Vacuum-Assisted Crystallization for Highly Efficient and Thermally Stable FA-Based Sn–Pb Perovskite Solar Cells

Narrow-bandgap Sn-Pb perovskites have shown considerable promise in both single-junction perovskite solar cells (PSCs) and all-perovskite tandem solar cells. However, most currently reported high-efficiency Sn-Pb PSCs incorporate a substantial fraction of methylammonium (MA) cations, which limits the long-term thermal stability of the perovskite films and the devices. MA-free, formamidinium (FA)-based Sn-Pb perovskites exhibit superior thermal stability but they still suffer from poor crystallization quality and high defect state density. Here, we rationally optimize the crystallization process of pure FA-based Sn-Pb perovskites by introducing a vacuum-assisted treatment during their fabrication. This strategy effectively enhances the crystallinity of the perovskite films, suppresses void formation, and mitigates defect-state density, thereby simultaneously boosting device efficiency and operational stability. Ultimately, the optimized FA-based Sn-Pb PSC delivers a power conversion efficiency of 23.15% and demonstrates outstanding thermal stability.