- Email: [email protected]
sodium lactate buffered electrolytes
Thin Solid Films 519 (2010) 725–728
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
Pulsed electrodeposition of oxygen-free tin monosulfide thin films using lactic acid/sodium lactate buffered electrolytes Feng Kang a,b,⁎, Masaya Ichimura b a b
Institute of Photo-Electronic Thin Film Devices and Technology, Nankai University, Tianjin 300071, China Department of Engineering Physics, Electronics and Mechanics, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466-8555, Japan
a r t i c l e
i n f o
Article history: Received 11 August 2009 Received in revised form 21 June 2010 Accepted 26 August 2010 Available online 21 September 2010 Keywords: Pulsed electrodeposition SnS thin films Lactic acid/sodium lactate pHydrian buffer
a b s t r a c t In this study, tin monosulfide (SnS) thin films have been prepared on indium-tin-oxide-covered glass substrates from an acidic electrolyte containing a pH buffer of lactic acid/sodium lactate using pulsed electrodeposition method. Results from Auger electron spectroscopy confirmed that nearly oxygen-free and stoichiometric SnS thin films were attained. X-ray diffraction and scanning electron micrograph studies indicated the formation of a smooth α-SnS thin film with the orthorhombic structure. Moreover, optical transmission spectroscopy showed a direct optical band gap of 1.39 eV and the photoelectrochemical measurements revealed the p-type conductivity. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Over the past decade, there has been a growing interest in the IV–VI compound semiconductor tin monosulphide (SnS). It has a direct band gap energy of 1.3–1.5 eV, a high absorption coefficient (α N 104 cm− 1) and a high theoretical conversion efficiency of 25%. In addition, it contains only abundant, non-toxic elements. These favorable features make it one of the most promising absorber materials for the fabrication of thin-film solar cells [1]. Up until now, a number of methods have been reported for depositing SnS thin films including vacuum evaporation [2,3], spray pyrolysis [4,5], chemical bath deposition [6] and electrodeposition. Among these methods, electrodeposition (ED) is more attractive since it offers the advantages of being non-vacuum, cost-effective and suitable for large scale deposition. This has motivated many research groups to study preparation of SnS thin films by employing different ED techniques [7–11]. In our previous work [9], we have shown that the surface morphology of SnS thin films can be highly improved by adopting three-step pulse ED. However, we also found that some oxygen was always included in the deposits, suggesting the formation of tin oxide phase during electrodeposition, which could be caused by pH changes in electrolytes. To overcome this problem, in this study, we introduced a pH buffer of lactic acid/ sodium lactate into electrolytes for three-step pulse ED of SnS thin films. This pH buffer system has already been proven to be an effective way to improve
electrolyte stability for one-step ED of CuInSe2 thin films [12]. The properties of as-deposited samples have been investigated by several techniques and compared with the ones prepared from non-pH buffer electrolytes under the same ED conditions as well. 2. Experimental details 2.1. Preparation A conventional three-electrode cell was used for ED, with an indium-tin-oxide (ITO)-coated glass substrate as the working electrode, a platinum sheet as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode, whose schematic diagram is shown in Fig. 1. The deposition area was limited to 1 cm × 1 cm by masking. The electrolyte solution contained 30 mM SnSO4 and 100 mM Na2S2O3 dissolved in a lactic acid/sodium lactate buffer system, maintaining the solution at a pH of about 2.3. The electrolyte temperature was kept at 60 °C in a water bath throughout the deposition. ED from the above electrolytes was performed under a periodic three-step pulse voltage [9] ( voltage “V1” = − 1 V, on time”t1” = 10 s, “V2” = − 0.6 V, on time”t2” = 10 s, “V3” = 0 V, on time “t3” = 10 s). The typical pulsed voltage cycles and current profile during deposition are shown in Fig. 2. The total deposition time was 15 min. 2.2. Characterization
⁎ Corresponding author. Institute of Photo-Electronic Thin Film Devices and Technology, Nankai University, Tianjin 300071, China. Tel.: +86 22 23497012; fax: +86 22 23498216. E-mail address: [email protected] (F. Kang). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.08.138
X-ray diffraction (XRD) patterns were recorded on the RIGAKU RINT-2000 diffractometer using CuKα radiation. The film thickness was measured by a profile meter Accretech, Surfcom-1400D. The
726
F. Kang, M. Ichimura / Thin Solid Films 519 (2010) 725–728
3. Results and discussion
Fig. 1. A schematic diagram of the SnS electrodeposition setup.
compositional analysis was carried out by Auger electron spectroscopy (AES) using the model JEOL JAMP 7800 Auger microprobe at probe voltage 10 kV and current 2 × 10− 8 A. The film morphology was investigated by scanning electron microscope (SEM) model HITACHI S2000 S, keeping the acceleration voltage at 10 kV and magnification 2000. The transmission spectra were measured using the JASCO U-570 UV/VIS /NIR spectrometer. Photoelectrochemical (PEC) measurements were carried out in an aqueous electrolyte consisting of 100 mM Na2S2O3. The applied voltage was swept at a speed of 5 mV/s, and the illumination was switched on and off for each 5 s. A Xenon lamp was used as a light source.
(a) 0.2
Voltage (V)
0.0 -0.2 -0.4 -0.6 -0.8 -1.0 0
20
40
60
80
100
Time (sec.)
All the SnS thin films prepared by three-step pulsed ED from lactic acid/sodium lactate buffered electrolytes show sufficient adherence to ITO substrates and grey in color. The thickness of the as-deposited films could be controlled simply by electrodeposition time. For 15 min, the thin film can be electrodeposited from buffered electrolytes to around 0.9 μm in thickness, which is a little thicker than the film deposited without buffer during the same time period (film thickness almost 0.8 μm). Fig. 3 (a) and (b) illustrate the AES spectra for the SnS samples obtained by three-step pulse ED from a lactic acid/sodium lactate buffered electrolyte and a non-pH buffered electrolyte, respectively. It is interesting to note that the oxygen peak is evidently absent for the sample grown from the buffered electrolyte and that only tin and sulphur elements are identified. The atomic ratio of Sn/S in this sample is evaluated to be about 1.09, close to stoichiometry. When deposited from the electrolyte without pH buffer, the sample always contains oxygen, whose signal is obviously seen in Fig. 3 (b). It was inferred that the inclusion of oxygen within the sample might be associated with hydrogen evolution process that is most likely to occur at the cathode during deposition and consequently causes a local pH increase, which could enable formation of tin oxide, while the presence of lactic acid/sodium lactate pH buffer agent can render the electrolyte pH more stable and thus can effectively repress tin oxide generation during the depositing process. For this reason, the oxygen content is sufficiently reduced in the deposits from the buffered electrolyte. Fig. 4 shows the XRD patterns of the as-deposited samples grown under the buffered and non-pH buffered conditions. From this figure, one can see that the film deposited from the buffered electrolytes shows better crystalline pattern as compared to the one from the nonpH buffered electrolytes. In Fig. 4 (a), beside the ITO substrate peaks, the major peaks of the SnS sample obtained from the buffered electrolytes can be clearly indexed as (101), (201), (210), ( 011), (111), (311) and (411) reflections corresponding to the α-SnS phase with an orthorhombic structure, by comparison with JCPDS card No.73-1859 [13]. In addition, no other phases or impurities were detected from the XRD studies. The SEM images in Fig. 5 contrast the typical surface morphology of the SnS samples deposited in both the buffered and non-buffered electrolytes. In the case of the film prepared from the electrolytes without pH buffer, there were many flower-like agglomerates of grains occurring on its surface, which induced an uneven and rough surface morphology, however, for the case of the buffered electrolytes,
(b) 10
Sn
S
(a)
0
Intensity
Current (mA)
5
-5
Sn
S
-10
O
(b)
-15 -20 0
20
40
60
80
100
Time (sec.) Fig. 2. (a) Pulsed-voltage profile and (b) pulsed-current profile during electrodeposition in lactic acid/sodium lactate buffered electrolytes.
0
200
400
600
800
1000
Kinetic Energy (eV) Fig. 3. AES spectra for SnS samples deposited from (a) lactic acid/sodium lactate buffered electrolytes and (b) non-pH buffered electrolytes.
F. Kang, M. Ichimura / Thin Solid Films 519 (2010) 725–728
sub
(411)
(311)
(111)
(a) sub
sub (101) (201) (210)
sub+(011)
6
( αhν)2109( cm-2eV2)
Intensity ( arb.units )
(b)
SnS (73-1859) 10
20
30
40
50
60
2 theta ( deg. )
727
4
2
0 1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
hν (eV) Fig. 6. (αhν)2 vs. hν plot of SnS sample deposited using three-step pulse from lactic acid/ sodium lactate buffered electrolytes.
Fig. 4. The XRD patterns of SnS films grown from (a) lactic acid/sodium lactate buffered electrolytes and (b) non-pH buffered electrolytes.
the film exhibited a relatively smooth microstructure as a result of only a small number of agglomerates formed on the surface. These observations indicate that the lactic acid/sodium lactate buffer can
contribute to improving the surface roughness of as-deposited SnS thin films. The optical transmittance of the SnS samples obtained by ED in the buffered electrolytes was measured in the wavelength range 300– 1400 nm at room temperature. The films showed a high absorption coefficient (higher than 104 cm− 1) above the fundamental absorption. The energy band gap of the film was determined from the plot of (αhν)2 vs. hν shown in Fig. 6. From the linear extrapolation, the direct band gap was evaluated to be 1.39 eV, in agreement with the literature value. Fig. 7 shows the photocurrent response in the PEC measurement for the SnS sample deposited from the buffered electrolytes. The negative current was enhanced during the illumination as shown in Fig. 7, while the photocurrent was negligibly small under the positive voltage scan. This indicates that the minority carrier in the sample is electron. Thus, the sample is photoactive, exhibiting p-type conductivity. 4. Conclusions In summary, a novel method for preparation of SnS thin films via three-step pulsed ED technique using a lactic acid/sodium lactate buffered electrolyte has been demonstrated. From characterization results, it is concluded that the proposed method can provide beneficial effects on the properties of deposits, leading to nearly oxygen-free and
50 0
Current (μA/cm2)
-50 -100 -150 -200 -250 -300 -350 -0.6
-0.4
-0.2
0.0
Voltage (V vs. SCE) Fig. 5. SEM images of SnS samples prepared from (a) buffered electrolytes and (b) nonpH buffered electrolytes.
Fig. 7. Photocurrent response in PEC measurements of the SnS sample deposited with three-step pulse from lactic acid/sodium lactate buffered electrolytes.
728
F. Kang, M. Ichimura / Thin Solid Films 519 (2010) 725–728
stoichiometric α-SnS thin films with smoothened surface morphology. In addition, the deposited film exhibits a direct band gap around 1.39 eV and shows p-type conductivity. Indeed, these results are encouraging, and the SnS films prepared by the illustrated method will be applied for solar cell fabrication in our future work. Acknowledgements The authors would like to thank Prof. M. Kato and Mr. A.M. Abdel Haleem for their useful discussion and invaluable research assistance. References [1] R.H. Bube, Photoconductivity of Solids, Wiley, New York, 1960 p. 233. [2] H. Noguchi, A. Setiyadi, H. Tanamura, T. Nagatomo, O. Omoto, Sol. Energy Mater. Sol. Cells 35 (1994) 325.
[3] B. Ghosh, M. Das, P. Banerjee, S. Das, Sol. Energy Mater. Sol. Cells 92 (2008) 1099. [4] N.K. Reddy, K.T. Ramakrishna Reddy, Thin Solid Films 325 (1998) 4. [5] M. Calixto-Rodriguez, H. Martinez, A. Sanchez-Juarez, J. Campos-Alvarez, A. Tiburcio-Silver, M.E. Calixto, Thin Solid Films 517 (2009) 2497. [6] D. Avellaneda, G. Delgadoa, M.T.S. Naira, P.K. Nair, Thin Solid Films 515 (2007) 5771. [7] Z. Zainal, M.Z. Hussein, A. Ghazali, Sol. Energy Mater. Sol. Cells 40 (1996) 347. [8] S.Y. Cheng, Y.Q. Chen, Y.J. He, G.N. Chen, Mater. Lett. 61 (2007) 1408. [9] K. Omoto, N. Fathy, M. Ichimura, Jpn. J. Appl. Phys. 45 (2006) 1500. [10] J.R.S. Brownson, C. Georges, C. Levy-Clement, Chem. Mater. 18 (2006) 6397. [11] G.H. Yue, W. Wang, L.S. Wang, X. Wang, P.X. Yan, Y. Chen, D.L. Peng, J. Alloy Compd. 474 (2009) 445. [12] F. Kang, J.P. Ao, G.Z. Sun, Q. He, Y. Sun, Mater. Chem. Phys. 115 (2009) 516. [13] S. Del Bucchia, J.C. Jumas, M. Maurin, Acta Crystallogr. B37 (1981) 1903.
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
Pulsed electrodeposition of oxygen-free tin monosulfide thin films using lactic acid/sodium lactate buffered electrolytes Feng Kang a,b,⁎, Masaya Ichimura b a b
Institute of Photo-Electronic Thin Film Devices and Technology, Nankai University, Tianjin 300071, China Department of Engineering Physics, Electronics and Mechanics, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466-8555, Japan
a r t i c l e
i n f o
Article history: Received 11 August 2009 Received in revised form 21 June 2010 Accepted 26 August 2010 Available online 21 September 2010 Keywords: Pulsed electrodeposition SnS thin films Lactic acid/sodium lactate pHydrian buffer
a b s t r a c t In this study, tin monosulfide (SnS) thin films have been prepared on indium-tin-oxide-covered glass substrates from an acidic electrolyte containing a pH buffer of lactic acid/sodium lactate using pulsed electrodeposition method. Results from Auger electron spectroscopy confirmed that nearly oxygen-free and stoichiometric SnS thin films were attained. X-ray diffraction and scanning electron micrograph studies indicated the formation of a smooth α-SnS thin film with the orthorhombic structure. Moreover, optical transmission spectroscopy showed a direct optical band gap of 1.39 eV and the photoelectrochemical measurements revealed the p-type conductivity. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Over the past decade, there has been a growing interest in the IV–VI compound semiconductor tin monosulphide (SnS). It has a direct band gap energy of 1.3–1.5 eV, a high absorption coefficient (α N 104 cm− 1) and a high theoretical conversion efficiency of 25%. In addition, it contains only abundant, non-toxic elements. These favorable features make it one of the most promising absorber materials for the fabrication of thin-film solar cells [1]. Up until now, a number of methods have been reported for depositing SnS thin films including vacuum evaporation [2,3], spray pyrolysis [4,5], chemical bath deposition [6] and electrodeposition. Among these methods, electrodeposition (ED) is more attractive since it offers the advantages of being non-vacuum, cost-effective and suitable for large scale deposition. This has motivated many research groups to study preparation of SnS thin films by employing different ED techniques [7–11]. In our previous work [9], we have shown that the surface morphology of SnS thin films can be highly improved by adopting three-step pulse ED. However, we also found that some oxygen was always included in the deposits, suggesting the formation of tin oxide phase during electrodeposition, which could be caused by pH changes in electrolytes. To overcome this problem, in this study, we introduced a pH buffer of lactic acid/ sodium lactate into electrolytes for three-step pulse ED of SnS thin films. This pH buffer system has already been proven to be an effective way to improve
electrolyte stability for one-step ED of CuInSe2 thin films [12]. The properties of as-deposited samples have been investigated by several techniques and compared with the ones prepared from non-pH buffer electrolytes under the same ED conditions as well. 2. Experimental details 2.1. Preparation A conventional three-electrode cell was used for ED, with an indium-tin-oxide (ITO)-coated glass substrate as the working electrode, a platinum sheet as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode, whose schematic diagram is shown in Fig. 1. The deposition area was limited to 1 cm × 1 cm by masking. The electrolyte solution contained 30 mM SnSO4 and 100 mM Na2S2O3 dissolved in a lactic acid/sodium lactate buffer system, maintaining the solution at a pH of about 2.3. The electrolyte temperature was kept at 60 °C in a water bath throughout the deposition. ED from the above electrolytes was performed under a periodic three-step pulse voltage [9] ( voltage “V1” = − 1 V, on time”t1” = 10 s, “V2” = − 0.6 V, on time”t2” = 10 s, “V3” = 0 V, on time “t3” = 10 s). The typical pulsed voltage cycles and current profile during deposition are shown in Fig. 2. The total deposition time was 15 min. 2.2. Characterization
⁎ Corresponding author. Institute of Photo-Electronic Thin Film Devices and Technology, Nankai University, Tianjin 300071, China. Tel.: +86 22 23497012; fax: +86 22 23498216. E-mail address: [email protected] (F. Kang). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.08.138
X-ray diffraction (XRD) patterns were recorded on the RIGAKU RINT-2000 diffractometer using CuKα radiation. The film thickness was measured by a profile meter Accretech, Surfcom-1400D. The
726
F. Kang, M. Ichimura / Thin Solid Films 519 (2010) 725–728
3. Results and discussion
Fig. 1. A schematic diagram of the SnS electrodeposition setup.
compositional analysis was carried out by Auger electron spectroscopy (AES) using the model JEOL JAMP 7800 Auger microprobe at probe voltage 10 kV and current 2 × 10− 8 A. The film morphology was investigated by scanning electron microscope (SEM) model HITACHI S2000 S, keeping the acceleration voltage at 10 kV and magnification 2000. The transmission spectra were measured using the JASCO U-570 UV/VIS /NIR spectrometer. Photoelectrochemical (PEC) measurements were carried out in an aqueous electrolyte consisting of 100 mM Na2S2O3. The applied voltage was swept at a speed of 5 mV/s, and the illumination was switched on and off for each 5 s. A Xenon lamp was used as a light source.
(a) 0.2
Voltage (V)
0.0 -0.2 -0.4 -0.6 -0.8 -1.0 0
20
40
60
80
100
Time (sec.)
All the SnS thin films prepared by three-step pulsed ED from lactic acid/sodium lactate buffered electrolytes show sufficient adherence to ITO substrates and grey in color. The thickness of the as-deposited films could be controlled simply by electrodeposition time. For 15 min, the thin film can be electrodeposited from buffered electrolytes to around 0.9 μm in thickness, which is a little thicker than the film deposited without buffer during the same time period (film thickness almost 0.8 μm). Fig. 3 (a) and (b) illustrate the AES spectra for the SnS samples obtained by three-step pulse ED from a lactic acid/sodium lactate buffered electrolyte and a non-pH buffered electrolyte, respectively. It is interesting to note that the oxygen peak is evidently absent for the sample grown from the buffered electrolyte and that only tin and sulphur elements are identified. The atomic ratio of Sn/S in this sample is evaluated to be about 1.09, close to stoichiometry. When deposited from the electrolyte without pH buffer, the sample always contains oxygen, whose signal is obviously seen in Fig. 3 (b). It was inferred that the inclusion of oxygen within the sample might be associated with hydrogen evolution process that is most likely to occur at the cathode during deposition and consequently causes a local pH increase, which could enable formation of tin oxide, while the presence of lactic acid/sodium lactate pH buffer agent can render the electrolyte pH more stable and thus can effectively repress tin oxide generation during the depositing process. For this reason, the oxygen content is sufficiently reduced in the deposits from the buffered electrolyte. Fig. 4 shows the XRD patterns of the as-deposited samples grown under the buffered and non-pH buffered conditions. From this figure, one can see that the film deposited from the buffered electrolytes shows better crystalline pattern as compared to the one from the nonpH buffered electrolytes. In Fig. 4 (a), beside the ITO substrate peaks, the major peaks of the SnS sample obtained from the buffered electrolytes can be clearly indexed as (101), (201), (210), ( 011), (111), (311) and (411) reflections corresponding to the α-SnS phase with an orthorhombic structure, by comparison with JCPDS card No.73-1859 [13]. In addition, no other phases or impurities were detected from the XRD studies. The SEM images in Fig. 5 contrast the typical surface morphology of the SnS samples deposited in both the buffered and non-buffered electrolytes. In the case of the film prepared from the electrolytes without pH buffer, there were many flower-like agglomerates of grains occurring on its surface, which induced an uneven and rough surface morphology, however, for the case of the buffered electrolytes,
(b) 10
Sn
S
(a)
0
Intensity
Current (mA)
5
-5
Sn
S
-10
O
(b)
-15 -20 0
20
40
60
80
100
Time (sec.) Fig. 2. (a) Pulsed-voltage profile and (b) pulsed-current profile during electrodeposition in lactic acid/sodium lactate buffered electrolytes.
0
200
400
600
800
1000
Kinetic Energy (eV) Fig. 3. AES spectra for SnS samples deposited from (a) lactic acid/sodium lactate buffered electrolytes and (b) non-pH buffered electrolytes.
F. Kang, M. Ichimura / Thin Solid Films 519 (2010) 725–728
sub
(411)
(311)
(111)
(a) sub
sub (101) (201) (210)
sub+(011)
6
( αhν)2109( cm-2eV2)
Intensity ( arb.units )
(b)
SnS (73-1859) 10
20
30
40
50
60
2 theta ( deg. )
727
4
2
0 1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
hν (eV) Fig. 6. (αhν)2 vs. hν plot of SnS sample deposited using three-step pulse from lactic acid/ sodium lactate buffered electrolytes.
Fig. 4. The XRD patterns of SnS films grown from (a) lactic acid/sodium lactate buffered electrolytes and (b) non-pH buffered electrolytes.
the film exhibited a relatively smooth microstructure as a result of only a small number of agglomerates formed on the surface. These observations indicate that the lactic acid/sodium lactate buffer can
contribute to improving the surface roughness of as-deposited SnS thin films. The optical transmittance of the SnS samples obtained by ED in the buffered electrolytes was measured in the wavelength range 300– 1400 nm at room temperature. The films showed a high absorption coefficient (higher than 104 cm− 1) above the fundamental absorption. The energy band gap of the film was determined from the plot of (αhν)2 vs. hν shown in Fig. 6. From the linear extrapolation, the direct band gap was evaluated to be 1.39 eV, in agreement with the literature value. Fig. 7 shows the photocurrent response in the PEC measurement for the SnS sample deposited from the buffered electrolytes. The negative current was enhanced during the illumination as shown in Fig. 7, while the photocurrent was negligibly small under the positive voltage scan. This indicates that the minority carrier in the sample is electron. Thus, the sample is photoactive, exhibiting p-type conductivity. 4. Conclusions In summary, a novel method for preparation of SnS thin films via three-step pulsed ED technique using a lactic acid/sodium lactate buffered electrolyte has been demonstrated. From characterization results, it is concluded that the proposed method can provide beneficial effects on the properties of deposits, leading to nearly oxygen-free and
50 0
Current (μA/cm2)
-50 -100 -150 -200 -250 -300 -350 -0.6
-0.4
-0.2
0.0
Voltage (V vs. SCE) Fig. 5. SEM images of SnS samples prepared from (a) buffered electrolytes and (b) nonpH buffered electrolytes.
Fig. 7. Photocurrent response in PEC measurements of the SnS sample deposited with three-step pulse from lactic acid/sodium lactate buffered electrolytes.
728
F. Kang, M. Ichimura / Thin Solid Films 519 (2010) 725–728
stoichiometric α-SnS thin films with smoothened surface morphology. In addition, the deposited film exhibits a direct band gap around 1.39 eV and shows p-type conductivity. Indeed, these results are encouraging, and the SnS films prepared by the illustrated method will be applied for solar cell fabrication in our future work. Acknowledgements The authors would like to thank Prof. M. Kato and Mr. A.M. Abdel Haleem for their useful discussion and invaluable research assistance. References [1] R.H. Bube, Photoconductivity of Solids, Wiley, New York, 1960 p. 233. [2] H. Noguchi, A. Setiyadi, H. Tanamura, T. Nagatomo, O. Omoto, Sol. Energy Mater. Sol. Cells 35 (1994) 325.
[3] B. Ghosh, M. Das, P. Banerjee, S. Das, Sol. Energy Mater. Sol. Cells 92 (2008) 1099. [4] N.K. Reddy, K.T. Ramakrishna Reddy, Thin Solid Films 325 (1998) 4. [5] M. Calixto-Rodriguez, H. Martinez, A. Sanchez-Juarez, J. Campos-Alvarez, A. Tiburcio-Silver, M.E. Calixto, Thin Solid Films 517 (2009) 2497. [6] D. Avellaneda, G. Delgadoa, M.T.S. Naira, P.K. Nair, Thin Solid Films 515 (2007) 5771. [7] Z. Zainal, M.Z. Hussein, A. Ghazali, Sol. Energy Mater. Sol. Cells 40 (1996) 347. [8] S.Y. Cheng, Y.Q. Chen, Y.J. He, G.N. Chen, Mater. Lett. 61 (2007) 1408. [9] K. Omoto, N. Fathy, M. Ichimura, Jpn. J. Appl. Phys. 45 (2006) 1500. [10] J.R.S. Brownson, C. Georges, C. Levy-Clement, Chem. Mater. 18 (2006) 6397. [11] G.H. Yue, W. Wang, L.S. Wang, X. Wang, P.X. Yan, Y. Chen, D.L. Peng, J. Alloy Compd. 474 (2009) 445. [12] F. Kang, J.P. Ao, G.Z. Sun, Q. He, Y. Sun, Mater. Chem. Phys. 115 (2009) 516. [13] S. Del Bucchia, J.C. Jumas, M. Maurin, Acta Crystallogr. B37 (1981) 1903.