Anchoring of halogen-cleaved organic ligands on perovskite surfaces

Abstract

Photoemission spectroscopy reveals halogen cleavage from bulky cation organics and the formation process of 2D-like perovskites on the parent perovskite as well as their impact on band alignment and device performance. Perovskite surface passivation is key to eliminating junction charge-carrier losses and is considered a critical step in fabricating solar cells with efficiencies close to the theoretical limit. Various halogenated organic ligands are found to produce efficient and stable solar cells when the perovskite surfaces are thermally activated. Photoemission spectroscopy shows that most of the ligands start to evaporate during heating. When annealed at high temperatures, halogens are cleaved off the ligands, leaving the organic cations anchored to the perovskite and causing charge transfer n-doping. The band-alignment measurement reveals that the formation of n-doped perovskites caused by strongly anchored ligands leads to the formation of ideal heterojunctions with energy offsets benefiting electron extraction and hole blocking, contributing to over 22% improvement in device performance without burn-in degradation.