**Low Cost Solar Cells Based on Cuprous Oxide**

Verka Georgieva, Atanas Tanusevski1 and Marina Georgieva

*Faculty of Electrical Engineering and Information Technology, 1Institute of Physics, Faculty of Natural Sciences and Mathematics, The "St. Cyril & Methodius"University, Skopje, R. of Macedonia* 

#### **1. Introduction**

54 Solar Cells – Thin-Film Technologies

Sőderstrőm T., F. –J. Haug, V. Terrazzoni-Daudrix, and C. Ballif, J. (2008). Optimization of

Yablonovitch E. and G. Cody. (1982). Intensity enhancement in textured optical sheets for solar cells. *IEEE Trans. Electron. Devices ED.,* Vol. 29, pp. 300., ISSN: 0018-9383 Zhou Dayu and Rana Biswas. (2008). Photonic crystal enhanced light-trapping in thin film

solar cells. *J. Appl. Phys.,* Vol. 103, pp. 093102. , ISSN: 1089-7550

103, pp. 114509-1., ISSN: 1089-7550

amorphous silicon thin film solar cells for flexible photovoltaics. *J. Appl. Phys*., Vol.

The worldwide quest for clean and renewable energy sources has encouraged large research activities and developments in the field of solar cells. In recent years, considerable attention has been devoted to the development of low cost energy converting devices. One of the most interesting products of photoelectric researches is the semiconductor cuprous oxide cell. As a solar cell material, cuprous oxide -Cu2O, has the advantages of low cost and great availability. The potential for Cu2O using in semiconducting devices has been recognized since, at least, 1920. Interest in Cu2O revived during the mid seventies in the photovoltaic community (Olsen et al.,1982). Several primary characteristics of Cu2O make it potential material for use in thin film solar cells: its non-toxic nature, a theoretical solar efficiency of about 9-11%, an abundance of copper and the simple and inexpensive process for semiconductor layer formation. Therefore, it is one of the most inexpensive and available semiconductor materials for solar cells. In addition to everything else, cuprous oxide has a band gap of 2.0 eV which is within the acceptable range for solar energy conversion, because all semiconductors with band gap between 1 eV and 2 eV are favorable material for photovoltaic cells (Rai, 1988).

A variety of techniques exist for preparing Cu2O films on copper or other conducting substrates such as thermal, anodic and chemical oxidation and reactive sputtering. Particularly attractive, however, is the electrodeposition method because of its economy and simplicity for deposition either on metal substrates or on transparent conducting glass slides coated with highly conducting semiconductors, such as indium tin oxide (ITO), SnO2, In2O3 etc. This offers the possibility of making back wall or front wall cells as well. We have to note that electrochemical preparation of cuprous oxide (Cu2O) thin films has reached considerable attention during the last years.

Electrodeposition method of Cu2O was first developed by Stareck (Stareck, 1937). It has been described by Rakhshani (Jayanetti & Dharmadasa, 1996, Mukhopadhyay et al.,1992, Rakhshani et al.1987, Rakhshani et al., 1996). In this work, a method of simple processes of electrolysis has been applied.

Electrochemical deposition technique is an simple, versatile and convenient method for producing large area devices. Low temperature growth and the possibility to control film thickness, morphology and composition by readily adjusting the electrical parameters, as well as the composition of the electrolytic solution, make it more attractive. At present,

Low Cost Solar Cells Based on Cuprous Oxide 57

time were changed. The Cu2O films were obtained under following conditions: 1) current density *j* = 1,26 mA/cm2, voltage between the electrodes *V* = 0,3 - 0,38 V and deposition time *t* = 55 min. Close to the value of current density, deposition time and Faraday's law, the

The potentiostatic mode was used for deposition the Cu2O films on glass coated with SnO2 prepared by spray pyrolisis method of 0.1 M water solution of SnCl2 complexes by NH4F . The applied potential difference between anode and cathode was constant. It was found that suitable value is *V* = 0,5 to 0,6 V. The deposition current density at the beginning was dependent on the surface resistance of the cathode. For a fixed value of the potential, the current decreased with increasing film thickness. The film thickness was dependent on deposition current density *j.* For current density of about 1 mA/cm2 at the beginning and deposition time of about 2 h, the film thickness was 5-6 m approximately. The thickness of

*s*, where *m* is the mass

Cu2O oxide layer thickness was estimated to be about 5 m.

deposited film was determined using a weighting method, as *d* = *m/*

m. All deposited films had reddish to reddish-gray color.

The deposition of Cu2O on a commercial glass coated with ITO was carried out under constant current density. The ITO/Cu2O films was obtained under the following conditions: current density *j* = 0,57 mA/cm2, voltage between the electrodes *V* = 1,1 - 1,05 V and deposition time *t* = 135 min. The Cu2O oxide layer thickness was estimated to be about 5

The structure of the films was studied by X – ray diffraction, using CuKradiation with a wavelength of 0.154 nm. The Bragg angle of 2was varied between 200 and 500. The XRD spectrums of the films samples, deposited on copper, glass coated by SnO2 and glass coated by ITO are shown in Fig.1, Fig.2 and Fig.3 respectively. It was found that all films are polycrystalline and chemically pure Cu2O with no traces of CuO. XRD peaks corresponded to Cu2O and the substrate material. The XRD spectrums indicate a strong Cu2O peak with

The surface morphology of the films was studied by a scanning electron microscope JEOL model JSM 35 CF. Fig.4, Fig.5 and Fig.6 show the scanning electron micrographs of Cu2O films deposited on copper, glass coated by SnO2 and glass coated by ITO respectively. The photographs indicate a polycrystalline structure. The grains are very similar to each other in size and in shape. They are about 1 m and less in size for the film deposited on copper, 1-2

The optical band-gap is an essential parameter for semiconductor material, especially in photovoltaic conversion. In this work it was determined using the transmittance spectrums of the films. The optical transmission spectrums were recording on Hewlett-Packard (model 8452 A) spectrophotometer in the spectral range 350-800 nm wavelength. Thin layers of a transparent Cu2O were preparing for the optical transmission spectrums recording. The optical transmission spectrum of about 1,5 m thick Cu2O film deposited on glass coated with SnO2 is presented in Fig.7. There are two curves, one (1) recorded before annealing

m for the film deposited on SnO2 and about 1 m for the film deposited on ITO.

, of 5.9 g/cm3 was used.

and s is the surface of the film. A density

**2.1.2 Structural properties** 

(200) preferential orientation.

**2.1.3 Morphological properties**

**2.1.4 Optical band-gap energy determination**

and the other one (2) after annealing of the film for 3h at 1300C.

electrodeposition of binary semiconductors, especially thin films of the family of wide bend gap II-IV semiconductors (as is ZnO), from aqueous solutions is employed in the preparation of solar cells. A photovoltaic device composed of a p-type semiconducting cuprous (I) oxide (Cu2O) and n-type zinc oxide (ZnO) has attracted increasing attention as a future thin film solar cell, due to a theoretical conversion efficiency of around 18% and an absorption coefficient higher than that of a Si single crystal (Izaki et al. 2007)

Therefore, thin films of cuprous oxide (Cu2O) have been made using electrochemical deposition technique. Cuprous oxide was electrodeposited on copper substrates and onto conducting glass coated with tin oxide (SnO2), indium tin oxide (ITO) and zinc oxide (ZnO). Optimal conditions for high quality of the films were requested and determined. The qualitative structure of electrodeposited thin films was studied by x-ray diffraction (XRD) analysis. Their surface morphology was analyzed with scanning electronic microscope (SEM). The optical band gap values Eg were determined. To complete the systems Cu/Cu2O, SnO2/Cu2O, ITO/Cu2O and ZnO/Cu2O as solar cells an electrode of graphite or silver paste was painted on the rear of the Cu2O. Also a thin layer of nickel was vacuum evaporated on the oxide layer. The parameters of the solar cells, such the open circuit voltage (*Voc)*, the short circuit current (*Isc*), the fill factor (*FF*), the diode quality factor (*n*), serial (*Rs*) and shunt resistant (*Rsh*) and efficiency () were determined. The barrier height (*Vb*) was determined from capacity-voltage characteristics.

Generally is accepted that the efficiency of the cells cannot be much improved. (Minami et al.,2004). But we successed to improve the stability of the cells, using thin layer of ZnO, making heterojunctions Cu2O based cells.

#### **2. Structural, morphological and optical properties of electrodeposited films of cuprous oxide**

#### **2.1 Experimental**

#### **2.1.1 Preparation of the films**

A very simple apparatus was used for electrodeposition. It is consisted of a thermostat, a glass with solution, two electrodes (cathode and anode) and a standard electrical circuit for electrolysis. The deposition solution contained 64 g/l anhydrous cupric sulphate (CuSO4), 200 ml/l lactic acid (C3H6O3) and about 125 g/l sodium hydroxide (NaOH), (Rakhshani et al.1987, Rakhshani & Varghese, 1987). Cupric sulphate was dissolved first in distilled water giving it a light blue color. Then lactic acid was added. Finally, a sodium hydroxide solution was added, changing the color of the solution to dark blue with pH = 9. A copper clad for printed circuit board, with dimension 50 m, 2.5 7 cm2, was used as the anode. Copper clad and conducting glass slides coated with ITO and SnO2 were used as a cathode. Experience shows that impurities (such as dirt, finger prints, etc.) on the starting surface material have a significant impact on the quality of the cuprous oxide. Therefore, mechanical and chemical cleaning of the electrodes, prior to the cell preparation, is essential. Copper boards were polished with fine emery paper. After that, they were washed by liquid detergent and distilled water. The ITO substrates were washed by liquid detergent and rinsed with distilled water. The SnO2 substrates were soaked in chromsulphuric acid for a few hours and rinsed with distilled water. Before using all of them were dried.

Thin films of Cu2O were electrodeposited by cathodic reduction of an alkaline cupric lactate solution at 600 C . The deposition was carried out in the constant current density regime. The deposition parameters, as current density, voltage between the electrodes and deposition

electrodeposition of binary semiconductors, especially thin films of the family of wide bend gap II-IV semiconductors (as is ZnO), from aqueous solutions is employed in the preparation of solar cells. A photovoltaic device composed of a p-type semiconducting cuprous (I) oxide (Cu2O) and n-type zinc oxide (ZnO) has attracted increasing attention as a future thin film solar cell, due to a theoretical conversion efficiency of around 18% and an

Therefore, thin films of cuprous oxide (Cu2O) have been made using electrochemical deposition technique. Cuprous oxide was electrodeposited on copper substrates and onto conducting glass coated with tin oxide (SnO2), indium tin oxide (ITO) and zinc oxide (ZnO). Optimal conditions for high quality of the films were requested and determined. The qualitative structure of electrodeposited thin films was studied by x-ray diffraction (XRD) analysis. Their surface morphology was analyzed with scanning electronic microscope (SEM). The optical band gap values Eg were determined. To complete the systems Cu/Cu2O, SnO2/Cu2O, ITO/Cu2O and ZnO/Cu2O as solar cells an electrode of graphite or silver paste was painted on the rear of the Cu2O. Also a thin layer of nickel was vacuum evaporated on the oxide layer. The parameters of the solar cells, such the open circuit voltage (*Voc)*, the short circuit current (*Isc*), the fill factor (*FF*), the diode quality factor (*n*), serial (*Rs*) and shunt resistant (*Rsh*) and efficiency () were determined. The barrier height

Generally is accepted that the efficiency of the cells cannot be much improved. (Minami et al.,2004). But we successed to improve the stability of the cells, using thin layer of ZnO,

**2. Structural, morphological and optical properties of electrodeposited films** 

A very simple apparatus was used for electrodeposition. It is consisted of a thermostat, a glass with solution, two electrodes (cathode and anode) and a standard electrical circuit for electrolysis. The deposition solution contained 64 g/l anhydrous cupric sulphate (CuSO4), 200 ml/l lactic acid (C3H6O3) and about 125 g/l sodium hydroxide (NaOH), (Rakhshani et al.1987, Rakhshani & Varghese, 1987). Cupric sulphate was dissolved first in distilled water giving it a light blue color. Then lactic acid was added. Finally, a sodium hydroxide solution was added, changing the color of the solution to dark blue with pH = 9. A copper clad for printed circuit board, with dimension 50 m, 2.5 7 cm2, was used as the anode. Copper clad and conducting glass slides coated with ITO and SnO2 were used as a cathode. Experience shows that impurities (such as dirt, finger prints, etc.) on the starting surface material have a significant impact on the quality of the cuprous oxide. Therefore, mechanical and chemical cleaning of the electrodes, prior to the cell preparation, is essential. Copper boards were polished with fine emery paper. After that, they were washed by liquid detergent and distilled water. The ITO substrates were washed by liquid detergent and rinsed with distilled water. The SnO2 substrates were soaked in chromsulphuric acid for a

few hours and rinsed with distilled water. Before using all of them were dried.

Thin films of Cu2O were electrodeposited by cathodic reduction of an alkaline cupric lactate solution at 600 C . The deposition was carried out in the constant current density regime. The deposition parameters, as current density, voltage between the electrodes and deposition

absorption coefficient higher than that of a Si single crystal (Izaki et al. 2007)

(*Vb*) was determined from capacity-voltage characteristics.

making heterojunctions Cu2O based cells.

**of cuprous oxide 2.1 Experimental** 

**2.1.1 Preparation of the films** 

time were changed. The Cu2O films were obtained under following conditions: 1) current density *j* = 1,26 mA/cm2, voltage between the electrodes *V* = 0,3 - 0,38 V and deposition time *t* = 55 min. Close to the value of current density, deposition time and Faraday's law, the Cu2O oxide layer thickness was estimated to be about 5 m.

The potentiostatic mode was used for deposition the Cu2O films on glass coated with SnO2 prepared by spray pyrolisis method of 0.1 M water solution of SnCl2 complexes by NH4F . The applied potential difference between anode and cathode was constant. It was found that suitable value is *V* = 0,5 to 0,6 V. The deposition current density at the beginning was dependent on the surface resistance of the cathode. For a fixed value of the potential, the current decreased with increasing film thickness. The film thickness was dependent on deposition current density *j.* For current density of about 1 mA/cm2 at the beginning and deposition time of about 2 h, the film thickness was 5-6 m approximately. The thickness of deposited film was determined using a weighting method, as *d* = *m/s*, where *m* is the mass and s is the surface of the film. A density , of 5.9 g/cm3 was used.

The deposition of Cu2O on a commercial glass coated with ITO was carried out under constant current density. The ITO/Cu2O films was obtained under the following conditions: current density *j* = 0,57 mA/cm2, voltage between the electrodes *V* = 1,1 - 1,05 V and deposition time *t* = 135 min. The Cu2O oxide layer thickness was estimated to be about 5 m. All deposited films had reddish to reddish-gray color.

#### **2.1.2 Structural properties**

The structure of the films was studied by X – ray diffraction, using CuKradiation with a wavelength of 0.154 nm. The Bragg angle of 2was varied between 200 and 500. The XRD spectrums of the films samples, deposited on copper, glass coated by SnO2 and glass coated by ITO are shown in Fig.1, Fig.2 and Fig.3 respectively. It was found that all films are polycrystalline and chemically pure Cu2O with no traces of CuO. XRD peaks corresponded to Cu2O and the substrate material. The XRD spectrums indicate a strong Cu2O peak with (200) preferential orientation.

#### **2.1.3 Morphological properties**

The surface morphology of the films was studied by a scanning electron microscope JEOL model JSM 35 CF. Fig.4, Fig.5 and Fig.6 show the scanning electron micrographs of Cu2O films deposited on copper, glass coated by SnO2 and glass coated by ITO respectively. The photographs indicate a polycrystalline structure. The grains are very similar to each other in size and in shape. They are about 1 m and less in size for the film deposited on copper, 1-2 m for the film deposited on SnO2 and about 1 m for the film deposited on ITO.

#### **2.1.4 Optical band-gap energy determination**

The optical band-gap is an essential parameter for semiconductor material, especially in photovoltaic conversion. In this work it was determined using the transmittance spectrums of the films. The optical transmission spectrums were recording on Hewlett-Packard (model 8452 A) spectrophotometer in the spectral range 350-800 nm wavelength. Thin layers of a transparent Cu2O were preparing for the optical transmission spectrums recording. The optical transmission spectrum of about 1,5 m thick Cu2O film deposited on glass coated with SnO2 is presented in Fig.7. There are two curves, one (1) recorded before annealing and the other one (2) after annealing of the film for 3h at 1300C.

Low Cost Solar Cells Based on Cuprous Oxide 59

Fig. 4. Micrograph obtained from a scanning electron microscope of Cu2O deposited on

Fig. 5. Micrograph obtained from a scanning electron microscope of Cu2O deposited on SnO

Fig. 6. Micrograph obtained from a scanning electron microscope of Cu2O deposited on ITO

copper

Fig. 1. X-ray diffraction spectrum of a Cu2O film deposited on copper

Fig. 2. X-ray diffraction spectrum of a Cu2O film deposited on SnO2

Fig. 3. X-ray diffraction spectrum of a Cu2O film deposited on ITO

Fig. 1. X-ray diffraction spectrum of a Cu2O film deposited on copper

Fig. 2. X-ray diffraction spectrum of a Cu2O film deposited on SnO2

Fig. 3. X-ray diffraction spectrum of a Cu2O film deposited on ITO

Fig. 4. Micrograph obtained from a scanning electron microscope of Cu2O deposited on copper

Fig. 5. Micrograph obtained from a scanning electron microscope of Cu2O deposited on SnO

Fig. 6. Micrograph obtained from a scanning electron microscope of Cu2O deposited on ITO

Low Cost Solar Cells Based on Cuprous Oxide 61

where d is the film's thickness determined using weighing method, and *A* is the

<sup>100</sup> ln (%)

The values of the optical absorption coefficient n dependence on wavelength are shown

absorbance determined from the values of transmittance, *T*(%) , using the equation

*A*

in Fig. 9 for Cu2O/SnO2 film and Fig. 10 for Cu2O/ITO film

Fig. 9. Coefficient vs wavelength for Cu2O/SnO2 film

Fig. 10. Coefficient vs wavelength for Cu2O/ITO film

*A d* 

, (2)

*<sup>T</sup>* . (3)

Fig. 7. Optical transmission spectrum of a 1,5 m thick Cu2O/SnO2 film

Fig. 8. Optical transmission spectrum of a 0,9 m thick Cu2O/ITO film

We can see that there is no difference in the spectrums. The absorption boundary is unchangeable. That means that the band gap energy is unchangeable with or without annealing. The little difference comes from different points recording, because the thickness of the film is not uniform. The transmittance spectrum of about 0,9 m thick Cu2O film, deposited on ITO, is presented in Fig. 8.

For determination of the optical band gap energy *E*<sup>g</sup> , the method based on the relation

$$
abla \nu = A(\ln \nu - E\_{\g})^{n/2},\tag{1}$$

has been used, where n is a number that depends on the nature of the transition. In this case its value was found to be 1 (which corresponds to direct band to band transition) because that value of n yields the best linear graph of *h* )2 versus *h*

The values of the absorption coefficient were calculated from the equation

Fig. 7. Optical transmission spectrum of a 1,5 m thick Cu2O/SnO2 film

Fig. 8. Optical transmission spectrum of a 0,9 m thick Cu2O/ITO film

deposited on ITO, is presented in Fig. 8.

that value of n yields the best linear graph of

We can see that there is no difference in the spectrums. The absorption boundary is unchangeable. That means that the band gap energy is unchangeable with or without annealing. The little difference comes from different points recording, because the thickness of the film is not uniform. The transmittance spectrum of about 0,9 m thick Cu2O film,

For determination of the optical band gap energy *E*<sup>g</sup> , the method based on the relation

 

has been used, where n is a number that depends on the nature of the transition. In this case its value was found to be 1 (which corresponds to direct band to band transition) because

> *h*

The values of the absorption coefficient were calculated from the equation

/2 ( ), *<sup>n</sup> g*

)2 versus *h*

*h Ah E* (1)

$$
\alpha = \frac{A}{d} \,\, \, \, \, \tag{2}
$$

where d is the film's thickness determined using weighing method, and *A* is the absorbance determined from the values of transmittance, *T*(%) , using the equation

$$A = \ln \frac{100}{T(\%)}.\tag{3}$$

The values of the optical absorption coefficient n dependence on wavelength are shown in Fig. 9 for Cu2O/SnO2 film and Fig. 10 for Cu2O/ITO film

Fig. 9. Coefficient vs wavelength for Cu2O/SnO2 film

Fig. 10. Coefficient vs wavelength for Cu2O/ITO film

Low Cost Solar Cells Based on Cuprous Oxide 63

determined from the spectral characteristics of the cells made with electrodeposited Cu2O films. The value of the energy band gap of Cu2O/ITO is little higher than the value of

Fig.11 shows that there is no different in optical band gap energy determined from the curve plotted before annealing and from the curve plotted after annealing. Also, Fig.11 and Fig.12 show that there is no shape absorption boundary in the small energy range of the photons.

The optical band-gap of the films was determined using the transmitance spectrums. It was

Cu2O Schottky barrier solar cells can be fabricated in two configurations, the so called back wall and front wall structures. By vacuum evaporating a thin layer of nickel on the Cu2O film, photovoltaic cells have been completed as back wall type cells (Fig.13), or by depositing carbon or silver paste on the rear of the Cu2O layers, photovoltaic cells have been completed as front wall type cells (Fig.14). Nickel, carbon or silver paste are utilized to form ohmic contacts with cuprous oxide films. From the energy band diagram (Fig.15)

(Olsen et al.,1982, Papadimitriou et al.,1990). That means that Cu2O will make ohmic contact with metals characterized with work function higher than 4,9 eV, as are Ni, C. Gold and silver essentially form ohmic contacts. A carbon or silver back contact was chosen because of simplicity and economy of the cell preparation. The rectifying junction exists at the interface between the cooper and Cu2O layers in the case of back wall cells. In the case of front wall cells the rectifying junction exists at the interface between the SnO2

s= +1,7 eV, ( is the electron affinity of Cu2O)

Cu2O/SnO2 film. The reason is maybe different size of the grains.

Probably defects and structural irregularities are present in the films.

found to be 2,33 eV for Cu2O/SnO2 film and 2,38 eV for Cu2O/ITO.

**3. Preparation of the Cu2O Schottky barrier solar cells** 

Fig. 13. Profile and face of Cu/Cu2O back wall cell structure

we can see that the Cu2O work function

(ITO) and Cu2O layers.

Fig.11 and Fig.12 show *h*2 versus *h* dependence for the Cu2O/SnO2 film and Cu2O/ITO film corresponding. The intersection of the straight line with the *h*axis determines the optical band gap energy *Eg*. It was found to be 2,33 eV for Cu2O/SnO2 film and 2,38 eV for Cu2O/ITO. They are higher than the value of 2 eV given in the literature and obtained for Cu2O polycrystals. These values are in good agreement with band gaps

Fig. 11. Graphical determination of the optical band gap energy for Cu2O/SnO2 film

( x - before annealing; · - after annealing)

Fig. 12. Graphical determination of the optical band gap energy for Cu2O/ITO film

determines the optical band gap energy *Eg*. It was found to be 2,33 eV for Cu2O/SnO2 film and 2,38 eV for Cu2O/ITO. They are higher than the value of 2 eV given in the literature and obtained for Cu2O polycrystals. These values are in good agreement with band gaps

dependence for the Cu2O/SnO2 film and

axis

Cu2O/ITO film corresponding. The intersection of the straight line with the *h*

Fig. 11. Graphical determination of the optical band gap energy for Cu2O/SnO2 film

*h*/eV

Fig. 12. Graphical determination of the optical band gap energy for Cu2O/ITO film

Fig.11 and Fig.12 show

( x - before annealing; · - after annealing)

*h*

2 versus *h*

determined from the spectral characteristics of the cells made with electrodeposited Cu2O films. The value of the energy band gap of Cu2O/ITO is little higher than the value of Cu2O/SnO2 film. The reason is maybe different size of the grains.

Fig.11 shows that there is no different in optical band gap energy determined from the curve plotted before annealing and from the curve plotted after annealing. Also, Fig.11 and Fig.12 show that there is no shape absorption boundary in the small energy range of the photons. Probably defects and structural irregularities are present in the films.

The optical band-gap of the films was determined using the transmitance spectrums. It was found to be 2,33 eV for Cu2O/SnO2 film and 2,38 eV for Cu2O/ITO.
