Homogeneous crystallization via sustained solvent-extraction channels for methylammonium-free all-perovskite tandem solar cells

Replacing volatile methylammonium (MA+) with formamidinium (FA+) or cesium (Cs+) cations in mixed Pb-Sn perovskite compositions improves thermal resilience. Nevertheless, the low-solubility Cs-based perovskite tends to preferentially crystallize into a dense Cs-rich surface layer during the AS-assisted crystallization process, which impedes the AS to extract the internal solvent. Here, we introduce a multi-Lewis-base modulator to maintain sustained solvent-extraction channels (SSC) open throughout the AS process, thereby homogenizing MA-free mixed Pb-Sn perovskite crystallization in depth. This approach yielded a PCE of 22.7% in FACs Pb-Sn perovskite solar cells. We fabricated monolithic all-perovskite tandem solar cell with a high PCE of 29.2% (certified PCE of 29.2%), which is the highest certified PCE for MA-free all-perovskite TSCs. The unencapsulated FACs-based mixed Pb-Sn perovskite solar cell remained over 80% of its initial PCE after 800 h of aging at 85 °C. This strategy enables the simultaneous realization of high-performance and thermal stability. Despite high efficiencies, all-perovskite tandem solar cells are hindered by thermal instability. Guo et al. address this issue by introducing a multi-Lewis-base modulator, which suppresses the formation of dense Cs-rich films in Pb-Sn perovskites to concurrently deliver high performance and stability.