Alleviating Interfacial Recombination of Heterojunction Electron Transport Layer via Oxygen Vacancy Engineering for Efficient Perovskite Solar Cells Over 23

Electron transport layer (ETL) is pivotal to charge carrier transport for PSCs to reach the Shockley–Queisser limit. This study provides a fundamental understanding of heterojunction electron transport layers (ETLs) at the atomic level for stable and efficient perovskite solar cells (PSCs). The bilayer structure of an ETL composed of SnO2 on TiO2 was examined, revealing a critical factor limiting its potential to obtain efficient performance. Alteration of oxygen vacancies in the TiO2 underlayer via an annealing process is found to induce manipulated band offsets at the interface between the TiO2 and SnO2 layers. In‐depth electronic investigations of the bilayer structure elucidate the importance of the electronic properties at the interface between the TiO2 and SnO2 layers. The apparent correlation in hysteresis phenomena, including current density–voltage (J–V) curves, appears as a function of the type of band alignment. Density functional theory calculations reveal the intimate relationship between oxygen vacancies, deep trap states, and charge transport efficiency at the interface between the TiO2 and SnO2 layers. The formation of cascade band alignment via control over the TiO2 underlayer enhances device performance and suppresses hysteresis. Optimal performance exhibits a power conversion efficiency (PCE) of 23.45% with an open‐circuit voltage (Voc) of 1.184 V, showing better device stability under maximum power point tracking compared with a staggered bilayer under one‐sun continuous illumination. This study demonstrates a simple and effective atomic reconstruction for an efficient TiO2/SnO2 heterojunction bilayer for the electron transport layer used in a perovskite solar cell. The fundamental understanding of this work may leverage the potential of heterojunction ETLs and further investigation in materials science, particularly for the development of an efficient charge transport layer in metal halide perovskite photovoltaics.