Low-Temperature Processed CsPbI₃ for Flexible Perovskite Solar Cells Through Cs─I bond Weakening

All-inorganic triiodide cesium lead (CsPbI₃) exhibits huge potential in perovskite solar cells (PSCs). However, the high-temperature crystallization process (≈340 or 180 °C) limits their further development, especially in flexible PSCs. Here, a Cs─I bond weakening approach is proposed to realize the low-temperature crystallization of CsPbI₃ by introducing organic sulfonate of 1-propylsulfonate-3-methylimidazolium chloride (SMCl). SMCl can strongly interact with CsI and weaken the Cs─I bond to dissociate free I⁻ ions for the effective transition of initial PbI₂ to [PbI₆]⁴⁻, which greatly decreases the crystallization temperature of black CsPbI₃ to 90 °C. As a result, flexible PSCs are realized with efficiency of 13.86%, which is the highest efficiency of flexible CsPbI₃ devices. Besides, SMCl will also help to release the tensile strain and stabilize CsPbI₃ phase, leading to good thermal and mechanical stability. Almost no efficiency loss is observed in flexible PSCs after 36000 bending cycles with a curvature radius of 5 mm.