Double perovskites (DPs) with Cs₂AgInCl₆ composition, as one of the lead-free perovskites, have been in the spotlight owing to their intriguing optical properties, namely, self-trapped exciton (STE) emission and dopant-induced photoluminescence. However, the current DPs still face the challenge of low photoluminescence efficiency and cannot be applied in practice. Herein, we synthesize the Bi³⁺ and Eu³⁺ codoped Cs₂AgInCl₆ DPs, which displays enhanced STE and Eu³⁺ ions characteristic emissions. Our results indicate that the Eu³⁺ ions mainly substitute the In sites and can increase the radiative recombination rate and exciton binding energy of STEs, which is discovered that Eu³⁺ ions can promote the localization of STEs by breaking the inversion symmetry of the Cs₂AgInCl₆ lattice. The existence of Bi³⁺ ions decreases the excitation (absorption) energy, provides a new absorption channel, and increases the energy transfer rate to Eu³⁺ ions. Through adjusting the Bi³⁺ and Eu³⁺ concentrations, a maximum photoluminescence quantum yield of 80.1% is obtained in 6% Eu³⁺ and 0.5% Bi³⁺ codoped Cs₂AgInCl₆ DPs. Finally, the high-quality single-component white-light-emitting diodes based on Bi³⁺ and Eu³⁺ codoped Cs₂AgInCl₆ DPs and a 410-nm commercial ultraviolet chip are fabricated with the optimum color rendering index of 89, the optimal luminous efficiency of 88.1 lm/W, and a half-lifetime of 1,493 h. This work puts forward an effective lanthanide and transition metals codoping strategy to design single-component white-light emitter, taking a big step forward for the application lead-free DPs.

The coupling of upconversion nanophosphors (UCNPs) with the surface plasmonic resonance (SPR) of noble metals is a promising way to improve luminescent efficiency of UCNPs; however, it is still a challenge to achieve stable, reproducible and effective upconversion luminescence (UCL) enhancement through such coupling. In this work, we present a novel strategy to improve UCL of NaYF₄: Yb³⁺, Er³⁺ UCNPs, by combining the near-field coupling of SPR of silver and the far-field coupling of poly(methyl methacrylate) (PMMA) opal photonic crystals (OPCs) with the UCNPs. In order to control the effective interaction distance between the UCNPs and the SPR, a porous silver film consisting of randomly distributed silver nanoparticles (NPs) (> 100 nm) was prepared which demonstrated strong SPR over a broad wavelength range, and its coupling to the UCNPs was found to be much stronger than that of a dense film. In the far-field coupling of OPCs, the photonic stop band (PSB) of the PMMA OPCs was tuned to 980 nm, matching exactly the excitation light. By modulating the particle size of the UCNPs, and the direction and excitation power of the incident light, a maximum enhancement of 60-fold was observed, which is an important advance for metal-induced UCL enhancement systems.