We report research on electrodeposition of indium sulfide films, with In2S3 a less hazardous alternative to CdS buffer layers in solar cells. Numerous organic and aqueous/organic electrolytes of InCl3, NaCl, and elemental sulfur were investigated, including several glycols and amides. Temperatures ranged from 80-170°C, and deposition voltages from -0.6 to -1.2 V (Ag/AgCl with organic filling solution). Substrates included indium tin oxide-on-glass, molybdenum, and titanium, with indium or graphite anodes. Rapid stirring was used. Deposition was sluggish in all baths. Uniformity and adherence were only moderate, with irregular coverage and cracking-and-flaking sometimes evident. The best baths were ethylene glycol or 1, 2-propanediol-based, with golden-yellow films, nominally In2S3-xOx, depositing typically heavier around the substrate edges. With low temperatures and/or large currents, brown films more rich in indium sometimes formed. Cyclic voltammetry elucidated onset potentials, secondary reactions, and photoactivity, with the greatest anodic photocurrents arising from In2S3's n-type conductivity occurring with mixed ethylene glycol/propionic acid/water baths. Scanning electron microscope photographs indicated a compact small grain microstructure for yellow films. Energy dispersive X-ray analysis and photoelectron spectroscopy data indicate up to 15% oxygen content.

1. ELEMENTAL SULFUR-BASED ELECTRODEPOSITION OF INDIUM SULFIDE FILMS
M. Jason Newell, Robert Engelken, J. Hall, M.A. Mughal, F. Felizco, J. Vangilder, S. Thapa, D. McNew, and Z. Hill
College of Engineering/Environmental Science Program, and B.R. Carroll, Department of Chemistry and Physics,
Arkansas State University, Jonesboro, AR, USA
ABSTRACT
We report research on electrodeposition of indium sulfide
films, with In2S3 a less hazardous alternative to CdS
buffer layers in solar cells. Numerous organic and
aqueous/organic electrolytes of InCl3, NaCl, and
elemental sulfur were investigated, including several
glycols and amides. Temperatures ranged from 80-170
o
C, and deposition voltages from -0.6 to -1.2 V (Ag/AgCl
with organic filling solution). Substrates included indium
tin oxide-on-glass, molybdenum, and titanium, with indium
or graphite anodes. Rapid stirring was used. Deposition
was sluggish in all baths. Uniformity and adherence were
only moderate, with irregular coverage and cracking-and-
flaking sometimes evident. The best baths were ethylene
glycol or 1, 2-propanediol-based, with golden-yellow films,
nominally In2S3-xOx, depositing typically heavier around
the substrate edges. With low temperatures and/or large
currents, brown films more rich in indium sometimes
formed. Cyclic voltammetry elucidated onset potentials,
secondary reactions, and photoactivity, with the greatest
anodic photocurrents arising from In2S3‟s n-type
conductivity occurring with mixed ethylene
glycol/propionic acid/water baths. Scanning electron
microscope photographs indicated a compact small grain
microstructure for yellow films. Energy dispersive X-ray
analysis and photoelectron spectroscopy data indicate up
to 15% oxygen content.
BACKGROUND
In2S3 has received attention as an alternative to CdS as
buffer layers in heterojunction solar cells with CdTe or
CuInxGa1-xSe2 absorber layers [1]. Progress has been
made, for example, the spray-ILGAR process being used
to produce In2S3-based solar cells, particularly in Europe
[2]. Reported research on indium sulfide
electrodeposition [3-6] has been minimal compared to that
on some other compound semiconductors. In 1998, de
Tacconi and Rajeshwar electrodeposited an extremely
thin layer of sulfur onto a gold substrate, followed by
underpotential deposition of indium in an In2(SO4)3
solution, and then transfer back to the solution containing
Na2SO4 and Na2S to sulfidize residual indium sites. In
2005, Asenjo, Chaparro, Gutierrez, Herrero, and Maffiotte
performed one-step cathodic deposition in an aqueous
solution of In2(SO4)3 and Na2SO3, on a molybdenum
substrate. In 2008, Halim and Ichimura electrodeposited
indium sulfide on ITO-coated glass using In2(SO4)3 and
Na2S2O3, with both DC and two-step pulse biasing. Also
in 2008, Todorov, Carda, Escribano, Grimm, Klaer, and
Klenk used an aqueous solution of Na2S2O3×5H2O, InCl3,
and HCl. Different substrates were tried: graphite sheet,
Mo- and ITO-coated glass, and CuInS2/Mo/glass
structures. We report the electrochemical deposition of
In2S3 using organic solvents of molecularly dissolved
elemental sulfur and ionically dissolved indium salts. This
basic process was first reported in 1980 by Baranksi and
Fawcett, who used it to plate CdS, HgS, PbS, Tl2S, Bi2S3,
Cu2S, NiS, CoS, and CdSe [7]. Many other researchers,
including our group [8], subsequently used the same basic
approach for electrodeposition of a variety of metal
chalcogenides, but, to our knowledge, no one has
previously published results using a similar technique for
electrodeposition of In2S3 thin films.
Although the bandgap of bulk indium (III) sulfide is
generally given as 2.0 eV, and Madelung gives a direct
bandgap of 2.3 eV and an indirect bandgap of 2.20 eV for
In2S3 thin films relative to 2.5 eV for CdS [9] (hence
potentially reducing open circuit voltage), which is
supported by experimental evidence, thin films can exhibit
higher bandgaps, depending upon synthesis conditions
and film compositions. Higher bandgaps are thought to
be a function of smaller grain sizes or the incorporation of
oxygen or hydroxide into the films. Some researchers
have reported bandgaps higher than 3 eV [10,11]. The
lower toxicity and environmental impact of indium relative
to cadmium, and significant photosensitivity, compel
ongoing research. CuInxGa1-xSe2 (CIGS) and CuInS2
(CIS) have the chalcopyrite crystal structure, whereas
CdS has a wurtzite crystal structure, and CdTe usually
has a zincblende (“cubic”) crystal structure, although a
wurtzite (“hexagonal”) structure can occur [9].
Mismatches in crystal structure lead to defect states at the
junction, particularly in the case of CIGS and CIS, which
increase recombination and can lead to Fermi level
pinning at the heterojunction [12]. In2S3 is tetragonal, and
there is no conduction band discontinuity between CIGS
and In2S3 [13]. Record efficiencies of 20.3% have been
achieved using the CdS buffer layer with CIGS absorber
layers [14], with module efficiencies of 15.7% [15]. The
use of In2S3 as a buffer layer in CIGS cells has only
produced efficiencies of 16.4% using atomic layer
deposition (ALD) and 15.7% using chemical bath
deposition (CBD), but CIGS cells using ALD In2S3 buffer
layers are comparable to CIGS cells using ALD CdS
buffer layers [1]. The spray-ILGAR technique also creates
In2S3-buffered cells that are comparable to CdS-buffered
cells prepared by the same technique [2]. The CBD
technique used to apply the thin CdS film is
environmentally unfriendly, due to the necessity of new
solutions for each batch. The window layer, ZnO,
requires an intrinsic layer to be sputtered before
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2. electroplating the remainder of the ZnO film to achieve
optimal results on CdS, whereas indium sulfide buffer
layers can be used as substrates for direct
electrochemical growth of the ZnO window layer [16-18].
The one significant disadvantage associated with In2S3 is
the relative scarcity and escalating cost of indium, which
is related to one of the advantages of electrodeposition;
that is, ions that do not plate remain in solution and can
be reused, lowering material requirements compared to
other deposition methods.
Electrodeposition has been used in solar cell production,
particularly with CdTe and CuInSe2-based cells [19]. Two
broad techniques are (1) near-simultaneous discharge of
all compound constituents and (2) sequential plating of
separate, stoichiometrically-matched layers, followed by
annealing and/or chalcogenization. The work reported
herein for indium sulfide electrodeposition involves the
first. Electrodeposition offers a variety of attributes: (1)
low temperature and atmospheric operation, (2)
straightforward scale-up to larger areas, (3) low cost and
simple apparatuses and reagents, (4) relatively low
hazard due to its non-gaseous nature, and (5) potential for
stoichiometry control through variation of deposition
voltage and current, and in-situ monitoring of
photocurrents and quasi-rest potential. Electrodeposition
also permits preferential growth of the buffer material on
defected areas with higher conductivity, reducing shunting
[6].
The Arkansas State University Optoelectronic Materials
Research Laboratory has much experience with
electrodeposition, and other liquid solution deposition
techniques such as CBD, of chalcogenide-based
semiconductor films, with particular emphasis on organic
and mixed deposition baths [20]. The work reported
herein for electrodeposition of indium sulfide builds upon
three decades of related experience, including significant
research on indium sulfide over the past 15 years [21].
DESCRIPTION OF THE WORK
Numerous organic, mixed organic, and organic/aqueous
baths were investigated. Organic solvents included
ethylene glycol, 1, 2-propanediol, diethylene glycol,
triethylene glycol, tetraethylene glycol, triethanolamine,
dimethylsulfoxide, propylene carbonate, formamide, N, N-
diethylformamide, N, N- dipropylformamide, and propionic
acid. Solutes consisted of 0.05 M InCl3 and 0.1 M NaCl,
and saturation levels of molecularly dissolved sulfur. The
gross sulfur molarity was 0.1, but only rarely, at
temperatures above 150
0
C, did it all dissolve to yield light
yellow solutions. With the pure organics, deposition
temperatures were 100-170
0
C, limited by solvent boiling
and charring temperatures, with the latter just below 200
0
C. Voltages ranged between -0.6 V and -1.2 V
(Ag/AgCl with a filling solution of ethylene glycol saturated
in KCl). Substrates included indium tin oxide-coated
glass, molybdenum, and titanium, with a graphite or
indium anode. Rapid stirring with Teflon-covered stirring
bar and hot plate was used.
Indium and indium sulfide deposition was surprisingly
sluggish in all solvent systems investigated. Some
yielded no deposition, while some yielded mixed indium
sulfide phases, delimited by yellow In2S3 and brown InS
(nominal compositions). Uniformity was typically poor,
with some regions on the cathode plated and others
devoid of deposit. Deposition typically was heavier near
the edges of the substrate. The best films, relative to
golden-yellow color and uniformity, were deposited from
formamide baths. However, these baths decompose at
temperatures above 150
0
C, yielding an In2S3 suspension
in-solution. Also, possible toxic products with formamide
were of concern. The next best, and safer, baths were
ethylene glycol- and 1, 2-propanediol-based, from which
yellow films of fair uniformity were deposited. Brown films
were sometimes deposited with temperatures near 100
0
C, or relatively large currents. Uniformity was improved
significantly with an initial transient (10-20 s) indium metal
“flash” at voltages just negative of -1.0 V to establish
nucleation sites. Figure 1 exhibits a photograph of a
brown and two yellow indium sulfide films
electrodeposited onto indium tin oxide-on-glass
substrates.
Figure 1. Electrodeposited InxS Films on ITO/Glass
Cyclic voltammetry elucidated reversible potentials for the
indium deposition, spurious reactions, and photoactivity
caused by chopped white light illumination. Although
uniformity and color were better with the pure organics,
the greatest photocurrents were observed with a mixed
bath containing ethylene glycol, propionic acid, and water.
Voltammograms indicated H2 generation and sluggish
indium/indium sulfide deposition on the forward, cathodic
sweep, but small anodic photocurrents on the reverse
sweep, consistent with In2S3‟s n-type conductivity.
Figure 2 shows a scanning electron microscope (SEM)
photograph of a yellow film at a magnification of 15 kX.
The film is compact, with small grains. Figure 3 exhibits
energy dispersive X-ray analysis (EDAX) data for the film.
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3. The indium-to-sulfur ratio, 0.804, is greater than 2:3 (as
would correspond to In2S3), but less than 1:1 (as would
correspond to InS). However, the ratio of indium to the
sum of the sulfur and oxygen atomic percentages is
smaller, 0.615, and a little below 2:3.
Figure 2. SEM Photograph of Yellow InxS Film
YELLOW Sample
Area 1 Area 2
Weight% Atomic% Weight% Atomic%
C K 1.5 6.6 C K 1.3 5.9
N K 1.6 6.1 N K 1.6 6.2
O K 3.6 12.1 O K 3.7 12.6
F K 0.6 1.7 F K 0.6 1.7
S K 24.0 40.4 S K 23.4 39.7
Cl K 0.8 1.2 Cl K 0.8 1.3
In L 67.9 31.9 In L 68.6 32.5
Figure 3. EDAX Data for Yellow InxS Film
Figure 4 exhibits photoelectron spectroscopy (XPS) data
for the yellow film. There is still oxygen in the film, even
down to 1100 angstroms from the surface, implying an
InSxOy composition, in this case In0.53S0.41O0.06. Figure 5
is a graph of optical absorbance versus wavelength for a
yellow film, showing an absorption edge in the 500 nm to
600 nm range. Figure 6 is a corresponding plot of (Ahʋ)
0.5
vs. hʋ (hʋ being the photon energy and A the
absorbance), which should present a linear plot for an
indirect bandgap material with a zero crossing equal to
the bandgap. The 2.1 eV zero crossing is consistent with
In2S3‟s reported ≈ 2.0 eV value.
Yellow
0
10
20
30
40
50
60
0 100 200 300 400 500 600 700 800 900 1000 1100
Sputter Depth, vs. SiO2
Concentratiion,at.%
C1s
O1s
S2p
Cl2p
In3d
Figure 4. XPS Data for Yellow InxS Film
Figure 5. Optical Absorbance vs. Wavelength for
Yellow InxS Film
Figure 6. Plot of (Ahᶹ)
0.5
vs. hᶹ for Yellow InxS Film,
Indicating an Indirect Bandgap ≈ 2.1 eV
FUTURE WORK
Research is ongoing, with plans to investigate additional
solvents and explore indium sulfamate as a solute, since
aqueous baths of indium sulfamate are among the best
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4. for indium metal plating. After further bath optimization,
electroplated indium sulfide films will be mated with CdTe
and CuInS2 absorber layers to form heterojunctions that
will be characterized for photovoltaic performance.
Innovations with indium sulfide electrodeposition may also
be translatable into improved electrodeposition of CuInS2
and CuInSe2.
This work is supported by ongoing EPSCoR grants from
both the NASA/Arkansas EPSCoR Office, and the
National Science Foundation/Arkansas Science and
Technology Authority (NSF Award #EPS-1003970).
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