Boosting the Self‐Trapped Exciton Emission in Alloyed Cs₂(Ag/Na)InCl₆ Double Perovskite via Cu⁺ Doping

Fundamental understanding of the effect of doping on the optical properties of 3D double perovskites (DPs) especially the dynamics of self-trapped excitons (STEs) is of vital importance for their optoelectronic applications. Herein, a unique strategy via Cu⁺ doping to achieve efficient STE emission in the alloyed lead-free Cs₂ (Ag/Na)InCl₆ DPs is reported. A small amount (1.0 mol%) of Cu⁺ doping results in boosted STE emission in the crystals, with photoluminescence (PL) quantum yield increasing from 19.0% to 62.6% and excitation band shifting from 310 to 365 nm. Temperature-dependent PL and femtosecond transient absorption spectroscopies reveal that the remarkable PL enhancement originates from the increased radiative recombination rate and density of STEs, as a result of symmetry breakdown of the STE wavefunction at the octahedral Ag⁺ site. These findings provide deep insights into the STE dynamics in Cu⁺ -doped Cs₂ (Ag/Na)InCl₆ , thereby laying a foundation for the future design of new lead-free DPs with efficient STE emission.