Preparation And Research On The Photovoltaic Properties Of Titania-Composite Photoanodes With Fluorine And Rare Earth Upconversion Materials

Dye-sensitized solar cells（DSSCs） have turned into one of the promising solar cells in renewable energy research field, owing to their novel design concept of photosynthesis, low production cost and relatively high conversion efficiency. TiO₂ based dye-sensitized solar cells mainly exist two issues: poor absorption intensity of visible light and the lacking energy use of infrared light, which lead to the mismatch existed between the absorption spectrum of solar cell and the solar irradiation spectrum. TiO₂ itself owning large amounts of surface states and oxygen vacancies, which cause serious interface recombination, and wide-band- gap will cause insufficient electron injection driving force. Therefore, the design and fabrication of highly-efficient multi-functional photoanodes are desired to further improve the performance of DSSCs.In order to solve these above mentioned problems, the conventional TiO₂ photoanode was modified by doping with fluorine or/and lanthanide ions or compounding with upconversion materials in this paper. After modification, the absorption scope of solar cell was broadened to the visible area and even to the near- infrared light area. The charge separation and electron injection efficiencies in photoanode films, as well as the interfacial charge recombination process in DSSCs were improved, ultimately improved the performance of DSSCs.We doped TiO₂ with fluorine ions, and studied the influence of F doping on the photogenerated charge separation process, electron injection and the charge transfer process in DSSCs. The result indicated that fluorine-doping could positively shift the conduction band of TiO₂, thus enlarge the driving force for efficient electron injection, and facilitate the charge transfer, consequently reducing the interfacial losses from the charge recombination. Finally, the conversion efficiency of DSSCs with fluorine-doped TiO₂ electrode was improved by 16.2% with 23.1% photocurrent gain.Based on the above results, in order to further improve the performance of DSSCs, we co-doped TiO₂ with nitrogen and fluorine ions. We investigated its electrical and optical properties, applied it in dye-sensitized solar cells as electrode materials and found that its photovoltaic performance was better than fluorine doping. Compared with the TiO₂ reference photoanode, the overall conversion efficiency of DSSC with N, F-TiO₂ photoanode was 8.20%. There were 52% increase in the photocurrent and 22% improvement in the conversion efficiency. Such improvement were mainly arising from the positive-shift flat band, increasing the driving force of electrons and then improving the electron injection efficiency from the LUMO of dye to the conduction band of TiO₂, therefore reducing the recombination odds between injected electrons and oxidized dye molecules and I3- ions; the narrowed band gap, expanding the response in visible region and increasing the utilization of visible light; the incorporation of the flower- like anatase N, F-TiO₂ crystals, serving as effective light scattering centers and improving the reactivity, both were important attributes for achieving a higher light harvesting and thus an improved Jsc and conversion efficiency.Upconversion material modified TiO₂ photoanode could increase the utility of near- infrared light in DSSCs. Upconversion@TiO₂ core-shell structure and upconversion/TiO₂ nanoheterostructure could effectively tackle the problem of upconversion materials（such as poor electronic conduction negatively affects the charge separation or acting as surface recombination center in DSSCs）. The results indicated that modification TiO₂ photoanode with upconversion material not only increased the utility of near- infrared light in DSSCs but also induced direct electron-injection from Yb F3-Ho to TiO₂ via hetero-junction interface; hetero-structured photoanode can facilitate the charge separation, suppress the recombination process of injected electrons with I3- in electrolyte at the photoanode/dye/electrolyte interface and prolong the electrons’ life time. Therefore, the overall conversion efficiency could be improved by 21²3%, the short-circuit photocurrent density could be increased by 17¹9%.Compared with upconversion material modified TiO₂ photoanode, lanthanides modified fluorine doped TiO₂ photoanode could convert the near- infrared light in situ to the dye-sensitive visible light, which could effectively reduce the distance between upconversion materials and sensitizers, thus minimizing the loss of the converted light and improving the energy transfer efficiency. We prepared Er3+/Ho3+, Yb3+, F- tri-doped TiO₂ and constructed them into double- layer structured photoanode. The results indicated that fluorine doping could enhance the photogenerated charge separation in TiO₂, reduce the TiO₂ surface defect（-OH） and enhance the upconversion emission; elevated flat-band potential increased the energy level difference between the electrons in the TiO₂ and the redox potential of the electrolyte, resulting in an improved Voc; double-layer structured photoanode could facilitate the photogenerated electrons’ transfer, retard the interface charge recombination and prolong the electrons’ life time; the green emission of Ho3+ ions could be more efficiently used by N719, thus the performance of Ho3+, Yb3+, F- tri-doped TiO₂ was better than that of Er3+, Yb3+, Ftri-doped TiO₂. Eventually the photoelectric conversion efficiency of DSSCs achieved 10%, compared with the reference TiO₂ photoanode cell, the conversion efficiency was increased by 37%.