Color selection and red fluorescence enhancement through the controllable energy transfer in Na_xCa_1-2xWO₄: Eu_x ³⁺ phosphor for UV converted LEDs

Phosphor-converted warm white light-emitting diodes (pc-wLEDs) have played a key role in our daily lighting life, but they are difficult to realize without adding red-emitting phosphors. Conventional Eu³⁺ doped red phosphors, however, suffer greatly from the intrinsic shortcomings such as low quantum efficiency, limited absorption of UV LED light and chemical and thermal stability. In this work, we design the Eu³⁺ doped Na_xCa_1-2xWO₄:Eu_x ³⁺ (0 ≤ x ≤ 0.20) phosphors, which can exhibit enhanced Eu³⁺ red fluorescence and the tunable color at the same time. Phase purity analysis, which was checked by X-ray powder diffraction (XRD), shows all samples crystallize in a scheelite structure with the space group of I4_1/a (No.88). Our findings reveal that increasing the Eu³⁺ content gives rise to moving the XRD position to low angle direction and, in the meanwhile, decreasing the charge transfer (CT) intensity of samples. Spectral results show that controlling the charge compensation content and energy transfer efficiency from host to Eu³⁺ enables us to achieve the enhanced Eu³⁺ red fluorescence and simultaneously the tunable color. Unlike the decrease change in the CT band intensity, the Eu³⁺ emission upon excitation at 393 nm and 464 nm exhibits a continuous increase in the emission intensity. Due to the inefficiency energy transfer between host and Eu³⁺ dopant, controlling the x value enables tuning the relative photoemission intensity between host and Eu³⁺ dopant and therefore the color tuning from blue to red. Our time-resolved PL results indicate that the energy transfer between the CaWO₄ host and Eu³⁺ dopant begins to occur at t = 5 μs. A feasible profile, which bases on the energy transfer, has been also established in the work. This work not only demonstrates simultaneous enhanced Eu³⁺ fluorescence and color tuning in CaWO₄ matrix through charge compensation method, but also can provide new insight into improving the PL properties in other phosphors for UV converted pc-wLEDs.