Organic Thin-film Transistors Based on Polythiophene Nanowires ...

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<strong>Organic</strong> <strong>Thin</strong>-<strong>film</strong> <strong>Transistors</strong> <strong>Based</strong> on Polythiophene
Nanowires Embedded in Insulating Polymer
By Longzhen Qiu, Wi Hyoung Lee, Xiaohong Wang, Jong Soo Kim, Jung
Ah Lim, Donghoon Kwak, Shichoon Lee, and Kilwon Cho*
<strong>Organic</strong> thin-<strong>film</strong> transistors (OTFTs) have attracted considerable
attention because of their potential applications in large-area,
flexible, and printed electronics. To achieve OTFT devices with
desirable properties, recent research has primarily focused on
molecular design, [1,2] dielectric–semiconductor interfacial engineering,
[3–5] and device optimization. [6–11] The use of conjugated
polymer blends as active materials has brought a new way to tune
and optimize the electronic properties of devices; for example,
ambipolar field-effect charge transport has been reported in
binary blends of p- and n-type conjugated polymers or
oligomers. [12,13] Semiconducting and insulating polymer blends
have also attracted increasing interest, because they can combine
the electronic properties of semiconducting polymers with the
low cost and excellent mechanical characteristics of insulating
polymers. However, the presence of the insulating component
tends to degrade the device performance by diluting the current
density of the <strong>film</strong>. [14,15]
To the best of our knowledge, the only effective approach to
overcome this drawback is controlling the blended <strong>film</strong>s to form
vertically phase-separated structures, to keep the connectivity of
the semiconducting layer in the presence of insulating
components. In recent works, the composites with this structures
have been used in OTFTs to fabricate low-voltage-driven devices,
to improve environmental stability or reduce semiconductor
cost. [16–21] However, the phase-separation process in polymer
blends is very complicated. The final morphology in the blend
<strong>film</strong>s is highly sensitive to many factors, including the solvent
evaporation rate, solubility parameters, <strong>film</strong>–substrate interactions,
the surface tension of the components, and the <strong>film</strong>
thickness. Vertical phase separation can only take place under
extreme conditions. [22,23] Therefore, to develop a more facile and
general method for realizing high-performance, low-semiconductor-cost
devices is of great technological and academic
significance.
In this paper, we show that the percolation threshold of
semiconducting/insulating polymer blends can be drastically
decreased by depositing them from a marginal solvent with
temperature-dependent solubility. Morphology and crystallinestructure
studies reveal that the excellent electronic performance
of the devices derives from the efficient charge transport and the
[*] Prof. K. Cho, Dr. L. Qiu, W. H. Lee, X. Wang, J. S. Kim, J. A. Lim,
D. Kwak, Dr. S. Lee
Department of Chemical Engineering
Pohang University of Science and Technology
Pohang 790-784 (Korea)
E-mail: kwcho@postech.ac.kr
DOI: 10.1002/adma.200802880
good connectivity observed in highly crystalline, interconnected
nanofibrillar networks of semiconductors embedded in an
insulator matrix.
Semiconductor/insulator-blend mother solutions were prepared
by blending poly(3-hexylthiophene) (P3HT) and amorphous
polystyrene (PS) in dichloromethane (CH2Cl2), which is a
marginal solvent for P3HT. [24] To completely dissolve P3HT, the
CH2Cl2 solution was kept at approximately 40 8C. For comparison,
chloroform (CHCl3), which is a good solvent for P3HT, was
used as a reference. <strong>Thin</strong> <strong>film</strong>s with different P3HTand PS ratios
were fabricated on a silicon substrate, using spin-casting blend
solutions with a total concentration of 0.5 vol%.
Field-effect characteristics were measured in the bottomcontact,
bottom-gate thin-<strong>film</strong> transistor (TFT) geometry, using a
SiO2 <strong>film</strong> 300 nm thick as the dielectric layer. The dielectric
surface was a bare surface without any organosilane treatment.
Figure 1 shows a comparison of the field-effect characteristics of
the TFTs based on P3HT/PS blend <strong>film</strong>s obtained from CH2Cl2
and from a reference CHCl3 solution. Small drain currents (IDS)
are recorded for the transistors based on P3HT/PS (20:80) <strong>film</strong>s
spin-cast from a good solvent, (CHCl3 solution, see Fig. 1a),
whereas in the case of the 10:90 P3HT/PS devices fabricated
using a marginal solvent (CH 2Cl 2) the saturation drain current is
about 10 4 times greater (see Fig. 1a and b), even though the P3HT
content is lower.
The dependence of the field-effect mobility on the P3HTcontent
of both types of P3HT/PS blend <strong>film</strong>s is shown in detail in Figure 1c.
The mobilities of pristine P3HTdevices spun-cast from CHCl3 and
CH2Cl2 are 7.0 10 4 and 6.0 10 3 cm 2 V 1 s 1 , respectively.
This result is very similar to that reported by Yang et al. [24] Using
atomic force microscopy (AFM) and grazing-incidence X-ray
diffraction (GIXRD), they found that the improved electronic
properties of the sample obtained from CH 2Cl 2 solution arose from
the formation of highly ordered edge-on P3HTcrystals in the <strong>film</strong>s.
In consistence with previously reported results, [15,18] the mobility of
the blend <strong>film</strong>s spin-cast from a CHCl 3 solution decreases
monotonically as the P3HT content is decreased. Finally, a
percolation threshold is observed at a concentration about
20 wt%. On the other hand, the P3HT/PS blends obtained from
a CH 2Cl 2 solution show a much higher mobility than those
prepared from a CHCl3 solution (see Fig. 1a–c). The former blends
show a mobility of 4.0 10 3 cm 2 V 1 s 1 —comparable to that of
homo-P3HT <strong>film</strong>s prepared under the same conditions (i.e.,
6.0 10 3 cm 2 V 1 s 1 )—even at semiconducting-polymer contents
as low as 3 wt%. A decrease in the charge-carrier mobility is
only observed for those devices with P3HT contents below 3 wt%.
Furthermore, the observed percolation-threshold concentration is
below 1 wt%. These results are very close to the best results reported
Adv. Mater. 2009, 21, 1349–1353 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1349
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Figure 1. Field-effect transistor performance of P3HT/PS blends obtained from CH2Cl2 and
CHCl3 solutions. IDS is the drain–source current, VDS is the drain–source voltage, and VGS is the
gate–source voltage. a) Typical transfer characteristics of devices based on a P3HT/PS (10:90)
blend from a CH2Cl2 solution and a P3HT/PS (20:80) blend from a CHCl3 solution. b) Output
characteristics of a device based on a P3HT/PS (10:90) blend from a CH2Cl2 solution. c) Average
field-effect mobility measured in the saturation region as a function of the P3HT content in the
blends. d) Effect of the CHCl 3 content in the mixed CH 2Cl 2/CHCl 3 solvent on the field-effect
mobility of the P3HT/PS (10:90) blend.
for TFT devices based on semiconducting/insulating polymer
blends. [18] To further study the effect of solubility on the electronic
properties of semiconducting/insulating polymer blends, a
mixture of CHCl 3 and CH 2Cl 2 was used as solvent to prepare
blend mother solutions. It is clearly shown in Figure 1d that the
field-effect mobility of the blends monotonically decreases with
increase in CHCl3 concentration in the mixed solvent.
Transport of charge carriers within the blend <strong>film</strong> can only
occur in the semiconductor component. Therefore, the TFT
characteristics are extremely dependent on the morphology of the
P3HT phase. To investigate the morphology of this phase, we
selectively removed the PS phase by immersing the sample in
cyclohexane for 10 min. Cyclohexane is a good solvent for PS, but
does not dissolve P3HT. Scanning electron microscopy (SEM)
studies were performed to elucidate the morphology of the
remaining P3HT phase obtained from both CHCl3 and CHCl2
solutions. Figure 2a and b show SEM images of the P3HT phase
in samples spin-cast from a CHCl 3 solution at different P3HT
contents. A bicontinuous network of P3HT and PS is observed in
the blend <strong>film</strong> containing comparable amounts of the two
components (60:40, Fig. 2a). [15,25] In the case of the blends
containing 10 wt% P3HT (Fig. 2b), isolated spherical P3TH
domains with a diameter of about 100–500 nm are observed. The
field-effect characteristics and morphology results clearly reveal
that a decrease in the connectivity of the P3HTphase is the reason
for the degradation of the electronic properties upon reducing the
P3HT content in the P3HT/PS blend obtained from the CHCl 3
solution.
A dramatic change occurs if CH2Cl2 is employed as the solvent
during the sample-preparation procedure (see Fig. 2c and d).
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Here, a network composed of very long fibers
(with a typical width of 25–40 nm and a length
of several micrometers) has been observed.
Although the density of nanofibers decreases
at lower P3HT contents, an interconnected
network can be obtained, even at the P3HT
concentrations as low as 1 wt%. This interconnected
network ensures the connectivity of
conducting channels between source and drain
electrodes. We also studied the morphologies
of P3HT/PS (10:90) <strong>film</strong>s spin-cast from
CHCl3/CH2Cl2 mixtures with different CHCl3
concentrations, by means of SEM (see Fig. 2e
and f). Comparing these results with those
obtained for <strong>film</strong>s prepared from neat CHCl3
(Fig. 2b) and CH2Cl2 (Fig. 2c) solutions, it can
be seen that with a decrease in the CHCl 3
concentration in the mixed solvent (which also
means a decrease in the solubility), the P3HT
phase slowly changes from isolated, spherical
particles to an interconnected, nanofibrillar
network. Upon comparing the results of
P3HT/PS blends from CHCl3, CH2Cl2, and
their mixture, in terms of electronic properties
and morphological characteristics, it is reasonable
to conclude that the excellent electronic
properties of the P3HT/PS blend cast from the
CH2Cl2 solutions are derived from the formation
of a network composed of P3HT nanofibers
with high aspect ratio. This result is further confirmed by the
relatively small change observed in the field-effect performance of
the blends upon etching of the PS phase (see Fig. S1, Supporting
Information).
We also carried out GIXRD measurements on the blend <strong>film</strong>s,
to further investigate the crystalline structure and molecular
orientation of their P3HTphase. Figure 3a shows a comparison of
the 2D GIXRD patterns of P3HT/PS (10:90) <strong>film</strong>s spin-cast from
both CH2Cl2 and CHCl3 solutions. It can be seen that the pattern
of the CHCl3-based blend exhibits a typical halo attributed to
amorphous PS, as well as a very weak and broad (100) diffraction
band from P3HT. Conversely, the CH2Cl2-based <strong>film</strong> shows
rather sharp and well-resolved P3HT (100) and (200) diffractions.
Clearly, the crystallinity of the P3HT phase in the CH2Cl2-based
<strong>film</strong>s is much higher than that of the CHCl 3-based <strong>film</strong>s. The
observed (l00) peaks of P3HT, with higher-order peaks along the
qz axis, suggest that the P3HTmolecules adopt edge-on structures
with their (100) axis normal to the substrate, which is extremely
beneficial for charge transport in the TFTdevices. [4,26] This results
agrees well with the nanofibrillar morphology observed using
SEM.
The formation of P3HT nanofibers by changing the solubility
has been previously reported in the literature. [27,28] It was reported
that these nanofibers showed excellent 1D charge-transport
characteristics along the nanofiber axis, because the P3HT chains
pack a lamellar structure, with 2D conjugated sheets formed by
interchain stacking perpendicular to the nanofiber axis. [29–33] In
ourcase, CH 2Cl 2 is amarginal solvent forP3HT, and itssolubility is
strongly temperature dependent. Figure 3b shows the temperature-dependent
UV–vis absorption spectra of this solution. At
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Figure 2. SEM images of P3HT/PS <strong>film</strong>s with various P3HT contents, prepared by means of spincasting
from solutions of CHCl3, CH2Cl2, and their mixture, after selectively dissolving PS with
cyclohexane.a,b)Filmsspin-castfromCHCl3withdifferentP3HTcontents:a)60wt%andb)10wt%.
c,d) Films spin-cast from CH2Cl2 with different P3HT contents: c) 10 wt% and d) 5 wt%, e,f) P3HT/
PS(10:90) blendsspin-castfromaCHCl3/CH2Cl2 mixture:e)5:95andf) 10:90.The scale baris1 mm.
room temperature, we observed obvious absorption bands at
l ¼ 515, 550, and 600 nm (these signals are typical of highly
ordered P3HT chains), [34,35] whereas the absorption band at
l ¼ 445 nm—attributed to dissolved individual P3HT chains—is
quite weak. These results suggest that large portions of the P3HT
molecules in the CH2Cl2 solution have solidified at room
temperature, to form nanofibers. As the temperature increases,
the intensities of the low-energy absorption bands decrease, and
the maximum absorption blue-shifts to l ¼ 445 nm, thus indicatingthattheP3HTchains
aregradually dissolved.Therefore, whena
warm CH 2Cl 2 solution (at 40 8C) containing P3HT and PS is
dropped onto a SiO2/Si substrate held at room temperature, the
P3HT immediately solidifies from the solution and self-assembles
(as a result of the change in solvent solubility), thereby forming
well-interconnected nanofibers. On the other hand, the PS
component still remains liquid at this stage (due to its good
dissolving ability in cold CH2Cl2), and can only solidify after
removal of most of the solvent during the spin-coating process. As a
result, the P3HT nanofibers are expected to be buried in a PS
matrix. AFM images of the top surface, the bottom surface, and the
interface (after PS etching) of P3HT/PS (10:90) clearly confirm this
structure (see Fig. 4a). Figure 4b shows a schematic representation
of the formation of embedded P3HT nanofibers in a PS matrix
during the fabrication process.
Environmental instability poses a crucial
restriction in the application of OTFTs. Encapsulating
the active semiconductor with an
insulating-polymer layer has proven to be an
effective method to improve the environmental
stability. [17,36] In the P3HT/PS blend obtained
from CH2Cl2 solutions, the P3HT nanofibers
are embedded in an insulating PS matrix, which
renders this structure extremely beneficial for
improving the environmental stability of the
devices. Figure 5a shows a comparison of the
transfer characteristics of TFTs based on
pristine P3HT and a P3HT/PS (10:90) blend
after different exposure times to air. The
degradation in the performance of the pristine
P3HT-based device is very pronounced. After
one week of exposure to air, the threshold
voltage increases from 17 to 68 V, and the
current on/off ratio decreases from 10 3 to 10.
On the contrary, the stability is dramatically
improved in the device based on P3HT/PS
(10:90). The on and off currents remained
almost unchanged after 7 days, and the shift in
the threshold voltage (from 10 to 20 V) was far
below that observed in the pristine P3HTdevice.
In contrast to the rigorous conditions
involved in the preparation of vertically
phase-separated structures (e.g., the need for
suitable substrate-surface energy, polymer
pairs, and/or processing temperature), [17,18,21]
the solubility-induced formation of embedded
P3HT nanofibrillar structures is facile and
reproducible. Therefore, these blends are
Figure 3. a) 2D GIXRD patterns of P3HT/PS (10:90) <strong>film</strong>s spin-cast from
CHCl3 (left) and CH2Cl2 (right) solutions. The inset shows 1D out-of-plane Xrayprofilesextractedfromthe2Dpatterns.b)Absorptionspectraofa0.5
vol%
P3HT/PS (5:95)solutioninCH2Cl2 as a functionofthe temperature.The inset
shows photographs of the solution at about 40 8C and at room temperature.
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Figure 4. a) AFM topography (top) and phase (bottom) images for a P3HT/PS (5:95) blend spincast
from a CH2Cl2 solution: top surface (left), bottom surface (middle), and interface after
selectively dissolving PS (right). The scale bar is 500 nm. b) Schematic representation of the
formation of a nanofibrillar network in the PS matrix.
suitable for the fabrication of flexible TFTs on plastic substrates
via solution processing (see Fig. 5b).The average field-effect
mobility of the flexible TFTs at the saturation regime is
4.8 10 3 cm 2 V 1 s 1 , which is similar to the mobility of the
devices obtained on silicon substrates.
In conclusion, this work demonstrates that the use of a
marginal solvent (namely, CH2Cl2) renders it possible to achieve
an electronic performance comparable to that found for pristine
P3HT in blends comprising P3HT and amorphous PS, at P3HT
content as low as 3 wt%. SEM and GIXRD studies disclose that
Figure 5. a) Changes in the transfer characteristics of the device based on pure P3HT (left) and
P3HT/PS (10:90, right) <strong>film</strong>s (spin-cast from CH2Cl2 solutions) upon exposure to air. b) Fieldeffect
characteristic of a flexible TFT device based on a P3HT/PS (10:90) blend spin-cast from
CH2Cl2. The insets at the left- and right-hand side of b) show a schematic cross-section and a
digital-camera image of a P3HT/PS TFT on a flexible substrate, respectively.
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these excellent electronic properties arise from
a network of highly crystalline P3HT nanofibers
embedded in an insulating PS matrix.
This structure allows good encapsulation of the
active P3HT nanofibers, which significantly
improves the environmental stability of the
devices. In addition, this simple, mild, and
reproducible method is suitable for the
fabrication of large-area, flexible electronic
devices. This nanofibrillar structure is quite
different to previously reported vertically
phase-separated structures, and opens a new
route to the fabrication of OTFTs with low
semiconductor cost, high environmental stability,
and good mechanical properties.
Experimental
Regioregular P3HT (Mw ¼ 40 kg mol 1 ) was
obtained from Rieke Metals Inc. Amorphous PS
(Mw ¼ 240 kg mol 1 ), poly-4-vinylphenol (PVP,
Mw ¼ 20,000 g mol 1 ), poly(melamine-co-formaldehyde), methylated
(PMF, Mw ¼ 511 g mol 1 ), propylene glycol monomethyl ether acetate
(PGMEA), chloroform, dichloromethane, and cyclohexane were purchased
from Aldrich Chemical Co. All materials were used as received without
further purification.
Field-effect transistors with bottom-gate bottom-contact configuration
were fabricated using heavily doped n-type Si wafers as the gate
electrodes—with a thermally grown silicon dioxide (SiO2) layer 300 nm
thick (capacitance ¼ 10.8 nF cm 2 ) as the gate dielectric. The gold source
and drain electrodes (100 nm on a 2 nm adhesion layer of titanium) were
deposited through shadow mask. The channel length and width were fixed
at 100 and 800 mm, respectively. P3HT/PS blended <strong>film</strong>s with different
P3HT content were achieved by spin-coating CHCl 3
and warm CH 2Cl 2 solutions (0.5 vol%). The samples
for GIXRD measurements were spun-cast on bare Si
wafers with about 2 nm SiO2 at the same conditions.
To obtain the flexible transistors, a water-based ink of
the conducting polymer, poly(3,4-ethylenedioxythiophene),
doped with PS sulfonic acid (PEDOT/PSS,
Bayer AG) was spin-coated as gate electrode onto a
polyarylate (PAR, Ferrania Technologies) sheet. Then,
a dielectric layer 600 nm thick (capacitance ¼ 6.6 nF
cm 2 ) was formed by spin-coating ( 2000 rpm) a
PGMEA solution comprising 11 wt% PVP and 8 wt %
PMF onto the PAR sheet, with subsequent crosslinking
for 1 h at 180 8C in a vacuum oven. After
evaporating the gold source and drain electrodes, a
<strong>film</strong> was spin-coated from a 0.5 vol% CH 2Cl 2 solution
of P3HT/PS (10:90) blend.
The electrical characteristics of the TFT devices
were measured in the accumulation mode using
Keithley 4200 and 236 source/measure units at room
temperature and under ambient conditions. The
capacitance was determined using an Agilent 4284
precision LCR meter. The <strong>film</strong> morphologies were
characterized using an atomic force microscope
(Digital Instruments Multimode) operating in the
tapping mode and a field-emission scanning electron
microscope (Hitachi S-4800). Two-dimensional
GIXRD measurements were performed on the 4C2
beamlines at the Pohang Accelerator Laboratory
(PAL) in Korea. Solution-state UV–vis absorption
spectra were recorded at various temperature with a
1352 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2009, 21, 1349–1353

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heating rate of 0.1 8C min 1 (from room temperature to 41 8C) using an
Agilent 8453(HP) UV–vis spectrophotometer equipped with a temperature-control
system.
Acknowledgements
This work was supported by a grant (F0004021-2008-31) from the
Information Display R&D Center under the 21st Century Frontier R&D
Program, Creative Research Initiative-Acceleration Research (R17-2008-
029-01000-0), the ERC Program of KOSEF (R11-2003-006-06004-0), the
Regional Technology Innovation Program (RTI04-01-04), and the POST-
ECH Core Research Program. The authors thank the Pohang Accelerator
Laboratory for providing the synchrotron radiation sources at 4C2, 8C1,
and 10C1 beam lines used in this study. Supporting Information is available
online from Wiley InterScience or from the author.
Received: September 29, 2008
Revised: November 13, 2008
Published online: December 30, 2008
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