence and X-ray beam induced current. This established that rather than directly matching phase transitions of pure compositions, chemical changes such as varying Cs-FA and I-Br ratios change the relationship between temperature, crystallinity, and phase purity of these mixed-halide compositions. These studies illuminate how and when crystalline phase formation, segregation, and degradation occur. This in turn allows for increased understanding of stability of these compounds from a molecular standpoint, particularly in previously established high performing compounds like Cs<sub>17%</sub>FA<sub>83%</sub>PbI<sub>3 </sub>and Cs<sub>17%</sub>FA<sub>83%</sub>PbI<sub>83%</sub>Br<sub>17%</sub>.