Charge transfer complex formation between organic interlayers drives light-soaking in large area perovskite solar cells

Light soaking (LS) is a well-known but poorly understood phenomenon in perovskite solar cells (PSCs) which significantly affects device efficiency and stability. LS is greatly reduced in large-area inverted PSCs when a PC₆₁BM electron transport layer (ETL) is replaced with C₆₀, where the ETL is commonly in contact with a thin bathocuproine (BCP) interlayer. Herein, we identify the key molecular origins of this LS effect using a combination of surface photovoltage, ambient photoemission spectroscopy, Raman spectroscopy, integrated with density functional theory simulations. We find that BCP forms a photoinduced charge-transfer (CT) complex with both C₆₀ and PC₆₁BM. The C₆₀/BCP complex accelerates C₆₀ dimer formation, leading to a favourable cascading energetic landscape for electron extraction and reduced recombination loss. In contrast, the PC₆₁BM/BCP complex suppresses PC₆₁BM dimer formation, meaning that PC₆₁BM dimerisation is not the cause of LS. Instead, it is the slow light-induced formation of the PC₆₁BM/BCP CT complex itself, and the new energetic transport levels associated with it, which cause the much slower and stronger LS effect of PC₆₁BM based PSCs. These findings provide key understanding of photoinduced ETL/BCP interactions and their impact on the LS effect in PSCs.