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

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) we can see that the Cu2O work function s= +1,7 eV, ( is the electron affinity of Cu2O) (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 (ITO) and Cu2O layers.

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

Low Cost Solar Cells Based on Cuprous Oxide 65

It should be stressed that this cells showed photovoltaic properties after heat treatment of the films for 3 hrs at 130 0C in a furnace. This possibly results in a decrease of sheet resistance value of the Cu2O films, which was not measured, or in transformation the Cu2O semiconductor from n to p type after heat treatment. Before heating *Voc* and *Isc* were about zero or negative. The serial resistance *Rs* and shunt resistance *Rsh* for all types of the cells

> *Rso* k

ITO/Cu2O/C 10 1,02 76 Ni/Cu2O/Cu 20 8,3 40 SnO2/Cu2O/C 14 3,3 25

The values are given in Table 1. *Rso* is evaluated from the dark characteristics (curve ∆) as *dV/dI* for higher values of forward applied voltage. *Rsh* is evaluated as *dV/dI* from the dark characteristics in reverse direction for lower values of the applied voltage (Olsen & Bohara, 1975). *Rs* is evaluated from the light I-V characteristics and it decreases with illumination. That means that *Rs* is photoresponse. The high series resistance *Rs* and low shunt resistance

Several cell parameters were evaluated from the *I-V* characteristics. Table 2 contains the optimal current and voltage values (*Im* and *Vm*), the open circuit voltage (*Voc*), the short

> *Isc* A

ITO/Cu2O/C 130 180 245 340 28 2,34 2,23 Ni/Cu2O/Cu 28 120 50 270 24 0,70 2,06 SnO2/Cu2O/C 46 90 74 225 25 0,41 2,20

The diode factor was evaluated from the logarithmic plot of the dependence of *Isc* versus *Voc*

*q V*

*kT I*

The performances of the cells depend on the starting surface material, the type of the junction, post deposition treatment and the ohmic contact material. From the I-V characteristics, we can see that the cells are with poor performances, low fill factor FF and

ln *oc sc*

*Voc* mV

*Rs* k *Rsh* k

*sc oc*

10–2 % <sup>n</sup>

, the efficiency

*I V FF I V* 

> *FF* %

(4)

were evaluated from I-V characteristics.

Table 1. Serial and shunt resistance

*Rsh* are one of the reasons for poor performance of the cell.

and diode factor *n*.

A

circuit current (*Isc*) and evaluated values of the fill factor *FF m m*

*Vm* mV

which were measured for different illumination. The diode factor defined as

*n*

Cell type

*sc oc in I V FF P*

Cell type *Im*

Table 2. Cell parameters

is about 2 for all type of the cells.

 

Fig. 14. Profile and rare of SnO2 (ITO)/Cu2O front wall cell structure

The evaporation of nickel has been made with Balzers apparatus under about 5,33 x 10-3 Pa pressure. The optical transmission of the nickel layer was 50% for 550nm wavelength. The total cell active area is 1.0 cm2. Antireflectance coating or any special collection grids have not been deposited.

Fig. 15. Energy band diagram for Cu2O

#### **4. Current-voltage characteristics of the cells**

The current-voltage characteristics of the best ITO/Cu2O/C, Ni/Cu2O/Cu and SnO2/Cu2O/C solar cells have been recorded in darkness and under 100 mW/cm2 illumination, point by point. The light intensity was measured by Solar Meter Mod.776 of Dodge Products. The measurement was carried out using an artificial light source with additional glass filter, 10 mm thick to avoid heating of the cells. I-V characteristics, Fig.16, Fig.17 and Fig.18, were recorded first with periodically illumination of the source (curve ) to avoid the heating of the cell. After that I-V characteristics were recorded with continually illumination (curve ). It is noted that the open circuit voltage *Voc* and the short circuit current density *Isc* decrease with increase in temperature. *Voc* drops because of increase reverse current saturation with temperature because minority carriers increase with increase in temperature. *Isc* decrease because of increase the recombination of the charges.

The evaporation of nickel has been made with Balzers apparatus under about 5,33 x 10-3 Pa pressure. The optical transmission of the nickel layer was 50% for 550nm wavelength. The total cell active area is 1.0 cm2. Antireflectance coating or any special collection grids have

1,7 eV

The current-voltage characteristics of the best ITO/Cu2O/C, Ni/Cu2O/Cu and SnO2/Cu2O/C solar cells have been recorded in darkness and under 100 mW/cm2 illumination, point by point. The light intensity was measured by Solar Meter Mod.776 of Dodge Products. The measurement was carried out using an artificial light source with additional glass filter, 10 mm thick to avoid heating of the cells. I-V characteristics, Fig.16, Fig.17 and Fig.18, were recorded first with periodically illumination of the source (curve ) to avoid the heating of the cell. After that I-V characteristics were recorded with continually illumination (curve ). It is noted that the open circuit voltage *Voc* and the short circuit current density *Isc* decrease with increase in temperature. *Voc* drops because of increase reverse current saturation with temperature because minority carriers increase with increase

in temperature. *Isc* decrease because of increase the recombination of the charges.

III II I

Fig. 14. Profile and rare of SnO2 (ITO)/Cu2O front wall cell structure

not been deposited.

Fig. 15. Energy band diagram for Cu2O

**4. Current-voltage characteristics of the cells** 

*h*

Cu O2 Ag(C)

D

L

*s*

Ag(C)

SnO (ITO) <sup>2</sup>

It should be stressed that this cells showed photovoltaic properties after heat treatment of the films for 3 hrs at 130 0C in a furnace. This possibly results in a decrease of sheet resistance value of the Cu2O films, which was not measured, or in transformation the Cu2O semiconductor from n to p type after heat treatment. Before heating *Voc* and *Isc* were about zero or negative. The serial resistance *Rs* and shunt resistance *Rsh* for all types of the cells were evaluated from I-V characteristics.


Table 1. Serial and shunt resistance

The values are given in Table 1. *Rso* is evaluated from the dark characteristics (curve ∆) as *dV/dI* for higher values of forward applied voltage. *Rsh* is evaluated as *dV/dI* from the dark characteristics in reverse direction for lower values of the applied voltage (Olsen & Bohara, 1975). *Rs* is evaluated from the light I-V characteristics and it decreases with illumination. That means that *Rs* is photoresponse. The high series resistance *Rs* and low shunt resistance *Rsh* are one of the reasons for poor performance of the cell.

Several cell parameters were evaluated from the *I-V* characteristics. Table 2 contains the optimal current and voltage values (*Im* and *Vm*), the open circuit voltage (*Voc*), the short circuit current (*Isc*) and evaluated values of the fill factor *FF m m sc oc I V FF I V* , the efficiency 

$$\left(\eta = FF \frac{I\_{sc} V\_{oc}}{P\_{in}}\right) \text{ and diode factor } n.$$


Table 2. Cell parameters

The diode factor was evaluated from the logarithmic plot of the dependence of *Isc* versus *Voc* which were measured for different illumination. The diode factor defined as

$$m = \frac{q}{kT} \frac{\Delta V\_{\text{oc}}}{\Delta \ln I\_{\text{sc}}} \tag{4}$$

is about 2 for all type of the cells.

The performances of the cells depend on the starting surface material, the type of the junction, post deposition treatment and the ohmic contact material. From the I-V characteristics, we can see that the cells are with poor performances, low fill factor FF and

Low Cost Solar Cells Based on Cuprous Oxide 67

Fig. 18. I-V characteristics for SnO2/Cu2O/C solar cell o-periodically illumination (100

Capacitance as a function of reverse bias voltage at room temperature of Ni/Cu2O/Cu, SnO2/Cu2O/graphite and ITO/Cu2O/graphite solar cells was measured by RCL bridge on

Results for 1/C2 versus reverse bias voltage for all these types of cells are shown in Fig 19, Fig 20 and Fig 21, before annealing (immediately after annealing () and after three months of annealing (). The dependence is straight line. The intercepts of the straight line with x-axis correspond to the barrier height *Vb*. Cu/Cu2O cell showed photovoltaic effect without post deposition heat treatment and their photovoltaic properties are almost unchangeable in time (fig.19). In contrast to this cell, the ITO/Cu2O (fig.20) and SnO2/Cu2O (fig.21) cells no showed photovoltaic properties and no potential barrier was found to exist (Georgieva &Ristov, 2002). Before annealing, the open circuit voltage *Voc* and the short

After annealing of the films for 3 h at 1300C, the devices exhibited good PV properties and the potential barrier excised. But this situation was not stationary. That is another essential factor in the properties of these cells indicating the possibility of chemical changes in

The values of barrier height *Vb* and the open circuit voltage *Voc* upon illumination by an artificial white light source of 100 mW/cm2 for all types of cells are presented in table 3. Also in this table are given their values after aging for 3 months (). Only Cu/Cu2O cell has stationary values of *Vb* and *Voc*. The values of barrier height *Vb* are great then the values of open circuit voltage *Voc*. The great *Vb* gives the great *Voc,* in consent with the photovoltaic

mW/cm2); -continually illumination(100 mW/cm2); ∆-dark characteristic

alternating current (HP type) with built source with 1000 Hz frequency.

**5. Potential barrier height determination of the cells** 

ITO/Cu2O and SnO2/Cu2O junction (Papadimitriou et al.,1981).

circuit current *Isc* were about zero.

theory.

very low efficiency. The high Rs and low Rsh (which is very far from ideally solar cell) are one of the reasons for poor performances. Because of high series resistance Rs, the values of the short circuit current density are very low. By depositing gold instead of nickel or graphite paste, the performance may be improved by decreasing of Rs.

Fig. 16. I-V characteristics for ITO/Cu2O/C solar cell -periodically illumination (100 mW/cm2); -continually illumination(100 mW/cm2); ∆-dark characteristic

Fig. 17. I-V characteristics for Ni/Cu2O/Cu solar cell o-periodically illumination (100 mW/cm2); -continually illumination(100 mW/cm2); ∆-dark characteristic

very low efficiency. The high Rs and low Rsh (which is very far from ideally solar cell) are one of the reasons for poor performances. Because of high series resistance Rs, the values of the short circuit current density are very low. By depositing gold instead of nickel or

Fig. 16. I-V characteristics for ITO/Cu2O/C solar cell -periodically illumination (100

Fig. 17. I-V characteristics for Ni/Cu2O/Cu solar cell o-periodically illumination (100

mW/cm2); -continually illumination(100 mW/cm2); ∆-dark characteristic

mW/cm2); -continually illumination(100 mW/cm2); ∆-dark characteristic

graphite paste, the performance may be improved by decreasing of Rs.

Fig. 18. I-V characteristics for SnO2/Cu2O/C solar cell o-periodically illumination (100 mW/cm2); -continually illumination(100 mW/cm2); ∆-dark characteristic
