Study of the physical properties of CuO thin films

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Superlattices and Microstructures xxx (2018) 1e7

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Study of the physical properties of CuO thin films grown by modified SILAR method for solar cells applications Othmane Daoudi*, Youssef Qachaou, Abderrahim Raidou, Khalid Nouneh, Mohammed Lharch, Mounir Fahoume Laboratory of Physics of Condensed Matter (LPMC), Department of Physics, Faculty of Sciences, IBN TOFAIL University, Kenitra, Morocco

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 September 2017 Received in revised form 24 January 2018 Accepted 2 March 2018 Available online xxx

CuO has attracted much interest owing to its appropriate material properties, inexpensive fabrication cost and potential applications including energy devices. In this study, copper oxide CuO thin films were deposited on glass slides substrates by the simple and economical modified successive ionic layer adsorption and reaction (modified SILAR). Properties of synthesized films were studied as a function numbers of deposition cycles and immersion time in the anionic solution (reaction time). The structural, surface morphology, chemical compositions and optical properties of the films have been investigated by X-ray diffraction analysis, scanning electron microscopy SEM, EDX and UV-vis spectrophotometry. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Copper oxide CuO SILAR method Thin films XRD Optical properties

1. Introduction Overall consideration on the readiness and portrayal of semiconductor oxide materials is increasing awesome force because of the developing novel applications including energy devices [1]. Copper oxide tenorite phase CuO is one of the vital semiconductors, profitable materials and has been considered for photovoltaic applications, since he has a high activity and selectivity in oxidation and reduction reactions [2]. This oxide is p-type semiconductor, the constituent elements are inexpensive and non-toxic [1,2]. It exhibits superior properties, including a direct band gap, from 1.2 to 2.1 eV [1,3], which is close to optimal band gap, 1.5 eV for solar cells and has high absorption coefficient in visible wavelengths of about 105 cm1 [4e7]. It is also having a monoclinic crystal structure (a ¼ 4.69 Å, b ¼ 3.42 Å, c ¼ 5.13 Å), which belongs to a crystallographic space group C2/c [7e9]. The synthesis of CuO thin films has been carried out by many physical and chemical techniques such as RF magnetron sputtering, pulsed laser deposition (PLD) [10,11], chemical spray pyrolysis (CSP) [4,12], spin-coating [13], Chemical Bath Deposition (CBD) [1,14], electrodeposition [15], successive ionic layer adsorption and reaction (SILAR) [16,17]. Among different advantages of SILAR: its a very simple technique, inexpensive, more efficient [18], the deposition rate and film thickness can be easily controlled [19]. In the normal SILAR method rinsing step is used which is avoided in modified SILAR method. Avoiding rinsing step increases rate of deposition and reduces deposition time [20]. In this research work, CuO thin films were deposited on glass slides substrates by modified SILAR method with variation of numbers of deposition cycles and reaction time. The structural, morphological and optical properties of CuO thin films optimized to be used in photovoltaic applications as an absorbent layer in solar cells.

* Corresponding author. E-mail address: [email protected] (O. Daoudi). https://doi.org/10.1016/j.spmi.2018.03.006 0749-6036/© 2018 Elsevier Ltd. All rights reserved.

Please cite this article in press as: O. Daoudi et al., Study of the physical properties of CuO thin films grown by modified SILAR method for solar cells applications, Superlattices and Microstructures (2018), https://doi.org/10.1016/j.spmi.2018.03.006

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2. Experimental details 2.1. Substrate cleaning Substrate cleaning plays an important role in the deposition of thin films, glass sildes substrates were used for the deposition of CuO thin film, were treated as follows before their use. Initially the substrates rinsed in a solution of distilled water and HCl acid for 15 min. Secondly were rinsed in acetone for 15min, following washed in an ethanol bath for 15 min finally dried before being used. 2.2. Copper oxide CuO thin films formation The copper oxide CuO thin films were deposited on glass slides substrates using the modified SILAR method. The cationic precursor was prepared with 0.1 M copper sulfide (CuSO4.5H2O) and 6 ml: 100 ml of ammonium hydroxide NH4OH. Then it was stirred in a magnetic stirrer for a few min, pH value of the solution was equalled  11, thus a copper-ammonia complex [Cu(NH3)4]2þ was obtained. The anionic precursor was hot deionized water maintained at 90  C. A SILAR cycle can be described as follows: the substrates were dipped into the complex [Cu(NH3)4]2þ solution. Then they were taken out from the bath and immediately dipped into hot water. For study the influence of deposition cycles this cycle repeated for 20, 30, 40 and 50 times, the adsorption time (in cationic solution) 30 s, the reaction time 30 s. On the other synthesis of CuO thin films the adsorption time 30 s, the reaction time was varied from 20 s to 40 s with an interval of 5 s. The deposition was carried out by the Holmarc's SILAR Coating System with Stirrer to achieve uniform film coating. For the first few deposition cycles CuO thin films exhibited pale brown color and the brown color darkened successively with the increase in deposition cycles. Finally the samples were rinsed in distilled water and were dried at room temperature for a day. 2.3. Characterizations of thin films The structural characterization of the CuO thin films was determined by X-ray diffraction (XRD) technique using a XPERT-3 X-ray diffractometer with a copper source of monochromatic Cu-Ka1 radiation; wavelength l ¼ 1:5406 A operating at 45 kV tube voltage and 40 mA current, with 2q ranging from 20 to 70 . The surface morphology and the composition of the films were examined by scanning electron microscopy (SEM), Energy Dispersive X-ray analysis (EDX). The optical characterizations, absorbance and transmittance spectra, were recorded using JENWEY-67 UV Visible spectrophotometer. 3. Results and discussion 3.1. Structural characterization Fig. 1 shows X-ray spectra of all the CuO thin films deposited on glass slides substrates by modified SILAR method with variation of (a) numbers of deposition cycles, (b) immersion time in the anionic solution (reaction time). The X-ray diffraction spectrum revealed that all the obtained films were polycrystalline nature. The diffraction peaks at 2q equal to 35.45 , 38.63 and 48.8 correspond to (111), (111) and (202) planes of monoclinic structure with tenorite phase CuO crystallites adjusted for PDF to better identify the card number (00-041-0254). Other less prominent peak corresponding to (020) was appeared as the number of deposition cycles increased. This is explained by the fact that the film is more thickened hence the improvement in the crystallinity. The texture is a term assigning a preferential orientation of the crystallites of a polycrystalline material. Depending on various factors, the crystallites can be oriented, not completely by random, but preferably in one or more particular direction.

Fig. 1. XRD patterns of CuO thin films deposited with variation of (a) numbers of deposition cycles (b) reaction time.

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The texture coefficient measures the relative degree of preferred orientation of specific (hkl) plane. This coefficient is calculated through the XRD spectrum by using the formula Eq (1).

TC ðhklÞ ¼

n IðhklÞ 1 X Iðhi ki li Þ I0 ðhklÞ n i¼1 I0 ðhi ki li Þ

!1 (1)

Where I0 represents the standard intensity taken from a powder diffraction file (PDF) card, n is the number of diffraction peaks, and I is the measured intensity of (hi ki li ) plane [7,21,22]. The reference estimation of TC is 1, it gives a random orientation of crystallites. The TC values greater than one (TC > 1) demonstrates the wealth of crystallites in a given (hkl) plane. For the TC (hkl) values that lie in vicinity zero, demonstrate the absence of crystallite orientation in this direction [7]. The crystallite size was calculated using the Scherer's equation Eq (2) [2].



0:9l bcosq

(2)

Where l is the wavelength of the X-rays, q is the Braggs diffraction angle, and b is the full width at half-maximum intensity (FWHM) of the diffraction peaks (in radians). To have more information on the measure of imperfections in the films. The dislocation density (d) was determined using the formula Eq. (3) [2].



1 D2

(3)

The strain ðεÞ was calculated using the relation Eq (4) [2].

ε¼

bcosq

(4)

4

The variations of structural parameters are given in Table 1. From this table it can be observed that the majority values of texture coefficients are greater than 1 along (111) (111) directions for all films which indicate that the number of grains oriented along these planes is more than the others and also indicate the good crystalline quality of the grown film [23]. The numbers of deposition cycles increase number of anions and cations increase these are responsible for the increase of the crystallite size [20]. The larger values of the crystallite size, the smaller the dislocation densities, strain indicate that the films are a better crystallization [2].

Table 1 The texture coefficient (TC), Full width at half maximum (FWHM), the crystallite size (D), dislocation density (d), and strain (ε) of CuO thin films. Samples

(hkl)

TC ðhklÞ

FWHM ( )

D (nm)

dð  103 nm2 Þ

εð  103 Þ

S1 ¼ 20 cycles S2 ¼ 30 cycles

_ (111) (111) (002) (111) (111) (002) (111) (111) (002) (111) (111) (202) (111) (111) (202) (111) (111) (202) (111) (111) (202) (111) (111) (202)

_ 1.00 0.99 _ 1.08 0.92 _ 1.13 0.89 _ 1.06 0.89 1.02 1.41 1.09 0.51 1.19 1.17 0.64 1.33 1.2 0.38 1.13 0.93 1.03

_ 0.8266 _ _ 0.5510 _ _ 0.3444 _ _ 0.5904 _ _ 0.8266 _ _ 0.4723 _ _ 0.5904 _ _ 0.5904 _ _

_ 10 _ _ 15 _ _ 24 _ _ 14 _ _ 10 _ _ 18 _ _ 14 _ _ 14 _ _

_ 10 _ _ 4.44 _ _ 1.74 _ _ 5.1 _ _ 10 _ _ 3.09 _ _ 5.1 _ _ 5.1 _ _

_ 3.43 _ _ 2.27 _ _ 1.42 _ _ 2.45 _ _ 3.44 _ _ 1.95 _ _ 2.45 _ _ 2.45 _ _

S3 ¼ 40 cycles

S4 ¼ 50 cycles

S5 ¼ 20s

S6 ¼ 25s

S7 ¼ 30s

S8 ¼ 35s

S9 ¼ 40s

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3.2. Surface morphology and chemical compositions Surface morphology of CuO thin films deposited on glass slides substrates was investigated by the scanning electron microscopy. The SEM images of (S2 ¼ 30 cycles), (S3 ¼ 40 cycles) and (S5 ¼ 50 cycles) are shown in Fig. 2. The SEM images show that the films are well covered and adhered to the substrate with no holes and good homogeneity. This type is important for the applications in thin films solar cells [24]. The inset of Fig. 2 shows a SEM of thin films at X 80,000 magnification, it can be seen from that the average size of the grain are in the range 44 nm-160 nm. Chemical compositions of the prepared samples (S2, S3 and S4) was identified using EDX analysis. Fig. 3 shows EDX spectra of CuO thin films revealed

Fig. 2. SEM images of CuO thin films deposited in 30 (S2), 40 (S3) and 50 (S4) cycles.

Fig. 3. EDX spectra of CuO thin films deposited in 30 (S2), 40 (S3) and 50 (S4) cycles.

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Table 2 Atomic concentration from EDX of CuO thin films deposited in 30 (S2), 40 (S3) and 50 (S4) cycles. Atomic concentration (%) samples

Cu

O

S2 ¼ 30 cycles S3 ¼ 40 cycles S5 ¼ 50 cycles

25.78 26.14 41.53

74.22 73.86 58.47

Fig. 4. Optical absorbance spectra of CuO thin films deposited in 30, 40 and 50 cycles.

the presence of copper and oxygen and other elements from the glass slides substrates. Atomic concentrations of Cu and O in the films were obtained through EDX analysis and the results are given in Table 2. 3.3. Optical properties Fig. 4 shows the absorbance spectra of CuO thin films deposited on glass slides substrates for different deposition cycles, 30, 40 and 50 cycles. From this figure, it is seen that a sharp drop in the absorbance and the absorption increases with the increase of deposition cycles, because there is increment in thickness [20]. These spectra do not show a step, which confirms that there is only one phase, this is in agreement with the results of X-ray diffraction which shows that there is no presence of secondary phase (see Fig. 1a). The optical absorbance-transmittance measurements were utilized to calculate the energy band gaps and to investigate the optical properties of the CuO thin films. The absorption coefficient ðaÞ was calculated using equation (5) [25].



lnT d

(5)

where d is the thickness of the film and T is the transmittance.As CuO is a direct band gap material, the absorption coefficient (a) can be related to the band gap energy (Eg ) through Tauc's relationship (6) [26].

  ðahnÞ2 ¼ B hn  Eg

(6)

Where B is a constant, hn is the energy of the photon. The optical gap Eg is determined by the intersection point of the extrapolation of the linear portion of the curve ðahnÞ2 with the photon energy axis ðhnÞ as shown in Fig. 5. The values of 1.92 eV, 1.89 eV and 1.69 eV were estimated for the 30, 40 and 50 cycles respectively. The decrease in band gap with deposition cycles can be attributed to the improvement in the crystallinity, morphological changes of the films, changes of atomic distances and grain size and structural defects in the film [17]. 4. Conclusion Thin films of copper oxide CuO were successfully deposited onto glass slides substrates by simple and inexpensive modified SILAR method. The preparation conditions such as concentration, pH, temperature and immersion period were optimized to obtain homogenous and good quality of CuO thin films. The X-ray diffraction spectroscopic study shows that Please cite this article in press as: O. Daoudi et al., Study of the physical properties of CuO thin films grown by modified SILAR method for solar cells applications, Superlattices and Microstructures (2018), https://doi.org/10.1016/j.spmi.2018.03.006

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Fig. 5. Plot of ðahnÞ2 versus hn for CuO thin films deposited at 30 cycles (a), 40 cycles (b), 50 cycles (c).

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deposited CuO are polycrystalline in nature with monoclinic structure. SEM images show films are dense and compact and EDX analysis confirm the presence of copper and oxygen. The optical band gap energy was found to decrease from 1.92 to 1.69 eV with the increase in number of deposition cycles. The most important conclusion of this study, show that the films are dense potential for the use in photovoltaic applications as an absorbent layer in solar cells. Acknowledgements This study was supported by the CNRST Projects (PPR-Project no: 2015/37). References [1] V. Ramya, K. Neyvasagam, R. Chandramohan, S. Valanarasu, A.M.F. Benial, Studies on chemical bath deposited cuo thin films for solar cells application, J. Mater. Sci. Mater. Electron. 26 (11) (2015) 8489e8496, https://doi.org/10.1007/s10854-015-3520-3. [2] Y. 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