Swapping field-effect transistor characteristics in polymeric diketopyrrolopyrrole semiconductors: Debut of an electron dominant transporting polymer

Article abstract of DOI:10.1039/c1jm14549e

A fact-finding study on thiophenyl diketopyrrolopyrrole (TDPP)-containing polymers for electronically convertible transport characteristics in organic field effect transistors (OFETs) is presented. In the subject of this consideration, a TDPP-based polymer with bis-benzothiadiazole (BisBT) units that serve as powerful electron-deficient building blocks, namely PDTDPP-BisBT, is prepared in order to achieve an n-channel transistor. The resulting polymer shows n-channel dominant ambipolar OFET characteristics and its electron mobility (1.3 × 10^-3 cm² V^-1 s ^-1) is found to be one order of magnitude higher than the hole mobility. Besides, the PDTDPP-BisBT OFET performance is independent of film-deposition conditions due to its completely amorphous microstructure, supported by the atomic force microscopy (AFM) and X-ray diffraction (XRD) analyses. Herein, we report an intriguing discovery in sync with our previous studies that TDPP-based polymers can function as a p-type, n-type, or ambipolar organic semiconductor in accordance with the degree of electron affinity of the comonomers. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c1jm14549e

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PAPER
Swapping field-effect transistor characteristics in polymeric
diketopyrrolopyrrole semiconductors: debut of an electron dominant
transporting polymer†
a
b
c
d
d
Junghoon Lee, Shinuk Cho, Jung Hwa Seo, Piyush Anant, Josemon Jacob and Changduk Yang
a
*
Received 14th September 2011, Accepted 2nd November 2011
DOI: 10.1039/c1jm14549e
A fact-finding study on thiophenyl diketopyrrolopyrrole (TDPP)-containing polymers for
electronically convertible transport characteristics in organic field effect transistors (OFETs) is
presented. In the subject of this consideration, a TDPP-based polymer with bis-benzothiadiazole
(BisBT) units that serve as powerful electron-deficient building blocks, namely PDTDPP–BisBT, is
prepared in order to achieve an n-channel transistor. The resulting polymer shows n-channel dominant
ambipolar OFET characteristics and its electron mobility (1.3 ꢀ 10ꢁ3 cm2 Vꢁ1 sꢁ1) is found to be one
order of magnitude higher than the hole mobility. Besides, the PDTDPP–BisBT OFET performance is
independent of film-deposition conditions due to its completely amorphous microstructure, supported
by the atomic force microscopy (AFM) and X-ray diffraction (XRD) analyses. Herein, we report an
intriguing discovery in sync with our previous studies that TDPP-based polymers can function as
a p-type, n-type, or ambipolar organic semiconductor in accordance with the degree of electron affinity
of the comonomers.
cm2 Vꢁ1 sꢁ1 with good ambient stability.5–7 However, solution-
processable n-type polymers remain largely elusive.8–11 To the
best of our knowledge, only few studies have been reported
focusing on polymers based on naphthalene,12–14 and perylene
diimide12,15,16 as well as the ladder-polymer BBL.17 In spite of
much effort in synthesizing novel n-type polymers,18,19 the
development of polymers having appreciable electron mobilities
that are soluble in conventional organic solvents and air-stable
under ambient conditions has met limited success.20
1. Introduction
Stimulated by the need for low-cost, flexible, and solution-pro-
cessed electronics, complementary integrated circuits from both
p- and n-channel organic field-effect transistors (OFETs) are
widely viewed to be the linchpin of next generation electronic
devices because of their low power consumption, easy patterning
technique, and large noise margin.1,2 Considering a key prereq-
uisite for inexpensive device assembly by spin-coating, casting, or
printing, polymeric materials are obvious candidates for satis-
factory OFETs and practicable logic circuits since they enable
ink formulation with tuned rheological properties.3,4 As far as
solution-processable polymeric semiconductors are concerned,
impressive progress has been achieved in developing p-type
conjugated polymers such as P3HT, F8T2, PBTTT, and PQT
of which OFETs exhibit hole mobilities (mh) of y 0.01–1
To pioneer an innovative new material that validates favorable
properties of charge carrier mobilities for either electrons or
holes, an original diketopyrrolopyrrole (DPP)-based materials-
design methodology is devised to evaluate structural modifica-
tion options. Herein, we demonstrate a new family of readily
processable n-type dominant polymers with an extended
optical absorption toward the IR region, namely poly[3,6-dithien-
2-yl-2,5-di(2-decyltetradecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-50,500-
aInterdisciplinary School of Green Energy and KIER-UNIST Advanced
Center for Energy, Ulsan National Institute of Science & Technology
(UNIST), Ulsan, 689-798, South Korea. E-mail: yang@unist.ac.kr;
Fax: +82-52-217-2909; Tel: +82-52-217-2920
diyl-co-4,40-bis-(2,1,3-benzothiadiazole)-7,70-diyl]
(PDTDPP–
BisBT), that integrates both DPP and bis-benzothiadiazole
(BisBT) units as powerful electron-deficient building blocks
(Fig. 1).
bDepartment of Physics and EHSRC, University of Ulsan, Ulsan, 680-749,
South Korea
cDepartment of Materials Physics, Dong-A University, Busan, 604-714,
South Korea
2. Experimental
dCentre for Polymer Science and Engineering, Indian Institute of
Technology, Hauz Khas, New Delhi, India
2.1. Materials and instruments
† Electronic supplementary information (ESI) available: Cyclic
voltammograms of PDTDPP–TPA and PDTDPP–BT, DFT
calculations of the polymers consisting of two units, and spectral data
for PDTDPP–BisBT. See DOI: 10.1039/c1jm14549e
All starting materials were purchased either from Aldrich or
Acros and used without further purification. All solvents are of
ACS grade unless otherwise noted. Anhydrous THF was
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12H). 13C NMR (CDCl3, 150 MHz): d ppm 161.71, 140.48,
137.62, 136.11, 135.63, 108.70, 84.55, 46.24, 37.77, 31.91, 31.27,
30.01, 29.80, 29.68, 29.66, 29.64, 29.62, 29.57, 29.35, 29.33, 26.32,
24.75, 22.67, 14.10. MALDI-TOF MS (m/z) 1225.68 (M+). Anal.
Calcd for C74H126B2N2O6S2: C, 72.52; H, 10.36; N, 2.29. Found:
C, 72.40; H, 10.59; N, 2.57.
Synthesis of poly[3,6-dithien-2-yl-2,5-di(2-decyltetradecanyl)-
pyrrolo[3,4-c]pyrrole-1,4-dione-50,500-diyl-co-4,40-bis-(2,1,3-benzo-
thiadiazole)-7,70-diyl] (PDTDPP–BisBT).
A mixture of 1
Fig. 1 Molecular structure of PDTDPP–BisBT.
(200 mg, 0.16 mmol), 7,70-dibromo-4,40-bis(2,1,3-benzothiadia-
zole) (69 mg, 0.16 mmol), and tris(dibenzylidenacetone)dipal-
ladium(0) (10 mg, 0.011 mmol) was taken in a Schlenk
flask. To this, tri(o-tolyl)phosphine (5 mg, 0.016 mmol)
and K3PO4 (220 mg) in toluene (5 ml) with demineralized
water (1 ml) were added and the reaction mixture was heated at
obtained by distillation from sodium/benzophenone prior to use.
Anhydrous toluene was used as received. 3,6-Dithien-2-yl-2,5-
dihydropyrrolo[3,4-c]pyrrole-1,4-dione,21
bromide,22 3,6-dithien-2-yl-2,5-di(2-decyltetradecanyl)-pyrrolo
[3,4-c]pyrrole-1,4-dione,23 7,70-dibromo-4,40-bis(2,1,3-benzo-
2-decyltetradecyl-
ꢂ
95 C under vigorous stirring for 72 h. The crude product was
poured into a mixture of methanol (300 ml) and water (100 ml).
The resulting solid was filtered off and subjected to sequential
Soxhlet extraction with methanol (1 d), acetone (1 d) and
hexane (1 d) to remove low molecular weight fraction of
the materials. The residue was extracted with chloroform to
give dark purplish product after precipitating again
from methanol and drying in vacuo. Isolated yield
thiadiazole)24 were prepared according to established literature
1
procedures. H NMR and 13C NMR spectra were recorded on
a VNMRS 600 (Varian, USA) spectrometer using CDCl3 as the
solvent and tetramethylsilane (TMS) as the internal standard and
MALDI-MS spectra were obtained from Ultraflex III (Bruker,
Germany). UV-Vis spectra were taken on a Cary 5000 (Varian
USA) spectrophotometer. Number-average (Mn) and weight
average (Mw) molecular weights, and polydispersity index (PDI)
of the polymer products were determined by gel permeation
chromatography (GPC) with Agilent 1200 HPLC Chemstation
using a series of monodisperse polystyrene as standards in THF
(HPLC grade) at 308 K. Cyclic voltammetry (CV) measurements
were performed on Solartron SI 1287 with a three-electrode cell
of polymer PDTDPP–BisBT
¼
100 mg (50%). GPC
analysis Mn ¼ 57 000 kg molꢁ1, Mw ¼ 199 500 kg molꢁ1, and
1
PDI ¼ 3.5 (against PS standard, see ESI† for GPC curve). H
NMR (CDCl3, 600 MHz) (see ESI† for full NMR
spectroscopy).
in
a 0.1 M tetra-n-butylammonium hexafluorophosphate
2.2. OFET device preparation and measurement
(n-Bu4NPF6) solution in acetonitrile at a scan rate of 50 mV sꢁ1
at room temperature under argon. A silver wire, a platinum wire
and a platinum disk were used as the reference electrode, counter
electrode and working electrode respectively. The Ag/Ag+
reference electrode was calibrated using a ferrocene/ferrocenium
redox couple as an external standard, whose oxidation
potential is set at ꢁ4.8 eV with respect to zero vacuum level. The
HOMO energy levels were obtained from the equation HOMO
All FETs were fabricated on heavily n-type doped silicon (Si)
wafers with a 200 nm thick thermally grown SiO2 layer. FETs
were fabricated in the top contact geometry. The n-type doped Si
substrate functioned as the gate electrode and the SiO2 layer
functioned as the gate dielectric. The semiconducting layer was
spin-cast from solution at 2500 rpm. All solutions were prepared
at 5 mg/ml concentration in chlorobenzene. Prior to deposition
of semiconducting layer, the SiO2 surface was modified with
polypropylene-co-1-butene (PPcB) to remove hydroxyl group
from the SiO2 surface. After semiconducting layer deposition,
the films were dried on a hot plate stabilized at 80 ꢂC for 30
minutes. The semiconducting layer deposition was carried out in
a controlled atmosphere glove box filled with N2. Source and
drain electrodes were deposited by thermal evaporation using
a shadow mask. The thickness of source and drain electrodes was
50 nm. Channel length (L) and channel width (W) were 50 mm
and 2.0 mm, respectively. Electrical characterization was per-
formed under N2 atmosphere using a Keithley semiconductor
parametric analyzer (Keithley 4200). The electron mobility (m)
was determined using the following equation in the saturation
regime:
onset
onset
(eV) ¼ ꢁ(E(ox)
ꢁ E(ferrocene)
+ 4.8). The LUMO levels
of polymers were obtained from the equation LUMO
onset
(eV) ¼ ꢁ(E(red)
ꢁ E(ferrocene)onset + 4.8).
Synthesis of 3,6-di(2-(4,4,5,5-tetramethyl-[1,3,2]dioxaborolan-
2-yl)thien-5-yl)-2,5-di(2-decyltetradecyl)-pyrrolo[3,4-c]pyrrole-1,
4-dione (1). To a solution of 3,6-dithien-2-yl-2,5-di(2-decylte-
tradecanyl)-pyrrolo[3,4-c]pyrrole-1,4-dione (1 g, 1.0 mmol) and
2-isopropyl-4,4,5,5-tetramethyl-1,3,2-dioxoborane (0.42 g, 0.46
ml, 2.5 mmol) in THF (30 ml) under argon at ꢁ40 ꢂC was slowly
added LDA solution (1.28 ml, 2.5 mmol) for 5 minutes. The
resulting mixture was stirred for 1 hour at 0 ꢂC and then
quenched with 200 ml of water. The compound was extracted in
CHCl3, washed, and dried over MgSO4. The solvent was evap-
orated under reduced pressure. The crude product was washed
with MeOH (200 ml) three times. Isolated yield ¼ 1.0 g (79%) as
Ids ¼ (mWCi/2L)(Vgs ꢁ VT)2,
1
a thick viscous dark purplish oil. H NMR (CDCl3, 600 MHz):
d ppm 8.91 (d, J ¼ 3.17, 2H), 7.71 (d, J ¼ 3.17, 2H), 4.05 (m, 4H),
where Ci is the capacitance per unit area of the SiO2 dielectric
(Ci ¼ 15 nF cmꢁ2) and VT is the threshold voltage.
1.91 (m, 2H), 1.36 (s, 24H), 1.27–1.20 (m, 80H), 0.87–0.86 (m,
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a yield of 79%. As shown in Scheme 1, the thiophenyl DPP units
3. Results and discussion
were incorporated into the polymer backbone by a 1 : 1 copo-
3.1. Synthetic strategies and chemical characterization
lymerization with
a
4,40-bis(2,1,3-benzothiadiazole) como-
nomer,24 affording PDTDPP–BisBT. The resulting polymer is
readily soluble in all organic solvents tested at room temperature
due to the long branched side chain substitutions. The number-
average molecular weight (Mn) of PDTDPP–BisBT is 57 000 kg
molꢁ1 with a polydispersity index of 3.5, as determined by using
GPC with a polystyrene standard calibration.
The 3,6-(2-thiophenyl)-substituted diketopyrrolopyrrole (thio-
phenyl DPP) unit has a well-conjugated structure, which leads to
strong p–p interaction,25 and the lactam part makes the DPP
unit exhibit a high electron-withdrawing effect, and hence the
DPP segment possesses high electron affinity.26 Although
significant progress has been made for application of thiophenyl
DPP-containing materials in both organic photovoltaic cells
(OPVs) and OFETs, all OFETs reported in the literature based
on these materials have shown either good p-channel properties
or ambipolar transport characteristics.23,27–32 Besides, it has been
recently reported that ambipolar transistors, integrated CMOS-
like inverters, and ring oscillators with poly(diketopyrrolo-
pyrrole-terthiophene) are used as semiconductors.33 Thus, the
thiophenyl DPP-based polymers without exception have been
applied as donor layers in OPV devices.27,30,34 Independently, we
found that thiophenyl DPP-based polymer, PDTDPP–TPA (see
Fig. 2) bearing electron-rich triphenylamine (TPA) building
blocks, exhibits high hole mobility.35 What is more, in our earlier
study, is that the replacement of TPA group in PDTDPP–TPA
by electron-deficient benzothiadiazole (BT) moiety resulted in an
excellent ambipolar polymeric semiconductor PDTDPP–BT.23
From such observations, we have designed a copolymer
PDTDPP–BisBT consisting of bis-benzothiadiazole (BisBT) and
thiophenyl DPP segments to possess an extended p electron
affinity system along the main chain, which would further
strengthen electron charge carrier transport and be anticipated to
afford n-channel OFETs.
To access a good deal of information of the electronic struc-
ture of the polymer, the photophysical properties of PDTDPP–
BisBT were investigated by UV-vis-NIR absorption spectros-
copy in dilute chloroform and as spin-coated film on quartz
substrate (Fig. 3). In solution, PDTDPP–BisBT shows an
absorption band centered at 780 nm with a vibronic absorption
shoulder at 865 nm which is associated with molecular rigidity
imposed molecular connectivity in the solution measurement,
indicating the rigid-rod nature of the polymer main backbone, as
a result of the coplanar structure and strong polarity of the DPP
The thiophenyl DPP as a starting compound was synthesized
according to the literature procedure.21 To overcome the solu-
bility issue, we chose long branched alkyl side chains, 2-decyl-
tetradecyl groups to substitute DPP at the nitrogen atoms. And
then, we treated the thiophenyl DPP containing two 2-decylte-
tradecyl solubilizing groups with lithium diisopropylamine
(LDA) solution at low temperature (ꢁ40 ꢂC), followed by the
addition of 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxabor-
olane, to generate the corresponding diboronic ester 1 with
Scheme 1 Synthetic route to PDTDPP–BisBT. Reagents and condi-
tions: (i) LDA, THF, ꢁ40 ꢂC under Ar, 24 h, 79%; (ii) Pd2(dba)3/P-(o-
Tol)3, K3PO4, toluene/H2O, 95 ꢂC for 72 h, 50%.
Fig. 2 Chemical structures of thiophenyl DPP-based polymers.
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Fig. 4 (a) Cyclic voltammogram of PDTDPP–BisBT thin film on the Pt
electrode in 0.1 M n-Bu4NPF6 acetonitrile solution at room temperature.
(b) Energy level diagrams of thiophenyl DPP derivatives.
same manner for both PDTDPP–TPA (ꢁ3.42 eV)35 and
PDTDPP–BT (ꢁ3.59 eV)23 while the HOMO energies of the
three polymers reveal remarkable similarities within experi-
mental error (ꢁ5.07 eV for PDTDPP–TPA, ꢁ5.07 for
PDTDPP–BT, and ꢁ5.04 for PDTDPP–BisBT), indicative of
the identical thiophenyl DPP counterparts (see Fig. 4b). The
discrepancy (0.01–0.27 eV) of the electrochemically and optically
determined bandgaps could be due to the exciton binding energy
of conjugated polymers, which can be 0.4–1.0 eV.38,39 Interest-
ingly, the low-lying LUMO position is comparable to well-
known n-type polymers containing rylene bisimides electroni-
cally conjugated along the backbone,12,14 approaching that of the
polymer based on strongly electron-depleted naphthalene
Fig. 3 (a) UV-vis-NIR absorption spectra of PDTDPP–BisBT in dilute
CHCl3 solution and thin film on quartz plate.
units. The absorption maximum is larger than our earlier
reported polymer PDTDPP–TPA (lmax ¼ 500–700 nm).35
A
rationale for the relatively wider optical bandgap of PDTDPP–
TPA (1.65 eV) is the minimized coplanarity in the polymer
backbone as a result of 3D propeller-like TPA units as well as the
weakness of intramolecular charge transfer (ICT) compared to
PDTDPP–BisBT.
The absorption spectrum of PDTDPP–BisBT in the film is
almost identical to that in solution and no significant bath-
ochromic shift is revealed. This observation is in contrast to most
DPP-based polymers, in which there are strong red-shifts from
the solutions to the films since they have the fused DPP ring units
for coplanarity, aggregation, and reducing p-stacking.21,29,36 This
is most likely attributed to the two bulky side chains impeding
a planar conformation of the DPP segment. In addition, we
cannot rule out that steric interaction between the adjacent BT
units in the BisBT segment can produce twisting around their
interring bonds, which probably leads to reduced p-conjugated
electronic structure along the chain.37 This assumption is sup-
ported by a previous report on the single crystal X-ray structure
of a BisBT derivative that showed a nonplanar anti-conforma-
tion with strong twist angles of about 43–54ꢂ between the two BT
units.24 As a consequence of the severe torsion between the two
BT units, despite the relatively stronger electron accepting
character of the BisBT versus the BT, the optical bandgap of
PDTDPP–BisBT is 1.27 eV, which is slightly higher than that of
PDTDPP–BT (1.20 eV) containing single BT with the same set
of DPP blocks.23
3.2. Electrochemical properties
Cyclic voltammetry (CV) was performed to determine the high-
est-occupied molecular orbital (HOMO) and lowest-unoccupied
molecular orbital (LUMO) energy levels of the conjugated
polymer. The CV curves were recorded referenced to an Ag/Ag+
(0.1 M of n-Bu4NPF6 in acetonitrile) electrode, which was cali-
brated by a ferrocene–ferrocenium (Fc/Fc+) redox couple (4.8 eV
below the vacuum level). As shown in Fig. 4a, PDTDPP–BT
shows quasi-reversible oxidation/reduction processes. The cor-
responding LUMO energy is estimated at ꢁ3.77 eV from the
onset reduction potential in cyclic voltammetry (CV). The
LUMO value is noticeably deeper than those calculated in the
Fig. 5 DFT-optimized geometries and charge-density isosurfaces for the
HOMO and LUMO levels and the side views of thiophenyl DPP-based
polymers.
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Fig. 6 (a) Output and (b) transfer characteristics of PDTDPP–BisBT FET. The output transfer characteristics of PDTDPP–TPA (c) and PDTDPP–
BT FETs (d) obtained from earlier works, respectively. Insets are chemical structures of thiophenyl DPP-based polymers and the schematic device.
diimide with the lowest LUMO of approximately ꢁ4.0 eV
reported to date.13,16 The low-lying LUMO level for PDTDPP–
BisBT is desirable to enable an electron-transporting semi-
conductor since the lower the LUMO energy, the lower the bias
required to inject the carriers and the less susceptible are the
electrons to trapping.16
replaced by methyl groups to reduce the time required for
calculation.
The optimized geometry of PDTDPP–BT reveals a highly
planar BT p core with negligible intramonomer torsion and
both the PDTDPP–TPA and PDTDPP–BisBT are predicted to
have large dihedral angles of the aromatic counterparts in the
settlement of thiophenyl DPP units. As depicted on the top of
Fig. 5a, the HOMO isosurface of PDTDPP–TPA is well delo-
calized along the conjugated chain whereas the LUMO is
disconnected at TPA moiety, showing only p-type OFET
reported in our previous study.35 Also, in the course of our
earlier experiment for PDTDPP–BT, the charge delocalizations
of both the HOMO and LUMO orbitals had been observed,
resulting in an effective ambipolar behavior. In the case of
PDTDPP–BisBT, to our surprise, the coefficients in the HOMO
orbital are positioned only on the core DTDPP unit, but the
effective coefficients in the LUMO are located on the entirely
lateral axis p-system. Thereby, the nature of PDTDPP–BisBT
3.3. DFT electronic structure calculation
To understand the position and energies of frontier orbitals for
the conjugated polymers, density functional theory (DFT)
calculations on their electronic states are performed using the
Gaussian 03 package at the B3LYP/6-31G* (Fig. 5). Both
PDTDPP–TPA and PDTDPP–BT data in this study are pre-
sented to help us better understand the changed electron-state-
density distributions of the HOMO and LUMO in the geometry
optimized structures by changing comonomers in the thiophenyl
DPP-based polymers. The alkyl chains in the polymers were
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material with achievable reproducibility in the OFETs. An
important point to pinpoint here is that thiophenyl DPP-con-
taining polymers can function as a p-type, n-type, or ambipolar
semiconductor depending on the counterpart comonomers.
4. Conclusions
Fig. 7 Topographic AFM images of PDTDPP–BisBT film (a) at room
temperature and (b) after annealing at 120 ꢂC. (c) X-ray diffraction
(XRD) patterns of PDTDPP–BisBT thin film.
In summary, we have demonstrated an approach to n-channel
transistor development, where our previous results can guide
materials design from initial conception to refinement of the final
semiconductor structure.
A narrow bandgap copolymer
OFETs is interesting considering the electron mobility achiev-
PDTDPP–BisBT comprised of a strong acceptor, BisBT, which
is relatively stronger than BT, and thiophenyl DPP moieties is
synthesized. The new polymer exhibits electron dominant
ambipolar characteristics, resulting in charge-carrier mobility of
up to 1.3 ꢀ 10ꢁ3 cm2 Vꢁ1 sꢁ1 for electrons, which is about one
order of magnitude higher than that for holes. While the
behavior of many high-performance semiconductors is sensitive
to deposition conditions, PDTDPP–BisBT polymer film exhibits
OFET performance predominately independent of fabrication
conditions because of the amorphous morphology in the solid
state as determined by AFM and XRD, evidenced by no scat-
tering patterns and non-fibrillar structural features in both as-
spun and annealed films. This study together with our previous
courses indicate that thiophenyl DPP-based polymers can
operate on either p-type, n-type, or ambipolar semiconductor
subjecting to tune the counterpart comonomers. As a result, this
work can serve as a beacon to further improve mobility of thi-
ophenyl DPP derivatives for either p-type, n-type, or ambipolar
OFETs. Besides, we believe that this discovery has answered
questions about effective electron transport capabilities of
polymeric semiconductors. A preliminary investigation on all-
polymers solar cells with PDTDPP–TPA as an electron donor
and PDTDPP–BisBT as an electron acceptor is in progress.
able (vide infra).
3.4. OFET performance
PDTDPP–BisBT OFETs with topcontactand a bottomgate were
fabricated by spin-coating the polymer solution in the SiO2/n++Si
substrate covered by a polypropylene-co-1-butene (PPcB) layer.
The PPcB layer was introduced as a passivation layer to remove
the hydroxyl group which acts as trap sites of electrons (Fig. 6a).
All fabrication and testing were performed in an inert nitrogen
atmosphere. The PDTDPP–BisBT device reveals a bipolar
feature with both p-type and n-type characteristics, such as
a diode-like current increase at low gate bias with current satu-
ration at high gate voltage in output curves (Fig. 6a) and V-shaped
transfer curves with narrow off-state (Fig. 6b), which is similar to
that of PDTDPP–BT with nearly balanced hole and electron
mobilities.23 In marked contrast, PDTDPP–BisBT predominately
displays electron transport behavior. The saturated charge carrier
mobility of PDTDPP–BisBT is calculated using the saturation
current equation: Ids ¼ (mWCi/2L)(Vgs ꢁ VT)2.40 A linear fit is
applied in the saturation region of the Ids1/2 versus Vgs curve of the
polymer in order to calculate the mobility. An electron mobility
(me) as high as 1.3 ꢀ 10ꢁ3 cm2 Vꢁ1 sꢁ1 and hole mobility as high as
6.5 ꢀ 10ꢁ4 cm2 Vꢁ1 sꢁ1 are estimated for OFETs produced from
PDTDPP–BisBT. Note that the electron mobility value is about
one order of magnitude higher than the hole mobility. Appar-
ently, this result is in accordance with the fact that BisBT has
better electron transport ability than BT unit, which is in good
agreement with both the aforementioned DFT study and the
electrochemical behavior.
Acknowledgements
This work was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education, Science and Technology
(2010-0002494) and the National Research Foundation of Korea
Grant funded by the Korean Government (MEST) (2010-
0019408), (2010-0026916), (2009-0093818), and (2011-0009148).
The first two authors contributed equally to this work.
3.5. Film surface microstructure
For OFET based on PDTDPP–BisBT, the deviation from
coplanarity caused by the interring bond between the two BT
units can account for the relatively reduced performance in
comparison with PDTDPP–BT. Both as-spun and annealed
PDTDPP–BisBT film microstructures assessed by atomic force
microscopy (AFM) and X-ray diffractions (XRD) studies
(Fig. 7a–c), as evidenced by the absence of order diffraction
peaks as well as detectable fibrillar structures, do not reveal the
distinctive crystalline features associated with other DPP-based
polycrystalline thin films.28,29 This is due to the combination of
the strong twist between consecutive BT groups and the bulky
branched side chains on the DPP. Therefore, one can conclude
that PDTDPP–BisBT has poorly defined, randomly ordered
lamellar structure, implying an amorphous p-conjugated
Notes and references
1 H. Klauk, U. Zschieschang, J. Pflaum and M. Halik, Nature, 2007,
445, 745–748.
2 E. J. Meijer, D. M. De Leeuw, S. Setayesh, E. Van Veenendaal,
B. H. Huisman, P. W. M. Blom, J. C. Hummelen, U. Scherf and
T. M. Klapwijk, Nat. Mater., 2003, 2, 678–682.
3 S. Sivaramakrishnan, P. J. Chia, Y. C. Yeo, L. L. Chua and
P. K. H. Ho, Nat. Mater., 2007, 6, 149–155.
4 F. Garnier, R. Hajlaoui, A. Yassar and P. Srivastava, Science, 1994,
265, 1684–1686.
5 H. L. Pan, Y. N. Li, Y. L. Wu, P. Liu, B. S. Ong, S. P. Zhu and G. Xu,
J. Am. Chem. Soc., 2007, 129, 4112–4113.
6 I. McCulloch, M. Heeney, C. Bailey, K. Genevicius, I. Macdonald,
M. Shkunov, D. Sparrowe, S. Tierney, R. Wagner, W. M. Zhang,
M. L. Chabinyc, R. J. Kline, M. D. McGehee and M. F. Toney,
Nat. Mater., 2006, 5, 328–333.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 1504–1510 | 1509

View Online
7 A. S. Dhoot, J. D. Yuen, M. Heeney, I. McCulloch, D. Moses and
A. J. Heeger, Proc. Natl. Acad. Sci. U. S. A., 2006, 103, 11834–11837.
8 H. Usta, A. Facchetti and T. J. Marks, J. Am. Chem. Soc., 2008, 130,
8580–8581.
9 J. A. Letizia, M. R. Salata, C. M. Tribout, A. Facchetti, M. A. Ratner
and T. J. Marks, J. Am. Chem. Soc., 2008, 130, 9679–9694.
10 X. W. Zhan, Z. A. Tan, B. Domercq, Z. S. An, X. Zhang, S. Barlow,
Y. F. Li, D. B. Zhu, B. Kippelen and S. R. Marder, J. Am. Chem.
Soc., 2007, 129, 7246–7247.
26 D. R. Cao, Q. L. Liu, W. J. Zeng, S. H. Han, J. B. Peng and S. P. Liu,
J. Polym. Sci., Part A: Polym. Chem., 2006, 44, 2395–2405.
27 H. Bronstein, Z. Y. Chen, R. S. Ashraf, W. M. Zhang, J. P. Du,
J. R. Durrant, P. S. Tuladhar, K. Song, S. E. Watkins, Y. Geerts,
M. M. Wienk, R. A. J. Janssen, T. Anthopoulos, H. Sirringhaus,
M. Heeney and I. McCulloch, J. Am. Chem. Soc., 2011, 133, 3272–
3275.
28 P. Sonar, S. P. Singh, Y. Li, M. S. Soh and A. Dodabalapur, Adv.
Mater., 2010, 22, 5409–5413.
11 A. Babel and S. A. Jenekhe, J. Am. Chem. Soc., 2003, 125, 13656–
13657.
29 Y. N. Li, S. P. Singh and P. Sonar, Adv. Mater., 2010, 22, 4862–
4866.
12 W. P. Wu, Y. Q. Liu and D. B. Zhu, Chem. Soc. Rev., 2010, 39, 1489–
1502.
30 J. C. Bijleveld, A. P. Zoombelt, S. G. J. Mathijssen, M. M. Wienk,
M. Turbiez, D. M. de Leeuw and R. A. J. Janssen, J. Am. Chem.
Soc., 2009, 131, 16616–16617.
13 H. Yan, Z. H. Chen, Y. Zheng, C. Newman, J. R. Quinn, F. Dotz,
M. Kastler and A. Facchetti, Nature, 2009, 457, 679–686.
14 X. G. Guo and M. D. Watson, Org. Lett., 2008, 10, 5333–5336.
15 M. M. Ling, P. Erk, M. Gomez, M. Koenemann, J. Locklin and
Z. N. Bao, Adv. Mater., 2007, 19, 1123–1127.
16 B. A. Jones, A. Facchetti, M. R. Wasielewski and T. J. Marks, J. Am.
Chem. Soc., 2007, 129, 15259–15278.
17 A. L. Briseno, S. C. B. Mannsfeld, P. J. Shamberger, F. S. Ohuchi,
Z. N. Bao, S. A. Jenekhe and Y. N. Xia, Chem. Mater., 2008, 20,
4712–4719.
€
31 L. Burgi, M. Turbiez, R. Pfeiffer, F. Bienewald, H. J. Kirner and
C. Winnewisser, Adv. Mater., 2008, 20, 2217–2224.
32 T. L. Nelson, T. M. Young, J. Y. Liu, S. P. Mishra, J. A. Belot,
C. L. Balliet, A. E. Javier, T. Kowalewski and R. D. McCullough,
Adv. Mater., 2010, 22, 4617–4621.
33 W. S. C. Roelofs, S. G. J. Mathijssen, J. C. Bijleveld, D. Raiteri,
T. C. T. Geuns, M. Kemerink, E. Cantatore, R. A. J. Janssen and
D. M. de Leeuw, Appl. Phys. Lett., 2011, 98, 203301.
34 J. C. Bijleveld, V. S. Gevaerts, D. D. Nuzzo, M. Turbiez,
S. G. J. Mathijssen, D. M. de Leeuw, M. M. Wienk and
R. A. J. Janssen, Adv. Mater., 2011, 22, E242–E246.
18 M. S. Liu, X. Z. Jiang, P. Herguth and A. K. Y. Jen, Chem. Mater.,
2001, 13, 3820–3822.
19 N. C. Greenham, S. C. Moratti, D. D. C. Bradley, R. H. Friend and
A. B. Holmes, Nature, 1993, 365, 628–630.
35 J. Lee, S. Cho and C. Yang, J. Mater. Chem., 2011, 21, 8528–
8531.
20 M. M. Alam and S. A. Jenekhe, Chem. Mater., 2004, 16, 4647–
4656.
36 L. J. Huo, J. H. Hou, H. Y. Chen, S. Q. Zhang, Y. Jiang, T. L. Chen
and Y. Yang, Macromolecules, 2009, 42, 6564–6571.
21 E. J. Zhou, S. P. Yamakawa, K. Tajima, C. H. Yang and
K. Hashimoto, Chem. Mater., 2009, 21, 4055–4061.
37 J. Kim, M. H. Yun, P. Anant, S. Cho, J. Jacob, J. Kim and C. Yang,
Chem.–Eur. J., 2011, DOI: 10.1002/chem.201101258.
€
22 M. Kastler, W. Pisula, D. Wasserfallen, T. Pakula and K. Mullen, J.
38 Y. Zhu, R. D. Champion and S. A. Jenekhe, Macromolecules, 2006,
39, 8712–8719.
39 E. Conwell, in Primary Photoexcitations in Conjugated Polymers:
Molecular Excitons versus Semiconductor Band Model, ed. N. S.
Sariciftci, World Scientific, Singapore, 1997, ch. 4.
Am. Chem. Soc., 2005, 127, 4286–4296.
23 S. Cho, J. Lee, M. Tong, J. H. Seo and C. Yang, Adv. Funct. Mater.,
2011, 21, 1910–1916.
24 P. Anant, N. T. Lucas and J. Jacob, Org. Lett., 2008, 10, 5533–
5536.
25 O. Wallquist and R. Lenz, Macromol. Symp., 2002, 187, 617–629.
40 S.-M. Kang and Y. Leblebici, CMOS Digital Integrated Circuits:
Analysis and Design, McGraw-Hill, New York, 1996.
1510 | J. Mater. Chem., 2012, 22, 1504–1510
This journal is ª The Royal Society of Chemistry 2012