Unveiling the Impact of C₆₀–O₂ Interaction on the Performance and Characterization of Perovskite Solar Cells

Abstract C₆₀ is the prevalent electron‐transport layer (ETL) in high‐efficiency p‐i‐n perovskite single‐junction and multi‐junction solar cells. Here, it is demonstrated that the exposure of the C₆₀ ETL to ambient O₂ results in significantly increased non‐radiative recombination, influencing results from commonly applied characterization techniques such as steady‐state and transient photoluminescence (PL), transient surface photovoltage, as well as current density‐voltage measurements. Based on PL and He‐I UV photoemission spectroscopy measurements and supported by density functional theory calculations and drift‐diffusion simulations, it is proposed that O₂ rapidly intercalates into the C₆₀ ETL, causing the formation of deep trap states and an altered charge carrier balance at the perovskite/C₆₀ interface. The findings reveal that the effect is reversible but can mislead experimental interpretations if disregarded, emphasizing the importance of O₂ management during device fabrication and characterization. Furthermore, it is demonstrated that this interaction enables simple PL measurements in air to serve as a novel sensing method for evaluating the barrier layer quality of the SnO_x buffer layer atop C₆₀. This study thereby not only highlights a critical deterioration mechanism in perovskite solar cells and provides a deeper understanding of the underlying interaction between the C₆₀ ETL and O₂ but also offers practical avenues for future selective contact optimizations.