Study of annealing effects of cuprous oxide grown by electrodeposition technique

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a~l Sol~,C~ls ELSEVIER

Solar Energy Materials and Solar Cells 44 (1996) 251-260

Study of annealing effects of cuprous oxide grown by electrodeposition technique W. Siripala a,*, L.D.R.D. Perera a K.T.L. De Silva b J.K.D.S. Jayanetti by I.M. Dharmadasa c a Dept. of Physics, UniversiO, ofKelaniya, Kelaniya, Sri Lanka b Dept. of Physics, UniL,ersitv of Colombo Colombo 3, Sri Lanka c Applied Physics Division, Sheffield Hallam Universi~, Sheffield S1 1 W, UK

Abstract

Low temperature electrochemical deposition of cuprous oxide from aqueous solutions has been investigated. X-ray diffraction, scanning electron microscopy, optical absorption, and photo-response of liquid/cuprous oxide junctions have been used to study the deposits' crystallographic, morphological, optical, and electrical properties. Effects of annealing in air have been studied using the above mentioned methods. As-deposited cuprous oxide exhibits a direct band gap of 2.0 eV, and shows an n-type behaviour when used in an liquid/solid junction. Annealing below 300°C enhances the n-type photocurrent produced by the junction. Type conversion occurs after heat treatments in air at temperatures above 300°C. No apparent bulk structure changes have been observed during annealing below this temperature, but heat treatments above this temperature produce darker films containing cupric oxide and its complexes with water. Keywords: Copper oxide; Electrodeposition; Thermal annealing; Spectral response

1. I n t r o d u c t i o n

Cuprous oxide (Cu20) is a low-cost and non-toxic semiconducting material with potential applications in solar energy converting devices [1-3]. The band gap of cuprous oxide is 2.0 eV, which is in the acceptable range of window materials for photovoltaic applications. Among the various techniques available for the preparation of cuprous oxide thin films, the method of electrodeposition is an attractive technique because of its simplicity and the possibility in making large area thin films [4,5]. Furthermore, it has

* Corresponding author. 0927-0248/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0927-0248(96)00043-8

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been reported earlier that electrodeposited cuprous oxide films behave as an n-type semiconductor material irrespective of the well established p-type conductivity of this material prepared by other methods [6,7]. This opens up a new area of investigation for the electrodeposited cuprous oxide thin films for the possibility of application as a window material in photovoltaic devices combined with another suitable p-type and low band gap absorbing semiconductors such as copper sulphide or copper indium diselenide. This paper reports the results of annealing effects on electrodeposited cuprous oxide thin films. Studies in particular have been focused on bulk structure, surface morphology, optical absorption, and spectral responses of liquid/CueO junctions. The results observed on as-deposited and annealed materials are presented and discussed in this communication.

2. Experimental Electrodeposition of cuprous oxide films on indium tin oxide (ITO) coated glass substrates were carried out in an electrochemical cell containing aqueous solutions of 0.1 M sodium acetate and 1.6 x 10 -2 M cupric acetate. The temperature of the electrolyte was maintained at 55°C and it was stirred continuously using a magnetic stirrer. The counter electrode was a platinum wire and the reference electrode was a saturated calomel electrode (SCE). Electrodeposition was carried out under potentiostatic condition of - 2 5 0 mV against SCE. Electrolytic solutions were prepared with double distilled water and reagent grade chemicals. Prior to the film deposition, the ITO substrates were cleaned with detergent and diluted hydrochloric acid and were then rinsed with distilled water. Electrodeposition was carried out for 1.5 hrs in order to obtain films of thicknesses in the order of 1 ~zm. The samples were annealed in air at different temperatures, for different durations. The temperatures were selected so that they do not exceed the temperature where the films become cupric oxide. The bulk structure of the films were determined in situ, as a function of annealing by X-ray diffraction (XRD) measurements using a SHIMADZU

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W. Siripala et al. / Solar Energy Materials and Solar Cells 44 (1996) 251-260

253

(model XD-D1) X-ray diffractometer. The surface morphology of the films was determined by scanning electron microscopy (SEM) using a Philips XL40 electron microscope. Optical absorption spectra of the films were obtained with a Hitachi U-2000 spectrophotometer. The spectral response measurements of the liquid/solid junctions formed with the Cu20 films were measured in a three electrode electrochemical cell containing an aqueous solution of 0.1 M sodium acetate. The counter electrode was a platinum wire and the reference was a SCE. The schematic diagram of the experimental arrangement is shown in Fig. 1, where the photoresponse is measured using the phase sensitive detection method to monitor the photocurrent signal produced by a chopped monochromatic light beam. The experimental setup consisted of a lock-in amplifier (NF electronic instruments, LI-570A), a monochromator (SOMA, S-50) and a potentiostat (Hukuto Denko, HA 301). The spectral response of the junctions were obtained at the rest potential ( - 0.075 V versus SCE) and the chopping frequency of the light beam was 40 Hz.

3. Results and discussion Fig. 2 shows the XRD spectra obtained in situ for Cu20 films as a function of annealing duration, at 300°C. Spectrum (a) corresponds to the as-deposited sample while

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254

W. Siripala et al./ Solar Energy Materials and Solar Cells 44 (1996) 251-260

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spectra (b) to (e) correspond to the annealing durations of 10, 20, 30, and 40 rain, respectively. It is clear from Fig. 2 that there are no significant changes of the XRD spectra due to these heat treatments. These observations are typical tbr all annealing temperatures carried out below 300°C. However, if the sample is annealed at temperatures above 300°C, additional peaks appear in the XRD spectrum indicating the formation of other compounds within the film. Fig. 3 shows the XRD spectrum of a thin film which was annealed in air at 400°C for 30 rain. Nine additional peaks have been observed in this spectrum together with the peaks correspond to CuzO, Seven of these new peaks correspond to cupric oxide (CuO) while the other two corresponds to a complex of CuO with water molecules (CuO: 3H20). The heat treatments in air at temperatures above 300°C therefore cause decomposition of the yellow-orange colour Cu20 film into a darker film containing black CuO and its complexes with water. Scanning electron micrographs obtained on both as-deposited and annealed film surfaces are shown in Fig. 4. As-deposited Cu20 thin films are polycrystalline in nature and the grain sizes are in the order of 1-2 t~m. Annealing at 200°C and 300°C considerably change the surface morphology by forming ring shaped structures introducing a more porous nature of the surface. These annealed surfaces therefore provide an effectively large area for liquid/CuzO junction devices. Fig. 5a shows the transmittance spectrum obtained for the long wavelength region for an as-deposited sample, Fig. 5b is the plot of (cthu) 2 versus photon energy (hv)

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W. Siripala et al. / Solar Energy Materials and Solar Cells 44 (1996) 251-260

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obtained from the data in Fig. 5a, where c~ is the absorption coefficient. This result verifies that the band gap o f the electrodeposited cuprous oxide is 2.0 eV and it is a direct gap material, which agrees well with the earlier reports [1]. Results for annealed samples are very similar to those shown in Fig. 5, and therefore indicates that the band gap of electrodeposited cuprous oxide is unaffected by the heat treatment in air.

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W. Siripala et a l . / Solar Energy Materials and Solar Cells 44 (1996) 251-260 55

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Fig. 6 shows the spectral response of the as-deposited Cu20 film electrode in a photoelectrochemical cell (PEC) containing 0.1 M sodium acetate as electrolyte. In Fig. 6a the spectral response was obtained by illuminating the samples through the Cu20/electrolyte interface (front illumination). In Fig. 6b the spectral response was obtained by illuminating through the ITO substrate (back illumination). The entire spectral range from 320 to 620 nm produces an n-type photocurrent signal for the front illumination. However, for the back illumination the shorter wavelengths produce p-type photocurrents while the longer wavelengths produce the n-type photocurrents. This behaviour has been reported earlier [8] and can be explained by using the band diagram shown in Fig. 6a. For the front illumination, the photogenerated electrons are swept away from the CuzO/electrolyte junction due to the presence of the large Schottky type junction at the interface. However, from the spectral curve shown in Fig. 6b it appears that there is an n-type Schottky junction at the I T O / C u 2 0 interface as well. This junction will then produce a p-type photocurrent signal for shorter wavelengths, as shorter wavelengths are absorbed more closer to I T O / C u 2 0 interface and then the photogenerated electrons are drifted towards the electrolyte producing the p-type photocurrent signal. However, for the back illumination the longer wavelengths are more absorbed deeper in the material closer to the Cu20/electrolyte interface and as a result n-type photocurrent signals are produced. Fig. 7 shows the effect of annealing on the spectral response for samples annealed at 200°C and 300°C for 30 rain. As shown in Fig. 7a the entire photocurrent spectrum for the front illumination is still n-type and the spectral response has been enhanced. On the other hand, the back illumination shown in Fig. 7b shows that in comparison with the result shown in Fig. 6b for the as-deposited sample, the shorter wavelength spectral response is considerably reduced and becomes n-type. However, the longer wavelength spectral response has been enhanced. This results suggest that due to the annealing in air, the Schottky barrier at the back contact ( I T O / C u 2 0 interface) has been removed. As a result, the p-type signal in the back wall illumination is completely eliminated. Therefore, as it is evident from the results shown in Fig. 6 and Fig. 7, the spectral responses are enhanced by the annealing. In Fig. 7c it shows that due to the annealing at

W. Siripala et al /Solar Energy Materials and Solar Cells 44 (1996) 251-260

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W. Siripala et al. / Solar Energy Materials and Solar Cells 44 (1996) 251-260

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w. Siripala et al./ Solar Energy Materials and Solar Cells 44 (1996) 251-260

259

the temperature 300°C, the entire photocurrent signal becomes p-type. No significant photocurrent from the back illumination was observed for this situation. This observation rules out the possibility of the existence of a non-ohmic barrier at the back contact after annealing at 300°C. Also the above observations suggest that the electrodeposited Cu20 becomes p-type after annealing at 300°C for 10 min. The enhancement of n-type photoresponse could arise due to various reasons, namely the improvement of the material quality, removal of a rectifying contact at the C u 2 0 / I T O interface and the increase of effective surface area due to the porous nature introduced by heat treatment.

4. Conclusions The following conclusions could be drawn from the results reported in this communication. (I) As revealed by the XRD measurements, the electrodeposited CueO films are polycrystalline and the bulk crystal structure is simple cubic. There is no apparent changes in the crystal structure when heat treated in air at or below 300°C. Annealing above 300°C causes decomposition of the yellow-orange coiour Cu20 film into a darker film containing black CuO and its complexes with water (CuO: 3H20). (II) As-deposited Cu20 films are polycrystalline with grain sizes in the order of 1-2 p~m. Annealing in air changes the morphology of the surface creating a porous nature with ring shaped structures on the surface. (III) Optical absorption measurements on as-deposited Cu20 films indicate that the deposit has a direct band gap of 2.0 eV. Heat treatments in air at temperatures below 300°C do not affect the observed band gap. (IV) As grown electrodeposited Cu20 films behave as an n-type semiconductor material when used as a liquid/solid junction in a photoelectrochemical cell. The n-type behaviour remains unchanged when heat treated in air at temperatures below 300°C. This heat treatment shows a considerable enhancement of the n-type photo-response, and type conversion occurs when annealed at temperatures above 300°C.

Acknowledgements The authors would like to thank Mr. John Davidson and Mr. Anura Samantilleke for helping in some experimental measurements. Financial assistance from Natural Resources, Energy and Science Authority of Sri Lanka (NARESA) and the British Council is greatly appreciated.

Fig. 7. Spectral photoresponses of Cu20/electrolyte junctions tbrmed with heat treated Cu20 and 0.1 M sodium acetate solution. Responsesobtainedby front (a), and back (b) illumination indicates the disappearance of the rectifying contact after annealing of Cu20 at 200°C. Annealing of Cu20 at 300°C causes the type conversion of the material (c).

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L.C. Olsen, F.W. Addis and W. Miller, Solar cells 7 (1982) 247. W.M. Sears and E. Fortin, Solar Energy Mater. 10 (1984) 93. R.P. Rai. Solar Cells 25 (1988) 265. A.E. Rakshani, Solid Stat. Electron. 29 (1987) 7. A.P. Chatterjee, A.K. Mukhopadhyay, A.K. Chakraborty, R.N. Samad and S.K. Lahiri, Mat. Lett. 11 (1991) 358. [6] F.D. Quarto and S. Piazra, Electrochemical Acta 30 (1985) 315. [7] W. Siripala and J.R.P. Jayakody, Solar Energy Mater. 14 (1986) 23. [8] W. Siripala, J. Nat. Sci. Council. Sri Lanka 23(1) (1995)49.