TiO₂ and pyrochlore Tm₂Ti₂O₇ based semiconductor as a photoelectrode for dye-sensitized solar cells

This study presents the use of TiO₂ nanoparticles with Tm as photoelectrodes in DSSCs. The nanoparticles were annealed at 1173 K and the predominant TiO₂ phase was rutile. XRD and Raman spectroscopy revealed the presence of a crystalline pyrochlore phase of the mixed oxide Tm₂Ti₂O₇. In turn, XPS confirmed the presence of Ti⁴⁺ and Tm³⁺, so the inclusion of Tm did not affect the oxidation state of the Ti. UV–Vis spectra showed that the presence of the pyrochlore phase led to new electronic states in the band gap. The use of the pyrochlore phase in the photoelectrode had a positive effect, improving the efficiency of the pure TiO₂ cells. The efficiency increased by between 2.32% and 3.16% when pure TiO₂ was replaced with a mixture of rutile TiO₂ and pyrochlore Tm₂Ti₂O₇, so the controlled use of a pyrochlore phase can produce good results in dye-sensitized solar cells. Another important effect of the pyrochlore phase was to increase the open-circuit voltage values by around 7% and can be explained by the flat band voltage values. The samples with Tm showed two flat band voltage values, which generated two possible electronic injection mechanisms in the cells. (paper)