Nonmetal-doped TiO₂Photoanodes For The Dye-sensitized Solar Cells

TiO₂is a wide band gap material that can hardly make use of sunlight as an energysource. This paper is focused on how to enhance visible spectral response and the chargeinjection through nonmetal doped TiO₂. The nonmetal doped TiO₂pastes weresynthesized by ball milling or hydrothermal. Nanocrystalline TiO₂films were fabricatedby doctor blading. The surface morphology, crystallinity and electrochemistryperformance was investigated.1. Nitrogen-doped TiO₂photoanodes by ball milling for the dye-sensitized solar cells.X-ray photoemission spectroscopy （XPS） was used to analyze the formation of nitrogen-doped TiO₂. The crystallinity of the nitrogen-doped TiO₂film was examined by X-raydiffraction analysis （XRD）. The UV-vis spectra of the nitrogen-doped film showed ahigher absorption in the scanned wavelength compared with the un-doped photoanode.The surface morphology was investigated by scanning electron microscopy （SEM）. Fromthe results it is concluded that nitrogen in the TiO₂lattice was doped mainly bysubstituting the sites of oxygen atoms in the experiments. The electrode made by urea:P25molar ratio of1:3showed the maximum conversion efficiency, which was26%higherthan that of the un-doped TiO₂dye-sensitized solar cells. However, further loading willdecrease the efficiency.2. Sulfur-doped TiO₂photoanodes by a mechanical process with ball milling thethiourea with commercial P25TiO₂nanoparticles and successfully used as photoanode ofdye-sensitized solar cells. The X-ray photoelectron spectroscopy （XPS） results indicatethat sulfur-doped TiO₂are locally distorted by incorporating S⁶⁺ species into TiO₂andsubstitutes for some of the lattice titanium (Ti⁴⁺), meanwhile a clear reduction in the bandgap energy of the sulfur-doped TiO₂photoanode compared to the band gap value for P25.The electrode made by thiourea:P25molar ratio of1:3showed the maximum conversionefficiency, which was24%higher than that of the un-doped TiO₂dye-sensitized solar cells. However, further loading will decrease the efficiency. Probably the doped sulfur athigher S:TiO₂（1:3） was formed sulfur oxide on the TiO₂surface, and most of the sulfurwill remove during the calcination process.3. Sulfur-doped TiO₂photoanodes by hydrothermal, the crystalline structure weredetermined by X-ray diffraction, and the XRD patterns indicated that all the films had ahomogeneous anatase phase. Study the optical performace found that the optimum contentof thiourea was0.770g. The X-ray photoelectron spectroscopy （XPS） results indicate thatsulfur-doped TiO₂are locally distorted by incorporating S6+species into TiO₂andsubstitutes for some of the lattice titanium (Ti⁴⁺), increasing the ratio from0g to0.770gsignificantly enhanced the peak intensities of the sulfur-doped TiO₂. However, furtherincrease reduced the peak intensities. Probably the doped sulfur at higher was formedsulfur oxide on the TiO₂surface, and most of the sulfur will remove during the calcinationprocess.The dye-sensitized solar cells of S-2showed the maximum conversion efficiencywith an open-circuit voltage of0.72V, a photocurrent of16.00mAcm^-2, a fill factor of0.648, and an efficiency of7.59%.