Oxygen-Mediated (0D) Cs₄PbX₆ Formation during Open-Air Thermal Processing Improves Inorganic Perovskite Solar Cell...

The desire to commercialize perovskite solar cells continues to mount, motivating the development of scalable production. Evaluations of the impact of open-air processing have revealed a variety of physical changes in the fabricated devices─with few changes having the capacity to be functionalized. Here, we highlight the beneficial role of ambient oxygen during the open-air thermal processing of metastable γ-CsPbI₃-based perovskite thin films and devices. Physiochemical-sensitive probes elucidate oxygen intercalation and the formation of Pb-O bonds in the CsPbI₃ crystal, entering via iodine vacancies at the surface, creating superoxide (O₂^-) through electron transfer reactions with molecular oxygen, which drives the formation of a zero-dimensional Cs₄PbI₆ capping layer during annealing (>330 °C). The chemical conversion permanently alters the film structure, helping to shield the subsurface perovskite from moisture and introduces lattice anchoring sites, stabilizing otherwise unstable γ-CsPbI₃ films. This functional modification is demonstrated in γ-CsPbI₂Br perovskite solar cells, boosting the operational stability and photoconversion efficiency of champion devices from 12.7 to 15.4% when annealed in dry air. Such findings prompt a reconsideration of glovebox-based perovskite solar cell research and establish a scenario where device fabrication can in fact greatly benefit from ambient oxygen.