Abstract
The performance of dye-sensitized solar cells (DSCs) critically depends on the efficiency of electron transport within the TiO2-dye-electrolyte interface. To improve the efficiency of the electron transfer the conventional structure of the working electrode (WE) based on TiO2 nanoparticles (NPs) was replaced with TiO2 nanotubes (NTs). Sol-gel method was used to prepare undoped and Nb-doped TiO2 NPs and TiO2 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 TiO2 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 TiO2 NTs-electrolyte interface compared to that at the TiO2 NPs-electrolyte interface. Moreover, Nb-doping of the TiO2 structures yields an additional 14% PCE increase. The application of Nb-doped TiO2 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 TiO2 NPs. Finally, NTs-based DSCs have demonstrated a 65% increase in the PCE value, when light intensity was decreased from 1000 to 10 W/m2 making such kind device be promising alternative indoor PV applications when the intensity of incident light is low.
Highlights
Nanocrystalline dye-sensitized solar cells (DSCs) based on titanium dioxide (TiO2) nanostructured layers have been the subject of intensive studies over the last decades [1]
TiO2 NTs-based DSCs have shown a better performance under low-light illumination conditions where they demonstrated a 65% increase of power conversion efficiency (PCE) value when light intensity was decreased from 1000 to 10 W/m2
We observed that Nb doping does not affect the absorption spectra of desorbed dye pointing out that doping has a negligible effect on the surface area of the electrodes
Summary
Nanocrystalline dye-sensitized solar cells (DSCs) based on titanium dioxide (TiO2) nanostructured layers have been the subject of intensive studies over the last decades [1]. It was shown that the Nb-doping of TiO2 electrodes in DSCs can improve the charge injection and transport by tailoring the electronic structure of the TiO2-dye-electrolyte interface [10]. In regards to the replacement of the TiO2 NP structure with alternative, TiO2 nanotubes (NTs) for DSCs offer several advantages due to increased light scattering, fast electron transportation and reduction of trap sites compared to TiO2 NPs [19,20]. The reported efficiencies of the DSCs fabricated with TiO2 NTs produced by anodization do not exceed those for TiO2 NPs-based devices due to lower surface area of the TiO2 NTs which limits the amount of the absorbed dye molecules and decreases light-harvesting ability [20,26]. TiO2 NTs-based DSCs have shown a better performance under low-light illumination conditions where they demonstrated a 65% increase of PCE value when light intensity was decreased from 1000 to 10 W/m2
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