Gradient Energy Alignment Engineering for Planar Perovskite Solar Cells with Efficiency Over 23

An electron‐transport layer (ETL) with appropriate energy alignment and enhanced charge transfer is critical for perovskite solar cells (PSCs). However, interfacial energy level mismatch limits the electrical performance of PSCs, particularly the open‐circuit voltage (VOC). Herein, a simple low‐temperature‐processed In2O3/SnO2 bilayer ETL is developed and used for fabricating a new PSC device. The presence of In2O3 results in uniform, compact, and low‐trap‐density perovskite films. Moreover, the conduction band of In2O3 is shallower than that of Sn‐doped In2O3 (ITO), enhancing the charge transfer from perovskite to ETL, thus minimizing VOC loss at the perovskite and ETL interface. A planar PSC with a power conversion efficiency of 23.24% (certified efficiency of 22.54%) is obtained. A high VOC of 1.17 V is achieved with the potential loss at only 0.36 V. In contrast, devices based on single SnO2 layers achieve 21.42% efficiency with a VOC of 1.13 V. In addition, the new device maintains 97.5% initial efficiency after 80 d in N2 without encapsulation and retains 91% of its initial efficiency after 180 h under 1 sun continuous illumination. The results demonstrate and pave the way for the development of efficient photovoltaic devices. A simple low‐temperature‐processed In2O3/SnO2 bilayer electron‐transport layer (ETL) is used for fabricating efficient perovskite solar cells (PSCs). The bilayer ETL with appropriate energy alignment is beneficial for charge transfer, thus minimizing open‐circuit voltage (VOC) loss. An optimized planar PSC with a power conversion efficiency (PCE) of 23.24% is obtained. In contrast, devices based on single SnO2 only achieve efficiency of 21.42%.