Increasing energy consumption and its environmental impacts make it necessary to look for alternative energy sources. Solar energy as huge energy source that can cover the terms sustainability is considered as a favorable alternative. Optoelectronic devices like solar cells and photoelectrochemical cells are in very high interest to provide the energy that we need. They can convert solar energy, as a sustainable energy source, to electricity and fuel. Transition metal oxides (TMOs) due to high chemical stability, abundance, facile production and low cost are favorable materials to be used in these optoelectronic applications. In TMOs, d orbitals electrons contribute in forming bonds that gives special magnetic, electronic and geometric characteristics to these materials. They can be synthesized with different types of chemical and physical deposition methods.

The electronic properties of TMOs varies from metallic characteristics to electrical insulators. Transition metal oxide semiconductors (TMOSs) are mostly used in optoelectronic devices. Tuning the properties of TMOSs like, composition, morphology, dimensions, crystal structure, improves the performance of the optoelectronic devices. The light absorption, charge carrier mobility, the time scale between charge injection, regeneration and recombination processes are some of the properties critical to exploitation of TMOSs in solar cells and solar fuel technology.

In this thesis, we explore the use of nanostructured TMOSs in all-oxide solar cells, photodetectors and photoelectrochemical cells. 1-dimentional heterojunctions of n-type transparent semi-conductive metal oxides (ZnO and TiO₂) in conjunction to p-type light absorbing semi-conductive metal oxides (Cu₂O and Co₃O₄) have been tested in all-oxide photodetectors and solar cells.  It is shown that the 1-dimentional nanostructured geometry (nanowires, nanotubes) improves the charge separation and charge transport properties. Increasing the surface to volume aspect ratio in nanowires and nanotubes improves the light absorption compare to the thin film geometry. Our ZnO-Cu₂O core-shell nanowire photodetector is the fastest visible light photodetector reported till now. Mesoporous NiO photocathode, sensitized with a biomimetic FeFe-catalyst and coumarin C343 dye, was tested in a photoelectrochemical cell for hydrogen production. This system is the first solar fuel device based on a biomimetic FeFe-catalyst and it shows a Faradic efficiency of 50% in hydrogen production. Cobalt catalysts have higher Faradic efficiency but their performance due to hydrolysis in low pH condition is limited. Nanostructured hematite/magnetite film as a photoanode was tested in a photoelectrochemical cell for water splitting. This hybrid electrode improved the photoactivity of the photoelectrochemical cell for water splitting. The main mechanism for the improvement of the functional properties relies with the role of the magnetite phase, which improves the charge carrier mobility of the composite system, compared to pure hematite, which acts as good light absorbing semiconductor.