A process for producing at least one epitaxial, nanostructured thin film of metal oxide and/or a hydrated metal oxide on at least one substrate placed in a container and a substrate/thin film thereof, comprises placing the substrate at any location in the container contg. a water soln., such that a covering area of the substrate being covered by the soln., contg. at least one metal salt and addnl. can contain other salts, pH-regulator(s), complexant(s), or mols., whereby from the substrate particles of a metal oxide of the metal from the metal salt is grown, forming the film on the covering area of the substrate. For a substrate (F-doped Sn oxide) for use as a solar cell, the oriented particles in the film have a photoactive component attached thereon, the fibers being arranged substantially perpendicular to the substrate.