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-sites. 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 four derivatives, which are classified into two categories: ZAA-1 (ZAA_allH and ZAA_CF) and ZAA-2 (ZAA_NF and ZAA_allF). ZAA-1 passivates undercoordinated Pb, reducing surface states and modulating the band gap to a value close to that of pristine MAPbI₃, while ZAA-2 with excessively long Pb⋯O distances introduces oxygen-derived impurity states near the Fermi level, causing severe band gap reduction. Furthermore, ZAA-1 increases the formation energies of 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 a bifunctional four-end ZAA to construct a protective layer that concurrently addresses defect passivation and stability enhancement in perovskite materials.