Nonsolute-Cation-based Bromide Induced Phase Transformation for Highly Crystalline Mixed-Halide Perovskite

【摘要】：The novel strategy by introducing nonsolute-cation-based bromide has been proposed for crystal modification and bromine incorporation. Such nonsolute-cation-based bromide successfully incorporated Br into perovskite without changing the stoichiometric ratio of Pb~(2+) and CH_3NH_3~+(MA~+) in perovskite precursor.The potential negative impacts on the perovskite crystallization, originated from the extra solute cations, are effectively avoided. More importantly, the NH_4~+ with small ionic radius preferentially diffused into the lead coordination complexes to form NH_4PbX_3(X=I, Br) intermediate phase. During the generation and consumption of intermediate phase, we not only dexterously offset the inevitable differences of the nucleation rates for PbI_2 and MAI, but also introduce extra heterogeneous nucleation sites and slowing down crystal growth, eventually obtaining the highly crystalline perovskite film. Fortunately, the introduced cation NH_4~+ disappeared in the form of NH_3 gas release. By judicious adjusting the amount of NH_4Br, a significant enhancement in the power conversion efficiency of the MAPbI_(3-x)Br_x cell to 18.79% was realized. The nonsolute-cation-based bromide, as ammonium halide additive, was incorporated to directly participate in the crystallization of MA-based perovskite via phase transformation process, which successfully improved the crystalline quality without harmful residue and stoichiometric changes, and given the flexibility to other mixed-cation perovskite systems. Such strategy demonstrates the feasibility of ammonium halide additive in the optimization of perovskite crystallization, which may give impetus to effectively promote the perovskite evolution.