Ternary diagrams of phase, stability, and optical properties of cesium lead mixed-halide perovskites

Cesium lead halides have been intensively studied as promising light absorption and emission materials. However, the complexity and instability of their phases has prevented a comprehensive investigation of the structural and optical properties of Cs-based mixed halides over a broad range of mixing ratios. Herein, we derive the ternary diagrams of phase, optical bandgap, and photoluminescence intensity of cesium lead halide perovskites (CsPbX3), with three vertices of CsPbI3, CsPbBr3 and CsPbCl3. For material property tests, mixed halide perovskite powders are synthesized using a facile solid-state reaction method. The orthorhombic perovskite (γ) phase, which is desired for photovoltaic and optoelectronic applications, is successfully obtained by quenching the powders from 350 °C to room temperature. By comparing the phase diagrams of as-ball-milled and heat-treated powders, we report a single-phase region (γ-phase) and a multi-phase region (I-rich and Cl-rich γ-phases). In aged powders having single-phase compositions, Cl-rich compositions presented considerably higher phase stability than I-rich ones. In the single-phase region, the lattice constant increases and optical bandgap decreases almost linearly with increase in average size of X-site ions, as per Vegard's law with extremely small bowing parameters (< 0.1 Å). Therefore, one can easily design an infinite number of composition combinations in the single-phase boundary to achieve a desired bandgap in the range of 1.73–2.96 eV. Ternary maps of photoluminescence intensity demonstrate potential compositions for high-performance optoelectronics. Our results provide a valuable database for developing desirable perovskite compositions for various optoelectronic applications.