Effective Passivation with Size‐Matched Alkyldiammonium Iodide for High‐Performance Inverted Perovskite Solar Cells

Abstract Organic ammonium salts have been widely used for defect passivation to suppress nonradiative charge recombination in perovskite solar cells (PSCs). However, they are prone to form undesirable in‐plane favored 2D perovskites with poor charge transport capability that hamper device performance. Herein, the defects passivation role of alkyldiammonium including 1.6‐hexamethylenediamine dihydriodide (HDAI 2 ), 1,3‐propanediamine dihydriodide (PDAI 2 ), and 1.4‐butanediamine dihydriodide (BDAI 2 ) for formamidinium‐cesium perovskite is systematically investigated. With help of density functional theory (DFT) calculations, BDA with suitable size can synergistically passivate two defect sites on perovskite surfaces, showing the best defect passivation effect among the above three alkyldiammonium salts. Perovskite films based on BDAI 2 modification are found to keep the 3D perovskite phase with considerably reduced trap‐state density, and enhanced carrier extraction. As a result, the BDAI 2 ‐modified devices deliver impressive efficiencies of 23.1% and 20.9% for inverted PSCs on the rigid and flexible substrates, respectively. Moreover, the corresponding encapsulated rigid devices maintain 92% of the initial efficiency after operating under continuous 1‐sun illumination with the maximum power point tracking for 1000 h. Furthermore, the mechanical flexibility of the BDAI 2 ‐modified flexible device is also improved due to the release of residual stress.