ARTICLE IN PRESS Physica B 405 (2010) 2009–2013
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Effect of temperature on structural, optical and photoluminescence properties of polycrystalline CuInS2 thin films prepared by spray pyrolysis C. Mahendran a, N. Suriyanarayanan b, a b
Department of Physics, Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore 641 020, Tamil Nadu, India Department of Physics, Government College of Technology, Coimbatore 641 013, Tamil Nadu, India
a r t i c l e in fo
abstract
Article history: Received 18 September 2009 Received in revised form 28 December 2009 Accepted 15 January 2010
Copper indium disulphide (CuInS2), is a good absorber material for photovoltaic applications. In this work, CuInS2 is deposited by chemical spray pyrolysis on heated glass substrates. It is observed that the film growth temperature and the ion ratio Cu/In affects the structural and optical properties of CuInS2 thin films. This paper presents the effect of temperature on the growth (for the ion ratio Cu/In=1.25), optical and photoluminescence properties of sprayed CuInS2 films. The XRD patterns confirm the well defined single phase composition of CuInS2 films grown from 300 to 350 1C (at Cu/In=1.25) as optimum temperature for depositing well defined crystallites along (1 1 2) oriented CuInS2 thin films with chalcopyrite structure. D2d point symmetry group is associated with the CuInS2 crystallites with energy gap of 1.53 eV at room temperature. The chemical nature and the presence of additional phases are discussed based on the EDAX measurements. The absorption coefficient of sprayed CuInS2 films is found to be in the order of 105– 106 cm 1 in the UV–visible region and the optical band gap decreases with increase in temperature. Defects-related photoluminescence properties are also discussed. CuInS2 polycrystalline films are prepared by the cost effective method of spray pyrolysis from the aqueous solutions of copper (II) chloride, indium (III) chloride and thiourea for synthesis on heated glass substrates. & 2010 Elsevier B.V. All rights reserved.
Keywords: Copper indium disulfide Thin films Optical properties Structural properties Photoluminescence Spray pyrolysis
1. Introduction Fabrication of thin film CuInS2 solar cells has drawn considerable attention in recent years. It has high absorption coefficient, direct band gap of 1.5 eV and contains non-toxic constituent’s making it suitable for photovoltaic applications. CuInS2 thin films have high absorptive layers and belong to semiconducting ternary compounds of I–III–VI2 with chalcopyrite structure. Various methods are used to deposit CuInS2 thin films [1–7]. Among them spray pyrolysis is an attractive, low cost method and the application that enables the deposition of thin films of larger area with good uniformity [8–12]. The present study reveals the variation of structural, optical and photoluminescence properties of as-deposited CuInS2 thin films (Cu/In=1.25) in the temperature range 300–400 1C on glass substrates.
2. Experimental CuInS2 thin films were deposited by spray pyrolysis on to glass substrates from aqueous solution of CuCl2, InCl3, and SC(NH2)2 using compressed air as the carrier gas. At first, aqueous solution (0.1 M) of these salts were prepared. Then they were mixed with Corresponding author. Tel.: + 91 9442843734.
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[email protected] (N. Suriyanarayanan). 0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.01.090
appropriate portions in order to have copper to indium molar ratio (Cu/In=1.25) and sulphur/copper ratio (S/Cu) fixed to 1 in the solution. The copper (II) chloride and indium (III) chloride were mixed and then thiourea solution was added. The solutions were prepared by dissolving in deionized water. Then the solution was sprayed using spray rates of 2 ml/m in air onto glass substrates (2.5 2.5 cm2) heated at different temperatures from 300 to 400 1C. The X-ray diffraction (XRD) patterns of sprayed films were recorded using the XPERT-PRO Gonio scan diffractometer with Cu Ka radiation. The phases were identified using JCPDS files. The optical transmittance spectra were recorded in the wavelength range 400–1500 nm using double beam Beckman Ratio Recording spectrophotometer. The surface morphology of the films was investigated using a Jeol, JSM-6390, JM-Spot size 35. The compositional analysis was carried out using energy dispersive X-ray spectroscopy (EDAX). A photoluminescence (PL) spectrum of the films was recorded using a Cary Eclipse instrument in fluorescence emission scan mode with excitation wavelength of 400 nm. 3. Results and discussion 3.1. Structural analysis The structural and the phase composition of sprayed CuInS2 films depend on growth temperature and Cu/In ratio in the
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spraying solution. XRD patterns obtained for as-deposited films with Cu/In= 1.25 at temperatures lower than 300 1C form very weak and broad reflections, indicating poor crystallinity of the films. The increase in temperature from 300 1C, facilitate the growth of polycrystalline CuInS2 (Fig. 1a). Additional reflections recorded at temperatures less than 300 1C, all disappeared with increase in growth temperature from 300 to 350 1C and well defined polycrystalline CuInS2 films with preferred orientation of (1 1 2) direction were formed [13]. The peak (1 1 2) on XRD becomes sharper at higher growth temperatures of 325 and 350 1C (Fig. 1b and c). The temperature required for single phase growth depends on the ratio of Cu/In in spraying solution. At Cu/In=1.25 the film
exhibits diffraction peak belonging to CuInS2 [PDF 27-159] at growth temperatures 300–350 1C (Fig. 1a–c). When the temperature is increased beyond 350 1C (Fig. 1d), the (1 1 2) preferred growth orientation of poly-crystals in the film gradually decreases and almost all the height of the reflections are suppressed at 400 1C (Fig. 1e) indicating the poor crystallinity of the film. CuInS2 films grown at temperatures 300–350 1C using Cu rich solutions have chalcopyrite structure. Increase in temperature in Cu excess spraying solutions have strong effect in the formation of chalcopyrite structure [14]. Our experiment clearly confirms that the temperature has a strong effect on crystalline structure of CuInS2 films deposited from Cu/In= 1.25 solutions. The films sprayed at substrate temperatures 300–350 1C exhibit a chalcopyrite structure (Fig. 1a–c), while at 375 and 400 1C the films are amorphous (Fig. 1d and e) [15]. The additional peaks in the XRD are formed due to the formation of intermediate complex compounds such as binary sulfides as a result of thermal decomposition containing metal chlorides and thiocarbamide [16–18]. The additional peaks at higher temperatures 375 and 400 1C are due to the formation of In2O3 and it is transparent at higher wavelengths (Fig. 3). XRD measurements show that there is a considerable growth of crystals in the temperature range 300–350 1C inside the film. The size of the crystallites is very much reduced in the temperature range 375–400 1C and form amorphous structure. EDAX measurements confirm the presence of Cu, In, S in the film (Fig. 2a–e). The increase in temperature to 375 1C and beyond leads to oxidation process which is also confirmed in EDAX measurements.
3.2. Optical properties Fig. 1. XRD of sprayed CuInS2 films prepared at different substrate temperatures: (a) 300 1C, (b) 325 1C, (c) 350 1C, (d) 375 1C, (e) 400 1C.
The optical transmittance of the sprayed CuInS2 films is presented in Fig. 3. There is no significant change in optical
Fig. 2. EDAX spectra of sprayed CuInS2 films and separate crystallites on the surface at different substrate temperature: (a) 300 1C, (b) 325 1C, (c) 350 1C, (d) 375 1C, (e) 400 1C.
ARTICLE IN PRESS C. Mahendran, N. Suriyanarayanan / Physica B 405 (2010) 2009–2013
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transmittance of CuInS2 films as the temperature of substrate is increased from 300 to 400 1C. The light scattering is low and in all the temperatures, about 70% of light is transmitted in the wavelength of range 800–1500 nm. The decrease in film thickness with increase in temperature from 300 to 400 1C has no significant effect on optical transmittance of CuInS2 thin films. Improved optical transmittance property is observed
Table 1 Variation of band gap and thickness of the film with the temperature.
Fig. 3. The transmittance spectra of CuInS2 film sprayed at different substrate temperatures 300–400 1C.
Temperature (1C)
Band gap (eV)
Thickness (nm)
300 325 350 375 400
1.66 1.63 1.62 1.60 1.58
694.4 672.3 652.3 621.0 609.4
Fig. 4. Variation of optical band gap energy values (Eg) of sprayed CuInS2 films prepared at different substrate temperatures: (a) 300 1C, (b) 325 1C, (c) 350 1C, (d) 375 1C, (e) 400 1C.
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Fig. 5. SEM micrographs of sprayed CuInS2 films prepared at different temperatures: (a) 300 1C, (b) 325 1C, (c) 350 1C, (d) 375 1C, (e) 400 1C.
at higher temperatures which are not reported by other researchers. The band gap energy (Eg) and absorption coefficient (a) for the sprayed CuInS2 films are determined from the optical transmission data. The absorption coefficient (a) can be calculated by
a ¼ ð2:303=tÞ½log ð1=TÞ
Fig. 6. Photoluminescence spectra of sprayed CuInS2 films excited by the 400 nm at various substrate temperatures: (a) 300 1C, (b) 325 1C, (c) 350 1C, (d) 375 1C, (e) 400 1C.
ð1Þ
where t is the thickness of the film and T is the transmittance. The absorption coefficient (a) is calculated for different wavelengths (l). The a value is found to be in the order of 105–106 cm 1 in the UV and visible region of the spectrum and reaches higher values as the temperature of the substrate is increased. Though such high value of a may be useful for the fabrication of high absorptive layers of solar cell, the spectral dependence of a may affect considerably the solar energy conversion efficiency. For the direct band gap semiconductors a can be related to through the equation: ðahuÞ2 ¼ AðhuEg Þ
ð2Þ
ARTICLE IN PRESS C. Mahendran, N. Suriyanarayanan / Physica B 405 (2010) 2009–2013
A is a constant, Eg is the Band gap energy and hu is the photon energy. A plot of ðahuÞ2 versus hu for the films deposited at various temperatures is presented in Fig. 4a–e. It is observed that the band gap energy (Eg) decreases as the temperature of the substrate increases (Table 1). The obtained Eg values are in good agreement for the sprayed CuInS2 films reported by other researchers [8,11,12,19]. The thickness of the film is found to decrease with increase in substrate temperature [20–22]. It is observed that, the influence of both the film thickness and substrate temperatures affect Eg values. The presence of unsaturated defects which increase the density of localized states in the band gap could be the cause for the reduction in band gap energies from 1.66 to 1.58 eV in the temperature range of 300–400 1C [23]. Further the dislocation density of the film increases as the temperature of the substrate is increased from 300 to 400 1C. Such investigated films are promising for the solid state modeling because these ternary materials possess large opportunity of the intrinsic defect states [24,25]. 3.3. Surface morphology The CuInS2 films deposited with Cu/In= 1.25 in the sprayed solution at temperatures 300–375 1C show a smooth surface. The films consisting of crystallites with sizes of about 100–200 nm (Fig. 5a–d) are observed. The scattering of the light from these surfaces is low and nearly 70% of transmission is observed in the wavelength range 800–1500 nm (Fig. 3). At temperatures of 300 and 350 1C, crystallites with sizes of about 200 nm are formed (Fig. 5a and c). The increase in substrate temperature leads to growth of the crystallites in the film and separate crystals on the surface. At 400 1C the crystal sizes are found to be about 700 nm (Fig. 5e) and the film surface is found to be smooth which results in no change in optical transmittance. 3.4. Photoluminescence Fig. 6(a)–(e) shows the emission spectra of the CuInS2 thin films grown in the temperature range (300–400 1C). Photoluminescence spectra have been recorded at room temperature with an excitation wave length of 400 nm for all the samples. About 6 emission peaks are observed in the wave length range 450–720 nm (Blue, Green, Yellow and Red band emission) when the substrate temperature is at 300 1C. This feature corresponds to Donor Acceptor pair transition between a sulfur vacancy and an In vacancy or Cu on an In site. [26–29]. The peaks seem to consist of two components due to the possibility of double structure in the sulfur vacancy, which may act as a doubly ionized donor. When the substrate temperature is in the range of 325–375 1C these peaks shift towards shorter wavelength and the film is found to consist of 3 peaks only, in the wave length region 440– 540 nm (Fig. 6b–d) (Blue and Green band emissions). In the photoluminescence spectra the peaks are found to broaden out with increase in substrate temperature and the broadening of peaks can be attributed to high concentration of defects. Further, the broadening of peak is due to the fact that the large crystals tend to produce more defects than small crystals. These defects may act as non-radiative recombination centres which quench the radiative band edge recombination [30]. Further increase in temperature (Fig. 6e) causes increase in number of peak emissions which may be due to the recombina-
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tion of deep donors and deep acceptors. All the emissions are associated with defects emerging during the growth of crystallites and are related to deformation of crystallinity due to dislocations and large vacancies [31].
4. Conclusions The (1 1 2) oriented chalcopyrite structures CuInS2 at (Cu/ In=1.25) are grown on glass substrates in the temperature range 300–400 1C. The increase in temperature affects the structure of the film. With increase in temperature to 400 1C and beyond, the film becomes amorphous. The EDAX measurements confirm the presence of Cu, In and S in the film. The films grown in the temperature range 300–400 1C have high absorption coefficient and their optical band gap energies (Eg) decrease with increase in temperature. There is no significant change in optical transmittance of the film as the substrate temperature is increased and almost about 70% light is transmitted in all the films. The photoluminescence spectra of the CuInS2 films exhibit Blue, Green, Yellow and Red band emissions corresponding to defect related luminescence emissions. References [1] L.L. Kazmerski, G.A. Sanborn, J. Appl. Phys. 48 (1977) 3178. [2] R. Scheer, T. Walter, H.W. Schock, M.L. Fearheily, H.J. Lewerenz, Appl. Phys. Lett. 63 (1993) 3294. [3] T. Walter, R. Menner, Ch. Koble, H.W. Schock, in: Proceedings of the 12th European Photovoltaic Solar Energy Conference, Amsterdam, 1994, p. 684. [4] T. Watanabe, M. Matsui, Jpn. J. Appl. Phys. 35 (1996) 1681. [5] Y. Ogawa, A. Jager-Waldau, Y. Hashimoto, K. Ito, Jpn. J. Appl. Phys. 33 (1994) L1775. [6] Y. Ogawa, S. Uenishi, K. Tohoyama, K. Ito, Sol. Energy Mater. Sol. Cells 35 (1994) 157. [7] C. Dzionk, H. Metzner, S. Hessler, H.-E. Mahnke, Cryst. Res. Technol. 31 (1996) 773. [8] M. Ortega-Lopez, A. Morales-Aveco, in: Proceedings of the 25th IEEE PVSC, Washington, May 13–17, 1996, p. 13. [9] H. Onnagawa, K. Miayashita, Jpn. J. Appl. Phys. 23 (1984) 965. [10] H. Bihri, C. Messaoudi, D. Sayah, A. Boyer, A. Mzerd, M. Abd-Lefdil, Phys. Status Solidi 129 (1992) 193. [11] R.P.V. Lakshmi, R. Venugopal, D.R. Reddy, B.K. Reddy, Solid State Commun. 82 (1992) 997. [12] C. Messaoudi, H. Bihri, D. Sayah, M. Cedene, M. Abd-Lefdil, J. Mater. Sci. Lett. 11 (1992) 1234. [13] Malle Krunks, Olga Bijakina, Tiit Varema, Valdek Mikli, Enn Mellikov, Thin Solid Films 338 (1999) 125–130. [14] S. Shirakata, T. Murakami, T. Kariya, S. Isomura, Jpn. J. Appl. Phys. 35 (1996) 191. [15] M. Sahal, B. Mari, M. Mollar, Thin Solid Films 517 (2009) 2202–2204. [16] B.J. Brown, C.W. Bates, Thin Solid Films 188 (1990) 301. [17] M. Krunks, J. Madarasz, L. Hiltunen, R. Mannonen, E. Mellikov, L. Niinisto, Acta Chem. Scand. 51 (1997) 294. [18] M. Krunks, E. Mellikov, O. Bijakina, Phys. Scr. T69 (1997) 189. [19] R. Rajaram, R. Thangaraj, A.K. Sharma, A. Raza, O.P. Agnihotri, Thin Solid Films 100 (1983) 111. [20] M. Krunks, E. Mellikov, Thin Solid Films 270 (1995) 33. [21] M. Krunks, E. Mellikov, E. Sork, Thin Solid Films 145 (1986) 105. [22] M. Krunks, E. Mellikov, O. Bijakina, T. Varema, D. Meissner, SPIE Proc. 2968 (1997) 129. [23] EI-Zahed, et al., Thin Solid Films 11 (2003) 19–27. [24] A.H. Reshak, S. Auluck, I.V. Kityk, Y. Al-Douri, R. Khenata, A. Bouhemadou, Appl. Phys. 94 (2009) 315. [25] A.H. Reshak, et al., J. Alloys Compud. 473 (2009) 20. [26] J. Van Gheluwe, J. Versluys, D. Poelman, J. Verschraegen, M. Burgelman, P. Clauws, Thin Solid Films 511–512 (2006) 304–308. [27] J. Eberhardt, H. Metzner, R. Goldhahn, F. Hudert, U. Reislohner, C. Hulsen, J. Cieslak, T.H. Hahn, M. Gossla, A. Dietz, G. Gobsch, W. Witthuhn, Thin Solid Films 480–481 (2005) 415. [28] J.J.M. Binsma, L.J. Giling, J. Bloem, J. Lumin. 27 (1982) 35. [29] M. Nanu, J. Schoonman, A. Goossens, Thin Solid Films 451–452 (2004) 193. [30] B.A. Kulp, H. Kelley, J. Appl. Phys. 31 (1960) 1057. [31] T. Onishi, K. Abe, Y. Miyoshi, K. Wakita, N. Sato, K. Mochizuki, J. Appl. Phys. Chem. Solids 66 (2005) 1947–1949.
