An Interdiffusion Method for Highly Performing Cesium/Formamidinium Double Cation Perovskites

The fabrication of high-quality cesium (Cs)/formamidinium (FA) double-cation perovskite films through a two-step interdiffusion method is reported. CsxFA1-xPbI3-y(1-x)Bry(1-x) films with different compositions are achieved by controlling the amount of CsI and formamidinium bromide (FABr) in the respective precursor solutions. The effects of incorporating Cs+ and Br− on the properties of the resulting perovskite films and on the performance of the corresponding perovskite solar cells are systematically studied. Small area perovskite solar cells with a power conversion efficiency (PCE) of 19.3% and a perovskite module (4 cm2) with an aperture PCE of 16.4%, using the Cs/FA double cation perovskite made with 10 mol% CsI and 15 mol% FABr (Cs0.1FA0.9PbI2.865Br0.135) are achieved. The Cs/FA double cation perovskites show negligible degradation after annealing at 85 °C for 336 h, outperforming the perovskite materials containing methylammonium (MA).