Research Article
www.acsami.org
Binary Oxide p‑n Heterojunction Piezoelectric Nanogenerators with
an Electrochemically Deposited High p‑Type Cu2O Layer
Seung Ki Baek,† Sung Soo Kwak,‡ Joo Sung Kim,† Sang Woo Kim,†,‡ and Hyung Koun Cho*,†
†
School of Advanced Materials Science and Engineering, Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon-si,
Gyeonggi-do 16419, Republic of Korea
‡
SKKU Advanced Institute of Nanotechnology (SAINT), Center for Human Interface Nanotechnology (HINT), Sungkyunkwan
University (SKKU), Suwon-si, Gyeonggi-do 16419, Republic of Korea
S Supporting Information
*
ABSTRACT: The high performance of ZnO-based piezoelectric nanogenerators (NGs) has been limited due to the
potential screening from intrinsic electron carriers in ZnO. We
have demonstrated a novel approach to greatly improve
piezoelectric power generation by electrodepositing a highquality p-type Cu2O layer between the piezoelectric semiconducting film and the metal electrode. The p-n heterojunction using only oxides suppresses the screening effect by
forming an intrinsic depletion region, and thus sufficiently
enhances the piezoelectric potential, compared to the pristine
ZnO piezoelectric NG. Interestingly, a Sb-doped Cu2O layer
has high mobility and low surface trap states. Thus, this doped
layer is an attractive p-type material to significantly improve piezoelectric performance. Our results revealed that p-n junction
NGs consisting of Au/ZnO/Cu2O/indium tin oxide with a Cu2O:Sb (cuprous oxide with a small amount of antimony) layer of
sufficient thickness (3 μm) exhibit an extraordinarily high piezoelectric potential of 0.9 V and a maximum output current density
of 3.1 μA/cm2.
KEYWORDS: cuprous oxide, zinc oxide, piezoelectric, nanogenerator, antimony
polymer.10−12 With the typical p-n junction structure, it is very
likely that the output performance of the nanogenerator (NG)
is improved by decreasing the screening effects. However, to
date, this approach has been limited by the expense of p-type
polymers and by severe material degradation under humid and
hot environmental conditions. Stable inorganic p-type oxide
materials, such as CuO and NiO, have been scarcely used to
form p-n junction piezoelectric NGs.13−15 These materials are
usually deposited using high-vacuum and high-temperature
processes. Such deposition conditions form a large technical
barrier for substituting p-type polymers, regarding the costeffectiveness and the use of flexible substrates. To enhance the
competitiveness of p-n based piezoelectric NGs, a low-cost
solution-based deposition process should be used. Among the
several possible methods, electrochemical deposition using a
liquid electrolyte provides excellent uniformity over a large area,
and is a low-cost processing method.16
In this study, we produced a Cu2O p-type layer by
electrodeposition to enhance the performance of ZnO-based
piezoelectric NGs. The Cu2O is intrinsically a p-type oxide
semiconductor with a band gap of 2.1 eV and a high hole
1. INTRODUCTION
The sudden development and use of various wireless sensors
and individual portable electric devices attests to their
attractiveness as power sources. Energy harvesters exploiting
thermal gradients, solar irradiation, and mechanical vibrations
make use of energy in the environment that otherwise goes to
waste.1−3 Among these various sustainable energy harvesters,
piezoelectric energy harvesters are potential devices, which
power electronic devices by capturing diverse types of energy,
such as the mechanical vibrations in human movement.4,5
Among the various materials available for high-performance
piezoelectric energy harvesters, the metal oxide ZnO has
attracted a great deal of attention due to the coupling of its
good piezoelectric and semiconducting properties.6,7 In
particular, ZnO has been considered as one of the most
promising candidates for piezoelectric devices because of its
relatively low toxicity and cost-effectiveness in comparison to
other piezoelectric materials.8 Nevertheless, the realization of
ZnO-based NGs with large output performance has unfortunately proven difficult due to the free carrier density of the ZnO
layer with intrinsically n-type characteristics, which oppositely
screens the piezoelectric potential generated by mechanical
deformation.9
To prevent this screening effect, several attempts have been
put forward to form a p-n junction structure with a p-type
© XXXX American Chemical Society
Received: March 30, 2016
Accepted: August 11, 2016
A
DOI: 10.1021/acsami.6b03649
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Research Article
ACS Applied Materials & Interfaces
mobility.17 Thus, it has been actively studied as a hole transport
layer or absorber in solar cells and in photocathodes for watersplitting systems.18−20 Nevertheless, electrodeposited Cu2O
films exhibit low electrical conductivity, limiting the application
of Cu2O. We confirmed a considerable enhancement in
piezopotential output and current in ZnO-based NGs by
introducing an electrodeposited p-type Cu2O layer in the p-n
junction. Moreover, we used a Cu2O:Sb (cuprous oxide with a
small amount of antimony) layer, reported in our previous
study, which has a higher hole mobility and a preferred grain
orientation compared to the undoped Cu2O layer.21 The
piezoelectric performances of ZnO, ZnO/Cu2O, and ZnO/
Cu2O:Sb structures were compared. Piezoelectric measurements revealed that the Sb-doped Cu2O layer dramatically
improves the piezoelectric output performance, compared with
pristine ZnO and undoped Cu2O, due to the formation of a
high-quality p-n junction.
2. EXPERIMENTAL SECTION
The p-type binary oxide Cu2O layer was synthesized on indium tin
oxide (ITO) (thickness of 200 nm)-coated polyethylene naphthalate
(PEN) (thickness of 200 μm) substrates by electrochemical deposition
in an aqueous Cu bath consisting of 0.4 M copper(II) sulfate (Junsei,
>98%, anhydrous), 3 M lactic acid (Sigma-Aldrich, 85%), and 4 M
sodium hydroxide solution to adjust the pH to 11. For the Cu2O:Sb
film, 2 mM antimony sulfate (Sigma-Aldrich, >98%) was added to the
Cu2O chemical bath. A Pt mesh and Ag/AgCl (saturated NaCl) were
used as the counter electrode and reference electrode, respectively.
The electrochemical deposition was potentiostatically progressed at
−0.4 V with a Princeton Applied Research Versatate 4. A ZnO thin
film was deposited by radio frequency (RF) magnetron sputtering.
During the deposition process, RF power, working pressure, and the
argon:oxygen mixture ratio were maintained at 80 W, 9 mTorr, and
20:2, respectively, at room temperature. An Au electrode of 200 nm
was coated on the surface of the ZnO layer to provide a Schottky
contact, as the top electrode of the piezoelectric NG.
The morphology of the ZnO/Cu2O structure was examined with a
field-emission scanning electron microscope (FE-SEM, JSM-6700F, 10
kV). The structural investigation of the samples was confirmed using
X-ray diffraction (XRD, Bruker AXS D8 discover). An oscilloscope
(Tektronix DPO 3052 Digital Phosphor) and low-noise current
preamplifier (SR570, Stanford Research Systems, Inc.) were used to
characterize the output voltage and current of the piezoelectric NG. A
bending tester was used to create the bending strain on the NG.
Figure 1. (a) Schematic of the oxide p-n junction ZnO/Cu2O-based
piezoelectric nanogenerator structure and (b) XRD pattern of Cu2O
and Cu2O:Sb films on an ITO-coated PEN substrate. (c) FE-SEM
image and (d) HRTEM image of the ZnO/Cu2O:Sb interface.
layers have considerably improved crystallinity even on the
ITO/PEN substrate, based on the narrow and intense [111]
Cu2O peak at 2θ = 36.5°. As explained in the previous study,
the addition of Sb element led to vertically well-aligned grains
with low-angle tilted boundaries, by inducing fast nucleation
and promoting lateral growth.21 As shown in Figure 1c, the
electrodeposition of the Cu2O:Sb clearly shows vertically wellaligned grain boundaries despite the use of a flexible PEN
substrate. On the other hand, the ZnO layers have a relatively
low degree of crystallinity with a weak ZnO (001) peak at 34.3°
due to deposition at room temperature (Figure S1 of the
Supporting Information). HRTEM images (Figure 1d and
Figure S2) show Moiré fringes arising from the overlap of the
lattice fringes at the Cu2O:Sb/ZnO interface, and these fringes
clearly indicate that the ZnO is crystalline phase and can
generate the piezopotential.
Considering the existence of a Schottky contact between
ZnO and Au, this structure is expected to have two resistances
in series consisting of p-n and Schottky junctions. Thus, the
total voltage drop between the Au and ITO electrodes can be
simply divided into two parts: a junction voltage applied
between p-Cu2O and n-ZnO and the voltage induced from the
Schottky contact. Resultantly, the Au/ZnO/Cu2O/ITO
structure clearly exhibits good rectifying diode characteristics
regardless of the Sb doping in the Cu2O, as shown in Figure
2a,b. The current density of the forward bias region in the
sample with Cu2O:Sb is 2.8 mA/cm2 at 3 V, which is over twice
that of the sample without Sb doping. In addition, the reverse
current density of the ZnO/Cu2O:Sb is very low, 9.1 × 10−6
mA/cm2 under −2 V, indicating that the Sb incorporation
produces a sample with a high-quality p-n junction. The carrier
recombination and transport mechanism can also be compared
by a semilog plot of current versus bias (0−0.5 V), as shown in
Figure 2b. The current−voltage characteristics of these curves
can be described by the sum of the p-n diode equation (Cu2O/
ZnO) and the thermionic field-emission Schottky diode
equation (ITO/Cu2O):22
3. RESULTS AND DISCUSSION
A schematic diagram of the oxide p-n junction ZnO/Cu2Obased piezoelectric nanogenerator structure and its FE-SEM
image are shown in parts (a) and (b), respectively, of Figure 1.
Here, the p-type Cu2O films were electrodeposited on the ITOcoated PEN substrate and slightly doped with Sb due to the
presence of 2 mM Sb2(SO4)3 in solution. In our previous study,
the p-type Cu2O:Sb film exhibited well-aligned vertical grain
boundaries and high conductivity compared to the undoped,
randomly oriented, and large-grained Cu2O film because the Sb
acted as an inorganic surfactant during electrodeposition. After
the electrodeposition of the p-type Cu2O and Cu2O:Sb layers,
the semi-insulating intrinsic ZnO thin films were subsequently
deposited using RF magnetron sputtering producing a layer
with 200 nm of thickness at room temperature. Then an Au
electrode was placed on the ZnO film to form a Schottky
contact with an energy barrier to charge transport. X-ray
diffraction patterns revealed the crystallinity of the Cu2O/ZnO
and Cu2O:Sb/ZnO on the PEN substrates, as shown in Figure
1b. These results show that the electrodeposited Cu2O:Sb
I1 = IS1[exp(qV1/n1kT ) − 1], I2 = IS2[exp(qV2/n2kT )]
B
DOI: 10.1021/acsami.6b03649
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Figure 3. NG performance of the ZnO NG without Cu2O (Au/ZnO/
ITO): (a) piezopotential and (b) piezocurrent. The sample was
slightly bent with a curvature of 0.5 cm on the bending stage. (c) Bend
cycle of ZnO NG and (d) a band diagram showing the energy band
bending under the deformed condition: positive piezoelectric potential
(V+) at the ZnO/ITO interface and negative piezoelectric potential
(V−) at the ZnO/Au interface.
Figure 2. I−V characteristics from the Au/ZnO/Cu2O/ITO structure:
(a) linear scale over a wide range and (b) semilog scale over a narrow
range (0−0.5 V).
where IS is the reverse saturation current, n is the junction
ideality factor, q is the electron charge, k is the Boltzmann
constant, and T is the absolute temperature. Here, the ideality
factor can be calculated from the slope of the current−voltage
semilog plot. Since the carrier density of the Cu2O:Sb sample
has similar order of magnitude as that of the Cu2O sample, the
effect of the Schottky junction is minor (similar n2 values) and
the difference in n1 from the p-n diodes is significant, indicating
that the ideality factor of the p-n diode in the Cu2O:Sb sample
was lower than that of the Cu2O sample. The relatively lower
ideality factor implies that the Cu2O:Sb film had a higher
conductivity and fewer interface trap states generating carrier
recombination, which thus justifies the good performance of
the ZnO/Cu2O:Sb p-n heterojunction upon Sb doping.
To demonstrate the influence of the p-n junction on NG
performance, the output performance of the ZnO NG without
Cu2O is first presented as a reference in Figure 3a,b. The NG
consisting of the simple Au/ZnO/ITO with only the n-type
oxide was slightly bent with a curvature of 0.5 cm on the
bending stage. The bending direction was applied with a tensile
strain in the ZnO/ITO interface, and the bending process was
composed of tensile strain (0.1 s) and a neutral state (1.2 s), as
shown in Figure 3c. A positive piezoelectric potential (V+) and
a negative piezoelectric potential (V−) are naturally created at
the ZnO−ITO interface and the ZnO−Au interface,
respectively, resulting in an energy band bending by
deformation (Figure 3d).23 Figure 3a shows the variation in
piezopotential caused by the bending strain, starting with a
negative voltage sign and current pulse under the forward
connection between two electrodes. Here, piezoelectrically
generated electrons are gradually driven from the Au to the
ITO electrode through an external circuit by a piezopotential,
generating the negative current pulse. Subsequently, these
electrons are accumulated at the ZnO/ITO interface until the
potential driven by these electrons is fully balanced with an
equilibrium in piezopotential. In contrast, as soon as the strain
is released, the piezoelectric potential immediately disappears.
Accumulated electrons appear, resulting in a positive current
signal, as shown in Figure 3b. The basic ZnO-based NG
without any p-type material demonstrates a maximum piezoelectric voltage of 0.15 V and a piezoelectric current density of
1.15 μA/cm2. Symmetric current density of a similar level is
usually observed under stress and release conditions. However,
the piezoelectric potential and current exhibit relatively low
values, arising from the native electrons in the ZnO having
intrinsically n-type characteristics, which screen the positive
piezoelectric potential of the ZnO/ITO interface.
An effective strategy to enhance the piezoelectric output of
the ZnO NG is to suppress the screening effect. We suggested
p-n heterojunction-based NGs, where the intrinsically p-type
Cu2O layers were electrochemically deposited to give a final
structure of Au/ZnO/Cu2O/ITO (Figure 4a). Here, the
bending direction of the NG applies strain on the ITO/ZnO.
When the NG is under tensile strain, a positive piezoelectric
potential (V+) is created at the ZnO/Cu2O interface instead of
at the ZnO/ITO interface present in the Au/ZnO/ITO. A
negative piezoelectric potential (V−) is still generated at the
ZnO/Au interface with the deformation of the energy band
diagram, as shown in Figure 4b. The potential gradient in the
internal ZnO layer gradually develops a depolarization electric
field (Edep) which leads to the movement of carriers (the
intrinsic electrons in the ZnO).24 This movement attenuates
the piezoelectric-induced polarization, as mentioned in the Au/
ZnO/ITO. However, unlike the Au/ZnO/ITO, it is very likely
that the p-n junction readily decreases the screening effect due
to two reasons: (i) according to the fundamental properties of
the p-n junction, the p-type Cu2O layer forms a depletion layer
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Figure 4. Band diagrams showing the energy band bending (a) from the Au/ZnO/Cu2O/ITO (inducing Ebi) and (b) under the deformed condition
(opposite direction between Edep and Ebi). The NG performance of the ZnO NG with undoped Cu2O (Au/ZnO/Cu2O/ITO): (c) piezopotential
and (d) piezocurrent.
Figure 5. Band diagrams showing the energy band bending (a) from the Au/ZnO/Cu2O:Sb/ITO (inducing enhanced Ebi in the ZnO) and (b)
under the deformed condition. The NG performance of the ZnO NG with Cu2O:Sb (Au/ZnO/Cu2O:Sb/ITO): (c) piezopotential and (d)
piezocurrent.
1
with built-in potential near the p-n junction interface. Here, the
free electrons existing in the ZnO layer are efficiently depleted
by the p-n junction, thus preventing the screening effect, which
generates the low piezopotential. The built-in electric field (Ebi)
in the p-n junction can reduce the strength of the negative
polarization-induced electric field (Edep), resulting in the
suppression of the screening effect. (ii) The piezopotential of
the p-n junction NG can be increased by the additional p-n
junction capacitance. Typically, the overall capacitance,
connected in series, is less than the value of the individual
capacitances, according to the following equation:
C total
=
1
Cpiezo
+
1
Cp‐n
As mentioned previously, the Au/ZnO/Cu2O/ITO structure
has two resistances in series, consisting of p-n and Schottky
junctions. Thus, the piezoelectric-induced capacitance in the
ZnO and the p-n junction capacitance together decrease the
overall capacitance. Finally, the increase in the piezopotential is
expected from the equation Q = CV under the same charge
density. Also, the interface roughness may influence the
piezoelectric potential, but can be excluded due to the
D
DOI: 10.1021/acsami.6b03649
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extremely different structural properties between undoped and
Sb-doped Cu2O layers.25 In addition, the enhancement in
piezoelectric-induced current density is also due to the higher
piezopotential upon insertion of the p-type Cu2O layer, which
can make more electrons flow from Au to ITO. Consequently,
the piezopotential of the Au/ZnO/Cu2O/ITO under tensile
strain has an asymmetric voltage pulse with a significant
enhancement from 0.15 to 0.45 V in negative voltage,
compared to that of the reference ZnO NG (Figure 4c). The
maximum piezocurrent density is ∼2.35 μA/cm2, twice that of
the NG without the Cu2O layer (Figure 4d). However, when
the strain is released, the slight enhancement in the positive
voltage pulse is due to the existence of the built-in potential
induced by the p-n junction. When the NG is bent with the
tensile strain in the ZnO/Cu2O interface, potential gradients
generated from both the built-in potential and the piezoelectric
potential coexist. On the other hand, when the strain is
released, only the piezoelectric potential vanishes, which causes
an asymmetric piezoelectric potential. In contrast, under the
compressive strain of the ITO/ZnO layers, the ZnO region of
the ZnO/Cu2O interface reveals a negative piezopotential,
opposite to that of the ZnO/Au interface, which shows a
positive piezopotential. In this case, the ITO electrode and the
negative piezopotential at the ZnO/Cu2O interface have a
separation corresponding to Cu2O thickness, and the Edep and
Ebi have the same direction. Thus, compressive strain leads to
insufficient suppression of the screening effect.
It was demonstrated that the electrical resistivity of the
electrochemically deposited Cu2O film is reduced by 1 order of
magnitude by adding 2 mM Sb2(SO4)3 to a cupric bath. The ptype Cu2O:Sb layer has a considerably high hole mobility of
28.6 cm2/(V s), which is 10 times higher than that of the
undoped Cu2O layer. Thus, we used a Cu2O:Sb layer (3 μm)
with high conductivity in the p-n junction NG consisting of
Au/ZnO/Cu2O/ITO. Under compressive strain at the Au/
ZnO interface, the piezoelectric potential is measured to be
extraordinarily high, 0.9 V, which is approximately twice as high
as the ZnO/Cu2O NG, as shown in Figure 5c. Similarly, the
maximum output current density also increased to 3.1 μA/cm2,
as a negative current pulse (Figure 5d). Generally, it is possible
to enhance the piezopotential of a p-n junction NG through the
formation of a space charge region. If the built-in electric field
exists due to the p-n junction, the effect of Edep on the actual
potential gradient can be reduced because of the opposing
directions of Ebi and Edep, as shown in Figure 4b and 5b. Free
electrons become depleted in this region of the p-n junction,
preventing the screening effect, which reduces the depolarization electric field. Thus, the degree of the screening effect is
strongly correlated with the magnitude of a built-in electric field
in the p-n junction, and the output voltage of p-n junction
piezoelectric nanogenerator is determined by the actual
potential gradient which is influenced by the Ebi and Edep,
and the origin of these factors is described in Figure S3. To
evaluate the carrier densities near the surface of the
electrodeposited Cu2O layer, Mott−Schottky plots of the
semiconductor electrolyte interface were obtained from ac
impedance measurements. An ac amplitude of 10 mV and an ac
frequency of 1 kHz were used in the potential range of 0.35−
0.60 V, as shown in Figure 6. The calculated surface carrier
density in the Cu2O:Sb layer is 4.7 × 1016 cm−2, a value 50%
higher than that of the undoped Cu2O layer. Thus, the built-in
potential of the ZnO/Cu2O:Sb junction can be estimated to be
∼0.5 V by the following equation,26
Figure 6. Mott−Schottky plot of the Cu2O and Cu2O:Sb films on
ITO substrate from electrochemical analysis in aqueous system with
1.0 M sodium sulfate with 0.1 M potassium phosphate monobasic (pH
5).
Vbi =
kBT ⎛ NdNa ⎞
ln⎜⎜ i i ⎟⎟
e
⎝ n pnn ⎠
where Nd and Na are the donor and acceptor concentrations of
n- and p-type layers, respectively, and ni is the intrinsic carrier
concentration. The Cu2O layers electrodeposited with Sb
dopants showed high crystallinity, high optical transmittance,
and high electrical conductivity, and resulted in dense and
defect-reduced p-type layers.21 Because of the high mobility and
high surface carrier concentration of the Cu2O:Sb layer, it is
expected that a high-quality ZnO/Cu2O:Sb p-n junction is
formed with low interface trap states. As a result, we believe this
contributes to the production of high piezopotential by
increasing the built-in potential, as shown in Figure 5c. If the
p-n junction has a significantly high density of defect states at
the junction interface, the charge carriers might be partially
removed from the depletion layer, resulting in the reduction of
built-in potential (Figure 4a).27 Consequently, the reduced
defects and high mobility/carrier concentration in the ZnO/
Cu2O:Sb junction are preferable to induce a higher built-in
potential. Furthermore, additional effective carrier transport via
the Cu2O:Sb layer can contribute to the higher piezocurrent.
Finally, we compared the output performance of ZnO/
Cu2O:Sb NGs with different Cu2O:Sb thicknesses to characterize the effect of the p-type thickness on NG performance.
Figure 7 shows the output piezopotential and piezocurrent
from a ZnO/Cu2O:Sb NG with a 300 nm Cu2O:Sb layer. The
output potential of the sample with a 300 nm Cu2O:Sb layer is
decreased to 40% of that of the NG with 3 μm Cu2O:Sb, and
piezo-induced current density is similar to that of the ZnO/
undoped Cu2O NG (60% of the value of the ZnO/Cu2O:Sb 3
μm). These variations in potential and current are involved
with the change of depletion width, which affects the junction
capacitance (c = ϵ/W). Typically, the depletion width in the p-n
junction can be estimated from the following equation,
⎡ 2ϵ(ND + Na)Vbi ⎤1/2
W=⎢
⎥
eNDNa
⎣
⎦
where ϵ is the dielectric constant of Cu2O. Here, the calculated
depletion width in the p-type Cu2O region is about 3 μm,
which is much larger than the Cu2O:Sb thickness of 300 nm.
Therefore, the real depletion layer (W) in the 300 nm Cu2O
layer is too narrow due to the incomplete generation of the
built-in potential. Subsequently the junction capacitance of the
sample with a 300 nm Cu2O:Sb layer is expected to be higher
E
DOI: 10.1021/acsami.6b03649
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Research Article
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■
generation mechanism of ITO/Cu2O:Sb/ZnO/Au
piezoelectric nanogenerators (PDF)
AUTHOR INFORMATION
Corresponding Author
*Tel.: +82 31 299 4733. Fax: +82 31 290 7410. E-mail:
[email protected] (H.K.C.).
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This research was supported by the Basic Science Research
Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Science, ICT and Future
Planning (Grant No. 2015R1A2A2A01007409).
■
Figure 7. NG performance of the ZnO NG with a relatively thin layer
of Cu2O:Sb (300 nm): (a) piezopotential and (b) piezocurrent.
than that of the thicker sample, Cu2O:Sb 3 μm. Thus, the ptype Cu2O with sufficient thickness is necessary to produce
high built-in potential and minimize total capacitance.
4. CONCLUSIONS
We developed binary oxide-based p-n heterojunction piezoelectric nanogenerators with electrochemically deposited high
p-type Cu2O layers. Here, we selected an intrinsically p-type
oxide, Cu2O, as the p-type material layer. Cu2O was formed by
a cost-effective electrodeposition process to enhance the
performance of ZnO-based piezoelectric NGs. The piezoelectric performance of these NGs was significantly improved
by introducing p-type Cu2O layers doped with Sb. The use of
the p-type Cu2O:Sb layer distinctly reduced screening effects
from the intrinsic electrons in the ZnO. Resultantly, the ZnO/
Cu2O:Sb heterojunction NGs showed a sixfold increase in
piezoelectric potential and 2.5 times higher current values,
compared to the NG with only ZnO. Additionally, we believe
that electrodeposition is a suitable process for flexible
piezoelectric NGs because it is possible to deposit thin films
at low temperature. Consequently, this work using high-quality
electrodeposited Cu2O:Sb will provide a plausible platform to
fabricate low-cost and high-performance flexible piezoelectric
NGs.
■
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ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsami.6b03649.
XRD result from the ITO/Cu2O:Sb/ZnO films;
HRTEM image of Cu2O/ZnO interface; power
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DOI: 10.1021/acsami.6b03649
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Research Article
ACS Applied Materials & Interfaces
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