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CdS thin films doped with metal-organic salts using chemical bath deposition
Thin Solid Films 518 (2010) 1791–1795
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
CdS thin films doped with metal-organic salts using chemical bath deposition J. Santos Cruz a,⁎, R. Castanedo Pérez b, G. Torres Delgado b, O. Zelaya Angel c a b c
Universidad Autónoma de Querétaro, Facultad de Química, Materiales, Querétaro, 76010, Mexico Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Unidad Querétaro A.P. 1‐798, Querétaro, Qro. 76001, Mexico Centro de Investigación y de Estudios Avanzados del IPN, Departamento de Física, A.P. 14-740, México 07360 D.F., Mexico
a r t i c l e
i n f o
Available online 19 September 2009 Keywords: Chemical bath deposition Semiconductor Thin films Cadmium sulfide
a b s t r a c t CdS thin films doped with metal-organic salts were grown on glass substrates at 90 °C by the chemical bath deposition technique. Metal-organic salts such as zinc acetate, chromium acetylacetonate, ammonium fluoride, aluminum nitrate, tin acetate and indium acetate were used. The chemical bath was prepared with cadmium acetate, ammonium acetate, thiourea and ammonium hydroxide. In the case of un-doped films, the S/Cd ratio was varied by changing the thiourea in the range 1–12. The best optical, structural and electrical properties were found for S/Cd=2. The doped films were prepared by always keeping the ratio S/Cd constant at 2. The band gap (Eg) of doped and un-doped films was evaluated from transmittance spectra, where films with lower sulfur concentration exhibited higher Eg. X-ray analysis showed that both un-doped and doped films were polycrystalline with preferential orientation along the (111) direction and with the zincblende structure in all cases. The dark electrical results showed that CdS doped with Zn (1at.%) exhibited the lowest resistivity values of 10Ω cm. © 2009 Elsevier B.V. All rights reserved.
1. Introduction CdS films are often grown by the chemical bath deposition (CBD) technique, and have shown good properties for device fabrication. CdS-CBD films are commonly used as a window or buffer layer material for solar cells of the type CdS/CdTe and CdS/CIGS [1–6]. CBD is a low-cost, relatively simple and practical method for covering complex substrates [7], and offers excellent control when depositing thinner films. For the application of CdS thin films in solar cells, it is necessary to have layers with the following characteristics: i) uniformity, ii) transparency, iii) crystallinity, and iv) good electrical properties. All of these properties are needed to produce high efficiency solar cells and other devices; unfortunately, in practice, the principal problem with CdS thin films grown by the CBD technique has been poor conductivity and uniformity [8]. It has been found that CdS films grown by CBD, when used as buffer layer in solar cells, produced high efficiency devices compared with those CdS films obtained by spraying and high-cost vacuum techniques [9,10]. The CdS used as the buffer layer plays a very important role, because in superstrate devices the CdTe film is grown on top of it, and for this reason, the morphological quality and electrical properties of CdS films are very important in these types of devices. The CdS buffer layers generally exhibit high resistivities. When thermal treatments in CdCl2 ambient is performed in order to decrease the resistivity, the films can show cracks and problems in the reproducibility of the devices [11]. In this work, we doped in situ the
⁎ Corresponding author. Tel.: +52 442 192 1200x5525; fax: +52 192 1302. E-mail address: [email protected] (J.S. Cruz). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.09.034
CBD prepared CdS in order to obtain films with low resistivity and good optical and structural properties. The effect of the S/Cd ratio on the properties of the un-doped CdS films was also studied, the S/Cd ratio was varied in the range 1–12. The best properties were obtained for S/Cd equals 2. Keeping this ratio, the CdS films were doped with metal-organic salts of Zn, Cr, F, Al, Sn and In. All the salts were dissolved in water before adding them to the growth solution.
2. Experimental procedure The CdS thin films were grown on Corning glass slides by the CBD technique at temperature of 90 ± 1 ºC and with a deposition time of 40 min. The samples were prepared by immersing the substrates vertically in the aqueous solution. The reagents used for the preparation of un-doped CdS were the following: cadmium acetate, ammonium acetate, ammonium hydroxide, and thiourea. The S/Cd ratios used for the bath were 1, 2, 3, 4, 5, 7, 9 and 12. For the doped CdS thin films, the ratio of S/Cd was kept constant at 2, and the concentration of doping agents used were 1, 3, 5, 10 and 30 at.%. The metal-organic salts used were: zinc acetate (CH3COOZn∙2H2O) as the source of Zn, Cr(III) acetyl acetonate (C15H21CrO6) as the source of Cr, ammonium fluoride (NH4F) as the source of F, aluminum nitrate [Al(NO3)3] as the source of Al, tin acetate [Sn(OOCCH3)2] as the source of Sn and indium acetate [In (COOCH3)3] as the source of In. Highly pure water (~ 18 MΩ) was used for the preparation of all the solutions. The temperature was controlled at 90 ± 1 °C by means of a hot plate equipped with magnetic stirring. Cadmium acetate and thiourea were the sources of Cd and S, respectively. The other components had the function of complexing the reaction process and keeping the pH at the
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desired level. The average thickness of the films was 100 ± 5 nm. After the deposition, the thin films were rinsed in high purity water in an ultrasonic bath for 15 min. In the case of doped CdS films, the doping solutions were fresh in all cases. The solutions containing the doping agents were prepared only few minutes prior to adding to the growth solution. The doping solutions with Zn, Cr, F and Al had good solubility and transparency, while the solutions with In and Sn were not totally soluble and transparent. In some cases, in order to achieve total solubility, it was necessary to add few microliters of acetic acid, lactic acid or nitric acid at moderate temperatures. The thickness of the films was measured by means of a surface profiler (Sloan Dektak II), X-ray diffraction (XRD) studies were carried out using a Rigaku D/max-2100 diffractometer (Co radiation, λCo = 1.78899 Å). The dark resistivity was measured in the conventional four probe method using a Loresta6P, model MCP-T600. The UV–VIS spectra were recorded with a Perkin-Elmer Lambda-2 spectrometer in the 250–1100 nm wavelength range using a glass substrate as reference. 3. Results and discussion
Fig. 2. Plot (αhυ)2 versus hυ to determine the direct band gap in CdS thin films.
The thickness of the CdS thin films was within in the 95–108 nm range. The maximum (minimum) values of the thicknesses were in accordance with the concentration of thiourea in the solution. The films were pin hole and crack free, uniform and with good adhesion to the Corning glass substrate. The optical transmission spectra of the un-doped CdS for the S/Cd ratios from 2 to 5 are shown in Fig. 1, in this range films showed best transmission (N80%) and for other ratios the transmission values are lower (b75%). For Eg estimation, the (αhυ)2 versus hυ plot was used, where α is the optical absorption coefficient and hυ is the photon energy. Eg values varied in the interval 2.37– 2.42 eV as can be observed in Fig. 2. The highest Eg value corresponds to the lowest thiourea concentration. This value is close to the Eg value
reported for single crystalline CdS (2.42 eV) [12], which shows the good stoichiometry of these films. The shift of Eg to lower values as the thiourea concentration increases could be attributed to defects generated due to the excess of S. Fig. 3 shows the X-ray diffractograms of the CdS samples for [S]/[Cd]= 2, 5 and 12, which are the most representative films. All the samples show cubic structure and a preferential orientation in the (111) plane. The grain size (GS), in the range 22–27 nm, was calculated using the full width at half maximum (FWHM) and Scherrer's formula. GS, Eg and average transmittance values were compiled in Table 1. It can be observed that samples with S/Cd ratios of 2, 3 and 4 had both higher grain sizes and transmission. The dark resistivity (ρ) values of the un-doped films, for the different S/Cd ratios were plotted in Fig. 4. The lowest resistivity values
Fig. 1. Optical transmission spectra of the CdS thin films with varying S/Cd ratios of 2–5.
Fig. 3. X-ray patterns of the CdS thin films with ratios [S]/[Cd] = 2, 5, and 12.
J.S. Cruz et al. / Thin Solid Films 518 (2010) 1791–1795
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Table 1 Grain size (D), band gap (Eg) and transmittance (%T) values of the CdS thin films with varying ratios of [S]/[Cd]. [S]/[Cd]
D (nm)
Eg (eV)
%T
2 3 4 5 7 9 12
27.03 25.10 22.70 21.36 21.53 21.72 22.09
2.415 2.410 2.400 2.390 2.374 2.360 2.370
80 80 80 82 75 70 68
(~80 Ω cm) were obtained for the films with S/Cd ratios in the 2–4 range. These values were within the lowest values reported in the literature (110–1 × 108 Ω cm) for CBD prepared CdS thin films [13–18]. These films showed both higher optical transmission and grain sizes. The highest ρ value was obtained for S/Cd = 1 and ρ increased as the sulfur concentration increased for S/Cd N 4. Doped films obtained were pin hole and crack free, uniform and adherent to the Corning glass substrate. The thickness was in the 98– 122 nm range. In these cases the ratio of S/Cd was kept constant at 2. The films were doped with the metal-organic salts at 1, 3, 5, 10 and 30 atomic percentages (at.%). The transmission spectra of the CdS films doped with Zn and Cr are shown in Fig. 5. The transmittance was in the 71–87% range. The films doped with F and Al show lower transmission, between 60 and 70%. The films doped with Sn and In show poor transmission. The band gap values of the doped CdS films are shown in Fig. 6, the values are in the 2.29–2.43 eV range. In general, higher band gaps were observed for the CdS films doped with Cr and Zn. This fact could be due to the incorporation of microcrystalline Cr2S3 and ZnS (Eg = 3.51 eV [19]) compounds, and related electronic effects. For the films doped with F and Al, the incorporation of these doping agents, as well as a possible sulfur deficiency, will give rise to donor levels in the band gap of CdS. As the doping content increases, the donor levels become degenerate and merge with the conduction band of CdS, causing the conduction band to extend into the forbidden region which reduces the band gap [20]. X-ray diffractograms of the CdS thin films doped with Zn and Cr are displayed in Fig. 7. All the samples showed a cubic structure and a preferential orientation along the (111) plane. In the XRD patterns, additional peaks associated with new compounds were not observed. This could be due to the lower concentration of the doping agent in solution (≤30 at.%). The grain size varied in the 7.5–26 nm range. The lower grain size values were for CdS films doped with Al and the highest for the ones doped with Cr. The tendency was to diminish the GS as the doping concentration was increased (see inset in Fig. 8).
Fig. 4. Electrical resistivity of the CdS films with varying [S]/[Cd] ratios.
Fig. 5. Transmission spectra of the CdS thin films doped with Zn and Cr.
The dark resistivity values versus doping concentrations of Zn, Cr and F, with S/Cd = 2 (kept constant) is plotted in Fig. 8. The lower ρ values, 9.7 and 80 Ω cm, were obtained respectively for Zn and F doping, with a doping level of 1 at.%. The ρ for the films doped with Cr did not show significant changes when the Cr concentration was varied. The In and Sn doped samples gave the highest resistivity
Fig. 6. Band gaps of the samples versus doping concentration for the films doped with different elements.
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Fig. 7. X-ray patterns of the doped CdS thin films with Zn and Cr (1, 5, 10 and 30 at.%).
values from 102 to 108 Ω cm and their transmittance was very low. This could be attributed to a poor incorporation of In and Sn in the lattice, probably due to the poor solubility of the doping salts in the growth solution. Studies using Hall measurements are in progress to understand the electrical properties of the doped films.
4. Conclusions Doped and un-doped CdS thin films prepared by CBD were studied to understand the structural, optical and electrical properties. Undoped CdS films with an S/Cd ratio equals to 2 have the best optical, structural and electric properties. In the case of doped CdS thin films, the lowest resistivity of 9.7 Ω cm was obtained for the films doped with Zn (1 at.%). These films could be used as buffer layers in solar cells and other optoelectronic devices due to their high transparency and low resistivity. Acknowledgements The authors wish to thank Rodrigo García Rodriguez and M. Sci. Joaquín Márquez Marín for their technical assistance. References
Fig. 8. Resistivity values of the CdS films doped with Zn, Cr and F versus doping concentration. Inset — grain sizes of the CdS samples doped with Zn, F, Cr and Al versus doping concentration.
[1] C.S. Ferekides, D. Marinskiy, V. Viswanathan, B. Tetali, V. Palekis, P. Selvaraj, D.L. Morel, Thin Solid Films 361–362 (2000) 520. [2] J. Britt, C. Ferekides, Appl. Phys. Lett. 62 (1993) 2851. [3] Alvin D. Compaan, Akhlesh Gupta, Sunghyun Lee, Shanli Wang, Jennifer Drayton, Sol. Energy 77 (6) (2004) 815. [4] R. Noufi, K. Zweibel, High-Efficiency CdTe and CIGS Thin-Film Solar Cells: Highlights and Challenges., Presented at the 2006 IEEE 4th World Conference on Photovoltaic Energy Conversion (WCPEC-4) Waikoloa, Hawaii, May 7–12, 2006. [5] D.L. Young, J. Abushama, R. Noufi, X. Li, J. Keane, T.A. Gessert, J.S. Ward, M. Contreras, M. Symko-Davies, T.J. Coutts, A New Thin-Film CuGaSe2/Cu(In,Ga)Se2 Bifacial,
J.S. Cruz et al. / Thin Solid Films 518 (2010) 1791–1795
[6] [7] [8] [9] [10] [11] [12]
Tandem Solar Cell with Both Junctions Formed Simultaneously., May 2002., NREL/CP520-31440., Presented at the 29th IEEE PV Specialists Conference New Orleans, Louisiana, May 20–24, 2002. T. Nakada, Thin Solid Films 361/362 (2000) 346. I. Kaur, D.K. Pandya, K.L. Chopra, J. Electrochem.Soc. 127 (1980) 943. R.S. Mane, C.D. Lokhande, Mater. Chem. Phys. 65 (2000) 1. T.L. Chu, S.S. Chu, C. Ferekides, C.Q. Wu, J. Britt, C. Wang, J. Cryst. Growth 117 (1–4) (1992) 1073. C. Ferekides, J. Britt, Sol. Energy Mat. Sol. Cells 35 (1994) 255. A. Romeo, D.L. Bätzner, H. Zogg, C. Vignali, A.N. Tiwari, Sol. Energy Mater. Sol. Cells 67 (2001) 311. S.M. Sze, Physics of Semiconductor Devices, second ed., Wiley-Interscience, New York, 1981, p. 849.
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[13] J. Hiie, K. Muska, V. Valdana, V. Mikli, A. Taklaja, A. Gavrilov, Thin Solid Films 516 (2008) 7008. [14] M. Altosaar, K. Ernits, J. Krustok, T. Varema, J. Raudoja, E. Mellikov, Thin Solid Films 480–481 (2005) 147. [15] Joel Pantoja Enriquez, Xavier Mathew, Sol. Energy Mat. Sol. Cells 76 (2003) 313. [16] O. Vigil, Y. Rodríguez, O. Zelaya-Angel, C. Vázquez-López, A. Morales-Acevedo, J.G. Vázquez-Luna, Thin Solid Films 322 (1998) 329–333. [17] R.K. Nkum, A.A. Adimado, H. Totoe, Mater. Sci. Eng., B 55 (1998) 102. [18] E. Çetinörgü, C. Gümüş, R. Esen, Thin Solid Films 515 (2006) 1688. [19] Qi Liu, Mao Guobing, Ao Jianping, Appl. Surf. Sci. 254 (2008) 5711. [20] Hani Khallaf, Guangyu Chai, Oleg Lupan, Lee Chow, S. Park, Alfons Schulte, J. Phys. D: Appl. Phys. 41 (185304) (2008) (10pp).
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
CdS thin films doped with metal-organic salts using chemical bath deposition J. Santos Cruz a,⁎, R. Castanedo Pérez b, G. Torres Delgado b, O. Zelaya Angel c a b c
Universidad Autónoma de Querétaro, Facultad de Química, Materiales, Querétaro, 76010, Mexico Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Unidad Querétaro A.P. 1‐798, Querétaro, Qro. 76001, Mexico Centro de Investigación y de Estudios Avanzados del IPN, Departamento de Física, A.P. 14-740, México 07360 D.F., Mexico
a r t i c l e
i n f o
Available online 19 September 2009 Keywords: Chemical bath deposition Semiconductor Thin films Cadmium sulfide
a b s t r a c t CdS thin films doped with metal-organic salts were grown on glass substrates at 90 °C by the chemical bath deposition technique. Metal-organic salts such as zinc acetate, chromium acetylacetonate, ammonium fluoride, aluminum nitrate, tin acetate and indium acetate were used. The chemical bath was prepared with cadmium acetate, ammonium acetate, thiourea and ammonium hydroxide. In the case of un-doped films, the S/Cd ratio was varied by changing the thiourea in the range 1–12. The best optical, structural and electrical properties were found for S/Cd=2. The doped films were prepared by always keeping the ratio S/Cd constant at 2. The band gap (Eg) of doped and un-doped films was evaluated from transmittance spectra, where films with lower sulfur concentration exhibited higher Eg. X-ray analysis showed that both un-doped and doped films were polycrystalline with preferential orientation along the (111) direction and with the zincblende structure in all cases. The dark electrical results showed that CdS doped with Zn (1at.%) exhibited the lowest resistivity values of 10Ω cm. © 2009 Elsevier B.V. All rights reserved.
1. Introduction CdS films are often grown by the chemical bath deposition (CBD) technique, and have shown good properties for device fabrication. CdS-CBD films are commonly used as a window or buffer layer material for solar cells of the type CdS/CdTe and CdS/CIGS [1–6]. CBD is a low-cost, relatively simple and practical method for covering complex substrates [7], and offers excellent control when depositing thinner films. For the application of CdS thin films in solar cells, it is necessary to have layers with the following characteristics: i) uniformity, ii) transparency, iii) crystallinity, and iv) good electrical properties. All of these properties are needed to produce high efficiency solar cells and other devices; unfortunately, in practice, the principal problem with CdS thin films grown by the CBD technique has been poor conductivity and uniformity [8]. It has been found that CdS films grown by CBD, when used as buffer layer in solar cells, produced high efficiency devices compared with those CdS films obtained by spraying and high-cost vacuum techniques [9,10]. The CdS used as the buffer layer plays a very important role, because in superstrate devices the CdTe film is grown on top of it, and for this reason, the morphological quality and electrical properties of CdS films are very important in these types of devices. The CdS buffer layers generally exhibit high resistivities. When thermal treatments in CdCl2 ambient is performed in order to decrease the resistivity, the films can show cracks and problems in the reproducibility of the devices [11]. In this work, we doped in situ the
⁎ Corresponding author. Tel.: +52 442 192 1200x5525; fax: +52 192 1302. E-mail address: [email protected] (J.S. Cruz). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.09.034
CBD prepared CdS in order to obtain films with low resistivity and good optical and structural properties. The effect of the S/Cd ratio on the properties of the un-doped CdS films was also studied, the S/Cd ratio was varied in the range 1–12. The best properties were obtained for S/Cd equals 2. Keeping this ratio, the CdS films were doped with metal-organic salts of Zn, Cr, F, Al, Sn and In. All the salts were dissolved in water before adding them to the growth solution.
2. Experimental procedure The CdS thin films were grown on Corning glass slides by the CBD technique at temperature of 90 ± 1 ºC and with a deposition time of 40 min. The samples were prepared by immersing the substrates vertically in the aqueous solution. The reagents used for the preparation of un-doped CdS were the following: cadmium acetate, ammonium acetate, ammonium hydroxide, and thiourea. The S/Cd ratios used for the bath were 1, 2, 3, 4, 5, 7, 9 and 12. For the doped CdS thin films, the ratio of S/Cd was kept constant at 2, and the concentration of doping agents used were 1, 3, 5, 10 and 30 at.%. The metal-organic salts used were: zinc acetate (CH3COOZn∙2H2O) as the source of Zn, Cr(III) acetyl acetonate (C15H21CrO6) as the source of Cr, ammonium fluoride (NH4F) as the source of F, aluminum nitrate [Al(NO3)3] as the source of Al, tin acetate [Sn(OOCCH3)2] as the source of Sn and indium acetate [In (COOCH3)3] as the source of In. Highly pure water (~ 18 MΩ) was used for the preparation of all the solutions. The temperature was controlled at 90 ± 1 °C by means of a hot plate equipped with magnetic stirring. Cadmium acetate and thiourea were the sources of Cd and S, respectively. The other components had the function of complexing the reaction process and keeping the pH at the
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desired level. The average thickness of the films was 100 ± 5 nm. After the deposition, the thin films were rinsed in high purity water in an ultrasonic bath for 15 min. In the case of doped CdS films, the doping solutions were fresh in all cases. The solutions containing the doping agents were prepared only few minutes prior to adding to the growth solution. The doping solutions with Zn, Cr, F and Al had good solubility and transparency, while the solutions with In and Sn were not totally soluble and transparent. In some cases, in order to achieve total solubility, it was necessary to add few microliters of acetic acid, lactic acid or nitric acid at moderate temperatures. The thickness of the films was measured by means of a surface profiler (Sloan Dektak II), X-ray diffraction (XRD) studies were carried out using a Rigaku D/max-2100 diffractometer (Co radiation, λCo = 1.78899 Å). The dark resistivity was measured in the conventional four probe method using a Loresta6P, model MCP-T600. The UV–VIS spectra were recorded with a Perkin-Elmer Lambda-2 spectrometer in the 250–1100 nm wavelength range using a glass substrate as reference. 3. Results and discussion
Fig. 2. Plot (αhυ)2 versus hυ to determine the direct band gap in CdS thin films.
The thickness of the CdS thin films was within in the 95–108 nm range. The maximum (minimum) values of the thicknesses were in accordance with the concentration of thiourea in the solution. The films were pin hole and crack free, uniform and with good adhesion to the Corning glass substrate. The optical transmission spectra of the un-doped CdS for the S/Cd ratios from 2 to 5 are shown in Fig. 1, in this range films showed best transmission (N80%) and for other ratios the transmission values are lower (b75%). For Eg estimation, the (αhυ)2 versus hυ plot was used, where α is the optical absorption coefficient and hυ is the photon energy. Eg values varied in the interval 2.37– 2.42 eV as can be observed in Fig. 2. The highest Eg value corresponds to the lowest thiourea concentration. This value is close to the Eg value
reported for single crystalline CdS (2.42 eV) [12], which shows the good stoichiometry of these films. The shift of Eg to lower values as the thiourea concentration increases could be attributed to defects generated due to the excess of S. Fig. 3 shows the X-ray diffractograms of the CdS samples for [S]/[Cd]= 2, 5 and 12, which are the most representative films. All the samples show cubic structure and a preferential orientation in the (111) plane. The grain size (GS), in the range 22–27 nm, was calculated using the full width at half maximum (FWHM) and Scherrer's formula. GS, Eg and average transmittance values were compiled in Table 1. It can be observed that samples with S/Cd ratios of 2, 3 and 4 had both higher grain sizes and transmission. The dark resistivity (ρ) values of the un-doped films, for the different S/Cd ratios were plotted in Fig. 4. The lowest resistivity values
Fig. 1. Optical transmission spectra of the CdS thin films with varying S/Cd ratios of 2–5.
Fig. 3. X-ray patterns of the CdS thin films with ratios [S]/[Cd] = 2, 5, and 12.
J.S. Cruz et al. / Thin Solid Films 518 (2010) 1791–1795
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Table 1 Grain size (D), band gap (Eg) and transmittance (%T) values of the CdS thin films with varying ratios of [S]/[Cd]. [S]/[Cd]
D (nm)
Eg (eV)
%T
2 3 4 5 7 9 12
27.03 25.10 22.70 21.36 21.53 21.72 22.09
2.415 2.410 2.400 2.390 2.374 2.360 2.370
80 80 80 82 75 70 68
(~80 Ω cm) were obtained for the films with S/Cd ratios in the 2–4 range. These values were within the lowest values reported in the literature (110–1 × 108 Ω cm) for CBD prepared CdS thin films [13–18]. These films showed both higher optical transmission and grain sizes. The highest ρ value was obtained for S/Cd = 1 and ρ increased as the sulfur concentration increased for S/Cd N 4. Doped films obtained were pin hole and crack free, uniform and adherent to the Corning glass substrate. The thickness was in the 98– 122 nm range. In these cases the ratio of S/Cd was kept constant at 2. The films were doped with the metal-organic salts at 1, 3, 5, 10 and 30 atomic percentages (at.%). The transmission spectra of the CdS films doped with Zn and Cr are shown in Fig. 5. The transmittance was in the 71–87% range. The films doped with F and Al show lower transmission, between 60 and 70%. The films doped with Sn and In show poor transmission. The band gap values of the doped CdS films are shown in Fig. 6, the values are in the 2.29–2.43 eV range. In general, higher band gaps were observed for the CdS films doped with Cr and Zn. This fact could be due to the incorporation of microcrystalline Cr2S3 and ZnS (Eg = 3.51 eV [19]) compounds, and related electronic effects. For the films doped with F and Al, the incorporation of these doping agents, as well as a possible sulfur deficiency, will give rise to donor levels in the band gap of CdS. As the doping content increases, the donor levels become degenerate and merge with the conduction band of CdS, causing the conduction band to extend into the forbidden region which reduces the band gap [20]. X-ray diffractograms of the CdS thin films doped with Zn and Cr are displayed in Fig. 7. All the samples showed a cubic structure and a preferential orientation along the (111) plane. In the XRD patterns, additional peaks associated with new compounds were not observed. This could be due to the lower concentration of the doping agent in solution (≤30 at.%). The grain size varied in the 7.5–26 nm range. The lower grain size values were for CdS films doped with Al and the highest for the ones doped with Cr. The tendency was to diminish the GS as the doping concentration was increased (see inset in Fig. 8).
Fig. 4. Electrical resistivity of the CdS films with varying [S]/[Cd] ratios.
Fig. 5. Transmission spectra of the CdS thin films doped with Zn and Cr.
The dark resistivity values versus doping concentrations of Zn, Cr and F, with S/Cd = 2 (kept constant) is plotted in Fig. 8. The lower ρ values, 9.7 and 80 Ω cm, were obtained respectively for Zn and F doping, with a doping level of 1 at.%. The ρ for the films doped with Cr did not show significant changes when the Cr concentration was varied. The In and Sn doped samples gave the highest resistivity
Fig. 6. Band gaps of the samples versus doping concentration for the films doped with different elements.
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Fig. 7. X-ray patterns of the doped CdS thin films with Zn and Cr (1, 5, 10 and 30 at.%).
values from 102 to 108 Ω cm and their transmittance was very low. This could be attributed to a poor incorporation of In and Sn in the lattice, probably due to the poor solubility of the doping salts in the growth solution. Studies using Hall measurements are in progress to understand the electrical properties of the doped films.
4. Conclusions Doped and un-doped CdS thin films prepared by CBD were studied to understand the structural, optical and electrical properties. Undoped CdS films with an S/Cd ratio equals to 2 have the best optical, structural and electric properties. In the case of doped CdS thin films, the lowest resistivity of 9.7 Ω cm was obtained for the films doped with Zn (1 at.%). These films could be used as buffer layers in solar cells and other optoelectronic devices due to their high transparency and low resistivity. Acknowledgements The authors wish to thank Rodrigo García Rodriguez and M. Sci. Joaquín Márquez Marín for their technical assistance. References
Fig. 8. Resistivity values of the CdS films doped with Zn, Cr and F versus doping concentration. Inset — grain sizes of the CdS samples doped with Zn, F, Cr and Al versus doping concentration.
[1] C.S. Ferekides, D. Marinskiy, V. Viswanathan, B. Tetali, V. Palekis, P. Selvaraj, D.L. Morel, Thin Solid Films 361–362 (2000) 520. [2] J. Britt, C. Ferekides, Appl. Phys. Lett. 62 (1993) 2851. [3] Alvin D. Compaan, Akhlesh Gupta, Sunghyun Lee, Shanli Wang, Jennifer Drayton, Sol. Energy 77 (6) (2004) 815. [4] R. Noufi, K. Zweibel, High-Efficiency CdTe and CIGS Thin-Film Solar Cells: Highlights and Challenges., Presented at the 2006 IEEE 4th World Conference on Photovoltaic Energy Conversion (WCPEC-4) Waikoloa, Hawaii, May 7–12, 2006. [5] D.L. Young, J. Abushama, R. Noufi, X. Li, J. Keane, T.A. Gessert, J.S. Ward, M. Contreras, M. Symko-Davies, T.J. Coutts, A New Thin-Film CuGaSe2/Cu(In,Ga)Se2 Bifacial,
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