Electrodeposition of CIS films on the Mo back electrodes with different crystallinities

Electrochimica Acta 75 (2012) 20–27

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Electrodeposition of CIS films on the Mo back electrodes with different crystallinities Hsien-Chung Huang a , Chao-Sung Lin a,∗ , Wei-Che Chang b a b

Department of Materials Science and Engineering, National Taiwan University, Taipei 10617, Taiwan, ROC Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 701, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 9 March 2012 Received in revised form 13 April 2012 Accepted 15 April 2012 Available online 14 May 2012 Keywords: Molybdenum Copper indium diselenide Solar cell Electrodeposition Hydrogen evolution reaction

a b s t r a c t Electrodeposition of copper indium diselenide (CuInSe2 ), which is an absorption layer for thin film solar cells, has been studied on a molybdenum (Mo)-coated glass with different crystallinities. Metastable FCC Mo and BCC Mo coatings were prepared by R.F. sputtering with varying R.F. power (100–170 W) and Ar pressure (3–11 mTorr). Experimental results indicated that the Mo coating deposited at lower power and higher pressure had smaller crystallite size. Cross-sectional transmission electron microscopy showed that the Mo coating deficient in crystallinities contained micro voids residing in the boundaries of the columnar grains and had higher oxygen content, as measured by energy dispersive spectroscopy. The crystallinity of Mo coatings strongly influenced the open circuit potential in the electrolyte for CIS electrodeposition. Consequently, the Cu/In ratio of CIS deposits plated at a constant potential (−0.7 vs. SCE) varied with the distinct Mo coatings. Moreover, the CIS deposit on the various Mo-coated glasses displayed a different morphology. The effect of the crystallinity of Mo coatings on hydrogen evolution reaction at pH 1.55 was also explored. Hydrogen evolution during the CIS electrodeposition may be one of the key factors to influence the CIS morphology. How the crystallinity of the Mo coating affects the composition and morphology of the CIS deposits can be useful for device fabrication and deserves for further study. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Sputtered molybdenum (Mo) coating is the back electrode commonly used for a vacuum-processed CIS/CIGS solar cell owing to its good electron conductivity, optical reflectance, high work function, and great ohmic contact behavior [1–4]. Furthermore, the sputtered Mo coating generally contains rapid diffusion paths for the diffusion of sodium from the soda lime glass substrate to the CIGS layer, which has been shown to be benefit for the CIGS electrical and structural properties [5]. For the electrodeposited CIS/CIGS process, the Mo coating is more essential because the back electrode acts as a conductive working electrode where the reduction of the various active species in the electrolyte occurs. And, most importantly, Mo is chemically stable in acid electrolytes for CIS/CIGS electroplating, which is commonly operated at pH 1.6–2.4 dependent on the different chelating agents [6]. Both the vacuum process and the electroplating technology are currently available for preparing CIS/CIGS absorption layers and have their own advantages and limitations [7–9]. Nevertheless, the structure and properties of

∗ Corresponding author at: No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan, ROC. Tel.: +886 2 33665240; fax: +886 2 3634562. E-mail address: [email protected] (C.-S. Lin). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.04.162

both the vacuum-processed and electroplated CIS/CIGS coatings are influenced by the structure of the Mo coating. For example, in the evaporation CIGS system, Schlenker et al. [10] reported the CIGS grew at a 112 preferred orientation on a polycrystalline Mo but at a 220/204 orientation on a 110-oriented Mo single crystal. Scofield et al. [4] employed a DC sputtered bi-layered Mo to improve the internal stress control and maintain a high electric conductivity. Ahn et al. [11] considered that the flaking of a spray deposited CIGS film from a Mo-coated glass during selenisation was attributed to the formation of a thick MoSe2 layer, which has the thermal expansion coefficient different from those of Mo and CIGS coatings. A thin MoSe2 layer is helpful for both the contact resistance and adhesion. Assmann et al. [12] reported an I–V curve characteristic of Mo/MoSe2 /CIGS at room temperature was linear and the contact resistance was approximately 0.08  cm2 , which is compatible for the solar cell technology (0.2  cm2 ). In the electroplating system, Jubault et al. [13] reported the CIGS solar cell with a R.F. sputtered Mo coating was more reflective in the region of visible spectrum than that with a DC sputtered Mo, although the cell with the DC sputtered Mo had a higher cell efficiency. Martinez et al. [14] found that low R.F. power or low Ar pressure resulted in a dense Mo coating with less stresses, which were essential to enhance the recrystallization of electrodeposited CIS during selenisation. Estela et al. [15] reported a two-step

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electroplating of CIGS films in a single electrolyte. The plating was first conducted at −0.5 V (vs. SCE) to avoid hydrogen accumulation at the Mo surface, followed by electrodeposition of CIGS at −0.6 V. Efforts have been made for a better understanding of the optical and electrical properties, as well as microstructure of Mo coatings and their effect on the structure of an adsorption CIGS layer and the efficiency of the CIGS solar cell. However, the electrochemical properties of the Mo coating and their effect on the electrocrystallization and growth of CIS/CIGS are less well studied. In this study, the open circuit potential (OCP) of the Mo coating deposited at different R.F. powers and pressures in a CIS electrolyte was measured and the composition as well as morphology of the resultant CIS were explored. The crystallinity of the CIS deposits was improved by annealing in selenium atmosphere using a semi-closed graphite box. Optimization of the processing conditions, including R.F. sputtering power and Ar pressure, resulted in a smooth, compact CIS adsorption layer. 2. Experimental 2.1. Sputtering of Mo and electroplating of CIS Mo coatings were deposited by R.F. magnetron sputtering using a Mo target (purity, 99.95%). Soda lime glass substrates of 20 mm × 12 mm × 7 mm were degreased ultrasonically in 0.01 M NaOH solution, rinsed using de-ionized water, and finally dried on a hot plate at 160 ◦ C. After cleaning, the glass substrates were loaded into a vacuum chamber with a base pressure of around 8 × 10−6 Torr. The working distance was fixed at 9 cm. The argon (Ar) pressure was varied between 3 and 11 mTorr by adjusting the Ar flow (purity, 99.9997%) and the R.F. power was varied between 100 and 170 W for a 4 in. Mo target. The average thickness of the Mo coating was set to be approximately 1.0 ␮m. Electroplating was performed on the Mo-coated glass with an exposed area of 5 mm × 10 mm. The electrolyte contained 0.013 M CuCl2 , 0.027 M InCl3 , 0.025 M H2 SeO3 , and 0.7 M KCl and had a pH of around 1.55. A three-electrode cell incorporating an EG&G 216 potentiostat was used for electroplating. The counter electrode was a platinum disc and the reference electrode was a saturated calomel electrode (SCE). The potential reported herein was with respect to the SCE. The electroplating was performed potentiostatically at −0.7 V for 600 s.

Fig. 1. (a) XRD patterns of the Mo coatings sputtered at 3 mTorr and sputtering powers ranging from 100 to 170 W; (b) XRD patterns of the Mo coatings sputtered at 115 W and Ar pressures ranging from 3 to 11 mTorr.

3. Results and discussion 3.1. Characterization of the R.F. sputtered Mo

2.2. Microstructural and electrochemical characterizations The surface morphology, cross-sectional structure, and chemical composition of the Mo and CIS coatings were investigated using a scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) equipped in SEM. X-ray diffractometry (XRD) was employed for the crystal structure identification and crystallinity evaluation. Hydrogen evolution reaction (HER) of the different Mo-coated glasses was studied in a pH 1.55 HCl solution to simulate the hydrogen discharge, a side reaction during CIS electroplating. Some as-deposited CIS samples were selenised to improve the crystallinity of the CIS. The selenisation was conducted using gaseous Se. The samples were placed in a semi-closed graphite box (50 mm × 50 mm × 30 mm) containing 2 g of Se powders. The graphite box was introduced into a quartz tube furnace with a base pressure of around 5 × 10−5 Torr. A two-step heat profile was employed. The first step was conducted at 230 ◦ C for 20 min to enhance the saturation of gaseous Se. The temperature was then elevated to 540 ◦ C for 30 min to accelerate the recrystallization and grain growth of the CIS, followed by air cooling to room temperature.

Fig. 1(a) and (b) show the XRD patterns of the Mo coatings sputtered at different powers and Ar pressures, respectively. The Mo coating sputtered at powers exceeding 115 W displayed a BCC structure and the typical diffraction peaks were associated with (1 1 0) and (2 1 1) planes. The presence of a strong (1 1 0) diffraction peak relative to the (2 1 1) peak indicates that the Mo coating exhibits a preferred orientation. The diffraction peak intensity of the Mo coating deposited at <115 W decreased significantly, indicating the poor crystallinity. The crystallite size, which was calculated from the full width at half-maximum (FWHM) of plane (1 0 0) by the Scherrer formula, showed that the decrease in the crystallite size with decreasing sputtering powers (Table 1). This trend is consistent with the study by Khatri and Marsillac [16]. The arriving adatoms ejected at high powers apparently have more energy and a high surface mobility, facilitating their diffusion to more energetically favorable sites. Well faceted grains with larger grain size can thus be obtained by the minimization of the shadowing influence and total free energy [17]. Moreover, the Mo coating deposited at 100 W showed a thermodynamically metastable FCC structure. It has been shown that Mo and tungsten coatings with the

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Fig. 2. Cross-sectional TEM micrographs of the Mo coating sputtered at (a) 115 W, 11 mTorr and (b) 170 W, 3 mTorr.

Table 1 Grain size of the Mo coatings sputtered at different powers and Ar pressures. Ar pressure set at 3 mTorr Power (W) 100 ˚ Grain size (A) 28

115 51

130 48

Target power set at 115 W Power (W) 3 ˚ Grain size (A) 80

7 55

11 51

150 66

170 72

FCC structure can be deposited at high temperatures, high pressure treatments, or by introducing a precursor gas containing impurity into a sputtering chamber [18,19]. The CIS electrodeposited on the metastable FCC Mo-coated glass electrode was found to have a poor adhesion (data not shown). The Mo coating for the CIS electroplating was thus prepared at sputtering powers ≥115 W. Fig. 1(b) shows the effect of Ar pressure on the crystallinity of the Mo coating deposited at 155 W. The crystallinity of the Mo coating decreased with increasing Ar pressures (Table 1). Furthermore, the d-spacing of Mo (1 1 0) was found to decrease with the Ar pressure. The residual stress of sputtered Mo coating has been shown to be associated with atomic voids, oxygen and argon impurity [20–22]. The decrease in the d-spacing may be due to a larger amount of codeposited oxygen resulting a compressive stress from high Ar pressure. The normalized electric resistivity of the sputtered Mo coating as a function of the sputtering power and Ar pressure is shown in Table 2(a). The Mo coatings with higher conductivity were

generally deposited at higher powers and lower pressures. This is due to the fewer amounts of grain boundaries because high power and low pressure conditions promote the crystallinity of Mo coating (Fig. 1 and Table 1). In addition, the electric resistivity increased in proportional to the oxygen content of the Mo coating measured by the EDS in the SEM (Table 2(b)). Fig. 2(a) and (b) shows the cross-sectional TEM micrographs of the Mo coatings deposited at 115 W, 11 mTorr and 170 W, 3 mTorr, respectively. Both coatings consisted of columnar grains perpendicular to the glass substrate. However, the Mo deposited at a lower power and higher pressure (Fig. 2(a)) displayed a more open structure with micro voids residing along the columnar boundaries, as compared to that deposited at a higher power and lower pressure (Fig. 2(b)). This is consistent with the result that high power and low Ar pressure conditions reduce the resistivity of the Mo coating (Table 2(a)). The Mo atoms sputtered at lower powers generally have lower mobility on the glass substrate and their bonding and stacking can result in more micro voids. Scofield et al. found that micro voids in the Mo back electrode were a rapid diffusion path for Na atom, which is one of the elements from the soda lime glass [4]. Moreover, Na element is beneficial to the electric properties of CIS/CIGS during evaporation or selenisation. 3.2. Characterization of the CIS electrodeposits To obtain a CIS deposit, electroplating was performed on the Mo-coated glass potentiostatically at −0.7 V for 10 min. Fig. 3(a)

Table 2 (a) Normalized electric resistivity and (b) oxygen content of the different Mo coatings. (a) Normalized resistivity (␮ cm) Power (W)

Ar pressure (mTorr)

3 mTorr 7 mTorr 11 mTorr

100 W

115 W

130 W

150 W

170 W

36 400 580

20 200 300

18 70 156

14 67 108

13 43 91

100 W

115 W

130 W

150 W

170 W

14.3 26.1 29.8

13.1 22.2 24.6

12.1 20.3 24.1

10.0 17.7 21.8

9.8 15.5 19.7

(b) Oxygen content (at% *molybdenum bal). Power (W)

Ar pressure (mTorr)

3 mTorr 7 mTorr 11 mTorr

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Fig. 3. (a) SEM surface morphology of the CIS on different Mo coatings and (b) the corresponding cross-sectional structure of the CIS in (a).

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and (b) show the surface morphology and cross-section microstructure of the CIS deposits on the various Mo-coated glasses. The CIS deposit on the 150 W, 3 mTorr Mo-coated glass showed a granular structure with pin holes resulting from hydrogen discharge, a side reaction frequently seen during electroplating (Fig. 3(a)). The CIS on the Mo coating deposited under lower powers and higher pressures consisted of smaller nodules with fewer pin holes. Specifically, a smooth, nearly pin-hole-free CIS deposit was electroplated onto the Mo deposited at 115 W, 11 mTorr. The nodules or so called cauliflower-like structure have been recognized as the secondary phase (Cux Se) that generally deteriorated the device efficiency due to their degenerate semiconductor characteristic [15,23]. Most hydrogen bubbles had penetrated through the CIS deposit, suggesting that the Mo had a higher catalysis toward hydrogen discharge than the CIS. Apparently, a great amount of hydrogen was formed at the beginning of electrodeposition. Fig. 3(b) shows the cross sections of the CIS deposits on the various Mo-coated glasses. In general, a more compact CIS deposit was electroplated on the Mo sputtered under lower power and higher Ar pressure conditions. The hydrogen evolution can depend on the composition and microstructure of the Mo coating, which determines the formation of hydrogen bubbles and their subsequent detachment from the growing CIS deposit. The cathodic polarization was thus used to study the hydrogen evolution reaction (HER) in pH 1.55 hydrochloride solutions, as shown in Fig. 4(a) and (b). The corresponding electrochemical parameters are listed in Table 4. Fig. 4(a) shows the effect of the sputtering power on the HER of the Mo sputtered at 11 mTorr. At −0.7 V, which was the same as the potential for CIS electroplating. The HER current density at −0.7 V increased with increasing the sputtering powers of the Mo coating. Furthermore, the overpotential corresponding to 1 × 10−3 A cm−2 decreased with increasing the sputtering powers. A comparison between Fig. 4(a) and Table 2 revealed that the HER rate was reduced with the oxygen content of the Mo coating. In addition, the difference in the ohmic resistivity of the Mo coating also contributed to the HER overpotential. Similar results are shown in Fig. 4(b) for the Mo coating sputtered at 115 W and the Ar pressure ranging from 3 to 11 mTorr. Specifically, the Mo coating sputtered at higher Ar pressures contained more oxygen and had lower HER activity. The HER was apparently retarded on the Mo coating with higher oxygen content and higher electric resistivity. Accordingly, the Mo coating tends to be passivated by the codeposited oxygen. And, most importantly, the CIS layers appeared to adhere well to the passive Mo coating. The development of nodular morphology may be due to the H2 bubbles presence during electrodeposition, which acts as a heterogeneous site for the nucleation and growth of the CIS deposit. Fig. 5(a) and (b) show the microstructure of the selenized CIS deposits on the Mo-coated substrate sputtered at 150 W, 3 mTorr and 115 W, 11 mTorr, respectively. The selenized CIS on the Mocoated substrate sputtered at 150 W, 3 mTorr displayed a porous structure and a thick MoSe2 was formed during selenisation. The formation of the thick MoSe2 was caused by the direct contact of the Mo coating with Se vapor through a large number of pores in the as-deposited CIS deposit. The MoSe2 layer was about 500 nm in

Fig. 4. Cathodic polarization of the various Mo coatings in pH 1.55 HCl solution: (a) the effect of the sputtering power at a Ar pressure of 11 mTorr; (b) the effect of the Ar pressure at a sputtering power of 115 W.

thickness and the electric resistivity was in the range of 104  cm. Moreover, local detachment of the CIS layer from the specimen was frequently observed. In contrast, the selenized CIS on the Mo-coated substrate sputtered at 115 W, 11 mTorr was relatively compact and the MoSe2 layer was relatively thin. Moreover, an obvious grain growth occurred during selenisation. As a result, the resultant CIS structure is very similar to the evaporation CIS/CIGS thin films and provides an electrical ohmic contact and sufficient adhesion. Fig. 6(a) shows the XRD patterns of the as-deposited CIS deposits on the different Mo-coated glass substrates. All the CIS deposits showed the characteristic of the CIS chalcopyrite phase (JCPDS 401487) and had insignificant difference in crystallinity. Moreover,

Table 3 The Se content and the ratio of Cu to In of the CIS electrodeposits on the various Mo-coated glass substrates. Power (W) 115 W

Ar pressure (mTorr)

3 mTorr 7 mTorr 11 mTorr

130 W

150 W

Cu/In

Se/(Cu + In + Se)

Cu/In

Se/(Cu + In + Se)

Cu/In

Se/(Cu + In + Se)

0.70 0.83 0.87

0.52 0.56 0.56

0.71 0.75 0.85

0.52 0.55 0.54

0.70 0.74 0.85

0.54 0.52 0.53

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Fig. 5. SEM surface morphology and cross-sectional image of the selenized CIS deposits on the Mo-coated glass sputtered at (a) 150 W, 3 mTorr and (b) 115 W, 11 mTorr.

a shoulder close to the (1 1 2) main peak was observed for all the as-deposited CIS deposits. This minor phase is commonly see in the literature and considered as the secondary phase mainly composed of Cux Se or Inx Se [13]. These minor phases are hard to avoid during constant polarization at −0.7 V, which covers the reduction potential of these secondary phases (Cux Se: −0.067 V to 0.314 V; Inx Se: −0.2 V to −0.016 V) [24]. For example, the CIS deposit on the Mo coating sputtered at 115 W, 11 mTorr still contained the secondary phase (Fig. 6(a)) although it was smooth and nearly free of nodules (Fig. 3(a)). These secondary phase can be removed by the optimize selenisation and KCN treatment, as shown in Fig. 6(b). The EDS results of the as-deposited CIS on the various Mocoated glass substrates are listed in Table 3. The ratio of Se of all the CIS deposits was controlled to be slightly above 0.5, which is generally accepted as a criterion for the CIS absorption layer with better solar cell efficiency [25]. The ratio of Cu to In increased with the Ar pressure and decreased with the sputtering power for the Mo coating process. Nevertheless, the ratio of Cu to In ranged from 0.70 to 0.87, indicating that the CIS of this study was a p-type semiconductor [1]. The different Cu/In ratios of the CIS deposits on the distinct Mo coatings indicate that the various Mo coatings cathodically polarized at −0.7 V had differ-

ent overpotentials during CIS electrodeposition. The OCP of the Mo-coated glass substrate was thus different in the electrolyte. Fig. 7 shows the OCP evolution in the CIS electrolyte, as measured at different electroplating intervals. At the beginning of electroplating, the OCP of the 11 mTorr, 115 W sputtered Mo and the 3 mTorr, 115 W sputtered Mo was 174 mV and 217 mV, respectively. The Mo metal has been shown to be attacked by HCl solution and becomes passive by an insoluble chloride layer [6]. The 3 mTorr, 115 W sputtered Mo, which contained fewer oxygen than the 11 mTorr, 115 W sputtered Mo, underwent a more complete passivation in the CIS electrolyte, giving rise to a nobler OCP. The OCP then displayed an abrupt increase during the early stages of electroplating, followed by a nearly constant value. Moreover, the 3 mTorr, 115 W sputtered Mo had OCP nobler than the 11 mTorr, 115 W sputtered Mo throughout the course of electroplating. When the Mo-coated glass substrate was polarized at −0.7 V, the substrate with higher OCP sample had a higher overpotential, which promoted the reduction In3+ over Cu2+ . Accordingly, the ratio of Cu to In was reduced (Table 3). This is consistent with the previous results showing that the deposition at a lower overpotential enhances the deposition of Cu and Se within the CIS deposit [26,27]. The steady OCP indicates the complete coverage of the CIS deposit on the glass substrate.

Table 4 Electrochemical parameters obtained from the cathodic polarization curves of the different Mo coatings in pH 1.55 HCl solution, including the Tafel slope, the exchange current density (J0 ), the overpotential for 1 × 10−3 A cm−2 (1 ), and the current density at −0.7 V (J−0.7 ). Mo substrate

Tafel slope (mV dec−1 )

J0 (mA cm−2 )

1 (mV)

J−0.7 (mA cm−2 )

11 mTorr, 115 W 11 mTorr, 130 W 11 mTorr, 150 W

115 112 100

0.20 0.39 0.51

71 35 27

4.65 10.66 17.08

11 mTorr, 115 W 7 mTorr, 115 W 3 mTorr, 115 W

105 95 66

0.15 0.19 0.26

69 56 55

4.61 10.32 50.29

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Although it is not immediately clear as why the steady OCP differs for the different Mo-coated glass substrate, it is likely that the different OCP results form the two CIS deposits with different composition. 4. Conclusions The electroplating of CIS deposits on the glass sputtered with Mo coatings at different sputtering powers and Ar pressures was studied. The results showed that the Mo sputtered at 115–170 W had s stable BCC structure and that sputtered at 100 W displayed a metastable FCC structure. The CIS electrodeposit had poor adhesion on the metastable FCC Mo-coated glass. In contrast, a smooth, compact CIS deposit was electroplated onto the Mo coating sputtered at 115 W and 11 mTorr. A selenisation treatment of the compact CIS resulted in a coarse-grained CIS and a relatively thin MoSe2 layer. The hydrogen evolution was commonly seen during CIS electroplating and resulted in the formation of pin holes. The nodular CIS with pin holes became porous with a relatively thick MoSe2 layer after selenisation. HER can be retarded by the oxygen in the Mo coating. TEM images also showed that the Mo coating sputtered at 115 W, 11 mTorr had micro voids along the columnar grain boundaries; conversely, the Mo coating sputtered at 170 W, 3 mTorr was relatively dense. This difference influenced the OCP during CIS electroplating and the Mo-coated glass substrate polarized at the same voltage relative to SCE had different overpotentials, which depended on the sputtering power and Ar pressure. As a result, the composition of the CIS deposit varied with the sputtering power and Ar pressure of the Mo coating. In general, a higher overpotential promoted In3+ reduction and the resulting CIS showed a granular structure. Moreover, the OCP evolution during electroplating was of great importance to control the composition, morphology, and defects of the CIS deposits, which can provide a basis for better control on the electroplating of the CIS absorption layer on a variety of Mo-coated glass substrates.

Fig. 6. (a) XRD patterns of the CIS electrodeposits on the Mo-coated glass substrates sputtered at 3 mTorr and different sputtering powers; (b) XRD patterns of the asdeposited and selenized CIS on the Mo coating sputtered at 150 W, 11 mTorr.

Fig. 7. Open circuit potential evolution of the Mo-coated glass substrate with the electroplating time in the CIS electrolyte.

References [1] L. Antonio, H. Steven, Handbook of Photovoltaic Science and Engineering, Wiley, England, 2003, Chapter 13: Cu(InGa)Se2 Solar Cells. [2] K. Ramanathan, M.A. Contreras, C.L. Perkins, S. Asher, F.S. Hasoon, J. Keane, D. Young, M. Romero, W. Metzger, R. Noufi, J. Ward, A. Duda, Progress in Photovoltaics: Research and Applications 11 (2003) 225. [3] J.H. Scofield, A. Duda, D. Albin, B.L. Ballard, P.K. Predecki, Thin Solid Films 260 (1995) 26. [4] J.H. Scofield, S. Asher, D. Albin, J. Tuttle, M. Contreras, D. Niles, R. Reedy, A. Tennant, R. Noufi, Proc. of the 24th IEEE Photovoltaic Specialists Conference, IEEE, New York, 1995, p. 164. [5] S. Niki, M. Contreras, I. Repins, M. Powalla, K. Kushiya, S. Ishizuka, K. Matsubara, Progress in Photovoltaics: Research and Applications 18 (2010) 453. [6] M. Pourbaix, Atlas of Electrochemical in Aqueous Solutions, National Association of Corrosion Engineerings, Houston, Texas, 1974. [7] D. Lincot, et al., Solar Energy 77 (2004) 725. [8] I. Repins, M.A. Contreras, B. Egaas, C. DeHart, J. Scharf, C.L. Perkins, B. To, R. Noufi, Progress in Photovoltaics: Research and Applications 16 (2008) 235. [9] K. Kushiya, Solar Energy Materials & Solar Cells 93 (2009) 1037. [10] T. Schlenker, V. Laptev, H.W. Schock, J.H. Werner, Thin Solid Films 480–481 (2005) 29. [11] S. Ahn, K.H. Kim, J.H. Yun, K.H. Yoon, Journal of Applied Physics 105 (2009) 113533. [12] L. Assmann, J.C. Bernède, A. Drici, C. Amory, E. Halgand, M. Morsli, Applied Surface Science 246 (2005) 159. [13] M. Jubault, L. Ribeaucourt, E. Chassaing, G. Renou, D. Lincot, F. Donsanti, Solar Energy Materials and Solar Cells 95 (2010) 1. [14] M.A. Martínez, C. Guillén, Surface and Coatings Technology 110 (1998) 62. [15] M.E. Calixto, K.D. Dobson, B.E. McCandless, R.W. Birkmire, Journal of the Electrochemical Society 153 (6) (2006) 1533. [16] H. Khatri, S. Marsillac, Journal of Physics: Condensed Matter 20 (2008) 055206. [17] R.F. Bunshah, Handbook of Deposition Technologies for Films and Coatings, William Andrew, New York, 2009.

H.-C. Huang et al. / Electrochimica Acta 75 (2012) 20–27 [18] Y. Shimizu, T. Sasaki, N. Koshizaki, K. Tereshima, US Patent Application, 20080206552 (2008). [19] J.H. Lee, J.K. Lee, C.O. Jeong, B.S. Cho, US patent, #7511302 (2009). [20] S.G. Malhotra, Z.U. Rek, S.M. Yalisove, J.C. Bilell, Journal of Vacuum Science and Technology A 15 (1997) 345. [21] T. Yamaguchi, R. Miyagawa, Japanese Journal of Applied Physics 30 (9A) (1991) 2069. [22] F.J. Pern, R. Noufi, A. Mason, A. Franz, Thin Solid Films 202 (1991) 299. [23] J.P. Roger, D. Fournier, A.C. Boccara, Thin Solid Films 128 (1985) 11.

27

[24] P. Carbonnelle, L. Lamberts, Journal of Electroanalytical Chemistry 340 (1992) 53. [25] H. Neumann, R.D. Tomlinson, Solar Cells 28 (1990) 301. [26] N.B. Chaure, A.P. Samantilleke, R.P. Burton, J. Young, I.M. Dharmadasa, Thin Solid Films 472 (2005) 212. [27] J. Kois, S. Bereznev, E. Mellikov, A. Öpik, Thin Solid Films 511 (2006) 420.