Luminescence color tuning and energy transfer properties in (Sr,Ba)2LaGaO5:Bi3+,Eu3+ solid solution phosphors: realization of single-phased white emission for WLEDs

Abstract

Recently, controllable luminescence tuning and single-phased white light emission have been extensively studied to improve the luminescence performances of phosphors for white light-emitting diodes (WLEDs). In this work, the controllable photoluminescence tuning from bluish-green to red was successfully achieved in Bi³⁺,Eu³⁺-codoped Sr_2−yBa_yLaGaO₅ solid solutions, combining the effects of crystallographic site engineering and energy transfer. The phase composition and structural variation of the designed phosphors were thoroughly analyzed through X-ray powder diffraction (XRD) and Rietveld refinement. The effects of cation substitutions on the photoluminescence properties were investigated in detail and the corresponding mechanism was proposed. A regular red-shift of Sr_2−yBa_yLaGaO₅:0.02Bi³⁺ in the bluish-green region could be observed when Sr²⁺ was substituted by Ba²⁺, which is attributed to the crystal field splitting and nephelauxetic effect. The red-shift of emission of Ba₂LaGaO₅:0.02Bi³⁺ phosphor with gradually increasing excitation wavelength reveals that Bi³⁺ ions occupy different cation sites to form multiple luminescent centers. Besides, the efficient energy transfer was designed and successfully realized in Sr_2−yBa_yLaGaO₅:0.02Bi³⁺,zEu³⁺ (y = 0–2.0; z = 0–0.20) samples. The energy transfer efficiency can reach up to 88%, 91% and 77% at z = 0.20 for y = 0, 1.0 and 2.0, respectively. Moreover, the thermal quenching behaviors of the studied phosphors reveal that the codoping of Eu³⁺ could efficiently improve the thermal stability of Sr₂LaGaO₅:0.02Bi³⁺ phosphor. Finally, the WLED devices were fabricated by composing 370 nm UV chips and single-phased Sr_2−yBa_yLaGaO₅:0.02Bi³⁺,0.05Eu³⁺ (y = 0, 1.0, 2.0) phosphors. The as-fabricated devices presented relatively excellent EL performances, the color coordinates of which all locate in the warm white light region. These results indicate the as-prepared Sr_2−yBa_yLaGaO₅:Bi³⁺,Eu³⁺ phosphors can be potential candidates for color tuning and efficient single-phased white emission in UV-based WLED lighting applications.