Perovskite grain fusion strategy via controlled methylamine gas release for efficient and stable formamide-based perovskite solar cells

The effectiveness of post-treatment on formamide (FA)-based perovskites by applying methylammonium (MA) has not been reported so far, due to the formation of fatal δ-phase. Herein, we select a series of carboxyl compounds to form their MA salts, utilizing the volatile nature of MA+, so as to control the release of MA gas via an MA-gas-releasing secondary grain growth (MGR-SGG) technique, stabilizing FA-based perovskite while inhibiting the formation of photo-inactive δ-phase. After such a treatment, perovskite grains fuse into larger ones, forming perovskites with better crystallinity, while the remaining compounds, including the carboxyl molecules and the undecomposed MA salts left on the perovskite surface, present passivation effect. The synergetic effects are tuned by varying the carboxyl acid backbones, and the moderate OPEM exhibits suitable MA gas release rate and strong interaction with perovskite, therefore sufficiently diminish defect density, contributing to increased open-circuit voltages by more than 20 mV, and enhanced maximum power conversion efficiency from 21.3% (untreated cells) to 22.4% (MGR-SGG treated cells), along with promoted device stability.