Hydrothermal growth of a composite TiO2 hollow spheres/TiO2 nanorods powder and its application in high performance dye-sensitized solar cells

In this research a composite powder including TiO2 hollow spheres (HSs) and TiO2 nanorods (NRs) was prepared and applied as the light scattering layer in the photoanode of the dye-sensitized solar cells (DSCs). TiO2 HSs with external diameters around 450 nm were synthesized using hydrothermally grown carbon spheres as the template and through a liquid phase deposition (LPD) technique. TiO2 NRs were also prepared by additional hydrothermal growth in the presence of sub-micronic HSs to form a composite mixture. The two different scattering structures i.e. the TiO2 HSs and NRs were well-mixed in the final composite powder. Besides, the number ratio of TiO2 NRs/HSs was calculated as 1.0 from the SEM images. TiO2 nanocrystals (NCs) were also grown by a hydrothermal method with dominant particle size of 25 nm. Three kinds of photoelectrodes composed of TiO2 NCs, TiO2 HSs and TiO2 NRs were fabricated and applied in DSCs. The first photoanode was made of a single nanocrystalline TiO2 layer with about 8 mu m thickness. The next two other kinds were composed of a TiO2 NCs layer covered by a TiO2 HSs or composite TiO2 HSs/NRs scattering layers. The photovoltaic characteristics of the corresponding DSCs were investigated and explained. According to the results, the energy conversion efficiency of the DSC with composite TiO2 HSs/NRs scattering layer was measured about 9.6%. This efficiency was increased about 35% and 6% compared to those of the cells with light scattering-free and TiO2 NCs/TiO2 HSs photoelectrodes. The reason was attributed to the increased dye adsorption and light scattering of the pioneer cell.