Synthesis of randomly directed inclined TiO2 nanorods on the nanocrystalline TiO2 layers and their optimized application in dye sensitized solar cells

In this research TiO2 nanocrystals of 20 nm size were synthesized through a hydrothermal method. These nanocrystals were deposited on FTO glass substrates with 6 μm thickness. A layer of TiO2 nanorods were over-grown on this nanocrystalline sub-layer by another hydrothermal process. The inclined TiO2 nanorods were well-formed and randomly directed on the top of the TiO2 nanocrystals layer. The length and diameter of the nanorods and corresponding thickness of this over-layer was controlled by the growth time. This bi-layer structure was applied as the photoanode of the dye sensitized solar cells (DSCs). The nanocrystalline TiO2 sub-layer could create enough dye adsorption and acceptable sensitization. Besides, the nanorods over-layer could increase the light scattering and light travelling distance within the photoanode. These two parameters resulted in an increased light absorption and energy conversion efficiency. A maximum efficiency of 6.2% was achieved for the optimized cell with bi-layer photoanode with over-grown nanorods in 20 h. This efficiency was increased about 47% compared to that of the DSC with just nanocrystalline photoelectrode. Final improvements were carried out on the thickness of nanocrystalline TiO2 sub-layer and its post-treatment before the nanorods growth. The energy conversion efficiency was increased to 7.5% due to the application of these modifications.