Elimination of light-induced degradation at the nickel oxide-perovskite heterojunction by aprotic sulfonium layers towards long-term operationally stable inverted perovskite solar cells

Abstract

Nickel oxide (NiO_x) is a promising hole-selective contact to produce efficient inverted p-i-n structured perovskite solar cells (PSCs) due to its high carrier mobility and high transparency. However, the light-induced degradation of the NiO_x–perovskite heterojunction is the main factor limiting its long-term operational lifetime. In this study, we used the time-resolved mass spectrometry technique to clarify the degradation mechanism of the NiO_x-formamidinium–methylammonium iodide perovskite (a common composition for high-performance PSCs) heterojunction under operational conditions, and observed that (1) oxidation of iodide and generation of free protons under 1-sun illumination, (2) formation of volatile hydrogen cyanide, methyliodide, and ammonia at elevated temperatures, and (3) a condensation reaction between the organic components under a high vapor pressure. To eliminate these multi-step photochemical reactions, we constructed an aprotic trimethylsulfonium bromide (TMSBr) buffer layer at the NiO_x/perovskite interface, which enables excellent photo-thermal stability, a matched lattice parameter with the perovskite crystal, and robust trap-passivation ability. Inverted PSCs stabilized with the TMSBr buffer layer reached the maximum efficiency of 22.1% and retained 82.8% of the initial value after continuous operation for 2000 hours under AM1.5G light illumination, which translates into a T₈₀ lifetime of 2310 hours that is among the highest operational lifetimes for NiO_x-based PSCs.