Mixed composition lead halide perovskites have emerged as successful light-absorbing materials for next-generation thin-film solar cell devices. For example, state-of-the-art triple cation Cs_0.05MA_0.17FA_0.78Pb(I_0.83Br_0.17)₃ perovskite solar cells have achieved higher efficiency and reproducibility than their single cation counterparts. The tunability of the band gap through compositional engineering is highly desirable for tandem devices and has helped such devices exceed efficiencies of 30%. However, the alloyed nature of these materials makes the thin films used in devices chemically and structurally heterogeneous, which can reduce device performance and long-term stability, creating a bottleneck in the development of perovskites into commercial devices. Additionally, increasing the band gap by increasing the bromide content has been shown to lead to halide segregation occurring upon illumination, reducing efficiency. Despite being integral to long term efficiency and stability, the mechanism behind halide segregation is not fully understood.

To elucidate the mechanism of halide segregation and the heterogeneous nature of alloyed perovskites thin films, we use photoluminescence (PL) spectroscopy, hyperspectral PL microscopy, and synchrotron X-ray techniques including X-ray absorption near edge spectroscopy (XANES) and XANES mapping under operando illumination conditions. This suite of approaches allows us to probe the optoelectronic performance, ion migration, elemental composition, and local chemical environment and structure both macroscopically and as a function of position across the thin film samples. By correlating these metrics, we gain a deeper understanding of the driving forces of performance-limiting halide segregation in these materials beyond what any single technique could deliver alone.