A highly efficient UV-to-red-light conversion by LuNbO4:Eu3+ phosphors through exciton-assisted host-activator energy transfer

Abstract

Broadband ultraviolet (UV) excitation and narrow red-emitting phosphors are employed extensively for spectral conversion, yet finding materials with a luminous efficiency high enough for applications is still challenging. Herein, extremely efficient red emission was realized by way of the exciton-assisted energy transfer when the LuNbO₄:xEu³⁺ phosphors were irradiated with UV light in the broadband region of 220–300 nm. When exciting the interband transition with UV light, self-trapped excitons (STEs) were generated and captured by Eu³⁺ ions to transfer the energy for lighting. The STE behaviors dominating the luminous properties could be optimized by tuning the Eu³⁺ doping. With increasing the doping levels x, more efficient energy transfer occurred, resulting in enhanced Eu³⁺ luminescence with the STE emission fading, and the luminescence external quantum efficiency reached an amazing value of 66.8% when x = 0.09. The STE-emission decay curves showed that the energy transfer was realized by exciton transmission from the NbO₄³⁻ groups to the Eu³⁺ ions. The Eu³⁺ doping resulted in more isolated NbO₄ tetrahedrons exerting a larger location force on the STEs, and then an orientational host–activator energy transfer occurred, accounting for the efficient light conversion. Since Eu³⁺ ions could capture more escaping STEs upon thermal stimulus and obtain extra exciting energy to resist the thermal quenching, the red emission also exhibited excellent thermal stability even at 478 K. These results suggest that the exciton-assisted energy transfer could initiate the exploration of new stably efficient red phosphors for applications in light conversion.