Enhancing self-trapped exciton emission in Cs2Ag0.6Na0.4In0.8Bi0.2Cl6-XBrX double perovskite by reducing trapped states through halide tuning.

Halide double perovskite (DP) nanocrystals (NCs) have emerged as promising lead-free alternatives for optoelectronic applications, yet their optical performance remains limited by surface and structural defects. In this work, we report the synthesis of Cs2Ag0.6Na0.4In0.8Bi0.2Cl6 colloidal NCs and systematically tune their photophysical properties via halide exchange by substituting Cl- with Br- using an antisolvent precipitation method. Photoluminescence (PL) spectroscopy, time-resolved PL, and femtosecond transient absorption spectroscopy reveal that moderate Br- incorporation optimizes the radiative recombination dynamics by significantly reducing trap states and enhancing self-trapped exciton (STE) emissions. Such effects are further supported by increased PL quantum yield up to 48%, prolonged STE lifetimes, and reduced nonradiative decay rates. X-ray diffraction and transmission electron microscopy further confirm that Br- substitution also influences lattice strain and distortion, affecting the emission behavior. Our findings reveal that halide composition tuning is an effective strategy for controlling trap-mediated dynamics and improving emission efficiency in DP NCs, offering valuable insights for developing broadband, stable white-light-emitting materials.