Effect of Formamidinium/Cesium Substitution and PbI 2 on the Long‐Term Stability of Triple‐Cation Perovskites

Altering cation and anion ratios in perovskites has proven an excellent means of tuning the perovskite properties and enhancing the performance. Recently, methylammonium/formamidinium/cesium triple‐cation mixed‐halide perovskites have demonstrated efficiencies up to 22 %. Similar to the widely explored methylammonium lead halide, excess PbI 2 is added to these perovskite films to enhance their performances. The excess PbI 2 is known to be beneficial for the performance. However, its impact on stability is less well known. Triple‐cation perovskites deploy excess PbI 2 up to 8 %. Thus, it is imperative to analyze the role of excess PbI 2 in the degradation kinetics. In this study, the amount of PbI 2 in the triple‐cation perovskite films is varied and the degradation kinetics monitored by X‐ray diffraction and optical absorption spectroscopy. The inclusion of excess PbI 2 is shown to adversely affect the stability of the material. Faster degradation kinetics are observed for samples with higher PbI 2 contents. However, samples with excess PbI 2 also showed superior properties such as enhanced grain sizes and better optical absorption. Thus, careful management of the PbI 2 quantity is required to obtain better stability and alternative pathways should be explored to achieve better device performance rather than adding excess PbI 2 .