**1. Introduction**

166 Solar Cells – New Aspects and Solutions

Macfarlane, G. G. McLean, T. P. Quarrington, J. E. & Roberts, V. (1957). Fine structure in the absorption-edge spectrum of Ge, *Physical Review* , Vol. 108, pp. 1377-1383. Maeda, Y. Tsukamoto, N. Yazawa, Y. Kanemitsu, Y. & Masumoto, Y. (1991). Visible

Mills, K. C. (1974). *Thermodynamic data for inorganic sulphide, selenides and Tellurides*.

Nelson, J. B. & Riley, D. P. (1945). An experimental investigation of extrapolation methods in

Nill, K. W. Sreauss, A. J. & Blum, F. A. (1973). Tunable cw Pb0.98Cd0.02S diode lasers emitting

Ohnuma, S. Fujimori, H. Mitani, S. & Masumoto, T. (1996). High-frequency magnetic

Oleinik, G.S. Mizetskii, P.A. & Nizkova, A.I. (1982). Nature of the interaction between lead and

Scherrer, P. (1918). Bestimmung der Größe und der inneren Struktur von Kolloidteilchen

Shannon, R. D. (1976). Revised effective ionic radii and systematic studies of interatomic

Takahasi, Y. Kitamura, K. Iyi, N. & Inoue, S. (2006). Phase-stability and photoluminescence of BaTi(Si, Ge)3O9, *Journal of Ceramic Society of Japan*, Vol. 114, pp. 313-317. Tang, H. Prasad, K. Sanjinès, R.P. Schmid, E. & Lévy F. (1994). Electrical and optical properties of TiO2 anatase thin films, *Journal of Appied Physics*, Vol. 75, pp. 2042-2047. Theis, D. (1977). Wavelength modulated reflectivity spectra of ZnSe and ZnS from 2.5 to 8 eV,

Vegard, L. (1921). Die Konstitution der Mischkristalle und die Raumerfüllung der Atome, *Z.* 

Weller, H.. (1991). Quantum sized semiconcuctor particles in solution in modified layers, *Berichte der Bunsengeselleschaft Physical Chemistry,* Vol. 95, pp. 1361-1365. Wise, F. W. (2000). Lead salts quantum dots: the limit of strong confinement, *Accounts of* 

Xue, D. (2009). A template-free solution method based on solid-liquid interface reaction towards dendritic PbSe nanostructures, *Modern Physics Letters B*, Vol. 23, pp. 3817-3823. Yang, W. Wan, F. Chen, S. Jiang, C. (2009). Hydrothermal growth and application of ZnO

Zaban, A. Micic, O. I. Gregg, B. A. & Nozik, A. J., (1998). Photosensitization of nanoporus TiO2

Zemel, J. N. Jensen, J. D. & Schoolar, R. B. (1965). Electrical and optical properties of epitaxial films of PbS, PbSe, PbTe, and SnTe, *Physical Review*, Vol. 140, pp. A330-A342. Zhu, G. Su, F. Lv, T. Pan, L. Sun, Z. (2010). Au nanoparticles as interfacial layer for CdS quantum dot-sensitized solar cells, *Nanoscale Research Letters*, Vol. 5, pp. 1749-1754.

electrodes with InP quantum dots, *Langmuir*, Vol. 14, pp. 3153-3156.

nanowire films with ZnO and TiO2 buffer layers in dye-sensitized solar cells,

photoluminescence of Ge microcrystals embeddded inSiO2, *Applied Physics Letters*,

the derivation of accurate unit-cell dimensions of crystals, *Proceedings of Physical* 

at 3.5 m: Applications to ultrahigh-resolution spectroscopy, *Applied Physics Letters*

properties in metal-nonmetal granular films, *Journal of Applied Physics,* Vol. 79, pp.

distances in halides and chalcogenides, *Acta Crystallography, Sect. A*, Vol. 32, pp. 751-767.

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

Nozik, A. J. (2002). Quantum dot solar cells, *Physics E*, Vol. 14, pp.115-120.

zic chalcogenides, *Inorganic Materials*, Vol. 18, pp. 734-735.

*The Merck Index* (Merck & Co, New Jersey, 1968) 8th ed., p. 1054.

*Physica Status Solidi* (B), Vol. 79, pp.125-130.

*Chemical Research*, Vol. 33, pp. 773-780.

*Nanoscale Research Letters*, Vol. 4**,** pp. 1486-1492.

*Phys*. Vol. 5, pp. 17-26.

mittels Röntgenstrahlen, *Göttinger Nachrichten*, Vol. 2, pp. 98-100.

Vol. 59, pp. 3168-3170.

*Society* Vol. 57, pp. 160.

Vol. 22, pp. 677-679.

Butterworth.

5130-5135.

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 most sustainable energy resources for future energy supplies.

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

Cuprous Oxide as an Active Material for Solar Cells 169

*r D* <sup>2</sup> <sup>2</sup>

(1)

where r is the grain radius and D is the diffusion coefficient of the carrier (Rothenberger et al., 1985, as cited in Tang et al., 2005). If the grains radius is reduced from micrometer dimensions to nanometer dimensions, the opportunities for recombination can be dramatically reduced. The preparation of nano crystalline Cu2O thin films is a key to improving the performance of solar application devices. Nanotechnologies in this area, therefore, given their full meaning. In the last decade the scientific literature, abounds with

works again showing progress in research related to obtaining the cuprous oxide.

(http://www.webelements.com/compounds/copper/dicopper\_oxide.html)

and investigations of Cu2O thin films using electrochemical techniques.

**2. Methodologies used for the synthesis of cuprous oxide** 

This chapter presents an overview of recent literature concerning cuprous oxide synthesis and application as an active material in solar cells, as well as our own results of synthesis

The optical and electrical properties of absorber materials in solar cells are key parameters which determine the performance of solar cells. Hence, it is necessary to tune these properties properly for high efficient device. Electrical properties of Cu2O, such as carrier mobility, carrier concentration, and resistivity are very dependent on preparation methods. Cuprous oxide thin films have been prepared by various techniques like thermal oxidation (Jayatissa et al., 2009; Musa et al., 1998; Sears & Fortin, 1984), chemical vapor deposition (Kobayashi et al. 2007; Maruyama, 1998; Medina-Valtierra et al., 2002; Ottosson et al., 1995; Ottosson & Carlsson, 1996), anodic oxidation (Fortin & Masson, 1982; Sears and Fortin, 1984; Singh et al., 2008), reactive sputtering (Ghosh et al., 2000), electrodeposition (Briskman, 1992; Daltin et al., 2005; Georgieva & Ristov, 2002; Golden et al., 1996; Liu et al., 2005; Mizuno et al., 2005; Rakhshani et al., 1987, Rakhshani & Varghese, 1987; Santra et al., 1999; Siripala et

Fig. 1. Crystal structure of Cu2O

used as anode material in thin film lithium batteries (Lee et al, 2004) as well as in solar cells (Akimoto et al., 2006; Musa et al., 1998; Nozik et al., 1978; Tang et al., 2005). Its semiconductor properties and the emergence of photovoltaic effect were discovered by Edmond Becquerel 1839th1 experimenting in the laboratory of his father, Antoine-César Becquerel.

Cu2O is a p-type semiconductor with a direct band gap of 2.0–2.2 eV (Grozdanov, 1994) which is suitable for photovoltaic conversion. Tang et al. (2005) found that the band gap of nanocrystalline Cu2O thin films is 2.06 eV, while Siripala et al. (1996) found that the deposited cuprous oxide exhibits a direct band gap of 2.0 eV, and shows an n-type behavior when used in a liquid/solid junction. Han & Tao (2009) found that n-type Cu2O deposited in a solution containing 0.01 M copper acetate and 0.1 M sodium acetate exhibits higher resistivity than p-type Cu2O deposited at pH 13 by two orders of magnitude. Other authors, like Singh et al. (2008) estimated the band gap of prepared Cu2O nanothreads and nanowires to be 2.61 and 2.69 eV, which is larger than the direct band gap (2.17 eV) of bulk Cu2O (Wong & Searson, 1999). The higher band gap can be attributed to size effect of the present nanostructures. Thus the increase of band gap as compared to the bulk can be understood on the basis of quantum size effect which arises due to very small size of nanothreads and nanowires in one-dimension.

Cuprous oxide attracts the most interest because of its high optical absorption coefficient in the visible range and its reasonably good electrical properties (Musa et al., 1998). Its advantages are, in fact, relatively low cost and low toxicity. Except for a thin film that can be electrochemically formed on different substrates (steel, TiO2), cuprous oxide can be obtained in the form of nano particles with all the benefits offered by nano-technology (Daltin et al., 2005; Zhou & Switzer, 1998). Nanomaterials exhibit novel physical properties and play an important role in fundamental research.

The unit cell of Cu2O with a lattice constant of 0.427 nm is composed of a body centered cubic lattice of oxygen ions, in which each oxygen ion occupies the center of a tetrahedron formed by copper ions (Xue & Dieckmann, 1990). The Cu atoms arrange in a fcc sublattice, the O atoms in a bcc sublattice. The unit cell contains 4 Cu atoms and 2 O atoms. One sublattice is shifted by a quarter of the body diagonal. The space group is Pn3m, which includes the point group with full octahedral symmetry. This means particularly that parity is a good quantum number. Figure 1 shows the crystal lattice of Cu2O. Molar mass of Cu2O is 143.09 g/mol, density is 6.0 g/cm3 and its melting and boiling points are 1235°C and 1800°C, respectively. Also, it is soluble in acid and insoluble in water.

Cuprous oxide (copper (I) oxide Cu2O) is found in nature as cuprite and formed on copper by heat. It is a red color crystal used as a pigment and fungicide. Rectifier diodes based on this material have been used industrially as early as 1924, long before silicon became the standard. Cupric oxide (copper(II) oxide CuO) is a black crystal. It is used in making fibers and ceramics, gas analyses and for Welding fluxes. The biological property of copper compounds takes important role as fungicides in agriculture and biocides in antifouling paints for ships and wood preservations as an alternative of Tributyltin compounds.

In solar cells, Cu2O has not been commonly used because of its low energy conversion efficiency which results from the fact that the light generated charge carriers in micron-sized Cu2O grains are not efficiently transferred to the surface and lost due to recombination. For randomly generated charge carriers, the average diffusion time from the bulk to the surface is given by:

<sup>1</sup> http://pvcdrom.pveducation.org/MANUFACT/FIRST.HTM

used as anode material in thin film lithium batteries (Lee et al, 2004) as well as in solar cells (Akimoto et al., 2006; Musa et al., 1998; Nozik et al., 1978; Tang et al., 2005). Its semiconductor properties and the emergence of photovoltaic effect were discovered by Edmond Becquerel

Cu2O is a p-type semiconductor with a direct band gap of 2.0–2.2 eV (Grozdanov, 1994) which is suitable for photovoltaic conversion. Tang et al. (2005) found that the band gap of nanocrystalline Cu2O thin films is 2.06 eV, while Siripala et al. (1996) found that the deposited cuprous oxide exhibits a direct band gap of 2.0 eV, and shows an n-type behavior when used in a liquid/solid junction. Han & Tao (2009) found that n-type Cu2O deposited in a solution containing 0.01 M copper acetate and 0.1 M sodium acetate exhibits higher resistivity than p-type Cu2O deposited at pH 13 by two orders of magnitude. Other authors, like Singh et al. (2008) estimated the band gap of prepared Cu2O nanothreads and nanowires to be 2.61 and 2.69 eV, which is larger than the direct band gap (2.17 eV) of bulk Cu2O (Wong & Searson, 1999). The higher band gap can be attributed to size effect of the present nanostructures. Thus the increase of band gap as compared to the bulk can be understood on the basis of quantum size effect which arises due to very small size of

Cuprous oxide attracts the most interest because of its high optical absorption coefficient in the visible range and its reasonably good electrical properties (Musa et al., 1998). Its advantages are, in fact, relatively low cost and low toxicity. Except for a thin film that can be electrochemically formed on different substrates (steel, TiO2), cuprous oxide can be obtained in the form of nano particles with all the benefits offered by nano-technology (Daltin et al., 2005; Zhou & Switzer, 1998). Nanomaterials exhibit novel physical properties and play an

The unit cell of Cu2O with a lattice constant of 0.427 nm is composed of a body centered cubic lattice of oxygen ions, in which each oxygen ion occupies the center of a tetrahedron formed by copper ions (Xue & Dieckmann, 1990). The Cu atoms arrange in a fcc sublattice, the O atoms in a bcc sublattice. The unit cell contains 4 Cu atoms and 2 O atoms. One sublattice is shifted by a quarter of the body diagonal. The space group is Pn3m, which includes the point group with full octahedral symmetry. This means particularly that parity is a good quantum number. Figure 1 shows the crystal lattice of Cu2O. Molar mass of Cu2O is 143.09 g/mol, density is 6.0 g/cm3 and its melting and boiling points are 1235°C and

Cuprous oxide (copper (I) oxide Cu2O) is found in nature as cuprite and formed on copper by heat. It is a red color crystal used as a pigment and fungicide. Rectifier diodes based on this material have been used industrially as early as 1924, long before silicon became the standard. Cupric oxide (copper(II) oxide CuO) is a black crystal. It is used in making fibers and ceramics, gas analyses and for Welding fluxes. The biological property of copper compounds takes important role as fungicides in agriculture and biocides in antifouling

In solar cells, Cu2O has not been commonly used because of its low energy conversion efficiency which results from the fact that the light generated charge carriers in micron-sized Cu2O grains are not efficiently transferred to the surface and lost due to recombination. For randomly generated charge carriers, the average diffusion time from the bulk to the surface is

paints for ships and wood preservations as an alternative of Tributyltin compounds.

1800°C, respectively. Also, it is soluble in acid and insoluble in water.

1 http://pvcdrom.pveducation.org/MANUFACT/FIRST.HTM

1839th1 experimenting in the laboratory of his father, Antoine-César Becquerel.

nanothreads and nanowires in one-dimension.

important role in fundamental research.

given by:

$$
\pi = r^2 \int \pi^2 \,\mathrm{D} \tag{1}
$$

where r is the grain radius and D is the diffusion coefficient of the carrier (Rothenberger et al., 1985, as cited in Tang et al., 2005). If the grains radius is reduced from micrometer dimensions to nanometer dimensions, the opportunities for recombination can be dramatically reduced. The preparation of nano crystalline Cu2O thin films is a key to improving the performance of solar application devices. Nanotechnologies in this area, therefore, given their full meaning. In the last decade the scientific literature, abounds with works again showing progress in research related to obtaining the cuprous oxide.

(http://www.webelements.com/compounds/copper/dicopper\_oxide.html)

Fig. 1. Crystal structure of Cu2O

This chapter presents an overview of recent literature concerning cuprous oxide synthesis and application as an active material in solar cells, as well as our own results of synthesis and investigations of Cu2O thin films using electrochemical techniques.
