Sponge-like structures for application in photovoltaics

Large surface areas at an interface between two different materials are desired in many research fields where the interaction between these materials significantly affects the performance of the physical system. This behaviour is illustrated on sponge-like structures, which assign for such a high surface area, and demonstrate the development from bulk material to thin films and a variety of applications. The focus is on sponge-like nanostructures consisting of a network of aggregated titania nanoparticles applied in hybrid structures for photovoltaics. Examples based on a sol–gel process for the preparation of titania nanostructures in thin films, mimicking the sponge morphology, are shown. In general, titania films are widely used in photovoltaics, contributing to a large surface area available for interfacial reactions, e.g. charge carrier transfer routes. Interpenetrating networks with dimensions matching exciton diffusion lengths in the polymer component of a hybrid organic–inorganic photovoltaic structure are highly desirable. To characterize the fabricated morphology, atomic force microscopy and field-emission scanning electron microscopy are employed in real space. The advanced scattering technique of grazing-incidence small-angle X-ray scattering complements the characterization in reciprocal space. From the obtained results, the sponge-like morphology is verified, a physical description of the morphology with statistical relevance is constructed and the successful complete filling of the network is shown. According to this description, the presented sponge-like titania nanostructures are well suited for use in hybrid organic–inorganic solar cells.