**1. Introduction**

88 Solar Cells – Thin-Film Technologies

Rostalsky M. & Mueller J. (2001). High rate deposition and electron beam recrystallization of

Shah A. V., Schade H., Vanecek M., Meier J., Vallat-Sauvain E., Wyrsch N., Kroll U., Droz

pp.84-87, ISSN: 0040-6090

ISSN: 1099-159X

silicon films for solar cells. *Thin Solid Films*, Vol.401, No. 1-2, (December 2001),

C. & Bailat J. (2004). Thin-film silicon solar cell technology. *Progress in Photovoltaics: Research and Applications*. Vol.12, No. 2-3, (March 2004), pp.113-142,

> Solar energy is considered as the most promising alternative energy source to replace environmentally distractive fossil fuel. However, it is a challenging task to develop solar energy converting devices using low cost techniques and environmentally friendly materials. Environmentally friendly cuprous oxide (Cu2O) is being studied as a possible candidate for photovoltaic applications because of highly acceptable electrical and optical properties. Cu2O has a direct band gap of 2 eV (Rakhshani, 1986; Siripala et al., 1996), which lies in the acceptable range of window material for photovoltaic applications. It is a stoichiometry defect type semiconductor having a cubic crystal structure with lattice constant of 4.27 Å (Ghijsen et al., 1988; Wijesundera et al., 2006). The theoretical conversion efficiency limit for Cu2O based solar cells is about 20% [5].

> Thermal oxidation was a most widely used method for the preparation of Cu2O in the early stage. It gives a low resistive, p-type polycrystalline material with large grains for photovoltaic applications. It was found that Cu2O grown at high temperature has high leakage-current due to the shorting paths created during the formation of the material, and it causes low conversion efficiencies. Therefore it was focused to prepare Cu2O at low temperature, which may provide better characteristics in this regard. Among the various Cu2O deposition techniques (Olsen et al., 1981; Aveline & Bonilla, 1981; Fortin & Masson, 1981; Roos et al., 1983; Sears & Fortin, 1984; Rakhshani, 1986; Rai, 1988; Santra et al., 1992; Musa et al., 1998; Maruyama, 1998; Ivill et al., 2003; Hames & San, 2004; Ogwa et al., 2005), electrodeposition (Siripala & Jayakody, 1986, Siripala et al., 1996; Rakhshani & Varghese, 1987a, 1988b; Mahalingam et al., 2004; Tang et al., 2005; Wijesundera et al., 2006) is an attractive one because of its simplicity, low cost and low-temperature process and on the other hand the composition of the material can be easily adjusted leading to changes in physical properties. Most of the techniques produce p-type conducting thin films. Many theoretical and experimental studies (Guy, 1972; Pollack & Trivich, 1975; Kaufman & Hawkins, 1984; Harukawa et al., 2000; Wright & Nelson, 2002; Paul et al., 2006) have been revealed that the Cu vacancies originate the p-type conductivity. However, electrodeposition (Siripala & Jayakody, 1986, Siripala et al., 1996; Wijesundera et al., 2000; Wijesundera et al., 2006) of Cu2O thin films in a slightly acidic aqueous baths produce n-type conductivity. Further it has been reported that the origin of this n-type behavior is due to oxygen vacancies and*/*or additional copper atoms. Recently, Garutara *et al*. (2006) carried out the photoluminescence (PL) characterisation for the electrodeposited n-type

Electrodeposited Cu2O Thin Films for Fabrication of CuO/Cu2O Heterojunction 91

Potentiostatic electrodeposition of Cu2O thin films on Ti substrates can be investigated using a three electrode electrochemical cell containing an aqueous solution of sodium acetate and cupric acetate. Cupric acetate are used as Cu2+ source while sodium acetate are added to the solution making complexes releasing copper ions slowly into the medium allowing a uniform growth of Cu2O thin films. The counter electrode is a platinum plate and reference electrode is saturated calomel electrode (SCE). Growth parameters (ionic concentrations, temperature, pH of the bath, and deposition potential domain) involved in the potentiostatic electrodeposition of the Cu2O thin films can be determined by the method of

voltammetric curves were obtained in a solution containing 0.1 M sodium acetate with the various cupric acetate concentrations, while temperature, pH and stirring speed of the baths were maintained at values of 55 oC, 6.6 (normal pH of the bath) and 300 rev./min respectively. Curve a) in Fig. 1 is without cupric acetate and curves b), c) and d) are cupric acetate concentrations of 0.25 mM, 1 mM and 10 mM respectively. Significant current increase can not be observed in absence with cupric acetate and cathodic peaks begin to form with the introduction of Cu2+ ions into the electrolyte. Two well defined cathodic peaks are resulted at –175 mV and –700 mV Vs SCE due to the presence of cupric ions in the electrolyte and these peaks shifted slightly to the anodic side at higher cupric acetate concentrations. First cathodic peak at –175 mV Vs SCE attributes to the formation of Cu2O

2Cu2+ + H2O + 2e- Cu2O + 2H+ Second cathodic peak at –700 mV Vs SCE attributes to the formation of Cu on the substrate

Cu2+ + 2e- Cu By examining the working electrode, it can be observed that the electrodeposition of deposits on the substrate is possible in the entire potential range. However, as revealed by the curves in Fig. 1, at higher concentrations the peaks are getting broader and therefore the formation of Cu and Cu2O simultaneously is possible at intermediate potentials (curve d of Fig. 1). The deposition current slightly increases and the peaks are slightly shifted to the

Fig. 2 shows the dependence of the voltammetric curves on the pH of the deposition bath. It is seen that cathodic peak corresponding to the Cu deposition is shifted anodically by about 500 mV and cathodic peak corresponding to the Cu2O deposition is shifted anodically by about 100 mV. This clearly indicates that acidic bath condition favours the deposition of copper over the Cu2O deposition and the possibility of simultaneous deposition of Cu and Cu2O even at lower cathodic potentials. This is further investigated in the following

The potential domain of the first cathodic peak gives the possible potentials for the electrodeposition of Cu2O films while second cathodic peak evidence the possible potential domain for the electrodeposition of Cu films. It is evidence that Cu2O can be electrodeposited in the range of 0 to -300 mV Vs SCE and Cu can be electrodeposited in the range of -700 to -900 mV Vs SCE. The potential domains of the electrodepostion of Cu2O and Cu are independent of the Cu2+ ion concentration and the temperature of the bath. However, the deposition rate is increased with the increase in the concentration or the

positive potential side as increasing the bath temperature range of 25 C to 65 C.

voltommograms.

sections.

temperature of the bath.

on the substrate according to the following reaction.

according to the following reaction.

polycrystalline Cu2O, and confirmed that the n-type conductivity is due to the oxygen vacancies created in the lattice. This n-type conductivity of Cu2O is very important in developing low cost thin film solar cells because the electron affinity of Cu2O is comparatively high. This will enable to explore the possibility of making heterojunction with suitable low band gap p-type semiconductors for application in low cost solar cells.

Most of the properties of the electrodeposited Cu2O were reported to be similar to those of the thermally grown film (Rai, 1988). The electrodeposition of Cu2O is carried out potentiostatically or galvanostatically (Rakhshani & Varghese, 1987a, 1988b; Mahalingam et al., 2000; Mahalingam et al., 2002). Dependency of parameters (concentrations, pH, temperature of the bath, deposition potential with deposits) had been investigated by several research groups (Zhou & Switzer, 1998; Mahalingam et al., 2002; Tang et al., 2005; Wijesundera et al., 2006). The results showed that electrodeposition is very good tool to manipulate the deposits (structure, properties, grain shape and size, etc) by changing the parameters. Various electrolytes such as cupric sulphate + ethylene glycol alkaline solution, cupric sulphate aqueous solution, cupric sulphate + lactic acid alkaline aqueous solution, cupric nitrate aqueous solution and sodium acetate+ cupric acetate aqueous solution, have been reported in the electrodeposition of Cu2O.

Cu2O-based heterojunctions of ZnO*/*Cu2O (Herion et al., 1980; Akimoto et al., 2006), CdO*/*Cu2O (Papadimitriou et al., 1981; Hames & San, 2004), ITO*/*Cu2O (Sears et al., 1983), TCO*/*Cu2O (Tanaka et al., 2004), and Cu2O*/*CuxS (Wijesundera et al., 2000) were studied in the literature, and the reported best values of Voc and Jsc were 300 mV and 2.0 mA cm−2, 400 mV and 2.0 mA cm−2, 270 mV and 2.18 mA cm−2, 400 mV and 7.1 mA cm−2, and 240 mV and 1.6 mA cm−2, respectively.

Cupric oxide (CuO) is one of promising materials as an absorber layer for Cu2O based solar cells because it is a direct band gap of about 1.2 eV (Rakhshani, 1986) which is well matched as an absorber for photovoltaic applications. It is also stoichiometry defect type semiconductor having a monoclinic crystal structure with lattice constants *a* of 4.6837 Å, *b* of 3.4226 Å, *c* of 5.1288 Å and of 99.54o (Ghijsen et al., 1988). CuO had been wildly used for the photocatalysis applications. However, CuO as photovoltaic applications are very limited in the literature. The photoactive CuO based dye-sensitised photovoltaic device was recently reported by the Anandan *et al*. (2005) and we reported the possibility of fabricating the p-CuO*/*n-Cu2O heterojunction (Wijesundera, 2010).
