Optimal ambipolar charge transport of thienylenevinylene-based polymer semiconductors by changes in conformation for high-performance organic thin film transistors and inverters

Article abstract of DOI:10.1021/cm303908f

We report the synthesis and characterization of thienylenevinylene-based donor-acceptor alternating copolymers (PTVPhI-Eh and PTVPhI-C12) as highly efficient ambipolar semiconductors in a thin film transistor. These polymers exhibit significantly improved hole and electron mobilities after thermal annealing. To determine the relationship between ambipolar charge transport and thermal annealing, we investigated these polymers using various analyses such as optical spectroscopy, Raman spectroscopy, computational quantum chemical calculation, X-ray diffraction, atomic force microscopy, and ambipolar charge mobility measurements. In pristine films, the polymer chains exhibited weak intra- and interchain ordering. However, when samples were annealed at sufficiently high temperatures, they exhibited a more ordered intra- and interchain conformation. As a result, we found a strong relationship between intra- and interchain conformational changes of the polymers and corresponding ambipolar charge transport properties during thermal annealing processes. Finally, we demonstrate complementary-like ambipolar inverters using a PTVPhI-Eh polymer. The largely shifted inverting voltage was improved for the thermally annealed inverters, which exhibited large voltage gains (～40).

Full text of DOI:10.1021/cm303908f

Article
pubs.acs.org/cm
Optimal Ambipolar Charge Transport of Thienylenevinylene-Based
Polymer Semiconductors by Changes in Conformation for High-
Performance Organic Thin Film Transistors and Inverters
Juhwan Kim,† Kang-Jun Baeg,∥ Dongyoon Khim,† David T. James,⊥ Ji-Seon Kim,⊥ Bogyu Lim,@
Jin-Mun Yun,† Hyung-Gu Jeong,† Paul S. K. Amegadze,§ Yong-Young Noh,*,§ and Dong-Yu Kim*,†,‡
†School of Materials Science and Engineering, Gwangju Institute of Science and Technology, 1 Oryong-Dong, Buk-Gu, Gwangju
500-712, Republic of Korea
‡Department of Nanobio Materials and Electronics, Gwangju Institute of Science and Technology, 1 Oryong-Dong, Buk-Gu,
Gwangju 500-712, Republic of Korea
§Department of Energy and Materials Engineering, Dongguk University, 26 Pil-dong, Jung-gu, Seoul 100-715, Republic of Korea
∥Nano Carbon Materials Research Group, Korea Electrotechnology Research Institute (KERI), 12 Bulmosan-ro 10beon-gil,
Seongsan-gu, Changwon, Gyeongsangnam-do 642-120, Republic of Korea
⊥Department of Physics and Centre for Plastic Electronics, Imperial College London, London SW7 2AZ, U.K.
@Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States
S
* Supporting Information
ABSTRACT: We report the synthesis and characterization of
thienylenevinylene-based donor−acceptor alternating copoly-
mers (PTVPhI-Eh and PTVPhI-C12) as highly efficient
ambipolar semiconductors in a thin film transistor. These
polymers exhibit significantly improved hole and electron
mobilities after thermal annealing. To determine the relation-
ship between ambipolar charge transport and thermal
annealing, we investigated these polymers using various
analyses such as optical spectroscopy, Raman spectroscopy,
computational quantum chemical calculation, X-ray diffraction, atomic force microscopy, and ambipolar charge mobility
measurements. In pristine films, the polymer chains exhibited weak intra- and interchain ordering. However, when samples were
annealed at sufficiently high temperatures, they exhibited a more ordered intra- and interchain conformation. As a result, we
found a strong relationship between intra- and interchain conformational changes of the polymers and corresponding ambipolar
charge transport properties during thermal annealing processes. Finally, we demonstrate complementary-like ambipolar inverters
using a PTVPhI-Eh polymer. The largely shifted inverting voltage was improved for the thermally annealed inverters, which
exhibited large voltage gains (∼40).
KEYWORDS: ambipolar polymer, charge transport, OTFTs, thienylenevinylene, polymer conformation
organic CMOS (complementary metal−oxide−semiconductor)
ICs via a simple blanket coating without the need for
sophisticated micropatterning processes of n- and p-channel
conjugated polymers. Therefore, many research groups are
actively developing ambipolar conjugated polymers to achieve
organic and flexible ICs via a simple large area blanket coating.4
To realize high-performance ambipolar OTFTs and organic
CMOS-based ICs, ambipolar conjugated polymers must meet
the following requirements: a high field effect mobility, a low
operating voltage, balanced p- and n-channel device character-
istics, and a suitable energy level for efficient hole and electron
injection from a single electrode, which is typically Au.
1. INTRODUCTION
Organic thin film transistors (OTFTs) based on conjugated
polymers have attracted much attention in recent years for
application as an active layer, because of solution processability
and an excellent compatibility with flexible substrates.1 These
unique properties make them useful in the fabrication of
various flexible electronics, such as organic integrated circuits
(ICs), radiofrequency identification (RFID) tags, and sensors,
and memory using cost-effective graphic art printing processes.2
Because of advances gained over the past two decades, state-of-
the-art printed OTFTs boast a charge carrier mobility that is
equal to or even higher than that of amorphous silicon TFTs in
an active matrix TFT-LCD.3 To maximize their advantage in
the manufacturing process, OTFTs and their circuitry should
be fabricated by a process that is as simple as possible.
Ambipolarity in charge transport allows the realization of
Received: December 5, 2012
Revised: March 12, 2013
Published: April 4, 2013
© 2013 American Chemical Society
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Recently, conjugated polymers with alternative donor−
acceptor (D−A) moieties in the backbone have been developed
for high-performance organic photovoltaics because of the
electron push−pull structure that can be used to make low-
band gap polymers and the ability to control the copolymer
highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) energies independ-
ently.5 In addition, D−A polymers can also enhance the
intermolecular interaction leading to a small π−π stacking
distance caused by the dipole interaction between donor and
acceptor moieties. Therefore, D−A polymers are expected to be
a new alternative for active materials in OTFTs with efficient
electron and hole ambipolar transport. However, most of the
existing polymers predominantly exhibit unipolar (p- or n-type)
transport behavior or lower-level and unbalanced ambipolar
transport. Among them, excellent ambipolar transport charac-
teristics have been reported for some D−A polymers
containing diketopyrrolopyrrole (DPP), rylene imide, and
benzobisthiadiazole (BBT) as electron deficient units.4,6
McCulloch and co-workers have reported the synthesis of
DPP and a thieno[3,2-b]thiophene-based copolymer (PDPP-
TT-T-TT), with hole and electron mobilities of 1.42 and 0.063
cm2 V−1 s−1, respectively.5d Naphthalenebiscarboximide-bithio-
phene copolymer (PNIBT) with ambipolar mobilities of 0.003
cm2 V−1 s−1 for holes and 0.04 cm2 V−1 s−1 for electrons and a
high gain of 30 from a PNIBT-based ambipolar inverter was
synthesized by Jenekhe and co-workers.4c Wudl and co-workers
also synthesized a BBT-based copolymer, in which BBT was
paired with dithienopyrrole, dithienocyclopentane, dithienosi-
lole, and fluorene. In these copolymers, a BBT-dithienocyclo-
pentane-paired copolymer showed evenly balanced ambipolar
properties, with hole and electron mobilities of 0.13 and 0.1
cm2 V−1 s−1, respectively.4d Herein, these previous results imply
the design approach for the achievement of excellent ambipolar
polymers, and consequently, the following design requirements
are suggested: (i) a structural highly planar π-conjugated unit
for efficient hole and electron transport by efficient π-orbital
delocalization, largely delocalized LUMO and HOMO orbitals,
and solid-state packing such as favorable intermolecular π−π
stacking; (ii) a decrease in LUMO energies by using an electron
deficient unit or an electron-withdrawing substituent to
enhance electron transport and air stability; (iii) alkyl side
chains for solubility and processability with little or no steric
hindrance to the backbone. To satisfy those design require-
ments, D−A-type conjugated polymers with fused aromatic
donor and acceptor moieties are ideal structures because the
fused structures allow highly planar conformations and strong
π−π interactions. Regrettably, it is relatively difficult to
synthesize fused aromatic compounds. In addition, the
solubility of such planar fused polymer systems decreases as
the size of the fused ring increases. Therefore, donor and
acceptor moieties that can be facilely synthesized and satisfy the
design requirements mentioned above are needed for successful
achievement of excellent ambipolar polymers.
conjugated polymers, it is not yet fully understood how the
process of controlling ambipolar transport affects device
performance. Therefore, the study of ambipolar transport
through new polymers is needed to gain insight into the
mechanisms of charge transport in ambipolar conjugated
polymers.
In this study, we synthesized and characterized ambipolar
D−A polymers using a new combination of a donor and an
acceptor based on the aforementioned design requirements.
First, a thienylenevinylene (TV) unit was introduced as an
electron-donating building block. The TV unit has good
planarity and an extended π-conjugation because of the
presence of a vinylene spacer between the thiophene units.
This was expected to induce a high hole mobility and a low
band gap in the polymer.8 Also, sufficient solubility was induced
as a result of alkyl substitution at position 3 or 4 or at both
positions of the thiophene in the TV unit. Our previous work
demonstrated the potential of an alkyl-substituted TV building
block that showed a high crystallinity and mobility as well as
excellent solubility.3a An imide-functionalized π-system, as an
electron deficient unit, was also introduced to complete the D−
A polymer. Electron deficient imide-functionalized π-systems
are gaining attention as a new approach to high-mobility n-type
or ambipolar materials, because of their planar structure and the
strong intermolecular interactions among the imide groups.9
Imide-functionalized π-systems with these properties have been
employed to impart strong electron transport characteristics
and also to alter the band gap when inserted into
polythiophene backbones. Among these imide-functionalized
moieties, N-alkyl-3,6-dibromophthalimides are a sufficiently
promising electron-accepting unit for the D−A copolymer.10
These moieties are very useful versatile building blocks because
their synthesis involves only two facile steps and because varied
alkyl substitution can be conducted on the imide nitrogen atom
to control the solubility, packing, and morphology. In addition,
it is well-known that phthalimides improve air stability and
reliability because they lead to a decrease in HOMO levels via
the electron withdrawing nature of an imide.11 Therefore, we
chose phthalimide as the accepting unit in a D−A polymer for
this study. The dodecyl-substituted TV and phthalimide-paired
copolymers (PTVPhI-Eh and PTVPhI-C12) exhibited lower
HOMO levels (−5.24 eV) and good ambipolar transport
characteristics.
Here, we describe the intra- and interchain conformational
changes of this ambipolar D−A polymer, PTVPhI polymers,
under various annealing conditions using optical spectroscopy,
Raman spectroscopy, computational quantum chemical calcu-
lation, X-ray diffraction (XRD), and atomic force microscopy
(AFM). OTFTs based on these polymers exhibited further
increased electron and hole mobilities via the annealing process,
which also resulted in balanced ambipolar mobility. We found a
relationship between ambipolar charge transport and intra- and
interchain conformational changes upon thermal annealing of
PTVPhI polymers. According to the results of this study, the
hole and electron mobilities of these polymers are affected by a
combination of intra- and interchain conformational changes.
In particular, electron mobility is more significantly affected by
interchain conformational changes than by changes of hole
mobility. Finally, complementary ambipolar inverters were
demonstrated by spin-coating without patterning, resulting in a
sharp switching signal and large voltage gains (∼40). In
addition, a large shift in inverting voltage in pristine OTFTs
was greatly improved for the thermally annealed devices
Material design and control of processing conditions are
important factors in obtaining well-balanced charge transport
properties in ambipolar conjugated polymers. Many previous
reports suggest several approaches to the process of improving
ambipolar properties such as high-temperature annealing and
charge injection engineering via chemical modification of the
electrode or the insertion of an interlayer between an organic
active layer and an electrode, to control electron and hole
mobility.7 Despite many related studies of ambipolar
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onto the sample for an exposure time of 1 s with five accumulations,
which resulted in a good signal-to-noise ratio with little or no
background fluorescence signal. All samples were measured in air and
required no special sample preparation.
because of the achievement of a more balanced hole and
electron mobility.
2. EXPERIMENTAL SECTION
Quantum Chemical Calculations. To interpret the Raman
spectra, quantum chemical calculations were performed on the
PTVPhI monomer in the virtual gas phase using GAUSSIAN09. To
save on computation cost without significantly altering the results of
the calculated properties, the long alkyl side chains were replaced with
methyl groups.12 Molecules were first structurally optimized followed
by vibrational frequency calculation using density functional theory
(DFT) at the B3LYP level of theory13 and using the 6-31G* basis set.
To evaluate the dependence of the vibrational modes on backbone
planarity, the thienylenevinylene-phthalimide dihedral angle was
frozen in a range of values, and the structure was reoptimized before
calculation of the vibrational frequencies of the twisted structures. An
empirical scaling factor of 0.96 was applied to the calculated
wavenumbers for comparison with the experimental spectra.14
Molecular orbital distributions along the conjugated backbone were
calculated by first structurally optimizing a PTVPhI dimer and trimer
in the gas phase using the Austin Model 1 (AM1) semiempirical
method. Excited-state energy calculations were then performed on the
optimized structures using Zerner’s Intermediate Neglect of Differ-
ential Orbital method (ZINDO/S), from which the HOMO and
LUMO isosurfaces were mapped.15
Fabrication and Characterization of OTFTs and Inverters.
OTFTs and inverters were fabricated on Corning Eagle 2000 glass
substrates with evaporation of gold/nickel (Au/Ni) (15/3 nm)
patterns (Ni was the adhesion layer) for the S/D electrodes [channel
width (W) = 1 mm, channel length = 20 μm, Wn/Ln = Wp/Lp] using a
conventional lift-off photolithography technique. The conjugated
polymers, PTVPhI-Eh and PTVPhI-C12, were dissolved in anhydrous
chlorobenzene to yield 10 mg/mL solutions. The conjugated polymer
solutions were spin-coated onto the as-cleaned substrates. The
polymer films were thermally annealed at 150, 200, 250, and 300 °C
for 30 min, in a nitrogen glovebox. For the polymer gate dielectric
layers, PMMA (Aldrich, molecular mass of 120 kDa) was used without
further purification. PMMA (80 mg/mL) was dissolved in n-
butylacetate and filtered with a 0.2 μm PTFE syringe filter before
spin-coating. The PMMA dielectric layer had a thickness of ∼520 nm,
as measured by an XP-1 Surface profiler system (Ambios Technology).
After the dielectric coating, the devices were finally annealed at 80 °C
for 2 h in the same glovebox. The top-gate transistors were completed
by forming gate electrodes on the active regions of the transistors via
the evaporation of thin Al films (∼30 nm) with a metal shadow mask.
The electrical characteristics of the fabricated individual polymer TFTs
and the static characteristics of the complementary inverter circuits
were measured using a Keithley 4200-SCS instrument in a glovebox
under a nitrogen atmosphere.
Materials. All chemicals were purchased from Aldrich and used
without further purification. 3-Dodecylthiophene, 2-bromo-3-dode-
cylthiophene, 3-dodecylthiophene-2-carboaldehyde, (E)-1,2-(3,3′-di-
dodecyl-2,2′-dithienyl)ethylene, 5,5′-ditrimethylstannyl-(E)-1,2-(3,3′-
didodecyl-2,2′-dithienyl)ethylene, 3,6-dibromophthalic anhydride,
and N-alkyl-3,6-dibromophthalimide were synthesized as reported
previously.3a,10 All reactions were conducted in a nitrogen atmosphere
1
using commercial solvents, unless otherwise stated. H NMR spectra
were recorded on a JEOL JNM-ECX400, 400 MHz spectrometer in
CDCl3. Chemical shifts are reported as parts per million relative to an
internal TMS standard. Elemental analysis was recorded on an EA1112
instrument (CE Instrument). The number-average molecular weight
(Mn) and weight-average molecular weight (Mw) were determined
using gel permeation chromatography (GPC) (YOUNGLIN Acme
9000 instrument) with chloroform as the eluent (flow rate of 1 mL/
min, at 35 °C) and were calibrated using a narrow polydispersity
polystyrene as the standard.
5,5′-Ditrimethylstannyl-(E)-1,2-(3,3′-didodecyl-2,2′-
1
dithienyl)ethylene (1). H NMR (400 MHz, CDCl3): δ 6.99 (s,
2H), 6.92 (s, 2H), 2.65 (t, 4H), 1.61 (m, 4H), 1.33 (br, 36H), 0.88 (t,
6H), 0.36 (s, 18H). Anal. Calcd (%): C, 56.22; H, 8.49; S, 7.50. Found
(%): C, 56.37; H, 8.43; S, 7.53.
N-(2-Ethylhexyl)-3,6-dibromophthalimide (2a). 1H NMR (400
MHz, CDCl3): δ 7.63 (s, 2H), 3.56 (d, 2H), 1.80 (m, 1H), 1.29 (m,
8H), 0.88 (m, 6H).
N-Dodecyl-3,6-dibromophthalimide (2b). 1H NMR (400
MHz, CDCl3): δ 7.62 (s, 2H), 3.66 (t, 2H), 1.65 (m, 2H), 1.27 (m,
18H), 0.85 (t, 3H).
Polymerization Procedure. Compound 1 (0.5 mmol) and
compound 2 (0.5 mmol) were dissolved in anhydrous chlorobenzene
and purged with nitrogen. The catalyst, Pd2(dba)3 (0.01 mmol), and
the ligand, P(o-tolyl)3 (0.08 mmol), were added to the resultant
mixture, and the mixture was refluxed at a constant temperature of 120
°C for 48 h. Subsequently, the resultant reaction mixture was cooled to
room temperature and precipitated from a solution in which 150 mL
of MeOH was mixed with 30 mL of HCl. The precipitate was filtered
to obtain the desired polymers. The final polymers were purified by
multiple Soxhlet extractions and vacuum drying.
PTVPhI-Eh. 1H NMR (400 MHz, CDCl3): δ 7.79 (s, 2H), 7.71 (s,
2H), 7.12 (s, 2H), 3.62 (br, 2H), 2.75 (br, 4H), 1.89 (br, 1H), 1.69
(br, 4H), 1.25 (m, 44H), 0.86 (m, 12H). Anal. Calcd (%): C, 76.38; H,
9.61; N, 1.78; S, 8.16. Found (%): C, 75.89; H, 9.38; N, 1.66; S, 7.98.
1
PTVPhI-C12. H NMR (400 MHz, CDCl3): δ 7.79 (br, 2H), 7.73
(s, 2H), 7.12 (s, 2H), 3.70 (br, 2H), 2.75 (br, 4H), 1.70−1.25 (br,
60H), 0.86 (m, 9H). Anal. Calcd (%): C, 76.99; H, 9.93; N, 1.66; S,
7.61. Found (%): C, 76.38; H, 9.77; N, 1.53; S, 7.80.
3. RESULTS AND DISCUSSION
Characterization. Thermogravimetric analyses (TGA) were
conducted with a TA-2050 instrument at a heating rate of 10 °C/
min in a nitrogen atmosphere. Differential scanning calorimetry
(DSC) analyses were performed on a TA2010 instrument at a heating
rate of 10 °C/min in a nitrogen atmosphere. UV−vis spectra were
measured using a Perkin-Elmer Lambada 750 instrument. Cyclic
voltammetry measurements of the polymers were performed using an
AUTOLAB Eco Chemie instrument at a scan rate of 50 mV/s. The
sample of polymers was measured using indium tin oxide (ITO) as the
working electrode, a Pt wire as the counter electrode, and a Ag wire as
the reference electrode in an acetonitrile solution with 0.1 M
tetrabutylammonium perchlorate (Bu4NClO4) as the supporting
electrolyte. XRD data of polymer thin films were recorded under
room-temperature conditions using a Rigaku RINT 2000 diffrac-
tometer with Cu Kα radiation. AFM data were measured using a
Digital Instruments multimode atomic force microscope.
Synthesis and Characterization of Polymers. To
establish synthetic motivation, we simulated the repeating
unit of PTVPhI polymers using the B3LYP density functional
theory method. This simulation result shows the planar
conformation of the PTVPhI repeating unit, as shown in
Figure 1. This can be attributed to the strong planarizing effect
of the vinylene linker. Also, the planar conformation of the
PTVPhI repeating unit shows that the conformation of the
molecule matched well with our synthetic intention.
Both polymers (PTVPhI-Eh and PTVPhI-C12) were
synthesized via a Stille coupling polymerization between 5,5′-
ditrimethylstannyl-(E)-1,2-(3,3′-didodecyl-2,2′-dithienyl)-
ethylene and N-alkyl-3,6-dibromophthalimide using Pd2(dba)3/
P(o-tolyl)3 in chlorobenzene via the synthetic route shown in
Scheme 1. Sufficient solubility of both polymers was achieved
by attaching branched 2-ethylhexyl and a sufficiently long n-
dodecyl chain. These polymers showed high solubility in
Raman Measurements. Raman spectra were recorded using a
Renishaw inVia Raman microscope with a 785 nm laser diode
excitation source. A 50× objective was used to focus the laser spot
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PTVPhI-Eh and PTVPhI-C12 were determined to be −5.24
and −5.18 eV, respectively, as shown in Figure S2 of the
Supporting Information. The LUMO level was estimated using
the optical band gap (∼1.83 eV) and HOMO levels. The
LUMO levels of PTVPhI-Eh and PTVPhI-C12 were calculated
as −3.39 and −3.33 eV, respectively. These results are
summarized in Table 1.
Optical Properties of Polymers. The UV−vis absorption
spectra of both polymers were measured to establish the effect
of thermal annealing on the optical properties. The absorption
spectra of both polymer films showed a red shift of ∼44 nm
compared with those in solution. This red shift is caused by
strong interchain interaction between the polymeric chains in
the thin film. As shown in Figure 2a, the absorption spectra of
the PTVPhI-Eh film showed significant changes after thermal
annealing. For the pristine film, the absorption spectrum
showed two peaks at 568 and 620 nm. After annealing had been
conducted from 150 to 200 °C, the spectra showed red shifts
and increased absorption intensities. The film annealed at 250
°C exhibited further increased absorption. It was noted that
strong reorganization of the polymer chains upon thermal
annealing had induced a red shift and an increased absorption
intensity of the annealed samples, either from strong interchain
contact or from an increased level of molecular ordering of the
conjugated backbone, i.e., either a polymer packing arrange-
ment or a polymer backbone planarity. With further increases
in the annealing or heating temperature from 250 to 300 °C,
the absorption spectra of PTVPhI-Eh films showed a blue shift
of absorption onset and a drastic diminution in intensity,
indicating a change in the conformation or intermolecular
aggregations. Similarly, PTVPhI-C12 films showed a red shift
and increased absorption intensity from pristine to 250 °C, as
shown in panels b and d of Figure 2. In addition, heating
(annealing above Tm) the film at 300 °C showed a blue shift
and a decrease in absorption. To obtain more detailed
information about the structural conformation of both
Figure 1. Structural simulation of a PTVPhI backbone: (a) front view
and (b) side view.
common organic solvents such as chloroform (∼30 mg/mL),
tetrahydrofuran (∼20 mg/mL), and chlorobenzene (∼30 mg/
mL) because of the presence of sufficient alkyl side chains
consisting of TV and phthalimide units. The final copolymers
had number-average molecular weights (Mn) of 18600 g/mol
(PTVPhI-Eh) and 17800 g/mol (PTVPhI-C12), as determined
by GPC versus a polystyrene standard. The thermal properties
of the polymers were measured by TGA and DSC, as shown in
Figure S1 of the Supporting Information. The TGA thermo-
grams of PTVPhI-Eh and PTVPhI-C12 revealed high
decomposition temperatures (5% weight loss) of 414 and
399 °C, respectively, under a nitrogen atmosphere, which
indicated good thermal stability because of the presence of a
phthalimide group. Interestingly, PTVPhI-Eh with a branched
ethylhexyl side chain showed a meltinglike thermal transition,
while common polymers with the branched side chain do not
exhibit such thermal transitions due to either poor molecular
ordering or the amorphous nature of the bulky chains.16
PTVPhI-Eh and PTVPhI-C12 showed meltinglike thermal
transitions (Tm) at 260 and 293 °C, respectively. This means
that PTVPhI-Eh can conduct chain motion or reorganization at
a temperature that is relatively lower than that for PTVPhI-
C12. The HOMO energetic levels for both polymers were
measured using cyclic voltammetry. The HOMO levels of
a
Scheme 1. Synthetic Routes to Alkyl-Substituted TV-Based Copolymers
a
Reagents and conditions: (i) NBS, CH3Cl/AcOH, 0 °C; (ii) Mg, DMF, THF, reflux; (iii) TiCl4, Zn, pyridine, THF, 0 °C to reflux; (iv) n-BuLi,
Me3SnCl, THF, −78 °C to room temperature; (v) bromine, iodine, oleum, 60 °C; (vi) R-amine, AcOH, reflux; (vii) Pd2(dba)3, chlorobenzene, 120
°C.
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Table 1. Properties of PTVPhI-Eh and PTVPhI-C12
a
b
c
d
polymer
Mn/Mw (g/mol)
Td (°C)
Tm (°C)
HOMO (eV)
LUMO (eV)
Egopt (eV)
PTVPhI-Eh
18600/35800
17800/35800
414
399
260
−5.24
−5.18
−3.39
−3.33
1.85
1.85
PTVPhI-C12
293
b
a
Determined by gel permeation chromatography vs a polystyrene standard. Electrochemically determined vs Fc/Fc+, where ferrocene E1/2 = 0.45 V,
c
d
PTVPhI-Eh Eonset = 0.89 V, and PTVPhI-C12 Eonset = 0.83 V. ELUMO = EHOMO + Egopt. Optical band gap estimated from the absorption edge of the
thin film.
Figure 2. UV−vis absorption spectra of PTVPhI-Eh (a and c) and PTVPhI-C12 (b and d) annealed at various temperatures. Panels c and d show
enlarged views of panel a and b, respectively.
polymers following thermal annealing, we measured the Raman
spectra and X-ray diffraction of a thin film at various thermal
annealing temperatures.
film at 300 °C had no further effect on the thiophene mode,
while the vinyl mode shifted to a slightly higher wavenumber at
300 °C. In the PTVPhI-C12 pristine film, both the thiophene
and vinyl modes shifted to a lower wavenumber relative to that
of the solution spectrum, with a trend (although a weaker
effect) similar to that of the PTVPhI-Eh films. Unlike that of
PTVPhI-Eh, further annealing caused a monotonic shift of the
PTVPhI-C12 thiophene and vinyl modes to progressively lower
wavenumbers.
Interpretation of the Raman Spectra. To explore the
significance of the shift in thiophene and vinyl Raman modes,
the vibrational spectra of a PTVPhI-methyl monomer with
varying torsion angles (40° to 0° in steps of 10°) were
calculated (Figure 3e,f). This had the effect of modulating the
degree of HOMO−LUMO delocalization along the backbone,
with the more planar structure having enhanced conjugation
relative to that of the more twisted structure. The results
showed unequivocally that for both the thiophene and vinyl
modes, the shift to a lower wavenumber signified an increase in
the planarity or degree of conjugation along the backbone.
Similar results for conjugated systems have been reported
previously.17 Using this analysis, it was possible to determine
that for PTVPhI-Eh the level of conjugation in pristine film was
greater than it was for PTVPhI-C12 (greater peak shift in
PTVPhI-Eh from solution to pristine film), which would
suggest better charge transport properties in the PTVPhI-Eh
pristine film compared to that in PTVPhI-C12. On subsequent
annealing, the PTVPhI-Eh film showed little change in
delocalization (small peak shifts) except at 300 °C, where
there was a small but noticeable decrease in the level of
Raman Spectra of Polymer Films. The initial Raman
spectra were obtained from solutions of PTVPhI-Eh and
PTVPhI-C12 in p-xylene to determine the nature of the Raman
peaks using calculated vibrational spectra. Previous work has
shown that the backbone conformation of conjugated systems
in the gas phase is insensitive to large alkyl groups while adding
greatly to the computational cost. Therefore, the Eh and C12
alkyl chains were substituted with methyl groups in all
calculations to optimize computation time.12 Both molecules
in a p-xylene solution exhibited almost identical Raman spectra,
supporting the idea that the conjugated cores of isolated
molecules are insensitive to long flexible alkyl groups.
Prominent Raman peaks at 1418 and 1594 cm−1 were observed.
The DFT-calculated methyl-substituted monomer showed
good agreement with the solution Raman spectra of both
PTVPhI-Eh and PTVPhI-C12, suggesting the gas-phase
monomer approximation was appropriate in this case. By
comparison of the experimental and calculated spectra, the two
main Raman modes were identified as belonging to the
thiophene (1418 cm−1) and vinyl (1594 cm−1) CC stretch
mode vibrations (Figure 3). For the PTVPhI-Eh polymer in a
pristine thin film, both the thiophene and vinyl peaks shifted to
lower wavenumbers (from 1418 to 1414.5 cm−1 and from
1593.5 to 1592 cm−1, respectively) relative to those of the
solution spectrum. Annealing the film at 110 °C further shifted
the thiophene peak to a slightly lower wavenumber, while the
vinyl mode remained stable. Heating (annealing above Tm) the
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Figure 3. Experimental Raman spectra of the (a) thiophene CC and (b) vinyl CC stretch mode for PTVPhI-Eh and (c) thiophene CC and
(d) vinyl CC stretch mode for PTVPhI-C12 under 785 nm excitation. Enlarged spectra are shown in the insets. Calculated Raman spectra of the
(e) thiophene CC and (f) vinyl CC stretch mode in the PTVPhI backbone.
conjugation (peak shift to a higher wavenumber). In the
PTVPhI-C12 pristine film, despite the fact that the Raman
spectrum suggested a smaller increase in the level of
conjugation from solution relative to pristine PTVPhI-Eh,
further heating (annealing above Tm) to 300 °C showed
continually improving conjugation (indicated by the continual
peak shifts to lower wavenumbers) with no evidence of a
diminished level of conjugation (peak shift to higher wave-
numbers) even at 300 °C.
Characterization of OTFTs. Top-gate/bottom-contact
(TG/BC) OTFTs were fabricated by spin-coating the
PTVPhI-Eh and PTVPhI-C12 solutions in chlorobenzene as
the active layer. Both devices showed typical ambipolar OTFT
characteristics, such as diodelike current−voltage characteristics
in the V-shaped transfer curve (Figure 4). The hole mobility
and electron mobility were calculated from the respective
saturation regime of the transfer curves using the standard
saturation regime equation of metal oxide field effect
transistors: Ids = (μWCo/2L)(Vg − Vt)2.18 Both polymers
exhibited a hole and electron mobility change as the annealing
temperature was increased as shown in Table 2. When the
samples were annealed at 200 °C, the electron mobility of both
polymers was greatly increased, causing both hole mobility and
electron mobility to be more balanced. Further heating
(annealing above Tm) at 300 °C led to hole and electron
mobilities of the PTVPhI-C12 device being further increased;
those of the PTVPhI-Eh device decreased upon heating above
250 °C. Pristine PTVPhI-Eh devices showed values of 0.18 cm2
V−1 s−1 for hole mobility and 0.004 cm2 V−1 s−1 for electron
mobility. The hole and electron mobilities significantly
improved on thermal annealing at 200 °C for 30 min, and
hole and electron mobilities of 0.75 and 0.03 cm2 V−1 s−1,
respectively, were achieved. When the annealing temperature
was increased to 250 °C, the hole mobility decreased from 0.75
to 0.4 cm2 V−1 s−1, whereas the electron mobility was increased
from 0.03 to 0.06 cm2 V−1 s−1. When the heating temperature
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Figure 4. Ambipolar transfer characteristics in the p-channel operation mode [(a) PTVPhI-Eh and (c) PTVPhI-C12] and n-channel operation mode
[(b) PTVPhI-Eh and (d) PTVPhI-C12] at various annealing temperatures.
Table 2. Top-Gate/Bottom-Contact (TG/BC) OTFT Performance for PTVPhI-Eh and PTVPhI-C12 at Various Annealing
Temperatures
PTVPhI-Eh
PTVPhI-C12
a
b
b
c
c
b
b
c
Ta
μhole
μelectron
Vth,hole
(V)
Vth,elec
(V)
Ta
(°C)
μhole
(cm2 V−1 s−1
μelectron
Vth,hole
(V)
c
(°C)
(cm2 V−1 s−1
)
(cm2 V−1 s−1
)
)
(cm2 V−1 s−1
)
Vth,elec (V)
none
150
200
250
300
0.18 0.02
0.34 0.14
0.75 0.18
0.4 0.10
0.09 0.02
0.004 0.002
0.007 0.002
0.03 0.011
0.06 0.023
−36
−41
−45
−40
−36
2
52
52
48
53
50
3
2
3
2
2
none
150
200
250
300
0.030 0.006
0.031 0.003
0.047 0.012
0.041 0.011
0.068 0.022
0.0028 0.0007
0.0060 0.001
0.0443 0.004
0.0493 0.0005
−33
−36
−40
−43
−40
2
56
56
55
1
3
5
2
1
2
2
5
1
2
1
56 0.8
52
0.04 0.018
0.0415 0.005
1
a
b
c
Thermal annealing temperature (Ta). Field effect mobilities (μFET) were averages of six to eight devices. Threshold voltage (Vth).
Figure 5. XRD spectra of (a) PTVPhI-Eh and (b) PTVPhI-C12 annealed at various temperatures.
was increased to 300 °C, both hole and electron mobilities were
decreased. The PTVPhI-C12 devices also showed typical
ambipolar characteristics but a mobility relatively lower than
that of the PTVPhI-Eh devices. The pristine PTVPhI-C12
devices showed a hole mobility of 0.03 cm2 V−1 s−1 and an
electron mobility of 0.0028 cm2 V−1 s−1.When the PTVPhI-C12
devices were annealed at temperatures up to 150 °C, no
significant change were observed in hole and electron
mobilities. However, the mobilities increased from 0.031 to
0.041 cm2 V−1 s−1 for holes and from 0.006 to 0.0493 cm2 V−1
s−1 for electrons when the annealing temperature was increased
from 150 to 250 °C. As a result, PTVPhI-C12 devices exhibited
well-balanced hole and electron mobilities of 0.041 and 0.0493
cm2 V−1 s−1, respectively. Unlike PTVPhI-Eh devices, the hole
and electron mobilities of PTVPhI-C12 devices increased
further when they were heat-treated (annealing above Tm) at
300 °C. The PTVPhI-Eh and PTVPhI-C12 devices showed
little threshold voltage (Vth) shift for both the p- and n-
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Figure 6. AFM images of pristine and annealed polymer thin films for (a, c, and e) PTVPhI-Eh and (b, d, and f) PTVPhI-C12. Images on the left
and right show topographic and phase mode images, respectively.
channels via the annealing process. The details of the
parameters as a function of thermal annealing temperature
for both polymers are summarized in Table 2.
it is for PTVPhI-C12. Both polymers showed more a diffraction
peak at 300 °C clearer than those of pristine films, despite the
large diminution in intensity that was observed for long
wavelength absorption. As a result, electron mobility was more
improved than those of pristine films due to the improvement
in molecular ordering and packing. Despite the increase in the
magnitude of the XRD peak by the thermal annealing process,
the shift in the position of the XRD peak was negligible, which
indicates no change in the π−π stacking distance between
intermolecules.
Microstructure and Morphologies of Polymer Thin
Films and Their Correlations with Charge Transport
Properties. To understand the effect of thermal annealing on
the device performance of both polymers, the microstructure
and surface morphology of thin films were investigated using
XRD and AFM. The ordering of both polymers in thin films
was investigated using XRD as shown in Figure 5. The XRD
spectra of the pristine films for both polymers showed weak
molecular ordering. This was because the polymer chains may
not have had a sufficient amount of time to form an ordered
molecular structure during the relatively fast spin-coating
process.19 After annealing had been conducted at 150 °C,
distinguishable diffraction peaks were observed at 2θ values of
3.5° and 3.2° for PTVPhI-Eh and PTVPhI-C12, respectively.
The intensity of the diffraction peaks in the PTVPhI-Eh film
changed with an increase in the annealing temperature. The
intensity of the diffraction peak gradually increased with
increases in temperature of as much as 200 °C, and then a
drastic increase was observed when the annealing temperature
reached 250 °C. However, the intensity of these diffraction
peaks decreased upon further heating (annealing above Tm) to
300 °C. The changes in diffraction peaks for the PTVPhI-Eh
film were strongly correlated with electron mobility. A maximal
electron mobility of 0.06 cm2 V−1 s−1 was obtained at 250 °C,
but hole mobility began to decrease drastically from 0.75 (200
°C) to 0.4 cm2 V−1 s−1 at 250 °C. This result means that the
strongest interchain ordering induces a favorable chain
conformation for electron hopping transport, such as the
alignment of transport sites, while strong interchain ordering
does not predominantly act on hole transport. The next section
provides a more detailed discussion about hole and electron
charge transport. On the other hand, in the case of the
PTVPhI-C12 film, there was no significant change in diffraction
peaks when the annealing/heating temperature was increased
from 150 to 300 °C. This means that chain reorganization is
more easily achieved by thermal annealing for PTVPhI-Eh than
Figure 6 shows the topography and phase images of both
polymers at various annealing temperatures. As shown in panels
a and b of Figure 6, pristine film of PTVPhI-Eh and PTVPhI-
C12 showed clustered nanofibrillar structures. Similar morpho-
logical changes in both thin films were observed when the films
were annealed at 200 °C. The widths and domain sizes of
clustered nanofibers were further increased compared with
those of the pristine samples. The surface roughness of
PTVPhI-Eh and PTVPhI-C12 significantly increased from 2.9
to 6.9 nm and from 2.9 to 6.7 nm, respectively. It is worth
noting that well-defined domains and clustered nanofibers are
favorable to the facilitation of domain interconnectivity, and,
thus, provide an efficient pathway for charge transport.
However, heating (annealing above Tm) films of both polymers
at 300 °C showed a different surface morphology. The
PTVPhI-Eh film showed disconnected nanorod-like structures
rather than clustered nanofibillar structures. In addition, surface
defects were formed after the film was annealed above the Tm
of the polymers (see Figure 7). These disconnected nanorod-
like structures and surface defects interrupted the charge
transport pathway and resulted in an increased level of charge
trapping. By contrast, the surface morphology of the PTVPhI-
C12 film at 300 °C showed the presence of nanofibrillar
structures, similar to reports of other semicrystalline polymers
such as rr-P3HT. These morphological dependencies on the
annealing temperature were in good agreement with ambipolar
mobility changes.
Correlation between Intra- and Interchain Conforma-
tion and Charge Transport Properties with the
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compared with that of the solution and pristine film. Increasing
the backbone planarity or level of conjugation could help with
intrachain hole and electron transport and lead to improve-
ments in the hole and electron mobilities of the polymer. In
addition, XRD spectra showed improvement of interchain
ordering by thermal annealing, and thus, an increase in the
extent of interchain hole and electron transport could be
observed. In particular, electron mobility showed a significant
improvement compared with that of hole mobility. Typically,
the main charge transfer route within the ordered domains is an
intrachain rather than an interchain one if the charge densities
of both HOMO and LUMO are well-delocalized over the
polymer backbone. However, intrachain electron transport of
these polymers is relatively limited and not expected to be a
very efficient transport compared to hole transport, because the
LUMO is less delocalized along the backbone than the HOMO,
as shown in Figure 8. Therefore, it would be necessary to create
another transport pathway for electrons to make up for limited
intrachain transport, and an increased interchain ordering by
annealing process could efficiently create another transport
pathway. For that reason, electron mobility is strongly affected
and more improved by interchain ordering. By contrast, the
effect was relatively small in hole transport because intrachain
hole transport is significantly predominant over interchain
hopping when the HOMO is strong delocalized over the
conjugated backbone without strong fluctuations in the torsion
angles between adjacent units.20
However, although the planarity and ordering of PTVPhI
polymers were improved by thermal annealing, the hole and
electron mobilities were not perfectly balanced. This effect can
be attributed to the energy level mismatch for hole and electron
injection between the polymer frontier molecular orbital and
the Au electrode. The difference in charge injection properties
can be corrected by the insertion of an interlayer between an
injection electrode and a conjugated polymer.7a Figure 9 shows
the schematic illustration of a PTVPhI polymer chain
conformation and hole and electron transport in a pristine
and thermally annealed film. Figure 9a is an illustration of weak
polymer chain ordering in pristine films, as observed in the
XRD and Raman data. Annealed polymer chains can reorganize
themselves in a more planar and ordered conformation of intra-
and interchains (Figure 9b). The reorganization of polymer
Figure 7. Optical microscope images of (a) pristine and (b) 300 °C
annealed PTVPhI-Eh thin films.
Annealing Process. The mechanisms of hole and electron
transport were studied using the electron-state density
distribution, Raman spectroscopy, and XRD. First, quantum
chemical calculations (ZINDO/S excited-state energy calcu-
lations on AM1 optimized geometries) were conducted for the
electron-state density distribution of the HOMO and LUMO of
the structurally optimized PTVPhI dimer and trimer to explain
the presence of ambipolarity in a PTVPhI backbone (Figure 8).
The side chains in a PTVPhI backbone were excluded in this
model because the alkyl chains have little or no impact on the
backbone conformation and the charge density isosurface.12
In the typical D−A polymer, F8BT, it is known that the
HOMO level is fully delocalized along the polymer backbone
and that the LUMO is strongly localized on the electron-
accepting unit of the polymer backbone.12b Therefore, hole
transport is expected to move efficiently along the polymer
backbone, while electron transport is limited. The localized
LUMO on the electron-accepting unit can obstruct intra- and
interchain charge transport, and for this reason, good alignment
or ordering of the localized LUMO is required. However, as
shown in Figure 8, the PTVPhI backbone shows that the charge
densities of HOMO and LUMO are delocalized across the
polymer backbone, even though the LUMO appears to be less
delocalized across the backbone than the HOMO. The
intrachain hole and electron transport are thus efficient along
the conjugated polymer backbone within an effective
conjugation length, particularly when the polymer chains are
well aligned in the transport direction and when there is an
absence of chemical defects. This result is consistent with the
results of Raman spectroscopy and XRD. From the Raman
analysis, either the planarity or the level of conjugation of the
PTVPhI backbone was increased with the annealing process,
Figure 8. Electron-state density distribution of the HOMO and LUMO in the structure-optimized dimer (a and c) and trimer (b and d),
respectively.
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PTVPhI-Eh ambipolar inverters showed typical Z-shaped-like
VTCs due to the noise of the pull-down transistor at a zero gate
bias. The inverting voltage of a PTVPhI-Eh inverter without
annealing was largely shifted in the positive direction from an
ideal value (half of VDD). This asymmetry of VTCs in the
pristine CMOS inverter resulted in the unbalanced character-
istics (μFET and Vth) of pull-up and pull-down transistors.21 The
largely shifted inverting voltages were greatly improved for
thermally annealed devices because of the more balanced hole
and electron mobilities. Panels c and d of Figure 10 show the
VTCs and output voltage gains, respectively, for various supply
voltages at 250 °C of an annealed PTVPhI-Eh ambipolar
inverter. The sharp switching curve of PTVPhI-Eh inverters
exhibited a maximal voltage gain of ∼40 at VDD = 70 V. In
addition, the inverting voltage and hysteresis of PTVPhI-C12
inverters showed trends similar to those of PTVPhI-Eh
inverters, as shown in Figure S6 of the Supporting Information.
Figure 9. Schematic illustration of the change in polymer chain
conformation by the annealing process: (a) nonannealed polymer film
and (b) annealed polymer film.
4. CONCLUSIONS
chains induces more efficient intra- and interchain charge
transport, resulting in increased hole and electron mobilities.
Characterization of a Complementary Inverter. We
demonstrated complementary-like ambipolar inverters using a
PTVPhI-Eh polymer. PTVPhI-Eh inverters consist of two
identical TG/BC OTFTs that are connected. They were
fabricated by spin-coating without a patterning process for the
conjugated polymer. Figure 10 shows the voltage transfer
characteristics (VTCs) and output voltage gains of a PTVPhI-
Eh ambipolar inverter at a supplied voltage (VDD) of 60 V
after annealing at various temperatures. As shown in Figure 10a,
We have synthesized and characterized a series of D−A
copolymers (PTVPhI-Eh and PTVPhI-C12) based on a
dodecyl-TV unit and an alkyl-substituted phthalimide unit.
These polymers showed energy levels that are suitable for
application in ambipolar OTFTs, and consequently, they
showed good ambipolar mobility. The maximal hole and
electron mobilities of both polymers were measured: for
PTVPhI-Eh, μh = 0.75 cm2 V−1 s−1 and μe = 0.06 cm2 V−1 s−1;
for PTVPhI-C12, μh = 0.068 cm2 V−1 s−1 and μe = 0.049 cm2
V−1 s−1. Our studies of the charge transport of PTVPhI
polymers upon thermal annealing have demonstrated that
Figure 10. (a) Voltage transfer characteristics (VTCs) and (b) output voltage gains of complementary inverters based on a PTVPhI-Eh transistor
after annealing at various temperatures. (c) VTCs and (d) output voltage gains of complementary inverters based on a PTVPhI-Eh transistor
(annealed at 250 °C) for various supplied voltages.
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changes in intra- and interchain conformation and packing
affect both hole and electron mobilities. The pristine films of
both polymers showed lower hole and electron mobilities as a
result of an inefficient charge transport pathway due to a
disordered conformation. Upon thermal annealing, increasing
the backbone planarity or level of conjugation could help with
intrachain hole and electron transport and lead to improve-
ments in the hole and electron mobilities of the polymer.
However, intrachain electron transport is relatively limited
compared to hole transport because the LUMO is less
delocalized along the backbone than the HOMO. Therefore,
after annealing had been conducted, electron transport is more
affected by interchain ordering, and that makes another
pathway for efficient electron transport to neighboring chains,
resulting in increased electron mobility. On the other hand,
intrachain hole transport is significantly predominant over
interchain hopping because the HOMO is strongly delocalized
over the backbone and consequently hole transport is less
affected by interchain ordering. Finally, we demonstrated
complementary-like ambipolar inverters using a PTVPhI-Eh
polymer. After annealing had been conducted at 250 °C, the
PTVPhI-Eh inverter showed improvement in inverting voltage
that is close to 0.5VDD and a sharp switching signal and a large
gain of ∼40 due to a better balance for hole and electron
mobility.
ASSOCIATED CONTENT
* Supporting Information
Thermal properties, cyclic voltammograms, XRD, and device
characterization. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
(6) (a) Sonar, P.; Singh, S. P.; Li, Y.; Soh, M. S.; Dodabalapur, A.
Adv. Mater. 2010, 22, 5409. (b) Li, Y.; Singh, S. P.; Sonar, P. Adv.
Mater. 2010, 22, 4862.
AUTHOR INFORMATION
Corresponding Author
*E-mail: kimdy@gist.ac.kr (D.-Y.K.) or yynoh@dongguk.edu
(Y.-Y.N.).
■
(7) (a) Baeg, K. J.; Kim, J.; Khim, D.; Caironi, M.; Kim, D. Y.; You, I.
K.; Quinn, J. R.; Facchetti, A.; Noh, Y. Y. ACS Appl. Mater. Interfaces
2011, 3, 3205. (b) Chen, Z. Y.; Lemke, H.; Albert-Seifried, S.; Caironi,
M.; Nielsen, M. M.; Heeney, M.; Zhang, W. M.; McCulloch, I.;
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Gu, Y.; Albert, S.; Nielsen, M. M.; Schroeder, B.; Anthopoulos, T. D.;
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by a National Research
Foundation of Korea (NRF) grant funded by the Korea
government (MEST) (Grant 2012-0008723) and the Dongguk
University Fund of 2013. We thank the Korea Basic Science
Institute (KBSI) and National Center for Inter-University
Research Facilities (NCIRF, Seoul National University) for
AFM and EA measurements.
̀
Chem. Mater. 1997, 9, 1720. (d) Elandaloussi, E. H.; Frere, P.;
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