Luminescence properties of Yb-doped SrTiO: the significance of the oxygen-titanium charge transfer state on photon downshifting.

Ceramic powders of Sr1-1.5xYbxTiO3 (x = 0.0, 0.0125, 0.025, 0.05 and 0.075) solid solutions were synthesized by the polymeric complex method. The crystal structure, microstructure and optical properties of the powders annealed at 800 °C for 1 h were investigated by X-ray diffraction, scanning electron microscopy, and diffuse reflectance and photoluminescence spectroscopy, respectively. All the solid solutions exhibit a cubic perovskite-like structure. The reflectance spectra show a broadband below 400 nm ascribed to the ligand-to-metal charge transfer (LMCT) O2- → Ti4+ fundamental state. The Yb3+ (λem = 980 nm) excitation spectra show a broadband being also compatible with the LMCT O2- → Ti4+ state, indicating the energy transfer from the host to the Yb3+. The sample with x = 0.025 presents the highest emission intensity upon near UV excitation, which is further enhanced when the powder is treated under an oxygen-rich atmosphere. The luminescence quenching of Yb3+ is explained as due to defects associated with O2- and Sr2+ vacancies. Finally, it is shown that the solid solutions may downshift photons from UV to wavelengths where a crystalline-silicon photovoltaic solar cell has a higher spectral responsivity.