Terbium Ion-Mediated Energy Transfer in WO₃:Tb³⁺ and Eu³⁺ Phosphors for UV-Sensitized White Light Emission

Incorporation of suitable lanthanide (Ln³⁺) ions into semiconducting WO₃ can be useful to produce host-sensitized luminescence for solid-state lighting applications. Codoping of another Ln³⁺ ion can assist in transferring energy from the host to the activator Ln³⁺ ion to produce bright luminescence depending upon the electronic structure of the doped system. As a case study, Eu³⁺ and Tb³⁺ ion–doped WO₃ phosphors (WO₃:Tb³⁺_xEu³⁺_y, x = 0–0.05 and y = 0–0.20) have been prepared over a wide range of doping concentrations to investigate the role of the Tb³⁺ ion as a sensitizer and realize host-sensitized emission from Eu³⁺ ions. The steady-state and time-resolved photoluminescence (TRPL) data for WO₃:Tb³⁺_xEu³⁺_y (x = 0–0.05 and y = 0–0.10) samples confirm that Tb³⁺ ions assist in excitation of Eu³⁺ ions via sequential energy transfers from the host to Tb³⁺ ions followed by Tb³⁺ to Eu³⁺ ions. The energy transfer process is controlled by optimizing their doping concentrations, and a single-phase white-light-emitting phosphor with a composition WO₃:Tb³⁺_0.05Eu³⁺_0.0005 has been developed. The electronic band structures and projected density of state plots for the WO₃:Tb³⁺_0.03125Eu³⁺_00.03125 system obtained using density functional theory (DFT)-based simulations confirm the formation of impurity states due to Eu³⁺ and Tb³⁺ ions within the forbidden gap of WO₃. Based on the TRPL and DFT data, it is confirmed that the Tb³⁺ ions act as a bridge between the conduction band edge of WO₃ and excited states of Eu³⁺ ions to transfer energy and facilitate characteristic emission from europium species.