Interfacial Contact Passivation for Efficient and Stable Cesium-Formamidinium Double-Cation Lead Halide Perovskite Solar Cells

Perovskite solar cells (PSCs) have achieved extremely high power conversion efficiencies (PCEs) of over 25%, but practical application still requires further improvement in the long-term stability of the device. Herein, we present an in situ interfacial contact passivation strategy to reduce the interfacial defects and extraction losses between the hole transporting layer and perovskite. The existence of PbS promotes the crystallization of perovskite, passivates the interface and grain boundary defects, and reduces the nonradiation recombination, thereby leading to a champion PCE of 21.07% with reduced hysteresis, which is one of the best results for the methylammonium (MA)-free, cesium formamidinium double-cation lead-based PSCs. Moreover, the unencapsulated device retains more than 93% and 82% of its original efficiencies after 1 year's storage under ambient conditions and thermal aging at 85°C for 1,000 h in a nitrogen atmosphere, likely due to the usage of MA-free perovskite and the enhanced surface hydrophobicity.