Below 200 °C Fabrication Strategy of Black-Phase CsPbI₃ Film for Ambient-Air-Stable Solar Cells

Cesium-based fully inorganic black-phase (BP) lead halide perovskites (such as α-, β-, and γ-CsPbI₃) with excellent thermal stability and a decently high photovoltaic performance have attracted increasing attention. However, a below 200 °C fabrication process of the desirable BP CsPbI₃ has rarely been reported. Herein, the detailed crystal structure evolution of ambient-air-stable BP CsPbI₃ prepared under low temperature conditions is investigated by exploiting the strong coordination bonding between CO in polyvinylpyrrolidone (PVP) and Pb in CsPbI₃ and inflection effect of PVP under annealing. It is found that ambient-air-stable BP CsPbI₃ films are formed and the energy barrier for the long-term stable BP CsPbI₃ formation is significantly reduced (the required annealing temperature is only 80 °C). After optimization, the highest power conversion efficiencies (PCEs) of ≈4.0% and 10.0% are recorded for the 3% PVP-added devices with light absorbers annealed at 80 and 160 °C, respectively. More importantly, the 3% PVP device annealed at 160 °C maintains ≈80% of its original PCE after 5 months storage under ambient-air conditions.