**4. Fabrication and characterisation of CuO/Cu2O heterojunction**

In order to fabricate CuO/Cu2O thin film hetorojunction, thin films of n-type Cu2O are potentiostatically electrodeposited on a Ti substrate in an acetate bath and are annealed at 500 oC for 30 min. in air for the growth of p-type CuO thin films. Thin films of Cu2O are potentiostatically electrodeposited on Ti*/*CuO electrodes at different deposition potentials Vs SCE while maintaining the same electrolytic conditions, which used to deposit Cu2O on the Ti substrate. Deposition period is varied form 240 min to 120 min in order to obtain sufficient thickness of the films. Film thickness was calculated by monitoring the total charge passed during the film deposition and it was 1 m.

Bulk structures of the electrodeposited films on Ti/CuO were studied by the XRD patterns. Fig. 13 shows the XRD patterns of the films deposited on Ti/CuO electrodes at the deposition potentials of -250, -400, -550 and -700 mV Vs SCE. XRD patterns evidence the formation of Cu2O for all deposition potentials on Ti/CuO electrodes while Cu deposition starts in addition to the Cu2O when the film deposited at -700 mV Vs SCE. Single phase Cu2O are possible at the deposition potentials less than -700 mV Vs SCE. XRD patterns further show that peak intensities corresponding atomic reflection of Cu2O increase with deposition potential. It indicates that amount of Cu2O deposit is increased by increasing deposition potential. This is further studied by using SEM.

Fig. 13. XRD pattern of thin films electrodeposited on Ti/CuO electrode at the potentials -250, -400, -550 and -700 mV Vs SCE

The surface morphology of the films prepared on the Ti/CuO electrode at the different deposition potentials was studied using the SEMs in order to identify the Cu2O thin film deposition conditions on Ti/CuO electrode. Figs. 14(a) to (c) show the SEMs of Cu2O films deposited on the Ti/CuO at -250 to -550 mV Vs SCE. Fig. 14(a) shows the cubic shape Cu2O

In order to fabricate CuO/Cu2O thin film hetorojunction, thin films of n-type Cu2O are potentiostatically electrodeposited on a Ti substrate in an acetate bath and are annealed at 500 oC for 30 min. in air for the growth of p-type CuO thin films. Thin films of Cu2O are potentiostatically electrodeposited on Ti*/*CuO electrodes at different deposition potentials Vs SCE while maintaining the same electrolytic conditions, which used to deposit Cu2O on the Ti substrate. Deposition period is varied form 240 min to 120 min in order to obtain sufficient thickness of the films. Film thickness was calculated by monitoring the total

Bulk structures of the electrodeposited films on Ti/CuO were studied by the XRD patterns. Fig. 13 shows the XRD patterns of the films deposited on Ti/CuO electrodes at the deposition potentials of -250, -400, -550 and -700 mV Vs SCE. XRD patterns evidence the formation of Cu2O for all deposition potentials on Ti/CuO electrodes while Cu deposition starts in addition to the Cu2O when the film deposited at -700 mV Vs SCE. Single phase Cu2O are possible at the deposition potentials less than -700 mV Vs SCE. XRD patterns further show that peak intensities corresponding atomic reflection of Cu2O increase with deposition potential. It indicates that amount of Cu2O deposit is increased by increasing

28 30 32 34 36 38 40 42 44 46 48 50

2 (deg)

Fig. 13. XRD pattern of thin films electrodeposited on Ti/CuO electrode at the potentials

The surface morphology of the films prepared on the Ti/CuO electrode at the different deposition potentials was studied using the SEMs in order to identify the Cu2O thin film deposition conditions on Ti/CuO electrode. Figs. 14(a) to (c) show the SEMs of Cu2O films deposited on the Ti/CuO at -250 to -550 mV Vs SCE. Fig. 14(a) shows the cubic shape Cu2O

CuO(111)

Ti

Cu O(111)

CuO(002)

Ti

2

Ti

Cu





Cu O(200)

2

CuO(210)

**4. Fabrication and characterisation of CuO/Cu2O heterojunction** 

charge passed during the film deposition and it was 1 m.

deposition potential. This is further studied by using SEM.

Cu O(110)

2

CuO(110)


Counts

grains on the CuO film and Figs. 14(a) to (c) show that the amount of Cu2O increases with increasing the deposition potential. The SEMs reveal that the well covered Cu2O layer can be deposited on Ti/CuO electrode under the potentiostatical condition of -550 mV Vs SCE and above. Grain size of the Cu2O deposited on Ti substrate is in the range of ~ 1-2 m as shown in Fig. 14(a) while it is lower to 1 m when Cu2O deposited on CuO at the deposition potential of -550 mV Vs SCE. The SEM with low magnification of Cu2O deposited at -700 mV Vs SCE clearly shows the existence of Cu on the surface of Cu2O as shown in the XRD pattern of the film deposited at -700 mV Vs SCE.

Fig. 14. Scanning electron micrograph of Cu2O thin films electrodeposited on Ti/CuO electrode at (a) -250 mV, (b) -400 mV and (c) -550 mV Vs SCE

XRD and SEM reveal that well-covered single phase polycrystalline Cu2O thin film on the Ti*/*CuO electrode can be possible at the deposition potential of −550 mV Vs SCE in an acetate bath. Structural matching of two semiconductors is very essential for fabricating a heterojunction. In general, the cubic-like Cu2O grains and the monoclinic-like CuO grains are not match with each other to make the CuO/Cu2O heterojunction. However, the electrodeposition technique produces the good matching of the Cu2O grains to the monoclinic-like CuO grains. The shape of the grains can be easily changed when the electrodeposition technique is used to grow a semiconductor. The electrodeposition is a very good tool to fabricate the heterojunctions as it does not depend on the grain shape of the material. Further, the SEMs of the Cu2O/CuO heterojunction suggested that the Cu2O polycrystalline grains are grown from the surfaces of the CuO polycrystalline grains and make the good contacts between two thin film layers. For the completion of the device, very

Electrodeposited Cu2O Thin Films for Fabrication of CuO/Cu2O Heterojunction 105

Cu2O(1,1,1) and CuO(1,1,-1) reflections. The results suggest that Cu2O grain layer can only be observed below φ = 01 degree. With increasing the grazing angle, CuO grain layer can be observed gradually, as Ti-reflections. The bulk structural information of Cu2O layer can be obtain for the grazing angles around 2.5 degree since it produces highest intensity of (1,1,1) reflection of Cu2O. It reveals that it is possible to obtain optimum structural information within the Cu2O/CuO junction region in addition to the Cu2O, CuO, and Ti for the grazing angles slightly grater than 2.5 degree. Further it is found that the intensity ratio of the (1,1,1) reflection and (1,1,-1) reflection is approximately constant above φ = 50 degree, in contrast to the intensity of the Ti-reflection. It shows that the bulk structural information of bi-layer (Cu2O

> Ti

> > CuO (111)

CuO (200)

Ti

Cu O (200)

Grazing

angle

(deg)

12.0 8.0 5.0 3.0 2.0 1.0 0.5 0.3 0.1

Intensity (a.u.)

2

and CuO) can be studied at the grazing angle 5 degree or greater

Cu O (111)

CuO (11-1)

Ti/CuO/Cu2O heterojunction

2

34 35 36 37 38 39 40 41 42 43

2 (deg)

Fig. 16. Grazing angle dependency of the X-ray diffraction patterns for the electrodeposited

The structural deformation localized around Cu ions of the CuO/Cu2O heterojunction can be investigated from EXAFS. Fig. 18 shows the expanded partial XAS at Cu-K edge of Ti/CuO/Cu2O heterojunction at φ = 03 to 100 deg. The electrodeposited CuO/Cu2O thin film heterojunction include Cu ions sited at different structures of Cu2O and CuO. The spectra result from a convoluted XAS induced by interference between the X-ray photoelectron waves emitted by X-ray absorbing Cu ions and the backscattering waves of its surrounding ions for both structures. The grazing angle dependency of the XAS suggests that the incident X-ray beam penetrate the thin films of Cu2O and CuO grains by the different path distance. It can be considered that the XAS measurements obtained at low grazing angles (0.3 and 0.5 deg) should be mainly the XAS of Cu2O thin film, which is the front layer of the heterojunction. Therefore, the XAS at φ = 0.5 and 3.0 deg are compared with the observed XAS of the electrodeposited thin films of Cu2O and CuO. Fig. 19 shows the comparsion of the expanded partial XAS at grazing angles of 0.5 and 3.0 deg and of the

thin (few angstroms) Au grid consists of 1 8 mm2 rectangular areas are deposited on Cu2O using a vacuum sputtering technique. The electrical contacts to the Cu2O surface (front contacts) is made using mechanically pressed transparent ITO plate to the Au grid, where the Ti substrate serves as back electrical contact to the CuO surface. The Ti*/*CuO*/*Cu2O*/*Au heterojunction gave the open circuit voltage (Voc) of 210 mV, short circuit current (Jsc) of 310 μA cm2, fill factor (FF) of 0.26 and efficiency (*η*) of 0.02% under the white light illumination of 90 mWcm−2. At the initial stage of fabrication the junction, the shape of the I–V characteristic as shown in Fig. 15 and values of Voc and Jsc are encouraging despite the low photoactive performance of the heterojunction.

Fig. 15. Dark and light current-voltage characteristics of Ti/CuO/Cu2O/Au heterojunction under the white light illumination of 90 mW/cm2

For the better performance, very thin Cu2O films should be used due to the high resistance of electrodeposited Cu2O and should be find out better ohmic contact to the Cu2O. Best omic contact to the n-type Cu2O may be Al but not the Au. Au is very good omic contact to p-type Cu2O. n-type electrodeposted Cu2O has high resistivity is due to low doping density. Growth of the n-type Cu2O with suitable dopent hasn't been achieved in the litterateur and it will be very important in developing Cu2O based solar cells.

CuO/Cu2O heterojunction was further investigated by means of X-ray diffractions and X-ray absorption spectra (XAS) at the Cu-K edge with grazing angle measurements. Layer by layer structural information of the CuO/Cu2O heterojunction can be studied with grazing angle measurements. Fig. 16 shows the grazing angle (φ) dependency of the X-ray diffraction patterns of the CuO/Cu2O heterojunction. The Ti peak of highest intensity at 2θ = 4023 degree is indexed by (0,1,1) and (1,1,1) reflections of hexagonal structure and are not observed below φ ~ 20 degree. On the other hand, the reflections of Cu2O and CuO structures are observed in all the grazing angles, though the reflections of Cu2O structure shows the different grazing angle dependence to those of the CuO ones. Fig. 17 shows grazing angle dependency of (1,1,1) reflection of Cu2O, (1,1,-1) reflection of CuO, (1,1,1) reflection of Ti, and intensity ratio of

thin (few angstroms) Au grid consists of 1 8 mm2 rectangular areas are deposited on Cu2O using a vacuum sputtering technique. The electrical contacts to the Cu2O surface (front contacts) is made using mechanically pressed transparent ITO plate to the Au grid, where the Ti substrate serves as back electrical contact to the CuO surface. The Ti*/*CuO*/*Cu2O*/*Au heterojunction gave the open circuit voltage (Voc) of 210 mV, short circuit current (Jsc) of 310 μA cm2, fill factor (FF) of 0.26 and efficiency (*η*) of 0.02% under the white light illumination of 90 mWcm−2. At the initial stage of fabrication the junction, the shape of the I–V characteristic as shown in Fig. 15 and values of Voc and Jsc are encouraging despite the


Fig. 15. Dark and light current-voltage characteristics of Ti/CuO/Cu2O/Au heterojunction

For the better performance, very thin Cu2O films should be used due to the high resistance of electrodeposited Cu2O and should be find out better ohmic contact to the Cu2O. Best omic contact to the n-type Cu2O may be Al but not the Au. Au is very good omic contact to p-type Cu2O. n-type electrodeposted Cu2O has high resistivity is due to low doping density. Growth of the n-type Cu2O with suitable dopent hasn't been achieved in the litterateur and

CuO/Cu2O heterojunction was further investigated by means of X-ray diffractions and X-ray absorption spectra (XAS) at the Cu-K edge with grazing angle measurements. Layer by layer structural information of the CuO/Cu2O heterojunction can be studied with grazing angle measurements. Fig. 16 shows the grazing angle (φ) dependency of the X-ray diffraction patterns of the CuO/Cu2O heterojunction. The Ti peak of highest intensity at 2θ = 4023 degree is indexed by (0,1,1) and (1,1,1) reflections of hexagonal structure and are not observed below φ ~ 20 degree. On the other hand, the reflections of Cu2O and CuO structures are observed in all the grazing angles, though the reflections of Cu2O structure shows the different grazing angle dependence to those of the CuO ones. Fig. 17 shows grazing angle dependency of (1,1,1) reflection of Cu2O, (1,1,-1) reflection of CuO, (1,1,1) reflection of Ti, and intensity ratio of

Bias Voltage (mV)

Light Current

Dark Current

low photoactive performance of the heterojunction.


under the white light illumination of 90 mW/cm2

it will be very important in developing Cu2O based solar cells.



Current Density (

A/cm2

)


0

200

Cu2O(1,1,1) and CuO(1,1,-1) reflections. The results suggest that Cu2O grain layer can only be observed below φ = 01 degree. With increasing the grazing angle, CuO grain layer can be observed gradually, as Ti-reflections. The bulk structural information of Cu2O layer can be obtain for the grazing angles around 2.5 degree since it produces highest intensity of (1,1,1) reflection of Cu2O. It reveals that it is possible to obtain optimum structural information within the Cu2O/CuO junction region in addition to the Cu2O, CuO, and Ti for the grazing angles slightly grater than 2.5 degree. Further it is found that the intensity ratio of the (1,1,1) reflection and (1,1,-1) reflection is approximately constant above φ = 50 degree, in contrast to the intensity of the Ti-reflection. It shows that the bulk structural information of bi-layer (Cu2O and CuO) can be studied at the grazing angle 5 degree or greater

Fig. 16. Grazing angle dependency of the X-ray diffraction patterns for the electrodeposited Ti/CuO/Cu2O heterojunction

The structural deformation localized around Cu ions of the CuO/Cu2O heterojunction can be investigated from EXAFS. Fig. 18 shows the expanded partial XAS at Cu-K edge of Ti/CuO/Cu2O heterojunction at φ = 03 to 100 deg. The electrodeposited CuO/Cu2O thin film heterojunction include Cu ions sited at different structures of Cu2O and CuO. The spectra result from a convoluted XAS induced by interference between the X-ray photoelectron waves emitted by X-ray absorbing Cu ions and the backscattering waves of its surrounding ions for both structures. The grazing angle dependency of the XAS suggests that the incident X-ray beam penetrate the thin films of Cu2O and CuO grains by the different path distance. It can be considered that the XAS measurements obtained at low grazing angles (0.3 and 0.5 deg) should be mainly the XAS of Cu2O thin film, which is the front layer of the heterojunction. Therefore, the XAS at φ = 0.5 and 3.0 deg are compared with the observed XAS of the electrodeposited thin films of Cu2O and CuO. Fig. 19 shows the comparsion of the expanded partial XAS at grazing angles of 0.5 and 3.0 deg and of the

Electrodeposited Cu2O Thin Films for Fabrication of CuO/Cu2O Heterojunction 107

9050 9100 9150 9200 9250

E(eV)

Fig. 18. Oscillating amplitudes IF/Io of the X–ray absorption spectra of the Ti/CuO/Cu2O

9040 9080 9120 9160 9200

Fig. 19. Oscillating amplitudes IF/Io of the X–ray absorption spectra of the Ti/CuO/Cu2O heterojunction at grazing angles of 0.5 and 3.0 degrees and of the electrodeposited Cu2O and

structure formed in the middle of the CuO/Cu2O heterojunction attributes better lattice matching between CuO and Cu2O interface. Further, it can be considered that formation of the smooth energy band lineup at the interface of CuO/Cu2O heterojunction without spikes at the conduction and valance bands (Ec and Ev). Band lineup between two

CuO thin films, in addition to the calculated one of (05Cu2O + 05CuO)

semiconductors is a crucial parameter leading to better photoactive properties.

E (eV)

 0.3 deg 0.5 deg 1.0 deg 3.0 deg 5.0 deg 10.0 deg

 0.5 deg 3 deg Cu2 O CuO 0.5Cu2

O+0.5CuO

0.98

heterojunction at φ = 03 to 100 degrees

0.96

0.98

1.00

I

F/IO

1.02

1.04

1.00

I

F/IO

1.02

Fig. 17. Grazing angle dependency of the (1,1,1) reflection of the Cu2O, (1,1,-1) reflection of the CuO structure, the (1,1,1) reflection of Ti and intensity ratio of Cu2O (1,1,1) and CuO (1,1,-1) reflections

electrodeposited Cu2O and CuO thin films, in addition to the calculated one of (05Cu2O + 05CuO). However, XAS (local structure around Cu ions) at low grazing angles are not similar with Cu2O. It shows that the XAS obtained at even low grazing angles are the convoluted spectra induced by the Cu2O and CuO structures. The convolution effect of the XAS can be studied by fitting the observed XAS at φ = 0.5 deg from a simple mathematical convolution of Cu2O-XAS and CuO-XAS. Fig. 19 shows that the observed XAS at φ = 0.5 deg is very similar to the calculated one of 0.5(Cu2O-XAS) + 0.5(CuO-XAS). However, XAS at low grazing angles can be analyzed by a simple mathematical convolution of Cu2O and CuO structures but not for the grazing angles higher than 0.5 deg. This reveals that the complex XAS results from the convoluted spectra induced from unknown structure in addition to the Cu2O and CuO structures. The XAS modulation due to the unknown structure depends on the grazing angles, and the maximum XAS modulation appears at the grazing angle of 3.0 deg. This suggests that the junction region has very complex structure. XAS of CuO/Cu2O hetrojunction with different grazing angles can be further compared by studying corresponding Fourier transformations of the oscillating EXAFS spectra. Fig. 20 shows the observed F(R) for the bi-layer thin film of Ti/CuO/Cu2O heterojunction at φ = 05 and 30 degrees and for the electrodeposited Cu2O and CuO thin films with calculated one of (05Cu2O + 05CuO). It is further confirmed that the F(R) obtained at φ = 05 and 30 degrees are not similar with that of Cu2O structure and CuO one, but more complex. Comparison between the F(R) of the bi-layer thin film obtained at φ = 05 degree and the calculated one of (05Cu2O + 05CuO) suggests that the F(R) of the bi-layer thin film is also convoluted by those of the Cu2O and CuO structures. As in Fig. 20 peak amplitudes are very small for the |F(R)| at φ = 3.0 deg compared to the others. It implies that structure in the junction region is diluted one (the surrounding ions around the Cu absorbing ion do not well arranged). Results reveal that the formation of amorphous structure in the interface of CuO/Cu2O heterojunction. It can be expected that amorphous

Cu2

Ti

Cu2

O(1,1,1) CuO(1,1,-1)

O(111)/CuO(11-1)

0 2 4 6 8 10 12

(deg)

Fig. 17. Grazing angle dependency of the (1,1,1) reflection of the Cu2O, (1,1,-1) reflection of the CuO structure, the (1,1,1) reflection of Ti and intensity ratio of Cu2O (1,1,1) and CuO

electrodeposited Cu2O and CuO thin films, in addition to the calculated one of (05Cu2O + 05CuO). However, XAS (local structure around Cu ions) at low grazing angles are not similar with Cu2O. It shows that the XAS obtained at even low grazing angles are the convoluted spectra induced by the Cu2O and CuO structures. The convolution effect of the XAS can be studied by fitting the observed XAS at φ = 0.5 deg from a simple mathematical convolution of Cu2O-XAS and CuO-XAS. Fig. 19 shows that the observed XAS at φ = 0.5 deg is very similar to the calculated one of 0.5(Cu2O-XAS) + 0.5(CuO-XAS). However, XAS at low grazing angles can be analyzed by a simple mathematical convolution of Cu2O and CuO structures but not for the grazing angles higher than 0.5 deg. This reveals that the complex XAS results from the convoluted spectra induced from unknown structure in addition to the Cu2O and CuO structures. The XAS modulation due to the unknown structure depends on the grazing angles, and the maximum XAS modulation appears at the grazing angle of 3.0 deg. This suggests that the junction region has very complex structure. XAS of CuO/Cu2O hetrojunction with different grazing angles can be further compared by studying corresponding Fourier transformations of the oscillating EXAFS spectra. Fig. 20 shows the observed F(R) for the bi-layer thin film of Ti/CuO/Cu2O heterojunction at φ = 05 and 30 degrees and for the electrodeposited Cu2O and CuO thin films with calculated one of (05Cu2O + 05CuO). It is further confirmed that the F(R) obtained at φ = 05 and 30 degrees are not similar with that of Cu2O structure and CuO one, but more complex. Comparison between the F(R) of the bi-layer thin film obtained at φ = 05 degree and the calculated one of (05Cu2O + 05CuO) suggests that the F(R) of the bi-layer thin film is also convoluted by those of the Cu2O and CuO structures. As in Fig. 20 peak amplitudes are very small for the |F(R)| at φ = 3.0 deg compared to the others. It implies that structure in the junction region is diluted one (the surrounding ions around the Cu absorbing ion do not well arranged). Results reveal that the formation of amorphous structure in the interface of CuO/Cu2O heterojunction. It can be expected that amorphous

Intensity (a.u.)

(1,1,-1) reflections

Cu

2

O(1,1,1)/CuO(1,1,-1)

Fig. 18. Oscillating amplitudes IF/Io of the X–ray absorption spectra of the Ti/CuO/Cu2O heterojunction at φ = 03 to 100 degrees

Fig. 19. Oscillating amplitudes IF/Io of the X–ray absorption spectra of the Ti/CuO/Cu2O heterojunction at grazing angles of 0.5 and 3.0 degrees and of the electrodeposited Cu2O and CuO thin films, in addition to the calculated one of (05Cu2O + 05CuO)

structure formed in the middle of the CuO/Cu2O heterojunction attributes better lattice matching between CuO and Cu2O interface. Further, it can be considered that formation of the smooth energy band lineup at the interface of CuO/Cu2O heterojunction without spikes at the conduction and valance bands (Ec and Ev). Band lineup between two semiconductors is a crucial parameter leading to better photoactive properties.

Electrodeposited Cu2O Thin Films for Fabrication of CuO/Cu2O Heterojunction 109

Science, Kyushu University, Japan are gratefully acknowledged for their invaluable advice,

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on copper and their application as a cathode in dye-sensitized solar cells. *Mater.* 

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77, 291-294

89, 1231-1233

1802-1807

Fig. 20. Amplitudes |F(R)| obtained from the Fourier transformation of the EXAFS spectra of the Ti/CuO/Cu2O heterojunction at φ = 05 and 30 degrees and of the electrodeposited Cu2O and CuO thin films and the calculated one of (05Cu2O + 05CuO)
