The optical properties and stability of metal halide perovskites can be improved by reducing their dimensionality. Because defects at the perovskite film grain body and boundaries cause significant energetic losses by nonradiative recombination, perovskite films with manageable crystal size and macroscopic grains are essential to improve the photovoltaic properties. Through theoretical calculation models and experiments, we show that the carboxyl group of 4-ammonium butyric acid-based cation (4-ABA+) can interact with the three-dimensional (3D) perovskite to produce in situ a secondary grain growth by post-treatment. It passivates the trap defects and broadens the light absorption. 4-ABA+ could induce a 2D capping layer on top of 3D mixed cation-based perovskite to construct a 2D/3D heterojunction. The 4-ABA+-modified perovskite film consists of large-sized grains with extremely low trap state densities and possesses a longer charge carrier lifetime and good stability, resulting in efficient perovskite solar cells with a champion efficiency of 23.16% and a VOC of 1.20 V. We show that the 4-ABA+-treated devices outperform the 3-ammonium propionic acid (3-APA+)- and 5-ammonium valeric acid (5-AVA+)-treated ones. Moreover, the devices exhibit high stability under high humidity and continuous light soaking conditions. This work gives a hint that our approach based on 4-ABA+ treatment is key to achieving better electrical properties, a controlled crystal growth, and highly stable perovskite solar cells.
Keywords: 2D/3D heterojunction; 4-ammonium butyric acid; defects passivation; hierarchical interface; perovskite solar cell.