Precursor Solution Annealing Forms Cubic-Phase Perovskite and Improves Humidity Resistance of Solar Cells

Solar cells with light-absorbing layers comprising organometal halide perovskites have recently exceeded 22% efficiency. Despite high power-conversion efficiencies, the stability of these devices, particularly when exposed to humidity and oxygen, remains poor. In the current study, a pathway to increase the stability of methylammonium lead iodide (CH₃NH₃PbI₃) based solar cells towards humidity is demonstrated, while maintaining the simplicity and solution-processability of the active layers. Thermal annealing of the precursor solution prior to deposition induces the formation of cubic-phase perovskite films in the solid state at room temperature. The experiments demonstrate that this improved ambient stability is correlated with the presence of the cubic phase at device operating temperatures, with the cubic phase resisting the formation of perovskite monohydrate—a pathway of degradation in conventionally processed perovskite thin films—on exposure to humidity.