Simultaneous Improvement of Hole and Electron Injection in Organic Field-effect Transistors by Conjugated Polymer-wrapped Carbon Nanotube Interlayers

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received: 24 November 2014

accepted: 13 April 2015

Published: 22 May 2015

# Simultaneous Improvement of Hole and Electron Injection in Organic Field-effect Transistors by Conjugated Polymer-wrapped Carbon Nanotube Interlayers

Seung-Hoon Lee 1 , Dongyoon Khim 2, † , Yong Xu 2 , Juhwan Kim 1 , Won-Tae Park 2 ,
Dong-Yu Kim 1 & Yong-Young Noh 2

Efficient charge injection is critical for flexible organic electronic devices such as organic light-emitting
diodes (OLEDs) and field-effect transistors (OFETs). Here, we investigated conjugated polymer-wrapped
semiconducting single-walled carbon nanotubes (s-SWNTs) as solution-processable charge-injection
layers in ambipolar organic field-effect transistors with poly(thienylenevinylene-co-phthalimide)s. The
interlayers were prepared using poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) or poly(9,9dioctylfluorene) (PFO) to wrap s-SWNTs. In the contact-limited ambipolar OFETs, the interlayer led
to significantly lower contact resistance ( _Rc_ ) and increased mobilities for both holes and electrons.
The resulting PTVPhI-Eh OFETs with PFO-wrapped s-SWNT interlayers showed very well-balanced
ambipolar transport properties with a hole mobility of 0.5 cm 2 V -1 S -1 and an electron mobility of
0.5 cm 2 V -1 S -1 in linear regime. In addition, the chirality of s-SWNTs and kind of wrapping of conjugated
polymers are not critical to improving charge-injection properties. We found that the improvements
caused by the interlayer were due to the better charge injection at the metal/organic semiconductor
contact interface and the increase in the charge concentration through a detailed examination
of charge transport with low-temperature measurements. Finally, we successfully demonstrated
complementary ambipolar inverters incorporating the interlayers without excessive patterning.

Charge injection from metal electrodes into organic semiconductors (OSCs) is one of the critical
issues in realizing high-performance organic electronic devices, because most of these devices contain
Schottky-type metal-semiconductor contacts 1,2 . In conventional silicon-based devices, heavily doping
in electrodes enables the fabrication of Ohmic contacts, permitting efficient charge injection. However,
such an efficient doping technique on an atomic scale is unfeasible for organic devices 3–5 . Therefore,
the performance of various organic devices (e.g., organic light-emitting diodes (OLEDs), and organic
field-effect transistors (OFETs)) is severely limited by the poor injection properties. 2,3,6,7 For instance, the
large Schottky barriers raise the threshold voltage (Vth) of OFETs and subsequently the operating voltage

1Department of Nanobio Materials and Electronics, School of Material Sciences and Engineering, Heeger Center
for Advanced Materials, Gwangju Institute of Science and Technology (GIST), 261 Cheomdan-gwagiro (Oryongdong), Buk-gu, Gwangju 500-712, Republic of Korea. 2 Department of Energy and Materials Engineering, Dongguk
University, 26 Pil-dong, 3-ga, Jung-gu, Seoul 100-715, Republic of Korea. † Current adress: Department of
Physics and Centre for Plastic Electronics, Blackett Laboratory, Imperial College London, London, SW7 2AZ, UK.
[Correspondence and requests for materials should be addressed to D.-Y.K. (email: kimdy@gist.ac.kr) or Y.-Y.N.](mailto:kimdy@gist.ac.kr)
[(email: yynoh@dongguk.edu)](mailto:yynoh@dongguk.edu)

and power consumption of the composed integrated circuits (ICs) 2,8 . With downscaling for high-density
integration and high-speed operation, the contact limitations become more and more serious than in
normal-sized devices 2,3,9 . As the channel length ( _L_ ) of transistors shrinks, the channel transport becomes
more “current demanding,” but the charge injection/extraction capability at the metal/semiconductor
interfaces does not increase in proportion to _L_ . As a result, injection-related limitations start to distort
the overall transport characteristics and gradually become a crucial problem for device downscaling. In
addition to the large Schottky barrier, inefficient charge injection could also arise from significant charge
trapping, structural OSC disorders, and additional effects induced by charge transfer at the contacts in
downscaled devices 10 .

A number of approaches have been developed to optimize the charge-injection properties at the
metal–OSC interface, most of them related to the modification of interface electrical or chemical properties 11 . The interface-modification techniques can be classified into three types.

The first involves tuning the work function (ΦWF) of metal electrodes through the interface dipole by
using self-assembled monolayers (SAMs) or other polymer interlayers. The SAMs of thiols and aliphatic
polymers with amine groups (e.g., polyethylenimine ethoxylated (PEIE) and branched polyethylenimine
(PEI)) as surface modifiers have been widely applied in this respect 12–15 . However, the SAM approach is
limited in its application to specific metal electrodes such as gold and silver, and it is not appropriate for
conjugated polymers that necessitate a high annealing temperature of over 200 °C to improve crystallinity. In addition, it is difficult to guarantee high device uniformity over a large area using polymer interlayers containing amine functional groups, because the optimum thickness of the polymeric interlayers
is very thin (~2 nm) due to their insulating properties 15,16 .

The second method involves doping OSCs selectively at the semiconductor–electrode interface.
Chemical doping at the interface increases the conductivity of OSCs locally and effectively reduces
charge-trapping effects, as the Fermi level is shifted toward the corresponding transport bands, filling the
resident trap states therein. Moreover, with greater charge concentrations, the thickness of the depletion
region within the OSC at the metal–OSC interface is significantly decreased. Thus, the more efficient field
emission (tunneling) replaces thermionic emission through a large Schottky barrier and starts to contribute predominately to the overall charge injection 17,18 . Note that doping increases the “OFF” current and
decreases the ON/OFF ratio of OFETs. In addition, the exact doping mechanism, and the concentration
and location of the dopant in OSCs are still not well understood. Thus, more comprehensive research is
needed to apply these techniques for further optimization of devices.

The last method involves introducing metal-oxide interlayers to the metal–OSC interface. The barrier
height of charge injection from the source/drain electrodes to the active OSC layers must be effectively
reduced by altering the interface dipoles or weakened by gap state-assisted hopping 19–22 . However, most
metal-oxide-based interlayers require high-vacuum-based deposition processes with high-temperature
annealing processes and degrade the ambient stability of devices in particular interlayers for electron
injection 20,21 .

Until now, conventional interlayers have generally been applied to improve charge injection for only
one-polarity charge carriers, either holes or electrons, since the interlayers change the ΦWF of metal electrodes in one direction, either high or low. Interlayers that can enhance both hole and electron injection
are very rare, because the underlying mechanisms of these interlayers are rather different from the conventional interlayers that tune ΦWF of metal electrodes by inducing interface dipoles and/or introducing
dopants close to the interface. However, a technique that improves the injection of both charge carriers
is needed for ambipolar-based sensors 23, ICs 24,25, and ambipolar light-emitting field-effect transistors
(FETs) 8 . Xie _et al._ recently reported enhanced injection of both holes and electrons in organic single-crystal
transistors using Au/single-walled carbon nanotube (SWNT) electrodes 10 . The one-dimensional (1D)
structure of SWNTs creates a strong electrostatic field that is known in SWNT-based FETs and has
been evidenced to facilitate charge tunneling through Schottky contacts without producing large energetic barriers. Therefore, the use of SWNT electrodes for ambipolar charge injection is attractive 26,27 .
Recently, several research groups reported a method of obtaining highly purified semiconducting SWNT
(s-SWNT) dispersion through the wrapping of conjugated polymers 28–32 . More recently, using this sorting method, Gwinner _et al._ reported ambipolar light-emitting FETs with enhanced light emission intensity by adding a small amount of s-SWNTs to the semiconducting layer 8 .

In this paper, we report a simple but effective method of improving both hole and electron injection in ambipolar OFETs with poly(thienylenevinylene-co-phthalimide)s (PTVPhI-Eh) (Fig. 1b)
using a conjugated polymer-wrapped s-SWNT as solution-processable charge-injection interlayers. The solution-processed interlayers are fabricated by selective wrapping of poly(9,9-di-n-octylfl
uorene-alt-benzothiadiazole (F8BT) or poly(9,9-dioctylfluorene) (PFO) on the surface of s-SWNT with
a specific chirality through π –π interaction. The hole and electron mobilities are significantly enhanced
(by approximately 3–100 times) by inserting an F8BT-wrapped s-SWNT interlayer (F8BT:s-SWNT)
and PFO-wrapped s-SWNT interlayer (PFO:s-SWNT) through much-reduced contact resistances ( _Rc_ ),
leading to well-balanced ambipolar transport. After a detailed investigation of charge transport by
low-temperature measurements and with mobility calculations based on a solid transport model, we
found that the Au–interlayer–OSC interface provides much better charge injection for both holes and
electrons as compared to the pristine Au–OSC interface, highlighting the importance of the interlayers
for ambipolar OFET applications.

Figure 1. ( a ) Schematic of TGBC OFET device structure, ( b ) Molecular structure of PTVPhI-Eh, ( c )
Optical absorption spectra of as-dispersed F8BT (5 mg/ml)- and ( d ) PFO (1 mg/ml)-wrapped s-SWNTs in
toluene solvent after mild centrifugation.

Results
A top-gate and bottom-contact (TGBC) staggered device structure (Fig. 1a) was used, because the interlayer can be applied on the pre-patterned Au source and drain electrodes by a simple solution process
and a semiconducting layer can be effectively passivated by the overlying gate dielectric and gate electrode layer to minimize degradation of the device by oxygen and moisture in the air. The interlayer
solution was prepared by selectively wrapping s-SWNTs using F8BT or PFO, similar to the previously
reported processes involving only simple sonication followed by a mild centrifugation step 33,34 . Next,
2 mg of SWNT produced by high-pressure carbon monoxide (HiPCO) method and 25 mg of F8BT or
5 mg of PFO were mixed in 5 ml toluene for the dispersion process. After tip sonication and the mild
centrifugation step (10,000 g for 30 min), the interlayer solutions were obtained. These procedures are
described in more detail in the method section. This method of s-SWNT sorting with a low-speed centrifugation is easy and efficient, but the application of this technique in the literature is very rare, because
the excess polymer is too much to have a negligent effect on device performance 35 . As a result, these
s-SWNTs sorted by conjugated polymer wrapping require further processes to remove the excess polymers and obtain high purity s-SWNT (i.e., ultracentrifugation or filtration) for FET applications as active
layers 8,28–30,35 . Note that unlike most studies, we directly apply SWNTs wrapped by F8BT or PFO solution using a mild centrifugation as contact interlayers between source/drain electrodes and OSC layers.
Fig. 1(c) and (d) show the absorption spectra of F8BT- and PFO-wrapped SWNTs dispersed in toluene
obtained using this approach. As previously reported, F8BT selectively wraps (9,4) and (10,5) s-SWNTs,
and PFO is selective for five different s-SWNT species corresponding to (7,5), (7,6), (8,6), (8,7), and
(9,7) 33,34 . In order to determine the concentrations of s-SWNTs in PFO:s-SWNT or F8BT:s-SWNT solution, we used the absorption cross-section ( _c.a._ 1.7 × 10 −17 cm 2 ) for the E 11 absorption peak of s-SWNT 36 .
The concentration of s-SWNTs was approximately 1.2 μ g/ml for both F8BT:s-SWNT and PFO:s-SWNT
solution, because the optical densities of the two interlayer solutions were similar ((10,5) s-SWNT in
F8BT:s-SWNT and (8,6) in PFO:s-SWNT ≈ 0.6). On the other hand, we intentionally applied different
concentrations of F8BT (5 mg/ml) and PFO (1 mg/ml) to obtain the similar concentrations of s-SWNTs
in the interlayer solutions, since the two polymers showed different wrapping efficiencies.

As shown in Fig. 2, the transfer curves of PTVPhI-Eh OFETs with PFO:s-SWNT and F8BT:s-SWNT
interlayers exhibited remarkably improved _p_ - and _n_ -channel characteristics in OFETs. The incorporation
of the interlayers effectively improved all basic parameters of transistors, including both hole and electron
mobility ( _μ_ h and _μ_ e), ON current and _V_ th, and small subthreshold swing ( _SS_ ). Details are summarized in
Table 1. In the linear regime ( _V_ ds = - 5 V) and saturation regime ( _V_ ds = - 60 V), the average _μ_ h was much

Figure 2. _P_ -channel and _n_ -channel transfer characteristics in linear ( a,b ) and saturation ( c,d ) regimes of
ambipolar PTVPhI-Eh OFETs with s-SWNTs wrapped by F8BT or PFO as a charge-injection interlayer
( _L_ = 20 μ m, _W_ / _L_ = 50, _C_ i = 6.2 nFCm −2 ).

higher than in devices with bare Au electrodes. More significant enhancements were found in _n_ -channel
characteristics. The linear _μ_ e of PTVPhI-Eh OFETs was increased by over 100 times by applying the
PFO:s-SWNT and F8BT:s-SWNT interlayers with respect to the reference pristine devices. Interestingly,
the linear mobilities show higher than saturation mobility, which indicates contact resistances were significantly reduced in the devices. In addition, the lower _V_ th and steeper _SS_ originated from the enhanced
charge injection. That is, the steeper _SS_ originated from the reduction of capacitance at contact caused
by the injection barrier between the metal electrode and the semiconductor 22 . The performance of the
best devices was comparable to that of high-performance _n_ -type polymer OFETs 37 . To check the generalizability of our interlayer functionality, we applied the interlayer to a commonly used _n_ -type OSC,
phenyl-C61-butyric acid methyl ester (PCBM), which showed similar enhancements in transistor characteristics (see Fig. S1 in Supplementary Information). This result indicates that our interlayers generally
can improve charge injection in OFETs.

To check that the interlayers themselves do not act as conductive channels, we fabricated interlayer-only
devices without an OSC layer. When F8BT:s-SWNT film (20 nm) of the same thickness as the OSC film
was applied as a semiconducting layer in TGBC transistors, the devices showed extremely poor characteristics with 2.1 × 10 −4 (for holes) and 3.4 × 10 −5 cm 2 /V −1 s −1 (for electrons) due to less dense percolation (see Fig. S2(a) and S2(b)) and the location of active channel. In the TGBC transistor, all charge
carriers transport close to the semiconductor-dielectric interface. That is located at the top surface of
the semiconductor film whereas s-SWNT network is located at the bottom of the semiconductor film.
Moreover, the very low “OFF” current of PTVPhI-Eh OFETs seen in Fig. 2(a),(b) confirms that there are
no percolation paths through s-SWNTs in the interlayer matrix. Note that the increased “OFF” current
with the interlayers often observed in the saturation regime is due to the more balanced ambipolarity
caused by the increase of _n_ -channel characteristics. In general, the ON/OFF ratios in ambipolar OFETs
are significantly low and strongly dependent on _V_ ds4,38. To explore the effects of metallic SWNTs on the

interlayers, we dispersed SWNTs using poly(9,9-dioctylfluorene-co-bithiophene (F8T2), which is known

P-channel N-channel

Interlayer μh (cm 2 V -1 S -1 ) Vth SS (V/dec.) μe(cm 2 V -1 S -1 ) Vth SS V/dec.)

Saturation Linear Saturation Linear

Aver. Max. Aver. Max.

Without interlayer 0.11 (± 0.01) 0.13 0.05 - 45.62 (± 5.3) 2.6 (± 0.28) 0.001 (± 0.0004) 0.002 6.5× 10 −3 47.2 (± 2.2) 4.98 (± 1.13)

F8BT:s-SWNT (5mg/ml) 0.24 (± 0.05) 0.3 0.06 - 21.65 (± 1.01) 0.94 (± 0.32) 0.16 (± 0.03) 0.22 0.26 35.28 (± 1.59) 2.88 (0.59)

PFO:s-SWNT (1mg/ml) 0.34 (± 0.03) 0.38 0.5 - 18.73 (± 1.25) 0.93 (± 0.3) 0.11 (± 0.03) 0.16 0.53 31.28 (± 2.08) 2.6 (± 0.56)

Table 1. Fundamental parameters of TGBC OFETs based on PTVPhI-Eh and after deposition of F8BT:sSWNT and PFO:s-SWNT onto Au S/D electrode*. *μh and _μ_ e refer to the field-effect mobility measured in the
linear ( _V_ ds =                  - 5 V for holes, and _V_ ds = 20 V for electrons) and saturation ( _V_ ds = ± 60 V for holes and electrons)
regimes. The threshold voltages (Vth) and subthreshold swing ( _SS_ ) were extracted from the linear regime.
The OFET saturation mobilities extracted from standard FET I-V equation (see experimental section) and
listed in this table are averages of at least five devices ( _L_ = 20 μ m and _W_ = 1 mm), and the error bars denote
the standard deviations. The OFET linear mobilities extracted at from M-TLM equation which in contact
resistances are reflected. The Vg-Vth in p-channel of bare, F8BT:s-SWNT and PFO:s-SWNT are each − 13, −9
and − 12 V. The Vg-Vth in n-channel of bare, F8BT:s-SWNT and PFO:s-SWNT are each 13, 15 and 19 V.

Figure 3. Output characteristics of PTVPhI-Eh OFETs with pristine gold electrodes ( a, b ), with F8BT:sSWNT contacts ( c, d ), and with PFO:s-SWNT contacts ( e, f ).

to disperse both metallic SWNTs and s-SWNTs without selectivity 34 . Most of the devices with SWNT
dispersed by F8T2 interlayer (F8T2:SWNT) exhibited metallic behaviors in the transfer characteristics
due to the abundance of metallic SWNTs (Fig. S3(c)). Although one or two of nine devices showed
enhanced characteristics with F8BT:s-SWNT and PFO:s-SWNT, the device stability in the cycling test
was much lower than those OFETs with F8BT:s-SWNT (Fig. S4).

Figure 4. Tapping mode AFM images of F8BT:s-SWNT ( a, b ) and PFO:s-SWNT ( c, d ) thin films on
Au after spin-coated chlorobenzene (CB) solvent (2000 rpm, 60 sec). Images on the left and right show
topographic and phase mode images, respectively.

Fig. 3 illustrates the output characteristics of ambipolar PTVPhI-Eh OFETs with Au or the interlayer
coated on Au contact. The well-balanced ambipolar characteristics were observed from the output symmetry between _p_ - and _n_ -channel operations by applying the interlayer (Fig. 3c–f). When Fermi energy
level of metal electrode was located in center position between HOMO and LUMO, the ambipolar injection become more balanced and efficient. Because of these reasons, it is difficult to be perfect Ohmic
behavior in one polarity. Therefore, some output curves show the non-linear behavior. However, the
current at small Vds (i.e., in linear region) is considerably enhanced with respect to the pristine devices,
indicating the much ameliorated charge injection and decreased Rc.

Fig. 4 shows the surface morphologies of the F8BT:s-SWNT and PFO:s-SWNT interlayers after the
spin-coating of chlorobenzene (CB), which was used as the solvent of OSCs. After deposition of the interlayers onto Au followed by thermal annealing at 200 °C, CB was spin-coated on top of the interlayers. To
check the wash off of F8BT and PFO in the interlayers during the deposition of the OSC, we compared the
thicknesses of the interlayers before and after spin-coating of the CB solvent. After spin-coating of the CB
solvent, the thicknesses of F8BT:s-SWNT and PFO:s-SWNT were slightly changed from 15 to 6 nm and
5 to 3 nm, respectively. Before washing by CB, the morphologies of the F8BT:s-SWNT and PFO:s-SWNT
films showed several aggregated features, probably due to the SWNTs in the interlayer films (Fig. S5).
In addition, we could not find any SWNT bundles at the sample surface before spin-coating of the CB
solvent (Fig. S5), but SWNT bundles were distinguishably observed at the surface after spin-coating
of the CB solvent (Fig. 4). The disclosure of SWNT at the surface is caused by the solubility difference
between SWNTs and wrapping conjugated polymers to CB solvent. SWNTs produced by the HiPco
process showed very low solubility to 1,2-dichlorobenzene and chloroform (95 and 31 mg·L −1 ) at room
temperature, much lower than the solubility of PFO and F8BT (5–10 mg·mL −1 ) 8,39–41 . Thus, during the
spin-coating of OSC, the F8BT and PFO on the s-SWNTs were naturally washed out at the interlayer
surface, and the over-coated PTVPhI-Eh semiconducting film could obtain direct contact with s-SWNTs,
which is crucial to improving both hole and electron injection. Furthermore, the morphological change

Figure 5. _P_ -channel and _n_ -channel transfer characteristics of OFETs with PFO:s-SWNT ( a, b ) and F8BT:sSWNT ( c, d ) depending on SWNT wrapping polymer concentration.

of OSC between before and after deposition of interlayer did not observed (Fig. S5). The improvement
of transistor characteristics is not due to morphological change of OSC.

To investigate the concentration effects of interlayer, F8BT or PFO solutions were prepared with
various concentrations in toluene solvent for wrapping s-SWNT (Fig. S6). In this process as the concentration of the polymer solution increased, the number of wrapped s-SWNTs also increased (Fig. S6).
However, higher concentrations of the polymer solution do not have much more s-SWNTs weight per
polymer weight. In this article, s-SWNTs ratio in the hybrid solution is simply defined as below;

_s_ − _SWNTs_ _weight_ ( _mg_ ) _in_ _solution_
_s_ - _SWNTs_ _ratio_ (%) = × 100

_polymer_ _weight_ ( _mg_ ) _in_ _solution_ (1)

_s_ − _SWNTs_ _weight_ ( _mg_ )

- _SWNTs_ _ratio_ (%) =

× 100

_polymer_ _weight_ ( _mg_ ) _in_ _solution_ (1

×
( _mg_ ) _in_ _solution_

Fig. 5 shows the transfer curves of the OFETs containing s-SWNT interlayers obtained from polymer
solution with various concentrations. The FET characteristics depend clearly on s-SWNTs ratio in the
interlayer solution. More details are summarized in Table 2. When the PFO:s-SWNT solution were
prepared with PFO of 1, 2, and 5 mg·ml −1, the s-SWNT ratio of PFO solution were 0.04, 0.02, and
0.03%, respectively. As the s-SWNT ratio increases in the solution compared to other solutions, the
OFET performance is systematically improved. Similar results are observed in the OFETs incorporating
F8BT:s-SWNT. For the most interlayers, the transistor’s contact properties are improved by increase of
the thickness of the semiconductor interlayer up to a certain thickness and then it turns to degrade 20 .
Unfortunately, in our system the exact interlayer thickness cannot be measured due to the washing off
of interlayer. Instead of thickness, we suggest that “s-SWNT ratio” determines the property of charge
injection. Interestingly, a very small number of s-SWNTs in the interlayer solution had a critical influence

s-SWNTs
ratio (%)

b _μ_ h (cm2V−1S−1)

Average

b _μ_ e (cm2V−1S−1)

Average

aSWNT
concentration (μg/

ml)

Interlayers

Concentration of

solution

Without interlayer - - - 0.11 (± 0.01) 0.001 (± 0.0004)

PFO solution

~0.4 0.04 0.343 (± 0.034) 0.11 (± 0.03)
1 mg/ml

PFO solution
PFO:s-SWNT ~0.5 0.02 0.361 (± 0.19) 0.034 (± 0.01)

2 mg/ml

PFO solution

~1.5 0.03 0.21 (± 0.025) 0.09 (± 0.011)
5 mg/ml

F8BT solution

~0.06 0.006 0.13 (± 0.045) 0.046 (± 0.045)
1 mg/ml

F8BT solution
F8BT:s-SWNT ~0.12 0.006 0.09 (± 0.021) 0.03 (± 0.008)

2 mg/ml

F8BT solution

~1.2 0.026 0.24 (± 0.03) 0.16 (± 0.03)
5 mg/ml

Table 2. Hole and electron saturation mobilities of PTVPhI-Eh OFET with or without PFO:s-SWNT and
F8BT:s-SWNT interlayers at various concentrations. a Calculated from UV-Vis-NIR spectrum according to
absorption cross-section of 1.7 × 10 −17 cm 2 for E 11 absorption peak 36 . b Calculated from curve of _I_ ds1/2 versus

_V_ g in saturation regime (at _V_ d = ± 60).

on OFET performance. For the F8BT:s-SWNT solution containing 1 and 2 mg·ml −1 F8BT, s-SWNT ratio
were almost identical (0.006%). Despite the extremely low concentration of s-SWNTs, the OFETs with
F8BT:s-SWNT, including 0.006% s-SWNT ratio, showed remarkably enhanced injection properties.
The electron mobility was increased by approximately 3 times. This result supports the assertion that
s-SWNTs play an important role, and even very small numbers of SWNTs significantly affect the charge
injection of OFETs.

In order to verify the effects of wrapping polymers, we introduced the F8BT film (20 nm) only without s-SWNTs to the interlayer for PTVPhI-Eh OFETs (Fig. S7). The OFETs with the F8BT interlayer did
not show any performance enhancement. Therefore, the observed improvement in the charge-injection
properties of the OFETs was due mainly to the s-SWNTs rather than the wrapping of conjugated polymers. Furthermore, the very similar amount of improvement in the charge injection for OFETs with
F8BT:s-SWNT and PFO:s-SWNT indicates that the chirality of s-SWNTs is not a dominant factor in
charge injection, since PFO is capable of being selectively dispersed with five different sorts of s-SWNTs,
whereas F8BT can wrap only two different s-SWNTs.

The modified transfer-line method (M-TLM) was employed to determine _Rc_ of the OFETs with diverse
channel lengths (Fig. 6). The M-TLM allows a more reliable evaluation of _Rc_ via a slight modification of
the conventional TLM 42 . In the M-TLM, _R_ c is estimated from the slope of 1 _/L_ with _L_ being the channel
length, as described by the following equation 42 :

_Rtotal_ × _W_ = 1
_L_ _μCi_ ( _V_ _g_ - _V_

_μCi_ ( _V_ _g_ - _V_ _th_ )

1 1

+ ( _Rc_ × )

_g_ - _V_ _th_ ) _W_ _L_

(2)

_total_

× _W_
=

+ ( _Rc_ × )

_W_

_c_

_i_ ( _V_ _g_ - _V_ _th_

where _R_ total is the total resistance, _W_ is the channel width, _C_ i is the unit area capacitance of the gate
dielectric, and _V_ G is the gate voltage. Fig. 6 shows M-TLM plots of _p_ - and _n_ -channel PTVPhI-Eh OFETs
with and without the interlayers. The extracted values of _R_ c from the pristine, F8BT:s-SWNT, and
PFO:s-SWNT devices are 14, 9.6, and 1.9 MΩ·cm at Vg = - 50 V in the _p_ -channel regime and 400, 1.3,
and 3.8 MΩ·cm at Vg = 50 V in the _n_ -channel regime, respectively. The _Rc_ was reduced by more than
one order of magnitude for _p_ -channel operation and by almost two orders of magnitude for _n_ -channel
operation. These results clearly indicate that charge injection is significantly improved by the insertion
of the interlayers, particularly for electrons.

In order to explore the underlying mechanism for the improved performance by using the interlayers,
we performed a low-temperature measurement for all devices. The temperature-dependent hole and electron mobilities are shown in Fig. 7. In pristine PTVPhI-Eh OFETs, we extracted activation energies ( _E_ a)
of 51 and 55 meV for hole and electron transport, respectively, by Arrhenius plotting. By incorporating
the interlayers, the _E_ a declined to 50 and 27 meV for holes and electrons, respectively. The improved values of Ea for electron transport are comparable with those of state-of-the-art high-performance diketopyrrolopyrrole (DPP) derivatives 43 . Even the _E_ a for hole transport was not changed by the insertion of
interlayers, yet the hole mobility was improved by nearly 10 times compared to that of pristine OFETs in
the low-temperature regime, while this difference became smaller in the high-temperature regime. This
is because at low temperatures, the macroscopic conductivity is primarily based on a single microscopic
conductivity at Fermi energy so that the mobility is constant, and different charge concentrations (i.e.,

Figure 6. Modified transfer-line method plots for _p_ - and _n_ -channel PTVPhI-Eh OFETs with pristine gold
electrodes ( a, d ), with F8BT:s-SWNT contacts ( b, e ), and with PFO:s-SWNT contacts ( c, f ). The slopes of
the linear fit denote the _R_ c.

Figure 7. Temperature dependence of hole ( a ) and electron ( b ) mobilities and their calculation for OFETs
with and without interlayers. Symbols represent experimental results, dashed lines at high temperatures
are least-square fittings for extracting activation energy, and solid lines denote calculated data based on a
previously reported mobility model.

different Fermi levels) lead to different mobility magnitudes. At high temperatures, the larger number of
microscopic conductivities spanning over a wide energy region concurrently contributes to the mobility,
often manifesting as thermal activation. In addition, due to the presence of energetic disorder and the
already-high Fermi level for hole transport (discussed below), the apparent _E_ a would not be very different
before and after incorporating the interlayers 44 . Regarding the _E_ a for electron transport, more dramatic
improvement was observed by inserting interlayers: Not only was the mobility magnitude improved over
the entire temperature region, but also the activation energy was significantly reduced from 55 to 27 meV.

The decreased _E_ a for electron transport by adding the interlayers can be explained by the better
electrical contacts between source/drain electrodes and the OSC. The _E_ a reflects the average barriers for
the whole process of charge transport mainly via hopping from the source contact to the drain. Several
factors could elevate the hopping barrier (e.g., traps at the contact region and in the channel that mainly
result from contact properties, OSCs, the gate dielectric, and the OSC–dielectric interface) 45,46 . In this
study the lowered _E_ a was due to the improved contact properties (i.e., reduced charge trapping at contacts) by the insertion of interlayers. It is logical that the enhanced charge injection supplies many more
charge carriers to the channel. Thus, as the charge concentration is increased by adding interlayers, the
Fermi level is accordingly raised, filling more gap states (traps) and freeing more injected carriers. This
behavior is very similar to the abovementioned contact doping 47 . The increased density of mobile carriers
by dopants raises the Fermi level and screens the lower energy states within the band gap of the OSC.
On the other hand, when the interlayer is inserted, the π –π interaction and similar conjugated structure
may reduce the trap states at metal–OSC interfaces 10,27,48 .

To comprehend how much the charge injection is improved more quantitatively and to understand
why the mobility magnitude changes so dramatically by adding the interlayers, we implemented mobility
calculations based on a solid mobility model that has been reported previously 44,47 . As there was only
one difference in the contact interlayer, we kept the transport profile constant and only varied the charge
concentration. Interestingly, the calculated data showed good agreement with our experimental results
for both hole and electron mobility, as shown in Fig. 7. Here, the hole concentrations ( _N_ h) and electron
concentrations ( _N_ e) of the pristine device and PTVPhI-Eh OFETs with the interlayers were 10 17.9 and
10 18.5 cm −3 and 10 16.7 and 10 18.2 cm −3, respectively. The data indicates that the application of the interlayers indeed increases the charge concentration owing to the improved charge injection. There are some
features worthy of note. i) The original _N_ e = 10 16.7 cm −3 is much lower than that of _N_ h = 10 17.9 cm −3 due
to the unbalanced charge injection in the pristine device. ii) The _N_ h is slightly increased to 10 18.5 cm −3
by adding the interlayers, which explains the slight change in _E_ a for hole transport, since the Fermi level
is already located at a high energy level. iii) Compared to _N_ h, _N_ e is greatly enhanced to 10 18.2, which is
comparable to _N_ h in devices with the interlayers and is highly desirable for balanced ambipolar charge
transport. The significantly increased electron density weakens the charge-trapping effects and lowers
the hopping barriers, as is commonly observed in OFETs for gate-voltage enhanced mobility. Moreover,
this improved charge injection for holes and electrons is accomplished by just one contact interlayer
rather than the conventional interlayers specifically for either hole or electron injection. Therefore, truly
high-efficient ambipolar transport can be expected in organic transistors.

We suggest two main mechanisms for effects of the interlayers on the improved hole and electron
injection in OFETs. One is the strong 1D electrostatic effect of SWNTs, which have a form factor that
associates with the 1D electrostatic field at the contact interfaces. The strong electric field induced by
SWNTs at the contact region enables both electrons and holes to tunnel from metal electrodes to OSCs.
Some research groups have reported such 1D electrostatic effects on the SWNT-included electrode and
the relevant transistors experimentally and theoretically. 49-52 In this context, the 1D electrostatic field
significantly reduced the depletion thickness to a few nanometers, facilitating efficient charge injection
by tunneling 8,26,49–52 . Second mechanism is the doping effect of the active layer in OFETs by SWNTs via
the charge transfer between the conjugated polymer and SWNT 53-56 . Silva _et al._ reported an electron
transfer from SWNT to P3HT, resulting to P3HT and SWNT were n-doped and p-doped respectively 56 .
We conjecture that more highly enhanced _n_ -type characteristics than _p_ -type may be originated from
combination of the doping and electrostatic effect. Here, although the s-SWNT interlayer was used with
conjugated polymers, a similar injection principle would also be applicable. The energetic levels of the
interlayers were checked by ultraviolet photoemission spectroscopy (UPS), but the data did not match
our device characteristics (Fig. S8).

Finally, we demonstrated ambipolar complementary inverters that were fabricated via a spin-coating
process without sophisticated patterning for _p_ - and _n_ -channel regions (Fig. 8). Each inverter comprised two identical ambipolar transistors. Fig. 8 shows the voltage transfer characteristics (VTCs)
of PTVPhI-Eh inverters with and without the interlayers. As the unique characteristics of ambipolar
inverter, the PTVPhI-Eh inverter operated not only positive but also negative input voltage (Vin) depending on the polarity of the supply voltage, which cannot be operated in unipolar inverter 57,58 . In conventional complementary inverter which is consist of _p_ -type and _n_ -type unipolar transistors, Vin alternatively
switches on and off the _p_ -and _n_ -type transistors, respectively, (i.e., the pull-up and pull-down transistors).
Whereas in an ambipolar complementary inverter, pull-up and pull-down transistor are consist of the
same ambipolar transistor. Therefore, our inverter worked both positive and negative Vin. The inverters
with the interlayers manifested much better switching characteristics at the input _V_ in very close to half

Figure 8. Voltage transfer characteristics of ambipolar complementary inverters based on PTVPhI-Eh
OFETs [ _W_ / _L_ ratios for _p_ - and _n_ -channel, _W_ / _L =_ 1 mm/20 μ m]: bare Au S/D electrode (black); and, with
F8BT:s-SWNT (green) and PFO:s-SWNT interlayer (yellow).

the power supply voltage ( _V_ dd/2). Furthermore, the significantly improved voltage gains of 40 and 30
from 10 for the pristine device were achieved by incorporation of the F8BT:s-SWNT and PFO:s-SWNT
interlayers, respectively. These results rely on the balanced _μ_ h and _μ_ e and matched _V_ th in the _p_ - and
_n_ -channel regimes, thanks to the improved ambipolar charge injection.

Conclusion
In summary, we achieved charge-injection improvement for both holes and electrons in ambipolar
OFETs by using interlayers fabricated by conjugated polymer -wrapping s-SWNTs with a low-speed centrifugation. Thus, no additional process was needed to remove excess conjugated polymers. The kinds of
conjugated polymers and the chirality of s-SWNTs were not important factors in improved OFETs with
the interlayers. In case of interlayer with abundant metallic SWNT, the operational stability was even
lower than those OFETs with interlayers containing s-SWNTs. s-SWNTs themselves played an important
role, and even a very small number of s-SWNTs significantly affected the charge injection of OFETs.
We concluded that interlayers provide much better interfacial contact than the Au–OSC interface and
increase the charge concentration. The improved injection of both holes and electrons was qualitatively
considered by the tunneling across the Schottky barrier because of the 1D electrostatic effects caused by
the form factor of the SWNTs. Finally, we demonstrated a well-balanced ambipolar CMOS-like inverter
with the interlayers, which achieved the high gain of 40. We believe that these interlayers prepared with
this simple method are not only effective for ambipolar charge injection in most organic optoelectronic
devices with contact problems, but also highly compatible with solution-processed manufacturing processes without micropatterning for fully printed organic optoelectronic devices.

Method
Preparation of Conjugated Polymer-wrapping SWNT Interlayer Solution. The polymers used
in this study were PFO (Sigma Aldrich, Mw ≤ 20, 000 g/mol) and F8BT (American Dye Source Inc.,
Mw ≈ 15,000 – 200,000). SWNTs grown by the HiPco process (diameter 0.8–1.2 nm, purified < 13 wt%
iron) were purchased from Unidym Inc. All other chemicals were purchased from Sigma Aldrich. The
toluene solution of PFO (1, 2, and 5 mg/ml) and F8BT solution (1, 2, and 5 mg/ml) were prepared and
heated at 80 °C for 2 hr for complete dissolution. After cooling, the SWNT powder was added to each
solution at 0.5, 1, and 2.5 mg/ml, and the solutions were homogenized in an ultrasonic bath (Branson
5510) for 30 min, followed by probe tip sonication (VCX 750 Ultrasonic Processor with Temperature
Probe) for 15 min. Then, these resulting solutions were centrifuged (VS-15000CFNII, Vision Scientific),
and the supernatants were collected.

Fabrication of OFETs
Interdigitated source/drain electrodes on Corning Eagle 2000 glass substrates were processed in sequence
by photolithography, thermal evaporation of 5 nm-thick nickel and 15 nm-thick gold, and lift-off (with
channel width _W_ = 1 mm, channel length _L_ = 20 μ m). The substrates were cleaned in an ultrasonic bath
with deionized water, acetone, and 2-propanol for 10 min each. The substrates and electrodes were
treated with UV/ozone for 20 min before the deposition of interlayers. The polymer/SWNT dispersion
solution was spin-coated onto substrates at 2000 rpm for 60 s, and the resulting films were annealed in
air at 200 °C for 20 min. An ambipolar semiconductor PTVPhI-Eh (Mw = 35,800 g/mol) was synthesized
for this work, as reported elsewhere 59 . PTVPhI-Eh and PC60BM were dissolved in anhydrous CB to
obtain 5 mg/ml and 10 mg/ml concentrations, respectively, and then they were spin-coated at 2000 rpm
in a dry nitrogen glovebox. The semiconducting films were annealed at 110 °C for 20 min on a hotplate
to remove residual solvent and moisture, and the film thickness measured by an atomic force microscope (AFM) was uniformly ~20 nm. For the dielectric of PTVPhI-Eh OFETs, poly(methyl methacrylate)
(PMMA) (Mw = 120kD) was used without further purification and dissolved in n-butyl acetate (80 mg/
ml). The dielectric solution was spin-coated on top of the semiconducting film at 2000 rpm for 60 s, and
the final thickness was approximately 500 nm. For the dielectric of PC60BM OFETs, the fluoropolymer
CYTOP (Asahi Glass Co.) was chosen, because the PC60BM film is a small molecular OSC that can
be dissolved by the nBA of PMMA solvent. Spin-coating CYTOP (CYTOP: solvent = 2:1) on top of
the PC60BM formed the dielectric layer (approximately 350 nm determined by a surface profiler (Surf
Cored ET 3000)). The devices were baked at 80 °C for 30 min under nitrogen conditions. The aluminum
top-gate electrodes (thickness of ~50 nm) were thermally evaporated through shadow masks in a high
vacuum chamber (~10 6 Torr). Reference devices without polymer/SWNT interlayers were processed in
the same way.

Characterizations
UV-Vis spectra were measured using a Perkin-Elmer Lambda 750 instrument. The FET electrical characterizations were carried out using a semiconductor parameter analyzer (Keithley 4200-SCS) in an
N2-filled glovebox. Linear charge carrier mobility values were extracted by the M-TLM plots 42, following
the expression (1). In the equation, the Vths of each devices were extrapolated from linear segment of
transfer curve. Saturation charge carrier mobility values were extracted by the transfer curves according
to the expression _I_ _d_ = _μsat_ × _Cdie_ × _W_ 2 _L_ × ( _V_ _g_ - _V_ _th_ )2, where Id is the drain current, _μsat_ is the satu

_W_

to the expression _I_ _d_ = _μsat_ × _Cdie_ × _W_ 2 _L_ × ( _V_ _g_ - _V_ _th_ )2, where Id is the drain current, _μsat_ is the satu
ration mobility. The height and phase images of the F8BT:s-SWNT and PFO:s-SWNT were obtained
using an AFM (Nanoscope III, Veeco Instruments, Inc.) at the Korea Basic Science Institute (KBSI).

Mobility Calculations
The mobility calculations were performed using numerical software with a previously reported mobility
model. It is based on the Kubo–Greenwood integral developed by Sir Nevill Francis Mott and coworkers,
which described the relationship between mobility and transport properties in non-crystalline semiconductors, including amorphous silicon conductors and OSCs. As the contact interlayers did not affect the
transport profiles of holes and electrons in the channel, the relevant parameters were kept identical in
calculating the mobility for the OFETs with and without interlayers, and only one factor was changed:
the charge concentration. The densities of states in the HOMO and LUMO of PTVPhI-Eh were supposed to be 10 21 cm −3 with a Gaussian distribution in energy, and their standard deviation ∆ _E_ reflects
the energetic disorder. Another parameter of the applied mobility model is _α_, which is a hybrid level of
delocalized and localized states in the band. A maximum value of _α_ = 1 indicates fully band-like transport, while a smaller _α_ indicates a greater hopping contribution to the overall transport process. In this
study, ∆ _E_ = 0.12 eV and _α_ = 0.56 for hole transport, and ∆ _E_ = 0.12 eV and _α_ = 0.6 for electron transport.
The HOMO and LUMO levels of PTVPhI-Eh were set to be 5.24 eV and 3.39 eV lower than the vacuum
level, respectively, and the relative permittivity of PTVPhI-Eh was configured as 2.1.

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Acknowledgement
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the
Korean Government (MSIP) (NRF-2014R1A2A2A01007159 and 2009-0093035).

Author Contributions
Y-Y.N. and D-Y.K. directed and coordinated the research. S-H.L. designed and fabricated the devices and
performed experiment. D.K., Y.X., W-T.P. and S.H.L. analyzed and interpreted the data. J.K. synthesized
the semiconducting polymer. S.H.L., Y.X., and Y-Y.N wrote the manuscript.

Additional Information
Supplementary information [accompanies this paper at http://www.nature.com/srep](http://www.nature.com/srep)

Competing financial interests: The authors declare no competing financial interests.

How to cite this article : Lee, S.-H. _et al._ Simultaneous Improvement of Hole and Electron Injection in
Organic Field-effect Transistors by Conjugated Polymer-wrapped Carbon Nanotube Interlayers.
_Sci. Rep._ 5, 10407; doi: 10.1038/srep10407 (2015).

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