Synergistic Passivation of Perovskite Surfaces Using a Multi-Functional Four-End Zwitterionic Amino Acid

Abstract

Passivating defects and enhancing the stability of perovskite materials are key focuses in perovskite solar cell research, particularly the simultaneous passivation of A-site vacancies and undercoordinated B-site. This study designs a four-end zwitterionic amino acid (ZAA), with two ammonium arms and two carboxylate arms around the central carbon symmetrically. Through NH₃⁺ occupying A-sites and COO⁻ passivating B-sites, ZAA forms a stable "quadrupedal anchoring" mode on the perovskite surface. Selective fluorination of ZAA yields three derivatives, which are defined as two categories: ZAA-1 (ZAA_allH , ZAA_CF ) and ZAA-2 (ZAA_NF , ZAA_allF ). ZAA-1 passivates undercoordinated Pb, reducing surface state and modulating the band gap close to that of pristine MAPbI₃ , while ZAA-2 with excessively long Pb•••O distances introduce oxygen-derived impurity states near the Fermi level, causing severe band gap reduction. Furthermore, the ZAA-1 increases the formation energies for both surface Pb and I vacancies, and provides robust resistance against water invasion. Compared to ZAA_allH , ZAA_CF exhibits superior overall protective performance, as it does not undergo significant displacement upon H₂O adsorption due to the absence of strong attractive interactions with water. Additionally, the ZAA_CF -modified system demonstrates comparable performance to the pristine material in photovoltaic device simulations. This work presents an effective strategy of employing bifunctional four-end ZAA to construct a protective layer that concurrently addresses defect passivation and stability enhancement in perovskite materials.