All-inorganic 3D lead-free double perovskites (DPs) with broadband self-trapped exciton (STE) emission have shown great promise as alternatives to lead halide perovskites in various optoelectronic applications such as light-emitting diodes (LEDs) and photodetectors. A fundamental understanding of the effect of doping on the optical properties of DPs, especially the STE dynamics, is of vital importance for their performance optimization and applications.