**5. Acknowledgment**

The present work was supported in part by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (No.21360346). The author is grateful to Dr. M. Ohnuma [National Institute for Materials Science (NIMS), Tsukuba, Japan], Dr. D. H. Ping (NIMS), and Dr. S. Ohnuma [Research Institute for Electromagnetic Materials (RIEM), Sendai, Japan] for collaborating in this work. The author gratefully acknowledges the valuable comments and continuous encouragement of President T. Masumoto (RIEM). The author is also grateful to Mr. N. Hoshi and Mr. Y. Sato (RIEM) for assisting in the experiments.

### **6. References**

164 Solar Cells – New Aspects and Solutions

Fig. 3.5. Optical absorption spectra for PbSe/ZnSe composite thin films (after Abe, 2011).

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

The present work was supported in part by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (No.21360346). The author is grateful to Dr. M. Ohnuma [National Institute for Materials Science (NIMS), Tsukuba, Japan], Dr. D. H. Ping (NIMS), and Dr. S. Ohnuma [Research Institute for Electromagnetic Materials (RIEM), Sendai, Japan] for collaborating in this work. The author gratefully acknowledges the valuable comments and continuous encouragement of President T. Masumoto (RIEM). The author is

also grateful to Mr. N. Hoshi and Mr. Y. Sato (RIEM) for assisting in the experiments.

**4. Conclusion** 

PbSe/ZnSe composite thin film.

**5. Acknowledgment** 


**8** 

*Serbia* 

**Cuprous Oxide as an** 

Zoran Stević2 and Vesna Grekulović2

**Active Material for Solar Cells** 

Sanja Bugarinović1, Mirjana Rajčić-Vujasinović2,

*1IHIS, Science and Technology Park "Zemun", Belgrade, 2University of Belgrade, Technical faculty in Bor, Bor* 

Growing demand for energy sources that are cleaner and more economical led to intensive research on alternative energy sources such as rechargeable lithium batteries and solar cells, especially those in which the sun's energy is transformed into electrical or chemical. From the ecology point of view, using solar energy does not disturb the thermal balance of our planet, either being directly converted into heat in solar collectors or being transformed into electrical or chemical energy in solar cells and batteries. On the other hand, every kilowatt hour of energy thus obtained replaces a certain amount of fossil or nuclear fuel and mitigates any associated adverse effects known. Solar energy is considered to be one of the

To make the energy of solar radiation converted into electricity, materials that behave as semiconductors are used. Semiconductive properties of copper sulfides and copper oxides, as well as compounds of chalcopyrite type have been extensively investigated (Rajčić-Vujasinović et al., 1994, 1999). One of the important design criteria in the development of an effective solar cell is to maximize its efficiency in converting sunlight to electricity. A photovoltaic cell consists of a light absorbing material which is connected to an external circuit in an asymmetric manner. Charge carriers are generated in the material by the absorption of photons of light, and are driven towards one or other of the contacts by the built-in spatial asymmetry. This light driven charge separation establishes a photo voltage at open circuit, and generates a photocurrent at short circuit. When a load is connected to the external circuit, the cell produces both current and voltage and can do electrical work. Solar technology, thanks to its advantages regarding the preservation of the planetary energy balance, is getting into an increasing number of application areas. So, for example, Rizzo et al. (2010) as well as Stević & Rajčić-Vujasinović (in Press) describe hybrid solar vehicles, while Vieira & Mota (2010) show a rechargeable battery with photovoltaic panels. The high cost of silicon solar cells forces the development of new photovoltaic devices utilizing cheap and non-toxic materials prepared by energy-efficient processes. The Cu–O system has two stable oxides: cupric oxide (CuO) and cuprous oxide (Cu2O). These two oxides are semiconductors with band gaps in the visible or near infrared regions. Copper and copper oxide (metal-semiconductor) are one of the first photovoltaic cells invented (Pollack and Trivich, 1975). Cuprous oxide (Cu2O) is an attractive semiconductor material that could be

most sustainable energy resources for future energy supplies.

**1. Introduction** 

Lopez-Otero, A. (1978). Hot wall epitaxy, *Thin Solid Films*, Vol. 49, pp. 3-57.

