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.