Chemistry of Materials
Article
CMOS devices that combine n-channel and p-channel
semiconducting materials have low power dissipation require-
ments, a high noise tolerance margin, and excellent robust-
ness.2,13,24−28 For the construction of CMOS-like inverters, it is
more desirable to use single-component ambipolar transistors
than a combination of p- and n-channel transistors. The use of
single-component transistors can simplify the design of circuits
and minimize the number of device fabrication steps. In this
regard, low band gap (typically, <1.5 eV) polymers with an
alternating donor−acceptor structure are particularly suited for
applications that require ambipolar transistors. The low energy
offset between a metal’s Fermi level and the frontier energy
levels (i.e., the HOMO and LUMO) means that low band gap
polymers have intrinsically low injection barriers for holes and
electrons at the interface. However, low band gap polymer-
based ambipolar transistors have not been systematically
studied,29−33 and the current state-of-the-art balanced hole and
electron mobilities are 0.35 cm2 V−1 s−1 and 0.40 cm2 V−1 s−1,
respectively.30 To further advance the development of
ambipolar polymer transistors, an in-depth investigation of
the key factors that affect charge transport characteristics is
required, including a study of the dependence of donor−
acceptor structures on polymer chain stacking and orientation,
the role of branched alkyl side chains, and the influence on
transport of the alignment of a polymer’s HOMO and LUMO
with respect to the contact metal Fermi level.
2. EXPERIMENTAL SECTION
2.1. Materials and Synthesis. Pd2dba3, Pd(PPh3)2Cl2, Aliquat
336, n-BuLi (1.6 M in hexane), thiophene-2-carbonitrile, dibutyl
succinate, and N-bromosuccinimide (NBS) were purchased from
Sigma-Aldrich, Acros, and TCI. Common organic solvents were
purchased from Daejung and J. T. Baker. Tetrahydrofuran (THF) was
dried over sodium and benzophenone prior to use. All other reagents
were used as received without further purification. The synthetic route
and chemical structures of polymers used in this study are shown in
Schemes 1 and 2. Compounds 1−8 were synthesized according to
literature procedures.18,32,34
2.1.1. Synthesis of pDPPT3-HD. To a degassed 7 mL toluene
solution of 2,5-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
thiophene (90 mg, 26.8 mmol), monomer 5 (0.242 g, 26.8 mmol),
K3PO4 (0.284 g, 1.34 mmol), and three drops of Aliquat 336 was
added 0.7 mL of degassed demineralized water. Pd2dba3 (9.8 mg,
0.0107 mmol) and triphenylphosphine (7.0 mg, 0.0268 mmol) were
then added to the reaction mixture. The reaction solution was stirred
for 1 h at 60 °C and 3 days at 90 °C under argon atmosphere. The
reaction mixture was poured into methanol/HCl (6/1 v/v%) solution.
The precipitated polymer was redissolved in chloroform and
reprecipited in MeOH. The collected polymer was further purified
by Soxhlet extraction using methanol, hexane, acetone, and chloro-
form. The chloroform fraction was collected and reprecipitated in
methanol and filtered. The polymer was dried under vacuum, yielding
176 mg (79.3%). 1H NMR (CDCl3, 400 MHz) δ (ppm) 9.05 (b, 2H),
6.97 (b, 4H), 4.03 (b, 4H), 2.20 (b, 2H), 1.28−1.20 (b, 48 H), 0.89
(b, 12H). Anal. Calcd (%): C, 72.41; H, 8.75; N, 3.38; O, 3.86; S,
11.60. Found (%): C, 72; H, 8.7; N, 3.3; O, 4.1; S, 11.7. GPC Mn = 33
275; PDI = 2.83.
In this study, we synthesized low band gap polymers based
on oligothiophene-alt-DPP units with a systematic variation of
oligothiophene units and branched side alkyl chain lengths.
Synthetic routes are shown in Scheme 1. In synchrotron X-ray
2.1.2. Synthesis of pDPPT3-OD. pDPPT3-OD was synthesized with
the same method used for pDPPT3-HD, using 2,5-bis(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)thiophene (90 mg, 26.8 mmol)
and monomer 7 (0.273 g, 26.8 mmol). The resulting polymer was
1
Scheme 1. Synthesis of Oligothiophene-alt-DPP Polymers of
pDPPT3-HD, pDPPT3-OD, pDPPT2TT-HD, and
pDPPT2TT-OD
obtained in 239 mg (94.7%). H NMR (CDCl3, 400 MHz) δ (ppm)
9.02 (b, 2H), 7.02 (b, 4H), 4.03 (b, 4H), 1.93 (b, 2H), 1.26−1.19
(b, 56 H), 0.84 (b, 12H). Anal. Calcd (%): C, 73.99; H, 9.42; N, 2.98;
O, 3.40; S, 10.22. Found (%): C, 73.8; H, 9.5; N, 2.8; O, 3.4; S, 10.2.
GPC Mn = 33 656; PDI = 6.08
2.1.3. Synthesis of pDPPT2TT-HD. Monomer 8 (200 mg,
42.9 mmol) and monomer 5 (388 mg, 42.9 mmol) were dissolved
in degassed 7 mL of anhydrous toluene. Subsequently, a catalyst,
Pd(PPh3)2Cl2 (9.04 mg, 0.0129 mmol) was added into the reaction
mixture and heated at 60 °C under argon atmosphere. After stirring for
1 h, the reaction solution was heated at 110 °C for 3 h under argon
atmosphere. The reaction mixture was poured into methanol/HCl
(6/1 v/v %) solution. The precipitated polymer was dissolved in
chloroform, and the polymer was reprecipitated in methanol. The
collected polymer was further purified by Soxhlet extraction using
methanol, hexane, acetone, and chloroform. The chloroform fraction
was collected and reprecipitated in methanol and filtered. The polymer
was dried under vacuum, yielding 181 mg (60.0%). 1H NMR (CDCl3,
400 MHz) δ (ppm) 8.98 (b, 2H), 7.17 (b, 4H), 4.10 (b, 4H), 2.01
(b, 2H), 1.31−1.27 (b, 48 H), 0.91 (b, 12H). Anal. Calcd (%): C,
70.54; H, 8.20; N, 3.16; O, 3.61; S, 14.49. Found (%): C, 70.4; H, 8.3;
N, 3.1; O, 3.39; S, 14.5. GPC Mn =20 657; PDI = 3.94.
diffraction (XRD) and atomic force microscopy (AFM)
measurements, we observed that longer branched alkyl side
chains and longer interdistances between alkyl chains in
the polymer backbone improve the crystalline structures in
the polymer film. Moreover, thermal treatment stimulates the
crystallinity in the polymer film. The polymer field-effect
transistors (PFETs) showed excellent ambipolar behavior with
high carrier mobilities of up to 2.2 and 0.2 cm2 V−1 s−1, respec-
tively. This work demonstrates that elaborate tuning of
chemical structure and thermal annealing significantly modulate
the interchain organization, which in turn affects the ambipolar
PFET device performance.
2.1.4. Synthesis of pDPPT2TT-OD. pDPPT2TT-OD was synthe-
sized with the same method used for pDPPT2TT-HD, using
monomer 8 (200 mg, 42.9 mmol) and monomer 7 (0.438 g,
42.9 mmol) with a longer polymerization period of 3 days at 110 °C.
1
The resulting polymer was obtained in 408 mg (95.0%). H NMR
(CDCl3, 400 MHz) δ (ppm) 9.02 (b, 2H), 7.03 (b, 4H), 4.05 (b, 4H),
2.06 (b, 2H), 1.29−1.25 (b, 56 H), 0.89 (b, 12H). Anal. Calcd (%): C,
72.24; H, 8.89; N, 2.81; O, 3.21; S, 12.86. Found (%): C, 72.3; H, 8.7;
N, 2.7; O, 3.3; S, 12.8. GPC Mn = 42 447; PDI = 4.34.
1
2.2. Material Characterization. H NMR spectra were recorded
on a Bruker advance 400 spectrometer (400 MHz). The molecular
weights of polymers were measured by gel permeation chromatog-
raphy (GPC) using chloroform as an eluent and polystyrene as a
1317
dx.doi.org/10.1021/cm2037487 | Chem. Mater. 2012, 24, 1316−1323