Study of the Heat-Induced Degradation Mechanism in PbI₂-Excess Perovskite Solar Cells

The photodecomposition of PbI2 in the perovskite halide structure has been identified as a primary reason for the rapid degradation of metal halide perovskite solar cells (PSCs) under continuous illumination. In particular, excess PbI2 on the surface of perovskite films reduces the stability of PSCs via several mechanisms that are still unelucidated. Herein, the influence of excess PbI2 on PSCs is investigated by varying the Au deposition rate, which results in different temperatures during the fabrication of PSCs, and then they are compared with stoichiometric PSCs. It is demonstrated that the anion exchange of Br- in perovskite and I- in excess PbI2 occurs at the grain boundary and interface of the perovskite layers induced by the high heating temperature, leading to a photoluminescence peak redshift and lower power conversion efficiency. Light illumination accelerates the exchange of Br- and I- between the perovskite layer and excess PbI2 induced by carrier accumulation at the interface and grain boundary, leading to the poor stability of the PSCs.