Performance and electron transport properties of TiO2 nanocomposite dye-sensitized solar cells

TiO2 nanowire (NW)/nanoparticle (NP) composite films have been fabricated by hybridizing various ratios of hydrothermal anatase NWs and TiO2 NPs for use in dye-sensitized solar cells (DSSCs). Scanning electron microscopy (SEM) images reveal that uniform NW/NP composite films were formed on fluorine-doped tin oxide (FTO) substrates by the dip-coating method. The NWs are randomly but neither vertically nor horizontally oriented within the composite film. The TiO2 NP DSSC possesses superior performance to those of the NW/NP composite and the pure NW cells, and the efficiency of the NW/NP composite DSSC increases on increasing the NP/NW ratio in the composite anode. All types of DSSC possess the same dependence of performance on the anode thickness that the efficiency increases with the anode thickness to a maximum value, then it decreases when the anode is thickened further. Electrochemical impedance spectroscopy analyses reveal that the NP DSSCs possess larger effective electron diffusion coefficients (Deff) in the photoanodes and smaller diffusion resistances of I-3(-) in electrolytes compared to those in the NW/NP and the NW DSSCs. Deff decreases when NWs are added into the photoanode. These results suggest that the vertical feature of the NWs within the anodes is crucial for achieving a high electron transport rate in the anode.