Aqueous chemical solution deposition of doped titanium dioxide films as Transparent Conducting Oxide

The development of transparent conducting oxides (TCOs) has led to the evolution of a variety of optoelectronic devices, such as photovoltaics, transparent thin film transistors and light emitting diodes. Owing to its excellent electrical and optical properties, indium tin oxide is the predominant TCO used in these kind of devices. There exists, however, a strong demand for inexpensive, and efficient alternatives. (Doped) titanium dioxide (TiO2) is such a promising TCO compound. The material is characterized by a wide band gap (3,2 eV), a high permittivity and large free electron mass, allowing for a high mobility combined with an optimal transparency in the visible spectral range. By doping TiO2 with group 5 elements, such as vanadium (V), niobium (Nb) or tantalum (Ta), additional charge carriers are introduced, improving the electrical conductivity of the material. The development of reliable synthesis routes for mass production at reasonable cost is a current challenge for the commercialization of TCO materials. In this respect, solution based processing and deposition is an appealing alternative to standard physical deposition methods that require a high vacuum. Available literature on solution based synthesis of TiO2 is quite extensive, with numerous examples of sol-gel, hydrothermal and precipitation methods. However, the vast majority of these publications focus on the application of titania in devices for gas sensing or (photo)catalysis. To our knowledge, reports on solution processing of anatase based TCOs are scarce. In the current work, we present time the preparation of group 5 doped anatase films by aqueous chemical solution deposition in view of TCO applications. The effect of the dopant on the crystallization, microstructure and electrical properties of the resulting films is examined. In all cases, fine grained polycrystalline films are obtained that display a uniform and homogeneous coverage of the substrates. All films, both doped and undoped, have a transparency in the visible range that is higher than 80%. TiO2:Nb annealed in a reducing atmosphere at high temperature demonstrates the best electrical properties. In fact, with 0,28 Ohm cm, these niobium doped samples show the lowest reported resistivity for a solution deposited anatase TCO so far. Although still higher in comparison with films prepared by physical deposition, these results demonstrate that, after optimization, solution based synthesis of anatase TCOs can provide an alternative to standard deposition techniques