Room-Temperature Perovskite Phase Transition of CsPbI for PV Manufacturing on Flexible Substrates.

Printing perovskite films typically involves a high-temperature treatment exceeding 150 °C, which limits the manufacturing of flexible devices. All inorganic CsPbI perovskite is particularly promising for commercialization due to its high thermal stability. Herein, we discovered that when using DMF precursors containing CsI and HPbI for fabricating CsPbI films, an isopropanol (IPA) antisolvent bath immersion treatment of the wet films can enable a direct and rapid formation of optically active perovskite black phases at room temperature without annealing. In situ photoluminescence and in situ transmission techniques were employed to monitor and characterize the transition from the wet film to the final perovskite phase. It can be concluded that the relatively fast nucleation and slow grain growth during the IPA-bath treatment result in films with small grains and pronounced pinholes on the surface. Furthermore, FTIR, Raman, and NMR techniques were used to investigate changes in the chemical bonds. The characterization results revealed that the hydrogen in HPbI can form a chemical bond with the oxygen in DMF, resulting in mutual attraction. As DMF is extracted by IPA, the DMF molecule simultaneously induces the hydrogen to leave its original position, and then free cesium easily fills the vacancy left by hydrogen, forming the black-phase CsPbI perovskite. This finding reveals the mechanism of the room-temperature phase transition of CsPbI facilitated by IPA post-treatment, and it explains why the use of HPbI instead of PbI in the precursor solution effectively lowers the reaction energy barrier for CsPbI in previous works.