A-site phase segregation in mixed cation perovskite

Mixed cation strategy greatly benefits the enhancement of device performance and chemical stability. However, adverse impact also accompanies the mixed cation system simultaneously. It brings the compositional instability, wherein the homogeneous film is likely to segregate into multi-phases during the fabrication and ageing process, thus resulting in the efficiency reduction of perovskite solar cells (PSCs) devices. This review focuses on the cation induced phase segregation, and elucidates the segregation mechanisms from the perspectives of film formation and ageing process, respectively. Furthermore, the influence of cation segregation on device performance and operational stability are discussed. And based on these understandings, viable strategies are proposed for the design of phase-stable mixed composition halide perovskites and for suppressing segregation to benefit its development towards commercial applications.