Synthesis and luminescence properties of LiY(WO4)2:Bi3+ broadband yellow-emitting phosphor for high-quality NUV pumped WLEDs

Abstract

Bi³⁺-activated luminescent materials have drawn escalating attention due to their intense excitation within the near-ultraviolet (NUV) range rather than the visible range. This distinctive characteristic enables them to evade the reabsorption phenomenon among phosphors, leading to their increasing prevalence in research and applications. In this work, a series of novel LiY(WO₄)₂:Bi³⁺ broadband yellow-emitting phosphors are successfully synthesized via the simple solid-state reaction method in air. The optimal preparation temperature, Bi³⁺ doping concentration, crystal structure, luminescence properties and lifetime of the LiY(WO₄)₂:Bi³⁺ phosphor are systematically investigated. The LiY(WO₄)₂:Bi³⁺ phosphor exhibits strong excitation in the NUV range due to the ¹S₀→³P₁ transition of Bi³⁺ and displays a broadband emission peaking at approximately 556 nm that covers the entire visible range from 400 to 800 nm with a full width at half maximum (FWHM) of 180 nm upon 350 nm excitation. The lifetimes of LiY(WO₄)₂:x%Bi³⁺ phosphors are on the order of a few hundred nanoseconds, verifying the emission stems from the partly allowed ³P₁→¹S₀ transition of Bi³⁺ instead of the forbidden ³P₀→¹S₀ transition which lifetime is on the order of several hundred microseconds. The quenching temperature of LiY(WO₄)₂:0.5%Bi³⁺ phosphor is approximately 420 K and the calculated activation energy $$ is 0.381 eV. Furthermore, its relative integrated emission intensity can recover to 99.06% of its initial integrated intensity at 300 K after one heating-cooling cycle, indicating good thermal degradation resistance. To explore the application of LiY(WO₄)₂:Bi³⁺ phosphor, the pc-WLED device was fabricated by depositing a mixture of this phosphor and two commercial phosphors on a 365 nm chip. The final pc-WLED device exhibits ideal photoelectric performance, featuring a low correlated colour temperature of 4206 K and a high colour rendering index of 92.2. These results provide clear evidence that LiY(WO₄)₂:Bi³⁺ phosphor would be a novel promising candidate for WLED applications.