Ternary diagrams of the phase, optical bandgap energy and photoluminescence of mixed-halide perovskites

Halide perovskites attract enormous attention as promising light absorption and emission materials for photovoltaics and optoelectronic applications. Here we report ternary diagrams of the phase, optical bandgap energy (E_g) and photoluminescence intensity of methylammonium lead halide (MAPbX₃, where X = I, Br and Cl) perovskites, with three vertices of MAPbI₃, MAPbBr₃ and MAPbCl₃. All the compositions were synthesized via a facile mechanochemical reaction at room temperature, to ensure the desired stoichiometries of the final products. Through structural study on MAPbX₃, the phase diagram comprising a single phase region and two multi-phase regions was obtained. In the single phase region, the a-axis lattice constant increases almost linearly with increasing the average size of the X site ions. Interestingly, E_g decreases almost linearly with increasing the average size of the X site ions, giving negligible deviation from Vegard's law. As the result, a certain bandgap value, in the range of 1.55 - 2.9 eV, can be easily designed with infinite numbers of compositions. For the last, the ternary diagram of the photoluminescence intensity reveals the effective compositions for red, green and blue light emission. The comprehensive structural and optical information reported in this study is useful for designing halide perovskites for various applications. In addition, our approach for compositional mapping various characteristics using a solid-state reaction is an efficient and robust way to studying halide perovskites.