Electrochemical approach for selenization of stacked Cu-In layers for formation of crystalline CuInSe2

Article abstract of DOI:10.1149/1.2957923

We report an electrochemical approach to form crystalline CuIn Se2 (CIS) films onto indium-tin-oxide substrates via thermal treatment to Se-coated Cu-In alloy. The simultaneous deposition of Cu-In alloy with optimum thickness was obtained by an electrochemical method from a mixture of aqueous solutions of CuS O4 and In2 (S O4)3 at constant potential. Further, the electrochemical method was used for deposition of elemental Se onto the priorly deposited Cu-In alloy film. To produce CIS films, Se-coated Cu-In alloy films were annealed in argon atmosphere at different temperatures ca. 350-450°C for 30 min. The Cu-In alloy, Se-coated Cu-In alloy, and thermally treated films were characterized using X-ray diffraction to identify the phases and scanning electron microscopy to observe the surface morphology.

Full text of DOI:10.1149/1.2957923

Journal of The Electrochemical Society, 155 ͑10͒ E131-E135 ͑2008͒
E131
0013-4651/2008/155͑10͒/E131/5/$23.00 © The Electrochemical Society
Electrochemical Approach for Selenization of Stacked Cu–In
Layers for Formation of Crystalline CuInSe2
T. P. Gujar,a,z V. R. Shinde,a Jong-Won Park,a,b Hyun Kyung Lee,b
Kwang-Deog Jung,a and Oh-Shim Jooa,z
aClean Energy Research Center, Korea Institute of Science and Technology, Seoul 130-650, Korea
bThe Industrial Chemistry, University of Sang-Myung, Seoul 130-650, Korea
We report an electrochemical approach to form crystalline CuInSe2 ͑CIS͒ films onto indium-tin-oxide substrates via thermal
treatment to Se-coated Cu–In alloy. The simultaneous deposition of Cu–In alloy with optimum thickness was obtained by an
electrochemical method from a mixture of aqueous solutions of CuSO4 and In2͑SO4͒ at constant potential. Further, the electro-
3
chemical method was used for deposition of elemental Se onto the priorly deposited Cu–In alloy film. To produce CIS films,
Se-coated Cu–In alloy films were annealed in argon atmosphere at different temperatures ca. 350–450°C for 30 min. The Cu–In
alloy, Se-coated Cu–In alloy, and thermally treated films were characterized using X-ray diffraction to identify the phases and
scanning electron microscopy to observe the surface morphology.
© 2008 The Electrochemical Society. ͓DOI: 10.1149/1.2957923͔ All rights reserved.
Manuscript submitted April 3, 2008; revised manuscript received June 18, 2008. Published July 29, 2008.
and Se, which are consecutively electrodeposited onto indium-tin-
oxide ͑ITO͒ substrates and annealed at high temperatures to form
CIS. The elemental selenium ͑Se͒ was deposited by the electro-
chemical route which provides a satisfactory deposition of elemental
Se from aqueous electrolytes like H2SeO2, and these electrolytes are
easy to handle and less toxic compared to H2Se gas.
It is well known that photovoltaic devices based on CuInGaSe2
͑CIGS͒ films by sequential coevaporation have reached an efficiency
up to 19.9% over small areas of around 0.5 cm2.1 However, the use
of this technique has limitations for commercial utilization of the
solar cells due to the problems associated with large-area
uniformity.2 Alternatively, several other techniques have been used
for the growth of CIGS and CuInSe2 ͑CIS͒ films such as metall-
organic vapor phase3 and molecular beam epitaxy,4 chemical bath
deposition,5 successive ionic layer adsorption and reaction,6
electrodeposition,7 etc. Because industrial applications need low-
cost processing methods for the large-scale production of materials
with necessary performance, efforts to find alternative methods are
still in progress. One such method, which is simple, low-cost, and
applicable for large-scale production, is electrodeposition. The elec-
trochemical methods to form CIS consist of either formation of
stacking layers ͑i.e., CuSe and InSe͒ and subsequent conversion to
CIS by annealing treatments or deposition of CIS from a single bath,
followed by thermal annealing in H2Se or elemental Se atmosphere
or etching process.8-20 Most papers reflect that the as-deposited CIS
film is either amorphous or has weak reflections of chalcopyrite CIS,
otherwise containing secondary phases of Cu2Se, In2Se3, etc.9-20
This can be attributed to the major problems encountered due to the
wide difference in the deposition potentials of Cu, In, and Se, which
result in a deposit with a nonuniform composition.16 Moreover, in
the aqueous electrolyte used for electrodeposition of CIS, H+ reduc-
tion causes composition inhomogeneity and hence limits the photo-
voltaic efficiency. For this reason, ionic liquids have also been stud-
ied for the deposition of CIS and CIGS films.21,22 Besides, to
overcome these problems, many researchers have used the annealing
treatment in H2Se or elemental Se atmosphere to get rid of un-
wanted phases. Along with being a highly toxic gas, selenization
using H2Se raises many problems, for example, inhomogeneity of
the films after the selenization process due to an inhomogeneous
Cu–In alloy layer, rapid volume expansion of CIS film during sele-
nization ͓leading to poor adhesion of the films onto molybdenum
back contacts͔, and phase separation of CIGS into CIS and
CuGaSe2.23 Therefore, several different approaches were explored
in order to overcome these issues. Recently, many researchers have
used Cu–In alloy films with consequent selenization to get CIS films
for photovoltaic application.23-28 Here we would like to add another
example to this list, where electrodeposited Se-coated Cu–In alloy
was used as a precursor to obtain crystalline and single-phase CIS
films.
Experimental
For the deposition of Cu–In alloy thin films, aqueous solutions of
͓analytical reagent ͑AR͒ grade͔ 10 mM copper͑II͒ sulfate ͑CuSO4͒
and 10 mM indium͑III͒ sulfate ͓In2͑SO4͒3͔ complexed with aque-
ous H2SO4 ͑0.2 M͒ were combined together in equal proportion.
For deposition of elemental Se thin films, aqueous solutions of ͑AR
grade͒ 10 mM selenic͑IV͒ acid ͑H2SeO3͒ were used. Electrodeposi-
tion of Cu–In alloy and Se thin films was carried out using a poten-
tiostat ͑EG&G 273 A͒ in potentiostatic mode onto the ITO substrate.
The substrate was thoroughly cleaned and subjected to ultrasonic
treatment prior to deposition. A three-electrode cell configuration
was formed from platinum plate as a counter electrode, the substrate
was a working electrode placed vertically, and silver/silver chloride
͑Ag/AgCl͒ was the reference electrode ͑all the potentials are quoted
with respect to Ag/AgCl͒. Prior to deposition, cyclic voltammetry
͑CV͒ curves at a scan rate of 20 mV/s were obtained in order to
study the electrochemical changes as well as to determine the depo-
sition potential. The preparative parameters were optimized to get
adherent Cu–In alloy films onto the ITO substrates. The deposition
of Cu–In films was carried out at applied constant potential
−750 mV from the unstirred bath for different deposition periods.
For deposition of the Se layer on formerly prepared Cu–In alloy
film, a constant potential of −600 mV was applied under potentio-
static control for a period of 30 min from the unstirred bath. To
convert Se-coated Cu–In films into CIS, the Se-coated Cu–In films
were annealed at different temperatures in argon ͑Ar͒ atmosphere.
The Cu–In alloy, Se-coated Cu–In, and CIS films were charac-
terized for their structural, and surface-morphological study. To
study the structural property of the films, X-ray diffraction ͑XRD͒
patterns were obtained using an X-ray diffractometer ͑RINT/PMAX
2500, Rigaku, Japan͒ in the range of scanning angle 10–80° ͑2␪͒
with Cu K␣ radiations. Surface morphological studies were carried
out using scanning electron microscopy ͑SEM͒ images obtained
with a field-emission SEM, ͑JSM-6340 F, Jeol, Japan͒.
Results and Discussion
In this paper we report on the electrochemical process of synthe-
sizing crystalline CIS thin films from stacked layers of Cu–In alloy
CV.— CV is a powerful tool for the determination of formal
redox potentials, detection of chemical reactions that precede or
follow the electrochemical reaction, and evaluation of electron
transfer kinetics.29,30 During the scan, for any species that can be
reduced through the range of potential scan, the current increases as
z E.mail: gujarគtp@yahoo.com; joocat@kist.re.kr
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Journal of The Electrochemical Society, 155 ͑10͒ E131-E135 ͑2008͒
been observed for Cu–Sn alloy deposition and have been attributed
to the adsorption-site competition between the different
precursors.34,35 The deposition of Cu–In begins at a potential of
−750 mV ͑Reactions 1-3͒. The steep rise in the cathodic current at a
potential of −850 mV can be attributed to the onset of the formation
of elemental In ͑cf. Fig. 1b͒. In the reversed scan the oxidation peak
is observed at a potential of +470 mV, which is near that observed
for a solution of CuSO4, and thus it is most likely due to the oxida-
tion of elemental Cu. Altogether, the above results lead to the con-
clusion that with these source concentrations stoichiometric Cu–In
can be deposited between potentials of −750 and −800 mV.
A CV curve of a solution containing HSeO−3 on the ITO sub-
strate, as explained in our previous report36 ͑not shown here͒, ex-
hibited the reduction point at a potential of −500 mV in forward CV
scan due to the irreversible Reactions 4 and 537
HSeO−3͑aq͒ + 5H+ + 4e− → Se + 3H2O
͓4͔
͑s͒
͑aq͒
Se + 2H+ + 2e− → H2Se
͓5͔
͑s͒
͑aq͒
͑aq͒
The corresponding oxidation point, usually above +1000 mV,37 is
not seen here. The formation of elemental Se can be seen by holding
the substrate at a constant overpotential of −500 mV and observing
the appearance of a red Se deposit. In addition to Reactions 4 and 5,
direct six-electron reduction of the HSeO−3 ion to H2Se has also been
reported.38 The difference in reduction potential from earlier reports
in the literature may be due to different substrates and other mea-
surement conditions, such as stirring, pH, and concentration.37,38 To
deposit an Se layer on Cu–In alloy, an overpotential of −600 mV
was applied for 30 min, and films were used as precursor for CIS
films.
Figure 1. ͑Color online͒ CV curves obtained on ITO substrates from aque-
ous solutions of ͑a͒ CuSO4, ͑b͒ In2͑SO4͒ and ͑c͒ a mixture of CuSO4
3
+ complexed In2͑SO4͒3.
the potential reaches the reduction potential of the species. Thus,
one can determine the reduction potential for a particular material to
be deposited.
Further, the growth of Cu–In alloy onto ITO substrate and Se on
Cu–In alloy was studied using chronoamperometry ͑CA͒, which is
commonly used to study the mechanism of electrodeposition of met-
als and alloys with current transients. The CA curves during growth
of Cu–In alloy and Se on Cu–In alloy are shown in Fig. 2a and b,
respectively. Both transient curves exhibited an initial rapid increase
in current with applied potential, further slowing current decay, fol-
lowed by a nearly constant current. The observed rapid surge and
exponential decay of the current are due to double-layer charge and
immediately afterward the typical response of nucleation and growth
by cluster formation, which is characteristic of diffusive controlled
growth.32,34 In the case of Cu–In alloy deposition, the current in the
next region is slightly increased with time, which can be a conse-
quence of low resistivity of Cu–In alloy. This graph indicates a
progressive nucleation which corresponds to slow growth of nuclei
on fewer active sites. For growth of Se on Cu–In alloy there is an
increase in current, reaching its maximum, followed by a slow de-
crease; this is typical behavior for three-dimensional nucleation
diffusion-limited growth. The CA graph for deposition of Se onto
Cu–In alloy indicates an instantaneous nucleation which corre-
sponds to fast growth of nuclei on many active sites, all activated
during the course of electroreduction.
The CV curves in solutions of CuSO4, In2͑SO4͒3 complexed
with dilute H2SO4, H2SeO3, and a mixture of Cu and In precursor
solutions onto the ITO substrate at room temperature were used to
monitor the electrochemical reactions as well as to find the suitable
deposition potentials/currents range. All voltammetry curves were
scanned first in the cathodic direction. Figure 1a shows the CV
curve obtained individually from the solution of CuSO4 on the ITO
substrates. The voltammogram clearly depicts the well-defined ca-
thodic ͑Ec͒ and anodic ͑Ea͒ peaks at −130 and +400 mV, respec-
tively. The former peak corresponds to the deposition of copper
metal and the latter to its dissolution. As discussed by Arenas et al.31
the Cu2+ reduction occurs in a global step via two electrons, because
the time for the existence of Cu+ is short, not detectable by CV
techniques, and can be considered to take place in the following
manner
Cu2+ + e− → Cu+
͓1͔
͑aq͒
Cu+ + e− → Cu
͓2͔
͑s͒
The voltammogram shows a crossover between cathodic current
branches, which is a characteristic of the nucleation process.32 Fig-
ure 1b shows CVs recorded from In2͑SO4͒3 solution onto the ITO
substrate. From In2͑SO4͒3 solution complexed with dilute H2SO4, a
reduction of In3+ occurred at a potential of −850 mV according to
Reaction 333
Investigations on Cu–In alloy.— Structural characterization of
the Cu–In alloy film deposited onto ITO substrate was carried out
using an XRD pattern within the 2␪ range of 10–80°. Figure 3
shows the XRD patterns of ͑a͒ Cu–In alloy films onto the ITO
substrates and ͑b͒ ITO glass substrate as a reference. The obtained
patterns were compared with the standard International Center for
Diffraction Data Powder Diffraction File ͑ICDD-PDF͒ Database
͑file no. 35-1150 and no. 41-0883͒ and the d values of CuIn and
Cu11In9 are observed to be in good agreement with those reported in
the standard PDF, possessing monoclinic structures. Along with dif-
fraction peaks corresponding to ITO substrate, a series of character-
istic peaks such as ͑101͒, ͑200͒, ͑102͒, ͑134͒, and ͑207͒ are identi-
fied for monoclinic CuIn phase, and ͑111͒, ͑312͒, and ͑511͒ are the
peaks of the monoclinic Cu11In9 phase. No elemental In or Cu
phases are observed. Volobujeva et al.27 deposited CuIn alloy using
magnetron sputtering of Cu/In alloy target, and they observed the
In3+ + 3e− → In
͓3͔
͑s͒
͑aq͒
The oxidation potential is seen at ca. −450 mV. The reduction po-
tential of elemental In was shifted from standard potential to
−850 mV ͑Fig. 1b͒. Here the complexation of In2͑SO4͒3 with
H2SO4 solution widens the deposition potential range of In. A cyclic
voltammogram measured from a mixture of solutions of CuSO4 and
In2͑SO4͒3 complexed with H2SO4 onto the ITO substrate at room
temperature is shown in Fig. 1c. In this CV curve the first reduction
point is observed at −230 mV, which corresponds to the reduction
of elemental Cu. The deposition potential of Cu has been shifted
compared with pure CuSO4 solution ͑Fig. 1a͒. Similar shifts have
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Journal of The Electrochemical Society, 155 ͑10͒ E131-E135 ͑2008͒
E133
Figure 3. ͑Color online͒ XRD patterns of ͑a͒ Cu–In alloy film on the ITO
substrates and ͑b͒ ITO glass substrate as a reference.
Investigations on Se-coated Cu–In alloy.— Cu–In alloy films
are used as a substrate to deposit the elemental Se film. The XRD
pattern of Se-coated Cu–In alloy is shown in Fig. 5a, and in Fig. 5b
ITO glass substrate is shown as a reference. In this pattern, along
with the peaks of Cu–In alloy and substrate peaks, the reflection
peak corresponding to elemental Se is seen to be evolved at 2␪
= 22.96° ͑ICDD-PDF file no. 24-0714͒, which corresponds to
monoclinic Se ͑deep red͒. Moreover, the XRD profile shows a back-
ground shape typical of an amorphous material, and it can be asso-
ciated to the presence of amorphous selenium on Cu–In alloy.36
Se films synthesized onto the Cu–In alloy show interesting mor-
phology, as is shown in Fig. 6. Figure 6a shows the low-
magnification SEM micrograph of Se-coated Cu–In film, and it is
seen that a deposit consists of elongated grains of nearly the same
size. In Fig. 6b the higher magnification image indicates that the
leaflike grains of Cu–In alloy are covered by an Se layer. These
structures were used as a precursor for the CIS layer via thermal
treatment to Se-coated Cu–In alloy.
Figure 2. ͑Color online͒ Potentiostatic current–time transients recorded on
͑a͒ ITO electrode for Cu–In alloy and ͑b͒ Cu–In alloy for Se deposition.
Investigations on CuInSe2 films.— Figure 7 shows the XRD pat-
terns of ͑a͒ CIS film obtained from the Se-coated Cu–In alloy layer
and ͑b͒ ITO glass substrate as a reference. From our experiments,
optimal temperature and the time of thermal annealing of Se-coated
CuIn2 and Cu11In9 phases, while Luo et al.28 deposited Cu–In alloy
using pulsed laser deposition, and they observed the dominant
Cu11In9 phase with secondary In phase. The formation of an elemen-
tal In or Cu phase may be disadvantageous, as this could lead to the
formation of binary intermediate phases like InSe or CuSe by depo-
sition of an Se layer.
The analysis of the surface morphology of the Cu–In alloy film
was carried out by SEM. Figure 4 shows the typical morphologies of
Cu–In alloy film was onto the ITO substrates. It can be clearly seen
that along with the heaps of interconnected grains, the symmetrical
dendritic structures have been grown. The central trunk consists of
two orderly and regular rows of particles as shown in Fig. 4a and b.
All the secondary branches consist of regular elongated particles,
almost parallel to each other. As discussed earlier, due to the pro-
gressive growth, the new deposited Cu–In alloy particles may stick
to any part of the formed clusters but are more likely to adhere to the
tips of the cluster than to penetrate deep into its inner regions, lead-
ing to outward growth from the initial location of seed particles.
Therefore, with an electrodeposition period, dendritic structures
consisting of orderly and regular particles are formed.
(a)
(b)
Figure 4. ͑a͒ Low- and ͑b͒ high-magnification SEM images of Cu–In alloy
film on ITO substrates.
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Journal of The Electrochemical Society, 155 ͑10͒ E131-E135 ͑2008͒
(a)
(b)
Figure 8. ͑a͒ Low- and ͑b͒ high-magnification SEM images of CuInSe2
films.
Cu–In alloy precursor had been optimized at about 450°C for
30 min. The XRD pattern of the thermally annealed Se-coated
Cu–In alloy was compared with the standard ICDD-PDF Database
͑file no. 40-1487͒, and the d values of the CIS obtained at all depo-
sition potentials are observed to be in good agreement with those
reported in the standard PDF, possessing chalcopyrite tetragonal
structure ͑a = 5.782 and c = 11.619 Å͒. A series of characteristic
peaks such as ͑112͒, ͑204͒, ͑312͒, and ͑325͒ are identified, and the
intensities of these reflections are in agreement with the standard
data from ICDD-PDF file no 40-1487. Secondary phases like Cu2Se
and In2Se3 or any elemental peaks were not observed in the XRD
patterns of the CIS films; these were mostly encountered by re-
searchers who deposited CIS films from the two-step process.
The analyses of the surface morphology of annealed films were
carried out by SEM. Figure 8 shows the morphology of CIS onto the
ITO substrates. Figures 8a and b show the lower- and higher-
magnification SEM micrograph of a CIS film. It is seen that a de-
posit consists of a well-defined agglomerated multigrain structure
with a size of ϳ450 nm. These grains are formed by incorporation
of Se into Cu–In alloy.
Figure 5. ͑Color online͒ XRD patterns of ͑a͒ Se-coated Cu–In alloy film and
͑b͒ ITO glass substrate as a reference.
Conclusions
Highly crystalline, single-phase CIS films were obtained by the
electrochemical approach from Se-coated Cu–In alloy. The elec-
trodeposited Cu–In alloy precursors showed the dominant CuIn
phase and leaflike structures formed by interconnection between
grains. The electrodeposition of an Se layer onto the Cu–In film
gave total coverage on the surface. All the thermally treated Se-
coated Cu–In alloy films at 450°C for 30 min gave CIS films with
high orientation along the ͑112͒ plane of the chalcopyrite structure,
and the agglomerated multigrain structure was formed on substrates.
In this study, stoichiometry and good quality of CIS films were
obtained by the electrodeposition method.
(a)
(b)
Figure 6. ͑a͒ Low- and ͑b͒ high-magnification SEM images of Se-coated
Cu–In alloy film.
Acknowledgment
This work was supported by the Hydrogen Energy Research and
Development Center, one of the 21st Century Frontier Research and
Development programs, funded by the Ministry of Science and
Technology of Korea.
Korea Institute of Science and Technology assisted in meeting the publi-
cation costs of this article.
References
1. I. Repins, M. A. Contreras, B. Egaas, C. DeHart, J. Scharf, C. L. Perkins, B. To,
and R. Noufi, Prog. Photovoltaics, 16, 235 ͑2008͒.
2. A. Ihlal, K. Bouabid, D. Soubane, M. Nya, O. A. T. Ali, Y. Amira, A. Outzourhit,
and G. Nouet, Thin Solid Films, 515, 5852 ͑2007͒.
3. S. H. Yoon, K. W. Seo, S. S. Lee, and W. Shim II, Thin Solid Films, 515, 1544
͑2006͒.
4. S. Niki, A. Yamada, R. Hunger, P. J. Fons, K. Iwata, K. Matsubara, A. Nishio, and
H. Nakanishi, J. Cryst. Growth, 237–239, 1993 ͑2002͒.
5. M. Dhanam, R. Balasundaraprabhu, S. Jayakumar, P. Gopalakrishnan, and M. D.
Kannan, Phys. Status Solidi A, 191, 149 ͑2002͒.
6. J. Yang, Z. Jin, T. Liu, C. Li, and Y. Shi, Sol. Energy Mater. Sol. Cells, 92, 621
͑2008͒.
Figure 7. ͑Color online͒ XRD patterns of ͑a͒ CuInSe2 film and ͑b͒ ITO glass
substrate as a reference.
7. D. Lincot, J. F. Guillemoles, S. Taunier, D. Guimard, J. S. Kurdi, A. Chaumont, O.
Downloaded on 2015-05-18 to IP 129.173.72.87 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 155 ͑10͒ E131-E135 ͑2008͒
E135
Roussel, O. Ramdani, C. Hubert, and J. P. Fauvarque, Sol. Energy, 77, 725 ͑2004͒.
8. S. Bereznev, J. Kois, I. Golovtsov, A. Opik, and E. Mellikov, Thin Solid Films,
511–512, 425 ͑2006͒.
9. J. Kois, S. Bereznev, O. Volobujeva, and E. Mellikov, Thin Solid Films, 515, 5871
͑2007͒.
23. M. Sugiyama, F. B. Dejene, A. Kinoshita, M. Fukaya, Y. Maru, T. Nakagawa, H.
Nakanishi, V. Alberts, and S. F. Chich, J. Cryst. Growth, 294, 214 ͑2006͒.
24. F. O. Adurodija, J. Song, S. D. Kim, S. H. Kwon, S. K. Kim, K. H. Yoon, and B.
T. Ahn, Thin Solid Films, 338, 13 ͑1999͒.
25. S. D. Kim, H. J. Kim, K. H. Yoon, and J. Song, Sol. Energy Mater. Sol. Cells, 62,
357 ͑2000͒.
26. T. Pisarkiewicz and H. Jankowski, Vacuum, 70, 435 ͑2003͒.
27. O. Volobujeva, M. Altosaar, J. Raudoja, E. Mellikov, M. Grossberg, L. Kaupmees,
and P. Barvinschi, Sol. Energy Mater. Sol. Cells, In press.
28. P. Luo, C. Zhu, and G. Jiang, Solid State Commun., 146, 57 ͑2008͒.
29. M. Noel and K. Vasu, Cyclic Voltammetry and the Frontiers of Electrochemistry,
Aspect, London ͑1990͒.
30. V. S. Bagotsky, Fundamentals of Electrochemistry, John Wiley & Sons, New York
͑2005͒.
31. J. V. Arenas, G. Vazquez, A. M. Melendez, and I. Gonzalez, J. Electrochem. Soc.,
154, D473 ͑2007͒.
32. S. B. Sadale and P. S. Patil, Solid State Ionics, 167, 273 ͑2004͒.
33. M. Pourbaix, Atlas of Electrochemical Equilibria, Pergamon Press, New York
͑1966͒.
34. A. N. Correia, M. X. Facanha, and P. de L. Neto, Surf. Coat. Technol., 201, 7216
͑2007͒.
10. S. Jost, F. Hergert, R. Hock, J. Schulze, A. Kirbs, T. Vox, and M. Purwins, Sol.
Energy Mater. Sol. Cells, 91, 1669 ͑2007͒.
11. S. Beyhan, S. Suzer, and F. Kadirgan, Sol. Energy Mater. Sol. Cells, 91, 1922
͑2007͒.
12. J. Araujo, R. Ortiz, A. Lopez-Rivera, J. M. Ortega, M. Montilla, and D. Larcon, J.
Solid State Electrochem., 11, 407 ͑2007͒.
13. M. Valdes, M. A. Frontini, M. Vazquez, and A. Goossens, Appl. Surf. Sci., 54, 303
͑2007͒.
14. S. M. Babu, A. Ennaoui, and M. C. Lux-Steiner, J. Cryst. Growth, 275, e1241
͑2005͒.
15. C. J. Huang, T. H. Meen, M. Y. Lai, and W. R. Chen, Sol. Energy Mater. Sol. Cells,
82, 553 ͑2004͒.
16. M. C. F. Oliveira, M. Azevedo, and A. Cunha, Thin Solid Films, 405, 129 ͑2002͒.
17. K. T. L. De Silva, W. A. A. Priyantha, J. K. D. S. Jayanetti, B. D. Chithrani, W.
Siripal, K. Blake, and I. M. Dharmadasa, Thin Solid Films, 382, 158 ͑2001͒.
18. F. Chraibi, M. Fahoume, A. Ennaoui, and J. L. Delplancke, Phys. Status Solidi A,
186, 373 ͑2001͒.
35. C. Natarajan, M. Sharon, C. Levy-Clement, and M. N. Spallart, Thin Solid Films,
237, 118 ͑1994͒.
36. T. P. Gujar, W. Y. Kim, K. D. Jung, and O. S. Joo, J. Electrochem. Soc., 155, E57
͑2008͒.
37. H. Saloniemi, T. Kanniainen, M. Ritala, M. Leskela, and R. Lappalainen, J. Mater.
Chem., 8, 651 ͑1998͒.
19. A. Kampmann, P. Cowache, D. Lincot, and J. Vedel, J. Electrochem. Soc., 146,
150 ͑1999͒.
20. C. Guille, M. A. Martm, and J. Herrero, Vacuum, 58, 594 ͑2000͒.
21. D. D. Shivagan, P. J. Dale, A. P. Samantilleke, and L. M. Peter, Thin Solid Films,
515, 5899 ͑2007͒.
22. S. Z. El-Abedin, A. Y. Saad, H. K. Farag, N. Borisenko, Q. X. Liu, and F. Endres,
Electrochim. Acta, 52, 2746 ͑2007͒.
38. M. S. Kazacos and B. Miller, J. Electrochem. Soc., 127, 869 ͑1980͒.
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