Fabrication of submicron/micron size cavities included TiO2 photoelectrodes and optimization of light scattering to improve the photovoltaic performance of CdS quantum dot sensitized solar cells

In this research sub-micron size carbon spheres were hydrothermally grown with sizes around 500-550 nm. TiO2 nanocrystals (NCs) were also prepared through a hydrothermal approach with dominant size of 20 nm. The TiO2 NCs and carbon spheres were applied to make carbon-TiO2 pastes with different weight percents of the included carbon spheres. The different pastes were deposited on FTO glass substrates and annealed to remove the carbon spheres from the layers. As a result some nanocrystalline TiO2 layers with included sub-micron and micron size cavities were fabricated. The density of cavities was higher and their corresponding size was larger for the layers formed of the carbon-TiO2 pastes with higher included carbon spheres. CdS nanocrystals were grown on the surface of these different TiO2 layers of various porosities as the light sensitizers. Finally these TiO2 layers with included large cavities were applied as the photoelectrode of the quantum dot sensitized solar cells (QDSCs). The effect of density/size of the cavities on the light scattering level of the photoelectrodes and the photovoltaic performance of corresponding QDSCs was investigated. The results demonstrated that the maximum energy conversion efficiency was belonged to the QDSC with a photoelectrode made of carbon-TiO2 paste with 8 wt% of the included carbon spheres. This efficiency was about 2.75% which was increased about 87% compared to that of the reference cell with cavity-free photoelectrode.