Annealing-free high-mobility diketopyrrolopyrrole-quaterthiophene copolymer for solution-processed organic thin film transistors

Article abstract of DOI:10.1021/ja1085996

A donor-acceptor polymer semiconductor, PDQT, comprising diketopyrrolopyrrole (DPP) and β-unsubstituted quaterthiophene (QT) for organic thin film transistors (OTFTs) is reported. This polymer forms ordered layer-by-layer lamellar packing with an edge-on orientation in thin films even without thermal annealing. The strong intermolecular interactions arising from the fused aromatic DPP moiety and the DPP-QT donor-acceptor interaction facilitate the spontaneous self-assembly of the polymer chains into close proximity and form a large π-π overlap, which are favorable for intermolecular charge hopping. The well-interconnected crystalline grains form efficient intergranular charge transport pathways. The desirable chemical, electronic, and morphological structures of PDQT bring about high hole mobility of up to 0.97 cm²/(V·s) in OTFTs with polymer thin films annealed at a mild temperature of 100 °C and similarly high mobility of 0.89 cm²/(V·s) for polymer thin films even without thermal annealing.

Full text of DOI:10.1021/ja1085996

ARTICLE
pubs.acs.org/JACS
Annealing-Free High-Mobility
Diketopyrrolopyrrole-Quaterthiophene Copolymer
for Solution-Processed Organic Thin Film Transistors
Yuning Li,†,‡,* Prashant Sonar,† Samarendra P. Singh,† Mui Siang Soh,† Martin van Meurs,§ and Jozel Tan§
†Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology, and Research (A*STAR),
3 Research Link, Singapore 117602
‡Department of Chemical Engineering and Waterloo Institute for Nanotechnology (WIN), University of Waterloo,
200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1
§Institute of Chemical and Engineering Sciences, A*STAR, 1 Pesek Road, Jurong Island, Singapore 627833
S Supporting Information
b
ABSTRACT: A donor-acceptor polymer semiconductor,
PDQT, comprising diketopyrrolopyrrole (DPP) and β-
unsubstituted quaterthiophene (QT) for organic thin film
transistors (OTFTs) is reported. This polymer forms orde-
red layer-by-layer lamellar packing with an edge-on orienta-
tion in thin films even without thermal annealing. The
strong intermolecular interactions arising from the fused
aromatic DPP moiety and the DPP-QT donor-acceptor
interaction facilitate the spontaneous self-assembly of the polymer chains into close proximity and form a large π-π overlap, which
are favorable for intermolecular charge hopping. The well-interconnected crystalline grains form efficient intergranular charge
transport pathways. The desirable chemical, electronic, and morphological structures of PDQT bring about high hole mobility of up
to 0.97 cm2/(V s) in OTFTs with polymer thin films annealed at a mild temperature of 100 °C and similarly high mobility of 0.89
3
cm2/(V s) for polymer thin films even without thermal annealing.
3
’ INTRODUCTION
or higher.4 To eliminate the thermal annealing step, we pre-
viously designed a polymer, poly(4,8-dihexyl-2,6-bis(3-hexylth-
iophen-2-yl)benzo[1,2-b:4,5-b0]dithiophene) (PBTBDT), which
comprises a fused aromatic ring structure, benzo[1,2-b:4,5-b0]-
dithiophene (BDT).5 Incorporation of BDT in this polymer
facilitated the self-assembly of the polymer chains into a highly
ordered crystalline structure through strong π-π interactions
exerted by the fused BDT units. In addition, the side chain
substitution of PBTBDT was engineered with adequately popu-
lated straight alkyl groups to avoid the interchain interdigitation,
which allowed the desirable polymer main chain packing to take
place preferentially. As a result, PBTBDT could readily form
molecularly orientated thin films comprising large crystal grains
(up to a few micrometers) after solution deposition without ther-
Printed organic thin film transistors (OTFTs) are of extensive
research interest mainly because they can be fabricated at low
costs by using printing techniques.1 In comparison to small mole-
cular or oligomeric materials, polymer semiconductors have
superior solution processability and mechanical robustness.
However, solution-processed polymer thin films always comprise
amorphous regions along with crystalline domains, which is the
primarily reason for their relatively poor charge carrier transport
properties.1a,1c Thermal annealing at around 150 °C or higher is a
commonly used practice to improve the molecular organization
of the polymer thin films to obtain higher charge carrier mobility.
Although high field effect mobility of up to ∼0.6 cm2/(V s) has
3
been achieved for certain polymers2,3 with the aid of thermal
annealing, the mobility for nonannealed polymer thin films is in
general significantly lower. The thermal annealing step is time-
consuming and usually needs to be conducted under vacuum or
in an inert atmosphere to prevent the polymer thin films from
adverse oxidative degradation. This poses a challenge in the cost-
effective production of printed OTFTs in a high-throughput,
roll-to-roll manufacturing manner. Another potential issue asso-
ciated with high-temperature thermal annealing is that common
plastic substrates such as poly(ethylene terephthalate) (PET)
films become dimensionally unstable at a temperature of 150 °C
mal annealing. High mobility of ∼0.25 cm2/(V s) was achieved
3
for nonannealed PBTBDT thin films in OTFTs.5 In this study,
we utilize the intermolecular donor-acceptor interaction as an
additional driving force to assist the self-assembly of the polymer
main chains from solution to thin films. Furthermore, long,
branched alkyl side chains are used to improve the solubility of
the resulting polymer as well as prevent the interference of the
Received: September 23, 2010
Published: January 27, 2011
r
2011 American Chemical Society
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Chart 1. Structures of Thieno[3,2-b]thiophene (X = S; Y =
CH), Thiazolo[3,2-b]thiazole (X = S; Y = N),
Selenopheno[3,2-b]selenophene (X = Se; Y = CH),
Diketopyrrolopyrrole (DPP), PDBT-co-TT, and
DPP-Oligothiophene Copolymers (m g 0)a
DPP, these polymers showed ambipolar transport character-
istics, i.e., capable of conducting both holes and electrons in
OTFTs.9d,9e Here, the use of a longer oligothiophene, β-
unsubstituted QT, in PDQT is expected to adequately com-
pensate for the excessive electron-accepting effect of DPP to
enhance the hole transport characteristics.
The synthetic route to PDQT is outlined in Scheme 1.
Compounds 1, 2, and 3 were prepared in moderate to good
isolated yields according to the established methods.10 Stille
coupling polymerization of 3 with 5,50-bis(trimethylstannyl)-
bithiophene in the presence of a catalytic amount of Pd-
(PPh3)2Cl2 in toluene at 90 °C for 48 h afforded polymer
PDQT. The polymer was purified by using Soxhlet extraction
with acetone, methanol, and hexane successively to remove the
low molecular mass fractions and other impurities. The final
polymer was obtained in a nearly quantitative yield (∼99%).
PDQT is readily soluble in tetrahydrofuran (THF), toluene,
chloroform, chlorobenzene, and a number of other organic sol-
vents. The molecular weights of PDQT were determined by gel
permeation chromatography (GPC) against polystyrene stan-
dards. It was found that the molecular weights (number average
molecular weight (Mn) of 106 000; weight average molecular
weight (Mw) of 365 000) measured using THF as the eluent at 40
°C are significantly higher than the values (Mn = 25 400; Mw =
60 600) obtained at a high temperature of 160 °C using 1,2,4-
trichlorobenzene as the eluent (see the Supporting Information).
Although PDQT appears soluble in THF at low temperatures, it
seems to readily form aggregates composed of several polymer
chains, manifesting the strong intermolecular interactions of this
polymer. On the other hand, the values obtained at 160 °C using
1,2,4-trichlorobenzene as the eluent should represent the mo-
lecular weights of the isolated single polymer chains. A similar
chain aggregation phenomenon was observed previously for
several other conjugated polymers when their molecular weights
were measured using THF as the eluent.11,12 PDQT displayed
absorption maxima, λmax, at 777 and 790 nm in the chloroform
solution and in the thin film, respectively (Figure 1). An optical
band gap of ∼1.2 eV was calculated from the onset absorption
(∼1000 nm) of the polymer thin film. The narrow band gap is
indicative of the extensive delocalization of π electrons, which is
the result of the extended π-conjugation and the intra- and
intermolecular donor (QT)-acceptor (DPP) interactions. The
electrochemical properties of PDQT thin films were character-
ized with cyclic voltammetry (CV) in 0.1 M Bu4NPF4 solution in
dry acetonitrile. A reversible oxidation process was observed
(Figure 2). The highest occupied molecular orbital (HOMO)
energy level of this polymer was calculated from the onset
oxidation potential (∼0.8 V) to be 5.2 eV. The reduction process,
on the other hand, showed significantly lower currents and was
less reversible, indicating that this polymer might favor hole
conduction over electron conduction. It is difficult to accurately
determine the onset reduction potential from the CV curves.
Therefore, the lowest unoccupied molecular orbital (LUMO)
energy level of PQT was obtained from the HOMO level
(5.2 eV) and the optical band gap (1.2 eV) to be 4.0 eV.
a R is a substituent such as an alkyl.
side chain crystallization with the desirable main chain packing.
Specifically, we have chosen 2,5-dihydropyrrolo[3,4-c]pyrrole-
1,4-dione or diketopyrrolopyrrole (DPP) as an electron acceptor
and β-unsubstituted quaterthiophene (QT) as an electron donor
to form a PDQT copolymer (m = 2 in Chart 1). This polymer can
spontaneously form crystalline thin films with an edge-on mole-
cular orientation after solution deposition without a subsequent
high-temperature annealing step. High hole mobility of up to
0.97 cm2/(V s) was achieved for OTFTs using PDQT thin films
3
annealed at a mild temperature of 100 °C. Similarly high mobility
of 0.89 cm2/(V s) was obtained for devices with nonannealed
3
polymer thin films.
’ RESULTS AND DISCUSSION
Among condensed aromatic structures, five-five-membered
fused heterocyclic aromatics such as thieno[3,2-b]thiophene,2,6
thiazolo[5,4-d]thiazole,7 and selenopheno[3,2-b]selenophene8
are useful building blocks for p-type polymer semiconductors
for OTFTs (Chart 1). Another analogous fused aromatic frame-
work, DPP (Chart 1), has also been found a good moiety for
constructing polymers for OTFT applications.9 Very recently, we
reported that the combination of the thieno[3,2-b]thiophene unit
as a donor and the DPP unit as an acceptor led to a donor-
acceptor copolymer, PDBT-co-TT (Chart 1), which showed high
hole mobility of up to 0.94 cm2/(V s).10 The enhanced inter-
3
molecular interactions due to the donor-acceptor interaction
are thought responsible for the observed high charge carrier
transport performance. However, PDBT-co-TT thin films re-
quire annealing at a high temperature of 200 °C to maximize the
mobility likely due to the rigid condensed thieno[3,2-b]thio-
phene moiety. In this study we use the unsubstituted bithiophene
(m = 2, Chart 1), instead of thieno[3,2-b]thiophene, to slightly
weaken the main chain interactions to enhance the molecular
ordering capability of the resulting polymer PDQT at lower
temperatures. Previously, DPP-bithiphiophene and DPP-ter-
thiophene copolymers (m = 0 and 1 in Chart 1) have been repor-
The crystallinity of PDQT was characterized by using differ-
ential scanning calorimetry (DSC). During the first heating scan,
an exothermic transition temperature at 300 °C was observed,
which is attributable to the melting of the polymer backbone (see
the Supporting Information). The expected absence of the
melting of the side chains indicates that the crystallization of
the long, branched 2-octyl-1-dodecyl side chains is prohibited,
ted, which showed hole mobility of less than ∼0.1 cm2/(V s) in
3
OTFTs.9c-9e Due to the strong electron-accepting nature of
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Scheme 1. Synthesis of Polymer PDQTa
a Reagents and conditions: (i) diisopropylsuccinate/t-C5H11ONa/tert-amyl alcohol/120 °C, 83%; (ii) 2-octyl-1-dodecyl bromide/
K2CO3/DMF/130 °C, 54%; (iii) Br2/chloroform/room temperature, 78%; (iv) 5,50-bis(trimethylstannyl)bithiophene/Pd(PPh3)2Cl2/
toluene/90 °C, 99%.
Figure 2. Cyclic voltammogram of a PDQT film on a platinum
electrode showing the first scan of cathodic and anodic cycles at a scan
rate of 100 mV/s in a 0.1 M tetrabutylammonium hexafluorophosphate
solution in anhydrous acetonitrile.
polymer, which showed one distinct diffraction peak at 2θ = 4.4°,
corresponding to a d spacing of 19.9 Å (Figure 3a). Upon thermal
annealing at temperatures of 100 and 200 °C, the primary
diffraction peak progressively intensified and the second- and
third-order peaks became visible. No diffraction originating from
the π-π stacking distance could be observed. This is reminiscent
of P3HT13 and several other polymers2,5,6,10,14 which form layer-
by-layer lamellar structures with the π-π stacking direction
parallel to the substrate in thin films as illustrated in Figure 3b. To
further substantiate the edge-on molecular orientation in the
polymer thin films, two-dimensional (2-D) transmission XRD
measurements were carried out on a stack of polymer thin films
with the incident X-ray parallel and perpendicular to the films. As
shown in Figure 3c,d, the XRD image and diffractogram obtained
with the incident X-ray parallel to the films exhibited diffraction
patterns similar to those obtained from the thin film cast on the
Si/SiO2 substrate (Figure 3a). On the other hand, data obtained
with the incident X-ray perpendicular to the film stack are
dramatically different (Figure 3e,f). A strong diffraction peak at
2θ = 23.7° appeared, while the diffraction at 2θ = 4.4° became
relatively weak. The peak at 2θ = 23.7° can be assigned to the
π-π stacking distance of ∼3.75 Å. This π-π stacking distance
Figure 1. UV-vis-NIR absorption spectra of PDQT and PDQT-a in
(a) chloroform solutions and (b) thin films.
while the crystallization of the polymer main chain dominates
during the molecular packing. A slightly lower melting point and
a smaller enthalpy of transition were observed in the second heat-
ing scan. This is likely caused by the partial degradation of the
polymer at high temperatures, because the thermogravimetric
analysis (TGA) results showed that the decomposition of PDQT
starts to occur at ∼350 °C (see the Supporting Information).
X-ray diffraction (XRD) analysis of an as-cast, nonannealed PDQT
thin film (∼100 nm) on 1-octyltrichlorosilane (OTS)-modified
Si/SiO2 substrate further confirmed the crystalline nature of this
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Figure 3. XRD intensity graphs of drop-cast PDQT thin films (∼100 nm) on OTS-modified Si/SiO2 substrates annealed at different temperatures (a).
Illustrative lamellar structures of PDQT in the thin film (b). Respective 2-D XRD image and pattern intensity graph obtained with the incident X-ray
parallel to the thin film stack (c, d). Respective 2-D XRD image and pattern intensity graph obtained with the incident X-ray perpendicular to the thin
film stack (e, f).
is slightly larger than that (3.71 Å) of PDBT-co-TT,10 indicating
the weaker intermolecular interactions of the former, but still
relatively small as compared with those (usually 3.8-3.9 Å) of
most other conjugated polymers5,6,15 The above XRD results
unambiguously revealed the formation of a highly ordered layer-
by-layer lamellar packing motif with an edge-on orientation
(Figure 3b) for the annealed and even the nonannealed polymer
thin films. This type of molecular organization is particularly
favorable for charge carrier hopping through π-π stacks in an
OTFT device where the source-drain current flows along the
semiconductor layer parallel to the substrate.13
PDQT was evaluated as a channel semiconductor in OTFTs
using a bottom-gate, top-contact configuration. Heavily doped
nþ-Si/SiO2 wafer was used as the substrate, wherein the con-
ductive Si wafer and the surface SiO2 layer (∼200 nm) function
as the common gate electrode and the gate dielectric, respec-
tively. Prior to use, the SiO2 surface was modified with a
monolayer of OTS. A PDQT thin film (∼35 nm) was deposited
on the substrate by spin-coating a solution of PDQT in chloro-
form (6 mg/mL) at room temperature and optionally annealed
at 100-200 °C on a hot plate under nitrogen in a glovebox.
OTFT devices with a channel length (L) of 100 μm and a chan-
nel width (W) of 1 mm were then obtained by thermally evapo-
rating gold through a shadow mask to pattern the source and
drain electrode pairs. OTFT devices were characterized in a
glovebox under nitrogen.
Figure 4 shows the output and transfer curves of two OTFT
devices having PDQT thin films without thermal annealing and
annealed at 100 °C, respectively. For the OTFT device with the
nonannealed thin film, the hole mobility extracted from the satu-
ration regime is 0.89 cm2/(V s), with a current on-to-off ratio
3
(Ion/Ioff) of ∼107 and a threshold voltage (VT) of -2.6 V. The
OTFT device with a PDQT thin film annealed at 100 °C exhi-
bited higher mobility of 0.97 cm2/(V s) (Ion/Ioff = ∼106;
3
VT = -2.7 V). No further improvements in mobility were
observed when the polymer thin films were annealed at a
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Figure 5. Transfer characteristics of an OTFT using a PDQT thin film
annealed at 100 °C with the drain current versus the forward (VG = þ20
to -70 V) and reverse (VG = -70 to þ20 V) sweep gate voltages under
a VD of -70 V at a sweep rate of 0.5 V/s. The inset shows the plots of the
square root of the drain current versus the gate voltage in forward and
reverse sweeps. Arrows indicate the sweeping directions of the gate
voltage.
under the same conditions. Surprisingly, Mn and Mw of PDQT-a
measured at 160 °C using 1,2,4-trichlorobenzene as the eluent
are respectively 21 100 and 50 700, only slightly lower than those
of PDQT (Mn = 25 400; Mw = 60 600). The significantly lower
molecular weights of PDQT-a measured at 40 °C as compared to
those of PDQT suggest that the slightly shorter polymer chains
of PDQT-a lead to notably reduced intermolecular interactions.
The UV-vis-NIR absorption profiles of PDQT-a in solution
and in the solid state are almost identical to those of the high
molecular weight PDQT (Figure 1). PDQT-a showed a margin-
ally lower melting temperature (Tm = 298 °C) and a smaller
enthalpy of transition (ΔH = 28.7 J/g), as compared with PDQT
(Tm = 300 °C; ΔH = 34.1 J/g), suggesting the slightly poorer
crystallinity of the former. The XRD patterns of PDQT-a indi-
cate that the polymer chains adopt an edge-on lamellar packing
motif similar to that of PDQT (see the Supporting Information).
The nonannealed and annealed (at 100 °C) PDQT-a thin films
Figure 4. Output (VG = 0 to -70 V) and transfer (VD = -70 V) cha-
racteristics of OTFTs with PDQT thin films without annealing (a, b)
and annealed at 100 °C (c, d) (L = 100 μm; W = 1 mm).
temperature beyond 100 °C (up to 200 °C), although the films
showed higher crystallinity at higher temperatures (Figure 3a).
The fact that PDQT requires a much milder annealing tempera-
ture of 100 °C to achieve the maximum mobility as compared
with most other high-mobility polymers2,3,6,7,10,14 indicates that
the PDQT polymer chains could readily establish efficient charge
transport structures at low temperatures. In our experiments,
exhibited mobility of 0.35-0.39 cm2/(V s) in OTFT devices,
3
saturation mobilities of 0.71-0.89 cm2/(V s) for nonannealed
substantially lower than the mobility observed for the high
molecular weight PDQT.
3
and 0.80-0.97 cm2/(V s) for annealed (100 °C) PDQT thin
3
films were obtained from 12 respective OTFT devices.
AFM height images of the nonannealed PDQT thin film show
randomly orientated nanorods which further comprise densely
packed grains (∼20-30 nm) (Figure 6a,d). Upon annealing at
100 °C, the nanorod texture remains similar (Figure 6b), but
small holes a few nanometers in diameter appeared on the grains
(Figure 6e). On the other hand, the thin film of the low molecular
weight PDQT-a shows less defined grains in its AFM images
(Figure 6c,f) as compared to that of the high molecular weight
PDQT. Furthermore, many pinhole-like voids are observed in
the nonannealed PDQT-a thin film, which implies the poor
connectivity between grains. Therefore, we believe that the sli-
ghtly higher molecular weights (or longer chains) of PDQT as
compared to PDQT-a could substantially strengthen the inter-
molecular interactions to form more interconnected crystalline
grains, resulting in excellent charge transport performance.15,17-19
The exceptionally high hole mobility observed for PDQT can
be attributed to a number of factors. First, the enhanced inter-
molecular interactions arising from (a) the fused aromatic DPP
moiety and (b) the donor (QT)-acceptor (DPP) interactions
The hysteresis characteristics of OTFTs using PDQT were
studied by sweeping the gate voltage in both the forward and
reverse directions. Figure 5 shows the transfer characteristics of
an OTFT with the drain current versus the forward and reverse
sweep gate voltages under a source-drain voltage (VD) of -70 V
at a sweep rate of 0.5 V/s. The device hysteresis, which is defined
as the difference between the forward and reverse threshold vol-
tages (ΔVT),16d is ∼8.0 V. The extent of the hysteresis effect
observed for PDQT is similar to that of other common organic
semiconductors.16 The device exhibited hole mobility of 0.91 and
1.06 cm2/(V s) in the saturation regime for the forward and the
3
reverse sweeps, respectively.
To investigate the influence of the molecular weight on the
mobility, PDQT-a with a lower molecular weight was prepared
by shortening the polymerization time. Mn and Mw of PQDT-a
were first determined with GPC at 40 °C using THF as the eluent
against polystyrene standards to be 45 200 and 97 200, much lower
than those (Mn = 106 000; Mw = 365 000) of PQDT measured
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Figure 6. AFM height images of the nonannealed PDQT (a, d), annealed (100 °C) PDQT (b, e), and annealed (100 °C) PDQT-a (c, f) thin films
(∼35 nm) on OTS-modified Si/SiO2 substrates.
bring polymer chains into close proximity and form a large π-π
overlap, which facilitate the intermolecular charge hopping thro-
ugh π-π stacks.1a,1c Second, the long electron-donating QT
sufficiently compensates the electron-accepting effect of DPP. In
comparison to PDQT, other DPP-oligothiophene copolymers
with fewer thiophene units such as DPP-bithiophene9c,9e and
DPP-terthiophene9d copolymers (Chart 1) showed consider-
a long, branched 2-octyl-1-dodecyl side chain afforded excellent
solubility of PDQT in various solvents. Thermal analysis and
XRD results suggested that PDQT is highly crystalline and could
readily form crystalline thin films with preferential edge-on mole-
cular orientation after solution deposition even without thermal
annealing. It was found that the molecular weight of the polymer
significantly influences the thin film morphology. The high poly-
mer weight PDQT formed thin films comprising densely packed
nanometer-sized grains, while the thin film of the low molecular
weight PDQT-a formed poorly interconnected structures with
populous pinholes. The high molecular weight PDQT showed
ably lower hole mobility of e∼0.1 cm2/(V s) and displayed pro-
3
nounced ambipolar transport characteristics in OTFTs. Third,
the use of the β-unsubstituted QT unit provides an adequate
spacing to avoid steric interactions between adjacent branched
side chains to maintain coplanarity of the polymer backbone. It
was reported that a structural analogue of PDQT, poly[3,6-
bis(40-dodecyl[2,20]bithiophenyl-5-yl)-2,5-bis(2-hexyldecyl)-2,5-
dihydropyrrolo[3,4-c]pyrrole-1,4-dione], which has side chain
substitution on the inner bithiophene moiety, exhibited much
high hole mobility of up to 0.89 cm2/(V s) in OTFTs with spin-
3
coated PDQT thin films without thermal annealing, which is
close to the maximum mobility of ∼0.97 cm2/(V s) achieved for
3
polymer thin films annealed at 100 °C. On the other hand, the
low molecular weight PDQT-a exhibited substantially lower
lower mobility (∼0.1 cm2/(V s)).9a The crowded side chain sub-
mobility of 0.35-0.39 cm2/(V s) under similar fabrication con-
3
3
stitution in this polymer might perturb the main chain coplanar-
ization and the intermolecular interactions in the solid state, resul-
ting inless ordered molecularorganization. Finally, thesufficiently
high molecular weight of PDQT enables the polymer chains to
self-assemble into denselypackednanometer-sizedgrains through
intensified intermolecular interactions, which interconnect the
crystalline grains to provide efficient pathways for intergranular
charge transport.15,17-19 The slightly weakened intermolecular
interactions as compared with PDBT-co-TT10 allow the sponta-
neous formation of well-interconnected thin film morphology with
a highly ordered, crystalline structure after solution deposition with-
out a subsequent high-temperature treatment, which accounts for
the high mobility achieved for the nonannealed PDQT thin films.
ditions. The significantly high hole mobility observed for PDQT
processed at mild temperatures might be contributable to (i) the
strong intermolecular interactions arising from the fused ring
DPP moiety and the intermolecular donor-acceptor interaction
that facilitate charge hopping through π-π stacks, (ii) the pro-
nounced hole transport capability by using a sufficiently long
electron-donating quaterthiophene, (iii) the spaced out side
chain arrangement by using β-unsubstituted quaterthiophene
to help polymer chains self-assemble into highly ordered crystal-
line structures, and (iv) the sufficiently high molecular weight
that enables polymer chains to form well-interconnected crystal-
line grains to establish efficient pathways for intergranular charge
transport. The high mobility, not requiring annealing, excellent
solution processability, and facile synthesis have rendered this
polymer potentially useful for high-throughput, roll-to-roll man-
ufacturing of low-cost OTFT circuits and arrays for a wide range
of electronic applications.
’ CONCLUSIONS
A donor-acceptor DPP-quaterthiophene copolymer PDQT
was designed and synthesized. Substitution of the DPP unit with
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’ ASSOCIATED CONTENT
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Phys. Rev. B 2006, 74, 115318.
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Supporting Information. Experimental details and addi-
b
tional graphs. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
yuning.li@uwaterloo.ca
(19) Singh, K. A.; Sauvꢀe, G.; Zhang, R.; Kowalewski, T.; McCullough,
R. D.; Porter, L. M. Appl. Phys. Lett. 2008, 92, 263303.
’ ACKNOWLEDGMENT
We are grateful to Mr. Poh Chong Lim for assistance in obtain-
ing XRD data, Ms. Lin Ting Ting for assistance in molecular
modeling, and Dr. Beng S. Ong for technical discussions.
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