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

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.