FA-Assistant Iodide Coordination in Organic-Inorganic Wide-Bandgap Perovskite with Mixed Halides

Mixed-halide wide-bandgap perovskites are key components for the development of high-efficiency tandem structured devices. However, mixed-halide perovskites usually suffer from phase-impurity and high defect density issues, where the causes are still unclear. By using in situ photoluminescence (PL) spectroscopy, it is found that in methylammonium (MA⁺ )-based mixed-halide perovskites, MAPb(I_0.6 Br_0.4 )₃ , the halide composition of the spin-coated perovskite films is preferentially dominated by the bromide ions (Br^- ). Additional thermal energy is required to initiate the insertion of iodide ions (I^- ) to achieve the stoichiometric balance. Notably, by incorporating a small amount of formamidinium ions (FA⁺ ) in the precursor solution, it can effectively facilitate the I^- coordination in the perovskite framework during the spin-coating and improve the composition homogeneity of the initial small particles. The aggregation of these homogenous small particles is found to be essential to achieve uniform and high-crystallinity perovskite film with high Br^- content. As a result, high-quality MA_0.9 FA_0.1 Pb(I_0.6 Br_0.4 )₃ perovskite film with a bandgap (E_g ) of 1.81 eV is achieved, along with an encouraging power-conversion-efficiency of 17.1% and open-circuit voltage (V_oc ) of 1.21 V. This work also demonstrates the in situ PL can provide a direct observation of the dynamic of ion coordination during the perovskite crystallization.