Oxygen‐Induced Reversible Degradation of Perovskite Solar Cells

After remarkable progress over the past decades, perovskite solar cells (PSCs) currently exhibit efficient solar power conversion efficiency. However, the environmental instability of perovskite materials and devices is still a serious issue, impeding the future commercialization of this technology. Herein, why PSCs degrade in air is investigated and it is found that one of the critical reasons for the air‐induced PSC degradation is the doping of the 2,2′,7,7′‐tetrakis( N,N‐ di ‐p ‐methoxyphenylamine)‐9,9′‐spirobi‐fluorene (spiro‐OMeTAD) hole‐transport layer with oxygen. Photoelectron yield spectroscopy reveals that the hole‐transport level of the spiro‐OMeTAD layer becomes deeper by oxygen doping, increasing an energy barrier for hole extraction. In other words, decreased hole extraction at the perovskite/spiro‐OMeTAD interface induces the degradation of PSCs in air. However, this oxygen‐induced degradation of PSCs is reversible to some extent by storing PSCs in a vacuum to remove oxygen. In contrast, no detectable degradation of the perovskite light absorber is observed after ≈600 h of air exposure from the results of morphological and structural characterizations. These aspects provide a deeper understanding of PSCs degradation, giving insight into improving long‐term durability in air in the future.