| Summary: | Free-standing TiO<sub>2</sub> nanotube (NT) electrodes have attracted much attention for application in solid- or quasi-solid-state dye-sensitized solar cells (DSSCs) because of their suitable pore structure for the infiltration of solid electrolytes. However, few studies have been performed on the relationship between nanostructures of these NT electrodes and the photovoltaic properties of the solid- or quasi-solid-state DSSCs. Here, we prepare vertically aligned and highly ordered TiO<sub>2</sub> NT electrodes via a two-step anodization method for application in quasi-solid-state DSSCs that employs a polymer gel electrolyte. The length of NT arrays is controlled in the range of 10–42 μm by varying the anodization time, and the correlation between NT length and the photovoltaic properties of quasi-solid-state DSSCs is investigated. As the NT length increases, the roughness factor of the electrode is enlarged, leading to the higher dye-loading; however, photovoltage is gradually decreased, resulting in an optimized conversion efficiency at the NT length of 18.5 μm. Electrochemical impedance spectroscopy (EIS) analysis reveals that the decrease in photovoltage for longer NT arrays is mainly attributed to the increased electron recombination rate with redox couples in the polymer gel electrolyte.
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