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

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

Cuprous Oxide as an Active Material for Solar Cells 171

elimination of CuO from the oxide layer was found to be 10400C. For thermal oxidation carried out below 10400C, Cu2O is formed first and it is then gradually oxidised to CuO depending on the temperature and time of reaction. Pure unannealed Cu2O layers grown thermally in air are observed to exhibit higher resistivity and low hole mobility. A significant reduction in resistivity and an increase in mobility values were obtained by oxidizing the samples in the presence of HCl vapour, followed by annealing at 5000C. Cu2O layers grown in air without the annealing process gave resistivities in the range 2x103 – 3x103 Ωcm. A substantial reduction in the resistivity of the samples was achieved by doping with chlorine during growth and annealing. An average mobility of 75 cm2 V-1 s-1 at room temperature was obtained for eight unannealed Cu2O samples. This average value increased to 130 cm2 V-1 s-1 after doping the samples with chlorine and annealing. The SEM studies indicate that the annealing process results in dense polycrystalline Cu2O layers of increased grain sizes which are appropriate for solar-cell fabrication. Figure 2 presents the micrograph of the surface morphology of a copper foil partially oxidised at 9700C for 2 min. The sample was neither annealed nor etched. The surface shows the black CuO coat formed on the violet-red Cu2O after the oxidation process. The surface morphology is porous and amorphous in nature. The structure formed by this

Jayatissa et al. (2009) prepared cuprous oxide (Cu2O) and cupric oxide (CuO) thin films by thermal oxidation of copper films coated on indium tin oxide (ITO) glass and non-alkaline glass substrates. The formation of Cu2O and CuO was controlled by varying oxidation conditions such as oxygen partial pressure, heat treatment temperature and oxidation time. Authors used X-ray diffraction, atomic force microscopy and optical spectroscopy to determinate the microstructure, crystal direction, and optical properties of copper oxide films. The experimental results suggest that the thermal oxidation method can be employed

to fabricate device quality Cu2O and CuO films that are up to 200–300 nm thick.

Fig. 2. SEM micrograph of unetched and unannealed sample oxidised at 9700C for 2 min

showing CuO coating (Musa et al., 1998)

oxidation process is of the form CuO/Cu2O/Cu/Cu2O/CuO.

al., 1996; Tang et al., 2005; Wang et al., 2007; Wijesundera et al., 2006), plasma evaporation (Santra et al., 1992), sol–gel-like dip technique (Armelao et al., 2003; Ray, 2001) etc. Each of these methods has its own advantages and disadvantages. In most of these studies, a mixture of phases of Cu, CuO and Cu2O is generally obtained and this is one of the nagging problems for non-utilizing Cu2O as a semiconductor (Papadimitropoulos et al., 2005). Pure Cu2O films can be obtained by oxidation of copper layers within a range of temperatures followed by annealing for a small period of time.

Results obtained using different methods, especially thermal oxidation and chemical vapor evaporation for synthesis of cuprous oxide thin films, are presented in next sections, with special emphasis on the electrochemical synthesis of cuprous oxide.

### **2.1 Thermal oxidation**

Polycrystalline cuprous oxide can be formed by thermal oxidation of copper under suitable conditions (Rai, 1988). The procedure involves the oxidation of high purity copper at an elevated temperature (1000–15000C) for times ranging from few hours to few minutes depending on the thickness of the starting material (for total oxidation) and the desired thickness of Cu2O (for partial oxidation). Process is followed by high-temperature annealing for hours or even days.

Sears & Fortin (1984) synthesized cuprous oxide films on copper substrates to a thickness of a few micrometers, using both thermal and anodic oxidation techniques. The measurements carried out on the anodic oxide layers indicate an unwanted but inevitable incorporation of other compounds into the Cu2O. They found that the photovoltaic properties of the resulting Cu2O/Cu backwall cells depend critically on the copper surface preparation, as well as on the specific conditions of oxidation. Backwall cells of the thermal variety with thicknesses down to 3 μm do not quite yet approach the performance of the best Cu2O front cells, but are much simpler to grow. Serious difficulties with shorting paths in the case of thermally grown oxide and with the purity of the Cu2O in the anodic case will have to be solved before a solar cell with an oxide layer thickness in the 1.5 to 2 μm range can be produced.

Musa et al. (1998) produced the cuprous oxide by thermal oxidation and studied its physical and electrical properties. The oxidation was carried out at atmospheric pressure in a hightemperature tube furnace. During this process the copper foils were heated in the range of 200 to 1050°C. Cu2O has been identified to be stable at limited ranges of temperature and oxygen pressure. It has also been indicated that during oxidation, Cu2O is formed first, and after a sufficiently long oxidation time CuO is formed (Roos & Karlson, 1983, as cited Musa et al., 1998). It has been suggested that the probable reactions that could account for the presence of CuO in layers oxidised below 1000 °C are:

$$2\text{Cu}\_2\text{O} + \text{O} \rightarrow 4\text{CuO} \tag{2}$$

$$\text{Cu}\_2\text{O} \to \text{CuO} + \text{Cu} \tag{3}$$

The unwanted CuO can be removed using an etching solution consisting of FeCl, HCl, and 8 M HNO3 containing NaCl. The results of the oxidation process as deduced from both XRD and SEM studies indicate that the oxide layers resulting from oxidation at 10500C consist entirely of Cu2O. Those grown below 10400C gave mixed oxides of Cu2O and CuO. It was observed that in general the lower the temperature of oxidation, the lower the amount of Cu2O was present in the oxide. Thermodynamic considerations indicate that the limiting temperature for the

al., 1996; Tang et al., 2005; Wang et al., 2007; Wijesundera et al., 2006), plasma evaporation (Santra et al., 1992), sol–gel-like dip technique (Armelao et al., 2003; Ray, 2001) etc. Each of these methods has its own advantages and disadvantages. In most of these studies, a mixture of phases of Cu, CuO and Cu2O is generally obtained and this is one of the nagging problems for non-utilizing Cu2O as a semiconductor (Papadimitropoulos et al., 2005). Pure Cu2O films can be obtained by oxidation of copper layers within a range of temperatures

Results obtained using different methods, especially thermal oxidation and chemical vapor evaporation for synthesis of cuprous oxide thin films, are presented in next sections, with

Polycrystalline cuprous oxide can be formed by thermal oxidation of copper under suitable conditions (Rai, 1988). The procedure involves the oxidation of high purity copper at an elevated temperature (1000–15000C) for times ranging from few hours to few minutes depending on the thickness of the starting material (for total oxidation) and the desired thickness of Cu2O (for partial oxidation). Process is followed by high-temperature annealing

Sears & Fortin (1984) synthesized cuprous oxide films on copper substrates to a thickness of a few micrometers, using both thermal and anodic oxidation techniques. The measurements carried out on the anodic oxide layers indicate an unwanted but inevitable incorporation of other compounds into the Cu2O. They found that the photovoltaic properties of the resulting Cu2O/Cu backwall cells depend critically on the copper surface preparation, as well as on the specific conditions of oxidation. Backwall cells of the thermal variety with thicknesses down to 3 μm do not quite yet approach the performance of the best Cu2O front cells, but are much simpler to grow. Serious difficulties with shorting paths in the case of thermally grown oxide and with the purity of the Cu2O in the anodic case will have to be solved before a solar cell with an oxide layer thickness in the 1.5 to 2 μm range can be

Musa et al. (1998) produced the cuprous oxide by thermal oxidation and studied its physical and electrical properties. The oxidation was carried out at atmospheric pressure in a hightemperature tube furnace. During this process the copper foils were heated in the range of 200 to 1050°C. Cu2O has been identified to be stable at limited ranges of temperature and oxygen pressure. It has also been indicated that during oxidation, Cu2O is formed first, and after a sufficiently long oxidation time CuO is formed (Roos & Karlson, 1983, as cited Musa et al., 1998). It has been suggested that the probable reactions that could account for the

2Cu2O + O2→ 4CuO (2)

 Cu2O → CuO + Cu (3) The unwanted CuO can be removed using an etching solution consisting of FeCl, HCl, and 8 M HNO3 containing NaCl. The results of the oxidation process as deduced from both XRD and SEM studies indicate that the oxide layers resulting from oxidation at 10500C consist entirely of Cu2O. Those grown below 10400C gave mixed oxides of Cu2O and CuO. It was observed that in general the lower the temperature of oxidation, the lower the amount of Cu2O was present in the oxide. Thermodynamic considerations indicate that the limiting temperature for the

followed by annealing for a small period of time.

presence of CuO in layers oxidised below 1000 °C are:

**2.1 Thermal oxidation** 

for hours or even days.

produced.

special emphasis on the electrochemical synthesis of cuprous oxide.

elimination of CuO from the oxide layer was found to be 10400C. For thermal oxidation carried out below 10400C, Cu2O is formed first and it is then gradually oxidised to CuO depending on the temperature and time of reaction. Pure unannealed Cu2O layers grown thermally in air are observed to exhibit higher resistivity and low hole mobility. A significant reduction in resistivity and an increase in mobility values were obtained by oxidizing the samples in the presence of HCl vapour, followed by annealing at 5000C. Cu2O layers grown in air without the annealing process gave resistivities in the range 2x103 – 3x103 Ωcm. A substantial reduction in the resistivity of the samples was achieved by doping with chlorine during growth and annealing. An average mobility of 75 cm2 V-1 s-1 at room temperature was obtained for eight unannealed Cu2O samples. This average value increased to 130 cm2 V-1 s-1 after doping the samples with chlorine and annealing. The SEM studies indicate that the annealing process results in dense polycrystalline Cu2O layers of increased grain sizes which are appropriate for solar-cell fabrication. Figure 2 presents the micrograph of the surface morphology of a copper foil partially oxidised at 9700C for 2 min. The sample was neither annealed nor etched. The surface shows the black CuO coat formed on the violet-red Cu2O after the oxidation process. The surface morphology is porous and amorphous in nature. The structure formed by this oxidation process is of the form CuO/Cu2O/Cu/Cu2O/CuO.

Jayatissa et al. (2009) prepared cuprous oxide (Cu2O) and cupric oxide (CuO) thin films by thermal oxidation of copper films coated on indium tin oxide (ITO) glass and non-alkaline glass substrates. The formation of Cu2O and CuO was controlled by varying oxidation conditions such as oxygen partial pressure, heat treatment temperature and oxidation time. Authors used X-ray diffraction, atomic force microscopy and optical spectroscopy to determinate the microstructure, crystal direction, and optical properties of copper oxide films. The experimental results suggest that the thermal oxidation method can be employed to fabricate device quality Cu2O and CuO films that are up to 200–300 nm thick.

Fig. 2. SEM micrograph of unetched and unannealed sample oxidised at 9700C for 2 min showing CuO coating (Musa et al., 1998)

Cuprous Oxide as an Active Material for Solar Cells 173

MgO single crystals are particularly effective substrates for the growth of Cu2O thin films. Authors found that the Cu2O films grow by an island-formation mechanism on MgO substrate. Films grown at 690°C uniformly coat the substrate except for micropores between grains. However, at a growth temperature of 790°C, an isolated, three-dimensional island

Kobayashi et al. (2007) investigated the high-quality Cu2O thin films grown epitaxially on MgO (110) substrate by halide chemical vapor deposition under atmospheric pressure. CuI in a source boat was evaporated at a temperature of 883 K, and supplied to the growth zone of the reactor by N2 carrier gas, and O2 was also supplied there by the same carrier gas. Partial pressure of CuI and O2 were adjusted independently to 1.24 x 10−2 and 1.25 x 103 Pa. They found that the optical band gap energy of Cu2O film calculated from absorption spectra is 2.38 eV. The reaction of CuI and O2 under atmospheric pressure yields high-

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

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

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.

morphology develops.

quality Cu2O films.

**2.3 Other methods** 

copper was identified.

3000C.

### **2.2 Chemical vapor deposition**

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 gas flow rate and the chamber geometry.

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 grayish black.

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 oxidation at different temperature gives different oxides.

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

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, MgO single crystals are particularly effective substrates for the growth of Cu2O thin films. Authors found that the Cu2O films grow by an island-formation mechanism on MgO substrate. Films grown at 690°C uniformly coat the substrate except for micropores between grains. However, at a growth temperature of 790°C, an isolated, three-dimensional island morphology develops.

Kobayashi et al. (2007) investigated the high-quality Cu2O thin films grown epitaxially on MgO (110) substrate by halide chemical vapor deposition under atmospheric pressure. CuI in a source boat was evaporated at a temperature of 883 K, and supplied to the growth zone of the reactor by N2 carrier gas, and O2 was also supplied there by the same carrier gas. Partial pressure of CuI and O2 were adjusted independently to 1.24 x 10−2 and 1.25 x 103 Pa. They found that the optical band gap energy of Cu2O film calculated from absorption spectra is 2.38 eV. The reaction of CuI and O2 under atmospheric pressure yields highquality Cu2O films.
