Electron diffusion at Sn perovskite/fullerene derivative interfaces and its influence on open-circuit voltage

Tin-based perovskite solar cells (PSCs) are the most promising alternatives to toxic lead-based ones. However, the loss in open-circuit voltage (V_OC) remains an important issue. Improvement of V_OC has been achieved by using a fullerene derivative, indene-C₆₀ bisadduct (ICBA), as the electron transporting layer (ETL). For further V_OC improvement, the V_OC improvement mechanisms must be clarified. Herein, we show, at a molecular level, V_OC improvement mechanisms by an ICBA ETL in tin-based PSCs. Electron spin resonance spectroscopy reveals that electron diffusion from perovskite to ETL occurs at perovskite/ETL interfaces, producing unfavorable upward band-bending of perovskite. Employing ICBA with a shallower LUMO level suppresses the upward band-bending as well as reduces the energy offset with the conduction band minimum of perovskite. Suppressing this unfavorable upward band-bending reduces interface recombination at perovskite/ETL interfaces and contributes to V_OC improvement. These insights support efficient optimization of the charge-transporting layer for additional improvement of V_OC.