High performance semiconducting polymers containing bis(bithiophenyl dithienothiophene)-based repeating groups for organic thin film transistors

Article abstract of DOI:10.1002/pola.24415

New dithienothiophene-containing conjugated polymers, such as poly(2,6-bis(2-thiophenyl-3-dodecylthiophene-2-yl)dithieno[3,2-b;2′, 3′-d]thiophene, 4 and poly(2,6-bis (2-thiophenyl-4-dodecylthiophene-2-yl) dithieno[3,2-b;2′,3′-d]thiophene, 8 have been successfully synthesized via Stille coupling reactions using dodecyl-substituted thiophene-based monomers, bistributyltin dithienothiophene, and bistributyltin bithiophene; these polymers have been fully characterized. The main difference between the two polymers is the substitution position of the dodecyl side chains in the repeating group. Grazing-incidence X-ray diffraction (GI-XRD) gave clear evidence of edge-on orientation of polycrystallites to the substrate. The semiconducting properties of the two polymers have been evaluated in organic thin film transistors (OTFTs). The two conjugated polymers 4 and 8 exhibit fairly high hole carrier mobilities as high as μ_ave = 0.05 cm ²/Vs (I_ON/OFF = 3.42 × 10⁴) and μ_ave = 0.01 cm²/Vs, (I_ON/OFF = 1.3 × 10⁵), respectively, after thermal annealing process. The solvent annealed films underwent reorganization of the molecules to induce higher crystallinity. Well-defined atomic force microscopy (AFM) topography supported a significant improvement in TFT device performance. The hole carrier mobilities of the solvent annealed films are comparable to those obtained for a thermally annealed sample, and were one-order higher than those obtained with a pristine sample. Copyright

Full text of DOI:10.1002/pola.24415

High Performance Semiconducting Polymers Containing
Bis(bithiophenyl dithienothiophene)-Based Repeating Groups
for Organic Thin film Transistors
KYUNG HWAN KIM,1 DAE SUNG CHUNG,2 CHAN EON PARK,2 DONG HOON CHOI1
1Department of Chemistry, Research Institute for Natural Sciences, Korea University, Seoul 136-701, South Korea
2Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 136-701, South Korea
Received 17 July 2010; accepted 21 September 2010
DOI: 10.1002/pola.24415
Published online 18 November 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: New dithienothiophene-containing conjugated poly-
as lave ¼ 0.05 cm2/Vs (ION/OFF ¼ 3.42 ꢀ 104) and lave ¼ 0.01
cm2/Vs, (ION/OFF ¼ 1.3 ꢀ 105), respectively, after thermal anneal-
ing process. The solvent annealed films underwent
reorganization of the molecules to induce higher crystallinity.
Well-defined atomic force microscopy (AFM) topography sup-
ported a significant improvement in TFT device performance.
The hole carrier mobilities of the solvent annealed films are
comparable to those obtained for a thermally annealed sample,
mers, such as poly(2,6-bis(2-thiophenyl-3-dodecylthiophene-2-
yl)dithieno[3,2-b;20,30-d]thiophene,
4 and poly(2,6-bis (2-thio-
phenyl-4-dodecylthiophene-2-yl)dithieno[3,2-b;20,30-d]thiophene,
8 have been successfully synthesized via Stille coupling reac-
tions using dodecyl-substituted thiophene-based monomers,
bistributyltin dithienothiophene, and bistributyltin bithiophene;
these polymers have been fully characterized. The main differ-
ence between the two polymers is the substitution position of
the dodecyl side chains in the repeating group. Grazing-inci-
dence X-ray diffraction (GI-XRD) gave clear evidence of edge-
on orientation of polycrystallites to the substrate. The semicon-
ducting properties of the two polymers have been evaluated in
organic thin film transistors (OTFTs). The two conjugated poly-
mers 4 and 8 exhibit fairly high hole carrier mobilities as high
and were one-order higher than those obtained with a pristine
C
V
sample.
2010 Wiley Periodicals, Inc. J Polym Sci Part A:
Polym Chem 49: 55–64, 2011
KEYWORDS: annealing; charge transport; conjugated polymer;
dithienothiophene; mobility; organic thin film transistor; semi-
conductor; synthesis; thin films
INTRODUCTION With the recent rapid progress in the devel-
opment of novel p-type organic semiconductors, high-per-
formance organic thin film transistors (OTFTs) with high
carrier mobility or poor device performance reproducibility
in the atmosphere. Desirable molecular ordering on the sub-
strate is not achieved to any great extent by conventional
thermal annealing or other post-treatment methods. In TFT
devices, the substrate temperatures are raised during deposi-
tion of the solution to increase crystallinity or induce a
particular crystalline phase. Post thermal annealing and
solvent annealing processes are also adopted to induce crys-
talline grain growth as well as morphological transformation,
in conjugated polymer thin films.14–17
hole mobilities of the order of
1
cm2/Vs have been
obtained.1–6 Most of the p-channel OTFTs with very high
mobility reported to date have been fabricated using the
vacuum deposition method.
Organic semiconductors that are soluble in organic solvents
can be employed for the fabrication of TFT devices via
conventional low-cost spin-coating or drop-casting techni-
ques.7–10 Compared to vacuum-processed TFT devices, solu-
tion-processed OTFTs usually have a much poorer device
performance. The development of solution-processable
p-type semiconducting materials is therefore an important
issue in the further development of OTFTs.
Recent achievements in the charge carrier mobilities and
stabilities of TFT devices fabricated with semiconducting
polymers have resulted in TFT characteristics as good as
those of vacuum-processed TFTs with semiconducting
oligomers.18–20
Extensive research has been carried out on polymer semi-
conductors because of their facile solution processability,
good film-forming properties, and good mechanical stability,
compared to small molar mass organic materials.8,11–13 How-
ever, most current polymeric semiconductors either have low
Very few polymer semiconductors, such as polythiophene
derivatives with regioregular long alkyl side chains, have
demonstrated good transistor characteristics (mobility >0.1
cm2/Vs and on/off current ratio >105) and device stability.
Ong and co-workers have reported the synthesis of
Correspondence to: D. H. Choi (E-mail: dhchoi8803@korea.ac.kr)
C
V
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 49, 55–64 (2011) 2010 Wiley Periodicals, Inc.
HIGH PERFORMANCE SEMICONDUCTING CONJUGATED POLYMERS, KIM ET AL.
55

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
poly(3,30-didodecylquaterthiophene) (PQT-12) with l ¼ 0.14
cm2/Vs and good oxidative doping stability.21 McCulloch and
co-workers have reported poly(2,5-bis(3-alkylthiophene-2-
the reaction was complete, the mixture was extracted with
methylene chloride (MC) and the organic phase was sepa-
rated, followed by drying over MgSO4. The product was
obtained by flash column chromatography using hexane as
the eluent. Yield: 92%, 3.6 g of yellow powder.
yl)thieno[3,2-b]thiophene)
(PBTTT),
which
includes
thieno[2,3-b]thiophene with sulfur atoms in the anti posi-
tion; PBTTT exhibits liquid crystalline behavior that aids
highly efficient crystallization of thin films. PBTTT-based
TFTs yield the highest mobility reported to date, l ¼ 0.01–
0.2 cm2/Vs.22,23 More recently, they also demonstrated a
new solution processable, annealing-free 4-(2-hexyldecan)-
4H-bisthieno[2,3-d:30,20-b]pyrrole and 4,40-dialkyl-2,2(-bithia-
zole-based copolymers (PBTzDTPs) with an excellent field-
effect transistor (FET) performance.24 These polymers have
high charge carrier mobilities (lFET ¼ 0.06–0.14 cm2/Vs, on/
off current ratio ¼ 105–106), excellent air stability, and good
solution processability. These polymers demonstrated the
new concept that higher crystallinity and long-range ordered
structures are not necessary to achieve great TFT device
performance.
1H NMR (CDCl3, 400 MHz) d (ppm) 7.28 (s, 2H), 7.22 (d, J ¼
4 Hz, 2H) 6.96 (d, J ¼ 4 Hz, 2H), 2.80 (t, J ¼ 16 Hz, 4H),
1.62–1.70 (m, 4H), 1.25–1.39 (m, 36H), 0.87 (t, J ¼ 14 Hz,
6H).
13C NMR (CDCl3, 100 MHz) d (ppm) 141.1, 140.6, 137.1,
130.8, 130.7, 130.3, 124.6, 119.5, 32.2, 31.0, 29.89, 29.88,
29.83, 29.76, 29.71, 29.6, 29.5, 22.9, 14.4; HRMS (m/z,
FABþ) Calcd. for C40H56S5 696.2986, found 696.2913.
Anal. Calcd C, 68.91; H, 8.10; S, 23.00; S, 23.09. Found C,
68.76; H, 8.10; S, 23.08.
Bis-5-(3-bromo-4-dodecyl)dithieno[3,2-b;20,30-d]
thiophene (6)
The analogous selenophene-based semiconducting polymer
was prepared because of its high electron donating pro-
perties. Poly(5,50-bis(3-dodecylthiophen-2-yl)-2,20-biseleno-
phene) (PDT2Se2) was employed to fabricate TFTs with a
high hole transporting mobility of 0.02 cm2/Vs resulting
from its excellent intermolecular ordering.25
NBS (1.3 g, 7.0 mmol) in DMF (50 mL) was added dropwise
to a solution of compound 5 (2.0 g, 2.8 mmol) in DMF
(75 mL) at 0 ꢁC. The reaction mixture was stirred at 90 ꢁC
for 10 h and poured into water. After extraction of the mix-
ture with MC, the organic phase was separated and dried
over MgSO4. The product was obtained by flash column
chromatography using hexane as the eluent. Yield: 90%,
2.2 g of yellow powder.
Dithienothiophene which is a thiophene-based heteroacene,
inherently has rigid and copalnar p-conjugated molecu-
lar frameworks.26 Although dithieno[3,2-b:20,30-d]thiophene
(DTT)-based small molecules have been studied, and
have shown high FET characteristics (lFET ¼ 0.42 cm2/Vs;
on/off current ratio ¼ 108) as vacuum-evaporated thin-film
semiconductors, few polymers prepared completely
by solution processing have been reported in the
literature.8,11,17,27
1H NMR (CDCl3, 400 MHz) d (ppm) 7.23 (s, 2H), 6.98 (d, J ¼
4 Hz, 2H) 2.72 (t, J ¼ 16 Hz, 4H), 1.51–1.64 (m, 4H), 1.15–
1.32 (m, 36H), 0.86(t, J ¼ 14 Hz, 6H); 13C NMR (CDCl3, 100
MHz) d (ppm) 145.2, 141.3, 133.0, 132.0, 120.0, 114.3,
111.5, 106.1, 32.1, 30.8, 30.6, 29.87, 29.8, 29.73, 29.57,
29.46, 29.39, 22.9, 14.4; HRMS (m/z, FABþ) Calcd. for
C40H54Br2S5 852.1196, found 851.8726; Anal. Cacld C, 56.19;
H, 6.37; S, 18.75. Found C, 55.39; H, 6.09; S, 17.95.
In this article, we present two different polymers bearing
2,6-bis (2-thiophenyl-3 (or 4-)dodecylthiophene-2-yl)di-
thieno[3,2-b;20,30-d]thiophene with two different dodecyl
substitution modes such as 4,40-substitution and 3,30-substi-
tution in the thiophene rings. We also investigated the opti-
cal, thermal, electrochemical, and photophysical properties of
new p-conjugated polymers. These solution processable poly-
mers were employed to fabricate OTFTs and their device
performances were investigated in detail. Thermal annealing
and solvent annealing processes were employed to improve
device performance.
Poly(2,6-bis(2-thiophenyl-4-dodecylthiophene-2-yl)
dithieno[3,2-b;20,30-d]thiophene (4)
Monomer 6 (1.13 g, 1.37 mmol), monomer 7 (1.06 g, 1.37
mmol), and dry toluene (40 mL) deoxygenated with nitro-
gen for 30 min were added to a 100-mL three-necked
round-bottom flask. Pd(PPh3)4 (79 mg, 5 mol %) was added
and the mixture was heated upto 90 ꢁC. The dark red solu-
tion was allowed to stand at 90 ꢁC for 2 days. The black
sticky solution was cooled down to room temperature. The
solution was dropped into 300 mL of methanol. The black
precipitate was filtered and washed with methanol. The
crude polymer was collected by filtration and then purified
by Soxhlet extraction with acetone, hexane, and chloroform,
successively. The final product was obtained by precipitation
in methanol as a dark red solid in a yield of 71%.
EXPERIMENTAL
Synthesis
Compounds 1, 2, 3, and 7 were synthesized using the litera-
ture method and a modified method.28–31
Bis-5-(4-dodecyl)dithieno[3,2-b;20,30-d]thiophene (5)
Compound 3 (4.38 g, 5.7 mmol) 2-bromo-3-dodecylthio-
phene(3.74 g, 11.4 mmol), and 75 mL DMF(75 mL) were
added to a 250-mL three-necked reaction flask under an
argon atmosphere. Pd(PPh3)4 (0.33 g, 5 mol %) was then
added and the mixture was heated at 90 ꢁC for 24 h. After
1H NMR (400 MHz, CDCl3): d 7.21 (br, 2H), 7.18 (br, 4H),
7.01 (br, 2H), 2.71 (br, 4H), 2.04 (br, 4H), 1.25 (br, 36H),
0.87 (br, 6H).
Anal. Calcd for (C48H60S7)n: C, 66.92; H, 7.02; S, 26.06.
Found: C, 64.72; H, 6.91; S, 24.75.
56
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
Poly(2,6-bis(2-thiophenyl-3-dodecylthiophene-2-yl)
dithieno[3,2-b;20,30-d]thiophene (8)
by spin-coating (1500 rpm) on silicon wafers followed by
drying at 60 C under vacuum (solvent: chloroform, concen-
ꢁ
Monomer 2 (1.5 g, 1.8 mmol), monomer 3 (1.3 g, 1.8 mmol),
and dry toluene (40 mL), deoxygenated with nitrogen for
30 min were added to a 100-mL three-necked round-bottom
flask. Pd(PPh3)4 (101 mg, 5 mol %) was added to the mix-
ture and the mixture was heated up to 90 ꢁC. The dark red
tration: 10 mg/mL). The surface topographies of pristine,
thermally annealed, and solvent-vapor-annealed films were
observed.
UV-Vis Absorption Spectroscopy
To study absorption behaviors, films of the two polymers
were fabricated on quartz substrates as follows. A chloro-
form solution (1 wt %) of each polymer was filtered through
an acrodisc syringe filter (Millipore 0.45 lm, Millipore, Bill-
erica, MA) and subsequently spin-cast on quartz glass. The
ꢁ
solution was allowed to stand at 90 C for 2 days. The highly
viscous black solution was cooled down to room tempera-
ture. The polymer was precipitated in methanol. The crude
polymer was collected by filtration and then purified by
Soxhlet extraction with acetone, hexane, and chloroform, suc-
cessively. The final product was obtained by precipitation in
methanol as a dark red solid in a yield of 76%.
ꢁ
films were dried overnight at 60 C for 12 h under vacuum.
Absorption spectra of the samples as films and as solutions
(chloroform, concentration 1 ꢀ 10ꢂ6 mol/L) were obtained
using a UV-Vis spectrometer (HP 8453, photodiode array
type) in the wavelength range 190–1100 nm.
1H NMR (400 MHz, CDCl3): d 7.22 (br, 2H), 7.07 (br, 4H),
7.02 (br, 2H), 2.78 (br, 4H), 1.67 (br, 4H), 1.24 (br, 36H),
0.86 (br, 6H).
OTFT Fabrication
Anal. Calcd for (C48H60S7)n: C, 66.92; H, 7.02; S, 26.06.
Found: C, 63.69; H, 6.86; S, 22.67.
Bottom-gate top-contact device geometry was employed for
characterization of TFT performance. Spin-coated films
(thickness ꢃ40–50 nm) of 4 and 8 were prepared on a heav-
ily n-doped Si/SiO2 substrate with monochlorobenzene
(MCB) and o-dichlorobenzene (ODCB) as solvent. Surface
modification was carried out with octyltrichlorosilane (OTS)
and octadecyltrichlorosilane (ODTS) to give hydrophobic
dielectric surfaces with more edge-on orientations in the
coated polymers. The OTFTs were fabricated by directly
spin-coating the polymer solution onto the dielectric sub-
strate at 1500 rpm. The fabricated thin films were heated in
air at 120 ꢁC to remove the solvent and to anneal them
thermally. Other devices were exposed to a solvent vapor
environment for 10 min. The solvent vapor environment was
prepared by pouring 10 mL of toluene into a glass dish. The
fabricated transistors were placed in the dish lid and placed
over toluene, followed by vacuum drying for 4 h.
Instrumental Analysis
1H NMR spectra were recorded on a Varian Mercury NMR
400 Hz spectrometer (Varian, Palo Alto, CA) using deuterated
chloroform purchased from Cambridge Isotope Laboratories
(Andover, MA). Elemental analyses were performed using an
EA1112 (Thermo Electron Corp., West Chester, PA) elemental
analyzer. High-resolution mass spectrometry (HRMS) was
performed on a JMS-700 MStation mass spectrometer (JEOL,
Tokyo, Japan; resolution 60,000, m/z range at full sensitivity
2400). The molecular weights of the polymers were deter-
mined by gel permeation chromatography (GPC, Tosoh GPC
HLC-8320GPC ecoxec) using polystyrene as the standard and
THF as the eluent.
The thermal properties were studied under a nitrogen
atmosphere on a Mettler DSC 821e instrument (Mettler, Grei-
fensee, Switzerland). Thermal gravimetric analysis (TGA) was
Source and drain electrodes were then thermally evaporated
(100 nm) through a shadow mask with a channel width and
length of 1500 lm and 100 lm, respectively. All the field
effect mobilities were extracted in the saturation regime
using the relationship lsat ¼ (2IDSL)/(WC(VgꢂVth)2), where
IDS is the saturation drain current, C is the capacitance of the
silicon oxide dielectric, Vg is the gate bias, and Vth is the
threshold voltage. The device performance was evaluated in
air using 4200-SCS semiconductor characterization system
under ambient conditions.
ꢁ
conducted on a Mettler TGA50 (temperature rate 10 C/min
under nitrogen). The redox properties of the two polymers
were examined using cyclic voltammetry (Model: EA161
eDAQ). Thin films were coated on a Pt plate using chloro-
form as the solvent. The electrolyte solution employed was
0.10 M tetrabutylammonium hexafluorophosphate (Bu4NPF6)
in freshly dried MC. Ag/AgCl and Pt wire (0.5 mm in dia-
meter) electrodes were used as the reference electrode and
the counter electrode, respectively. The scan rate was 50 mV/s.
The GI-XRD measurements were performed at the 10C1 and
3C2 beam lines (wavelength k ¼ 1.54 Å) at the Pohang
Accelerator Laboratory (PAL). The measurements were
obtained in a scanning interval of 2h between 3ꢁ and 35ꢁ.
The film samples were fabricated by drop-casting on silicon
wafer, followed by drying at 60 ꢁC under vacuum (solvent:
chloroform, concentration: 10 mg/mL).
RESULTS AND DISCUSSION
Synthesis
We report here the facile and high-yield synthesis of new
p-type DTT-based semiconducting polymers. Scheme 1 illus-
trates the synthetic routes for the two polymers, 4 and 8.
Bis (5-(3-bromo-4-dodecyl))-(2,20)-bithiophene, 2 and bis(-5-
(3-bromo-4-dodecyl) dithieno[3,2-b;20,30-d]thiophene, 6 were
used as the monomers with bistributyl tin DTT, 3 and bistri-
butyl tin (2,20)-bithiophene 7, respectively. As solubilizing
groups and crystallization promoting moieties, two dodecyl
groups were substituted in one repeating group with the
AFM (Digital Instruments Multimode equipped with a nano-
scope IIIa controller) operating in the tapping mode with a
silicon cantilever was used to characterize the surface mor-
phologies of the samples. The film samples were fabricated
HIGH PERFORMANCE SEMICONDUCTING CONJUGATED POLYMERS, KIM ET AL.
57

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Synthesis of the conjugated polymers. (i) NBS, DMF, 0 ꢁC, 12 h and (ii) Pd(PPh3)4, toluene, 90 ꢁC, 2 days.
positions of two dodecyl groups. Stille coupling of 2 and 6
and the tributyl-tin compounds of 3 and 7 produces the con-
jugated-polymers, 4 and 8, respectively, in yields of 71–76%.
peratures. (Tg of 4 ¼ 105 ꢁC, Tg of 8 ¼ 100 ꢁC). TGA mea-
surements at a heating rate of 10 ꢁC/min under nitrogen
revealed that the polymers had good thermal stabilities and
ꢁ
enhanced onset decomposition temperatures (ꢃ350–400 C)
The resulting compounds were then purified by silica-gel
column chromatography or, in the case of the two polymers,
by Soxhlet extraction and reprecipitation into methanol. The
identity and purity of the synthesized materials were
confirmed by 1H NMR, HRMS, and elemental analysis. The
number-average molecular weights (Mn) of 4 and 8 were
determined gel permeation chromatography (GPC) using a
(see Table 1). Although the temperature was raised to
900 ꢁC, some residual weights of 4 and 8 were measured;
this is attributed to char formation from the fused dithieno-
thiophene ring structure in the repeating group.
UV-Vis Absorption Spectroscopy
The absorption spectra of the solutions in chloroform (con-
centration 1 ꢀ 10ꢂ5 mol/L) and of the thin films were
obtained (see Fig. 1) The absorption maxima of 4 and 8 in
solution are located at 472 and 461 nm, respectively. 8
Exhibits a lower kmax because of disruption of planarity
along the polymer chains arising from the geometrical hin-
drance between the dodecyl side chains. We observed a
drastic spectral change in the film states of 4 and 8, which is
attributed to a high degree of intermolecular interaction.
From the significant red-shift of kmax, it can be conjectured
that a localized J-aggregation behavior is predominant in the
polymer films. For instance, in the as-spun thin film of 4, the
absorption spectrum was shown to be structureless and
significantly red-shifted, which is attributed to the formation
of an ordered structure in a localized domain with an inter-
molecular p-stacking (see Fig. 1).
polystyrene standard and found to be 16,100 (Mw/Mn
1.84) and 14,700 (Mw/Mn ¼ 1.93), respectively.
They were found to have good self-film forming properties
and were readily soluble in various organic solvents such as
chloroform, xylene, MC, monochlorobenzene, and THF. In
particular, more than 10 mg of 4 and 8 can be dissolved in
1 mL monochlorobenzene at room temperature.
¼
Thermal Analysis
When we use the polymers for OTFT applications, their
thermal stabilities and dynamic behaviors are particularly
important for device fabrication. The thermal properties of
the polymers were characterized by differential scanning cal-
orimetry (DSC) and thermogravimetry (TGA). DSC measure-
ments were performed at a heating (cooling) scan rate of
10 (ꢂ10) ꢁC/min under nitrogen, with the highest tempera-
ture limited to be below the decomposition temperature. 4
and 8 exhibit no distinct crystalline-isotropic transitions in
After thermal annealing, a less-significant change was ob-
served in the spectrum of 4, which indicates that the spin-
coated films already contained nanocrystallites and chain
ꢁ
the range of 25–250 C but show clear glass transition tem-
TABLE 1 Measured and Calculated Parameters of the Two Polymers
Energy Level
HOMO (eV)b LUMO (eV)b
abs
abs
onset
Tg
(ꢁC)
Td
(ꢁC)
k
k
Eogpt
(eV)b
E
ox
max
max
Polymer
(nm)a
(nm)b
(V)
4
8
a
105
100
400
350
472
461
522 (518)c
512 (502)c
1.87
1.90
0.77
0.81
ꢂ5.17
ꢂ5.21
ꢂ3.30
ꢂ3.31
Solution sample in chloroform.
Film sample.
After annealing.
b
c
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ARTICLE
FIGURE 1 Optical absorption spectra of 4 (A) and 8 (B). *Solid line: solution, dash-dot-dashed line: as-spun film, dashed line:
annealed film.
arrangement was fully developed. 8 showed a 10 nm hypso-
chromic shift of the absorption maximum, induced by ther-
mal annealing. In the case of polythiophene derivatives, it
was reported that s-trans and s-cis forms can be mixed in
the backbone structure.32–34 On the basis of the experimen-
tal and theoretical results, one might expect that polythio-
phene derivatives would tend to have the predominant
s-trans conformation of the interlinked thiophene rings. It is
assumed that all the single bonds between thiophene
moieties in two conjugated polymers 4 and 8 must exist in
s-trans configurations to show energetically favorable poly-
mer chain structures. Interchain interactions between the
polymer chains of 4 is therefore more robust in governing
the degree of order of the polymer chains.
angle of the insulator surface, the scattering from the sub-
strate was reduced relative to the scattering from the films.
Polymer 4 showed a small degree of crystallinity even in an
as-spun film, indicating the ease of crystallization through
the interactions between the long alkyl chains (see Fig. 2).
The as-cast film of polymer 8 displayed no crystallinity in
the diffractogram (see Fig. 3). After thermal annealing, both
polymer samples showed enhanced crystalline behavior due
to ordered reorganization of the polymer chains.
Significantly, anisotropic edge-on chain orientation of the
nanocrystalline polymer domains with respect to the insula-
tor substrates treated with OTS was clearly observed in the
annealed films. The edge-on orientations of the polymer
chains are evident from the different intensity distributions
of the (100) reflections due to the lamellar layer structure
Electrochemical Analysis
Cyclic voltammograms (CV) of a film sample were recorded
and the redox potentials relative to an internal ferrocene
reference (Fc/Fcþ) were obtained. These CV scans in the
range ꢂ2.0 V to þ2.0V (vs. Ag/Agþ) showed quasi-reversible
oxidation peaks. Unfortunately, the reduction behaviors were
irreversible; we were therefore unable to estimate their
HOMO and LUMO energies accurately. To determine the
LUMO levels, we combined the oxidation potential in the CV
with the optical energy bandgap (Eogpt) resulting from the
absorption edge in the absorption spectrum. Voltammograms
of 4 and 8 in the film state show that their lowest oxidative
waves are at þ0.77 and þ0.81 V, respectively. As shown in
Table 1, 4 and 8 have HOMO levels of ꢂ5.17 and ꢂ5.21 eV,
respectively. The relatively higher HOMO level of 8 should
endow the material with better environmental stability. In
addition, 4 and 8 have almost identical LUMO energy levels
of ꢂ3.30 and ꢂ3.31 eV, respectively.
d
(100)-spacing: 26.0 Å for 4 and 25.3 Å for 8 and the (010)
reflections due to pꢂp interchain stacking d(010)-spacing 4.4
(3.7) Å for 4, d(010)-spacing 4.3 (3.7) Å for 8 resulting from
out-of-plane and in-plane geometric modes.
Briefly, the degree of crystallinity of the spin-coated film is
likely to be affected by the tethering positions of the pendant
alkyl chains in the repeating group. The phenomena
observed in annealed films of the two polymers induce the
polymer chains to orient perpendicular to the insulator
substrate in the desired TFT devices and thus to become sta-
bilized at the interface between the polymer films and the
insulator.
Properties of OTFTs Made from the Polymers
Bottom-gate top-contact OTFT devices were fabricated using
a gold source and drain electrodes which were thermally
evaporated using the conventional method. p-Doped poly-
crystalline silicon was used as the gate electrode, with an
OTS-treated (or ODTS-treated) SiO2 surface layer used as the
gate dielectric insulator. A 40-nm thin film of the semicon-
ductor was deposited by spin-coating a 1 wt % solution of
the polymers in monochlorobenzene. After spin coating, the
polymer solution onto the insulator surface, gold was
Grazing-Incidence X-Ray Diffraction
To determine the chain orientation and investigate the crys-
tallinity of the thin films synchrotron GI-XRD measurements
were carried out. By using a fixed grazing incidence angle
(0.18ꢁ) that is above the critical angle for total internal
reflection from the polymer thin films but below the critical
HIGH PERFORMANCE SEMICONDUCTING CONJUGATED POLYMERS, KIM ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 2 Out-of-plane (A) and in-plane (B) GI-XRD intensities as a function of the scattering angle 2h for polymer, 4 films depos-
ited on an insulator surface modified with OTS. The inset shows the crystallography of the nano-crystallite with respect to insula-
tor substrate schematically. *Solid line (a): as-spun film, dotted line (b): after annealing at 120 ꢁC for 20 min. [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.com.]
deposited via thermal evaporation. To achieve the optimal
performance, the OTFT devices maꢁde of 4 and 8, respec-
tively, were further annealed at 120 C, for 20 min.
played a mobility of the order of 10ꢂ4 cm2/Vs, indicating
that a more significant annealing effect could be observed in
the device made of polymer 8. These results are in good
agreement with the GI-XRD results.
The output characteristics showed very good saturation
behaviors and clear saturation currents that were quadratic
to the gate bias (see Fig. 4). The saturated field-effect mobili-
ties, lFET can be calculated from the amplification character-
istics, by using the classical equations describing FETs. The
mobility values obtained by measuring five different devices
are listed in Table 2.
Our optical spectroscopy and GI-XRD results could provide
direct evidence of the strong intermolecular interactions in,
and preferred molecular orientations of, 4 and 8 in thermally
annealed crystalline films. The predominant edge-on orienta-
tions of 4 and 8 can therefore explain the higher value of
the field effect mobilities. Polymer 4 exhibited a slightly
higher carrier mobility of around 0.05 cm2/Vs (ION/OFF
¼
Figure 4 shows typical drain current (IDC) versus source-
drain voltage (VDS) plots at various gate voltages in accumu-
lation mode for TFTs based on polymers 4 and 8. When
using OTS or ODTS treated SiO2 insulator, the transistor
devices made of 4 and 8 provided a field effect mobility of
0.01–0.05 cm2/Vs, together with a high current on/off ratio
(>104) and a threshold voltage (Vth < ꢂ15 V). The devices
made of unannealed polymer 4 exhibited a field effect mobil-
ity of around 0.005–0.007 cm2/Vs; however, the unannealed
films of polymer 8 which had much less crystallinity dis-
3.42 ꢀ 104) when used in the fabricating of a semiconduct-
ing layer on ODTS-treated SiO2. It can be conjectured that
the 4,40-substitution mode of the dodecyl group can aid the
formation of a compact structure of polymer chains more
efficiently and that there is strong geometrical interaction
with the alkyl chains in the surface-treating agent (OTS
or ODTS). This behavior correlates well with the anisotro-
pic molecular arrangement in a localized area indicated by
GI-XRD.
FIGURE 3 Out-of-plane (A) and in-plane (B) GI-X-ray diffraction intensities as a function of the scattering angle 2h for polymer, 8
films deposited on an insulator surface modified with OTS. The inset shows the crystallography of the nanocrystallites with
respect to the insulator substrate schematically. Solid line (a): as-spun film, dotted line (b): after annealing at 120 ꢁC for 20 min.
[Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
60
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ARTICLE
FIGURE 4 Output (upper) and transfer (lower) characteristics of OTFT made by spin coating of 4 (A) and 8 (B) from monochloro-
benzene solution, annealed at 120 ꢁC for 20 min. *Insulator: ODTS-treated SiO2. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
Finally, we prepared TFT devices and carried out solvent
annealing rather than thermal annealing. Low-cost solvent-
vapor annealing has advantages as it can increase the molec-
ular crystallinity under the desired organic vapor without
the application of heat. The solvent type can affect the
molecular ordering and resultant crystallinity. Our polymers
are composed of heteroaromatic moieties which induce
higher conjugative effects. To aid the intermolecular stacked
aggregation uniformly, the aromatic solvents might be more
favorable as they behave as mediators for binding neighbor-
ing polymer chains. As with thermal annealing, solvent
annealing with toluene, acetone, 1,2-dichloroethane, and hex-
ane yielded polymer thin films that were highly crystalline,
and devices with comparable moderate saturation mobilities.
Toluene solvent-vapor partitions into the thin films, enabling
molecular rearrangement to take place without involving
subsequent chemical reactions. The extent of morphological
transformation and improvement in electrical characteristics
depends on the partitioning ability of the solvent vapor into
the organic semiconductor.
acetone and dichloroethane are used as solvents, the device
performances are not satisfactory. This suggests that the aro-
matic solvent molecules are easily penetrated and swell the
polymer surface relieving the strain between the polymer
chains. As a result, the aromatic ring in the solvent mole-
cules and the fused heteroaromatic rings strongly interact to
promote crystallization. Toluene could therefore act as a
‘‘crystallization-promoting-plasticizer’’ (CPP) in the conju-
gated polymer structures. When the CPP was removed
during drying, the reorganized densely packed crystalline
TABLE 2 TFT Characteristics: lFET (cm2/Vs) is the Carrier
Mobility, Ion/off is the On/Off Current Ratio, and Vth(V) is the
Threshold Voltage
Polymer Solvent SAM
l (cm2/Vs) Vth (V)
Ion/Ioff
4a
4b
4a
8a
8b
8a
a
MCB
MCB
ODCB
MCB
MCB
ODCB
OTS
OTS
0.022
0.014
ꢂ7.851
2.26 ꢀ 105
ꢂ41.51 V 2.08 ꢀ 103
ODTS 0.050
ꢂ3.214
ꢂ14.63
ꢂ40.58
ꢂ11.43
3.42 ꢀ 104
1.30 ꢀ 105
1.04 ꢀ 104
1.20 ꢀ 104
OTS
OTS
0.011
0.010
Figure 5, shows transfer curves, obtained from the toluene-
vapor annealed 4 and 8 film-based TFTs. Under identical
surface treatment with OTS, the device made of 4 showed a
mobility of ꢃ10ꢂ2 cm2/Vs with a 2.08 ꢀ 103 on/off current
ratio. Although these values are inferior to those of the
thermally annealed sample, they are much better than the
values achieved with a device made of pristine film. When
ODTS 0.010
Thermal annealing.
Solvent annealing.
b
*MCB, monochlorobenzene, ODCB, o-dichlorobenzene, OTS, octyltri-
chlorosilane, ODTS, octadecyltrichlorosilane.
HIGH PERFORMANCE SEMICONDUCTING CONJUGATED POLYMERS, KIM ET AL.
61

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 5 Transfer characteristics of OTFTs made of 4 (A) and 8 (B). Spin-coated film using monochlorobenzene solution. The
films were treated by toluene-vapor annealing, for 10 min. *Insulator: OTS-treated SiO2.
structure was almost retained with only a minute level of
shrinkage, to complete the anisotropic crystalline structures.
Atomic Force Microscopy
To explain why the mobility of 4 is higher, we investigated
the topography of the polymers and the surface coverage of
the dielectric layer using AFM. In addition to the long-range
molecular structural organization confirmed by GI-X-ray
studies, the AFM micrographs display well-resolved surface
structures on the dielectric layer. We prepared a solution of
each polymer, and covered the OTS-treated dielectric surface
with each solution by spin-coating. After complete drying,
we took three sets of micrographs as shown in Figure 6.
Polymer 8, showed an almost identical performance for the
thermally annealed sample (see Table 2). Toluene vapor is
therefore quite effective in relieving the strain in the solid-
state crystalline polymer chain and inducing a thermo-
dynamically favorable stable state of the reorganized poly-
mer structure. Thermal and solvent annealing processes
are both able to improve device performance in solution
processed TFT devices.
FIGURE 6 AFM images of 4 and 8 (5 ꢀ 5 lm). (A) pristine film, (B) thermally annealed film, and (C) solvent annealed film.
62
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ARTICLE
3 Jurchescu, O. D.; Baas, J.; Palstra, T. T. M. Appl Phys Lett
We analyzed the topography of the semiconducting layer on
the OTS-SiO2 layer. The two polymer films showed a compact
structure on the surface, with the smallest crystallites, with-
out annealing treatment. After thermal annealing, we could
observe crystallites of larger grain size, and a smoother
surface (roughness <0.8 nm) for the polymer 4 film, indicat-
ing highly connected and compact structures compared
to those in the polymer 8 film (roughness <3.5 nm). This
could explain why the device based on thermally annealed 4
demonstrated the highest mobility.
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The two solvent-annealed films showed a quite similar surface
topography and the roughness. A high coverage of the sub-
strate was achieved even after solvent annealing. The crystal-
lite grain size was larger than in pristine films. It may be that
the degree of crystallization becomes higher and the highly
packed crystalline molecules on the dielectric layer can
improve carrier transport with good connection behavior.
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CONCLUSIONS
We have successfully synthesized and characterized new
DTT-based conjugated polymers that are solution process-
able. Polymers 4 and 8 not only form smooth films on large
surfaces but also show better homogeneous layer formation
with relatively small crystallites via thermal annealing and
solvent vapor annealing. GI-XRD showed strong intermolecu-
lar interactions and preferred edge-on orientation of the
polymer chains in the thermally annealed crystalline film. In
brief, the two polymers incorporating a dithienothiophene
moiety not only offers high carrier mobility of the order of
10ꢂ2 cm2/Vs (lFET ¼ 0.01–0.05 cm2/Vs) but also have high
on/off current ratios, after both thermal annealing and sol-
vent annealing. In particular, the 4,40-substitution of the do-
decyl side chain in polymer, 4 drives higher crystallization,
even before thermal annealing, and more efficient edge-on
molecular arrangement, as shown by GI-XRD, resulting in
higher TFT performances. In the samples treated under
toluene vapor, the corresponding TFT devices also displayed
device performances comparable to those of devices pre-
pared from thermally annealed polymers. The process has
advantages for use in assembly of OTFTs in low cost fabrica-
tion of practical devices.
11 Zhan, X.; Tan, Z. a.; Domercq, B.; An, Z.; Zhang, X.; Barlow,
S.; Li, Y.; Zhu, D.; Kippelen, B.; Marder, S. R. J Am Chem Soc
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This research work was supported by 21st Century Frontier
Research Program (F0004091-0000-00, 2010), and Korea
Science and Engineering, (KOSEF R0120070001128402008),
and Priority Research Centers Program through the National
Research Foundation of Korea (NRF) funded by the Ministry of
Education, Science and Technology (NRF20100020209). Mr.
K. H. Kim thanks the financial support by the Seoul R&BD
Program (10543).
19 Yang, H.; Shin, T. J.; Yang, L.; Cho, K.; Ryu, C. Y.; Bao, Z.
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