Luminescence properties of Yb3+ and Er3+ co-doped into Gd2O3:Bi3+ phosphor powder

Yb3+ and Er3+ co-doped Gd2O3:Bi3+ phosphor were prepared via a combustion method. The structural properties and surface morphology of the phosphors were studied with X-ray diffraction and a scanning electron microscope. UV–visible absorption, photoluminescence and cathodoluminescence (CL) spectra were also investigated. The optical bandgap of Gd2O3 increased with additional doping. The visible emission confirmed that the Gd2O3:Bi3+ co-doped with Yb3+ is located in the blue and green band regions related to the Bi3+ in the S6 and C2 symmetry sites. The near-infrared (NIR) down-conversion (DC) emission intensity increased upon increasing the Yb3+ concentration. The visible Bi3+ emission was reduced when adding a higher Yb3+ ion concentration while the NIR emission increased due to the energy transfer from the Bi3+ to the Yb3+ ions. UV-Vis energy excitation photons were successfully converted into NIR photons. The up-conversion (UC) visible emission spectra of Gd2O3:Bi3+ co-doped Er3+ consisted of two strong green, weak red and NIR emission bands. The 980 nm infrared excitation was successfully converted into UV-Vis emission. The CL emission spectra confirmed blue emission bands assigned to the 3P1 → 1S0 transitions of the Bi3+ ions and the green-yellow emission bands corresponded to an overlap between the 3P1 → 1S0 transitions of the Bi3+ ions and the 2H11/2, 4S3/2 → 4I15/2 transitions of the Er3+ ions. A significant enhancement of the emission intensities of Yb3+ and Er3+ ions was observed after Bi3+ doping. The results indicated that Gd2O3 materials based on the Bi3+–Yb3+ and Bi3+–Er3+ couples may be possible phosphors for improving photovoltaic conversion efficiency via spectral modification utilizing the DC and UC processes.