Near infrared photon-downshifting in Yb3+-doped titanates: The influence of intrinsic defects

Powder samples of Yb3+-doped TiO2 (0.5 mol%) -exhibiting anatase and rutile phases- CaTiO3, SrTiO3 and Na0.5Bi0.5TiO3 were synthesized by the sol-gel and polymeric complex methods. The crystal structure, microstructure and optical properties of the powders annealed at 800 °C and 1000 °C for 1 h were investigated by X-ray diffraction, scanning electron microscopy, diffuse reflectance and photoluminescence spectroscopies, respectively. X-ray diffraction patterns show the presence of the expected crystalline phase for each compound. Reflectance spectra for all compounds exhibit a broad absorption band below 425 nm ascribed to the ligand-to-metal charge transfer (LMCT) O2− → Ti4+ fundamental state. The Yb3+ excitation spectra for all titanates show a broadband being also compatible with the LMCT O2− → Ti4+ state, indicating the energy transfer from the host to the Yb3+. Interestingly, the excitation band edges follow the same order as that of the optical bandgap energy of the investigated titanates. A conspicuous excitation band in the CaTiO3:Yb3+ provides evidence about the importance of intrinsic defects in the energy transfer process from the host to Yb3+. Photoluminescence emission and diffuse reflectance spectra in the visible and NIR, reveal an emission band that overlaps with the absorption band of the Yb3+, explaining reasonably well the energy transfer process from the host to Yb3+. According to each compound, the integrated intensity of the Yb3+ emission follows the order Na0.5Bi0.5TiO3 > CaTiO3 > SrTiO3 > TiO2. Finally, it is shown that titanates may downshift photons from UV to wavelengths where a crystalline-silicon photovoltaic solar cell has its higher spectral responsivity.