Efficient Near-Infrared Luminescence with Near-Unity Photoluminescence Quantum Yield in Erbium-Doped Double Perovskites Cs₂NaYCl₆ under Green Light Excitation

Lanthanide (Ln(3+))-doped double perovskites to achieve near-infrared (NIR) emission have been demonstrated to be an effective strategy. However, these emitters generally exhibit low emission efficiencies, which limits further applications. Herein, we synthesized Er3+-doped Cs2NaYCl6 via a hydrothermal reaction. Steady-state and transient fluorescence spectra show that there is a pronounced cross-relaxation between neighboring Er3+, thus boosting the NIR emission with a photoluminescence quantum efficiency (PLQE) of 93 +/- 2% (NIR-II region similar to 87%). Moreover, the ferromagnetic coupling can enhance the absorption intensity of Er3+; thus, the strongest emission of Er3+-doped Cs2NaYCl6 can be witnessed via direct excitation at 520 nm. The remarkable PLQE and ideal excitation wavelength make Cs2NaYCl6:Er3+ an ideal candidate for the next generation of NIR light sources. In addition, Nd3+ and Yb3+ were also doped in Cs2NaYCl6 successfully, and their respective PLQE values are 51 +/- 3% and 30 +/- 2%, respectively. Particularly, the emission intensity of Ln(3+)-doped Cs2NaYCl6 at 440 K can maintain 103% (Er3+), 91% (Nd3+), and 112% (Yb3+) of the initial emission intensity at 300 K. Their admirable properties enable us to demonstrate the application of as-synthesized compounds in night vision and nondestructive testing. Therefore, our findings make Ln(3+)-doped Cs2NaYCl6 one of the most promising NIR emitters.