Synthesis and characterization of Ho3+-doped strontium titanate downconversion nanocrystals and its application in dye-sensitized solar cells

Ho3+-doped strontium titanate (SrTiO3:Ho3+) downconversion (DC) nanocrystals are synthesized by the solid state interaction of titanium dioxide, strontium nitrate, holmium oxide and sodium chloride and then used as a photoanode in dye-sensitized solar cells (DSSCs) to investigate the effect of DC nanocrystals in DSSCs. Differential thermal analysis, X-ray diffraction, scanning electronic microscope, energy dispersive spectroscopy and Brunauer-Emmet-Teller analysis confirmed the formation of cubic structured SrTiO3:Ho3+ nanocrystals with diameters of 40-400 nm, pore size of ∼45 nm, sintering temperature of 950 °C. The photofluorescence and UV-Vis absorption spectra of the SrTiO3:Ho3+ nanocrystals revealed strong emission intensity and visible light absorption when doped content of holmium oxide was between 1 wt% and 3 wt%. Compared with the pure SrTiO3 photoanode, SrTiO3:Ho3+ DC photoanode showed a greater photovoltaic efficiency. The photoelectric conversion efficiency (η) of the DSSCs with a SrTiO3:Ho3+ photoanode doped with 1 wt% holmium oxide was 59% higher than that with a pure SrTiO3 photoanode. This phenomenon could be explained by SrTiO3:Ho3+ nanocrystals' ability to absorb ultraviolet light and downconvert it to visible light, which extends spectral response range of DSSC to the ultraviolet region and increased the short-circuit current density (Jsc) of DSSCs.