Research Article
www.acsami.org
Thermally Stable Silver Nanowires-Embedding Metal Oxide for
Schottky Junction Solar Cells
Hong-Sik Kim,†,⊥ Malkeshkumar Patel,†,⊥ Hyeong-Ho Park,‡ Abhijit Ray,§ Chaehwan Jeong,#
and Joondong Kim*,†
†
Photoelectric and Energy Device Application Lab (PEDAL) and Department of Electrical Engineering, Incheon National University,
119 Academy Road Yeonsu, Incheon 406772, Republic of Korea
‡
Applied Device and Material Lab., Device Technology Division, Korea Advanced Nano Fab Center (KANC), Suwon 443270,
Republic of Korea
§
Solar Research and Development Center, Pandit Deendayal Petroleum University, Gandhinagar 382007, Gujarat, India
#
Applied Optics and Energy Research Group, Korea Institute of Industrial Technology, Gwangju 500480, Republic of Korea
S Supporting Information
*
ABSTRACT: Thermally stable silver nanowires (AgNWs)embedding metal oxide was applied for Schottky junction solar
cells without an intentional doping process in Si. A large scale
(100 mm2) Schottky solar cell showed a power conversion
efficiency of 6.1% under standard illumination, and 8.3% under
diffused illumination conditions which is the highest efficiency
for AgNWs-involved Schottky junction Si solar cells. Indium−
tin−oxide (ITO)-capped AgNWs showed excellent thermal
stability with no deformation at 500 °C. The top ITO layer
grew in a cylindrical shape along the AgNWs, forming a
teardrop shape. The design of ITO/AgNWs/ITO layers is
optically beneficial because the AgNWs generate plasmonic
photons, due to the AgNWs. Electrical investigations were
performed by Mott−Schottky and impedance spectroscopy to reveal the formation of a single space charge region at the interface
between Si and AgNWs-embedding ITO layer. We propose a route to design the thermally stable AgNWs for photoelectric
device applications with investigation of the optical and electrical aspects.
KEYWORDS: transparent conductor, thermal stable AgNWs, metal oxide, Schottky junction, solar cells, surface plasmon,
impedance spectroscopy
1. INTRODUCTION
Photoelectric devices convert electric energy to photon energy
or vice versa. Most photoelectric devices require transparent
conductors (TCs) to transmit light to generate electric energy
of solar cells and to pass the emission of LEDs, displays, and
lighting devices. TCs possess high ability for use in efficient
energy conversion devices.1−4 Metal oxides (AZO, FTO, ITO,
graphene oxide) and metal nanoscaled networks (graphene,
silver nanowires (AgNWs), CNT) have been explored as TCs
for use in functional semiconductor applications and are
envisioned to have a significant impact on the realization of
TC-employed Schottky junction devices for solar energy
conversion.1,4−12 Inarguably, the formation of a Schottky
junction using TC will offer deep insight on making solar
energy conversion technology affordable. Tremendous interest
has been attracted by graphene, AgNWs, and their hybrids due
to the high optical transmittance and the excellent electrical
conductivity of these materials.13−16 Meanwhile, the difficulties
of large-scale processes and thermal instability have retarded
practical applications of these materials. To resolve these
© 2016 American Chemical Society
concerns, AgNW networks have been used in various metal
oxides such as AZO,15−19 ITO,20,21 graphene oxide,22−24 and
polymers,14,15,25−28 which has led to interesting results in this
field. However, these approaches have been investigated only
for p−n junction devices, which are inherently intensive
compared to Schottky junction devices.14−19,21,23 Although
graphene/Si Schottky devices (or solar cells) have shown
relatively high efficiency, the degradation of graphene-based
solar cell has been a cause of profound concern.13,29,30 Recently,
light trapping via AgNWs networks with AZO and ITO
materials has been reported to show crucial enhancement of
photoelectric devices, including solar cells.31−34 AgNWs
networks would provide advantages of light manipulation, in
the aspects of (1) scattering from NWs to increase effective
optical path, (2) excitation of localized surface plasmon for
Received: December 28, 2015
Accepted: March 14, 2016
Published: March 14, 2016
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Figure 1. (a) Fabrication process flow. (b) Cross-sectional view of the FIB processed sample. (c) Cross-sectional TEM image. (d) Cross-sectional
geometry of teardrop shaped ITO cylinder along the AgNWs. (e) High resolution/low resolution TEM images. (f) XRD patterns of AgNWs and
AgNWs-embedding ITO film.
layer was demonstrated with commercial p-Si wafer without
causing deformation of AgNWs or the use of chemical etching.
Hence, to make solar cells affordable and efficient, we are
proposing an inline Schottky junction fabrication process flow
combining sputtering and spin coating. In addition, the present
study may have profound potential for deploying the proposed
devices for all classes of solar cells.13,15,16,37,38
effective electron−hole generation, and (3) plasma polariton at
an interface to propagate in the semiconductor layer.31,32,35,36
In this article, we report a fascinating TC design realized by
combining solution-processed AgNWs and large-scale applicable ITO layers. Using the presence of functional TC layers on
a Si substrate, an excellent Schottky junction solar cell was
realized. The ITO capping spontaneously ensured the thermal
stability of the AgNWs and signified that this was a functional
TC design for Schottky junction solar cells. Systematic
investigation has revealed that these devices are high-temperature processable and stable, and that they have optical and
electrical functional capabilities attributed to the embedding of
AgNWs in the conventional metal oxide. Moreover, Schottky
junction formation using a design of AgNWs-embedding ITO
2. EXPERIMENTAL SECTION
2.1. Preparation of AgNWs. AgNWs were deposited according to
the reported procedure.9 Material was dispensed onto spinning ITO
coated p-Si wafer using micropipet at room temperature and thereafter
annealed at 150 °C for 10 min to form Ohmic contact. AgNWs were
purchased from PlasmaChem GmbH (Berlin, Germany); AgNWs had
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average diameter of 50 ± 40 nm and length 0.5−50 μm (Cat. No. PLAgNW50-IP-25m). AgNW suspension (10 mg/mL in ethanol as
purchased) was diluted with isopropyl alcohol (IPA) to vary the
concentration.
2.2. Deposition of ITO Films. ITO bottom and top layers of
AgNW networks were deposited at room temperature by 4-in. DC
magnetron sputtering system. During the sputtering, a DC power
source (3.7 W/cm2) was applied to a 4-in. ITO target (In2O2
containing 10 wt% SnO2) at room temperature under an Ar flow
condition of 50 sccm.4 Rapid thermal processing (RTP) treatment was
applied to AgNWs-embedding ITO films.39,40
2.3. Device Fabrication Steps. The solar cell had a structure of
ITO/AgNWs/ITO/p-Si/Al. The proposed low temperature and inline
device fabrication process flows is shown in Figure 1a; we used a
Czochralski (CZ) grown 4-in. p-type Si wafer (resistivity, ρ = 1−10 Ω
cm; orientation (100) ± 0.5°; thickness = 525 ± 20 μm; front side
polished) as a light-absorbing semiconductor material. Al metal was
coated by DC sputtering on the back side of Si. A bottom ITO layer
was deposited by DC sputtering on the Si substrate. AgNWs
containing ink were dispersed onto the (bottom) ITO-coated Si
wafer using spin coating at a speed of 1000 rpm for 30 s; this was
followed by baking at 150 °C to obtain the transparent conducting
AgNW network. The top ITO layer was deposited to cap the AgNWs
from air exposure, which also provides a high thermal stability.
Thereafter, RTP treatment was performed at 500 °C for 10 min in
order to improve the optical and electrical properties. During this RTP
process, the back Al metal diffuses into Si to form a back surface field,
resulting in an Ohmic contact. Then a front Al grid was formed onto
the structure of top-ITO/AgNWs/bottom-ITO/p-Si. The solar cells
were tailored to 100 mm2 size by diamond scribing to isolate electrical
shunt along to the side.
2.4. Characterization. The planar morphologies were analyzed
using a field emission scanning electron microscope (FESEM, JEOL,
JSM_7800F) with 15 kV of field voltage, using an SE2 secondary
detector. The crystal structures of the AgNWs-embedding ITO film
were characterized by X-ray diffraction (XRD, Rigaku, SmartLab) with
Cu Kα radiation (λKα = 1.540598 Å) in grazing mode with a glancing
angle of 0.5°. In order to observe the crystal structures of the AgNWs,
the ITO, and the interfaces, a field-emission transmission electron
microscope (FETEM, JEOL, JEM-2100F) was used. Cross-sectional
TEM samples were prepared using a focused ion beam system (FIB,
FEI, Quanta 3D FEG). The elemental compositions, as line profiles of
the cross-sectional layers, were determined using an energy dispersive
spectroscopy (EDS) attachment to the FETEM. The thickness and
average surface roughness of the deposited films were determined
using a surface profiler (Vecco, Dektak XT-E). Optical characterization
was carried out using a UV−visible spectrophotometer (Shimadzu,
UV-1800); characterization was performed by recording the transmission and absorbance spectra of the thin films in the range 300−
1100 nm. The sheet resistivity values were measured using the fourpoint probe method (CMT-100S, Advanced Instrument Technology)
at room temperature.
A simulator system (McScience-K3000, Korea) was employed to
measure solar cell performances. A photovoltaic power meter
(McScience-K101) was used to monitor the I−V characteristics
under one sun (100 mW cm−2) illumination. Carrier collection
efficiencies of solar cells were profiled using a quantum efficiency
measurement system (McScience-K3100, Korea) coupled with a
monochromator (Oriel Cornerstone 130 1/8 m), source measurement
unit (2440, Keithley), and lock in amplifier (K102, McScience).
Mott−Schottky analyses (C−V characteristics) and impedance spectra
of the Schottky junction devices were obtained using the potentiostat/
galvanostat (ZIVE SP1, WonA Tech, Korea). The high-temperature
stability was studied by vacuum-compatible probe station under the
monochromatic illumination coupled with temperature controller
(TC-200P, Misung Scientific), digital oscilloscope (TBS 1102B-EDU,
Tektronix), and function generator (MFG-3013A, MCH Instruments).
3. RESULTS AND DISCUSSION
Figure 1a showed the fabrication processes of the AgNWsembedding Si (ITO/AgNWs/ITO/p-Si) Schottky devices. A
bottom ITO layer was previously formed before the solutionprocessed AgNWs were coated above it. A top ITO layer was
coated over the AgNWs. This process does not utilize any toxic
chemicals and does not require doping. The various device
structures, employing combinations of transparent conductors
(ITO, AgNWs, and AgNWs-embedding ITO), are shown in
Figure S1; all samples have active areas of 100 mm2.
Microstructural aspects of the cross sections were studied
using a dual beam instrument, combining transmission electron
microscopy (TEM) and focused ion beam (FIB) technologies.
The cross-sectional specimen, fabricated using FIB technology,
is shown in Figure 1b. Figure 1b shows the functional structures
of the ITO/AgNWs/ITO on a Si substrate. We observed for
the first time a distinct morphological arrangement of bottom
and top ITO layers after identical sputtering conditions.
Teardrop-shaped compact ITO cylinders were found to have
formed, along with the top ITO layer growth above AgNWs
(Figure 1c). The formed ITO cylinder has an equal,
geometrical distribution above the 100 nm-thick ITO layer
(Figure 1d). The interface between the bottom ITO and the pSi wafer was found to be very smooth and defect free.
Meanwhile, a noticeably different growth preference was
observed from the top ITO layer. The top ITO growth
above the AgNWs is about 2 times faster than that above the
bottom ITO layer (Figure 1c−e); this preferential growth
direction induces the teardrop-shaped ITO cylinders surrounding AgNWs. The geometrical parameters, estimated using
HRTEM analysis, revealed that the teardrop cylinder has a
diameter of ∼125 nm and a length ∼150 nm; AgNWs (Figure
1c−e) with a diameter of 25−30 nm were found. This
geometrical arrangement of the ITO material may have the
capability of offering better transmittance of incoming light for
generation of free carriers in the absorbing materials. This
material has shown the great potential of the sputtering
deposition method, in which the AgNW surfaces facilitate the
growth of incoming sputtered In2O3 particles, which have
profound hosting ability compared to that of the ITO aperture.
This increased hosting ability is probably attributable to the
electrical conductivity of AgNWs, which is higher than that of
the ITO surface in the present case of the DC magnetron
sputtering deposition method.
HRTEM analysis focused on the center location at which the
AgNWs were embedded has revealed that one-dimensional
AgNWs are surrounded by the ITO film, as shown in Figure 1e.
We can see two different orientations of the ITO atoms at the
top and bottom surfaces of the AgNWs. The top ITO has a
higher order of crystallinity than that of the bottom ITO. No
void formation was found in the contacts of the AgNWs and
the ITO layers. It is desirable for there to be minimum
recombination of free carriers, which can eventually lead to
better probability of their collection, which is preferable for the
long-term stability of AgNWs. As voids may cause the oxidation
of AgNWs, there is higher probability for deformation of
AgNWs and degradation of carrier collection.41−43
Furthermore, in order to understand the crystalline properties of the observed one-dimensional AgNWs and the AgNWsembedding ITO film on Si, glancing angle high-resolution XRD
analysis was carried out (Figure 1f). The AgNWs on the ITO
film are found to be preferentially oriented in the (111)
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Figure 2. Observations of AgNWs after RTP processes: (a) deformation of bare AgNWs, and (b) shape-maintained AgNWs of ITO/AgNWs/ITO
sample. Optical properties of various transparent electrodes: (c) transmittance spectra of AgNWs, ITO, and AgNWs-embedding ITO films, (d)
reflectance spectra of AgNWs, ITO, and AgNWs-embedding ITO films, and (e) effect of top ITO layer thickness on the reflectance caused by
AgNW embedded ITO films on Si substrate. (f) Room temperature J−V characteristics under dark and one sun illumination conditions. (g) Solar
cell performance parameters. (h) Quantum efficiency spectra for Schottky junction solar cell.
retained their morphologies. Moreover, the ITO cylinder was
found to become compact under the influence of the RTP
treatment, likely due to the improved packing density.
To examine the transmittance properties, we prepared
several samples of the AgNWs, single ITO-film, and AgNWsembedding ITO film on quartz wafers (Figure 2c). In order to
evaluate the optoelectronic performance of the transparent
conductors, the ratio of direct current σdc to optical
conductivity σop can be used as a figure of merit (FOM),44
which can be expressed as
direction along with lattice plans (002), (022), (113), and
(222). In contrast, the AgNWs-embedding ITO layers showed
strong preferential growth in the (222) lattice directions. The
magnitude of the X-ray diffraction intensity of the ITO material
was found to be very close to the value estimated for AgNWs;
this confirms good crystalline nature of the ITO cylinders,
showing it to be similar to that of the AgNWs. The selected
area electron diffraction patterns of the one-dimensional
AgNWs and of the ITO cylinders are shown in Figure S4;
distinct diffraction spots confirm the crystalline nature of this
material.
In addition, FESEM analysis of the topography of the
samples reveals the uniform dispersion of the AgNWs network
on the p-Si and ITO coated p-Si surfaces (Figure S2). The top
ITO layer deposition provided better integration of the AgNWs
network and offered isolation from an ambient condition,
eventually preventing any possible deformation processes such
as corrosion, which is one of the critical issues in the area of
nanostructured metal electrodes.41,42 As a result, integrated and
highly compact AgNWs-embedding ITO cylinders can be
observed on the ITO coated p-Si, shown in Figure S3.
In order to assess their thermal feasibility and stability, ITOcapped AgNW (ITO/AgNWs/ITO) TCs were thermally
treated at 500 °C using RTP for 10 min. As a comparator, a
bare AgNWs sample was also treated in the identical RTP
condition. Interesting results were observed, as the AgNWs
networks deformed and agglomerated oblong Ag particles
(Figure 2a). However, the AgNWs-embedding ITO film
σopt(λ)
σdc(λ)
=
⎞
2R sh ⎛ 1
1
−
⎜
⎟
273 ⎝ T (λ)2
⎠
(1)
where T and Rsh are the transmittance (quoted normally at λ =
550 nm) and the sheet resistance, respectively. The σop/σdc
values of the transparent conductors can be obtained using eq
1, and are shown in the inset of Figure 2c. The estimated
FOM550 nm (at 550 nm) values of the AgNW networks and of
the ITO films were found to be 28.2 and 35.4, respectively. On
the other hand, AgNWs-embedding ITO layer led to an
improved FOM550 nm value of more than 120, which is
important for the application of such a material in highperforming photoelectric devices such as solar cells, photodetectors, light emitting diodes, and display devices. The FOM
values, which correspond to the values of maximum transmittance in visible region for the AgNWs, ITO, and AgNWembedding ITO films, were estimated at 35, 122, and 250,
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Figure 3. (a) Mott−Schottky characteristics and estimated energy band diagram of AgNWs-embedding ITO films at p-Si interface. (b) Estimated
band edges of the AgNWs-embedding ITO film at the Si interface. (c) Schottky junction parameters estimated from equivalent circuit fitting of
impedance spectra. (d) Photograph of custom designed homemade vacuum compatible high-temperature characterization setup for solar cell, and
(e) high temperature J−V characteristics under monochromatic (λ = 850 nm, 20 mW cm−2) illumination condition.
diode properties. The estimated diode parameters of the
devices before and after RTP are summarized in Table S1,
where F (1.26) and Jo (8.5 × 10−5 mA cm−2) can be seen to
have improved. These improvements indicate the formation of
a high quality junction using an AgNWs-embedding ITO film
with p-Si, which confirms the formation of a Schottky junction.
In order to investigate the AgNWs-embedding ITO/p-Si
device for use as a solar cell, the prepared devices were
characterized using a simulator system under one sun (AM1.5,
100 mW cm−2) illumination in connection with a power meter.
The AgNWs-embedding ITO/p-Si device provided the
conversion efficiency (η) of about 5%, with a short circuit
current density (JSC) value of 24 mA cm−2 and an open circuit
voltage (VOC) value of 550 mV. The estimated fill factor (FF)
value of 37.3% shows the upper bound of the efficiency. After
the RTP treatment, this device showed an improvement in the
conversion efficiency of 6.1% due to the enhanced FF value of
50%. Figure 2g shows the J−V characteristics under AM1.5
illumination; solar cell parameters are summarized in the inset.
The improved η is mainly attributed to the enhanced FOM
property and the diode characteristics, which were influenced
by the RTP treatment,39,40 and elevated the FF value without
compromising the values of JSC or VOC. This important
observation demonstrates the retention of junction quality
under high-temperature RTP conditions.
To examine the carrier collection performance, external
(EQE) and internal (IQE) quantum efficiencies were measured
for all samples (Figure 2h). The AgNWs-embedding ITO/p-Si
junction showed an EQE on the order of 60% for the entire
visible and near-infrared (NIR) region. This clearly shows that
the AgNWs-embedding ITO network on p-Si is very effective at
collecting the photogenerated carriers. Furthermore, the IQE
spectra were recorded to assess the conceivable surface
plasmon effect resulting from the presence of the AgNW-
respectively; these values are much larger than those of
monolayer graphene (26), graphene-AgNW hybrid (53−
168),13 or composite AgNW-resin (71).25 Moreover, the
transmittance values in the visible region are not much
changed compared to that of single ITO film. The obtained
state of the art FOM values using the AgNW and ITO
combined transparent electrodes make these materials
attractive and affordable for application in photovoltaic devices
with high-temperature processing stability.
In addition, reflectance profiles of the various transparent
conductors on p-Si are shown in Figure 2d. AgNWs-embedding
ITO film has the possibility of reducing the reflectance of
incoming photo flux in comparison to those of AgNW
networks or a conventional metal oxide layer. Even more, the
design of AgNWs-embedding ITO film can be modulated in an
effective manner. When the top ITO layer thickness was varied
from 25 to 100 nm on the AgNWs/100 nm-thick bottom ITO
layer, we found that the thickness of the top ITO layer and of
the cylinders has profound impacts on the ability to tailor the
reflectance (Figure 2e). Among all the samples, the reflectance
caused by the 100 nm-thick top ITO film on a 100 nm-thick
bottom ITO film on the p-Si wafer was found to be the
minimum value over a spectrum range from 300 to 1100 nm.
Hence, we have chosen this combination of transparent
conductors for further device level investigation.
To realize the Schottky devices, a rectifying current flow
should be confirmed.39,40,45−47 In order to investigate the diode
properties, current−voltage (J−V) characteristics were obtained
at room temperature, as shown in Figure 2f (please refer to
Figure S5 for diode analysis). An AgNWs-embedding ITO layer
on p-Si device provided a diode-ideality factor (F) of 1.3, with a
low saturation current density (Jo) value of 4.2 × 10−4 mA
cm−2, and a significantly high rectification ratio (RR) of about
1000. RTP treatment was found to be effective to improve the
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CSCR are the series resistance, shunt resistance, and space charge
capacitance, respectively, under dark condition. IS analysis
revealed a very low RS value of 37 Ω cm−2, which was caused by
the front and back contact of the device. Figure 3c shows the
dependence of the values of RSH and CSCR on the applied bias
condition, which is governed by a single RS−RSH∥CSCR circuit
representing the Schottky junction formed between the
AgNWs-embedding ITO layer and p-Si. In addition, the
minority carrier lifetime (τ), which is represented by the
electron (τn) value of 0.155 ms, was estimated using
exponential fitting of the open circuit voltage decay (OCVD)
(Figure S9), giving a diffusion length (Ld) of 747 μm
(considering the diffusion constants Dn = 36 cm2 s−1 and τ =
0.155 ms, where Ld = Dnτn indicates the average distance
that photogenerated electrons travel before recombining).56
Meanwhile, in the p−n junction Si solar cells, the value of τn =
25 μs resulted from the surface recombination because of the
inevitable surface texturing necessary to reduce the surface
reflectance and to increase the junction surface area. Inarguably,
conventional Si solar cells require expensive surface passivation
treatment, which can be disregarded when using the proposed
AgNWs-embedding ITO film with p-Si designed in the present
study.
The high-temperature stability and operational aspects of the
developed AgNWs-embedding ITO transparent conductor
were studied using a custom designed vacuum setup, as
shown in Figure 3d. Further, the developed Schottky junction
device was evaluated (from room temperature to 433 K) under
monochromatic illumination conduction (λ = 850 nm, intensity
= 20 mW cm−2) by recording the J−V characteristics (Figure
3e) and the OCVD value (Figure S10). The acquired data were
systematic and consistent at high temperature, revealing the
durability and stability of the AgNWs-embedding ITO with pSi, which showed no breakdown. These measurements
demonstrate for the first time that the AgNWs-embedding
metal oxide transparent conductor can be used to establish a
Schottky junction with an active area 100 mm2. The obtained
solar cell performance parameters, such as η, JSC, VOC, FF, and
τ, are shown in Figures S10−S12. Furthermore, analysis
revealed that the AgNWs-embedding ITO transparent
conductor offers a high conversion efficiency value of more
than 8.34% for a low level of injection, which makes these
devices more attractive for diffused-illumination condition, such
as cloudy days. We found that the efficiency drops down in a
linear fashion from 8.34% to 0.5% because of the higher
operational device temperature, which reduces the built in
potential height and, consequently, reduces the VOC from 500
mV to 50 mV in a linear manner. In addition, recombination
rate profoundly deteriorates the value of FF which dropped
from 65% to 28%. Moreover, the value of τn was found to
decrease exponentially from 0.225 to 0.05 ms as the device
operational temperature was raised from 300 to 450 K,
respectively.
embedding teardrop-shaped ITO cylinders. Interestingly, the
short wavelength region (480−560 nm) of the visible spectrum
showed a higher IQE than that of the other regions of the
visible wavelengths. More interestingly, the IQE values of the
visible photons in range of 500−550 nm are found to be less
than the IQE values of the 650−900 nm region for the Si solar
cell, due to the potential absorption at the surface and the
presence of a serious Auger recombination effect resulting from
the presence of a heavily doped emitter region (>10 20
cm−3).4,8,48 This observation provides us with two important
facts about the developed device. First, AgNW-embedding ITO
film can potentially be used to establish a Schottky junction
with p-Si. Second, and more importantly, the AgNWsembedding ITO film induces a surface plasmonic effect due
to the combination of the teardrop structure of the dielectric
ITO and the metallic AgNWs networks on the Si substrate; this
teardrop structure should potentially allow the concentration of
light at a subwavelength volume beyond the optical diffraction
limit.12,31,49−51
To study the junction properties at the interface of the
AgNWs-embedding ITO film and p-Si, Mott−Schottky (MS)
analyses were carried out by applying various frequencies with
small AC bias (10 mV). The typical MS plot (1/C2 vs V) of the
developed Schottky junction is shown in Figure 3a. The
negative slope of the 1/C2 vs V relation shows the holes to be
the majority carrier, with an acceptor carrier (NA) concentration on the order of 6 × 1015 cm−3. The obtained parameters
(slope, built-in potential: ϕbi, and NA) are provided in the inset
table in Figure 3a. Devices showed consistent ϕbi values of ∼0.7
V for the AgNW-embedding ITO film with p-Si. MS analysis
confirmed the high quality of the Schottky junction properties,
which were consistent during the application of various
frequencies (1 kHz to 20 kHz). The procedure for estimating
the band edges in the MS analysis is detailed elsewhere.52,53
Estimated energy band parameters of the AgNWs-embedding
ITO film at the Si interface, determined using MS analysis, are
shown in Figure 3b, where EO, EC, EF, EV, ϕbi, and SCR are the
vacuum level, conduction band, Fermi level, valence band, built
in potential, and space charge region, respectively. The present
MS analyses reveal that the formation of a 950 nm wider SCR
led to a value of ϕbi of 0.7 eV because of the 4.23 eV value of
work function of the AgNWs-embedding ITO film on the p-Si;
this work function value in turn influenced the polarization of
the free charges. In order to examine surface states, MS analyses
were employed to profile 1/C2 vs V for ITO/AgNWs/ITO
(Figure 3a). The capacitance value is stable to frequency
variations compared to that of the AgNWs/ITO (Figure S6).
This indicates that the ITO-capping on AgNWs is effective to
passivate the surface.
Impedance spectroscopy (IS) was carried out to evaluate the
interface aspect of the developed device; it is inarguable that
this technique emphasizes simplifying the integrated junction
capacitance and can bring out fine details of the space charge
junction capacitance.54−56 Therefore, the obtained impedance
spectra in forward and reverse bias conditions were determined
and revealed the high quality of the junction, which was found
to have considerably high shunt resistance (RSH) and space
charge capacitance (CCSR) values of 14.23 kΩ cm−2 and 30.9 nF
cm−2, respectively, giving a relaxation RC time constant of 0.4
ms for the zero bias condition. These parameters were
estimated by considering transmittance line model (equivalent
circuit (RS−RSH∥CSCR) is shown in Figure S7) fitting using the
impedance spectra, as shown in Figure S8, where RS, RSH, and
4. CONCLUSIONS
In summary, using a large scale sputtering method with spin
coating inline solar cell production, we have for the first time
demonstrated a high-temperature processable and operational
AgNW-embedding metal oxide transparent conductor with the
6.1% efficiency (8.34% in low light injection, 20 mW cm−2)
Schottky junction. In-depth systematic structural and physical
analysis revealed that the solution-processed AgNWs enable an
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■
interesting teardrop network of the ITO to form, which reduces
the inevitable surface reflection of the polished Si wafer and
simultaneously yields a high quality Schottky junction with
superior diode properties. AgNWs, embedded at the bottom
center of the ITO cylinder, enable the generation of surface
plasmonic subwavelength photons; this process was confirmed
by IQE measurement and offers a very high figure of merit
value of 250. An all-inclusive, device level investigation
estimated work function value of 4.23 eV for the AgNWsembedding ITO has the potential to yield a value of 0.7 eV for
the built in potential, which should lead to a 747 μm diffusion
length of the photogenerated electrons in the p-Si wafer. Mott−
Schottky analysis combined with impedance spectroscopy
revealed the formation of a single space charge region at the
interface. The AgNWs-embedding ITO transparent conductor,
which has the attributes of high temperature processing and
operational functionality, was demonstrated and should bring
widespread benefits to the field of transparent conductors for
the improved efficiency of energy conversion devices.
■
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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.5b12732.
Photographs of developed Schottky junction solar cells,
various surface morphology including AgNWs and
AgNWs embedded ITO, HRTEM analysis including a
selected area electron diffraction pattern of one-dimensional AgNW and teardrop ITO. Dark J−Vmeasurement.
Impedance spectroscopy including transmission line
equivalent circuit, and cole−cole plot for forward and
reverse bias conditions. Minority carrier lifetime analysis
including open circuit voltage decay measurement. Solar
cell performance analysis and their temperature dependence. Device statistics in tables/plots as well as detailed
experimental description (PDF)
■
Research Article
AUTHOR INFORMATION
Corresponding Author
* Email: joonkim@incheon.ac.kr
Author Contributions
⊥
J.K. conceived this research. H.K. performed the device
fabrication. M.P. and H.K. investigated the photoelectric
devices. H.P. performed TEM analyses. All the authors
contributed to prepare this manuscript. All authors have
given approval to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
The authors acknowledge the financial support of the Korea
Institute of Energy Technology Evaluation and Planning, in a
grant funded by the Ministry of Knowledge Economy (KETEP20133030011000) and Basic Science Research Program
through the National Research Foundation (NRF) of Korea
by the Ministry of Education (NRF-2015R1D1A1A01059165).
Dr. Chaehwan Jeong is grateful for the support through the
Korea Institute of Industrial Technology through Breeding of
Hidden Champion program.
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DOI: 10.1021/acsami.5b12732
ACS Appl. Mater. Interfaces 2016, 8, 8662−8669
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