High–Quality and Stable Quasi‐2D Perovskite Film Prepared by Introducing Alkali Metal Ion Additive

Abstract Due to the excellent stability, researchers have shown great interest in quasi two–dimensional (2D) perovskite in recent years. However, the poor film quality and low photoelectric conversion efficiency (PCE) of 2D perovskite solar cells are still a major issue that restricts them from catching up with three–dimensional (3D) perovskite. Herein, we introduce sodium iodide as an additive to improve the surface coverage of 2D perovskite ((BA) 2 (MA) 3 Pb 4 I 13 ) films and reduce the number of grain boundaries. Steady–state photoluminescence (PL) and ultraviolet photoelectron spectroscopy (UPS) indicate that non–radiative defects are suppressed and the band alignment is regulated. After modified by NaI, the film surface coverage enhances and the number of holes decreases. As a result, the PCE of device with NaI increases from 13.63 % to 16.40 % at optimal concentration, and the hysteresis effect is also suppressed. Besides, the humidity and thermal stability of perovskite are enhanced. The device with NaI modification can keep 80 % of initial efficiency, compared to 30 % for reference device after stored 30 days under ambient air condition. This work provides a simple method to improve the quality of quasi 2D perovskite films.