Semiconductor Nano-Morphologies for Energy Applications
Titanium dioxide is a semiconductor with great potential in dye-sensitized solar cells, energy storage and photocatalysis. These applications depend on control of surface area, porosity and morphology. We have recently developed a highly flexible new preparation strategy for the formation of various titania nano-morphologies, based on fusing preformed ultrasmall titania nanocrystals with surfactant-templated sol-gel titania acting as a structure-directing matrix and as a chemical glue. In this “brick & mortar” approach, the “mortar” acts as a reactive precursor for the further growth of the crystalline phase seeded by the nanocrystalline “bricks”. This synergy leads to a significantly lowered temperature needed for crystallization and the preservation of the mesoporous structure. It also allows us to build various hierarchical structures such as titania inverse opals penetrated by titania mesopores, because we can significantly reduce shrinkage effects. Coatings with a broad variety of periodic mesostructures can be tuned by varying the surfactant and the fraction of the “bricks”, and thicknesses ranging from a few nanometers to several micrometers are accessible. These mesostructured and crystalline films are being employed as active layers in thin dye-sensitized solar cells exhibiting high conversion efficiency due to short diffusion paths. We also investigate additional sensitizing approaches such as extremely thin absorber layers (ETA) and alternative solid hole conductors. Moreover, we find that the ultrathin crystalline walls of the mesoporous brick & mortar titania feature extremely fast lithium insertion kinetics.