Red-emitting phosphors NaLiXF6:Mn4+ (X = Ti, Si) for white LEDs with high color rendering index and low correlated color temperature

Abstract

White light-emitting diodes (WLEDs) are widely used in various lighting applications due to their high brightness and energy efficiency. However, commercial WLEDs, which typically combine YAG:Ce3+ phosphors with blue light-emitting diode chips, often lack red components, resulting in a high correlated color temperature (CCT) and a low color rendering index (CRI). To address this issue, two novel Mn4+-doped red-emitting phosphors, NaLiTiF6 and NaLiSiF6, were synthesized using an ion exchange method. Both phosphors exhibit strong red light with a pronounced zero-phonon line (ZPL) emission under blue light excitation. Optimal photoluminescence emission intensities were observed at Mn4+ concentrations of 6% for NaLiTiF6 and 9% for NaLiSiF6. When the temperature increased from 303 K to 413 K, both phosphors demonstrated outstanding brightness and chromatic stability. Integrated into phosphor-converted light-emitting diodes (pc-LEDs), the device incorporating NaLiTiF6:6%Mn4+ achieved a CRI of 95.1 and a CCT of 3172 K, while the device with NaLiSiF6:9%Mn4+ showed a CRI of 87.9 and a CCT of 3784 K. In addition, the luminous efficacy, CRI, and CCT of both prototype pc-LEDs maintained minimal variation with increasing driving current. These results indicate the stability and suitability of the synthesized phosphors for enhancing the performance of warm WLEDs applications.