Impact of the valence band energy alignment at the hole-collecting interface on the photostability of wide band-gap perovskite solar cells

This work discusses the need to enhance charge carrier collection to minimize halide segregation in wide band-gap (WBG) perovskites. Here, we systematically elucidate the impact of valence band maximum (VBM) offsets and energetic barriers formed at the hole transport layer (HTL)/perovskite interface on charge accumulation, its influence on halide segregation, and ultimately on perovskite solar cell (PSC) long-term photostability. To this end, we precisely tune the VBM-HTL energetic levels by employing blends of self-assembled monolayers (SAMs; MeO-2PACz and Br-2PACz) to fabricate customized HTLs for PSCs with three different WBG perovskite photoabsorbers (1.69, 1.81, and 2.00 eV), commonly used in various tandem configurations. We find that optimized energetic alignment at the SAM HTL/perovskite interface significantly enhances the long-term photostability of the WBG PSCs. Our results show that photostability of devices can be predicted when comparing HTL/perovskite interfaces using photoluminescence’s evolution and transient surface photovoltage spectroscopies of half-stacks (glass/metal oxide/HTL/perovskite) in correlation with halide segregation.