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

Tin fluoride (SnF 2 ) 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 SnF 2 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 SnF 2 . The hydrogen bonding between PEACl and SnF 2 , coupled with steric hindrance effects, enables uniform dispersion of SnF 2 at grain boundaries, effectively suppressing SnF 2 phase segregation and promoting homogeneous crystallization of Sn–Pb perovskites. Moreover, it is revealed that the precise regulation of SnF 2 distribution through PEACl effectively releases local strain in perovskite thin films. Single‐junction Sn–Pb devices treated with the SnF 2 +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.