Broadly tuning Bi3+ emission via crystal field modulation in solid solution compounds (Y,Lu,Sc)VO4:Bi for ultraviolet converted white LEDs

Abstract

For phosphor-converted white LEDs (pc-WLEDs) based on UV chips, it is essential to search for highly efficient phosphors that have broadly tunable emissions and minimal excitation in the visible range. This would allow the designer of pc-WLEDs to select the desirable emission wavelength and, thereby, fabricate the device with color perception comfortable for the human eye. Conventional rare earth schemes, for instance Ce³⁺ or Eu²⁺ doped phosphors, can be tuned in emission wavelengths, but they cannot overcome the severe intrinsic excitation in the range from blue to green. We report here that Bi³⁺, the valence of which is confirmed by X-ray photoelectron spectra, exhibits almost no excitation in the visible range and has an extremely broad emission covering the full visible spectral range with internal quantum efficiency up to 75% when substituted into the solid solution compounds (Y,Lu,Sc)VO₄:Bi. When the crystal field around Bi³⁺ is tuned by gradual substitution of smaller lanthanide ions for larger lanthanide ions in solid solution, the emission peak position can be modulated linearly between 566 nm and 635 nm due to the susceptibility of bismuth naked 6s electrons to the local environment. In accordance with Rietveld refining results, static and transient photoluminescence spectra reveal the existence of a single type of Bi³⁺ emission center in the sample, and energy transfer from VO₄³⁻ group to Bi³⁺ upon excitation into the charge transfer state of VO₄³⁻ groups. The mechanism of concentration quenching of Bi³⁺ emission occurs possibly due to dipole–quadrupole rather than exchange interactions. This work shows the potential application of bismuth doped samples in UV converted pc-WLEDs.