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