Preparation Of Flexible Oriented TiO₂ Micro-nano Composite Photoanode And Its Photoelectric Conversion Property

Flexible oriented TiO₂ nanoarrays/TiO₂ nanoparticles composites have been widely applied as photoanodes of dye-sensitized solar cells （DSSCs） due to their special nanostructure features. In this dissertation, hierarchical TiO₂ nanowire arrays （NWAs）/TiO₂ nanosheet nanostructures, TiO₂ NWAs/Nb5+-doped TiO₂ nanoparticles （NPs）, and TiO₂ NWAs/Er³⁺-Yb³⁺-codoped TiO₂ NPs composites photoanodes were prepared on flexible Ti-mesh substrates via hydrothermal and spin-coating approaches. The effects of various experimental parameters on the growth of TiO₂ micro-nano structures and their related photovoltaic properties were investigated in detail. Controllable preparation of the above nanostructures was realized and the enhanced photovoltaic performances were achieved.Well-aligned TiO₂ NWAs with uniform size were synthesized on flexible Ti-mesh substrate by one-step hydrothermal method and the effects of hydrothermal conditions on the controllably synthesis of TiO₂ nanoarrays and its related photovoltaic performnce were systematically investigated. The growth mechanism of the Ti mesh-supported TiO₂ nanostructures was discussed, and the TiO₂ NWAs morphologies can be controlled. Moreover, a parametric study was performed to determine the optimized temperature and time of the dye sensitized process, relationship between the nanostructures and photoelectric properties was preliminary illuminated. It was shown that the well oriented single crystalline TiO₂ NWAs were prepared on Ti mesh substrates when the optimum conditions were:NaOH solution was adjusted at 1.0 mol L-1, hydrothermal reaction was carried out at 220℃ for 20 h, after obtained TiO₂ NWAs was sensitized at 40℃ for 16 h, an overall power conversion efficiency of 3.42%was achieved.Hierarchical TiO₂ NWAs （HNWAs）/TiO₂ nanosheet photoanode were synthesized on flexible Ti-mesh substrate by using two-step hydrothermal approach in a K2TiO·C4O8 solution. Small short nanosheet branches with varying sizes （50±5 nm in length） were formed on the surface of the TiO₂ nanowires backbone. Then, TiO₂ HNWAs were coated with a thin Nb2O5 layers. It was indicated that compared to the TiO₂ NWAs, TiO₂ HNWAs with enlarged internal surface area and strong light scattering properties improved the photovoltaic conversion efficiency from 3.42% to 3.85%. The Nb2O5 coating layer decreased the electron recombination rate and increased electron collection efficiency and injection efficiency, resulting in enhanced power conversion efficiency of 4.55%.TiO₂ NWAs/Nb-doped TiO₂ NPs composite structures were firstly synthesized on flexible Ti meshes by using hydrothermal and spin-coating approaches. The effect of different amounts of Nb doping （0-9.6 mol%） on the microstructures and photoelectrical properties were studied. Three key factors consisting of sunlight harvesting efficiency （η1h）, electron injection efficiency （ηinj） and electron collection efficiency （ηec） which determine the conversion efficiency of flexible DSSCs were discussed in detail. Different amounts of Nb-doping have no effect on the surface area of photoanode. With increasing Nb doping amount, the Vtb, exhibited a positive shift from -0.771 to -0.665 V, thus, the electron injection efficiency ηinj from the lowest unoccupied molecular orbital （LUMO） of dye to flat band energy level of Nb-doped TiO₂ increased. Appropriate Nb doping can efficiently quicken the electron transfer rate and reduce the energy trap state amounts in the TiO₂ crystal lattice. The fully flexible DSSCs assembled by TiO₂ NWAs/2.4 mol% Nb-doped TiO₂ NPs photoanode exhibited a well photovoltaic efficiency of 7.20%.TiO₂ NWAs/Er³⁺-Yb³⁺-codoped TiO₂ NPs composites were synthesized on Ti mesh by hydrothermal and spin-coating process. The effect of different amount of Er³⁺-Yb³⁺-codoping on the upconversion luminescence intensity and photovoltaic efficiency were investigated. Excited with a laser at 980 nm, the strong upconversion luminescence spectra was obtained when doping amount of Er and Yb was 2.0 and 1.0 mol%, respectively. The luminescence spectra exhibited emission peaks centered at 489,526,549, and 658 nm. So, the composites enlarged the response region of DSSCs from an ultraviolet-visible region to the whole region of the solar spectrum. The conversion efficiency of flexible DSSC based on TiO₂ NWAs/2.0 mol% Er³⁺-1.0 mol% Yb³⁺-codoped TiO₂ NPs was improved to 7.29%. Moreover, the introduction of ND2O5 coating effectively inhibited electron recombination losses, thereby improving electron lifetime τe and collection efficiency ηec, and an enhanced conversion efficiency of 8.10% was achieved.