Boosting characteristic emissions of lanthanide in Cs2Ag0.6Na0.4InCl6–xBrx double perovskites via mixing

Lanthanide ions (Ln3+) doping provides a potential strategy to control over the luminescent properties of lead-free halide double perovskite nanocrystals (DP NCs). However, due to the low energy transfer efficiency between self-trapped exciton (STE) and Ln3+ ions, the characteristic emissions of Ln3+ ions are not prominent. Furthermore, the energy transfer mechanism between STE and Ln3+ ions is also elusive and requires in-depth study. We chose trace Bi3+-doped Cs2Ag0.6Na0.4InCl6–xBrx as a representative DP matrix to demonstrate that by tuning the bromide concentration, the Ln3+ emission can be greatly enhanced. Such enhanced STE and Ln3+ ions energy transfer originates from the high covalency of Ln–Br bond, which contributes to improvement of the characteristic emission of Ln3+ ions. Furthermore, optical spectroscopy reveals that the energy transfer mechanism from DP to Eu3+ ions is different from all the other doped Ln3+ ions. The energy transfer from DP to Eu3+ ions is mostly through Eu–Br charge transfer while the other Ln3+ ions are excited by energy transfer from STE. The distinct energy transfer mechanism has resulted from the energy separation between the excited energy level of Ln3+ ions and the bottom of conduction band of DP. With increasing the energy separation, the energy transfer from STE to Ln3+ ions is less efficient because of the generation of a larger number of phonons and finally becomes impossible for Eu3+ ions. Our results provide new insight into tuning the energy transfer of Ln3+-doped DP NCs.