**10. Conclusion**

The importance of heat-treatment in the MPM was demonstrated through fabrication of thin films of anatase and rutile with unprecedentedly high photoreactivities. This is due to a photoreactive mechanism via O deficiency in the oxide thin films. Based on the excellent miscibilities of the molecular precursors in the SrTiO3 thin film fabrication, heat-treatment was shown to be an essential step. It eliminates organic ligands in the precursor metal complexes and provides important functions to the metal oxides in the chemical fabrication of Ag-nanoparticle/titania composite thin films with high conductivity.

The chemical fabrication of the first p-type Cu2O transparent thin film was also recently achieved using the MPM, although the heat-treatment of the spin-coated substrates resulted in the deposition of a large amount of powder on the substrates in previous sol–gel studies [96]. The electrical and optical properties of the Cu2O thin film fabricated using the MPM were consistent with those of similar thin films fabricated using physical procedures. It is very interesting that the charge on copper changes stepwise from +2 to +1 through 0 during heat-treatment of the precursor film involving a Cu–EDTA complex in an Ar gas flow. Based on these results, a transparent dry-type solar cell of area 20 × 20 mm2 with a combination of Vis-responsive anatase thin films was examined. This film is mentioned in the section **Vis-responsive anatase thin film fabricated using the MPM**. The structure of the solar cell was FTO electrode/n-Vis-responsive anatase/p-Cu2O/conductive polymer/Ag on a glass substrate, and the photovoltaic nature of the solar cell could be successfully measured under the light from a solar simulator. Thus, the present MPM is useful for fabricating Vis-responsive dry solar cells. The MPM coupled with heat-treatment of various precursor films allows transparent thin films of metal oxides such as Co3O4, ZnO, Ga2O3, and ZrO2 *etc.* to be examined and fabricated on glass and/or metallic substrates. The MPM has great potential as a fundamental technology for thin film fabrication by chemical processes.
