**2. Growth and characterisation of electrodeposited Cu2O**

Electrodeposition is a simple technique to deposit Cu2O on the large area conducting substrate in a very low cost. Electrodeposition of Cu2O from an alkaline bath was first developed by Starek in 1937 (Stareck, 1937) and electrical and optical properties of electrodeposited Cu2O were studied by Economon (Rakhshani, 1986). Rakshani and coworkers studied the electrodeposition process under the galvanostatic and potentiostatic conditions using aqueous alkaline CuSO4 solution, to investigate the deposition parameters and properties of the material. Properties of the electrodeposited Cu2O were reported to be similar to those of the thermally grown films (Rai, 1988) except high resistivity. Siripala *et al*. (Siripala & Jayakody, 1986) reported, for the first time, the observation of n-type photoconductivity in the Cu2O film electrodes prepared by the electrodeposition on various metal substrates in slightly basic aqueous CuSO4 solution in 1986. However, we have reported that electrodeposited Cu2O thin films in a slightly acidic acetate bath attributed n-type conductivity.

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

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

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

Electrodeposition is a simple technique to deposit Cu2O on the large area conducting substrate in a very low cost. Electrodeposition of Cu2O from an alkaline bath was first developed by Starek in 1937 (Stareck, 1937) and electrical and optical properties of electrodeposited Cu2O were studied by Economon (Rakhshani, 1986). Rakshani and coworkers studied the electrodeposition process under the galvanostatic and potentiostatic conditions using aqueous alkaline CuSO4 solution, to investigate the deposition parameters and properties of the material. Properties of the electrodeposited Cu2O were reported to be similar to those of the thermally grown films (Rai, 1988) except high resistivity. Siripala *et al*. (Siripala & Jayakody, 1986) reported, for the first time, the observation of n-type photoconductivity in the Cu2O film electrodes prepared by the electrodeposition on various metal substrates in slightly basic aqueous CuSO4 solution in 1986. However, we have reported that electrodeposited Cu2O thin films in a slightly acidic acetate bath attributed

been reported in the electrodeposition of Cu2O.

p-CuO*/*n-Cu2O heterojunction (Wijesundera, 2010).

**2. Growth and characterisation of electrodeposited Cu2O** 

1.6 mA cm−2, respectively.

n-type conductivity.

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 voltommograms.

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 on the substrate according to the following reaction.

$$\rm 2Cu^{2+} + H\_2O + 2e^- \Rightarrow Cu\_2O + 2H^+$$

Second cathodic peak at –700 mV Vs SCE attributes to the formation of Cu on the substrate according to the following reaction.

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

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 sections.

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 temperature of the bath.

Electrodeposited Cu2O Thin Films for Fabrication of CuO/Cu2O Heterojunction 93

Cu2O film deposition potential domain can be further verified by the X-ray diffraction (XRD) spectra obtained for the films electrodeposited at various potentials (-100 to -900 mV Vs SCE). Fig. 3 shows the XRD spectra of the films deposited at a) -200 mV Vs SCE, b) -600 mV Vs SCE and c) -800 mV Vs SCE on Ti substrates in a bath containing 0.1 M sodium acetate and 0.01 M cupric acetate aqueous solution. Fig. 3(a) shows five peaks at 2 values of 29.58, 36.43, 42.32, 61.39 and 73.54 corresponding to the reflections from (110), (111), (200), (220) and (311) atomic plans of Cu2O in addition to the Ti peaks. Fig. 3(b) exhibits three additional peaks at 2 values of 43.40, 50.55 and 74.28 corresponding to the reflection from (111), (200) and (220) atomic plans of Cu in addition to the peaks corresponding to the Cu2O and Ti substrate. It is evident that the intensity of Cu peaks increases with increase of the deposition potential with respect to the SCE while decreasing the intensities of Cu2O peaks. Peaks corresponding to the Cu2O disappeared with further increase in deposition potential. XRD of Fig. 3(d) exhibits peaks corresponding to Cu and Ti only. Thus, in the acetate bath single phase polycrystalline Cu2O thin films with a cubic structure having lattice constant 4.27 Å are possible only with narrow potential domain of 0 to -300 mV Vs SCE while Cu thin films having lattice constant 3.61 Å are possible at

25 30 35 40 45 50 55 60 65 70 75 80

2 angle (deg)

Fig. 3. XRD spectra obtained for the films deposited on Ti substrate at the potentials (a) -200

**O**


**O**



potential –700 mV and above Vs SCE.

12

 **O Ti Cu Cu2 O**

**O**

**O**

**O**

**O**

0

4

b

mV Vs SCE, (b) -600 mV Vs SCE and (c) -800 mV Vs SCE

a

8

Intensity (kcps)

c

Fig. 1. Voltammetric curves of the Ti electrode (4 mm2) obtained in a solution containing 0.1 M sodium acetate and cupric acetate concentrations of a) 0 mM, b) 0.25 mM, c) 1 mM and d) 10 mM

Fig. 2. Voltammetric curves of the Ti electrode (4 mm2) in an electrochemical cell containing 0.1 M sodium acetate and 0.01 M cupric acetate solutions at two different pH values (pH was adjusted by adding diluted HCI).

d c b a


pH = 6.6

pH = 5.6

Deposition potential (mV) Vs SCE

Fig. 1. Voltammetric curves of the Ti electrode (4 mm2) obtained in a solution containing 0.1 M sodium acetate and cupric acetate concentrations of a) 0 mM, b) 0.25 mM, c) 1 mM and

Fig. 2. Voltammetric curves of the Ti electrode (4 mm2) in an electrochemical cell containing


Deposition potential (mV) vs SCE

0.1 M sodium acetate and 0.01 M cupric acetate solutions at two different pH values

(pH was adjusted by adding diluted HCI).


Deposition current (A)


d) 10 mM





Deposition current (A)




0

Cu2O film deposition potential domain can be further verified by the X-ray diffraction (XRD) spectra obtained for the films electrodeposited at various potentials (-100 to -900 mV Vs SCE). Fig. 3 shows the XRD spectra of the films deposited at a) -200 mV Vs SCE, b) -600 mV Vs SCE and c) -800 mV Vs SCE on Ti substrates in a bath containing 0.1 M sodium acetate and 0.01 M cupric acetate aqueous solution. Fig. 3(a) shows five peaks at 2 values of 29.58, 36.43, 42.32, 61.39 and 73.54 corresponding to the reflections from (110), (111), (200), (220) and (311) atomic plans of Cu2O in addition to the Ti peaks. Fig. 3(b) exhibits three additional peaks at 2 values of 43.40, 50.55 and 74.28 corresponding to the reflection from (111), (200) and (220) atomic plans of Cu in addition to the peaks corresponding to the Cu2O and Ti substrate. It is evident that the intensity of Cu peaks increases with increase of the deposition potential with respect to the SCE while decreasing the intensities of Cu2O peaks. Peaks corresponding to the Cu2O disappeared with further increase in deposition potential. XRD of Fig. 3(d) exhibits peaks corresponding to Cu and Ti only. Thus, in the acetate bath single phase polycrystalline Cu2O thin films with a cubic structure having lattice constant 4.27 Å are possible only with narrow potential domain of 0 to -300 mV Vs SCE while Cu thin films having lattice constant 3.61 Å are possible at potential –700 mV and above Vs SCE.

Fig. 3. XRD spectra obtained for the films deposited on Ti substrate at the potentials (a) -200 mV Vs SCE, (b) -600 mV Vs SCE and (c) -800 mV Vs SCE

Electrodeposited Cu2O Thin Films for Fabrication of CuO/Cu2O Heterojunction 95

film deposited at –600 mV Vs SCE shows the photoactivity but magnitudes of the photovoltage and photocurrent were very small. The best photoresponse we have obtained for the Cu2O thin film deposited at –400 mV Vs SCE. This may be due to the better charge transfer process between Cu2O and electrolyte due to the randomly distributed Cu spheres

The optical absorption measurements of the Cu2O thin films on indium doped tin oxide (ITO) substrate deposited at -100 mV to -600 mV Vs SCE indicate that the electrodeposited Cu2O has a direct band gap of 2.0 eV, and the band gap of the material is independent of the

Photoactivity of the films was further studied by the dark and light current-voltage measurements. Fig. 5 shows the dark and light current-voltage characteristics in a PEC of the films deposited at (a) –200 mV and (b) –400 mV Vs SCE. Current-voltage measurements were obtained in three electrode electrochemical cell. The change of the sign of the photocurrent with the applied voltage shows the evidence for the existence of two junctions within the Ti/Cu2O/electrolyte system. Particularly with the positive applied bias voltage, the Cu2O/electrolyte junction become dominant and thereby the n-type photosignal is produced, when negative bias voltage is applied the Ti/Cu2O junction become dominant and therefore a p-type signal is produced. Similar results have been reported earlier on the ITO/Cu2O/electrolyte system (Siripala et al., 1996) and ITO/Cu2O/CuxS system (Wijesundera et al., 2000). It has been reported earlier that both n- and p-type photosignals can be obtained in the currant–voltage scans due to the existence of Ti/Cu2O and Cu2O/electrolyte Schottky type junctions. The enhancement of n-type signal could be due to the enhancement of Cu2O/electrolyte junction as compared with the Ti/Cu2O junction.

Current (


Fig. 5. Dark and light current-voltage characteristics for the films deposited at (a) -200 mV and (b) -400 mV Vs SCE in a PEC containing 0.1 M sodium acetate under the white light

Single phase polycrystalline n-type Cu2O thin films can be potentiostatically electrodeposited on conducting substrates selecting proper deposition parameters and these




0

10

20

30

40

light off

light on

µA)

light on

light off

(b) Voc = 210 mV Isc = 15 µA)

Applied voltage (mV) vs SCE


light on

light off

on top of Cu2O thin films as shown in Fig. 4.

(a) Voc = 170 mV Isc = 6.5 µA)

light off

light on

Applied voltage (mV) vs SCE

illumination of 90 W/m2 (effective area of the film is 1 mm2).


deposition potential.

Current (





0

10

20

30

40

µA)

Fig. 4 shows the scanning electron micrographs (SEMs) of the above set of samples. It is evident that the surface morphology depends on the deposition potential and the films grown on Ti substrate are uniform and polycrystalline. Grain size of Cu2O is in the range of ~1-2 m. It is observed that the Cu2O thin film deposited at –200 mV Vs SCE exhibit cubic structure (Fig. 4(a)) and deviation from the cubic structure can be observed when deposition potential deviate from the -200 mV Vs SCE. Thus, polycrystalline Cu2O thin films with cubic grains are possible only within a very narrow potential domain of around –200 mV Vs SCE. Fig. 4(b) shows the existence of spherical shaped Cu on top of Cu2O when film deposited at –400 mV Vs SCE. The co-deposition of Cu with Cu2O is evident in the XRD spectra, too. This small grains of Cu distributed over the Cu2O surface will be useful in some other applications. It is clear from XRD and SEM results that Cu2O, Cu2O + Cu, and Cu microcrystalline thin films can be separately electrodeposited on Ti substrate by changing the deposition potential from –100 mV to –900 mV Vs SCE using the same electrolyte.

Fig. 4. Scanning electron micrographs of thin films electrodeposited at (a) -200 mV Vs SCE, (b) -400 mV Vs SCE and (c) -800 mV Vs SCE

Cu2O thin films produce negative photovoltages in a photelectrochemical cell (PEC) containing 0.1 M sodium acetate under the white light illumination of 90 W/m2. Active area of the film in a PEC was ~1 mm2. The magnitudes of the photovoltage and the photocurrent of Cu2O films deposited at –100 mV to –500 mV Vs SCE were 125 mV and 5 A, 168 mV and 6.5 A, 172 mV and 8 A, 210 mV and 15 A and 68 mV and 1 A respectively. Also Cu2O

Fig. 4 shows the scanning electron micrographs (SEMs) of the above set of samples. It is evident that the surface morphology depends on the deposition potential and the films grown on Ti substrate are uniform and polycrystalline. Grain size of Cu2O is in the range of ~1-2 m. It is observed that the Cu2O thin film deposited at –200 mV Vs SCE exhibit cubic structure (Fig. 4(a)) and deviation from the cubic structure can be observed when deposition potential deviate from the -200 mV Vs SCE. Thus, polycrystalline Cu2O thin films with cubic grains are possible only within a very narrow potential domain of around –200 mV Vs SCE. Fig. 4(b) shows the existence of spherical shaped Cu on top of Cu2O when film deposited at –400 mV Vs SCE. The co-deposition of Cu with Cu2O is evident in the XRD spectra, too. This small grains of Cu distributed over the Cu2O surface will be useful in some other applications. It is clear from XRD and SEM results that Cu2O, Cu2O + Cu, and Cu microcrystalline thin films can be separately electrodeposited on Ti substrate by changing the deposition potential from –100 mV to –900 mV Vs SCE using the same electrolyte.

(a) (b)

(c)

Fig. 4. Scanning electron micrographs of thin films electrodeposited at (a) -200 mV Vs SCE,

Cu2O thin films produce negative photovoltages in a photelectrochemical cell (PEC) containing 0.1 M sodium acetate under the white light illumination of 90 W/m2. Active area of the film in a PEC was ~1 mm2. The magnitudes of the photovoltage and the photocurrent of Cu2O films deposited at –100 mV to –500 mV Vs SCE were 125 mV and 5 A, 168 mV and 6.5 A, 172 mV and 8 A, 210 mV and 15 A and 68 mV and 1 A respectively. Also Cu2O

(b) -400 mV Vs SCE and (c) -800 mV Vs SCE

film deposited at –600 mV Vs SCE shows the photoactivity but magnitudes of the photovoltage and photocurrent were very small. The best photoresponse we have obtained for the Cu2O thin film deposited at –400 mV Vs SCE. This may be due to the better charge transfer process between Cu2O and electrolyte due to the randomly distributed Cu spheres on top of Cu2O thin films as shown in Fig. 4.

The optical absorption measurements of the Cu2O thin films on indium doped tin oxide (ITO) substrate deposited at -100 mV to -600 mV Vs SCE indicate that the electrodeposited Cu2O has a direct band gap of 2.0 eV, and the band gap of the material is independent of the deposition potential.

Photoactivity of the films was further studied by the dark and light current-voltage measurements. Fig. 5 shows the dark and light current-voltage characteristics in a PEC of the films deposited at (a) –200 mV and (b) –400 mV Vs SCE. Current-voltage measurements were obtained in three electrode electrochemical cell. The change of the sign of the photocurrent with the applied voltage shows the evidence for the existence of two junctions within the Ti/Cu2O/electrolyte system. Particularly with the positive applied bias voltage, the Cu2O/electrolyte junction become dominant and thereby the n-type photosignal is produced, when negative bias voltage is applied the Ti/Cu2O junction become dominant and therefore a p-type signal is produced. Similar results have been reported earlier on the ITO/Cu2O/electrolyte system (Siripala et al., 1996) and ITO/Cu2O/CuxS system (Wijesundera et al., 2000). It has been reported earlier that both n- and p-type photosignals can be obtained in the currant–voltage scans due to the existence of Ti/Cu2O and Cu2O/electrolyte Schottky type junctions. The enhancement of n-type signal could be due to the enhancement of Cu2O/electrolyte junction as compared with the Ti/Cu2O junction.

Fig. 5. Dark and light current-voltage characteristics for the films deposited at (a) -200 mV and (b) -400 mV Vs SCE in a PEC containing 0.1 M sodium acetate under the white light illumination of 90 W/m2 (effective area of the film is 1 mm2).

Single phase polycrystalline n-type Cu2O thin films can be potentiostatically electrodeposited on conducting substrates selecting proper deposition parameters and these

Electrodeposited Cu2O Thin Films for Fabrication of CuO/Cu2O Heterojunction 97

400 oC for 15 min. Fig. 6 shows that the intensities of the peaks correspondent to the CuO structure increases while intensities of the peaks correspondent to the Cu2O structure decreases with the increasing of annealing temperature and duration. The reflections from the Cu2O structure disappear when the film is annealed at 500 C for 30 min. in air. It is reveled that the single phase CuO thin films on Ti substrate can be prepared by annealing

The surface morphology of the annealing Cu2O thin films is studied with SEMs. Fig. 7 shows SEMs of (a) as grown, and annealed in air at (b) 175 C, (c) 400 C and (d) 500 C. Results reveal that, by increasing the annealing temperature, the size of the cubic shape polycrystalline grain gradually increase up to 200 C, change to the different shape at 400 oC and converted to the monoclinic like shape polycrystalline grain at 500 oC. Cu2O thin films have the cubic-like polycrystalline grains. SEMs clearly show that structural phase transition take place from Cu2O, Cu2O-CuO, CuO as reveal by the XRD patterns. CuO crystallites are

(a) (b)

(c) (d)

Fig. 7. Scanning electron micrographs of the electrodeposited semiconductor Cu2O thin

Photosensitivity (Voc and Isc) of the annealed electrodeposited Cu2O thin films in a two electrode PEC cell containing 0.1 M sodium acetate aqueous solution, under white light illumination of 90 W/m2, shows that initial n-type photoconductivity changes to the p-type after annealing 300 oC. Type of the photoconductivity of the Cu2O thin films can be converted from n- to p-type with annealing because of Cu2O structure remain same even if

films a) as grown and annealed in air at (b) 175 C, (c) 400 C and (d) 500 C

films annealed at 300 oC as revealed by XRD patterns.

Cu2O in air.

in the order of 250 nm.

films are uniform and well adhered to substrate. Garutara *et al*. (Garuthara & Siripala, 2006) carried out the photoluminescence (PL) characterisation for the electrodeposited n-type polycrystalline Cu2O. They showed the existence of the donor energy level of 0.38 eV below the bottom of the conduction band due to the oxygen vacancies and confirmed that the n-type conductivity is due to the oxygen vacancies created in the lattice. Previously reported electrodeposited Cu2O in a various deposition bath, except slightly acidic acetate bath, attribute p-type conductivity due to the Cu vacancies created in the lattice as thermally grown films.
