Effect of Formamidinium/Cesium Substitution and PbI₂ 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 PbI₂ is added to these perovskite films to enhance their performances. The excess PbI₂ is known to be beneficial for the performance. However, its impact on stability is less well known. Triple-cation perovskites deploy excess PbI₂ up to 8 %. Thus, it is imperative to analyze the role of excess PbI₂ in the degradation kinetics. In this study, the amount of PbI₂ 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 PbI₂ is shown to adversely affect the stability of the material. Faster degradation kinetics are observed for samples with higher PbI₂ contents. However, samples with excess PbI₂ also showed superior properties such as enhanced grain sizes and better optical absorption. Thus, careful management of the PbI₂ quantity is required to obtain better stability and alternative pathways should be explored to achieve better device performance rather than adding excess PbI₂.