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

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

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 circuit current *Isc* were about zero.

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

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 theory.

Low Cost Solar Cells Based on Cuprous Oxide 69

Table 3**.** Values of barrier height *Vb* and open circuit voltage *Voc* for all types of cells after

Evaluation of the barrier height, before annealing (); after annealing (); after 3 months of annealing ().

Until now, we have made Schottky barrier solar cells. As we could not improve their efficiency and their stability, we decided to make heterojunction p-n solar cells based on a ptype Cu2O thin films. We selected ZnO as an n-type semiconductor. ZnO is a transparent oxide that is widely used in many different applications, including thin film solar cells. The p-n junction was fabricated by potentiostatic deposition of the ZnO layer onto SnO2 conducting glass with a sheet resistance of 14 and potentiostatic deposition of Cu2O onto

ZnO/Cu2O heterojunction solar cells were made by consecutive cathodic electrodeposition of ZnO and Cu2O onto tin oxide covered glass substrates. Zinc oxide (ZnO) was cathodically deposited on a conductive glass substrate covered with SnO2 as cathode by a potentiostatic method (Dalchiele et al.,2001, Izaki et al.,1998, Ng-Cheng-Chin et al.,1998). Conducting glass slides coated with SnO2 films are commercial samples. The electrolysis takes place in a

Cell type Cu/Cu2O ITO/Cu2O SnO2/Cu2O

*Voc* (mV) 310 249 118

annealing and after aging for 3 months ().

Fig. 21**.** 1/C2 vs applied voltage of SnO2/Cu2O cell.

**6. ZnO/Cu2O heterojunction solar cells** 

**6.1 Electrochemical depositing of ZnO** 

ZnO, Fig.22.

(mV) 370 150 60

(mV) 310 105 30

*Vb* 

*Voc*

*Vb* (mV) (mV) 378 330 180

Evaluation of the barrier height, before annealing (); after annealing (); after 3 months of annealing ().

Fig. 19**.** 1/C2 vs applied voltage of Cu/Cu2O cell.

Evaluation of the barrier height, before annealing (); after annealing (); after 3 months of annealing ().

Fig. 20. 1/C2 vs applied voltage of ITO/Cu2O cell.

Evaluation of the barrier height, before annealing (); after annealing (); after 3 months of annealing ().

Evaluation of the barrier height, before annealing (); after annealing (); after 3 months of annealing ().

Fig. 19**.** 1/C2 vs applied voltage of Cu/Cu2O cell.

Fig. 20. 1/C2 vs applied voltage of ITO/Cu2O cell.


Table 3**.** Values of barrier height *Vb* and open circuit voltage *Voc* for all types of cells after annealing and after aging for 3 months ().

Evaluation of the barrier height, before annealing (); after annealing (); after 3 months of annealing (). Fig. 21**.** 1/C2 vs applied voltage of SnO2/Cu2O cell.
