α-FAPbI3 phase stabilization using aprotic trimethylsulfonium cation for efficient perovskite solar cells

Abstract

Stabilization of the black-phase formamidinium lead triiodide (alpha-FAPbI3) perovskite by inhibiting its phase transition to the yellow non-perovskite phase (delta-FAPbI3) under ambient conditions is one of the major concerns for developing stable and highly efficient perovskite solar cells (PSCs). Moreover, the protic characteristic of the formamidinium (FA+) cation leads to the facile degradation of the corresponding alpha-FAPbI3 induced by depro-tonation in moisture. This work demonstrates the phase stabilization of alpha-FAPbI3 with enhanced moisture sta-bility by introducing a moisture-stable and aprotic organosulfonium cation, namely trimethylsulfonium (TMS+), to form a mixed-cation (TMS)x(FA)1-xPbI3 perovskite. The optimized doping of TMS+ into the FAPbI3 increased the crystallinity with compact and pinhole-free morphology and reduced the defect states and leakage current. Concurrently, the power conversion efficiency (PCE) of the optimized (TMS)0.02(FA)0.98PbI3-based PSCs reached 17.69%, compared to 15.41% for the FAPbI3-based PSC. Further, the mixed-cation (TMS)x(FA)1-xPbI3 PSCs showed improved device stability under continuous one-sun illumination and storage in ambient conditions. This study paves the way for developing new cations that can be applied to PSCs to minimize the phase transition of alpha-FAPbI3 with high moisture stability.