A Novel Solvent for Multistep Solution‐Processed Planar CsPbBr3 Perovskite Solar Cells Using In2S3 as Electron Transport Layer

All‐inorganic CsPbBr3 perovskite solar cells (PSCs), which show potential applications in the semitransparent top cell of tandem solar cells, have attracted tremendous attention owing to their outstanding thermal and moisture stability in an air atmosphere. However, the common solution‐processed CsPbBr3 perovskite films usually suffer from poor phase purity due to the local uncontrolled ratio of CsBr and PbBr2. Herein, a novel 2‐methoxyethanol solvent for CsBr is used in place of methanol to yield high‐quality CsPbBr3 films with better phase purity as well as larger and more uniform grains. Meanwhile, a facile and controllable reflux condensation technique is proposed to prepare In2S3 thin films for the electron transport layer (ETL) of CsPbBr3 PSCs. Therefore, the power conversion efficiency (PCE) of all‐inorganic planar CsPbBr3 PSCs with the architecture of FTO/In2S3/CsPbBr3/carbon is increased from 5.83% to 6.42%. By adjusting the reflux condensation temperature (80, 100, 120, or 140 °C) and tailoring the aggregation of In2S3 colloidal particles, In2S3 thin films with different surface roughnesses are synthesized to optimize the ETL of the devices, which further enhances the short‐current density and boosts the PCE to 6.54%. 2‐methoxyethanol is reported as a solvent for CsBr in the place of methanol, and is used to yield high‐quality CsPbBr3 films with better phase purity as well as larger and more uniform grains. A facile and controllable reflux condensation technique is adopted to prepare In2S3 thin film. The all‐inorganic planar CsPbBr3 perovskite solar cells with an architecture of FTO/In2S3/CsPbBr3/carbon deliver a champion power conversion efficiency of 6.54%.