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

All‐inorganic CsPbBr 3 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 CsPbBr 3 perovskite films usually suffer from poor phase purity due to the local uncontrolled ratio of CsBr and PbBr 2 . Herein, a novel 2‐methoxyethanol solvent for CsBr is used in place of methanol to yield high‐quality CsPbBr 3 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 In 2 S 3 thin films for the electron transport layer (ETL) of CsPbBr 3 PSCs. Therefore, the power conversion efficiency (PCE) of all‐inorganic planar CsPbBr 3 PSCs with the architecture of FTO/In 2 S 3 /CsPbBr 3 /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 In 2 S 3 colloidal particles, In 2 S 3 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%.