Multicolor and Warm White Emissions with a High Color Rendering Index in a Tb³⁺/Eu³⁺-Codoped Phosphor Ceramic Plate

A series of Tb³⁺/Eu³⁺-codoped phosphor ceramic plates with a high color rendering index (CRI) for a near-ultraviolet light emitting diode (LED) were fabricated. Color emission can be tuned from green to reddish as a function of Eu³⁺ concentration. By doping only 0.15 mol% of Eu³⁺ concentration, a comfortable warm white emission is promoted as a result of simultaneous emissions of Tb³⁺ and Eu³⁺ ions. A theoretical model is proposed to calculate the contributions of the emitted color of the donor (Tb³⁺) and acceptor (Eu³⁺) ions in terms of europium concentration. The energy transfer from Tb³⁺ to Eu³⁺ ions is corroborated by the luminescence spectra and decay time of Tb³⁺, with a maximum energy transfer efficiency of 76% for 28 mol% of Tb³⁺ and 14 mol% of Eu³⁺. Warm white LEDs were constructed using a 380 nm UV chip and showed a CRI of 82.5, which was one of highest values reported for Tb³⁺/Eu³⁺-codoped samples. Color-correlated temperature (CCT), color coordinate (CC), and luminous efficacy (LE) were utilized to know the potentials as a phosphor converter in solid-state lighting.