Over 21% Efficient Cesium Lead Triiodide Solar Cell Enabled by Molten Salt Accelerating the Crystallization Processes.

High-quality CsPbI with low defect density is indispensable for acquiring excellent photoelectric performance. Meticulous regulation of the CsPbI crystal growth processes is both feasible and efficacious in enhancing the quality of perovskite films. In this study, the cesium formate (CsFo) is introduced. On one hand, its low melting point can induce the crystallization processes at a low level of energy consumption. On the other hand, the pseudo-halide anion can participate in the passivation of iodide vacancies, as the formate anion exhibits a relatively higher affinity with iodide vacancies compared to other halides. Consequently, the introduction of CsFo enhances the quality of CsPbI thin films by altering the crystallization process and curbing defect formation. As a result, a steady-state output efficiency of 21.23% and an open-circuit voltage (V) as high as 1.25 V are achieved, with both parameters ranking among the highest for this type of solar cell.