Rare-Earth Substitution of La₃Si₂S₈I for Efficient and Color-Tunable Phosphor Converted White Light Emitting Diodes

Mixed-anion compounds enable properties to be controlled to a greater degree by chemical substitution than single-anion compounds. La₃Si₂S₈I was investigated as a host to develop phosphors based on solid solutions (La_1-xRE_x)₃Si₂S₈I (RE = Ce, Pr, Tb, Dy, Ho; x = 0-0.33) and (La_1-x-yCe_xRE_y)₃Si₂S₈I (RE = Tb, Dy; x, y = 0-0.33) that exhibit wide color tunability and white emission. Upon excitation with a 364 nm UV laser source, the phosphors emit through a down-conversion process over a wide visible color gamut. Depending on the RE³⁺ concentration, the spectra featured broad emission (cyan for Ce³⁺) or sharp 4f-4f emission peaks (red for Ho³⁺; green for Tb³⁺; cyan-green and red for Pr³⁺; yellow and cyan for Dy³⁺) which can be fine-tuned to a wider gamut. The cosubstituted (La_1-x-yCe_xRE_y)₃Si₂S₈I phosphors showed even more intense photoluminescence and wider color palettes within the cyan to green regions (Ce³⁺-Tb³⁺) and cool to warm white (Ce³⁺-Dy³⁺). The photometric parameters were appropriate for backlighting display and near UV-pumped phosphor-converted white light-emitting diodes (pc-wLEDs). Photoluminescence decay profiles follow biexponential behavior with lifetimes ranging from a few ns (for Ce³⁺) to 3-450 μs (for other RE³⁺). Most of the optimized phosphors were moderately stable at 80-100 °C before the onset of thermal quenching effects. They gave absolute quantum yields up to 50%, which are favorable for efficient energy conversion. Prototype pc-wLEDs fabricated from a blend of three phosphors or from single-phase phosphors showed promising performance.