Electron Transport and Back Reaction in Nanocrystalline TiO₂ Films Prepared by Hydrothermal Crystallization

The electron transport and back reaction in nanocrystalline TiO₂ films prepared at low temperature using a new hydrothermal crystallization method on conductive glass and plastic substrates have been investigated by intensity modulated photocurrent spectroscopy (IMPS) and intensity modulated photovoltage spectroscopy (IMVS). The hydrothermal method enables the preparation of crack-free TiO₂ thick films and at the same time enhances the electron transport compared to films prepared by low-temperature sintering, providing a path towards efficient photoelectrode materials for flexible dye-sensitized solar cells. UV/ozone treatment of the films enables the removal of residual organics left from the hydrothermal preparation process. Since these organics represent surface states that mediate the back reaction of electrons, the electron lifetime is increased by their removal, while the electron transport is not enhanced significantly. High-temperature sintering of the hydrothermally prepared films leads to both a removal of the surface states and a significant enhancement of the electron transport properties. Interestingly, the electron lifetimes are not changed by high-temperature sintering, since faster electron transport and less surface states have opposite effects on the back reaction process. These films showed improved electron transport properties and efficiencies even if compared with films prepared by conventional high-temperature methods, which shows the high potential for the further development of the hydrothermal method.