Electrochemical deposition characteristics of p-CuSCN on n-ZnO rod arrays films

Article abstract of DOI:10.1016/j.electacta.2008.01.111

p-CuSCN/n-ZnO rod array heterojunctions were electrodeposited with a weak basic (pH ～9) aqueous electrolyte solution. I-V characteristics showed the heterostructure had clear rectification, indicating good electrical contacts between ZnO rod arrays and the embedded CuSCN. The energy band model for the electrodeposition of CuSCN on ZnO rod arrays was proposed based on linear sweep voltammetric (LSV) measurements, which indicated that the electrodeposition process was the prior growth of CuSCN on bare ZnO rods according to a conduction process, followed by compact filling in the gaps of the arrays based on the thermal activation mechanism of surface states. The diode properties of the heterojunctions revealed that although deposition was dominated by thermal activation mechanism of surface states, the electrodeposition should be performed at a lower temperature in order to reach fine filling of the gaps of ZnO rod arrays.

Full text of DOI:10.1016/j.electacta.2008.01.111

Available online at www.sciencedirect.com
Electrochimica Acta 53 (2008) 6048–6054
Electrochemical deposition characteristics of p-CuSCN
on n-ZnO rod arrays films
Yong Ni, Zhengguo Jin∗, Ke Yu, Yanan Fu, Tongjun Liu, Tao Wang
Key Laboratory for Advanced Ceramics and Machining Technology of Ministry of Education,
School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China
Received 8 October 2007; received in revised form 23 January 2008; accepted 29 January 2008
Available online 4 March 2008
Abstract
p-CuSCN/n-ZnO rod array heterojunctions were electrodeposited with a weak basic (pH ∼9) aqueous electrolyte solution. I–V characteristics
showed the heterostructure had clear rectification, indicating good electrical contacts between ZnO rod arrays and the embedded CuSCN. The
energy band model for the electrodeposition of CuSCN on ZnO rod arrays was proposed based on linear sweep voltammetric (LSV) measurements,
which indicated that the electrodeposition process was the prior growth of CuSCN on bare ZnO rods according to a conduction process, followed
by compact filling in the gaps of the arrays based on the thermal activation mechanism of surface states. The diode properties of the heterojunctions
revealed that although deposition was dominated by thermal activation mechanism of surface states, the electrodeposition should be performed at
a lower temperature in order to reach fine filling of the gaps of ZnO rod arrays.
© 2008 Published by Elsevier Ltd.
Keywords: p-CuSCN; Electrochemical deposition; ZnO rod arrays; Aqueous weak basic electrolyte; Energy band model
1. Introduction
photo-generated carriers. As a p-type hole conducting material,
CuSCN is thought to be most promising due to its transparence
in the visible light spectrum range, reasonable hole conductivity
(≥5 × 10−4 S cm−1) [6] and chemical stability [7].
Using heterogeneous deposition on highly structured semi-
conductor substrates to form heterojunctions has been an
attractivecomponentinmanyelectrical, photoelectrical, andcat-
alytic applications where an enlargement of the interface area
is generally desirable. This is especially valuable for nanocrys-
tal photovoltaic cells (NPCs), such as dye-sensitized solar cells
(DSSCs) [1] and extremely thin absorber solar cells (ETAs)
[2], because of the enhancement of light trapping for photo-
generated carriers and the separation efficiency of the excited
carriers. The n-type electrodes of NPC solar cells have suc-
ceeded in using porous TiO2 layers [3,4], and nanostructured
columnar ZnO arrays [2,5] have also been studied for their use
as the n-type electrodes supplying better electron carries trans-
port with less grain boundaries. Generally, to effectively make
use of the high internal surface of the n-type layer, the pores
or gaps in the non-planar structure need to be filled completely
with a p-type semiconductor layer to ensure the transport of
The deposition methods of CuSCN films, such as solution
deposition [8], CBD [9], SILAR [9], and electrochemical depo-
sition [1,6,10] have been reported, in which electrochemical
deposition was considered to be a feasible method for coating the
insidesofcomplexshapes. Untilnow, studiesonelectrochemical
deposition of CuSCN were generally performed in organic sol-
vents because of the instability of Cu(II) cations and SCN anions
in aqueous solvents [10]. Rost et al. studied electrochemical
deposition of CuSCN on porous n-type TiO2 films using puri-
fied ethanol solvent [11], and obtained a spatially distributed
p–n heterojunction. However, using organic solvents led to poor
conductivity of the deposition solution, dye desorption on the
dye-absorbed n-type layer and poor contact with the sensitiza-
tion layer [12]. Wu et al. deposited p-type CuSCN films through
an EDTA-chelated copper aqueous electrolytes with pH 2–2.6
and demonstrated the advantages of the aqueous electrolytes,
such as high conductivity, strong dissolvability of solutes, high
deposition efficiency, and prevention of desorption of dyes [13].
More recently, Wu et al. has also investigated the electrode-
∗
Corresponding author.
E-mail address: zhgjin@tju.edu.cn (Z. Jin).
0013-4686/$ – see front matter © 2008 Published by Elsevier Ltd.
doi:10.1016/j.electacta.2008.01.111

Y. Ni et al. / Electrochimica Acta 53 (2008) 6048–6054
6049
position of CuSCN on porous TiO2 films using such aqueous
electrolytes [14]. However, the acidic aqueous electrolytes do
not allow their employment on the ZnO rod array substrates due
to its poor resistance to acidic etching.
figuration was applied, in which the as-prepared ZnO rod arrays
functioned as working electrodes, Pt foil as a counter electrode
and an Ag/AgClsat as a reference electrode.
Previously, we have studied the electrochemical deposition of
CuSCN on ITO substrates using mild aqueous electrolytes with
weakly basic pH value (∼9) [15]. In this paper, we will focus on
fine filling of CuSCN on ZnO rod arrays by the electrodeposi-
tion method to form p-CuSCN/n-ZnO rod array interpenetrating
heterojunctions for the solar cell application. The morphology,
junction property, and electrochemical deposition characteris-
tics will be described.
2.3. Characterization
The electrochemical measurements of CuSCN and I–V char-
acteristics of the deposited heterostructure films were recorded
on the Potentiostat/Galvanostat. Thin Au layers were vacuum-
evaporated on the obtained CuSCN films for I–V measurements.
The I–V measurements were performed in the sweep direction
from −1.0 V to +1.0 V with a sweep speed of 50 mV/s. The film
morphology was observed by field emission scanning electron
microscope (FESEM, JEOL JSM-6700).
2. Experimental
2.1. Preparation of ZnO rod electrode
3. Results and discussion
ZnO rod arrays were grown on ITO glass substrates with ZnO
crystalline seed layers by an aqueous chemical growth method,
as explained previously [16]. First, a sol solution for ZnO seed
layers was prepared by dissolving 8.2 g Zn(CH3COO)2·2H2O
into a mixed solution of 47.7 ml 2-methoxyethanol and 2.3 ml
ethanolamine (MEA, NH2CH2CH2OH) at room temperature.
ThecleanedITOsubstrateswerecoatedwiththesolsolutionbya
dip-coating technique and subsequently dried in an oven at 80 ◦C
for 5 min. This dip-coating was repeated three times to increase
the seed layer thickness, and the final films were annealed in air
at 550 ◦C for 1.5 h to obtain crystalline seed layers. The aque-
ous growth solution for ZnO rod arrays was a mixture of 6.3 g
Zn(NO3)2·6H2O, 13 ml ammonia (25%) and 337 ml deionized
water. The ZnO seed layer coated substrates were dipped in the
growth solution at 90 ◦C for 4 h for the ZnO rod growth. Next,
the substrates were rinsed with deionized water and air dried.
To examine the chemical stability of the ZnO rod arrays in the
electrolyte solution, the ZnO film was dipped into the electrolyte
solution for 24 h. The surface FESEM images of the ZnO rod
arrays prior to and after dipping are shown in Fig. 1. From the
FESEM observation, it can be concluded that the as-prepared
ZnO rods were uniform with a length of about 2 ␮m and mean
diameter of about 100 nm and vertically aligned to the substrate.
After dipping in the electrolyte solution, the rods maintained
their original morphology, suggesting that the weak basic elec-
trolyte had little effect on the ZnO rod array morphology in spite
of the long duration of dipping. This further indicates that the
weak basic CuSCN electrolyte solution could be used in the fol-
lowing research of CuSCN electrodeposition on the ZnO rod
arrays.
CuSCN was electrodeposited on ZnO rod arrays substrates
at the potential of −500 mV with different deposition times and
temperatures, andtheFESEMimagesareshownin Fig. 2. Fig. 2a
and b is the cross-sectional plane and the surface morphology
of the sample deposited for 5 min, respectively. It can be seen
that a few CuSCN grains grew on the lateral portions of ZnO
rods, and there were no CuSCN grains grown on the top of
ZnO rods. For the sample deposited for 15 min (Fig. 2c and d),
we can clearly see the CuSCN deposit had wrapped around the
ZnO rods, particularly in the area denoted by an arrow. After
1 h of deposition (Fig. 2e), the gaps among the ZnO rods had
2.2. Electrodeposition of CuSCN
The electrodeposition of CuSCN films was performed on
a Potentiostat/Galvanostat (TD3691, Tianjin Zhonghuan Co.,
China). A stable weak basic (pH ∼9) aqueous solution, con-
sisting of 0.01 M CuSO4·5H2O, 0.05 M KSCN and 0.10 M
triethanolamine (TEA, N(CH2CH2OH)3) [15], was used as the
electrolyte. A standard three-electrode electrochemical cell con-
Fig. 1. FESEM images of the ZnO rod arrays: (a) as-grown and (b) following dipping in the electrolyte solution for 24 h.

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Y. Ni et al. / Electrochimica Acta 53 (2008) 6048–6054
Fig. 2. FESEM images of cross-sectional and top views of electrodeposited CuSCN on ZnO rod arrays with different deposition time (a) and (b) 5 min, (c) and (d)
15 min, (e) and (f) 1 h, at the potential of −500 mV and temperature 0 ◦C. (g) and (h) were prepared at −500 mV and 20 ◦C for 1 h deposition.
been fully filled by stacked small CuSCN grains. From these
FESEM images, it is clear that that every single ZnO rod works
as an electrode, on which the reduction of cupric ions (Cu2+) to
cuprous ions (Cu+) and the deposition of CuSCN occurs locally.
Moreover, it can be seen from Fig. 2f that CuSCN, having an
intrinsic trigonal pyramid shape [15], covered the top of ZnO
rod arrays only when the gaps of the arrays had been totally
filled. Fig. 2g and h is the samples deposited at 20 ◦C, it can
be found that the CuSCN deposition at 0 ◦C was more com-
pactly filled with smaller CuSCN grains, comparing with those
deposited at 20 ◦C. However, from the above FESEM observa-
tion, the CuSCN growth behavior still maintained its intrinsic

Y. Ni et al. / Electrochimica Acta 53 (2008) 6048–6054
6051
Fig. 3. Current density as a function of time for potentiostatic deposition of
−500 mV on the ZnO rod arrays substrates at different deposition temperatures.
Fig. 4. Linear sweep voltammetric curves on (I) bare ITO, (II) ZnO rod arrays
and (III) CuSCN-coated ZnO rod arrays substrate after 1 h deposition at 0 ◦C.
The scan rate is 50 mV s−1
.
trigonal pyramid shape, which would deteriorate the compact
filling of CuSCN in the gaps of ZnO rod arrays, especially at
a high temperature. So, the present electrodeposition system of
CuSCN should be improved by the optimization of the elec-
trolyte solution to improve the interface chemical behavior so
as to get finer grains and restrain the free growth of trigonal
pyramid grains, and finally achieving the total filling of ZnO
rod arrays.
increase of cathodic current densities is in the potential range of
−180 mV to −700 mV, indicating that the reduction reaction of
Cu2+ + e− → Cu+ occurred, which is a crucial step for CuSCN
deposition. Compared with curve II on ZnO rod arrays, the
cathodic threshold potential on ITO substrate is more negative
and about −220 mV. The slight shift of the cathodic threshold
of ZnO rod arrays toward a little more positive potential may be
attributed to the surface states existing on ZnO rods. It is known
that the as-grown ZnO is an n-type semiconductor with intrin-
sic defects, such as oxygen vacancies and Zn interstitials [17].
These defects will be easy to form some surface states on ZnO
rods. In addition, when a semiconductor electrode is placed in
an electrolyte solution, adsorption of the ions in the electrolyte
also contributes to the formation of surface states [18]. The sur-
face states noticeably improve the transfer of the electrons on
the interface of the semiconductor electrode/electrolyte solu-
tion during the deposition reaction, causing the earlier cathodic
reduction current. Furthermore, it can be seen from Fig. 4 that
the cathodic current density on ZnO rod arrays in the range of
−200 mV to −450 mV is slightly higher than that of the ITO
substrate, which is caused by the earlier threshold potential of
the reduction reaction on ZnO rod arrays. After the potential of
−450 mV, the cathodic current density becomes smaller, which
is likely due to the space barrier in interface region consisting
of ZnO rod arrays and liquid electrolyte, leading to a potential
drop.
Fig. 4 also shows the LSV curve on the CuSCN-coated ZnO
rod arrays following a 1 h deposition (curve III). It can be deter-
mined that there is not a cathodic reduction reaction current
until −700 mV, showing a negative shift of −520 mV compared
with that on the bare ZnO rod arrays. In this circumstance, the
electrons transfer from the conduction band of the deposited
CuSCN layer to the reaction interface or the holes transfer from
the reaction interface to the valence band of CuSCN as the
reduction of Cu2+ + e− → Cu+ has been restricted in the poten-
tial range of −180 mV to −700 mV. Actually, the appreciated
potential range of sole CuSCN deposition is in the range from
−180 mV to −500 mV. While over −500 mV, Cu co-deposition
Fig. 3 shows the curves of current density as a function
of time for potentiostatic deposition of −500 mV on the ZnO
rod arrays substrates at different deposition temperatures. As
the chronoamperometry measurements are shown in Fig. 3, the
current densities rapidly decreased at the onset of the applied
cathodic potentials on the four curves, which may be attributed
to the induction process (the charge–discharge process of dou-
ble layer) at the interface of electrode/electrolyte solution. It
should be noted that the initial value of current density beyond
the induction process was rather different at different temper-
atures, and it changed from about 0.11–0.27 mA/cm2 with the
increasing of temperature from 0 ◦C to 45 ◦C, indicating a strong
temperaturedependence. Comparedbetweendepositionsattem-
peratures 0 ◦C and 20 ◦C, the current density of the sample
deposited at 20 ◦C was higher than that of the sample deposited
at 0 ◦C, especially in the early 10 min. Moreover, both current
densities became rather low beyond 40 min, indicating a low
rate and steady state growth of CuSCN. For deposition at higher
temperatures, suchas30 ◦Cand45 ◦C, thechangeofcurrentden-
sity depending on temperature is more remarkable. Obviously,
high current densities tended to result in high growth rate of
CuSCN with large grain size [15]. On the other hand, the samples
deposited at 45 ◦C apparently presented brown-colored points
dispersed on the film surface, indicating the Cu co-deposition
[15].
Fig. 4 shows the electrochemical behaviors in the deposition
systems at the temperature of 0 ◦C. Curve II is linear sweep
voltammetric (LSV) characteristic of ZnO rod arrays and curve
I is performed on a bare ITO substrate, shown as reference.
We can see that the cathodic threshold potential for deposition
of CuSCN on ZnO rod arrays is about −180 mV. Subsequent

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Y. Ni et al. / Electrochimica Acta 53 (2008) 6048–6054
Fig. 5. Schematic of the energy band model of (a) the initial deposition on ZnO rod arrays and (b) ZnO/CuSCN heterojunction/electrolyte solution at negative
external potential.
will occur [15]. It implied that the increase of current density
over −700 mV on curve III is caused by the deposition of metal
Cu. Thus, according to the above LSV measurements, we pro-
posed an energy band model for the electrodeposition of CuSCN
on ZnO rod arrays. Fig. 5a is the energy band model concerning
the initial deposition on the bare ZnO rod arrays under a nega-
tive external potential. In the initial stage, the growth of CuSCN
was a conduction process, in which the electrons could easily
transfer from ZnO rods to electrolyte solution. Once the CuSCN
layer covered the surface of ZnO rods, the carrier transfer mech-
anism was invalid. The successive deposition of CuSCN should
be based on the energy band model concerning the ZnO/CuSCN
junction/electrolyte solution at the negative applied potential, as
shown in Fig. 5b. The energy band contact at the reaction inter-
face converted from n-ZnO/electrolyte to p-CuSCN/electrolyte.
As mentioned above, the carrier transfer through the formed
CuSCN layer for successive growth of CuSCN was restricted.
Thus, the following deposition mechanism could be explained
according to the thermal activation of surface states. Wu et al.
[13] has suggested a thermal activation mechanism of surface
states to describe the electrodeposition of CuSCN on conduct-
ing ITO substrate. In fact, the adsorption of SCN− anions and
Cu2+ cations at the electrode interface, especially the adsorption
of Cu2+ cations due to the applied negative potential, served
as a key role in building of the surface states [18], as well as
the temperature-dependent electron state of the surface state
also played another important role. Both affected by temper-
ature dominated the successive electrodeposition process on
the CuSCN-coated ZnO rod arrays. In terms of this deposition
mechanism, it could conclude that the original nucleation and
grain growth of CuSCN occurred on bare ZnO rods during the
conduction band process were prior. The following growth of
CuSCN was slow and based on the thermal activation of sur-
face states controlled by temperature. Fig. 2a–f confirmed the
above growth mechanism that the growth of CuSCN was prior
to filling of the gaps of the ZnO rod arrays. And the signifi-
cant increases of current density with deposition temperatures,
as shown in Fig. 3, also demonstrated the function of thermal
activation governed by temperature. So, according to the ther-
malactivationgrowthmechanism, ahighdepositiontemperature
could lead to a faster growth rate of CuSCN on ZnO rod arrays.
Nevertheless, the intrinsic growth of trigonal pyramid grains
and Cu co-deposition at higher temperature must be avoided
in order to achieve the total filling and promise the quality of
the junction. On the other hand, the diffusion of Cu2+ ions in the
formoftheCu(II)–TEAcomplexesatdifferenttemperatureswill
also impact the deposition, which needs specific investigation in
further.
Fig. 6 shows the electrical measurements of the prepared
CuSCN/ZnO rod array interpenetrating junctions. I–V char-
acteristics of the p-CuSCN/n-ZnO rod arrays heterojunctions
formed within 1 h of deposition under 0 ◦C and 20 ◦C are shown
in Fig. 6a. The inset exhibited linear I–V characteristic of the
contact between ITO and p-CuSCN film, indicating an ohmic
contact formed between ITO and electrodeposited CuSCN film.
So the rectification in Fig. 6a was naturally attributed to the junc-
tion properties of the electrodeposited CuSCN/ZnO rod array
interpenetrating heterojunctions. In Fig. 6a, the two I–V curves
clearly exhibit the almost exponential increase of current den-
sity under forward bias. For the heterojunction deposited at
20 ◦C, the turn-on voltage is approximately 0 V and the reverse
current does not saturate in the reverse bias range. The rec-
tification ratio, measured at the applied voltage of 0.5 V, is
only about 6, implying a weak-rectifying effect. For the hetero-
junction deposited at 0 ◦C, the turn-on voltage is about 0.3 V.
The reverse bias current density saturates in a reverse bias
range of 0 V to −0.5 V and is about 5 × 10−3 mA/cm2. The
rectification ratio, also measured at 0.5 V, is about 19. The
larger rectification ratio of the heterojunction formed at 0 ◦C,
compared with the one deposited at 20 ◦C, indicates that the het-
erojunction deposited at low temperature has a better electrical
contact.
Fig. 6b shows the semilog plot of current density versus for-
ward bias voltage of the two heterojunctions. Based on Fig. 3b,
the diode ideality factor and series resistance were evaluated
according to the empirical equation [19]:
ꢀ
ꢁ ꢁ
ꢂ
ꢃ
V − IRS
I = I0 exp
q
− 1
(1)
nkT
where I0 is the reverse bias saturation current, and k, T, n and RS
are Boltzmann’s constant, temperature, diode ideality factor and
series resistance, respectively. For the heterojunction deposited
at 20 ◦C, in the low forward bias voltage range, it seems that
there is not a straight line region, making it difficult to estimate
the diode ideality factor. Nevertheless, we still estimated the
series resistance of this junction to be about 240 ꢀ. However,

Y. Ni et al. / Electrochimica Acta 53 (2008) 6048–6054
6053
ratio of the rod arrays as well as the surface modification also
needs to be tailored carefully.
4. Conclusions
p-CuSCN/n-ZnO rod array heterostructures were success-
fully electrodeposited by using a weakly basic electrolyte
solution to avoid the acidic etching of the ZnO substrates. From
the FESEM observation, CuSCN was able to sufficiently embed
in the gaps of the ZnO rod arrays, and grow thicker to cover the
ZnO rod arrays substrate. The chronoamperometry measure-
ments revealed that a high deposition temperature leaded to a
higher current density, implying faster growth of CuSCN on
ZnO rod arrays with larger grain size. However, the high tem-
perature also resulted in the emergence of Cu co-deposition.
According to the LSV analysis, the energy band models for
the electrodeposition of CuSCN on ZnO rod arrays suggested
that the initial growth of CuSCN on bare ZnO substrate was
a conduction band process, which caused the prior filling in
the gaps of ZnO rod arrays. Successive growth of CuSCN on
the formed ZnO/CuSCN heterojunction was dominated by the
thermal activation mechanism of surface states, which could
be directly controlled by the deposition temperature. The p-
CuSCN/n-ZnO interpenetrating heterojunctions deposited at
both 0 ◦C and 20 ◦C showed clear rectification and the hetero-
junction formed at 0 ◦C exhibited better diode properties. Lower
deposition temperature is preferred to better electrical contact
of the heterojunction so far as the thermal activation mechanism
at such low temperature is available. The further study should
be focused on the optimization of the electrodeposition solution
system and the deposition parameters so as to obtain high quality
heterojunctions.
Fig. 6. (a) I–V characteristics of CuSCN/ZnO rod arrays p–n heterojunctions
prepared in weak basic electrolyte solution at deposition temperature of 0 ◦C and
20 ◦C under −500 mV, and (b) Semilog plot of current density versus forward
bias voltage of the p–n heterojunctions. The inset in (a) is I–V characteristics of
ohmic contact between ITO/CuSCN/Au junction and ITO/Au junction, shown
as a reference in which the CuSCN film was prepared in the same weak basic
electrolyte solution at deposition temperature of 20 ◦C under −500 mV after 1 h.
Acknowledgement
The authors wish to acknowledge the financial support of Key
Basic Research of TSTC (Project No. 07JCZDJC009900).
for the heterojunction formed at 0 ◦C, the diode ideality factor
is about 3.3 in the low forward bias voltage range of 0.15–0.4 V,
and the series resistance is about 130 ꢀ. Thus, these result sug-
gest that the electrical contact of the heterojunction formed at
0 ◦C is better than that of the one formed at 20 ◦C. The bet-
ter heterojunction properties imply that the CuSCN has been
better filled in the ZnO rods, and the contact between electrode-
posited CuSCN and ZnO rods was compact, which was also
confirmed by the FESEM observation. In addition, according
to the Sah–Noyce–Shockley theory [20], in a p–n junction, the
value of the ideality factor is 1.0 at a low voltage, and 2.0 at a
higher voltage. The ideality factor of the rod-type heterojunction
is quite large, which may be due to: (1) the total filling among the
ZnO rod arrays did not achieve completely; (2) the large series
resistance of ZnO rod arrays embedded heterostructures [21];
(3) the existence of the surface states, which could also lead to
a large diode ideality [22]. So in order to improve the electrical
properties of the interpenetrating heterojunctions, the total fill-
ing of the ZnO rod arrays should be firstly promised. The aspect
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