A Multifunctional Polar Amino Acid for Mixed Tin‐Lead Perovskites and All‐Perovskite Tandems

Abstract Over the past years, the performance of all‐perovskite tandem solar cells has skyrocketed. However, mixed tin‐lead (Sn‐Pb) perovskites, which are pivotal in tandem cells, face challenges such as inherent Sn 2+ oxidation and p‐type self‐doping. In this study, A molecular engineering strategy is introduced to address these issues by identifying key properties for effective additive designing: a high highest occupied molecular orbital level, a high boiling point, a large dipole moment, and a large electrostatic surface potential. Guided by these principles, the polar amino acid asparagine hydrochloride (AsnCl) is selected as a multifunctional additive. AsnCl, with its distinctive properties, effectively enhances the orientation and crystalline quality of perovskite films, suppresses harmful Sn 4+ and PbI 2 residues, realizes larger grains, and significantly extends carrier lifetimes while reducing non‐radiative recombination. As a result, the best‐performing single‐junction mixed Sn‐Pb perovskite solar cell achieves a power conversion efficiency (PCE) of 22.54% with significantly enhanced operational and storage stability. Furthermore, the two‐terminal all‐perovskite tandem solar cells based on AsnCl‐treated Sn‐Pb perovskites show high PCEs, and the highest steady‐state PCE is up to 28.24%. This work highlights the potential of additive molecular engineering strategies and their systematic selection principles in developing high‐performance perovskite tandem solar cells.