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

## TL;DR

This paper introduces a new amino acid additive that improves the efficiency and stability of perovskite solar cells and tandem devices.

## Contribution

A multifunctional polar amino acid, AsnCl, is proposed to enhance mixed Sn-Pb perovskite performance through molecular engineering.

## Key findings

- AsnCl-treated perovskite solar cells achieved a 22.54% power conversion efficiency with improved stability.
- Tandem solar cells using AsnCl reached a steady-state efficiency of 28.24%.
- AsnCl suppresses defects and enhances crystallinity, carrier lifetimes, and film orientation.

## 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 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.

AsnCl improves film orientation and crystallinity, suppresses Sn4+ and PbI2 defects, and prolongs carrier lifetimes, enabling a 22.54% efficiency and enhanced stability in Sn–Pb perovskite solar cells. The AsnCl‐treated tandem device achieves a steady‐state PCE of 28.24%, underscoring the effectiveness of additive molecular engineering for high‐performance PSCs.