Molecule Anchoring Strategy Promotes Vertically Homogeneous Crystallization and Aligned Interfaces for Efficient Pb–Sn Perovskite Solar Cells and Tandem Device

Abstract Narrow‐bandgap (NBG) Pb–Sn perovskites are ideal candidates as rear subcell in all‐perovskite tandem solar cells. Because Pb–Sn perovskites contain multiple components, the rational regulation of vertical structure and both interfaces of the film is primarily crucial to achieve high‐performing NBG perovskite solar cells (PSCs). Herein, a molecule anchoring strategy is developed to in situ construct Cs 0.1 MA 0.3 FA 0.6 Pb 0.5 Sn 0.5 I 3 perovskite film with vertically aligned crystals and optimized interfaces. Specifically, l ‐alanine methyl ester is developed as an anchoring additive to induce the vertical crystal growth, while PEA 2 PbI 3 SCN film is introduced to promote the homogeneous crystallization at the buried interface via SCN− anchoring with cations. Further ethylenediamine dihalides (EDA(I/Cl) 2 ) post‐treatment leads to the gradient energy level alignment on the film surface. Pb–Sn PSCs based on such film show efficient charge transport and extraction, producing a champion power conversion efficiency (PCE) of 22.3% with an impressive fill factor of 82.14%. Notably, combining with semitransparent 1.78 eV wide‐bandgap PSCs, the four‐terminal all‐perovskite tandem device achieves a PCE of 27.1%. This work opens up a new pathway to boost the performance of Pb–Sn PSCs and their tandem devices.