Homogeneous SnF ₂ ‐Induced Strain Modulation in Sn–Pb Perovskites for High‐Performance Monolithic All‐Perovskite Tandem Solar Cells

Tin fluoride (SnF₂) serves as an indispensable antioxidant in Sn-Pb perovskite solar cells, playing a critical role in the development of efficient all-perovskite tandem architectures. However, excessive SnF₂ often suffers from aggregation challenges, inevitably causing phase separation and tensile strain within the perovskite films, severely compromising device efficiency and stability. In this study, we report a precise regulation strategy by employing phenylethylammonium chloride (PEACl) to modulate the distribution of excess SnF₂. The hydrogen bonding between PEACl and SnF₂, coupled with steric hindrance effects, enables uniform dispersion of SnF₂ at grain boundaries, effectively suppressing SnF₂ phase segregation and promoting homogeneous crystallization of Sn-Pb perovskites. Moreover, it is revealed that the precise regulation of SnF₂ distribution through PEACl effectively releases local strain in perovskite thin films. Single-junction Sn-Pb devices treated with the SnF₂+PEACl demonstrate an exceptional power conversion efficiency (PCE) of 23.52%, substantially outperforming control devices at 20.83%. The optimized two-terminal (2-T) monolithic all-perovskite tandem solar cells achieve a remarkable PCE of 28.89%. Notably, these tandem devices maintain over 80% of their initial efficiency after continuous operation at maximum power point under one-sun illumination for 670 h, exhibiting excellent long-term stability.