Electrodeposition of p-type CuSCN thin films by a new aqueous electrolyte with triethanolamine chelation

Article abstract of DOI:10.1111/j.1551-2916.2007.01832.x

A stable aqueous electrolyte solution containing Cu²⁺ and SCN^- was prepared by adding triethanolamine (TEA, N(CH ₂CH₂OH)₃) to chelate with Cu(II) cations. The electrolyte solutions were basic, with pH values in the range of 8.5-9, and could be used in the electrodeposition of CuSCN as a hole-conducting layer on a ZnO substrate and as an electron-conducting layer for nanocrystal photovoltaic cells because it could prevent the ZnO layer from acidic etching. CuSCN films were potentiostatically deposited on indium tin oxide glass substrates through the aqueous solutions, and the deposition potential for the sole CuSCN phase layer was determined by a linear sweep voltammetry measurement. The influence of applied potentials, electrolyte components, and deposition temperatures on the stoichiometry, phase, and particle morphology of the CuSCN films was investigated by X-ray photoelectron spectra, X-ray diffraction, and a field-emission scanning electron microscope. The results showed that the morphology of the dense CuSCN films was trigonal pyramid and the stoichiometric portions of SCN/Cu were excess of SCN. The current-voltage (I-V) characteristic of the junction between electrodeposited CuSCN and ZnO nanostructured layer displayed p-type semiconductor characteristics of CuSCN. The transmittance measurements detected high transmittance (≥87%) in the visible wavelength range, and the direct transition band gap calculated was 3.88 eV.

Full text of DOI:10.1111/j.1551-2916.2007.01832.x

J. Am. Ceram. Soc., 90 [9] 2966–2973 (2007)
DOI: 10.1111/j.1551-2916.2007.01832.x
r 2007 The American Ceramic Society
ournal
J
Electrodeposition of p-Type CuSCN Thin Films by a New Aqueous
Electrolyte With Triethanolamine Chelation
Yong Ni, Zhengguo Jin,w and Yanan Fu
Key Laboratory for Advanced Ceramics and Machining Technology of Ministry of Education, School of Materials
Science and Engineering, Tianjin University, Tianjin 300072, China
A stable aqueous electrolyte solution containing Cu21 and
SCNÀ was prepared by adding triethanolamine (TEA,
N(CH2CH2OH)3) to chelate with Cu(II) cations. The electro-
lyte solutions were basic, with pH values in the range of 8.5–9,
and could be used in the electrodeposition of CuSCN as a hole-
conducting layer on a ZnO substrate and as an electron-
conducting layer for nanocrystal photovoltaic cells because it
could prevent the ZnO layer from acidic etching. CuSCN films
were potentiostatically deposited on indium tin oxide glass sub-
strates through the aqueous solutions, and the deposition poten-
tial for the sole CuSCN phase layer was determined by a linear
sweep voltammetry measurement. The influence of applied
potentials, electrolyte components, and deposition temperatures
on the stoichiometry, phase, and particle morphology of the
CuSCN films was investigated by X-ray photoelectron spectra,
X-ray diffraction, and a field-emission scanning electron micro-
scope. The results showed that the morphology of the dense
CuSCN films was trigonal pyramid and the stoichiometric por-
tions of SCN/Cu were excess of SCN. The current–voltage (I–
V) characteristic of the junction between electrodeposited
CuSCN and ZnO nanostructured layer displayed p-type semi-
conductor characteristics of CuSCN. The transmittance mea-
surements detected high transmittance ( ꢀ 87%) in the visible
wavelength range, and the direct transition band gap calculated
was 3.88 eV.
conductors, such as p-type semiconductors, ionic liquid electro-
lytes, and polymer electrolytes.18–22 A p-type semiconductor is a
good candidate for a solid-state electrolyte, which has some
advantages in improving the long-term durability and stability
of NPC cells. There are some wide band gap materials, such
as CuI,16,20 CuAlO2,23 NiO,24 and CuSCN,4,5,25 which can
meet the requirements for the p-type layer of NPC solar cells.
Among these, CuSCN is thought to be the most promising
for its stability and possibility in filling the pores of the highly
structured n-type layer by solution deposition,8,26 CBD,27
SILAR,27 and electrodeposition,5,19,25 in which electrodeposition
is a feasible method for coating the insides of complex shapes.
Recently, there have been some studies on ZnO used as an
n-type anode for NPC solar cells. Compared with the TiO2 an-
ode, ZnO has a higher electron mobility (200 cm2 Á (VÁ s)À1 28
)
than anatase TiO2 (10 cm2 Á (vÁ s)À1).29 O’Regan et al.5 had
studied the cell structure and performance of ZnO/dye/CuSCN
devices and achieved higher quantum efficiency and energy ef-
ficiency than TiO2/dye/CuSCN devices. Levy-Cle
´ ´
ment et al.8
succeeded in achieving an innovative composite nanostructure
ZnO/CdSe/CuSCN, based on free-standing ZnO nanowire
arrays. In these studies, preparation of CuSCN layers by elec-
trodeposition using organic solvents had been reported.5,19,25
However, organic solvents had a few technical disadvantages
in the electrodeposition of the p-type semiconductors applied
in NPC cells. Typically, use of organic solvents led to poor
conductivity of the deposition solution, dye desorption on the
n-type layer, and poor contact with the sensitization layer.19,30
To avoid these technical disadvantages, electrodeposition of
CuSCN films in an aqueous solution should be a better choice
because there are many advantages for electrodeposition in
an aqueous solution, such as high conductivity, strong dis-
solvability to solutes, high deposition efficiency, and especially
in preventing desorption of dyes. Nevertheless, it is difficult
to electrodeposit CuSCN by an aqueous solution because of
the poor stability of Cu(II) cations and SCN anions in
water.25 Wu et al.31 had succeeded in preparing a stable
aqueous electrolyte solution using ethylenediamine tetraacetic
acid (EDTA) disodium salt as the chelating agent for Cu(II)
cations, and obtaining dense CuSCN films by electrodeposition
in acidic electrolyte solutions with a pH value of about 2. How-
ever, the pH value of the electrolyte is so low that ZnO
as a bottom substrate for NPC devices is easy to be etched.
Hence, a neutral or weak basic aqueous electrolyte solution is
required to deposit CuSCN films on ZnO substrates for NPC
solar cells.
I. Introduction
ANOCRYSTAL photovoltaic cells (NPC), such as dye-sensi-
N
tized solar cells (DSSCs)1–7 and extremely thin absorber
(ETA) solar cells,8–14 have been receiving increasing attention
because of their advantages of low cost and high efficiency,
which presage commercial applications.1 Generally, NPC cells
mainly consist of a nano- or microstructured layer of an n-type
wide-band-gap semiconductor, a sensitization layer, and an elec-
trolyte layer as a hole-conducting layer. The n-type semicon-
ductor serves as electron conductors, and in DSSCs, it also
serves as a supporting layer for the adsorbed dyes. The most
1,2,15
widely used n-type semiconductors are porous TiO2
and
ZnO nanowire films.8,11,13,14 The nano- or microstructured char-
acter of these films results in great surface area enlargement. To
make use of the high internal surface of the n-type layer, the
pores must be filled completely with absorbers and hole con-
ductors. Liquid electrolytes are well suited to this role. However,
problems associated with the liquid electrolytes, such as dye
desorption, solvent evaporation and degradation, seal imper-
fections, and reaction of the sealant with electrolyte,16,17 have
encouraged studies on solid-state or quasi-solid-state hole
In the present paper, the research work is mainly focused
on the electrodeposition and characterization of CuSCN
films through new triethanolamine (TEA, N(CH2CH2OH)3)-
chelated aqueous electrolytes with weak basic. Based on the
electrodeposition feature determined by linear sweep voltamme-
try (LSV) measurement, the influence of the applied potentials,
electrolyte component, and deposition temperature on the sto-
ichiometric proportion and morphology of the as-prepared
CuSCN films is primarily investigated, and its optical property
is characterized.
M. P. Paranthaman—contributing editor
Manuscript No. 22779. Received February 7, 2007; approved April 24, 2007.
This work was financially supported by Key Basic Research of TSTC.
wAuthor to whom correspondence should be addressed. e-mail: zhgjin@tju.edu.cn
2966

September 2007
Electrodeposition of p-Type CuSCN Thin Films
II. Experimental Procedure
2967
CuSO410.10M TEA10.05M KSCN, 0.01M CuSO410.10M
TEA, 0.01M CuSO4, and 0.05M KSCN, respectively. On com-
paring curve with curve , the cathodic threshold potential for
the reduction of Cu21 to Cu1 ions in a CuSO41TEA solution is
about À200 mV, while about 50 mV for the CuSO4 solution,
and at the same time, curve has a less cathode reaction current
density, indicating that Cu(II)–TEA complexes were formed in
the CuSO41TEA solution and resulted in a shift of the reduc-
tion potential to a more negative range as well as a decrease in
the cathode reaction current density. Hence, it can be concluded
that the stable mechanism of the Cu(II) cations and SCN anions
in an aqueous solution is the formation of Cu(II)–TEA com-
plexes, which is a feasible method to achieve the electrode-
position of CuSCN in a neutral aqueous electrolyte solution.
For curve , the current density began to increase at the poten-
tial of À200 mV, displaying the cathodic threshold for elec-
trodeposition of CuSCN, and subsequently increased gradually
with potential in this diffuse-controlled region.32 In the potential
range of À550 to À900 mV, the current density becomes gently,
showing that the reaction reached a saturated state accompanied
by a minor co-deposition of copper.33 Beyond À900 mV, the
current density ascended again, showing that dominant deposi-
tion changed from the formation of CuSCN to copper reduc-
tion. For the curve of , the cathode reaction began at an
applied potential of about À1350 mV, which could be attributed
to the reduction of H1 in the aqueous solution to form H2. The
high cathodic threshold potential indicates that KSCN did not
undergo any reaction in the CuSCN deposition process and just
supplied SCN anions. Hence, the growth process of CuSCN
from the TEA-chelated aqueous solution could be suggested to
be such that the cupric ions of the Cu(II)–TEA complexes that
stably existed in the electrolyte solutions were first reduced to
cuprous ions (Cu211eÀ-Cu1), and then CuSCN was formed
by the reaction between a cuprous ion and a thiocyanate ion
(Cu11SCNÀ-CuSCN). Such a sole deposition could take
place in a potential window between À200 and À550 mV. Af-
ter this, Cu co-deposition would occur. Hence, it was concluded
that the appropriate deposition potentials in the sole deposition
of CuSCN in the TEA-chelated aqueous solution could be se-
lected to be in the potential range of À200 to À550 mV.
Cupric sulfate pentahydrate (CuSO4 Á 5H2O) and potassium
thiocyanate (KSCN) as precursors and TEA as a chelating
reagent for Cu(II) cations were used. They were all analytical-
grade reagents. Aqueous electrolytes were firstly prepared by
mixing cupric sulfate solutions with TEA solutions at the molar
ratio [Cu21]:[TEA] 5 1:10 and the concentration of Cu21 was
maintained at 0.01M. Then, KSCN solutions with the concen-
tration range of 0.005–0.1M were added to the CuSO4/TEA
complex solutions under stirring for 1 h and then stored for 24 h
before being electrodeposited. The pH value of the final solution
was 8.5–9, varying with the concentration of KSCN.
The electrodeposition was carried out in a normal three-
electrode cell. The electrochemical measurement was performed
on a potentiostat/galvanostat (TD3691, Tianjin Zhonghuan
Electronic Instrument Company, Tianjin, China). Indium tin
oxide (ITO)-coated glass (10 O/cm) was used as a working elec-
trode, a large area Pt foil as a counter electrode, and an Ag/
AgClsat as a reference electrode. Before deposition, ITO sub-
strates were cleaned ultrasonically in acetone, 0.1 mol/L NaOH,
and deioned water, respectively. A CuSCN film was electrode-
posited potentiostatically ranging from À200 to À500 mV
versus Ag/AgClsat at varied temperatures. The potentials men-
tioned below were all in volts versus the Ag/AgClsat reference
electrode. Typically, the lower 1 cm of a 1 cm 2.5 cm substrate
was vertically placed in electrolyte solutions and about 2 cm
from the counter electrode. The voltage was applied after sta-
bilization for 30 s and the electrolyte was not stirred. The elec-
trodeposition time was 30 min for all samples.
The phase composition of as-prepared films was analyzed by
X-ray diffraction (XRD) on a Rigaku D/Max-2500 X-ray dif-
˚
fractometer (Tokyo, Japan) with CuKa radiation (l 5 1.541 A).
The microstructures of the films were observed by a JSM-6700
field-emission scanning electron microscope (FESEM) from
JEOL (Tokyo, Japan). X-ray photoelectron spectra (XPS) of
films were obtained on an Esca Multiplex Philips-1600 X-ray
photoelectron spectrometer (Eindhoven, the Netherlands). The
current–voltage (I–V) characteristic was determined on the po-
tentiostat/galvanostat (TD3691). The transmittance spectrum of
the film was recorded on a Beckman DU-8B (Fullerton, CA)
ultraviolet–visible (UV–Vis) absorption spectrometer.
To further study the influence of the applied potentials in the
range of À200 to À550 mV on the nucleation and growth of
electrodeposited CuSCN films, chronoamperometry measure-
ments based on the current transient recording during the elec-
trodeposition process were performed on bare ITO electrodes in
the electrolyte solutions consisting of 0.01M Cu21 and 0.05M
SCNÀ at room temperature. All measurements were carried out
by stepping the potential of the working electrode from an open-
circuit potential of 150 mV to potentials of À200, À300, À400,
III. Results and Discussion
LSV was used to investigate the role of TEA in the electrolyte
solution and survey CuSCN electrodeposition from the pre-
pared solutions. The linear sweep voltammetric curves
Fig. 1 represent the electrolyte solutions consisting of 0.01M
–
in
–0.5
0.0
0.5
1.0
1.5
2.0
0.3
–200mV
–300mV
–400mV
–500mV
0.2
0.1
0.0
2.5
CuSO +TEA+KSCN
3.0
CuSO +TEA
CuSO
KSCN
3.5
4.0
–1800 –1600 –1400 –1200 –1000 –800 –600 –400 –200
Potential /(mV vs. Ag/AgCl
0
200 400
0
200
400
600
800
1000 1200 1400 1600 1800
)
Time/s
Fig. 1. Linear sweep voltammetric curves of the electrolyte solutions
consisting of 0.01M CuSO410.10M TEA10.05M KSCN, 0.01M Cu-
SO410.10M TEA, 0.01M CuSO4, and 0.05M KSCN, in the range of 500
to À1800 mV on the indium tin oxide electrode at a rate of 50 mV/s.
TEA, triethanolamine; KSCN, potassium thiocyanate.
Fig. 2. Current density transients as a function of time for potentio-
static deposition at different potentials at room temperature in electro-
lytes consisting of 0.01M Cu21 and 0.05M SCNÀ. The inset shows a
magnified view of the time range from 0 to 100 s.

2968
Journal of the American Ceramic Society—Ni et al.
Vol. 90, No. 9
Fig. 3. Field-emission scanning electron microscopic images of the CuSCN films prepared in electrolytes consisting of 0.01M Cu21 and 0.05M SCNÀ,
at different potentials: (a) À200 mV, (b) À300 mV, (c) À400 mV, (d) À500 mV, at room temperature.
and À500 mV to initiate the nucleation and growth. The re-
corded curves are shown in Fig. 2. In all transients, the rapid
surge of current density at the onset of the potential was due to
the double-layer charging, and then the current density decayed
gradually because of an increase in the electric resistance of the
electrochemical cell system, especially including the nucleation
and growth of CuSCN crystals as semiconductor characteristics.
In this current-decay stage, the magnitudes of the current den-
sity and the degradation were not the same for different applied
potentials. A higher potential led to a larger current density and
a slower degradation. Figure 3 shows the FESEM images of the
resultant CuSCN films prepared during the chronoamperometry
measurements. As shown in Fig. 3(a), when the applied poten-
tial was À200 mV, only sparse CuSCN grains could be seen on
the surface of the ITO substrate due to the small amount of
CuSCN nuclei formed on the ITO substrate at the low applied
potential. The grain size grown at À200 mV was about 1 mm.
When the potential increased to À300 mV, the relatively higher
current density accelerated the nucleation and growth of
CuSCN. As can be seen in Fig. 3(b), more CuSCN grains
were formed on the ITO substrate, but the bare areas on the
substrate were still present. The film grown at this potential had
a smaller grain size (about 500 nm). At potentials of À400 and
À500 mV, the surface of the ITO substrate was completely
covered with well-grown CuSCN grains, and the grain size de-
creased to about 200 and 100 nm, respectively. Furthermore,
compared with these images, the particle morphology was also
different at different applied potentials. The sample grown at a
lower potential (À200 mV) was not well grown, with some de-
fects in the grain figure. With an increase in the potentials, the
intrinsic morphology of a trigonal pyramid34 appeared more
obviously. The above-mentioned result indicates that the in-
creased potential may lead to a high nucleation rate and more
uniform nucleation on a large area and smaller grain size due to
a high nuclei density. Hence, eventually a dense film with a fine
grain size can be obtained at a potential as high as possible for
sole deposition of CuSCN. This potential was À500 mV using
the TEA-chelated aqueous electrolyte solution.
It can be observed from Fig. 2 that the current density tran-
sients plateau in the later stage, but still oscillate in the hori-
zontal range. The plateau current density is defined as a
saturated current density, which is determined by the electric
resistance of the electrochemical cell.35 Firstly, at different ap-
plied potentials, the consumption rates of the cations and anions
in solutions were different so that the thickness of the diffusion
layers in solutions differed. And secondly, from the FESEM
observation, the distinct morphology of films deposited at dif-
ferent applied potentials led the layers of CuSCN obtained to
attain different electric resistance. As a result, the saturated cur-
rent density curves varied according to both main effects, where
high electric resistance retained stable current density. In addi-
tion, the oscillation of the current density seems to be related to
the rough surface morphology of the deposits. The electrode-
posited surfaces did not appear to be smooth, and hence the
oscillations or unstable behaviors of the current density could be
seen. The current density transient curve at À500 mV had the
lowest fluctuation as a vivid contrast against the one at À200 mV.
The stoichiometric proportions of the CuSCN films prepared
in the electrolyte solutions with the molar ratios [Cu21]:[SCNÀ]
of 1:10, 1:5, and 1:1 at potentiostatic À500 mV and room tem-
perature were measured by XPS. The results are shown in Table
I and Fig. 4, in which the content of S represents the SCN con-
tent in the film because S will not be imported as absorbed im-
purities from ambient. It can be seen from Table I, from
Fig. 4(a), that the stoichiometric proportions of SCN/Cu in
films were all above 1 whenever the molar ratios of electrolyte

September 2007
Electrodeposition of p-Type CuSCN Thin Films
2969
I-C1:S10
II-C1:S5
III-C1:S1
I
II
III
I-C1S10
III-C1S1
Atom %
O1s 18.2
C1s 69.2
N1s 5.5
S2p 3.8
II-C1S5
Atom %
Atom %
O1s 11.8
C1s 51.2
N1s 14.5
S2p 13.8
Cu2p 8.8
O1s 10.7
C1s 48.8
N1s 14.4
S2p 14.5
Cu2p 11.6
Cu2p 3.4
1000
800
600
400
200
0
(a)
Binding Energy (eV)
Cu 2p
I-C1:S10
II-C1:S5
III-C1:S1
I-C1:S10
II-C1:S5
III-C1:S1
S2p
I
I
II
II
III
III
970
960
950
940
930
920
172
170
168
166
164
162
160
158
156
Binding Energy (eV)
Binding Energy (eV)
(b)
(c)
Fig. 4. X-ray photoelectron spectra (XPS) spectra of CuSCN films prepared from electrolyte solutions with different molar ratios [Cu21]:[SCNÀ] of
1:10, 1:5, and 1:1, at À500 mV and room temperature. (a) Full spectra, (b) Cu 2p core-level XPS spectra, (c) S 2p core-level XPS spectra. The inset shows
the spectrum of the sample with molar ratio [Cu21]:[SCNÀ] 5 1:1. (C indicates Cu21, and S presents SCNÀ in the electrolyte solutions; the followed
numbers are molar ratios.)
solutions fluctuated. A higher molar ratio [SCNÀ]/[Cu21]510 in
the electrolyte solution led to a relatively higher stoichiometric
proportion SCN/Cu 5 1.57 in the film. Even though the molar
ratio [SCNÀ]/[Cu21] 5 1, the stoichiometric proportion SCN/
Cu still reached 1.12. In known reference, it was reported that
when SCN was in stoichiometric excess, Cu(SCN)11x would
show p-type semiconductor characteristics.36 However, verifica-
tion is difficult in our experiments because the thin CuSCN films
were electrodeposited on n-type ITO substrates only to form
an ohmic contact, which disturbed the electrical measurements
such as Hall measurements to determine the mobility type of the
film. Despite this, we can still give an indirect evidence. Figure 5
gives a I–V characteristic of the junction between electrode-
posited CuSCN and ZnO nanostructured layers. The junction
clearly displays a nonlinear behavior, showing rectification. It is
well known that ZnO is a natural n-type semiconductor and an
n–n junction generally will not show a rectifying action. Hence,
the result indicates that the CuSCN films electrodeposited in
TEA-chelated aqueous solutions should be p-type, which is re-
quired for the application of NPC cells.
10
8
6
4
2
0
–2
–4
Table I. Stoichiometric Proportion of CuSCN Films from
Different Electrolyte Solutions
Molar ratio [SCNÀ]/[Cu21] in electrolyte solutions
SCN/Cu in films
–1500
–1000
–500
0
500
1000
1500
10
5
1
1.57
1.25
1.12
Voltage (mV)
Fig. 5. Current–voltage (I—V) characteristic of the junction between
CuSCN deposit and ZnO nanostructured layer.

2970
Journal of the American Ceramic Society—Ni et al.
Vol. 90, No. 9
Fig. 6. Field-emission scanning electron microscopic images of the CuSCN films prepared with molar ratios [Cu21]:[SCNÀ] of (a) 1:10, (b) 1:5, (c) 1:3,
and (d) 1:1, at –500 mV and room temperature.
Figure 4(b) shows the Cu 2p core level XPS spectra of the
CuSCN films prepared at different molar ratios [SCNÀ]/[Cu21].
The binding energy of Cu 2p3/2 is 932.24 eV when the molar ratio
[SCNÀ]/[Cu21] is 10 or 5. But when the molar ratio [SCNÀ]/
[Cu21] is 1, the binding energy of Cu 2p3/2 shifts to 932.49 eV.
Both values are in good agreement with the values reported for
CuSCN.37 The shift of the binding energy may be attributed to a
higher oxidation state of S existing in the film prepared at the
molar ratio [SCNÀ]/[Cu21] 5 1. This is confirmed by the S 2p
core-level XPS spectra shown in Fig. 4(c). The S 2p3/2 shows a
main peak at 163.24 eV and a small shoulder at 168.24 eV, which
is clearly shown in the inset. The main peak corresponds to S in
SCN and the small shoulder may be due to S in SO24À as one of
the binding energy values of S 2p3/2 in SO24À is 168.15 eV,38
which is perhaps introduced from the use of CuSO4 Á 5H2O as
a raw material. However, at the molar ratios [SCNÀ]/[Cu21] of
10 or 5, such a small shoulder could not be found. In addition,
it can be concluded that high atomic content of O and excess
content of C are due to the absorption of O2, CO2 in air.
Figure 6 shows FESEM images of the CuSCN films derived
from the electrolytes with molar ratios [Cu21]:[SCNÀ] of 1:10,
1:5, 1:3, and 1:1 at À500 mV and room temperature. It clearly
shows that the intrinsic morphology of CuSCN is a trigonal
pyramid. The growth with the molar ratio [Cu21]:[SCNÀ] of
1:10 showed random aggregates of trigonal pyramids (Fig. 6(a)).
The smaller trigonal pyramid conjunct crystals with the same
crystal orientation grew together at molar ratios [Cu21]:[SCNÀ]
of 1:5 and 1:3 as shown in Figs. 6(b) and (c). When the molar
ratio [Cu21]:[SCNÀ] decreased to 1:1, the defined trigonal pyr-
CuSCN
Cu
C1:S0.5
C1:S1
C1:S5
C1:S10
ITO
JCPDS No 29-581
10
20
30
40
50
60
70
2 Theta/degree
Fig. 7. X-ray diffraction (XRD) patterns of the CuSCN films prepared
in different molar ratios [Cu21]:[SCNÀ] in electrolyte solutions, at À500
mV and room temperature. (C indicates Cu21, and S indicates SCNÀ;
the followed numbers are molar ratios.) The XRD pattern of bare in-
dium tin oxide glass and the standard diffraction pattern of b-CuSCN
(JCPDS card number 29-581) are also shown as references.
amid could not be seen in Fig. 6(d). Combined with the XPS
analysis, the grain not well-grown perhaps existed as SO24À de-
rived from the molar ratio [Cu21]:[SCNÀ] of 1:1, indicating that
Fig. 10. Field-emission scanning electron microscopic images of the CuSCN films prepared at different temperatures: (a) 01C, (b) 201C, (c) 351C, (d)
451C, and (e) 551C, in electrolytes consisting of 0.01M Cu21 and 0.05M SCNÀ at potentiostatic À500 mV.

September 2007
Electrodeposition of p-Type CuSCN Thin Films
2971
1.0
CuSCN
Cu
0.8
0.6
(b)
(b)
0.4
0.2
(a)
(a)
0.0
0
200
400
600
800 1000 1200 1400 1600 1800
10
20
30
40
50
60
70
Time/s
2 Theta/degree
Fig. 9. Current density transients as a function of time for potentio-
static deposition at À500 mV at (a) room temperature and (b) 551C in
the electrolytes consisting of 0.01M Cu21 and 0.05M SCNÀ.
Fig. 8. X-ray diffraction patterns of the CuSCN films prepared at (a)
room temperature and (b) 551C, in the electrolyte consisting of 0.01M
Cu21 and 0.05M SCNÀ, at À500 mV.

2972
Journal of the American Ceramic Society—Ni et al.
Vol. 90, No. 9
100
80
60
40
20
0
200
300
400
500
600
700
800
900
200
300
400
500
600
700
800
900
Wavelength, λ (nm)
Wavelength, λ (nm)
(a)
(b)
Absorbance
Transmittance
Fig. 11. Optical spectra of the CuSCN film prepared in the electrolyte consisting of 0.01M Cu21 and 0.05M SCNÀ, at potentiostatic À500 mV and
room temperature. The inset shows the (ahv)2 versus hv plot used for the determination of optical band gap of the CuSCN film.
this molar ratio was not an appropriate component of the
electrolyte solution for CuSCN electrodeposition. Furthermore,
it can be clearly concluded that the size of grains considerably
decreases with a decline in the SCNÀ concentrations in electro-
lyte solutions.
the peak intensities of CuSCN at (003) and (101) planes at room
temperature and 551C are different. The relatively higher inten-
sity of (101) plane at 551C further provides information regard-
ing the preferred orientation of the crystal along this plane.
The optical property of a typical CuSCN film with a film
thickness of B230 nm is shown in Fig. 11. The film was pre-
pared in an electrolyte solution consisting of 0.01M Cu21 and
0.05M SCNÀ, at potentiostatic À500 mV and room tempera-
ture. The transmittance spectrum Fig. 11(a)) showed a high op-
tical transmission value above 87% in the visible wavelength
range. A significant increase in the absorption wavelength range
lower than 320 nm can be assigned to the intrinsic band gap
absorption of b-CuSCN. For direct band gap semiconductors,
the absorption coefficient can be expressed as ahv 5 A(hvÀEg)1/
2. According to the formula and the graph of (ahv)2 versus hv as
illustrated in the inset of Fig. 11(b), the optical band gap Eg of
the typical CuSCN film equals to 3.88 eV. The band gap value is
slightly higher, 0.28 eV, than that of the reported CuSCN films
on a copper plate.36 The crystal size of the sample measured
optical band gap was estimated by the Debye–Scherrer formula;
along the (003) peak, the size was B21.8 nm while along the
(101) peak it was B18.2 nm. Hence, the blue shift of the band
gap may be attributed to the stoichiometric excess of SCN in the
electrodeposited film31 and the trigonal pyramid conjunct crys-
tals on a nanometer scale.
The XRD patterns of the CuSCN films electrodeposited with
molar ratios [Cu21]:[SCNÀ] of 1:10, 1:5, 1:1, and 1:0.5 at À500
mV and room temperature are shown in Fig. 7, and that of the
bare ITO substrate is also shown as a reference. It can be seen
that when the molar ratio [Cu21]:[SCNÀ] is 1:10 or 1:5, all char-
acteristic peaks of the electrodeposited films (C1:S10 and C1:S5)
are well consistent with b-CuSCN (JCPDS card number 29-
581), except those of the ITO-conducting layer. However, when
the molar ratio [Cu21]:[SCNÀ] was 1:1 (C1:S1), it failed to yield
diffraction peaks of CuSCN. With a further decrease in the
concentration of SCNÀ such as sample C1:S0.5, the diffraction
peaks showed Cu co-deposition in the CuSCN films and no
CuSCN characteristic peaks. This may be caused by the insuf-
ficiency of SCNÀ ions in the electrolyte so that when Cu21 ions
are reduced to Cu1, there are not enough SCNÀ ions to react
with Cu1 to form CuSCN. Hence, the redundant Cu1 ions
continue to be reduced to Cu. In conclusion, an appropriate
excess of SCNÀ concentration in the electrolyte solution is nec-
essary to avoid Cu co-deposition, the existence of SO24À, and not
well-grown grains.
The changes in XRD patterns and FESEM images of the
CuSCN films with temperature are shown in Figs. 8 and 10,
respectively. Other processing conditions were the electrolytes of
0.01M Cu21 and 0.05M SCNÀ and a potential of À500 mV.
When the temperature was above 451C, there were apparently
some brown-colored points dispersed on the film surface, clearly
indicating the co-deposition of Cu. The typical sample prepared
at 551C also shows the characteristic peak of copper in Fig. 8(b).
It shows that Cu is easier to co-deposit at a higher temperature.
Figure 9 shows the current density transients at room temper-
ature and 551C, and the curve of 551C showed a metal copper
deposition, clearly ascending in the early stage of the current
density. The evolution of the morphology of the CuSCN films
with the temperature shows that the grain size of CuSCN is en-
hanced with an increase in temperature. At a low temperature
(01 and 201C), the as-prepared films are small trigonal pyramid
conjunct crystals, and the orientation of the conjunct crystals is
random. When the temperature increased, the crystals grew like
single trigonal pyramids. At temperatures of 451 and 551C, the
bottom of more single trigonal pyramids could be seen in the
FESEM images, likely indicating preferred crystal orientation at
an elevated temperature. The preferred orientation of CuSCN
has also been reported in the upended taper-shaped CuSCN
arrays prepared by a liquid–solid reaction between a copper
substrate and an aqueous solution of NH4CNS at room tem-
perature.39 From the XRD patterns in Fig. 8, it can be seen that
IV. Conclusions
A novel technique was developed to prepare a b-CuSCN film
using a basic aqueous solution with TEA as a chelating agent.
Mild electrolytes with pH values in the range of 8.5–9 were
available for those deposition substrates that need to prevent
acid etching such as with ZnO. Experimental results showed that
the optimal electrodeposition potential was À500 mV for the
sole CuSCN electrodeposition in the TEA-chelated aqueous so-
lutions, and above it Cu would be co-deposited, and below it
growth of CuSCN would be impaired. The electrodeposited
films were hexagonal b-CuSCN with trigonal pyramid and sto-
ichiometric excess of SCN by XRD and XPS characterization.
The I–V characteristic of the junction between electrodeposited
CuSCN and ZnO nanostructured layer indicated p-type semi-
conductor characteristics of the CuSCN films prepared. At the
applied potential of À500 mV, the XPS spectrum of the film
from the molar ratio [Cu21]:[SCNÀ] 5 1:1 showed that SO24À
may be present. Cu co-deposition appears in the film with a
molar ratio [Cu21]:[SCNÀ] 5 1:0.5 due to the insufficiency of
SCNÀ. The grain size could be controlled by the component of
electrolyte, and dense films with small trigonal pyramid conjunct
crystals were achieved when the molar ratio [Cu21]:[SCNÀ] was
1:5 and 1:3. At deposition temperature ꢀ 451C, Cu could be
co-deposited more easily, and the films obtained tended to have

September 2007
Electrodeposition of p-Type CuSCN Thin Films
2973
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a larger grain size and preferred orientation along with the (101)
plane. The as-prepared CuSCN film had high transmittance
ꢀ 87% in the visible wavelength range and its direct band gap
was 3.88 eV.
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Acknowledgments
The authors wish to acknowledge Fei He, Haiyan Du, Yuqing Duan, and
Dongqing Jiang, for their kind assistance in the XPS, XRD, FESEM, and UV-Vis
measurements, respectively.
22J. Kruger, R. Plass, L. Cevey, M. Piccirelli, and M. Gra
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Solid-State Photovoltaic Device Due to Inhibition of Interface Charge Recombi-
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