Oxygen vacancy content drives self-reduction and anti-thermal quenching

Abstract

The widespread use of high-power LEDs has driven higher requirements for the thermal stability of luminescent materials. Here, we propose an effective synthesis strategy to achieve excellent performances of yellow-emitting phosphors LiZnPO₄:xMn²⁺ through modulating the contents of lattice defects. For the samples prepared in air, the characteristic transitions of Mn²⁺ ions are observed and the photoluminescent intensity becomes stronger with the increasing temperature until reaching a maximum at 200 °C. Based on electron paramagnetic resonance and defect formation energy, oxygen vacancies are responsible for the self-reduction process from Mn⁴⁺ to Mn²⁺ ions and anti-thermal quenching. The corresponding deep trap levels release captured electrons under thermal stimulus to the conduction band minimum, which non-radiatively relax to the lowest excited state and then withdraw back to the ground state to compensate for the luminous loss. Then, the oxygen vacancy content is optimized by a reducing atmosphere, and the high-temperature behaviours of the targeted phosphor are improved, which further verifies the positive response of properties to defect control. Finally, a fabricated WLED exhibits a good color rendering index (R_a = 89.4), demonstrating the prominent application potential of the as-prepared sample. This study opens up a new gateway for exploring novel anti-TQ optical functional materials, by virtue of the subjective structure design in self-reduction systems.