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

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 Sn²⁺ 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 Sn⁴⁺ and PbI₂ 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.