Drastic enhancement of blue-to-orange color conversion efficiency in heavily-doped ZnS:Mn2+ phosphor and its application in white LEDs

Heavily-doped ZnS:Mn2+ phosphors showed an extremely enhanced photoluminescence excitation intensity at 464 nm peak wavelength originated from 6A1(6S)→4A1(4G) forbidden transition of Mn2+ ions. The best ZnS: Mn2+ phosphors showed the intensive orange color emission of 584 nm peak wavelength from 4T1(4G) → 6A1(6S) forbidden transitions with an excellent lumen maintenance of 97% at 200 °C, and their quantum efficiency of 75% under the 464 nm excitation wavelength. The photoluminescence enhancement was explained in terms of the relaxation of selection rule on the forbidden intra-transitions of Mn2+ ion. It was caused by the spin-spin and the electron-phonon interactions supported by electron spin resonance signal and Raman spectroscopy, respectively. The excellent lumen maintenance was explained in terms of resonance-assistant direct intra-transition caused by the coupling between an excited electron (e*) on the 4A1(4G) state and multi-phonon energy. To evaluate its feasibility for white light-emitting diodes, the optimized ZnS:Mn2+ phosphor and its mixture with a green Lu3Al5O12:Ce3+ phosphor were applied to a 460 nm blue-light-emitting diode. It showed high luminous efficiency of 68 lm/W, suggesting a possibility on the application of Mn-doped ZnS phosphor for pc-WLED.