Inherent Compositional Heterogeneity in Alloyed Perovskite Photovoltaics

The metal halide perovskite photovoltaic field has witnessed rapid advances in compositional diversity, where alloying perovskites improves structural stability and allows broad bandgap tunability. However, in addition to illumination-, bias-, and thermally induced ion migration and phase segregation during operation, alloyed perovskites exhibit inherent compositional heterogeneity formed during processing. Such heterogeneity couples with structural disorder and lattice stress, ultimately degrading device performance. This review summarizes the multiscale distribution and underlying origins of inherent composition inhomogeneity in alloyed perovskites during film-formation, elucidates its impacts on film quality and device performance, and discusses emerging strategies for compositional homogenization. During processing, nanoscale clusters and microscale phase-separated domains can form and persist into the final alloyed perovskite films, where they introduce deep traps, perturb band edges, and accelerate degradation, thereby limiting efficiency and stability. To address these issues, homogenization strategies across different film formation stages are discussed, including nucleation engineering during the initial nucleation stage, mass transfer modulation in gel-state intermediates, and crystallization control to balance the crystallization kinetics of different components. Finally, we highlight the challenges and potential strategies for homogenizing the composition of alloyed perovskites toward achieving efficient and stable perovskite photovoltaics.