**4. Conclusion**

This chapter has been focused on one-step physical synthesis of Ge/TiO2 and PbSe/ZnSe composite thin film as candidate materials for quantum dot solar cell. It should be pointed out in the Ge/TiO2 that the anatase-dominant structure appears in the restricted composition range as a result of optimization of Ge chip numbers and additional oxygen ratio in argon. Furthermore, their optical absorption edge is obviously shifted to vis-NIR region. The solubility range of Ge in the Ti1-*x*Ge*x*O2 powder is estimated to be 0.23 0.01 at 1273 K. In addition, their optical absorption edge is obviously shifted to the UV region as *x* increases. Thus, the Ti1-*x*Ge*x*O2 solid solution does not exhibit the vis-NIR absorption. In contrast, SAXS and HREM results clearly indicated that the Ge nanogranules were embedded in the matrix. The size was sufficiently small to appear the quantum size effect. Thus, the both valuable characteristics are simultaneously retained in the Ge/TiO2 composite films. In the PbSe/ZnSe, the solubility limit of Pb in ZnSe is quite narrow, less than 1mol% in the film form, indicating that an atmosphere near thermal equilibrium is achieved in the apparatus used. Elemental mapping indicates that isolated PbSe nanocrystals are dispersed in the ZnSe matrix. The optical absorption edge shifts toward the lower-photon-energy region as the PbSe content increases. In particular, onset absorption can be confirmed at approximately 1.0eV with 16mol%PbSe, favorably covering the desirable energy region for high conversion efficiency. The insolubility material system and the HWD technique enable a one-step synthesis of PbSe/ZnSe composite thin film.
