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Metal-based semiconductor nanomaterials for thin-film solar cells   173

           Moreover, the film thickness, pipe diameter, and morphology of NT arrays can be
           easily adjusted by changing the input power, reaction time, and oxidizing solution.
              In 2005 first-generation TiO 2  NTs were used as electrodes for DSSCs, and photo-
           electrochemical tests of Ru-dye (N3)-sensitized TNARs with a diameter of 100 nm
           and length ranging from 500 to 2.5 mm were carried out [78]. The maximum incident
           photon-to-current efficiency (IPCE) conversion value in the visible range was on the
           order of 1.6%–3.3%, yielding a PCE of only 0.036% for such NT-based DSSCs. To
           improve the output of this kind of DSSC, second-generation NTs produced in glyc-
           erol or ethylene glycol electrolytes were applied in DSSCs, and both the thickness of
           TiO 2  NTs and the PCE were greatly enhanced. Karthik and colleagues [79] fabricated
           highly ordered TiO 2  NT arrays with a thickness of up to 220 μm in an organic reagent
           and applied them in DSSCs, yielding a PCE of 6.9%. Although many μm NTs have
           been successfully fabricated and applied in DSSCs, 1D TiO 2  NT-based DSSCs still
           struggle to reach the high photovoltaic performance of the mesoporous TiO 2 -based
           DSSCs. Compared with traditional NP-based DSSCs, the 1D TiO 2  DSSCs’ lower
           efficiency is mainly attributed to the semiconducting electrode’s surface area’s lim-
           ited capacity for dye adsorption. Recently, various strategies, including surface en-
           gineering and electrode architecture, were developed to address this issue. Schmuki
           and colleagues  [80] fabricated conical-shaped TiO 2  NT arrays as the photoanodes
           for backside-illumination DSSCs, yielding a PCE as high as 8.09% (as shown in
           Fig. 8.16). Despite the loss of light absorption with the backside-illumination con-
           figuration, the significant PCE improvement was due mainly to the efficient use of
           light managed by the conical-shaped tips of the tubes, which were prepared via a two-
           step anodization process using an electrolyte containing 0.2 M NH 4 F, 50 vol% lactic
           acid, and 50 vol% triethyleneglycol. Applying a hierarchical structure on TNARs is
           another strategy to improve the DSSCs’ PCEs. In this regard, Lin et al. combined a
           two-step electrochemical anodization with a hydrothermal process to fabricate novel
           hierarchical TiO 2  NT arrays as photoanodes for DSSCs [81]. First, highly ordered
           and smooth TiO 2  NT arrays were created through a typical two-step anodic oxidation
           process, and then these smooth NTs were transformed into rough NTs by an additional
           hydrothermal treatment, leading to greatly increased dye loading. Subsequently, these
           NTs were used to produce DSSCs in a backside-illumination mode, which yielded a
           significantly high-power conversion efficiency of 7.12%, which was further increased
           to 7.75% upon exposure to O 2  plasma.
              Although modification of the 1D TiO 2  NTs can enhance light utilization in thin-
           film solar cells, the backside-illumination configuration based on Ti foil/TiO 2  NTs
           still limits overall light-harvesting efficiency; in this case, the frontside-illumination
           configuration based on TCO glass/1D TiO 2  is more likely to enhance the overall pho-
           tovoltaic performances of the cells. To this end, Grimes and colleagues took another
           alternative strategy to fabricate long transparent  TiO 2  NT arrays (33 μm) on FTO
           transparent conducting oxide glass for frontside-illuminated configuration DSSCs
           [10]. In this case, instead of Ti foil substrates, Ti metal was deposited on FTO glass
           by magnetron sputtering and was utilized as the source material for the anodization;
           thus the TiO 2  NT arrays grew robustly on the FTO substrates, yielding a PCE of 6.9%
           and an IPCE ranging from 70% to 80% for wavelengths between 450 nm and 650 nm,
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