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Simple Bar-Coating Process for Large-Area, HighPerformance Organic Field-Effect Transistors and
Ambipolar Complementary Integrated Circuits
Dongyoon Khim, Hyun Han, Kang-Jun Baeg, Juhwan Kim, Sun-Woo Kwak,
Dong-Yu Kim,* and Yong-Young Noh*
Solution-processed organic semiconductors are of great potential for large-area, inexpensive, lightweight, and flexible electronic applications. With respect to these materials, tremendous
effort has recently been focused on developing several types of
organic electronic and optoelectronic devices, such as organic
light-emitting diodes (OLEDs), organic photovoltaics (OPVs),
organic field-effect transistors (OFETs), and organic memory
and sensors, using graphic-art printing methods on flexible
substrates.[1–5] OFETs are a fundamental building block of integrated circuits (ICs) and drivers for active-matrix flat-panel displays. Accordingly, they are a promising candidate to replace the
vacuum-processed amorphous inorganic ICs; this would enable
the use of drivers in printed and flexible radio-frequency identification tags, memories, sensors, and display backplanes.[6,7]
To realize high-speed organic printed ICs, the complementary
IC geometry, which consists of p- and n-channel transistors,
has an advantage over those that comprise unipolar transistors
because of reduced transition delays, higher noise immunity,
and negligible power consumption in the static state.[8] In solution-processed devices, p- and n-type active channels have been
patterned at resolutions as low as a few micrometers using a
variety of printing methods such as inkjet, spray, and gravure
printing.[9] However, these printing processes typically result
in device-to-device performance deviations because of difficulties inherent in controlling the morphology (e.g., roughness
H. Han,[+] S.-W. Kwak, Prof. Y.-Y. Noh
Department of Energy and Materials Engineering
Dongguk University
26 Pil-dong, 3-ga, Jung-gu
Seoul 100-715, Republic of Korea
E-mail: [email protected]
D. Khim,[+] J. Kim, Prof. D.-Y. Kim
Heeger Center for Advanced Materials
School of Materials Science and Engineering
Gwangju Institute of Science and Technology (GIST)
261 Cheomdan-gwagiro, Buk-gu
Gwangju 500-712, Republic of Korea
E-mail: [email protected]
Dr. K.-J. Baeg
Nano Carbon Materials Research Group
Korea Electrotechnology Research Institute (KERI)
70 Boolmosangil, Changwon, Gyeongsangnam-do 642-120
Republic of Korea
[+] Both authors contributed equally to this work.
DOI: 10.1002/adma.201205330
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and crystallinity) of micrometre-sized deposits. For example,
organic semiconductor droplets that are deposited via ink jet
onto non-absorbing substrates typically show significant coffeestain effects leading to a non-uniform film morphology.[10] In
addition, precise patterning processes are not very compatible
with fast-moving roll-to-roll (R2R) printing processes.
Recently, several research groups have reported alternative
approaches to the construction of complementary circuitry
without additional patterning processes using ambipolar
OFETs, which can efficiently transport both electrons and
holes in devices depending on the applied voltage polarity.[11,12]
Even though charge transport ambipolarity is an intrinsic
behaviour of most conjugated polymers, a few critical properties such as balanced charge injection and transport must
be optimized to enable practical applications.[13] In previous
reports, we successfully demonstrated high-performance complementary ambipolar OTFTs and circuits obtained by chargeinjection engineering and semiconductor–dielectric interface
modifications.[14,15] The insertion of a cesium-based interlayer
between the Au injection metal and conjugated polymers effectively reduced both the contact resistance for electron injection and the leakage current through the pull-down transistor
in a complementary inverter.[14] In addition, well balanced
mobility was achieved by using a high-k polymer blend (i.e.,
poly(vinylidenefluoride-trifluoroethylene) P(VDF-TrFE) and
poly(methyl methacrylate) PMMA) as a gate dielectric layer.[15]
The primary advantage of ambipolar circuits is that complicated ICs can be achieved with a simple blanket coating without
requiring the patterning of semiconducting layers. Spin-coating
is the most commonly used method for forming blanket thin
films for a variety of solution-processable electronic devices.
Although the spin-coating process is a useful technique on
the laboratory-scale, there are significant drawbacks to this
process including a large amount of chemical waste, difficulty
in adapting it to an R2R continuous process flow, and limited
substrate sizes.[16] Moreover, the strong centrifugal force during
the spinning process inherently causes undesirable aggregation, spherical grooves, and uncontrollable film morphology.[17]
Alternatively, a variety of printing processes, such as doctorblade, slot-die, and spray-coating, have been suggested as
suitable film deposition methods for the high-speed R2R process.[16,18–22] Although these approaches have been reported for
large-area OLED displays and lighting and OPVs, there is no
report on the application of large-area blanket-coating methods
for large-area ambipolar printed ICs and OFET arrays for flexible display backplanes.
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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will pass through as it moved along the substrate. Figure 1b
shows an optical microscope image of the wires in the bar. The
thickness and quality, i.e., roughness and uniformity, of the
deposited films are mainly determined by a variety of experimental factors including the viscosity and surface tension
of the solvent, solid contents of the coating solution, surface
energy of the substrate, distance between the coating bar and
substrate, and speed of the bar.[19,23] When the bar is lowered
to achieve the most proximate contact between the bar and
substrate, a small amount of solution passes through the gaps
between the wound wires (Figure 1c). Therefore, the thickness
of the wet film is directly proportional to the diameter of the
wire. A ratio of 1:10 between the coated wet-film thickness and
wire diameter is generally followed when selecting the wire
size.[23] In this study, we used a bar with a ∼1.3 cm diameter
and ∼70 μm diameter wire coils since they are the smallest that
are commercially available. To achieve an adequate thickness
for the active and dielectric layers in OFETs, we also varied
the solution concentration and speed of the bar. After the barcoating process, the initial surface morphologies of the films
consist of a series of strips with gaps that correspond with the
spacing between the wound wires. These film stripes immediately merge together because of the surface tension of the
solution to form a flat and smooth surface. At this point, the
films start to dry. Schematic diagrams of the film-drying process with corresponding photographs and a video clip of the
bar-coating process are presented in Supporting Information
(see Figure S1).
After verifying the parameters for the bar-coating process,
optimized thin films as semiconductors and dielectrics for
top-gated OFETs were obtained (see Figures 1c–e). A variety
of π-conjugated polymer semiconductors, including PTVPhIEh, F8BT, MEH-PPV, DPPT-TT, and the polymer dielectric,
PMMA, were deposited via the bar-coating process on glass or
poly(ethylene naphthalate) (PEN; Dupont Tenjin Films) plastic
substrates. For the semiconductor layer, a relatively thin film
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Herein, we report a large-area, simple wire-bar–coating process as a suitable method for deposition of conjugated and insulating polymer films in OFET arrays and complementary ICs.
Both highly crystalline conjugated polymer and very smooth
insulating polymer layers were formed by consecutive wirebar–coating processes on a 4 inch glass or plastic substrate.
The film thickness was precisely controlled between a few
tens of nanometers and several micrometers via the concentration of the polymer solution, diameter of wound metal wires
on the bar, and coating speed. Interestingly, the highly crystalline polymer semiconductor, i.e., poly(thienylenevinylene-cophthalimide) functionalized with dodecyl at the imide nitrogen
(PTVPhI-Eh), showed better crystalline morphology and the
amorphous polymer semiconductor, i.e., poly(2-methoxy-5-(2′ethylhexyloxy)-p-phenylene vinylene) (MEH-PPV) or PMMA,
exhibited smoother film morphology when applied via the barcoating process than by the spin-coating process. The best barcoated top-gate/bottom-contact (TG/BC) OFETs with PTVPhIEh or DPPT-TT, comprising diketopyrrolopyrrole (DPP),
thieno[3,2-b]thiophene (TT) and two thiophene moieties in
the repeat unit showed a charge carrier mobility for holes as
high as 0.46 or 2.83 cm2 V−1 s−1 and excellent device-to-device
performance uniformity with standard deviation of 0.05–0.06
in 4 inch transistor arrays. Finally, we fabricated bar-coated
ambipolar complementary inverters (voltage gain >40) and ring
oscillators (ROs) (oscillation frequency (fosc) of ∼25 KHz) with a
PTVPhI-Eh and CsF interlayer.
Figure 1a shows a schematic diagram of the bar-coating process, which comprises three major steps: (i) Deposition of the
polymer solution just ahead of the coating bar, (ii) wet coating
of the polymer solution onto the substrate by the horizontal
movement of the coating bar along a fixed substrate, and (iii)
gradual drying of the wet film from the edge to the centre. The
overall process is also displayed in a video clip available in Supporting Information. The grooves between the wire coils wound
around the bar determine the amount of coating solution that
Figure 1. (a) Schematic description of the bar-coating process with (b) optical microscope image and (c) illustration of the coating-bar used in this
study. (c) TG/BC OFET structure. Molecular structures of the (d) polymer semiconductors [PTVPhI-Eh] and (e) [DPPT-TT].
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Figure 2. Top-surface AFM images of the PTVPhI-Eh semiconductor thin films deposited using (a) spin-coating and (b) bar-coating methods. (c) Normalized absorbance of UV/VIS/NIR spectra for the spin-coated and bar-coated PTVPhI-Eh films. (d) PMMA gate dielectric layer thickness depending
on solution concentration and bar-coating speed. (e) Surface roughness profiles of the spin-coated or bar-coated PMMA gate dielectric films [PMMA,
80 mg/mL solution in n-butyl acetate].
thickness of 30–40 nm was obtained using a solution concentration of 15 mg/mL in chlorobenzene and a bar-coating speed of
10 mm/s. Figures 2a and 2b show the atomic force microscopy
(AFM) images of PTVPhI-Eh thin films prepared by spin- and
bar-coating processes, respectively. The surface morphology of
both PTVPhI-Eh films featured fibril-like microstructures. Interestingly, the bar-coated film contained larger nano-fibrils than
the spin-coated film. It is notable that the bar-coated PTVPhIEh thin film showed a higher root-mean-square (rms) surface
roughness (∼4.38 nm) than the spin-coated film (∼1.91 nm);
this is attributed to the larger nano-fibrils that are present on
the bar-coated PTVPhI-Eh film surface. In addition, UV/Vis
absorption spectra of both films were collected: The spectrum of
the bar-coated film is broader than that of the spin-coated film,
as shown in Figure 2c. This is presumably due to enhanced
molecule crystallinity in the bar-coated PTVPhI-Eh thin film.
In contrast, for amorphous semiconductors, such as MEH-PPV,
the bar-coated films (rms = ∼0.573 nm) had smoother surfaces
than the spin-coated films (rms = ∼0.805 nm) (see Figure S3).
Therefore, it is evident that excellent film quality can be easily
obtained for both crystalline and amorphous semiconductor
active layers using a bar-coating process.
The improved crystallinity of the bar-coated polycrystalline
films is mainly due to the relatively slow evaporation rate of
solvents from the wet films compared with that in the spincoated films, which occurs more quickly because of the large
external centrifugal force. Note that the bar-coating process
typically takes longer (∼30–60 s) for solvent evaporation during
the formation of the solid thin film from the liquid state; for
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comparison, most solvents evaporate within ∼5–10 s during
the spin-coating process under these experimental conditions.
Many previous papers reported that the crystallinity of conjugated polymer films is strongly dependent on the speed of
evaporation of the solvent; therefore, drop-casted or dip-coated
OFETs exhibited better charge-carrier mobility than those that
were spin-coated.[24] Moreover, the bar-coating process provides superior film uniformity and smoother surfaces on largearea rigid or flexible substrates. As can be seen in Figure S2,
PTVPhI-Eh and F8BT thin films that were bar-coated onto a
flexible PEN substrate were very uniform and homogeneous
and did not have any dewetting or aggregation problems after
a mild O2 plasma treatment. Therefore, the bar-coating process
is a suitable method for forming uniform and large-area thin
films without undesired fluidic phenomena such as the coffeestain effect, which frequently occurs during drop-casting or inkjet printing.[10]
After the deposition of the semiconducting layer, a PMMA
gate dielectric layer was bar-coated onto the dry conjugatedpolymer surface. The thickness of the PMMA dielectric films
was controlled by adjusting the solution concentrations and
speed of the bar. As shown in Figure 2d, the PMMA film thickness increased with increasing speed of the bar: When the
speed of the bar increased from 10 to 100 mm/s, the film thickness linearly increased from ∼10 to ∼150 nm using 20 mg/mL
PMMA solutions and from ∼100 to ∼500 nm using 40 mg/mL
PMMA solutions. Interestingly, the surface profiles of the spincoated and bar-coated PMMA films showed significantly different film qualities (Figure 2e): The surface of the bar-coated
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variations in the OFET characteristics over large areas. In
contrast, the bar-coated PMMA film had a quite uniform and
smooth surface free from distinct striations (see the surface
profiles in Figure 2e); this is mainly attributed to the gradual
evaporation of the solvent from the wetted film that occurs
without external forces, such as the centrifugal force during
the spin-coating process, that cause striation defects. To investigate the quality of the bar-coated gate dielectric layer, the current density versus the applied electric field characteristics of
metal-insulator-semiconductor (MIS) diodes (Al/bar- or spincoated PMMA (500 nm)/PTVPhI-Eh (30 nm)/Au) were measured (Figure S4). The bar- and spin-coated PMMA films on
PTVPhI-Eh exhibited similar leakage currents (∼10 μA mm−2 at
1.5 MV cm−1), which suggests that the bar-coated dielectric layer
can provide sufficient insulation for high-performance OFETs.
Figure 3 shows characteristics of bar-coated PTVPhI-Eh or
DPPT-TT OFETs. For the fabrication of OFETs, PTVPhI-Eh
or DPPT-TT as an active and PMMA as a gate dielectric layer
are sequentially bar-coated on a glass or plastic substrate with
patterned Au/Ni source and drain electrodes, respectively.
Fundamental transistor parameters, such as the field-effect
mobility (μFET) and the threshold voltage (VTh), were calculated
at the saturation region using gradual channel approximation and are summarized in Table 1.[26] The OFETs based on
spin-coated PTVPhI-Eh films with Au S/D contacts typically
showed ambipolar characteristics with dominant hole transport
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PMMA thin film on the conjugated polymer was very smooth
and uniform over the large area; in contrast, the surface of
the spin-coated PMMA film had large grooves with an average
peak-to-valley roughness of ∼25 nm. These striations are part of
the radial ridges and thickness undulations that generally result
from fluid motions due to capillary instability at the surface of
wetted thin films during the spin-coating process.[17] During the
spin-coating process, if two adjacent regions in the film have
different surface tensions, the region with higher surface tension actively pulls solutes from the other region. Therefore,
a film morphology develops with the higher surface-tension
regions becoming hills and the lower surface tension regions
becoming valleys. Generally, the phenomenon is more severe
when poor solvents are used; thus, this unfavourable surface
tension situation can be prevented by using properly selected
solvents. However, in this study, the choice of solvents for the
PMMA layer is strictly limited to marginal or slightly poor
solvents, such as n-butylacetate (nBA), in order to minimize
undesirable dissolution or swelling of the under-laid conjugated
polymer layer.[25] It should be noted that nBA is also not a good
solvent for PMMA; as a result, overnight heating at ∼80 °C with
strong agitation was required to completely dissolve the PMMA
in nBA. This can be one of the critical drawbacks for the formation of polymer dielectric layers using the spin-coating process.
Further, undesirable fluctuations of the dielectric surface cause
differences in the film thicknesses leading to device-to-device
Figure 3. Transfer characteristics of the bar-coating processed PTVPhI-Eh or DPPT-TT OFETs; (a) P-channel properties with bare Au, (b) N-channel
properties with a CsF-coated Au S/D electrodes, and (c) P-channel properties of DPPT-TT with bare Au. (d) Digital camera images of the bar-coated
PTVPhI-Eh thin film and (e) field-effect mobilities of the DPPT-TT OFET array on 4 inch size glass substrate. Statistics of the field-effect mobilities
measured for (f) 240 PTVPhI-Eh OFETs at the saturation region at Vd = -60 V and (g) 60 DPPT-TT OFETs at the saturation region at Vd = –80V.
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Table 1. Fundamental OFET characteristics of the TG/BC OFETs with glass substrate based on PTVPhI-Eh or DPPT-TT as an active layer and PMMA
(∼500 nm) as gate dielectric. The field-effect mobility measured in the saturation regime [PTVPhI-Eh: at Vd = ±60V, W/L = 1 mm/ 20 μm, DPPT-TT: at
Vd = -80V, W/L = 1 mm/ 10 μm].
Semiconductor
PTVPhI-Eh
DPPT-TT
Electrodes
Printing Method for
Active/Dielectric
μFET,h
[cm2V−1s−1]
VTh,h
[V]
μFET,e
[cm2V−1s−1]
VTh,e
[V]
Au
Bar-coating/
Bar-coating
0.24 (±0.057)
–32.3 (±2.5)
0.0081
55.9
CsF/Au
Bar-coating/
Bar-coating
0.078 (±0.07)
–54.8 (±3.5)
0.23
45.6
Au
Spin-coating/
Spin-coating
0.13 (±0.05)
–35.8 (±5.1)
0.005
54.6
Au
Bar-coating/
Bar-coating
1.64 (±0.41)
–41.6 (±2.5)
–
–
Au
Spin-coating/
Spin-coating
0.72 (±0.27)
–39.0 (±4.2)
–
–
(μFET,h = ∼0.13 cm2 V−1 s−1) and weak electron transport (μFET,e =
∼0.005 cm2 V−1 s−1). This unbalanced mobility is mainly attributed to the injection barrier from the Au electrode being larger
for the electron (LUMO: ∼3.5 eV) than for the hole (HOMO:
∼5.2 eV).[14] Figures 3a and 3b show the transfer characteristics
of the bar-coated PTVPhI-Eh OFETs at Vd = ±60 V; their output
characteristics are shown in Supporting Information (see
Figure S5). The bar-coated PTVPhI-Eh OFETs exhibited a significantly improved μFET,h (max. ∼0.46 cm2 V−1 s−1) and slightly
improved μFET,e (∼0.008 cm2 V−1 s−1) in the saturation region
at Vd = ±60 V. These improved characteristics of the bar-coated
PTVPhI-Eh OFETs devices are strongly correlated with the
improved surface crystallinity of the conjugated polymer, which
is verified by the AFM images in Figures 2a and 2b.
In order to investigate the yield and device-to-device performance uniformity we fabricated 1440 OFETs on 4 inch glass
or PEN plastic substrates with one bar-coating application of
the active and dielectric layers. The digital camera image of
the array of 1440 bar-coated transistors revealed excellent and
uniform film quality without any aggregation or uncoated
areas (Figure 3d). As shown in Figures 3e and 3f, the spatial
and statistical distributions of the μFET,h values were measured for 240 OFETs at a regularly spaced interval of 2.2 mm.
Regardless of the bar-coating direction, the OFETs had a high
and uniform μFET,h of ∼0.2–0.5 cm2 V−1 s−1 with standard deviation ∼23.8% (0.057 cm2 V−1 s−1) over the large area. The yields
of the bar-coated OFETs exceed ∼99% and negligible device
failure was observed. Using DPPT-TT as a state-of-the-art
polymer semiconductor,[27] we also fabricated a 1440 OFET
array on a 4 inch wafer substrate using a single bar-coating.
The bar-coated DPPT-TT OFETs showed very high and uniform μFET,h values with a maximum of 2.83 cm2 V−1 s−1 and
an average of 1.64 cm2 V−1 s−1 with standard deviation ∼25%
(0.41 cm2 V−1 s−1) from 60 OFETs in 10 × 10 cm2 substrate. In
addition, their corresponding Vth values exhibited 41.6 V with
standard deviation ∼6% (2.5 V). Statistical distributions of the
μFET,h and the Vth values randomly measured for 60 OFETs
are shown in Figure 3g and S7. The μFET,h values of bar-coated
DPPT-TT OFETs are much higher than those of spin-coated
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devices (an average of μFET,h = 0.72 cm2 V−1 s−1 with standard
deviation ∼37.5% (0.27 cm2 V−1 s−1) from 9 devices in 1.5 ×
1.5 cm2 substrate). In a consideration of printed area and the
number of devices, large area uniform, high performance
OFETs can be easily produced by simple bar-coating process.
Finally, we fabricated ambipolar complementary inverters
and ring oscillators (ROs) with bar-coated PTVPhI-Eh and
PMMA layers. Solution-processed ambipolar circuits can easily
be fabricated via a simple one-time coating without sophisticated micro-patterning of each p- and n-channel region.
Recently, our group successfully demonstrated complementary ambipolar inverters and ROs by selectively spray coating
of Cs-salt based interlayers, such as Cs2CO3 and CsF, in the
n-channel region between the semiconductor and the BC Au
S/D electrodes.[14] The Cs-salt interlayers enhances electron
injection as well as blocking hole injection; this enables fully
complementary inverters without Z-shaped voltage transfer
characteristics (VTCs), which are typically observed in ambipolar inverters because of current leakage through the pulldown transistors. To generate high-performance bar-coated
circuits, a 1.5 nm thick CsF interlayer was applied to the Au
bottom electrodes at only the n-channel transistor. Thereafter,
the semiconductor (PTVPhI-Eh, ∼30–40 nm thick) and gate
dielectric (PMMA, ∼500 nm thick) were sequentially applied via
bar-coating, followed by thermal deposition of an Al gate electrode. Figures 3c and S5 show the transfer and output characteristics of the PTVPhI-Eh OFETs after inserting the CsF interlayers (at Vd = 60 V). The n-channel characteristics dramatically
improved after incorporation of the CsF: The μFET,e increased by
∼30 times from 0.078 to 0.23 cm2 V−1 s−1, and the VTh,e significantly decreased by ∼10.3 V from 55.9 to 45.6 V. The bar-coated
ambipolar PTVPhI-Eh inverters with CsF interlayers exhibited
excellent VTCs without Z-shaped characteristics including an
inverting voltage (Vinv) near the theoretical switching point at
½VDD, high gain of more than ∼40, and high noise immunity
of more than 70% of the maximum value (½VDD) (Figure 4a–c).
Complementary ROs were fabricated to accurately analyze
the dynamic characteristics of bar-coated ambipolar ICs. Figures 4d and 4e show a CCD camera image of a five-stage RO
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Figure 4. (a) Voltage transfer characteristics, (b) corresponding voltage gains at various VDD from 60 V to 100 V, and (c) noise margin at VDD = 100 V
of the complementary inverter circuits using bar-coated ambipolar PTVPhI-Eh active layer and Au S/D contacts for p-channel TFTs and Au/CsF S/D
contacts for the n-channel TFTs. (d) CCD camera image of the fabricated 5-stage complementary RO and its circuit symbol. (e) Corresponding output
voltage oscillation at VDD = −100 V and oscillation frequency (fosc) and corresponding stage delay time of the ROs.
device fabricated using the bar-coating process and its VOUT
oscillation characteristics, respectively. The bar-coated ambipolar RO showed an fosc of ∼6 KHz at VDD = −100 V and a maximum fosc of ∼25 KHz at VDD = −160 V. The minimum stage
delay time (τ) between each complementary inverter stage was
determined to be ∼4 μs from the following equation: τ = 1/
(2nfosc), where n is the number of inverting stages (Figure 4e).
The fosc values of bar-coated PTVPhI-Eh ROs are two times
higher than those of spin-coated ROs;[14] this is due to the
higher μFET,e and μFET,h of the bar-coated OFETs.
In conclusion, we successfully demonstrated the large-area
TFT arrays and ambipolar complementary circuits by a simple
bar-coating process for depositing a large-area conjugated and
insulating polymer thin film for low-cost organic electronic
and optoelectronic devices. The bar-coated polycrystalline and
amorphous polymer films exhibited better crystallinity and
smoother morphology, respectively, than spin-coated ones. Further, bar-coated PTVPhI-Eh and DPPT-TT OFETs showed high
μFET,h values of 0.46 and 2.73 cm2 V−1 s−1, respectively, and good
device performance uniformity in a large-area TFT array. Furthermore, complementary organic inverters and ROs based
on the ambipolar PTVPhI-Eh semiconductor were fabricated;
the inverters exhibited high voltage gains (>40) and the ROs
showed a maximum fosc of ∼25 KHz. We believe that this simple
printing method is not only very effective for producing high
Adv. Mater. 2013, 25, 4302–4308
performance device and circuits, but also highly compatible
with roll-to-roll continuous manufacturing processes, which
will enable fully printed large-area electronic and optoelectronic
devices.
Experimental Section
Field-Effect Transistor Fabrication: The Au/Ni (15 nm:3 nm thick)
patterns used for the source and drain electrodes were fabricated using
a conventional photolithography procedure on Corning Eagle 2000 glass
and PEN (Tenjin Dupont Films) plastic substrates. The substrates
were sequentially cleaned in an ultrasonic bath with deionized water,
acetone, and isopropanol for 10 min each. Before coating with the
polymer solutions, the substrates were treated with oxygen plasma.
Ambipolar semiconductor PTVPhI-Eh was synthesized in our laboratory
using a previously published procedure,[14] and dissolved in anhydrous
chlorobenzene to obtain a ∼15 mg mL−1 solution. This solution was then
dropped onto one edge of the patterned substrate (15 cm × 15 cm).
After a meniscus formed on the solution as the bar was lowered, the
bar was horizontally transported at a constant velocity (10–100 mm s−1)
over the substrate to achieve a uniform coating of the polymer solution;
the solution was then dried in air. The semiconductor film was thermally
annealed at 110 °C for 20 min in a nitrogen-filled glove box. Polymer
dielectric PMMA was purchased from Aldrich and, without further
purification, was dissolved in nBA to generate 20, 40, and 80 mg mL−1
solutions. A PMMA thin film was formed on the PTVPhI-Eh layer via
the same bar-coating process followed by thermal annealing at 80 °C
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for 30 min in a nitrogen-filled glove box. The transistor fabrication was
completed by depositing Al top-gate electrodes (∼50 nm) via thermal
evaporation using a metal shadow mask.
Complementary Inverters and ROs Fabrication: The electrical contacts
(Au/Ni) of the inverters and ROs were patterned using photolithography,
as above. CsF was then thermally evaporated with a metal shadow mask
for selective patterning only onto the n-channel transistor regions.
Following the same procedure described above, the semiconductor and
PMMA gate dielectric layers were deposited via sequential bar-coating
and thermal annealing. To define the via-holes in the RO devices, pure
chlorobenzene was inkjet-printed to dissolve both the PTVPhI-Eh and
PMMA in the selected area. The top-gated complementary inverters
and ROs were completed by the application of gate electrodes on the
active regions of the transistors via the evaporation of a thin Al film
(50–500 nm thick) using a metal shadow mask.
Characterization: The surface morphology of the thin film was
investigated using tapping-mode atomic force microscopy (Nanoscope
III, Veeco Instrument, Inc) at the Korea Basic Science Institute. The
surface profile was measured using a surface profiler (Ambios, XP-1).
The absorption spectra were measured using a Lamda 750 UV/vis
spectrophotometer (PerkinElmer). The field-effect transistor electrical
characteristic and static characteristics of the complementary inverters
were measured using a Keithley 4200-SCS instrument in a nitrogen-filled
glove box. The μFET and VTh values were calculated at the saturation
region using the gradual channel approximation equation.[21] The VOUT
oscillation was measured using a built-in Keithley 4200-SCS oscilloscope.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
This research was financially supported by the Dongguk University
Research Fund of 2013 and a grant (code No. 2011-0031639) from the
Center for Advanced Soft Electronics under the Global Frontier Research
Program of the Ministry of Education Science and Technology, Korea,
the Industrial Strategic Technology Development Program (10041957,
the Design and Development of Fiber-Based Flexible Display) funded
by the Ministry of Knowledge Economy (MKE, Korea), the National
Research Foundation of Korea(NRF) grant funded by the Korea
government(MEST) (No. 2012-0008723)
Received: December 28, 2012
Published online: April 12, 2013
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Adv. Mater. 2013, 25, 4302–4308