5-Ammonium Valeric Acid Iodide to Stabilize MAPbI₃ via a Mixed-Cation Perovskite with Reduced Dimension

5-Ammonium valeric acid iodide (AVAI) has been widely known as a stabilizer to enhance the stability of MAPbI₃ perovskite, but its role and function is still under exploration. The typical 2D perovskites of AVA₂PbI₄ have been proposed as the capping layer for stabilization. Here, a novel AVA-MA mixed-cation perovskite of AVAMAPbI₄ is found to show a more even and compact coverage than the typical 2D perovskite of AVA₂PbI₄. A simple post-treatment on MAPbI₃ films by using AVAI isopropanol solution can fabricate such a mixed-cation 2D perovskite capping layer on the MAPbI₃ sample. This AVAMAPbI₄ capping layer effectively passivates surface defects of MAPbI₃ perovskite films and reduces the charge-carrier recombination, enabling AVAI-MAPbI₃ perovskite films to exhibit improved stability against thermal and moisture stress. Finally, the AVAI-MAPbI₃-based perovskite solar cells also show an enhanced photovoltaic performance with a champion PCE up to 20.05% with enhanced stability.