Modulating Self-Trapped Exciton Emission via Structural Phase Transitions in Lead-Free Double Perovskites

Elucidating the structural phase transitions and luminescence mechanisms of metal halides is a challenging, yet critical endeavor. Their full potential is in developing materials with tailorable optoelectronic properties and multifunctional applications. In this study, we synthesized two distinct lead-free double perovskites Cs₂NaInCl₆ and Cs_3.4Na_0.6InCl₇ using identical reactants under different synthesis conditions. We found that the phase transition between these two perovskites can be precisely controlled by adjusting the N, N-Dimethylformamide (DMF) concentration in the solvent. The optical properties and emission mechanisms of these materials were systematically elucidated through a combination of experimental analysis and density functional theory (DFT) calculations. The Sb³⁺-doped perovskites exhibit remarkable photoluminescence quantum yield (PLQY), with the yellow green-emitting Cs_3.4Na_0.6InCl₇ reaching 90.1% and the blue-emitting Cs₂NaInCl₆ achieving an impressive 90.3%. By introducing more water solvents, we successfully synthesized orange-emitting crystal Cs₂InCl₅·H₂O:Sb³⁺. Finally, we fabricated white-light-emitting diodes (WLEDs) by integrating the synthesized blue-, yellow-green-, and orange-emitting crystals with a UV chip. The resulting device demonstrated excellent white light performance, exhibiting a high color rendering index (Ra) of 92.2, a correlated color temperature (CCT) of 5119 K, and CIE coordinates of (0.34, 0.38). This work provides new insights into the luminescence mechanisms of halide perovskites and demonstrates that crystal structure engineering is a feasible and promising approach for tuning their optical properties.