Electronic properties of Nano-porous TiO₂- and ZnO-Thin Films-comparison of simulations and experiments

The electronic properties of anatase, rutile and ZnO thin films are of interest for many applications such as photocatalysis, or dye-sensitised solar cells. This paper describes the correlation between microstructure and electronic properties of photoactive TiO₂ and ZnO thin films, which are based on experimental data and are interpreted by ab-initio simulations. This study considers the influence of three main factors on the optical and electrical properties: Strain of epitaxially grown thin films, nano-particle- and nano-pore- structures, and finally the interface to adsorbed dye molecules. The first part describes the search for a suitable substrate to grow titania layers with a smaller band-gap and ab-initio simulations showed excellent agreement to the experimental data. A narrow band gap is predicted for strained lattice constants in the a-direction and compressed in the c-direction. The second part describes the fabrication of nano-porous thin films by sol-gel processing using appropriate precursor solutions (titanium-tetra-iso-propanol (TTIP), di-ethanol-amine (DEA)) applied on suitable nano-templates. Due to the surface curvatures the nano-porous TiO₂ has a narrow band gap, while nano-particles of titania show a larger band-gap. In the third part Dvxα ab-initio simulations were applied to dye-sensitized solar cells (DSSC), based on ZnO and TiO₂. The results are compared to experimental data obtained for different dyes on nanoporous electrodeposited ZnO films. The ranking in efficiency found experimentally for the different combinations could be explained by the calculation results. This finding is discussed in the view of further optimization of solar cells.