Non-equivalent Mn⁴⁺ doping into A₂NaScF₆ (A = K, Rb, Cs) hosts toward short fluorescence lifetime for backlight display application

Mn⁴⁺-doped fluoride red-emitting phosphors have been developed for backlighting white light-emitting diodes (WLEDs). However, the long fluorescence lifetime (τ > 5 ms) leads to the image-retention phenomenon for the human eye when Mn⁴⁺ is doped into equivalent X sites in A₂XF₆ (A = Na, K, Rb, Cs; X = Si, Ge, Ti, Zr). Herein, we report on novel red-emitting A₂NaScF₆:Mn⁴⁺ (A = K, Rb, Cs) (ANSF) phosphors with short fluorescence lifetimes (τ < 4 ms) and characteristic zero phonon line (ZPL) emission, which can be realized via non-equivalent doping of Mn⁴⁺ into Sc³⁺ sites. Density functional theory (DFT) calculations verified the formation of a slightly distorted environment of Mn⁴⁺ and an Na⁺ vacancy to balance the charge with Mn⁴⁺ non-equivalent doping. The WLED backlight using ANSF as a red-light component can reach a wide color gamut above 105% of the National Television Standards Committee (NTSC) standard, and the image-retention on the backlighted screen can be effectively avoided compared to the backlight obtained using commercial K₂SiF₆:Mn⁴⁺. Such a Mn⁴⁺ non-equivalent doping strategy opens a new perspective for the modification of local symmetry and enhancement of the luminescence properties of Mn⁴⁺-doped red-emitting fluoride phosphors.