Enhancing the Phase Stability of Formamidinium Lead Triiodide by Addition of Calcium Chloride

Formamidinium lead triiodide (FAPbI3) with stable black perovskite phase and without significantly changing its bandgap is demonstrated by the incorporation of CaCl2. More incorporated CaCl2 in FAPbI3 creates a desirable crystal structure, satisfying the Goldschmidt tolerance factor, and thereby stabilizing black perovskite phase under ambient conditions. However, more incorporated CaCl2 in FAPbI3 is likely to degrade the perovskite due to its hygroscopic characteristics. In addition, the incorporated CaCl2 in FAPbI3 solution reduces solubility, causing the formation of smaller crystal grains and pinholes due to quick precipitation during the non-solvent dripping crystallization process. Accordingly, 5 % CaCl2 incorporated FAPbI3 films have the best phase stability and solar cell performance with 18.0 % of power conversion efficiency at 1 sun condition.