| Summary: | The performance of dye-sensitized solar cells (DSCs) critically depends on the efficiency of electron transport within the TiO<sub>2</sub>-dye-electrolyte interface. To improve the efficiency of the electron transfer the conventional structure of the working electrode (WE) based on TiO<sub>2</sub> nanoparticles (NPs) was replaced with TiO<sub>2</sub> nanotubes (NTs). Sol-gel method was used to prepare undoped and Nb-doped TiO<sub>2</sub> NPs and TiO<sub>2</sub> NTs. The crystallinity and morphology of the WEs were characterized using XRD, SEM and TEM techniques. XPS and PL measurements revealed a higher concentration of oxygen-related defects at the surface of NPs-based electrodes compared to that based on NTs. Replacement of the conventional NPs-based TiO<sub>2</sub> WE with alternative led to a 15% increase in power conversion efficiency (PCE) of the DSCs. The effect is attributed to the more efficient transfer of charge carriers in the NTs-based electrodes due to lower defect concentration. The suggestion was confirmed experimentally by electrical impedance spectroscopy measurements when we observed the higher recombination resistance at the TiO<sub>2</sub> NTs-electrolyte interface compared to that at the TiO<sub>2</sub> NPs-electrolyte interface. Moreover, Nb-doping of the TiO<sub>2</sub> structures yields an additional 14% PCE increase. The application of Nb-doped TiO<sub>2</sub> NTs as photo-electrode enables the fabrication of a DSC with an efficiency of 8.1%, which is 35% higher than that of a cell using a TiO<sub>2</sub> NPs. Finally, NTs-based DSCs have demonstrated a 65% increase in the PCE value, when light intensity was decreased from 1000 to 10 W/m<sup>2</sup> making such kind device be promising alternative indoor PV applications when the intensity of incident light is low.
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