Robust dual cationic ligand for stable and efficient warm-white light emission in lead-free double perovskite nanocrystals

• Bi-doped Cs 2 Ag 1-x Na x InCl 6 double perovskite NCs were synthesized using strongly coordinated silver-trioctylphosphine complex. • Utilization of silver-trioctylphosphine complex effectively suppressed the early reduction of silver ions by oleylamine. • The synthesized NCs shows high photoluminescence quantum yield with high tolerance against polar solvents during washing. • The high colloidal and photophysical stability of NCs was attributed to the tight binding of dual cationic ligand of benzoyl trioctylphosphonium and oleylammonium cations together with oleate anion. Bismuth-doped alloyed Cs 2 Ag 1-x Na x InCl 6 double perovskite nanocrystals (DP NCs) emerged as a new class of lead-free alternative for single-emitter-based solution-processed white-light-emitting devices (WLEDs). However, their thin-film processing and device fabrication have been limited due to low photoluminescence (PL) quantum yield (QY) and degradation during purification from the as-synthesized crude solution. Moreover, easy reduction of a silver ions by oleylamine also raised concern regarding stability issues of silver based DP NCs. Here, we report a facile synthesis and purification method for shape-pure and monodispersed bismuth-doped alloyed Cs 2 Ag 1-x Na x InCl 6 DP NCs with high PL QY and long-term stability. At optimized bismuth-doping, alloyed Cs 2 Ag 0.4 Na 0.6 InCl 6 DP NCs showed warm white-light-emission with a PL QY of 40% due to suppressed non-radiative recombination. The synthesis of highly emissive and stable DP NCs was achieved by utilizing a combination of strongly coordinating silver-trioctylphosphine (Ag-TOP) complex along with additional TOP ligand. The Ag-TOP complex served as a highly reactive silver precursor and prevented the reduction of silver ions into metallic silver. While TOP facilitated the nucleophilic reaction with benzoyl chloride during nucleation and growth stage and forms benzoyl trioctylphosphonium chloride intermediate that served as both halide source and surface capping ligand which enabled the formation of high-quality DP NCs. The high tolerance of DP NCs against common antisolvents such as methyl acetate and isopropanol was attributed to the tight binding of dual cationic ligand benzoyl trioctylphosphonium and oleylammonium cations together with oleate anion to the surface of DP NCs.