Electrodeposition can enable stoichiometric control of deposited samples through variation of electroplating potential. We demonstrate an in-situ technique for deposit analysis and stoichiometric control by interspersing periods of open-circuit during deposition. Opening the circuit in an organic Cu-In-S plating bath allows greater incorporation of Cu, In, and/or S into deposited films, based upon the open-circuit voltage the film/electrolyte interface is allowed to achieve. With the same deposition potential, samples can be made to vary from highly Cu-rich to highly In-rich through selection of an appropriate open-circuit voltage limit.

1. Stoichiometric Control via Periods of Open-circuit During
Electrodeposition
M. Jason Newell, Maqsood Ali Mughal, Joshua Vangilder, Shyam Thapa, Kayla Wood, Steven A. Hoke,
Clay Kardas, J. Bruce Johnson, B. Ross Carroll, and Robert Engelken
Arkansas State University, Jonesboro, AR, 72467, USA
Abstract — Electrodeposition can enable stoichiometric
control of deposited samples through variation of electroplating
potential. We demonstrate an in-situ technique for deposit
analysis and stoichiometric control by interspersing periods of
open-circuit during deposition. Opening the circuit in an organic
Cu-In-S plating bath allows greater incorporation of Cu, In,
and/or S into deposited films, based upon the open-circuit voltage
the film/electrolyte interface is allowed to achieve. With the same
deposition potential, samples can be made to vary from highly
Cu-rich to highly In-rich through selection of an appropriate
open-circuit voltage limit.
Index Terms — electrodeposition, stoichiometry, open-circuit,
in-situ, chalcogenide, CuInS2, CIS.
I. INTRODUCTION
With the growing recognition of the need for sustainable
energy production, solar energy is well-poised to gain market
share. Roof-mounted thin-film photovoltaics (PV) scores
among the most sustainable in terms of key sustainability
indexes such as land [1] and water [2] use, as well as harmful
emissions [3-5]. Electrodeposited PV materials offer even
greater sustainability, as well as reduced production cost,
through high material utilization efficiency, and operation at
low temperatures and atmospheric pressures. However, due to
the high-quality material requirements for use in solar cells,
electrodeposition has rarely achieved industrial-scale
production.
One of the primary advantages of electrodeposition is the
ability to control deposition stoichiometry through variation of
deposition potential. We show that the inclusion of open-
circuit periods allows control of deposit stoichiometry without
adjustment of deposition potential, and that by varying the
allowed open-circuit voltage limit between the film and
solution, stoichiometry of deposited films can be further
controlled. This can enable in-situ self-correction, as the
open-circuit voltage is based upon the interface between the
film and ions present in the solution.
Cu(In,Ga)(S,Se)2 (CIGS) has emerged as a promising
material for thin-film PV, and has been shown to enable the
transition from highly Cu-rich to highly In-rich material by
modulation of the deposition voltage [6]. Its sister compound,
CuInS2 (CIS) has an excellent absorption coefficient of 105
/cm
and optimal direct bandgap around 1.5 eV [7] without the
inclusion of gallium, is more environmentally friendly without
the selenium, and is somewhat less worrisome in terms of
material scarcity concerns. The Vertically-Integrated Center
for Transformative Energy Research (VICTER), through
which the work is funded by the National Science Foundation
through the Arkansas Science and Technology Authority, aims
to achieve environmentally responsible energy production in
Arkansas.
II. EXPERIMENTAL DETAILS
Experiments were conducted at room temperature in an
ethylene glycol bath consisting of dissolved elemental sulfur,
CuI, InCl3, and NaCl, the latter added for enhanced
conductivity (see Fig. 1a and b). Sulfur was stirred into
ethylene glycol at 150 ˚C until molten and thoroughly
dissolved; the solution was then allowed to cool before the
addition of the remaining solutes. Deposition and
measurements were performed in a three-electrode setup with
the Princeton Applied Research Parstat 4000 potentiostat,
using a double-junction Ag/AgCl reference electrode with
potassium nitrate buffer solution in the outer sleeve and KCl
saturated with AgCl in the inner sleeve. The anode and
cathode, as well as the substrate, were made from graphite.
Substrates were scrubbed with Comet®
and water, rinsed with
distilled water, and dried under flowing air. Immediately after
deposition, samples were thoroughly rinsed with acetone and
allowed to dry overnight before being characterized.
Characterization was performed via INCA x-sight EDS from
Oxford Instruments, attached to a Tescan Vega scanning
electron microscope.
Constant potential depositions were carried out for 40
minutes. Depositions with periods of open-circuit remained at
constant potential for one minute before the circuit was
opened, after which the potential drifted positively as
dissolved sulfur molecules continued to react with plated
copper and indium atoms, and/or copper ions displaced indium
atoms. An open-circuit voltage limit was set, and the film was
allowed to remain at open-circuit until that threshold was
crossed, at which point the deposition potential was again held
constant for another minute. This continued for approximately
40 cycles.

2. III. RESULTS
EDS data for deposited films are shown in Table 1.
Deposition at -0.85 V produced films that are Cu-rich and S-
poor, relative to stoichiometric CuInS2 ratios, which would be
one for both In/Cu and S/(In+Cu). Deposition at -1 V
produced In-richness in the film, but it is even more deficient
in S than the film grown at -0.85 V. In the case of the Cu-rich
film, open-circuit periods interspersed between constant
potential depositions resulted in greater incorporation of In,
while in the case of the In-rich material obtained at the higher
plating voltage, open-circuit periods resulted in significant
inclusion of both Cu and S. Allowing the open-circuit voltage
to drift to a more positive value allowed the incorporation of
more Cu into the deposit.
IV. DISCUSSION
The open-circuit voltage transient (VocT) can give an
indication of compound formation when an equilibrium
reaction occurs between the deposited material and ions and/or
molecules in the electrolyte. Fig. 2a) shows the VocT for a
single, one-minute deposition at -1.2 V in a bath of CuI, InCl3,
and NaCl; Fig. 2b) shows a series of one-hour VocTs from 0.6
V to -1.2 V in the same bath. Note that there was no sulfur in
the bath. Potentials hit plateaus during open-circuit periods if
the deposition potential is sufficiently negative to deposit
material (that presumably causes the plateaus). It appears that
the most positive plateau, at approximately -0.2 V and
presumably the result of Cu, is achieved with depositions more
negative than -0.4 V. The next plateau, at approximately -0.3
V, only appears for depositions whose quasi-rest potential
(QRP)—the open-circuit potential immediately upon
interruption of the current, before exchange reactions begin to
take place (see Fig. 3)—is above the indium plateau. This
could possibly indicate that there is an indium-copper alloy
that cannot form on the substrate until indium has been
deposited, but that can be deposited through copper
exchanging with indium at the film-electrolyte boundary
during open-circuit periods. If that is the case, it would be
TABLE 1
STOICHIOMETRIC RATIOS OF DEPOSITED FILMS
Deposition at -0.85 V Deposition at -1 V
constant
potential
Voc limit of
-0.5 V
constant
potential
Voc limit of
-0.54 V
Voc limit of
-0.25 V
In/Cu 0.42 0.63 1.8 0.94 0.64
S/(In+Cu) 0.38 0.36 0.22 0.86 0.46
Fig. 1. Cyclic voltammetry in the Cu-In-S system. a) InCl3 in
ethylene glycol, and b) with NaCl added, demonstrating enhanced
conductivity with no additional features (note the scale); c) was run
in the full solution, on an ITO-coated glass substrate and with
pulsed light, showing photo-electrochemical activity immediately
prior to, or concurrently with, onset of indium deposition.
a)
b)
c)

3. possible to deposit a (possibly preferred) indium-copper alloy
by allowing the film to sit in solution at open-circuit. Fig. 4
shows additional VocTs in the Cu-In-S system that, along with
cyclic voltammetry (Fig. 1), informed the depositions reported
herein. Together with the stoichiometric data, they confirm
the usefulness of open-circuit voltage measurements as an in-
situ marker for deposited material, as well as being an in-situ
method for controlling stoichiometry.
Fig. 2. a) VocT from a single one-minute deposition at -1.2 V, in a
bath with dissolved CuI, InCl3, and NaCl. After the initial drop to
the QRP, three distinct plateaus can be seen before it begins to
approach the rest potential between graphite and the solution.
Differing bath solutions, in conjunction with cyclic voltammetry,
suggest the plateau around -0.6 V corresponds to indium (see Fig.
4). b) One-hour VocTs from multiple one-minute depositions with
deposition potentials ranging from 0.6 to -1.2 V.
Fig. 3. First ten milliseconds of open-circuit, demonstrating the
immediate drop to the QRP, which is relatively constant for the
length of time shown: a) sulfur and NaCl, b) added InCl3, and c)
added CuI.
a)
b)
a)
b)
c)

4. V. CONCLUSIONS
We have demonstrated the ability to vary the stoichiometry
of films deposited from an organic Cu-In-S bath through an
electrodeposition cycle that allows the depositing films to
undergo periods of open-circuit whereby exchange/reaction
processes are allowed to occur between the film and adjacent
species in the solution. The ability for in-situ control of
deposition stoichiometry and a method for in-situ analysis and
characterization of deposited compounds are promising for the
field of electrodeposition. Especially critical for CuInS2
deposition is increased incorporation of sulfur into the film
while at open-circuit, as electrodeposition with CIGS generally
involves deposition of metal stacks, followed by annealing in a
selenized and sulfidized atmosphere, which requires vacuum
and raises toxicity and on-site job safety concerns.
We are currently optimizing parameters for CuInS2
deposition, and annealing and XRD studies are underway,
including laser annealing. Beyond the immediate goal of
producing CuInS2, the project seeks to produce a working
heterojunction device with CuInS2 as the absorber layer and
In2S3 [8] as the buffer layer.
ACKNOWLEDGEMENTS
The authors acknowledge the gracious support provided by
Arkansas State University, National Science Foundation grant
EPS-1003970 administered by the Arkansas Science and
Technology Authority, and NASA grant NNX09AW22A
administered by the Arkansas Space Grant Consortium. Dr.
Alan Mantooth, Kathy Kirk, Dr. Greg Salamo, Dr. Omar
Manasreh, Dr. Alex Biris, Dr. Tansel Karabacak, Dr. Hyewon
Seo, and other collaborators at the University of Arkansas
(Fayetteville, Little Rock, and Pine Bluff campuses) are also
thanked, as are Dr. Keith Hudson and Laura Holland at ASGC,
and Dr. Gail McClure, Cathy Ma, and Marta Collier at ASTA.
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Fig. 4. One-minute VocTs with deposition potentials ranging from
0.6 V to -1.2 V in 10 mV increments from a) sulfur and NaCl in
ethylene glycol, b) added InCl3, and c) added CuI, i.e., the full
solution.
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