Synthesis and characterization of new thermally stable poly(naphthodithiophene) derivatives and applications for high-performance organic thin film transistors

Article abstract of DOI:10.1021/ma300540f

A series of new p-type polymers, PNDT-T and PNDT-TT, with enforced coplanar structure for effective χ-electron delocalization, having naphtho[2,1-b:3,4-b′]dithiophene and thiophenes as main core units, were successfully synthesized by Stille coupling reaction. The naphtho[2,1-b:3,4- b′]dithiophene unit of the polymer main chain enhances charge carrier mobility by extending χ-conjugation length and rigidly enforced coplanar structure. Both polymers, PNDT-T and PNDT-TT, have high thermal stability up to 250 °C with a high T_g of 402 °C. On the basis of AFM and XRD results, it was found that PNDT-TT showed relatively more highly ordered intermolecular structures than did PNDT-T, with thiophene unit and high field-effect mobility, because the bithiophene unit provides crystallinity with increasing planarity and enough space for interdigitation of the long alkyl side chains for high order. These new p-type polymers PNDT-T and PNDT-TT exhibit high carrier mobilities of 0.01 and 0.076 cm²/(V s) and on/off ratios of 4 × 10⁵ and 7 × 10⁶, respectively. The above results indicate that the plate structure with a sulfur-containing fully aromatic system, which has the upper direction extended, could enhance the thermal stability and charge transport characteristics for OTFT applications.

Full text of DOI:10.1021/ma300540f

Article
pubs.acs.org/Macromolecules
Synthesis and Characterization of New Thermally Stable
Poly(naphthodithiophene) Derivatives and Applications for High-
Performance Organic Thin Film Transistors
Moon Chan Hwang,† Jae-Wan Jang,† Tae Kyu An,‡ Chan Eon Park,*,‡ Yun-Hi Kim,*,§
and Soon-Ki Kwon*,†
†School of Materials Science & Engineering and Research Institute for Green Energy Convergence Technology(REGET),
Gyeongsang National University, Jin-ju 660-701, Republic of Korea
‡Organic Electronics Laboratory, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang,
790-784, Republic of Korea
§Department of Chemistry & Research Institute of Natural Science, Gyeongsang National University, Jinju 660-701, Republic of
Korea
S
* Supporting Information
ABSTRACT: A series of new p-type polymers, PNDT-T and
PNDT-TT, with enforced coplanar structure for effective π-
electron delocalization, having naphtho[2,1-b:3,4-b′]-
dithiophene and thiophenes as main core units, were
successfully synthesized by Stille coupling reaction. The
naphtho[2,1-b:3,4-b′]dithiophene unit of the polymer main
chain enhances charge carrier mobility by extending π-
conjugation length and rigidly enforced coplanar structure.
Both polymers, PNDT-T and PNDT-TT, have high thermal
stability up to 250 °C with a high Tg of 402 °C. On the basis of
AFM and XRD results, it was found that PNDT-TT showed relatively more highly ordered intermolecular structures than did
PNDT-T, with thiophene unit and high field-effect mobility, because the bithiophene unit provides crystallinity with increasing
planarity and enough space for interdigitation of the long alkyl side chains for high order. These new p-type polymers PNDT-T
and PNDT-TT exhibit high carrier mobilities of 0.01 and 0.076 cm2/(V s) and on/off ratios of 4 × 105 and 7 × 106, respectively.
The above results indicate that the plate structure with a sulfur-containing fully aromatic system, which has the upper direction
extended, could enhance the thermal stability and charge transport characteristics for OTFT applications.
reported a thieno[3,2-b]thiophene unit in a polymer (0.3 cm2/
INTRODUCTION
With increased interest among both academic and industrial
institutions in organic semiconductors, considerable research
efforts have been devoted to the improvement of polymer-
based organic thin film transistors (OTFTs) produced by
solution processes, which have potential advantages of low cost,
light weight, and flexibility for electronic circuit; potential fields
of use include such area as drivers for electronic paper displays,
■
(V s));10 Ong and co-workers have reported a poly(2,6-bis(3-
alkylthiophen-2-yl)dithieno-[3,2-b;2′,3′-d]thiophene)
(PBTDT) (0.3 cm2/(V s)) with processing stability.11 Malliaras
and co-workers reported poly(2,5-bis(thiophene-2-yl)-(3,7-
didecanyltetrathienoacene) (P2TDC10FT4) (0.087 cm2/(V
s)) with varied conjugation sizes of fused thiophene rings and
side chain effects.12,13 More recently, naphthodithiophene
derivatives are being intensively studied because of their
radio-frequency identification (RFID) tags, sensors, and flexible
displays on flexible substrates.1−8
extended conjugation system, which leads to strong inter-
molecular orbital overlap.14 Takimiya and co-workers reported
Among the organic materials being considered for use in
naphthodithiophene (NDT)-based polymers (0.77 cm2/(V s))
active layers in solution-processed OTFTs, polythiophenes,
which have been extensively investigated to date, have provided
essential guidance for designing materials. Polythiophene is
attractive due to its extended π-conjugated length and its
with enhanced coplanarity, π-stacking, and steric impact of
isomeric polymer.14a In this study, we aimed to develop a new
p-type OTFT material that would have a sulfur-containing fully
aromatic system and upper direction extended π-conjugation
coplanar structure, which allows for effective π-electron
delocalization.9 Recently, to enhance the charge carrier mobility
along the main chain for effective π-electron delocalization.
of OTFTs based on polythiophene derivatives, many research
groups have focused on the study of the relationship between
OTFT performance and the molecular structure of poly-
thiophene derivative. For example, Toney and co-workers have
Received: March 19, 2012
Revised: May 7, 2012
Published: May 30, 2012
© 2012 American Chemical Society
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Figure 1. Structures of P3HT and thiophene-based polymeric semiconductors.
Recently, interest in air-stable materials for solution-
processable organic semiconductors has increased due to issues
of oxidative doping by atmospheric oxygen and water, which
processes limit polymer performance and application.15
Generally regioregular poly(3-hexylthiophene) (rr-P3HT),
which is a widely studied prototype conjugated semiconducting
many interesting characteristics, and the OTFT fabricated using
PNDT-TT polymer showed a high hole mobility of 0.076 cm2/
(V s) and on/off ratio of 7.6 × 106. Moreover, the high thermal
characteristics of PNDT-TT were stable after exposure to
temperatures as high as 250 °C.
polymer, has provided guidance for oxidative stability, owing to
its high highest occupied molecular orbital (HOMO) level
EXPERIMENTAL SECTION
■
1
Measurements. H NMR and 13C NMR spectra were recorded
(about −4.9 eV). Many research groups have reported new
materials to enhance the stability of thiophene-based
conjugated polymers, through the use of more fused thiophene,
which lowers the HOMO.15,16 An additional requirement of
OTFT materials is that they must have sufficient thermal
stability because OTFT performance is degraded by the
relatively low glass transition temperature of the polymers
and by possible morphology changes at the process temper-
ature (150 °C).17,18 However, stability at high temperatures has
received less attention because conjugated polymers are
generally believed to be incapable of withstanding high
temperatures. However, semiconducting polymers capable of
withstanding high temperatures will be needed to enable the
fabrication of novel organic−inorganic hybrid devices because,
typically, inorganic electronic devices carried out at high
temperature as well as the heat generated during operation of
integrated devices.19
Here, we designed new poly(4,5-bis(2-octyldodecyloxy)-
naphtho[2,1-b:3,4-b′]dithiophene−thiophene) (PNDT-T) and
poly(4,5-bis(2-octyldodecyloxy)naphtho[2,1-b:3,4-b′]-
dithiophene−bithiophene) (PNDT-TT), which are alternating
copolymers composed of thiophenes and naphthodithiophene,
for use in high-performance solution-processed OTFTs. The
rigidly enforced planarity of naphthodithiophene confers
effective π-electron delocalization with the extended π-
conjugation in the upper direction when incorporated into
the conjugated polymer backbone. Furthermore, the enlarged
aromatic form will lead to the further enhancement of air and
thermal stability. In addition, the introduction of electron-
donating 2-octyldodecyloxy groups can tune the electronic
properties of naphthodithiophene and increase the solubility of
the polymer. Although linear alkyl groups generally perform
better than large branch alkyl groups at increasing the stacking
characteristics through easy interdigitation, we found that
polymers with linear alkoxy (dodecyloxy, 2,6-dimethylheptoxy)
groups in the naphthodithiophene with thiophene units showed
very poor solubility in common organic solvents. In order to
increase the solubility, we used a long branched alkoxy side
chain. To enhance the intermolecular π−π stacking, unsub-
stituted thiophene units were introduced. This property led to
with a Bruker Advance-300 spectrometer. The thermal analysis were
performed on a TA TGA 2100 thermogravimetric analyzer in a
nitrogen atmosphere at a rate of 10 °C/min. Differential scanning
calorimeter (DSC) was conducted under nitrogen on a TA
Instruments 2100 DSC. The sample was heated with 10 °C/min
from 30 to 300 °C. UV−vis absorption spectra were measured by UV-
1650PC spectrophotometer. Molecular weights and polydispersities of
the copolymers were determined by gel permeation chromatography
(GPC) analysis with polystyrene standard calibration (Waters high-
pressure GPC assembly model M515 pump, u-Styragel columns of
HR4, HR4E, HR5E, with 500 and 100 Å, refractive index detectors,
solvent CHCl3). Cyclic voltammetry (CV) was performed on an EG
and G Parc model 273 Å potentiostat/galvanostat system with a three-
electrode cell in a solution of 0.1 M tetrabutylammonium perchlorate
(Bu4NClO4) in acetonitrile at a scan rate of 50 mV/s. The polymer
films were coated on a square carbon electrode by dipping the
electrode into the corresponding solvents and then dried under
nitrogen. A Pt wire was used as the counter electrode, and an Ag/
AgNO3 (0.1 M) electrode was used as the reference electrode. The
atomic force microscope (AFM) (Multimode IIIa, Digital Instru-
ments) was operated in tapping mode to acquire images of the surfaces
of conjugated polymer. X-ray measurements were carried out using D8
DISCOVER with GADDS (Bruker AXS). DFT calculations were
carried out with the Spartan 08 computational programs.20
OTFT Device Fabrication. To fabricate PNDT-T- and PNDT-
TT-based OTFTs, we used heavily N-doped silicon substrate and
thermally deposited SiO2 300 nm as dielectric layer. To determine the
device performance for each material, we have employed bottom-gate
top-contact geometry. Prior to treating the silicon oxide surface with
ODTS (octadecyltrichlorosilane), the wafer was cleaned in piranha
solution for 15 min at 200 °C and rinsed with abundant distilled water.
PNDT-T semiconducting materials usually deposited onto each
modified dielectric surface via spin-casting at 2000 rpm from 0.5 wt
% chloroform solution. Films of PNDT-TT organic semiconductors
were spin-coated at 4000 rpm from 0.5 wt % chloroform solution. Au
as source and drain electrodes was thermally evaporated onto the
semiconducting layer of 100 nm thickness, and to make pattern and
channel region of 1000 μm length and 100 μm width, a shadow mask
was used. All the OTFTs based on the polymers PNDT-T and PNDT-
TT were found to exhibit typical p-channel characteristics. The
electrical characteristics of the fabricated FETs were measured in air
using both Keithley 2400 and 236 source/measure units. Field-effect
mobilities were extracted in the saturation regime from the slope of the
source-drain current.
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Materials. 2,5-Bis(trimethylstannyl)thiophene and 5,5′-bis-
(trimethylstannyl)-2,2′-bithiophene were prepared according to the
reports.27 3-Thienylboronic acid (4) was purchased from Sigma-
Aldrich, and tetrakis(triphenylphosphine)palladium (Pd(PPh3)4) was
purchased from Strem Chemicals Inc. The other materials were
common commercial level and used as received. All solvents used were
further purified prior to use.
4,5-Bis(2-octyldodecyloxy)naphtho[2,1-b:3,4-b′]dithiophene
(6) (NDT). A solution of 3,3′-(4,5-bis(2-octyldodecyloxy)-1,2-
phenylene)dithiophene (5) (5 g, 6 mmol) in dichloromethane (200
mL) was stirred vigorously under nitrogen as a suspension of iron(III)
chloride (FeCl3) (2.14 g, 13.2 mmol) in nitromethane (MeNO2) (100
mL) was added dropwise. After the solution was stirred under nitrogen
purging for 0.5 h, anhydrous methyl alcohol (MeOH) was added and
stirred for 0.5 h. The solvents were removed, and the residue was
taken up in dichloromethane and stirred vigorously with ammonium
hydroxide NH4OH(aq). The aqueous layer was washed with
dichloromethane, and the combined organics were washed with
NH4OH and ammonium chloride (NH4Cl), dried, and removed.
Purification on silica (n-hexane/dichloromethane 4:1) provided the
product as a light yellow oil (40%). 1H NMR (300 MHz, CDCl3): δ =
7.91 (2H, d), 7.69 (2H, s), 7.49 (2H, d), 4.09 (4H, d), 1.93−1.95 (2H,
m), 1.28−1.63 (64H, m), 0.89 (12H, t). 13C NMR (300 MHz,
CDCl3): δ = 149.79, 134.35, 130.74, 123.73, 123.12, 122.99, 107.03,
72.35, 38.65, 32.34, 31.92, 30.55, 30.15, 30.13, 30.09, 29.78, 27.42,
23.08, 14.48. HRMS (FAB+) m/z calcd for (C54H88O2S2) 832.6226;
found 832.7335.
2-Octyldodecyl 4-Methylbenzenesulfonate (1). p-Toluenesul-
fonyl chloride (63.8 g, 0.33 mmol) was added to 2-octyldodecanol
(100 g, 0.33 mol) and pyridine (300 mL) at 0 °C under a nitrogen
atmosphere. The reaction mixture was then stirred for 1 h in ice/water
bath, and the mixture was stirred for another 2 h at room temperature.
Afterward, the mixture was poured into aqueous 2 N HCl (300 mL)
and extracted with ethyl acetate (EA). The combined organic layer was
dried over magnesium sulfate (MgSO4) and concentrated. The crude
oil was purified on silica gel (n-hexane/ethyl acetate 50:1). Yield: 90%
1
as colorless oil. H NMR (300 MHz, CDCl3): δ = 7.82 (2H, d), 7.37
(2H, d), 3.94 (2H, d), 2.47 (3H, s) 1.57−1.61 (1H, m), 1.27 (32H,
m), 0.92−0.88 (6H, t). 13C NMR (300 MHz, CDCl3): δ = 144.90,
133.71, 130.13, 128.32, 73.30, 38.02, 32.30, 32.27, 31.02, 30.18, 30.01,
29.92, 29.88, 29.72, 29.65, 26.86, 23.04, 21.98, 14.47. HRMS (FAB+)
m/z calcd for (C27H48O3S) 452.3324; found 452.3745.
2,9-Dibromo-4,5-bis(2-octyldodecyloxy)naphtho[2,1-b:3,4-
b′]dithiophene (7). N-Bromosuccinimide (NBS) (0.88 g, 4.96
mmol) in dichloromethane was added 2-octyldodecyloxynaphtho-
[2,1-b:3,4-b′]dithiophene (6) (2 g, 2.48 mmol) in dichloromethane
(100 mL) under nitrogen, and the mixture was stirred at room
temperature for 12 h. After aqueous sodium thiosulfate (Na2S2O3) was
added to the mixture, the organic layer was separated. The aqueous
layer was extracted with dichloromethane. The combined organic
phases were washed with brine, dried over MgSO4, and concentrated.
The crude product was purified by silica gel column chromatography
(n-hexane/dichloromethane 3:1). The compound was further purified
by recrystallization from acetone. Yield: 1.71 g, 72% as a white solid.
1H NMR (300 MHz, CDCl3): δ = 7.85 (2H, s), 7.50 (2H, s), 4.06
1,2-Bis(2-octyldodecyloxy)benzene (2). From a mixture of
catechol (15 g, 0.13 mol), 2-octyldodecyl 4-methylbenzenesulfonate
(135.7 g, 0.3 mol), potassium hydroxide (30.6 g, 0.545 mol), and N,N-
dimethylformamide (DMF) (400 mL), oxygen was removed by
repeated evacuation followed by admission of nitrogen. The mixture
was heated at 100 °C in a dry nitrogen atmosphere with stirring. After
24 h, water and diethyl ether were added. The organic layer was
separated and dried over MgSO4. After removal of solvent, the crude
oil was purified on silica gel (n-hexane/ethyl acetate 10:1). Yield: 69%
as a colorless oil. 1H NMR (300 MHz, CDCl3): δ = 6.89 (4H, s), 3.87
(4H, d), 1.82 (2H, s), 1.49−1.28 (64H, m), 0.90 (12H, t). 13C NMR
(300 MHz, CDCl3): δ = 150.16, 121.26, 114.51, 72.46, 38.67, 32.33,
31.81, 30.53, 30.14, 30.11, 30.07, 29.79, 27.34, 23.09, 14.48. HRMS
(FAB+) m/z calcd for (C46H86O2) 670.6628; found 670.7289.
1,2-Dibromo-4,5-bis(2-octyldodecyloxy)benzene (3). Bro-
mine (9.7 mL, 0.19 mol) in dichloromethane (DCM) (15 mL) was
added to 1,2-bis(2-octyldodecyloxy)benzene (2) (63 g, 0.09 mol) in
200 mL of DCM. The mixture was stirred overnight at room
temperature. After aqueous Na2S2O3 was added to the mixture, the
organic layer was separated. The combined organic phases were
washed with brine, dried over MgSO4, and concentrated. The crude oil
was purified by silica gel column chromatography (n-hexane/
(4H. d), 1.91−1.93 (2H, m), 1.27−1.61 (64H, m), 0.89 (12H, t). 13
C
NMR (300 MHz, CDCl3): δ = 150.21, 134.15, 130.54, 125.91, 122.13,
112.49, 106.26, 72.21, 38.61, 32.33, 31.90, 30.53, 30.15, 30.12, 30.08,
29.80, 27.40, 23.08, 14.48. HRMS (FAB+) m/z calcd for
(C54H86Br2O2S2) 990.4416; found 990.5056.
Poly(4,5-bis(2-octyldodecyloxy)naphtho[2,1-b:3,4-b′]-
dithiophene−thiophene), PNDT-T (8). 2,9-Dibromo-2-
octyldodecyloxynaphtho[2,1-b:3,4-b′]dithiophene (7) (500 mg, 0.504
mmol) and 2,5-bis(trimethylstannyl)thiophene (199 mg, 0.504 mmol)
were dissolved into toluene (20 mL) in a flask. Nitrogen was bubbled
through this flask for 0.5 h. Tetrakis(triphenylphosphine)palladium(0)
(29.19 mg, 0.025 mmol) was added to the mixture, and the reaction
was maintained at 90 °C for 48 h. 2-Bromothiophene (0.2 mL) was
injected to the reaction mixture for end-capping, and the mixture was
stirred for 12 h followed by the addition of 2-(tributylstanny)-
thiophene (0.4 mL), after which it was stirred overnight. The reaction
mixture was precipitated into a mixture of methanol (200 mL) and 2
N HCl hydrochloric acid (30 mL) and stirred for 1 h. The precipitate
was filtered. The polymer was purified by silica gel column
chromatography (toluene) and concentrated polymer was precipitated
in methanol. The polymer was further purified by washing via Soxhlet
extraction with methanol (24 h), acetone (24 h), and hexanes (24 h),
then dissolved in chloroform and reprecipitated into methanol, and
then dried in vacuo to give product PNDT-T as a red solid (0.39 g,
85%). 1H NMR (500 MHz, CDCl3): δ = 7.39 (4H, br), 6.97 (2H, br),
4.02 (4H, br s), 1.92 (2H, br s), 1.24 (64H, br), 0.81 (12H, br s).
Poly(4,5-bis(2-octyldodecyloxy)naphtho[2,1-b:3,4-b′]-
dithiophene−bithiophene), PNDT-TT (9). 2,9-Dibromo-2-
octyldodecyloxynaphtho[2,1-b:3,4-b′]dithiophene (7) (500 mg, 0.504
mmol) and 5,5′-bis(trimethylstannyl)-2,2′-bithiophene (248 mg, 0.504
mmol) were dissolved into toluene (20 mL) in a flask. Nitrogen was
bubbled through this flask for 0.5 h. Tetrakis(triphenylphosphine)-
palladium(0) (29.13 mg, 0.025 mmol) was added to the mixture, and
the reaction was maintained at 90 °C for 48 h. 2-Bromothiophene (0.2
mL) was injected to the reaction mixture for end-capping, and the
mixture was stirred for 12 h followed by the addition of 2-
(tributylstanny)thiophene (0.4 mL), after which it was stirred
1
dichloromethane 10:1). Yield: 87% as a light yellow oil. H NMR
(300 MHz, CDCl3): δ = 7.05 (2H, s), 3.82 (4H, d), 1.80 (2H, m),
1.46−1.28 (64H, m), 0.90 (12H, t). 13C NMR (300 MHz, CDCl3): δ
= 149.90, 118.27, 114.85, 72.67, 38.47, 32.32, 31.72, 30.45, 30.11,
30.07, 30.02, 29.76, 27.27, 23.08, 14.48. HRMS (FAB+) m/z calcd for
(C46H84Br2O2) 828.4818; found 828.6124.
3,3′-(4,5-Bis(2-octyldodecyloxy)-1,2-phenylene)dithiophene
(5). 1,2-Dibromo-4,5-bis(2-octyldodecyloxy)benzene (3) (68 g, 0.08
mol), 3-thiopheneboronic acid (23.1 g, 0.18 mol), Na2CO3 (69.6 g,
0.65 mol), toluene (300 mL), ethyl alcohol (EtOH) (75 mL), and
H2O (75 mL) were added to the flask, and the solution was vigorously
stirred during 1 h purge with nitrogen. Under a gentle nitrogen stream,
Pd(PPh3)4 (5.68 mg, 4.92 mmol) was added to the solution and
heated at reflux for 18 h. After cooled to room temperature, the
aqueous layer was extracted with ethyl acetate. The combined organic
phases were washed with brine, dried over MgSO4, and concentrated.
Chromatography of the crude material on silica gel (4:1 n-hexane/
dichloromethane) provided the product as colorless oil. Yield: 79%. 1H
NMR (300 MHz, CDCl3): δ = 7.17−7.19 (2H, dd), 7.04 (2H, d), 6.96
(2H, s), 6.78−6.80 (2H, dd), 3.92 (4H, d), 1.83−1.85 (2H, m), 1.28−
1.50 (64H, m), 0.90 (12H, t). 13C NMR (300 MHz, CDCl3): δ =
149.26, 142.60, 129.53, 128.29, 124.82, 122.61, 116.03, 72.55, 38.68,
32.36, 31.86, 30.56, 30.17, 30.14, 30.10, 29.80, 27.36, 23.10, 14.48.
HRMS (FAB+) m/z calcd for (C54H90O2S2) 834.6382; found
834.7550.
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Scheme 1. Synthetic Route of PNDT-T and PNDT-TT
overnight. The reaction mixture was precipitated into a mixture of
methanol (200 mL) and 2 N HCl hydrochloric acid (30 mL) and
stirred for 1 h. The precipitate was filtered. The polymer was purified
by silica gel column chromatography (toluene) and concentrated
polymer was precipitated in methanol. The polymer was further
purified by washing via Soxhlet extraction with methanol (24 h),
acetone (24 h), and hexanes (24 h), then dissolved in chloroform and
reprecipitated into methanol, and then dried in vacuo to give product
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The structures and the
synthetic route of the polymers are outlined in Scheme 1. The
monomer was obtained by various organic reactions such as
bromination, Suzuki coupling reaction, and oxidative cycliza-
tion. Especially, NDT was synthesized by thiophene-centered
oxidative cyclization21,22 to be a sulfur-containing fully aromatic
and confirmed by 1H NMR, 13C NMR, and MS. Both polymers
were synthesized by Stille coupling reaction of 5,5′-bis-
(trimethylstannyl)-2,2′-bithiophene or 2,5-bis-
(trimethylstannyl)thiophene with dibrominated NDT. The
number-average molecular weights (Mn) of PNDT-T and
1
PNDT-TT as a dark red solid (0.46 g, 91%). H NMR (500 MHz,
CDCl3): δ = 7.49 (4H, br), 7.08 (4H, br), 4.12 (4H, br s), 2.06 (2H,
br s), 1.34 (64H, br), 0.89 (12H, br s).
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Table 1. Polymerization Results and Thermal Properties of
Polymers
a
a
Mn
Mw
a
b
polymer
yield [%]
[kg/mol]
[kg/mol]
PDI
Td [°C]
PNDT-T
85
91
18.5
23
40.7
57.9
2.20
2.51
402
402
PNDT-TT
a
Determined by GPC in chloroform (CHCl3) using polystyrene
b
standards. The temperature of degradation corresponding to a 5%
weight loss determined by TGA at a heating rate of 10 °C/min.
Figure 3. UV−vis absorption spectra of PNDT-TT thin films at
various temperatures measured after exposure to elevated temper-
atures in a N2 atmosphere.
Figure 2. Normalized UV−vis absorption spectra of polymers in
solution (CHCl3) and film. The PNDT-T and PNDT-TT films were
annealing at 200 and 250 °C, respectively.
Table 2. Optical and Electrochemical Properties of PNDT-T
and PNDT-TT
UV−vis absorption spectrum
cyclic voltammogram
λmax (nm) λmax (nm) λonset
Egopt
HOMO LUMO
c
a
b
polymer
solution
film
(nm) (eV)
(eV)
(eV)
PNDT-T
523, 554
546, 579
532, 578
546, 590
618
642
2.01
1.93
−5.33
−5.26
−3.32
−3.33
PNDT-
TT
Figure 4. Cyclic voltammetry (CV) of PNDT-T and PNDT-TT films
−
deposited in 0.1 M Bu4N+ClO4 /acetonitrile solution at scan rate of
a
b
Dissolved in CHCl3. Calculated from the absorption edge
50 mV/s: PNDT-T (red line) and PNDT-TT (blue line), ferrocene
curve for calibration (black line).
c
wavelength in the films. Calculated from HOMO and optical band
gap.
PNDT-TT are 18.5 and 23 kg mol−1, with corresponding PDI
of 2.20 and 2.51, respectively, determined by gel permeation
chromatography (GPC) using CHCl3 as eluent and the
calibration curve of polystyrene as standards. Both polymers
have excellent solubility in common organic solvents such as
toluene, chloroform, chlorobenzene (CB), and dichloroben-
zene (DCB) due to their bulky side chain. The good solubility
of both the polymers makes possible the solution processed
thin films for electronics such as OTFTs.
The thermal properties of PNDT-T and PNDT-TT were
investigated by thermogravimetric analysis (TGA) and differ-
ential scanning calorimetry (DSC) under a nitrogen atmos-
phere. As shown in Figure S1 (Supporting Information) and
Table 1, both polymers exhibit good thermal stability with 5%
weight-loss temperatures (Td) of 402 °C. The glass transition
temperatures (Tg) or melting peaks of the copolymers were not
observed up to 280 °C, as shown in Figure S2. TGA and DSC
results revealed that neither PNDT-T nor PNDT-TT degrades
Figure 5. HOMO and LUMO of the oligomers at ab initio Hartree−
Fock (HF) and density functional theory (DFT-B3LYP) methods with
6-31+G* and 6-311+G** basis sets using Spartan 08.
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Table 3. Field Effect Transistor Device Characteristics upon
Annealing Temperature
annealing
temp (°C)
mobility
threshold
voltage (V)
polymer
(cm2/(V s))
on/off ratio
PNDT-T
fresh
100
150
200
250
300
fresh
100
150
200
250
300
0.000709
0.000735
0.00151
0.00490
0.00812
0.0102
−2.37
−22.5
−24.7
−19.2
−18.5
−17.3
−2.52
−7.72
−8.86
−9.75
−10.3
−13.0
2.13 × 104
8.11 × 104
7.20 × 104
3.39 × 105
9.14 × 105
4.19 × 105
6.29 × 104
1.65 × 106
8.77 × 105
2.30 × 106
7.61 × 106
1.11 × 106
PNDT-TT
0.0227
0.0364
0.0453
0.0579
0.0760
0.0689
Figure 6. SAM images of PNDT-T (a, b, c) and PNDT-TT (d, e, f):
(a, d) as-prepared samples; (b, e) annealed at 200 °C; (c, f) annealed
at 250 °C. All annealing processes were taken for 15 min.
in N2. The π−π* absorption spectrum of PNDT-TT is not
affected by annealing temperatures up to 300 °C because of the
rigidly enforced planarity of naphthodithiophene and bithio-
phene, while PNDT-T is stable up to 200 °C, as shown in
Figure S3. These results indicate that the electronic band
structure of PNDT-TT is stable up to an annealing temperature
as high as 300 °C.17
below 280 °C nor melt, which are properties desirable for
polymer-based OTFT applications.
Optical and Electrochemical Properties. The UV−vis
absorption spectra of both polymers in chloroform solution,
and in thin films produced by spin-coating using polymer
solutions (5 wt % in chloroform), are shown in Figure 2 and
summarized in Table 2. In dilute chloroform solution, PNDT-T
and PNDT-TT respectively show absorptions at 523 and 546
nm; the shoulder peaks appear at 554 and 579 nm, which
appearances are associated with interchain interactions.23 In the
solid state, the shoulder peaks of PNDT-T and PNDT-TT
show a significant red-shift at around 578 and 590 nm,
respectively. This particular enhanced shoulder absorption in
films generally indicates an effective π-conjugation length
elongation and a strong intermolecular packing in the solid
state, caused by the planar and rigid backbones of NDT and the
thiophene unit.24 Notably, although PNDT-T exhibited similar
absorption shapes in thin film, PNDT-TT showed more
distinctively strong absorption shoulder at 590 nm and a
broader absorption spectrum. Evidently, compared to PNDT-
T, PNDT-TT exhibited pronounced aggregation in the solid
film because of the rigid bithiophene unit, which might further
enhance its mobility.
The cyclic voltammetry measurements for PNDT-T and
PNDT-TT thin films are shown in Figure 4, and the results are
summarized in Table 2. The HOMO levels of both polymers
can be estimated from the equation EHOMO = −(EOX
−
ferroceneOX) − 4.8 eV, where EOX is the onset oxidation
potential of both polymers (PNDT-T: 0.97 eV; PNDT-TT: 0.9
eV) and ferroceneOX is the onset oxidation potential of the
ferrocene (0.44 eV) as reference.25,26 The lowest unoccupied
molecular orbital (LUMO) values were calculated using the
optical band gap (Egopt) and the calculated highest occupied
molecular orbital (HOMO). HOMO energy levels of PNDT-T
and PNDT-TT were estimated to be −5.33 and −5.26 eV,
respectively, which values are about 0.4 eV higher than that of
regioregular P3HT (about 4.9 eV), thus showing greater
stability against oxidation doping. In addition, the HOMO
value of PNDT-TT (−5.26 eV) is higher than that of PNDT-T
(−5.33 eV) because the bithiophene unit in PNDT-TT is more
electron-rich, while the LUMO values of both polymers are
quite similar.27,28
To confirm the thermal stability of the polymers, Figure 3
and Figure S3 show the UV−vis absorption spectra of the
polymer thin films annealed for 15 min at various temperatures
In addition, we calculated the electron distribution in the
molecular structure using the density functional theory (DFT)
Figure 7. X-ray diffraction patterns of (a) PNDT-T and (b) PNDT-TT: as-spun (i), and annealed film at 100 °C (ii), and 150 °C (iii), and 200 °C
(iv), and 250 °C (v).
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Figure 8. Field effect transistor device characteristics under ambient air with PNDT-T (a, b, c) and PNDT-TT (d, e, f) semiconductors: (a, d)
output characteristics; (b, e) transfer characteristics; (c, f) transfer curves at fresh, 100 °C, 150 °C, 200 °C, 250 °C, and 300 °C.
in order to study the electron delocalization on a molecule,
which is two repeating units of the polymers; for the
simplification of the calculation, long alkyl chains were replaced
by a CH3 group in the molecule. The calculated electron
density distributions of HOMO and LUMO are shown in
Figure 5. Inspection of the HOMO densities for these systems
shows that π-orbitals are delocalized along the NDT unit as
well as along the conjugated backbone.
Structural and Morphological Studies. In order to
investigate the microstructure of the polymer thin films in detail
at different thermal annealing temperatures, we conducted
tapping-mode atomic force microscopy (TM-AFM) and X-ray
diffraction measurements. The films of PNDT-T and PNDT-
TT were prepared using 0.5 wt % chloroform solution; they
were then spin-cast at 2000 and 4000 rpm on molecularly
functionalized ODTS-treated SiO2 substrates, respectively.
Before annealing, the as-cast films of both PNDT-T and
PNDT-TT show no clear crystalline surface morphology, as can
be seen in Figure 6 (and Figure S4).
However, after annealing at 150, 200, 250, and 300 °C, the
AFM phase images of both polymers clearly demonstrate
pronounced film morphology, and the rms roughness also
increased for both. It is likely that the morphology of the
polymers is indicative of the effective packing formation of
polymer chains in the π−π stacking direction. In addition, as
shown in the UV−vis spectra of the as-cast and annealed
polymer films at different temperatures, the morphology of the
PNDT-TT film in the AFM image is stable up to as high as 250
°C and PNDT-T is stable up to 200 °C. The structural study of
the polymers was further confirmed with X-ray diffraction
(XRD) by analyzing the relative crystallinities of the PNDT-T
and PNDT-TT in the solid state at various annealing
temperatures, as shown in Figure 7. The XRD spectra of
both polymers show no visible peak up to 100 °C. However,
PNDT-T clearly shows a crystalline diffraction peak at the
annealing temperature of 150 °C, and PNDT-TT shows a
crystalline diffraction peak at the annealing temperature of 200
°C. Both polymers exhibited obvious diffractions at 2θ = 3.8°
(PNDT-T) and 3.5° (PNDT-TT), which correspond to the
interlayer d-spacings of 23.2 and 25.2 Å, respectively. The
primary diffraction peak of PNDT-TT was observed to exhibit
the relatively higher intensity. This indicates that PNDT-TT
has better crystallinity than PNDT-T because bithiophene
increases the planarity and provides enough space for the
interdigitation of the long alkyl side chains.29
OTFT Properties. Thin film transistors (TFT) based on
both polymers were fabricated using bottom gate−top contact
device geometry by spin-casting the polymer solution in
chloroform. The carrier mobility was measured in the
saturation regime using the relationship μsat = (2IDSL)/
(WC(Vg − Vth)2), where IDS is the saturation drain current, C
is the capacitance of the oxide dielectric, Vg is the gate bias, and
Vth is the threshold voltage.30,31 The slope of the plot was
applied in the saturation region of the drain current1/2 vs gate
voltage curve of each of the polymers in order to calculate the
mobility. The mobilities and on/off ratios of both polymers are
summarized in Table 3. As shown in Figure 8, PNDT-T and
PNDT-TT were found to exhibit typical p-type characteristics,
and the output curves of the polymers showed linear shapes
and very good saturation in each range of the drain voltage,
which is common behavior for conjugated polymers.30,32
PNDT-TT exhibited high hole mobility of 0.076 cm2/(V s)
and on/off ratio of 7.61 × 106. PNDT-T showed a hole
mobility of 0.01 cm2/(V s) and on/off ratio of 4.19 × 105.
Interestingly, as shown in Table 3, for both polymers the
annealing temperature tends to affect the mobility. All devices
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based on both polymers were annealed at 100, 150, 200, 250,
and 300 °C, and both polymers showed thermal stability on
each device. PNDT-T and PNDT-TT devices at 250 °C were
found to possess higher charge carrier mobility of 0.01 and
0.076 cm2/(V s), respectively. This behavior is explained by the
enhanced intensity and the second-order peak in X-ray
diffraction, indicating a higher order in the polymer films, as
well as the morphological differences in the AFM images at the
various annealing temperatures.
Research Program through the National Research Foundation
of Korea by the Ministry of Education, Science and Technology
(2011-0027499).
REFERENCES
■
(1) Braga, D.; Horowitz, G. Adv. Mater. 2009, 21, 1−14.
(2) Forrest, S. R. Nature(London) 2004, 428, 911−918.
(3) Baude, P. F.; Ender, D. A.; Haase, M. A.; Kelley, T. W.; Muyres,
D. V.; Theiss, S. D. Appl. Phys. Lett. 2003, 82, 3964.
The FET devices using these polymers showed high on/off
ratio (about 106). In addition, PNDT-TT exhibited a higher
mobility compared to that of PNDT-T. This result shows that,
compared to that of PNDT-T, the high mobility of PNDT-TT
resulted from the increased degree of crystallinity produced by
the thermal annealing, as verified by AFM and X-ray diffraction,
compared to those of PNDT-T. PNDT-TT polymer can be
thought to be capable of developing thin film crystallinity
during thermal treatment because the bithiophene provides
enough space for the long alkyl side chains interdigitation and
increases the planarity with enhanced π-stacking.
(4) Kim, H. S.; Kim, Y. H.; Kim, T. H.; Noh, Y. Y.; Pyo, S. M.; Yi, M.
H.; Kim, D. Y.; Kwon, S.-K. Chem. Mater. 2007, 19, 3561−3567.
(5) Kim, S.-O.; Tea, K. A.; Chen, J.; Kang, il.; Kang, S. H.; Chung, D.
S.; Park, C. E.; Kim, Y.-H.; Kwon, S.-K. Adv. Funct. Mater. 2011, 21,
1616−1623.
(6) Yan, H.; Chen, Z.; Zheng, Y.; Newman, C.; Quinn, J. R.; Dotz, F.;
Kastler, M.; Facchetti, A. Nature 2009, 457, 679.
(7) Virkar, A. A.; Mannsfeld, S.; Bao, Z.; Stingelin, N. Adv. Mater.
2010, 22, 3857−3875.
(8) Zhang, W.; Smith, J.; Watkins, S. E.; Gysel, R.; McGehee, M.;
Salleo, A.; Kirkpatrick, J.; Ashraf, S.; Anthopoulos, T.; Heeney, M.;
McCulloch, I. J. Am. Chem. Soc. 2010, 132, 11437−11439.
(9) Liu, Y.; Liu, Y.; Zhan, X. Macromol. Chem. Phys. 2011, 212, 428−
443.
(10) McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.;
Macdonald, I.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.;
Zhang, W.; Chabinyc, M. L.; Kline, R. J.; McGehee, M. D.; Toney, A.
M. F. Nat. Mater. 2006, 5, 328.
(11) Li, J.; Qin, F.; Li, C. M.; Bao, Q.; Chan-Park, M. B.; Zhang, W.;
Qin, J.; Ong, B. S. Chem. Mater. 2008, 20, 2057−2059.
(12) He, M.; Li, J.; Sorensen, M. L.; Zhang, F.; Hancock, R. R.; Fong,
H. H.; Pozdin, V. A.; Smilgies, D.-M.; Malliaras, G. G. J. Am. Chem.
Soc. 2009, 131, 11930−11938.
(13) Fong, H. H.; Pozdin, V. A.; Amassian, A.; Malliaras, G. G.;
Smilgies, D.-M.; He, M.; Gasper, S.; Zhang, F.; Sorensen, M. J. Am.
Chem. Soc. 2008, 130, 13202−13203.
(14) (a) Osaka, I.; Abe, T.; Shinamura, S.; Takimiya, K. J. Am. Chem.
Soc. 2011, 133, 6852−6860. (b) Osaka, I.; Abe, T.; Koganezawa, T.;
Takimiya, K. ACS Macro Lett. 2012, 1, 437−440. (c) Loser, S.; Bruns,
C. J.; Miyauchi, H.; Oritz, R. P.; Faccetti, A.; Stupp, A. I.; Marks, T. J. J.
Am. Chem. Soc. 2011, 133, 8142−8145. (d) Dutta, P.; Yang, W.; Eom,
S. H.; Lee, W. H.; Kang, I. N.; Lee, S. H. Chem. Commun. 2012, 48,
573−575.
(15) Chung, D. S.; Park, J. W.; Kim, S.-O.; Heo, K.; Park, C. E.; Ree,
M.; Kim, Y.-H.; Kwon, S.-K. Chem. Mater. 2009, 21, 5499−5507.
(16) Park, J. W.; Lee, D. H.; Chung, D. S.; Kang, D.-M.; Kim, Y.-H.;
Park, C. E.; Kwon, S.-K. Macromolecules 2010, 43, 2118−2123.
(17) Jang, J.; Seung, H. H. Curr. Appl. Phys. 2006, 6, e17−e21.
(18) Guo, D.; Ikeda, S.; Saiki, K. J. Appl. Phys. 2006, 99, 094502.
CONCLUSION
■
We have synthesized and characterized two p-type conjugated
polymers, PNDT-T and PNDT-TT, for use as active materials
in OTFTs, by incorporating naphthodithiophene for more
effective π-electron delocalization with upper direction
extended π-conjugation. The PNDT-T polymer has a molecular
weight of 18.5 kDa with a polydispersity index of 2.2, and the
PNDT-TT has a molecular weight of 23 kDa with a
polydispersity index of 2.51. The obtained polymers have
good solubility with long alkyl side chains and HOMO levels of
−5.33 eV (PNDT-T) and −5.26 eV (PNDT-TT), resulting in
good oxidation stability. Both polymers have high thermal
stability up to 250 °C with a high Tg of 402 °C, from TGA, UV,
and AFM. These new polymers, PNDT-T and PNDT-TT,
exhibit higher field-effect mobilities of 0.01 cm2/(V s) (Ion/off
=
4.19 × 105) and 0.076 cm2/(V s) (Ion/off = 7.61 × 106),
respectively. The higher mobility of PNDT-TT was found to
result from the increasing thiophene content of PNDTs, which
effectively increases polymer crystallinity, as evident from XRD
and AFM studies. Based on these results, main chain
modifications for high crystallinity with NDTs are also expected
to yield higher thermal stability and enhanced charge carrier
mobility for high OTFT performance.
(19) Cho, S.; Seo, J. H.; Park, S. H.; Beaupre,
A. J. Adv. Mater. 2010, 22, 1253−1257.
́
S.; Leclerc, M.; Heeger,
ASSOCIATED CONTENT
■
S
* Supporting Information
(20) Freeman, F.; Derek, E. J. Comput. Chem. 2003, 24, 909−919.
(21) Tovar, J. D.; Swager, T. M. Adv. Mater. 2001, 23, 1775−1780.
(22) Tovar, J. D.; Rose, A.; Swager, T. M. J. Am. Chem. Soc. 2002,
124, 7762−7769.
Figures S1−S4. This material is available free of charge via the
Internet at http://pubs.acs.org.
(23) Price, S. C.; Stuart, A. C.; Yang, L.; Zhou, H.; You, W. J. Am.
Chem. Soc. 2011, 133, 4625−4631.
AUTHOR INFORMATION
Corresponding Author
*E-mail: skwon@gnu.ac.kr (S.-K.K.); ykim@gnu.ac.kr (Y.-
H.K.); cep@postech.ac.kr (C.E.P.).
■
(24) Wang, M.; Hu, X.; Liu, P.; Li, W.; Gong, X.; Huang, F.; Cao, Y.
J. Am. Chem. Soc. 2011, 133, 9638−9641.
(25) Li, Y.; Cao, Y.; Gao, J.; Wang, D.; Yu, G.; Heeger, A. J. Synth.
Met. 1999, 99, 243−248.
Notes
(26) Asadi, K.; Gholamrezaie, F.; Smits, E. C. P.; Blom, P. W. M.;
Boer, B. J. Mater. Chem. 2007, 17, 1947−1953.
The authors declare no competing financial interest.
(27) Zhou, H.; Yang, L.; Stoneking, S.; You, W. ACS Appl. Mater.
Interfaces 2010, 2, 1377−1383.
ACKNOWLEDGMENTS
■
This research was supported by a grant (F0004010-2010-33)
from Information Display R&D Center, one of the Knowledge
Economy Frontier R&D Program funded by the Ministry of
Knowledge Economy of Korean government and the Basic
(28) Hou, J.; Park, M.-H.; Zhang, S.; Yao, Y.; Chen, L.-M.; Li, J.-H.;
Yang, Y. Macromolecules 2008, 41, 6012−6018.
(29) Durban, M. M.; Kazarinoff, P. D.; Luscombe, C. K.
Macromolecules 2010, 43, 6348−6352.
4527
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Article
(30) Zaumseil, J.; Sirringhaus, H. Chem. Rev. 2007, 107, 1296−1323.
(31) Lee, M.-J.; Kang, M. S.; Shin, M.-K.; Park, J. W.; Chung, D. S.;
Park, C. E.; Kwon, S.-K.; Kim, Y.-H. J. Polym. Sci., Part A: Polym. Chem.
2010, 48, 3942−3949.
(32) Lim, B. L.; Baeg, K.-J.; Jeong, H.-G.; Jo, J.; Kim, H.; Park, J.-W.;
Noh, Y.-Y.; Vak, D.; Park, J.-H.; Park, J.-W.; Kim, D.-Y. Adv. Mater.
2009, 21, 2808−2814.
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