Anti‐Thermal Quenching of Sb‐Doped Cs ₂ ZnCl ₄ : Electronic Mechanism and Device Applications

Abstract High‐power near‐infrared (NIR) light‐emitting diodes (LEDs) are central in a wide range of applications. However, their high operating temperature (T) tends to quench the emission intensity (thermal quenching, TQ). In this work, a NIR metal halide phosphor, Sb‐doped Cs ₂ ZnCl ₄ , displaying zero emission quenching at T >500 K is reported. Single crystals of Cs ₂ ZnCl ₄ :Sb ³⁺ are synthesized, treated by an annealing procedure to disperse the Sb ³⁺ ions homogeneously throughout the crystals, and fully characterized. The samples thus produced can sustain multiple thermal cycles while retaining their luminescence properties and crystal structure. The anti‐TQ behavior occurs only for excitation wavelengths above 370 nm. A thorough computational investigation based on (time‐dependent) density functional theory reveals that the anti‐TQ behavior and its wavelength dependence stem from a T‐induced transition between dark and bright excited states of dopants. Other insights into the optical properties of Cs ₂ ZnCl ₄ :Sb ³⁺ emerge, such as the mechanism of the observed large Stokes shift. Finally, it is shown that further Rb alloying of Cs ₂ ZnCl ₄ :Sb ³⁺ boosts the photoluminescence quantum yield to ≈100%. These Cs _2₋y Rb _y ZnCl ₄ :xSb ³⁺ crystals are used to fabricate LEDs with stable emission at high operating temperatures (up to 200 °C).