Broad visible emission from a blue-light-excitable single-phase NaLaS2:Bi3+ phosphor for high-CRI white LEDs

Abstract

Blue-light-excitable single-phase phosphors with broadband visible emission are essential for high-quality phosphor-converted white LEDs (pc-WLEDs). Here we report a NaLaS₂:Bi³⁺ phosphor that exhibits broadband visible emission spanning 500–750 nm, enabling full-visible-spectrum WLEDs when combined with a blue LED chip. Compared with NaLaS₂, Bi³⁺-doped NaLaS₂ shows an additional absorption peak extending up to 520 nm, attributed to the allowed S²⁻ → Bi³⁺ charge transfer (CT) transition. Through this CT sensitization route, the NaLaS₂:Bi³⁺ phosphor exhibits a broadband emission centered at 570 nm, arising from the Bi^{3+ 3}P₁ → ¹S₀ transition. This broad and red-shifted emission results from a pronounced nephelauxetic effect and an enhanced crystal field around the Bi³⁺ activators in the NaLaS₂ host. Furthermore, time-resolved luminescence measurements indicate that the broadband emission in the NaLaS₂ matrix originates from the site-selective occupation behavior of Bi³⁺ ions. Based on this single-phase NaLaS₂:Bi³⁺ phosphor, a full-spectrum pc-WLED was fabricated by coupling a 450 nm blue LED, achieving a high color-rendering index of 87.36 and a low correlated color temperature of 5171 K. This superior performance arises from the effective compensation of spectral gaps in the cyan and red regions. Furthermore, NaLaS₂:Bi³⁺,Yb³⁺ enables simultaneous visible (570 nm) and near-infrared (990 nm) emissions, indicating its potential for multifunctional lighting.