Li Substitution Strategy for Enhancing Quantum Efficiency and Improving Thermal Stability of Red‐Light Mn⁴⁺‐Activated Fluoride for Wide‐Gamut Displays

Mn 4+ ‐activated red‐light fluorides are widely used in high‐quality displays, due to the narrow‐spectral emission. However, achieving exceptional chromaticity coordinates and appropriate quantum efficiency (QE) at the same time is a great challenge. Herein, a Li substitution strategy is proposed for enhancing QE and improving the thermal stability of red‐light Mn 4+ ‐activated Cs 2 NaGaF 6 (CNGF). The external quantum efficiency (EQE) of optimal Cs 2 NaGaF 6 : Mn 4+ , Li + (CNGF: Mn 4+ , Li + ) is improved from 15.79% to 33.61% and thermal stability increases from 68.4% to 85.5% (intensity at 150 °C relative to room temperature). EQE enhancement mechanism on the strategy is explored that Li substitution increases the local structural distortion of Mn 4+ for relieving the parity forbidden, and promotes Mn 4+ doping in the host lattice, which provides new insights for improving the QE of Mn 4+ non‐equivalent doped fluorides. Moreover, with exceptional chromaticity coordinates of (0.7029, 0.2970), optimal sample CNGF: 4.8 mol% Mn 4+ ,0.9 mol% Li + can be used to fabricate white light‐emitting diode (w‐LED) backlight, which obtains wide color gamut of 110.68% National Television Standards Committee (NTSC) and high luminous efficacy of 110.57 lm W −1 , indicating that CNGF: Mn 4+ , Li + is a promising candidate for the next generation of wide gamut and efficient displays.