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

Ceramic powders of Sr 11.5 x Yb x TiO 3 ( 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) O 2 Ti 4+ fundamental state. The Yb 3+ ( em = 980 nm) excitation spectra show a broadband being also compatible with the LMCT O 2 Ti 4+ state, indicating the energy transfer from the host to the Yb 3+ . 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 Yb 3+ is explained as due to defects associated with O 2 and Sr 2+ 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. Photon downshifting in SrTiO 3 :Yb 3+ to wavelengths where a PV cell has its higher spectral responsivity is achieved upon UV excitation through an energy transfer process from a ligand-to-metal-charge transfer O 2 Ti 4+ state to the Yb 3+ excited state.