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Spray pyrolysis deposition of p-CdTe films: Structural, optical and electrical properties
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ScienceDirect Solar Energy xxx (2013) xxx–xxx www.elsevier.com/locate/solener
Spray pyrolysis deposition of p-CdTe films: Structural, optical and electrical properties S.D. Gunjal a, Y.B. Khollam b, S.R. Jadkar a, T. Shripathi c, V.G. Sathe c, P.N. Shelke d, M.G. Takwale a, K.C. Mohite a,e,⇑ a
School of Energy Studies, Department of Physics, University of Pune, Pune 411 007, India b Department of Physics, Anantrao Pawar College, Pirangut, Pune 412 115, India c UGC–DAE–CSR, University Campus, Khandwa Road, Indore 452 017, India d Department of Physics, B.R. Gholap College, New Sangvi, Pune 411 027, India e Department of Physics, H.V. Desai College, Budhwar Peth, Pune 411 002, India Available online xxxx Communicated by: Associate Editor Smagul Zh. Karazhanov
Abstract The present communication reports the structural, optical and electrical properties of p-CdTe films deposited by using spray pyrolysis technique on thoroughly cleaned sodalime glass substrates at 350 °C in inert N2 atmosphere. The films were characterized by using XRD, Raman spectroscopy, XPS, thickness profiler, SEM, EDAX and UV–Visible spectroscopy and Hall parameters measurement. The structural studies showed the formation of pure cubic CdTe in resultant films with crystallite size = 32 nm. The texture coefficient TC(h k l) indicated preferential orientation of films along [3 1 1] and [5 1 1] directions. The SEM image showed almost spherical granules with average size of 0.25 lm. From UV–Visible spectral data, the maximum absorbance and direct band gap obtained using Tauc plot are found to be 0.94 and Eg = 1.44 eV respectively for films having thickness, t = 3.2 lm. The Hot probe experiment, Hall coefficient RH = 1.478 103 cm3/C, and resistivity q = 2.228 101 X cm confirmed the p-type semiconducting behavior of CdTe films. The mobility of majority charge carriers – holes is found to be l = 6.63 101 cm2/Vs. The films processed using present work can be treated as possible p-type candidate for fabrication of hetero-junction solar cell. Ó 2013 Elsevier Ltd. All rights reserved. Keywords: CdTe films; Spray pyrolysis; Optical properties; Hall parameters
1. Introduction Cadmium telluride (CdTe) is one of the most useful semiconducting compounds for the various applications in c-ray detectors, infrared windows, solar cells, and other optoelectronic devices (Ison et al., 2009; Chopra et al., 2004; Zanio, 1978). The deposition methods and their preparation conditions are very important for the preparation ⇑ Corresponding author at: School of Energy Studies, Department of Physics, University of Pune, Pune 411 007, India. Tel.: +91 20 25695201 (O), +91 9890094577 (M). E-mail address: [email protected] (K.C. Mohite).
of CdTe films giving high conversion efficiency to solar cell. Different deposition methods have been used to obtain the photovoltaic quality p-type cadmium telluride (p-CdTe) films (Oliva et al., 2003; Soliman et al., 2001; Seth et al., 1999; Abdalla et al., 2009; Colombo et al., 2009; Razykov et al., 2009) giving high solar cell conversion efficiency upto 17.3% (Flores et al., 2012). The deposition of phase pure and stoichiometric CdTe layer is one of the critical stages in solar cell fabrication. The chemical spray pyrolysis method having low equipment cost and easy to operate is more suitable technique for the depositions of thin polycrystalline films of different materials (Patil, 1999; Chapman et al., 2002). The important advantage of this
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Please cite this article in press as: Gunjal, S.D., et al. Spray pyrolysis deposition of p-CdTe films: Structural, optical and electrical properties. Sol. Energy (2013), http://dx.doi.org/10.1016/j.solener.2013.11.029
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method is that the properties of films can easily be changed by varying the deposition conditions. In view of this, the present work focuses on the preparation of phase and material pure p-CdTe films using spray pyrolysis method. The resultant films are characterized by using XRD, Raman spectroscopy, XPS, thickness profiler, SEM and EDAX. The optical and electrical properties are obtained by using UV–Visible spectroscopy and Hall measurement set-up respectively. The results pertaining to this are discussed in this report. 2. Experimental The CdTe films were prepared by using home-built spray pyrolysis deposition system (Fig. 1) having glass spray gun with nozzle diameter = 0.1 mm. The 0.04 M precursor solutions of (i) CdCl2H2O (CDH Chem.) in 25 ml double distilled water (DDW) and (ii) TeO2 in 20 ml ammonia solution, 2 ml HCl and 3 ml hydrazine hydrate (reducing agent) were prepared for the preparation of CdTe films. These two precursor solutions were added together ultrasonically and the pH of final solution was noted to be 11.5. The sodalime glass substrates used for deposition of CdTe films were thoroughly cleaned by using treatments: (i) soap solution washing, (ii) rinsing with DDW and then with acetone (ultrasonically) for 10 min and finally (iii) dipping in isopropyl alcohol for 5 min. The Cd–Te based films were deposited on thoroughly cleaned glass substrates by using parameters: (i) spray gun nozzle-substrate distance = 25 cm, (ii) carrier N2 gas
Fig. 1. Schematic diagram for home-built spray pyrolysis system.
flow rate = 15 lpm, (iii) deposition time = 10 min, (iv) solution flow rate = 5 ml/min and (v) substrate temperature = 350 °C. Before deposition, the chamber was purged with Grade-I N2 gas for 30 min. After deposition, the films were cooled naturally to the room temperature (RT). The resultant films were characterized by using different physical techniques: (i) X-ray diffraction (XRD) (Bruker D8 Advance, Cu Ka X-ray line is k = 1.5406 1010 m), (ii) Raman spectroscopy (OLYMPUS BX41, HORIBA JOBIN YVON’ LABRAM HR800 Laser: He–Ne, Power: 20 mW, excitation wavelength: 632.81 nm), (iii) X-ray photoelectron spectroscopy (VSW Scientific Instruments Ltd., Manchester, England, Source: Al Ka radiation with photon energy = 1486.6 eV), (iv) scanning electron microscopy (SEM, JEOL JSM-6360A), (v) energy dispersive X-ray (EDAX) analysis, (vi) UV–visible spectroscopy (JASCO Model: V670) and (vi) surface profiler (KLA Tencor P_16+). The UV–Visible spectra were used to estimate optical properties: absorptance (a) and optical band gap (Eg) of resultant films. The electrical properties: sheet and bulk charge carrier concentration (n), mobility (l), resistivity (q), conductivity (r) and Hall coefficient (RH) were obtained by using Hall Effect measurement set-up (ECOPIA HMS-3000).
3. Results and discussion 3.1. X-ray diffraction (XRD) Fig. 2 gives the XRD pattern for the resultant film. All the observed peaks are found to be matching with the reflections given in JCPDS data for cubic phase CdTe (file No. 15-0770). Further, the ‘d’ values calculated by using Bragg’s condition, 2d sin h ¼ nk are found to be very close to standard data given in JCPDS data file No. 15-0770 for cubic phase CdTe. This indicates the formation of single cubic phase CdTe in resultant films. The average crystallite size is obtained by using classical Debye–Scherrer’s formula (Cullity and Stock, 2001):
Fig. 2. XRD pattern for the resultant film.
Please cite this article in press as: Gunjal, S.D., et al. Spray pyrolysis deposition of p-CdTe films: Structural, optical and electrical properties. Sol. Energy (2013), http://dx.doi.org/10.1016/j.solener.2013.11.029
S.D. Gunjal et al. / Solar Energy xxx (2013) xxx–xxx
D¼
3
0:9k b cos h
where k = 1.54 1010 m, b = FWHM in radian and h = Bragg’s angle for the different planes of cubic phase CdTe. The average crystallite size is found to be 32 nm indicating thereby the nanocrystalline nature of films. The data for the degree of orientation for different planes i.e. texture coefficient (Agashe et al., 1988; Barret and Massalski, 1980) is obtained by using the relation, ½Iðh k lÞ=I 0 ðh k lÞ TCðh k lÞ ¼ 1 P N Iðh k lÞ=I 0 ðh k lÞ N where I(h k l) = observed intensity of given plane, I0(h k l) = intensity of corresponding plane from JCPDS data file, N = number of reflections. The texture coefficient data is presented in Table 1. The data for TC(h k l) shows the preferred orientation of the film along the [3 1 1] and [5 1 1] directions. This is clearly supported by the value of standard deviation rg = 0.3567 (Holland and Kaelble, 1967) obtained by using the relation, " #1=2 1 X 2 rg ¼ ðTCðh k lÞÞ 1 N N
3.2. Raman spectroscopy Fig. 3 gives the Raman spectrum for the resultant film. The Raman peaks at 140 cm1 and 163 cm1 are due to the fundamental transverse optic (1TO) and longitudinal optic (1LO) phonon modes of CdTe. The peak at 122 cm1 corresponds to Te–O bond confirming the presence of oxide in the resultant film (Frausto-Reyes et al., 2006; Ison et al., 2009). It is reported that, the Raman spectroscopy can be used as a non-contact technique to detect the mean surface roughness of film sample (Frausto-Reyes et al., 2006). The Raman spectroscopy with an excitation wavelength of 632.8 nm can be used to detect changes in the surface roughness or as an on-line mean surface roughness monitor of film (Frausto-Reyes et al., 2006). At given excitation wavelength of 632.8 nm, with increasing the surface roughness, the optical phonon modes/peaks at 273 cm1 and 331 cm1 start appearing (Frausto-Reyes et al., 2006). In Fig. 3, the presence of optical phonon Table 1 Texture coefficient data. (h k l)
2h
I
I0
TC(h k l)
rg
111 220 311 400 331 422 511
23.7 39.3 46.4 56.8 62.4 71.2 76.2
100 70 42 7 11 12 7
100 60 30 6 10 10 4
0.7970 0.9298 1.1157 0.9298 0.8767 0.9564 1.3947
0.3567
Fig. 3. Raman spectrum for the resultant film.
modes at 273 cm1 and 331 cm1 might be due to the high surface roughness of resultant film qualitatively. 3.3. X-ray photoelectron spectroscopy (XPS) The X-ray photoelectron spectroscopy (XPS) can be used for the elemental analysis, identification of valence state of element, stoichiometric findings and phase symmetry analysis in the films. The XPS spectra of resultant film were recorded at a pressure >1 109 Torr. The general scan and C1s, O1s, Cd3d and Te3d core level spectra were recorded with non-monochromatized Al Ka radiation (photon energy = 1486.6 eV) at a pass energy of 50 eV and an electron takeoff angle (angle between electron emission direction and surface plane) of 55°. For XPS measurements, the overall resolution was 1.0 eV. The background corrections of core level spectra were done by using the Shirley + linear algorithm (Shirley, 1972). The baseline corrections and peaks fitting for all the spectra were done by using Xpspeak41 software. The core level binding energies (BEs) were aligned with respect to the C1s binding energy of 284.6 eV. Fig. 4(a) gives the general survey scan XPS spectrum of binding energy (BE) from 0 to 1200 eV for resultant film. It shows the presence of the peaks corresponding to only C1s, O1s, Cd3d and Te3d only. This confirms the purity of materials of resultant films prepared in present work. Further, in order to identify the nature of resultant film, O1s, Cd3d and Te3d core levels were recorded systematically at high resolution. The raw spectra of O1s, Cd3d and Te3d were fitted using the Xpspeak41 software program with Shirley + linear type background (Shirley, 1972) Fig. 4(b) and (c) gives the slow scan high resolution XPS raw spectra-fitted curves–resultant curves for Te3d core levels. Fig. 4(d) and (e) gives the slow scan high resolution XPS raw spectra-fitted curves-resultant curves for Cd3d and O1s core levels respectively. Due to spin-orbit coupling, the 3d peaks split into two components of 3d5/2 and 3d3/2. Fig. 4(b) shows the core level Ti 3d5/2 peaks at binding energies equal to 572.85 eV and 576.54 eV. Fig. 4(c) shows the core level Ti 3d3/2 peaks at binding energies equal to 583.22 eV and 586.91 eV. These two
Please cite this article in press as: Gunjal, S.D., et al. Spray pyrolysis deposition of p-CdTe films: Structural, optical and electrical properties. Sol. Energy (2013), http://dx.doi.org/10.1016/j.solener.2013.11.029
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Fig. 4. (a) General survey scan XPS spectrum, (b) XPS Te 3d5/2 spectrum, (c) XPS Te 3d3/2 spectrum, (d) XPS Cd 3d5/2 and Cd 3d3/2 spectra and (e) XPS O1s spectrum.
peaks can be assigned to Te2+ in CdTe at 572.7 ± 0.2 eV and Te4+ in TeO2 at 576.1 ± 0.1 eV (Briggs and Seah, 1983). Fig. 4(d) shows the core level Cd 3d5/2 peaks at binding energies equal to 405.19 eV and 411.98 eV. These two peaks can be assigned to the elemental Cd in CdTe at 405.2 ± 0.1 eV (Briggs and Seah, 1983). Fig. 4(e) gives the slow scan high resolution XPS raw spectra-fitted curves-resultant curves for O 1s core level. The peak is enhanced for clarity. Fig. 4(e) shows the core level O 1s peak at binding energies equal to 531.65 eV. This peak can be assigned to O4 in TeO2 at 531 ± 0.2 eV. All these
observations indicate the formation of phase pure CdTe in resultant films with minor oxidation corresponding to Te4+ (TeO2) at the surface. 3.4. Scanning electron microscopy The scanning electron microscopy (SEM) image of resultant film given in Fig. 5 shows the spherical morphology of CdTe dense particles without any agglomeration. The graph generated (Fig. 6) for particle size distribution from SEM image shows the uniform size distribution of
Please cite this article in press as: Gunjal, S.D., et al. Spray pyrolysis deposition of p-CdTe films: Structural, optical and electrical properties. Sol. Energy (2013), http://dx.doi.org/10.1016/j.solener.2013.11.029
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Fig. 7. EDAX plot for the resultant film.
hR
Fig. 5. SEM image for the resultant film.
a¼
1:0lm 0:4lm
aðkÞI k dk hR i 1:0lm I dk 0:4lm k
i
where Ik = intensity at wavelength k (Cullity and Stock, 2001; Duffie and Beckman, 2006). The average value of ‘a’ is found to be 0.94 for the resultant film. Fig. 8(b) gives the plot for the variation of (ahm)2 versus hm for the resultant film obtained by using Tauc relation, ðahmÞ
Fig. 6. The particle size distribution for the resultant film.
spherical particles with average particle size of 0.25 lm. The SEM image also exhibits porous nature with high roughness at the surface of film. This is consistent with the observations from Raman spectroscopy studies of resultant film. 3.5. Energy dispersive X-ray analysis Fig. 7 gives the energy dispersive X-ray (EDAX) analysis plot for the resultant film. It shows the presence of Cd and Te only indicating thereby the purity of material of resultant film. The EDAX data is given Table 2. The EDAX data confirms the stoichiometric (Cd:Te ratio = 1:0.9543) formation of resultant CdTe film. The presence of very small atomic% of O is due to the oxidation in the film corresponding to TeO2 phase during the processing. This is consistent with observations from the Raman spectroscopy studies of resultant film. 3.6. UV–Visible spectroscopy Fig. 8(a) gives the UV–visible spectrum for the resultant film. The value of absorptance (a) is obtained by using the equation,
1=2
¼ Aðhm Eg Þ
where a = absorption coefficient, A = constant independent of photon energy and hm (eV) = energy of excitation (Tauc et al., 1966). The value of direct band gap energy (Eg) obtained by extrapolation of straight-line portion of the plot to zero absorption edge is found to be 1.44 eV for the resultant film. The thickness of the resultant film measured by using thickness-surface profiler is found to be 3.2 lm. 3.7. Hall effect measurements The electrical properties of the resultant film were obtained by using Hall measurement set-up at current (I) = 10 nA and magnetic field (H) = 0.54 T. Table 3 gives the electrical characterization data for the same. The positive sign of average Hall coefficient (RH) and value of resistivity (q) indicate the p-type semiconducting nature of the resultant film deposited at 350 °C. The negative sign of current obtained in Hot Probe experiment also confirmed the p-type nature of resultant film. There are two important aspects in the preparation of CdTe films by using the spray pyrolysis deposition technique in present work. Table 2 EDAX data. Element
Mass%
Atomic%
Cd:Te ratio
Cd Te O
47.96 51.96 00.08
50.85 48.53 00.63
1:0.9543
Please cite this article in press as: Gunjal, S.D., et al. Spray pyrolysis deposition of p-CdTe films: Structural, optical and electrical properties. Sol. Energy (2013), http://dx.doi.org/10.1016/j.solener.2013.11.029
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formation of oxide phases corresponding to TeO2 and/or CdTeO3 (Krishna et al., 2003; Rao and Dutta, 2004). Further, the crystallization of CdTe films with wide particle size distribution is also reported in literature (Enriquez and Mathew, 2004; Fritsche et al., 2001). The reported particle/grain sizes are found to be higher and in micrometer range (Seto et al., 2001). The several reports have shown the deposition of CdTe films by using different wet-chemical and physical routes. However, p-type nature of the films is not confirmed by using the measurement Hall Effect parameters and/or Hot probe method. In view of these points, the CdTe films prepared in present work are phase pure with a very minor oxidation corresponding to the TeO2 at the surface. This might be due to use of lower concentration of the hydrazine hydrate as reducing agent or processing of films in N2 nitrogen atmosphere having minor percentage of molecular oxygen. Further, the CdTe films deposited films in present work have shown the uniform size distribution of spherical particles with average particle size of 0.25 lm from SEM studies. Additionally, the positive sign of average Hall coefficient (RH) and value of resistivity (q) indicated the p-type semiconducting nature of the resultant CdTe film deposited at 350 °C. The negative sign of current obtained in Hot Probe experiment also confirmed the p-type nature of resultant films. Fig. 8. (a) UV–Visible spectrum and (b) Tauc plot for the resultant film.
4. Conclusions Table 3 Electrical characterization data. Parameter
Value
Bulk concentration, n (cm3) 1015 Sheet concentration, n (cm2) 1011 Mobility, l (cm2/V s) 101 Resistivity, q (X cm) 101 Conductivity, r (X1 cm1) 102 Hall coefficient, RH (cm3/C) 103
4.224 9.529 6.634 2.228 4.489 1.478
(1) The spray pyrolysis route is simple, time saving, easy to operate and having less number of processing steps as compared to the other wet-chemical and physical deposition methods. It requires less instrumentation as compared to the other wet chemical and physical deposition methods. Further, it is low cost method and can be used for the deposition of large area films as compared to other methods. It does not require the post processing treatment (e.g. annealing) after deposition of films like electrochemical deposition. By using the spray pyrolysis processing, the crystallite size, grain size, stoichiometry in resultant films can be easily controlled by optimizing the various parameters of this method. (2) The deposition and characterization of CdTe films are given by several reports in literature. However in most of reports the CdTe films are deposited with
Spray pyrolysis is simple and low cost technique for the deposition of CdTe films. The CdTe films deposited by using spray pyrolysis at nozzle-substrate distance = 25 cm, N2 gas flow rate = 15 lpm, deposition time = 10 min, solution flow rate = 5 ml/min and substrate temperature = 350 °C are highly phase pure, material pure and stoichiometric. The resultant films have very good optical and p-type semiconducting properties. The CdTe films processed using present work could be treated as possible p-type candidate for the fabrication of heterojunction solar cell. Acknowledgements The author SDG is thankful to Prof. S. V. Ghaisas, Director, School of Energy Studies, University of Pune, India for providing the research facilities to the present research work. The author SDG is also grateful to the BCUD, University of Pune and Department of Science and Technology (DST), New Delhi, India for providing the financial assistance to the present research work. References Abdalla, A., Alnajjar Maysoon, F.A., Alias Rasha, A., Almatuk, A., Ala Al-Douri, A.J., 2009. The characteristics of anisotype CdS/CdTe heterojunction. Renew. Energy 34, 2160–2163.
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S.D. Gunjal et al. / Solar Energy xxx (2013) xxx–xxx Agashe, C., Marathe, B.R., Takwale, M.G., Bhide, V.G., 1988. Structural properties of SnO2: F films deposited by spray pyrolysis technique. Thin Solid Films 164, 261–264. Barret, C., Massalski, T.B., 1980. Structure of Metals. Pergamon, Oxford. Briggs, D., Seah, M.P., 1983. Practical Surface Analysis. Wiley, Chichester. Appendix 4. Chapman, A.J., Lane, D.W., Rogers, K.D., Ozsan, M.E., 2002. Microstructural changes of CdTe during the annealing process. Thin Solid Films 403, 522–525. Chopra, K.L., Paulson, P.D., Dutta, V., 2004. Thin film solar cells: an overview. Prog. Photovoltaics Res. Appl. 12, 69–92. Colombo, E., Bosio, A., Calusi, S., Giuntini, L., Lo Giudice, A., Manfredotti, C., Massi, M., Olivero, P., Romeo, A., Romeo, N., Vittone, E., 2009. IBIC analysis of CdTe/CdS solar cells. Nucl. Instrum. Methods Phys. Res. Sect. B: Beam Interact. Mater. Atoms 267, 2181–2184. Cullity, B.D., Stock, S.R., 2001. Elements of X-ray Diffraction, third ed. Princeton Hall. Duffie, J.A., Beckman, W.A., 2006. Solar Engineering of Thermal Processes, third ed. John Wiley & Sons Inc., pp. 8–184. Enrıquez, J.P., Mathew, X., 2004. XRD study of the grain growth in CdTe films annealed at different temperatures. Sol. Energy Mater. Sol. Cells 81, 363–369. Flores, A.R., Ares, O., Rejon, V., Camacho, J.M., Pena, V.J.L., 2012. Procedure to obtain higher than 14% efficient thin film CdS/CdTe solar cells activated with HCF2Cl gas. Sol. Energy 86, 780–785. Frausto-Reyes, C., Rafael Molina-Contreras, J., Medina-Gutierrez, C., Sergio, C., 2006. CdTe surface roughness by Raman spectroscopy using the 830 nm wavelength. Spectrochim. Acta Part A 65, 51–55. Fritsche, J., Gunst, S., Golusda, E., Lejard, M.C., Thißen, A., Mayer, T., Klein, A., Wendt, R., Gegenwart, R., Bonnet, D., Jaegermann, W., 2001. Surface analysis of CdTe thin film solar cells. Thin Solid Films 387, 161–164. Holland, J.R., Kaelble, E.F. (Eds.), 1967. Handbook of X-rays. McGrawHill, New York.
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Krishna, K.V., Dutta, V., Paulson, P.D., 2003. Effect of electric field on spray deposited CdTe thin films. Thin Solid Films 444, 17–22. Ison, V.V., Ranga Rao, A., Dutta, V., 2009. Characterization of spray deposited CdTe films grown under different ambient conditions. Solid State Sci. 11, 2003–2007. Oliva, A.I., Castro-Rodrguez, R., Sols-Canto, O., 2003. Comparison of properties of CdS thin films grown by two techniques. Appl. Surf. Sci. 205, 56–64. Patil, P.S., 1999. Review versatility of chemical spray pyrolysis technique. Mater. Chem. Phys. 59, 185–198. Rao, R., Dutta, V., 2004. Hexagonal CdTe nanorods in spray deposited CdTe nanoparticle thin films. Phys. Status Solidi (a) 201 (11), R72– R74. Razykov, T.M., Contreras-Puente, G., Chornokur, G.C., Dybjec, M., Emirov, Yu., Ergashev, B., Ferekides, C.S., Hubbimov, A., Ikramov, B., Kouchkarov, K.M., Mathew, X., Morel, D., Ostapenko, S., Sanchez-Meza, E., Stefanakos, E., Upadhyaya, H.M., Vigil-Galan, O., Vorobiev, Yu. V., 2009. Structural, photoluminescent and electrical properties of CdTe films with different compositions fabricated by CMBD. Sol. Energy 83, 90–93. Seth, A., Lush, G.B., McClure, J.C., Singh, V.P., Flood, D., 1999. Growth and characterization of CdTe by close spaced sublimation on metal substrates. Sol. Energy Mater. Sol. Cells 59, 35–49. Seto, S., Yamada, S., Suzuki, K., 2001. Structural and optical characterizations of CdTe on CdS grown by hot-wall vacuum evaporation. Sol. Energy Mater. Sol. Cells 67, 167–171. Shirley, D.A., 1972. High-resolution X-Ray photoemission spectrum of the valence bands of gold. Phys. Rev. B 5, 4709–4714. Soliman, M., Kashyout, A.B., Shabana, M., Elgamal, M., 2001. Preparation and characterization of thin films of electrodeposited CdTe semiconductors. Renew. Energy 23, 471–481. Tauc, J., Grigorovic, R., Vancu, A., 1966. Optical properties and electronic structure of amorphous germanium. Phys. Status Solidi 15, 627–637. Zanio, K., 1978. In: Semiconductors and Semimetals, 13. Cadmium Telluride Academic, New York.
Please cite this article in press as: Gunjal, S.D., et al. Spray pyrolysis deposition of p-CdTe films: Structural, optical and electrical properties. Sol. Energy (2013), http://dx.doi.org/10.1016/j.solener.2013.11.029
ScienceDirect Solar Energy xxx (2013) xxx–xxx www.elsevier.com/locate/solener
Spray pyrolysis deposition of p-CdTe films: Structural, optical and electrical properties S.D. Gunjal a, Y.B. Khollam b, S.R. Jadkar a, T. Shripathi c, V.G. Sathe c, P.N. Shelke d, M.G. Takwale a, K.C. Mohite a,e,⇑ a
School of Energy Studies, Department of Physics, University of Pune, Pune 411 007, India b Department of Physics, Anantrao Pawar College, Pirangut, Pune 412 115, India c UGC–DAE–CSR, University Campus, Khandwa Road, Indore 452 017, India d Department of Physics, B.R. Gholap College, New Sangvi, Pune 411 027, India e Department of Physics, H.V. Desai College, Budhwar Peth, Pune 411 002, India Available online xxxx Communicated by: Associate Editor Smagul Zh. Karazhanov
Abstract The present communication reports the structural, optical and electrical properties of p-CdTe films deposited by using spray pyrolysis technique on thoroughly cleaned sodalime glass substrates at 350 °C in inert N2 atmosphere. The films were characterized by using XRD, Raman spectroscopy, XPS, thickness profiler, SEM, EDAX and UV–Visible spectroscopy and Hall parameters measurement. The structural studies showed the formation of pure cubic CdTe in resultant films with crystallite size = 32 nm. The texture coefficient TC(h k l) indicated preferential orientation of films along [3 1 1] and [5 1 1] directions. The SEM image showed almost spherical granules with average size of 0.25 lm. From UV–Visible spectral data, the maximum absorbance and direct band gap obtained using Tauc plot are found to be 0.94 and Eg = 1.44 eV respectively for films having thickness, t = 3.2 lm. The Hot probe experiment, Hall coefficient RH = 1.478 103 cm3/C, and resistivity q = 2.228 101 X cm confirmed the p-type semiconducting behavior of CdTe films. The mobility of majority charge carriers – holes is found to be l = 6.63 101 cm2/Vs. The films processed using present work can be treated as possible p-type candidate for fabrication of hetero-junction solar cell. Ó 2013 Elsevier Ltd. All rights reserved. Keywords: CdTe films; Spray pyrolysis; Optical properties; Hall parameters
1. Introduction Cadmium telluride (CdTe) is one of the most useful semiconducting compounds for the various applications in c-ray detectors, infrared windows, solar cells, and other optoelectronic devices (Ison et al., 2009; Chopra et al., 2004; Zanio, 1978). The deposition methods and their preparation conditions are very important for the preparation ⇑ Corresponding author at: School of Energy Studies, Department of Physics, University of Pune, Pune 411 007, India. Tel.: +91 20 25695201 (O), +91 9890094577 (M). E-mail address: [email protected] (K.C. Mohite).
of CdTe films giving high conversion efficiency to solar cell. Different deposition methods have been used to obtain the photovoltaic quality p-type cadmium telluride (p-CdTe) films (Oliva et al., 2003; Soliman et al., 2001; Seth et al., 1999; Abdalla et al., 2009; Colombo et al., 2009; Razykov et al., 2009) giving high solar cell conversion efficiency upto 17.3% (Flores et al., 2012). The deposition of phase pure and stoichiometric CdTe layer is one of the critical stages in solar cell fabrication. The chemical spray pyrolysis method having low equipment cost and easy to operate is more suitable technique for the depositions of thin polycrystalline films of different materials (Patil, 1999; Chapman et al., 2002). The important advantage of this
0038-092X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.solener.2013.11.029
Please cite this article in press as: Gunjal, S.D., et al. Spray pyrolysis deposition of p-CdTe films: Structural, optical and electrical properties. Sol. Energy (2013), http://dx.doi.org/10.1016/j.solener.2013.11.029
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method is that the properties of films can easily be changed by varying the deposition conditions. In view of this, the present work focuses on the preparation of phase and material pure p-CdTe films using spray pyrolysis method. The resultant films are characterized by using XRD, Raman spectroscopy, XPS, thickness profiler, SEM and EDAX. The optical and electrical properties are obtained by using UV–Visible spectroscopy and Hall measurement set-up respectively. The results pertaining to this are discussed in this report. 2. Experimental The CdTe films were prepared by using home-built spray pyrolysis deposition system (Fig. 1) having glass spray gun with nozzle diameter = 0.1 mm. The 0.04 M precursor solutions of (i) CdCl2H2O (CDH Chem.) in 25 ml double distilled water (DDW) and (ii) TeO2 in 20 ml ammonia solution, 2 ml HCl and 3 ml hydrazine hydrate (reducing agent) were prepared for the preparation of CdTe films. These two precursor solutions were added together ultrasonically and the pH of final solution was noted to be 11.5. The sodalime glass substrates used for deposition of CdTe films were thoroughly cleaned by using treatments: (i) soap solution washing, (ii) rinsing with DDW and then with acetone (ultrasonically) for 10 min and finally (iii) dipping in isopropyl alcohol for 5 min. The Cd–Te based films were deposited on thoroughly cleaned glass substrates by using parameters: (i) spray gun nozzle-substrate distance = 25 cm, (ii) carrier N2 gas
Fig. 1. Schematic diagram for home-built spray pyrolysis system.
flow rate = 15 lpm, (iii) deposition time = 10 min, (iv) solution flow rate = 5 ml/min and (v) substrate temperature = 350 °C. Before deposition, the chamber was purged with Grade-I N2 gas for 30 min. After deposition, the films were cooled naturally to the room temperature (RT). The resultant films were characterized by using different physical techniques: (i) X-ray diffraction (XRD) (Bruker D8 Advance, Cu Ka X-ray line is k = 1.5406 1010 m), (ii) Raman spectroscopy (OLYMPUS BX41, HORIBA JOBIN YVON’ LABRAM HR800 Laser: He–Ne, Power: 20 mW, excitation wavelength: 632.81 nm), (iii) X-ray photoelectron spectroscopy (VSW Scientific Instruments Ltd., Manchester, England, Source: Al Ka radiation with photon energy = 1486.6 eV), (iv) scanning electron microscopy (SEM, JEOL JSM-6360A), (v) energy dispersive X-ray (EDAX) analysis, (vi) UV–visible spectroscopy (JASCO Model: V670) and (vi) surface profiler (KLA Tencor P_16+). The UV–Visible spectra were used to estimate optical properties: absorptance (a) and optical band gap (Eg) of resultant films. The electrical properties: sheet and bulk charge carrier concentration (n), mobility (l), resistivity (q), conductivity (r) and Hall coefficient (RH) were obtained by using Hall Effect measurement set-up (ECOPIA HMS-3000).
3. Results and discussion 3.1. X-ray diffraction (XRD) Fig. 2 gives the XRD pattern for the resultant film. All the observed peaks are found to be matching with the reflections given in JCPDS data for cubic phase CdTe (file No. 15-0770). Further, the ‘d’ values calculated by using Bragg’s condition, 2d sin h ¼ nk are found to be very close to standard data given in JCPDS data file No. 15-0770 for cubic phase CdTe. This indicates the formation of single cubic phase CdTe in resultant films. The average crystallite size is obtained by using classical Debye–Scherrer’s formula (Cullity and Stock, 2001):
Fig. 2. XRD pattern for the resultant film.
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S.D. Gunjal et al. / Solar Energy xxx (2013) xxx–xxx
D¼
3
0:9k b cos h
where k = 1.54 1010 m, b = FWHM in radian and h = Bragg’s angle for the different planes of cubic phase CdTe. The average crystallite size is found to be 32 nm indicating thereby the nanocrystalline nature of films. The data for the degree of orientation for different planes i.e. texture coefficient (Agashe et al., 1988; Barret and Massalski, 1980) is obtained by using the relation, ½Iðh k lÞ=I 0 ðh k lÞ TCðh k lÞ ¼ 1 P N Iðh k lÞ=I 0 ðh k lÞ N where I(h k l) = observed intensity of given plane, I0(h k l) = intensity of corresponding plane from JCPDS data file, N = number of reflections. The texture coefficient data is presented in Table 1. The data for TC(h k l) shows the preferred orientation of the film along the [3 1 1] and [5 1 1] directions. This is clearly supported by the value of standard deviation rg = 0.3567 (Holland and Kaelble, 1967) obtained by using the relation, " #1=2 1 X 2 rg ¼ ðTCðh k lÞÞ 1 N N
3.2. Raman spectroscopy Fig. 3 gives the Raman spectrum for the resultant film. The Raman peaks at 140 cm1 and 163 cm1 are due to the fundamental transverse optic (1TO) and longitudinal optic (1LO) phonon modes of CdTe. The peak at 122 cm1 corresponds to Te–O bond confirming the presence of oxide in the resultant film (Frausto-Reyes et al., 2006; Ison et al., 2009). It is reported that, the Raman spectroscopy can be used as a non-contact technique to detect the mean surface roughness of film sample (Frausto-Reyes et al., 2006). The Raman spectroscopy with an excitation wavelength of 632.8 nm can be used to detect changes in the surface roughness or as an on-line mean surface roughness monitor of film (Frausto-Reyes et al., 2006). At given excitation wavelength of 632.8 nm, with increasing the surface roughness, the optical phonon modes/peaks at 273 cm1 and 331 cm1 start appearing (Frausto-Reyes et al., 2006). In Fig. 3, the presence of optical phonon Table 1 Texture coefficient data. (h k l)
2h
I
I0
TC(h k l)
rg
111 220 311 400 331 422 511
23.7 39.3 46.4 56.8 62.4 71.2 76.2
100 70 42 7 11 12 7
100 60 30 6 10 10 4
0.7970 0.9298 1.1157 0.9298 0.8767 0.9564 1.3947
0.3567
Fig. 3. Raman spectrum for the resultant film.
modes at 273 cm1 and 331 cm1 might be due to the high surface roughness of resultant film qualitatively. 3.3. X-ray photoelectron spectroscopy (XPS) The X-ray photoelectron spectroscopy (XPS) can be used for the elemental analysis, identification of valence state of element, stoichiometric findings and phase symmetry analysis in the films. The XPS spectra of resultant film were recorded at a pressure >1 109 Torr. The general scan and C1s, O1s, Cd3d and Te3d core level spectra were recorded with non-monochromatized Al Ka radiation (photon energy = 1486.6 eV) at a pass energy of 50 eV and an electron takeoff angle (angle between electron emission direction and surface plane) of 55°. For XPS measurements, the overall resolution was 1.0 eV. The background corrections of core level spectra were done by using the Shirley + linear algorithm (Shirley, 1972). The baseline corrections and peaks fitting for all the spectra were done by using Xpspeak41 software. The core level binding energies (BEs) were aligned with respect to the C1s binding energy of 284.6 eV. Fig. 4(a) gives the general survey scan XPS spectrum of binding energy (BE) from 0 to 1200 eV for resultant film. It shows the presence of the peaks corresponding to only C1s, O1s, Cd3d and Te3d only. This confirms the purity of materials of resultant films prepared in present work. Further, in order to identify the nature of resultant film, O1s, Cd3d and Te3d core levels were recorded systematically at high resolution. The raw spectra of O1s, Cd3d and Te3d were fitted using the Xpspeak41 software program with Shirley + linear type background (Shirley, 1972) Fig. 4(b) and (c) gives the slow scan high resolution XPS raw spectra-fitted curves–resultant curves for Te3d core levels. Fig. 4(d) and (e) gives the slow scan high resolution XPS raw spectra-fitted curves-resultant curves for Cd3d and O1s core levels respectively. Due to spin-orbit coupling, the 3d peaks split into two components of 3d5/2 and 3d3/2. Fig. 4(b) shows the core level Ti 3d5/2 peaks at binding energies equal to 572.85 eV and 576.54 eV. Fig. 4(c) shows the core level Ti 3d3/2 peaks at binding energies equal to 583.22 eV and 586.91 eV. These two
Please cite this article in press as: Gunjal, S.D., et al. Spray pyrolysis deposition of p-CdTe films: Structural, optical and electrical properties. Sol. Energy (2013), http://dx.doi.org/10.1016/j.solener.2013.11.029
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Fig. 4. (a) General survey scan XPS spectrum, (b) XPS Te 3d5/2 spectrum, (c) XPS Te 3d3/2 spectrum, (d) XPS Cd 3d5/2 and Cd 3d3/2 spectra and (e) XPS O1s spectrum.
peaks can be assigned to Te2+ in CdTe at 572.7 ± 0.2 eV and Te4+ in TeO2 at 576.1 ± 0.1 eV (Briggs and Seah, 1983). Fig. 4(d) shows the core level Cd 3d5/2 peaks at binding energies equal to 405.19 eV and 411.98 eV. These two peaks can be assigned to the elemental Cd in CdTe at 405.2 ± 0.1 eV (Briggs and Seah, 1983). Fig. 4(e) gives the slow scan high resolution XPS raw spectra-fitted curves-resultant curves for O 1s core level. The peak is enhanced for clarity. Fig. 4(e) shows the core level O 1s peak at binding energies equal to 531.65 eV. This peak can be assigned to O4 in TeO2 at 531 ± 0.2 eV. All these
observations indicate the formation of phase pure CdTe in resultant films with minor oxidation corresponding to Te4+ (TeO2) at the surface. 3.4. Scanning electron microscopy The scanning electron microscopy (SEM) image of resultant film given in Fig. 5 shows the spherical morphology of CdTe dense particles without any agglomeration. The graph generated (Fig. 6) for particle size distribution from SEM image shows the uniform size distribution of
Please cite this article in press as: Gunjal, S.D., et al. Spray pyrolysis deposition of p-CdTe films: Structural, optical and electrical properties. Sol. Energy (2013), http://dx.doi.org/10.1016/j.solener.2013.11.029
S.D. Gunjal et al. / Solar Energy xxx (2013) xxx–xxx
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Fig. 7. EDAX plot for the resultant film.
hR
Fig. 5. SEM image for the resultant film.
a¼
1:0lm 0:4lm
aðkÞI k dk hR i 1:0lm I dk 0:4lm k
i
where Ik = intensity at wavelength k (Cullity and Stock, 2001; Duffie and Beckman, 2006). The average value of ‘a’ is found to be 0.94 for the resultant film. Fig. 8(b) gives the plot for the variation of (ahm)2 versus hm for the resultant film obtained by using Tauc relation, ðahmÞ
Fig. 6. The particle size distribution for the resultant film.
spherical particles with average particle size of 0.25 lm. The SEM image also exhibits porous nature with high roughness at the surface of film. This is consistent with the observations from Raman spectroscopy studies of resultant film. 3.5. Energy dispersive X-ray analysis Fig. 7 gives the energy dispersive X-ray (EDAX) analysis plot for the resultant film. It shows the presence of Cd and Te only indicating thereby the purity of material of resultant film. The EDAX data is given Table 2. The EDAX data confirms the stoichiometric (Cd:Te ratio = 1:0.9543) formation of resultant CdTe film. The presence of very small atomic% of O is due to the oxidation in the film corresponding to TeO2 phase during the processing. This is consistent with observations from the Raman spectroscopy studies of resultant film. 3.6. UV–Visible spectroscopy Fig. 8(a) gives the UV–visible spectrum for the resultant film. The value of absorptance (a) is obtained by using the equation,
1=2
¼ Aðhm Eg Þ
where a = absorption coefficient, A = constant independent of photon energy and hm (eV) = energy of excitation (Tauc et al., 1966). The value of direct band gap energy (Eg) obtained by extrapolation of straight-line portion of the plot to zero absorption edge is found to be 1.44 eV for the resultant film. The thickness of the resultant film measured by using thickness-surface profiler is found to be 3.2 lm. 3.7. Hall effect measurements The electrical properties of the resultant film were obtained by using Hall measurement set-up at current (I) = 10 nA and magnetic field (H) = 0.54 T. Table 3 gives the electrical characterization data for the same. The positive sign of average Hall coefficient (RH) and value of resistivity (q) indicate the p-type semiconducting nature of the resultant film deposited at 350 °C. The negative sign of current obtained in Hot Probe experiment also confirmed the p-type nature of resultant film. There are two important aspects in the preparation of CdTe films by using the spray pyrolysis deposition technique in present work. Table 2 EDAX data. Element
Mass%
Atomic%
Cd:Te ratio
Cd Te O
47.96 51.96 00.08
50.85 48.53 00.63
1:0.9543
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formation of oxide phases corresponding to TeO2 and/or CdTeO3 (Krishna et al., 2003; Rao and Dutta, 2004). Further, the crystallization of CdTe films with wide particle size distribution is also reported in literature (Enriquez and Mathew, 2004; Fritsche et al., 2001). The reported particle/grain sizes are found to be higher and in micrometer range (Seto et al., 2001). The several reports have shown the deposition of CdTe films by using different wet-chemical and physical routes. However, p-type nature of the films is not confirmed by using the measurement Hall Effect parameters and/or Hot probe method. In view of these points, the CdTe films prepared in present work are phase pure with a very minor oxidation corresponding to the TeO2 at the surface. This might be due to use of lower concentration of the hydrazine hydrate as reducing agent or processing of films in N2 nitrogen atmosphere having minor percentage of molecular oxygen. Further, the CdTe films deposited films in present work have shown the uniform size distribution of spherical particles with average particle size of 0.25 lm from SEM studies. Additionally, the positive sign of average Hall coefficient (RH) and value of resistivity (q) indicated the p-type semiconducting nature of the resultant CdTe film deposited at 350 °C. The negative sign of current obtained in Hot Probe experiment also confirmed the p-type nature of resultant films. Fig. 8. (a) UV–Visible spectrum and (b) Tauc plot for the resultant film.
4. Conclusions Table 3 Electrical characterization data. Parameter
Value
Bulk concentration, n (cm3) 1015 Sheet concentration, n (cm2) 1011 Mobility, l (cm2/V s) 101 Resistivity, q (X cm) 101 Conductivity, r (X1 cm1) 102 Hall coefficient, RH (cm3/C) 103
4.224 9.529 6.634 2.228 4.489 1.478
(1) The spray pyrolysis route is simple, time saving, easy to operate and having less number of processing steps as compared to the other wet-chemical and physical deposition methods. It requires less instrumentation as compared to the other wet chemical and physical deposition methods. Further, it is low cost method and can be used for the deposition of large area films as compared to other methods. It does not require the post processing treatment (e.g. annealing) after deposition of films like electrochemical deposition. By using the spray pyrolysis processing, the crystallite size, grain size, stoichiometry in resultant films can be easily controlled by optimizing the various parameters of this method. (2) The deposition and characterization of CdTe films are given by several reports in literature. However in most of reports the CdTe films are deposited with
Spray pyrolysis is simple and low cost technique for the deposition of CdTe films. The CdTe films deposited by using spray pyrolysis at nozzle-substrate distance = 25 cm, N2 gas flow rate = 15 lpm, deposition time = 10 min, solution flow rate = 5 ml/min and substrate temperature = 350 °C are highly phase pure, material pure and stoichiometric. The resultant films have very good optical and p-type semiconducting properties. The CdTe films processed using present work could be treated as possible p-type candidate for the fabrication of heterojunction solar cell. Acknowledgements The author SDG is thankful to Prof. S. V. Ghaisas, Director, School of Energy Studies, University of Pune, India for providing the research facilities to the present research work. The author SDG is also grateful to the BCUD, University of Pune and Department of Science and Technology (DST), New Delhi, India for providing the financial assistance to the present research work. References Abdalla, A., Alnajjar Maysoon, F.A., Alias Rasha, A., Almatuk, A., Ala Al-Douri, A.J., 2009. The characteristics of anisotype CdS/CdTe heterojunction. Renew. Energy 34, 2160–2163.
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Please cite this article in press as: Gunjal, S.D., et al. Spray pyrolysis deposition of p-CdTe films: Structural, optical and electrical properties. Sol. Energy (2013), http://dx.doi.org/10.1016/j.solener.2013.11.029