Preparation Of TiO₂ Nanoarrays And Their Application In Dye-sensitized Solar Cells

Dye-sensitized solar cells?DSSCs?have been recognized a promising third generation photovoltaic devices and have attracted a tremendous attention due to their simple assembly,low cost and relatively high power conversion efficiency.Recent advances in the designing of dyes and electrolytes have improved the power conversion efficiency of DSSCs up to 14.3%.A three-dimensional network film of randomly organized and disordered TiO₂ nanoparticles is used in photoanode of conventional DSSCs due to their large surface area for dye loading.However,the TiO₂ nanoparticles flim with numerous grain boundaries limits the electron transport and accelerates the charge recombination.Trying to overcome the limintation of TiO₂ nanoparticles film,there have been numerous efforts to improve the electron transport and reduce electron recombination rate by the use of alternative structure and morphology of the photanodes.One-dimensional TiO₂ nanostructures such as nanotube arrays,nanowire arrays and nanorod arrays have been extensively investigated as photoanode materials in DSSCs due to their superior photo-generated charge separation and transport properties.However,the one dimensional nanostructures with poor dye adsorption and low light harvesting ability resulted in poor photovoltaic performance,ascribing to the large diameter and the large space between the one dimensional nanostructure trunks.Two methods of solving the problem of low dye-adsorption and low light scartteing were to extend the length of TiO₂ nanowires and synthesize the hierarchical nanostructures.Therefore,this theis based on these two methods to improve the power convesion efficiency of DSSCs.The main work is concentrated on the following aspects:1.The ultra-long hierarchical bud-like branched TiO₂ nanowire arrays on flurine-doped tin oxide glass substrate have been prepared via a two-step solution route,which involves the growth of the anatase TiO₂ nanowires with the length of 48 ?m on flurine-doped tin oxide glass substrate,followed by a secondary growth of the rutile TiO₂ nanorods on the as-grown TiO₂ nanowires backbone.The dye-sensitized solar cell based on the hierarchical bud-like branched TiO₂ nanowire arrays shows a power conversion efficiency of 6.00%,which is nearly 2.88 times as high as that of bare TiO₂ nanowire arrays because of the preferable nanostructure,which not only retains the efficient charge separation and transport properties of one-dimensional TiO₂ nanostructures,but also improves the amount of dye adsorption and light-scattering ability due to the branched structure.2.The bottom-straight and top-bent dual anatase TiO₂ nanowires directly on flurine-doped tin oxide?FTO?glass substrate are successfully synthesized for the first time via a two-step solvothermal reaction,which obtains single anatase TiO₂ nanowires with the diameter of 250-450 nm and the length of ⁴9 ?m in the first growth step,then the slender TiO₂ nanowires with the diameter of 40-80 nm intertwining larger nanowires to form dual nanowires network in the secondary growth step.Dye-sensitized solar cell based on dual TiO₂ nanowires exhibits the power conversion efficiency of 5.30%,which is much higher than those of DSSCs based on the pristine single TiO₂ nanwires?2.36%?due to the twining network like dual nanowires structure maintaining excellent electron transport and simultaneously enhancing the specific surface area and light scattering ability.Moreover,by TiO₂ sol treatment,the performance of dual TiO₂ nanowires based DSSC can be further enhanced,reaching an impressive PCE of 7.65%.3.The large-scale hierarchical anatase titania nanotube arrays on transparent conductive substrate are fabricated via in situ conversion from anatase titania nanowire arrays.The first-step hydrothermal reaction is the growth of ultra-long anatase titania nanowire arrays,and the second-step hydrothermal reaction is the conversion of titania nanowire arrays to titania nanotube arrays modified with a large number of nanosheets.The resultant hierarchical titania nanotube array film provides a large surface area and superior light scattering ability.Dye-sensitized solar cell based on the hierarchical titania nanotube array photoanode obtains a power conversion efficiency as high as 5.96% and shows a 181% increase compared to the pristine titania nanowire array photoanode.In addition,the most interesting result is that an optimized efficiency of 7.54% is achieved for the cell based on the hierarchical titania nanotube array photoanode with titania sol modification.