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. Triplecation 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.