**2.3 Other methods**

172 Solar Cells – New Aspects and Solutions

Chemical vapor deposition is a chemical process used to produce high-purity, highperformance solid materials. The films may be epitaxial, polycrystalline or amorphous depending on the materials and reactor conditions. Chemical vapor deposition has become the major method of film deposition for the semiconductor industry due to its high throughput, high purity, and low cost of operation. Several important factors affect the quality of the film deposited by chemical vapor deposition such as the deposition temperature, the properties of the precursor, the process pressure, the substrate, the carrier

Maruyama (1998) prepared polycrystalline copper oxide thin films at a reaction temperature above 2800C by an atmospheric-pressure chemical vapor deposition method. Copper oxide films were grown by thermal decomposition of the source material with simultaneous reaction with oxygen. At a reaction temperature above 2800C, polycrystalline copper oxide films were formed on the borosilicate glass substrates. Two kinds of films, i.e., Cu2O and CuO, were obtained by adjusting the oxygen partial pressure. Also, there are large differences in color and surface morphology between the CuO and Cu2O films obtained. Author found that the surface morphology and the color of CuO film change with reaction temperature. The CuO film prepared at 3000C is real black, and the film prepared at 5000C is

Medina-Valtierra et al. (2002) coated fiber glass with copper oxides, particularly in the form of 6CuO•Cu2O by chemical vapor deposition method. The authors' work is based on design of an experimental procedure for obtaining different copper phases on commercial fiberglass. Films composed of copper oxides were deposited over fiberglass by sublimation and transportation of (acac)2Cu(II) with a O2 flow (oxidizing agent), resulting in the decomposition of the copper precursor, deposition of Cu0 and Cu0 oxidation on the fiberglass over a short range of deposition temperatures. The copper oxide films on the fiberglass were examined using several techniques such as X-ray diffraction (XRD), visible spectrophotometry, scanning electronic microscopy (SEM) and atomic force microscopy (AFM). The films formed on fiberglass showed three different colors: light brown, dark brown and gray when Cu2O, 6CuO•Cu2O or CuO, respectively, were present. At a temperature of 320°C only cuprous oxide is formed but at a higher temperature of about 340°C cupric oxide is formed. At a temperature of 325°C 6CuO-Cu2O is formed. The decomposition of precursor results in the formation of a zero valent copper which upon

Ottosson et al. (1995) deposited thin films of Cu2O onto MgO (100) substrates by chemical vapour deposition from copper iodide (CuI) and dinitrogen oxide (N2O) at two deposition temperatures, 650°C and 700°C. They found that the pre-treatment of the substrate as well as the deposition temperature had a strong influence on the orientation of the nuclei and the film. For films deposited at 650°C several epitaxial orientations were observed: (100), (110) and (111). The Cu2O(100) was found to grow on a defect MgO(100) surface. When the substrates were annealed at 800°C in N2O for 1 h, the defects in the surface disappeared and only the (110) orientation was developed during the deposition. The films deposited at

Markworth et al. (2001) prepared cuprous oxide (Cu2O) films on single-crystal MgO(110) substrates by a chemical vapor deposition process in the temperature range 690–790°C. Cu2O (*a=*0.4270 nm) and MgO (*a=*0.4213 nm) have cubic crystal structures, and the lattice mismatch between them is 1.4%. Due to good lattice match, chemical stability, and low cost,

700°C (without annealing of the substrates) displayed only the (110) orientation.

**2.2 Chemical vapor deposition** 

gas flow rate and the chamber geometry.

oxidation at different temperature gives different oxides.

grayish black.

Several novel methods for the synthesis of cuprous oxide (i.e. reactive sputtering, sol-gel technique, plasma evaporation,) and some results obtained using these techniques are presented in this part. For example, Santra et al. (1992) deposited thin films of cuprous oxide on the substrates by evaporating metallic copper through a plasma discharge in the presence of a constant oxygen pressure. Authors found two oxide phases before and after annealing treatment of films. Before annealing treatment, cuprous oxide was identified and after annealing in a nitrogen atmosphere, cuprous oxide changes to cupric oxide. The results of optical absorption measurement show that the band gap energies for Cu2O and CuO are 2.1 eV and 1.85 eV, respectively. Thin films prepared in the absence of a reactive gas and plasma were also deposited on glass substrates and in these films the presence of metallic copper was identified.

Ghosh et al. (2000) deposited cuprous oxide and cupric oxide by RF reactive sputtering at different substrate temperatures, namely, at 30, 150 and 3000C. They used atomic force microscopy for examination of the properties of the prepared oxides films related to surface morphology. It was found for the film deposited at 300C, that, 8-10 small grains of size ~40 nm diameter agglomerate together and make a big grain of size ~120 nm. At the temperature of 1500C the grain size becomes 160 nm. The grain size decreases to 90 nm at 3000C. From thickness and deposition time, the deposition rates of the films are found to be 8, 11.5 and 14.0 nm/min for substrate temperature corresponding to 30, 150 and 3000C, respectively. Optical band gap of the films deposited at 30, 150 and 3000C are 1.75, 2.04 and 1.47 eV, respectively. Different phases of copper oxides are found at different temperatures of deposition. CuO phase is obtained in the films prepared at a substrate temperature of 3000C.

Sol gel-like dip technique is a very simple and low-cost method, which requires no sophisticated specialized setup. For example, Armelao et al. (2003) used a sol-gel method to synthesize nanophasic copper oxide thin films on silica slides. They used copper acetate monohydrate as a precursor in ethanol as a solvent. Authors observed formation of CuO crystallites in the samples annealed under inert atmosphere (N2) up to 3 h. A prolonged treatment (5 h) in the same environment resulted in the complete disappearance of tenorite and in the formation of a pure cuprite crystalline phase. Also, under reducing conditions, the formation of CuO, Cu2O and Cu was progressively observed, leading to a mixture of Cu(II) and Cu(I) oxides and metallic copper after treatment at 9000C for 5 h.

Cuprous Oxide as an Active Material for Solar Cells 175

μm in size and preferentially oriented along (100) planes parallel to the substrate surface.

Mukhopadhyay et al. (1992) deposited Cu2O films by galvanostatic method on copper substrates. An alkaline cupric sulphate (about 0.3 M) bath containing NaOH (about 3.2 M) and lactic acid (about 2.3 M) was used as the electrolyte at pH 9. The bath temperatures were 40, 50 and 60°C. XRD analysis indicated a preferred (200) orientation of the Cu2O deposited film. The deposition kinetics was found to be independent of deposition temperature and linear in the thickness range studied (up to about 20 μm). The electrical conductivity of Cu2O films was found to vary exponentially with temperature in the 145-

Golden et al. (1996) found that the reflectance and transmittance of the electrodeposited films of cuprous oxide give a direct band gap of 2.1 eV. Namely, authors used electrodeposition method for obtaining the films of cuprous oxide by reduction of copper (II) lactate in alkaline solution (0.4 M cupric sulfate and 3 M lactic acid). Films were deposited onto either stainless steel or indium tin oxide (ITO) substrates. Deposition temperatures ranged from 25 to 65 °C. They found that the cathodic deposition current was limited by a Schottky-like barrier that forms between the Cu2O and the deposition solution. A barrier height of 0.6 eV was determined from the exponential dependence of the deposition current on the solution temperature. At a solution pH 9 the orientation of the film is [100], while at a solution pH 12 the orientation changes to [111]. The degree of [111]

Siripala et al. (1996) deposited cuprous oxide films on indium tin oxide (ITO) coated glass substrates in a solution of 0.1 M sodium acetate and 1.6 x 10-2 M cupric acetate and the effect of annealing in air has been studied too. Electrodeposition was carried out for 1.5 h in order to obtain films of thicknesses in the order of 1 μm. Authors concluded that the electrodeposited Cu2O films are polycrystalline with grain sizes in the order of 1-2 μm and the bulk crystal structure is simple cubic. They concluded that there is no apparent change in the crystal structure when heat treated in air at or below 300°C. Annealing in air changes the morphology of the surface creating a porous nature with ring shaped structures on the surface. Annealing above 300°C causes decomposition of the yellow-orange colour Cu2O

Zhou & Switzer (1998) deposited Cu2O films on stainless steel disks by the cathodic reduction of copper (II) lactate solution (0.4 M cupric sulfate and 3 M lactic acid). The pH of the bath was between 7 and 12 and the bath temperature was 60°C. Authors concluded that the preferred orientation and crystal shape of Cu2O films change with the bath pH and the applied potential. They obtained pure Cu2O films at bath pH 9 with applied potential

Mahalingam et al. (2000) deposited cuprous oxide thin films on copper and tin-oxide-coated glass substrates by cathodic reduction of alkaline cupric lactate solution (0.45 M CuSO4, 3.25 M lactic acid and 0.1 M NaOH). The deposition was carried out in the temperature range of 60-800C at pH 9. Galvanostatic deposition on tin-oxide-coated glass and copper substrates yields reddish-grey Cu2O films. All the films deposited are found to be polycrystalline having grains in the range of 0.01 - 0.04 μm. The deposition kinetics is found to be linear and independent of the deposition temperature. From the optical absorption measurements, authors found that the deposit of cuprous oxide films has a refractive index of 2.73, direct band gap of 1.99 eV, and extinction coefficient of 0.195. After deposition on temperature of 700C, cuprous oxide films were annealed in air for 30 min at different temperatures (150, 250

texture for the films grown at pH 12 increased with applied current density.

film into a darker film containing black CuO and its complexes with water.

between -0.35 and -0.55 (SCE) or at bath pH 12.

A band gap was found and it was 1.90-1.95 eV.

3000C range with associated activation energy of 0.79 eV.

All the obtained films have nanostructure with an average crystallite size lower than 20 nm.

Nair et al. (1999) deposited cuprous oxide thin films on glass substrate using chemical technique. The glass slides were dipped first in a 1 M aqueous solution of NaOH at the temperature range 50-90°C for 20 s and then in a 1 M aqueous solution of copper complex. X-ray diffraction patterns showed that the films, as prepared, are of cuprite structure with composition Cu2O. Annealing the films in air at 3500C converts these films to CuO. This conversion is accompanied by a shift in the optical band gap from 2.1 eV (direct) to 1.75 eV (direct). The films show p-type conductivity, ~ 5 x 10-4 Ω-1 cm-1 for a film of thickness 0.15 μm.
