High-performance amorphous donor-acceptor conjugated polymers containing x-shaped anthracene-based monomer and 2,5-bis(2-octyldodecyl)pyrrolo[3,4-c] pyrrole-1,4(2H,5H)-dione for organic thin-film transistors

Article abstract of DOI:10.1002/pola.26078

New diketopyrrolopyrrole (DPP)-containing amorphous conjugated polymers, such as poly(3-(5-((9,10-bis((4-hexylphenyl)ethynyl)-6-(prop-1-ynyl)anthracen-2- yl)ethynyl) thiophen-2-yl)-5-(2-hexyldecyl)-2-(2-octyldodecyl)-6-(thiophen-2-yl) pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione) (4), and poly(3-(5-((2,6-bis((4- hexylphenyl)ethynyl)-10-(prop-1-ynyl)anthracen-9-yl)ethynyl)thiophen-2-yl)-2, 5-bis(2-octyldodecyl)-6-(thio phen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione) (7), were successfully synthesized via Sonogashira coupling reactions under microwave conditions. Copolymer 7, incorporating a DPP moiety at the 9,10-position of the anthracene ring through a triple bond, showed a much lower bandgap energy (E_g = 1.81 eV) than copolymer 4 (E_g = 2.13 eV). Tuning of the molecular frontier orbital energies was achieved by only changing the anchoring position of dithiophenyl-DPP from the 2,6- to the 9,10-position in the anthracene ring. Because of the donor-acceptor (D-A) interaction and the two-dimensional planar structure of the X-shaped donor monomer, the resulting polymers showed good interchain π-π stacking in the thin-film state, despite being amorphous polymers. When the newly synthesized polymer 7 was used as a semiconductor material in an organic thin-film transistor, the best mobility of up to 0.12 cm² V^-1 s ^-1 (I_on/off = ～ 4.4 × 10⁶) was observed, which is one of the highest values recorded for amorphous polymer films reported to date.

Full text of DOI:10.1002/pola.26078

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High-Performance Amorphous Donor–Acceptor Conjugated
Polymers Containing X-Shaped Anthracene-Based Monomer
and 2,5-Bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione
for Organic Thin-Film Transistors
Joo Bin Lee, Kyung Hwan Kim, Chang Seop Hong, Dong Hoon Choi
Department of Chemistry, Research Institute for Natural Sciences, Korea University, 5 Anam-dong, Sungbuk-gu,
Seoul 136-701, South Korea
Correspondence to: D. H. Choi (E-mail: dhchoi8803@korea.ac.kr)
Received 25 December 2011; accepted 13 March 2012; published online
DOI: 10.1002/pola.26078
ABSTRACT: New diketopyrrolopyrrole (DPP)-containing amor-
phous conjugated polymers, such as poly(3-(5-((9,10-bis((4-hexyl-
phenyl)ethynyl)-6-(prop-1-ynyl)anthracen-2-yl)ethynyl) thiophen-
2-yl)-5-(2-hexyldecyl)-2-(2-octyldodecyl)-6-(thiophen-2-yl)pyrrolo
[3,4-c]pyrrole-1,4(2H,5H)-dione) (4), and poly(3-(5-((2,6-bis((4-
hexylphenyl)ethynyl)-10-(prop-1-ynyl)anthracen-9-yl)ethynyl)
thiophen-2-yl)-2,5-bis(2-octyldodecyl)-6-(thio phen-2-yl)pyr-
rolo[3,4-c]pyrrole-1,4(2H,5H)-dione) (7), were successfully syn-
thesized via Sonogashira coupling reactions under microwave
conditions. Copolymer 7, incorporating a DPP moiety at the
9,10-position of the anthracene ring through a triple bond,
showed a much lower bandgap energy (Eg ¼ 1.81 eV) than co-
polymer 4 (Eg ¼ 2.13 eV). Tuning of the molecular frontier orbital
energies was achieved by only changing the anchoring position of
dithiophenyl-DPP from the 2,6- to the 9,10-position in the anthra-
cene ring. Because of the donor–acceptor (D–A) interaction and the
two-dimensional planar structure of the X-shaped donor mono-
mer, the resulting polymers showed good interchain pꢀp stacking
in the thin-film state, despite being amorphous polymers. When
the newly synthesized polymer 7 was used as a semiconductor
material in an organic thin-film transistor, the best mobility of up
to 0.12 cm2 Vꢀ1 sꢀ1 (Ion/off ¼ ꢁ 4.4 ꢂ 106) was observed, which is
one of the highest values recorded for amorphous polymer films
C
V
reported to date. 2012 Wiley Periodicals, Inc. J Polym Sci Part A:
Polym Chem 000: 000–000, 2012
KEYWORDS: charge transport; conjugated polymer; copolymer-
ization; synthesis; heteroatom-containing polymers; noncrystal-
line polymers
INTRODUCTION Over the past few years, a number of prom-
ising candidates for organic semiconducting materials have
been developed; therefore, high-performance organic thin-
film transistors (OTFTs) with high carrier mobilities of the
order of 1 cm2 Vꢀ1 sꢀ1 have been obtained.1–5 Moreover,
soluble semiconducting materials have been strongly desired
to achieve low-cost, large-area electronic device fabrication.
Organic semiconductors that are soluble in organic solvents
can be used for the fabrication of thin-film transistor (TFT)
devices via conventional low-cost, spin-coating, or drop-cast-
ing techniques.6–9 Compared with vacuum-processed TFT
devices, solution-processed OTFTs usually have significantly
worse device performance. The development of high-perform-
ance solution-processable semiconducting materials is there-
fore an important issue in the further development of OTFTs.
chanical stability compared with small molar mass organic
materials.7,10–12 However, most current polymeric semicon-
ductors have low carrier mobility or poor device-perform-
ance reproducibility in the atmosphere.
In TFT devices, desirable molecular ordering on the sub-
strate is difficult to achieve to any great extent by conven-
tional thermal annealing or other post-treatment meth-
ods.13,14 However, recent achievements in the charge-carrier
mobility and stability of TFT devices fabricated with crystal-
line semiconducting polymers have resulted in TFT charac-
teristics comparable with those of vacuum-processed
TFTs.15,16
Ong et al.17 reported the synthesis of poly(3,30-didodecylqua-
terthiophene) (PQT-12) with l ¼ 0.14 cm2 Vꢀ1 sꢀ1 and good
oxidative doping stability. McCulloch and coworkers18,19
reported the synthesis of poly(2,5-bis(3-dodecylthiophene-2-
Extensive research has recently been performed on polymer
semiconductors in particular because of their facile solution
processability, good film-forming properties, and good me-
yl)thieno[3,2-b]thiophene)
(PBTTT),
which
includes
C
V
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thieno[2,3-b]thiophene with sulfur atoms in the antiposition;
state. We demonstrate that the ‘‘unannealed’’ polymer 7 not
only offers a high carrier mobility of 0.12 cm2 Vꢀ1 sꢀ1 but
also has a high on/off current ratio (Ion/off ¼ ꢁ 4.4 ꢂ 106)
in TFT devices.
PBTTT-based TFTs yield the highest mobility, l ¼ 0.01–0.2
cm2 Vꢀ1 sꢀ1
.
Thus far, ‘‘homogeneous’’ polymers composed of donor and
acceptor units in the repeating group have been reported to
EXPERIMENTAL
have an excellent mobility of 0.94–3.3 cm2 Vꢀ1 sꢀ1 20
How-
.
Synthesis
ever, mobilities of the aforementioned conjugated polymers
are very sensitive to the molecular weight and fabrication
methods. Moreover, the reproduction of films with identical
degrees of crystallinity and molecular ordering remains
extremely challenging.15,21–23
Compounds 3, 5, and 6 were synthesized using the modified
literature method.27–30
((9,10-Bis((4-hexylphenyl)ethynyl)anthracene-2,6-diyl)
bis(ethyne-2,1-diyl))bis (trimethylsilane), (1)
An oven dried, mag.-stirred, 250-mL round-bottom flask
(RBF) was charged with a solution of 2,6-dibromo-9,10-
bis((4-hexylphenyl) ethynyl)anthracene (1.02 g, 1.45 mmol),
PdCl2(PPh3)2 (0.05 g, 0.072 mmol), and CuI (0.014 g, 0.072
mmol) in a mixture of freshly distilled tetrahydrofuran (THF)
(70 mL), triethylamine (20 mL), and diisopropylamine (15
mL). Ethynyltrimethylsilane (0.21 g, 4.34 mmol) was then
added and the mixture was allowed to stir at 80 ꢃC for 5 h. Af-
ter completing the reaction, the solution was poured into
methanol to collect the precipitates. The crude solid was puri-
fied by silica-gel column chromatography using the mixture of
methylene chloride (MC) and hexane (1:5). Further precipita-
tion afforded a pure compound, 1. (Yield 0.84 g, 78%).
1H NMR (300 MHz, CDCl3): d (ppm) 8.76 (s, 2H), 8.58 (d, J
¼ 8.79 Hz, 2H), 7.71 (d, J ¼ 8.25 Hz, 4H), 7.61 (d, J ¼ 8.79
Hz, 2H), 7.30 (m, 4H), 2.71 (t, J ¼ 8.46 Hz, 4H), 1.69 (m,
4H), 1.34 (m, 12H), 0.93 (t, 9H), 0.33 (m, 18H). 13C NMR
(100 MHz, CDCl3): d (ppm) 144.29, 131.74, 131.69, 131.58,
129.43, 128.72, 127.40, 121.59, 120.21, 118.43, 105.47,
103.28, 96.64, 85.23, 36.03, 31.71, 31.29, 28.95, 22.61,
14.12. Low-resolution mass spectrometry [LR-MS (FAB)] m/z
(Mþ): Calcd. for C52H58Si2, 738.41; found, 739.42. Anal.
Calcd. for C52H58Si2: C, 84.49; H, 7.91; Si, 7.60, found: C,
84.51; H, 7.77.
The thermal annealing process is always complicated and
impractical in real device fabrication. In addition, it is diffi-
cult to use organic semiconductors for continuous device
processing such as roll-to-roll processing for elaborating
large circuit devices. Until now, amorphous-conjugated poly-
mers have attracted little interest because of their intrinsi-
cally poor semiconducting properties resulting from a rela-
tively low degree of molecular ordering leading to a less
efficient carrier-hopping process between the polymer
chains. Recently, promising TFT performances were demon-
strated when amorphous polymers such as poly(triaryl-
amine),24 alkoxy-substituted poly(p-phenylene vinylene),15
and fluorene-containing polymers25 were used in OTFT
devices.
More recently, McCulloch’s group also demonstrated a new
solution for 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 with excellent field-effect transistor
(FET) performance.26 These polymers have high charge-car-
rier mobilities (l ¼ 0.06–0.14 cm2 Vꢀ1
s
ꢀ1, Ion/off ¼ 105–
106), excellent air stability, and good solution processability.
The aforementioned polymers demonstrated the new con-
cept that higher crystallinity and long-range ordered struc-
tures are not always necessary to achieve great TFT device
performance. Briefly, amorphous polymers can be highly ad-
vantageous because they provide constant film morphology,
which improves the reproducibility of the device perform-
ance during the continuous device processing. In addition,
the manufacturing process can be simplified and less costly
because device performance is less sensitive to environmen-
tal and processing conditions compared with TFT devices
made of crystalline-conjugated polymers.
2,6-Diethynyl-9,10-bis((4-hexylphenyl)ethynyl)anthracene, (2)
A 250-mL, oven dried, mag.-stirred RBF was charged with a
solution of ((9,10-bis((4-hexylphenyl)ethynyl)anthracene-2,6-
diyl)bis(ethyne-2,1-diyl))bis(trimethylsilane), 1 (0.84 g, 1.13
mmol) and potassium carbonate (0.62 g, 4.52 mmol) in a
mixture of freshly distilled THF (20 mL) and methanol (50
mL). The mixture was allowed to stir at room temperature
for 2 h and then neutralized with diluted hydrochloric
acid. The solution was extracted with MC. The crude
solid was purified by silica-gel column chromatography
using the mixture of MC and hexane (1:3) as an eluent.
Further precipitation afforded a pure compound, 2 (Yield
0.55 g, 82%).
Herein, we report the synthesis of two different amorphous
D–A alternating copolymers in which X-shaped electron-rich
monomers and diketopyrrolopyrrole (DPP)-based electron-
deficient monomers are incorporated into the polymer back-
bone. In addition, to compensate for the lack of crystallinity,
the uniform D–A alternating copolymer structure was chosen
to improve the interchain interaction. It could be envisaged
that the carrier transport phenomenon would be enhanced
by the van der Waals interaction for p–p molecular stacking
and for the D–A interaction between the anthracene and
DPP moieties. It should be noted that the DPP-tethering
mode at the 9,10-position in the anthracene ring of 7
induced the reduction of the bandgap energy in the film
1H NMR (300 MHz, CDCl3): d (ppm) 8.82 (s, 2H), 8.61 (d,
J ¼ 8.79 Hz, 2H), 7.70 (d, J ¼ 8.25 Hz, 4H), 7.64 (d, J ¼ 8.79
Hz, 2H), 7.29 (m, 4H), 3.29 (s, 2H), 2.71 (t, J ¼ 8.46 Hz, 4H),
1.69 (m, 4H), 1.34 (m, 12H), 0.91 (t, 9H). 13C NMR (100
MHz, CDCl3): d (ppm) 144.31, 131.85, 134.69, 131.64,
131.57, 129.23, 128.71, 127.58, 120.63, 120.10, 118.52,
103.51, 85.05, 84.12, 79.08, 36.03, 31.72, 31.28, 28.95,
22.62, 14.12. LR-MS (FAB) m/z (Mþ): Calcd. for C46H42
,
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594.33; found, 595.34. Anal. Calcd. for C46H42: C, 92.88; H,
7.12, found: C, 90.16; H, 6.46.
which can be exposed to a maximum of 85 ꢃC and 1.03 bar
internal pressure.
1H NMR spectra were recorded on a Varian Mercury NMR 300
Hz spectrometer (Varian, Palo Alto, CA) using deuterated chloro-
form purchased from Cambridge Isotope Laboratories (Andover,
MA). Elemental analyses were performed using an EA1112
(Thermo Electron, West Chester, PA) elemental analyzer. LR-MS
was performed on a JMS-700 MStation mass spectrometer
(JEOL, Tokyo, Japan; resolution 60,000, m/z range at full sensitiv-
ity 2400). The molecular weights of the polymers were deter-
mined by gel permeation chromatography (GPC, Waters GPC,
Waters 515 pump, Waters 410 RI, 2ꢂ Plgel Mixed-B) using poly-
styrene as the standard and chloroform as the eluent at 35 ꢃC.
Poly[3-(5-((9,10-bis((4-hexylphenyl)ethynyl)-6-
(prop-1-yn-1-yl) anthracen-2-yl)ethynyl)thiophen-2-yl)-
5-(2-hexyldecyl)-6-(5-methylthiophen-2-yl)-2-(2-octyldo-
decyl) pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione], (4)
Monomer 2 (0.2 g, 0.34 mmol), monomer 3 (0.34 g, 0.34
mmol), PdCl2(PPh3)2 (0.012 g, 0.017 mmol), CuI (0.0032 g,
0.017 mmol), 0.7 mL of triethylamine and 0.3 mL of diiso-
propylamine, and 1.5 mL of dry toluene were charged in a
sealed 10-mL microwave vial. The reaction mixture was
ꢃ
stirred at 85 C under microwave (ꢁ 50 W) irradiation for 1
h. The highly viscous dark green solution was cooled down
to room temperature. The polymer was precipitated in meth-
anol/H2O (10:1 v/v). The crude polymer was collected by fil-
tration and then purified by Soxhlet extraction with metha-
nol, acetone, hexane, and chloroform, successively. The final
product was obtained by precipitation into methanol. Then,
the product was dried under vacuum for 1 day to get the co-
The thermal properties were studied under a nitrogen
atmosphere on a Mettler DSC 821e instrument (Mettler, Grei-
fensee, Switzerland). Thermal gravimetric analysis (TGA) was
conducted on a Mettler TGA50 (temperature rate 10 ꢃC
minꢀ1 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
chloroform as a solvent. The electrolyte solution used was
0.10 M tetrabutylammonium hexafluorophosphate (Bu4NPF6)
in freshly dried MC. Ag/AgCl and Pt wire (0.5 mm in diame-
ter) electrodes were used as the reference electrode and the
polymer, 4 as dark green solid in a yield of 69% [Mn
29078, Mw ¼ 118,901, polydispersity index (PDI) ¼ 4.09].
¼
1H NMR (300 MHz, CDCl3): d (ppm) 8.45 (broad, 4H), 7.90
(broad, 6H), 7.73 (broad, 8H), 3.91 (broad, 4H), 2.75 (broad,
4H), 1.75 (broad, 4H), 1.18 (m, 40H), 0.82 (m, 11H). Anal.
Calcd. for C102H132N2O2S2: C, 82.65; H, 8.98; N, 1.89; S, 4.33;
found: C, 81.95; H, 8.72; N, 2.33; S, 4.34.
counter electrode, respectively. The scan rate was 50 mV sꢀ1
.
The X-ray diffraction (XRD) measurements were performed
using the Rigaku D/Max Ultimata 3 at Nano Chemistry Labo-
ratory in Korea University. The measurements were obtained
in a scanning interval of 2y between 2ꢃ and 40ꢃ. The film
samples were fabricated by spin casting the polymer solu-
tions on n-octyltrichlorosilane (OTS)-treated SiO2 wafer, fol-
lowed by drying at 80 ꢃC under vacuum (solvent: chloro-
form, concentration: 10 mg mLꢀ1).
Poly[3-(5-((2,6-bis((4-hexylphenyl)ethynyl)-10-(prop-1-
yn-1-yl)anthracen-9-yl)ethynyl)thiophen-2-yl)-6-
(5-methylthiophen-2-yl)-2,5-bis(2-octyldodecyl)
pyrrolo[3,4-c] pyrrole-1,4(2H,5H)-dione], (7)
Monomer 6 (0.2 g, 0.34 mmol), monomer 3 (0.34 g, 0.34
mmol), PdCl2(PPh3)2 (0.012 g, 0.017 mmol), CuI (0.0032 g,
0.017 mmol), 0.7 mL of triethylamine and 0.3 mL of diiso-
propylamine, and 1.5 mL of dry toluene were charged in a
sealed 10-mL microwave vial. The reaction mixture was
stirred at 85 ꢃC under microwave (ꢁ 50 W) irradiation for
30 min. The highly viscous dark green solution was cooled
down to room temperature. The polymer was precipitated in
methanol/H2O (10:1 v/v). The crude polymer was collected
by filtration and then purified by Soxhlet extraction with
methanol, acetone, hexane, and chloroform, successively. The
final product was obtained by precipitation in methanol.
Then, the product was dried under vacuum for 1 day to get
the copolymer, 7 as dark green solid in a yield of 55% (Mn
¼ 42,727, Mw ¼ 575,820, PDI ¼ 13.48).
Atomic force microscopy (AFM, Advanced Scanning Probe
Microscope, XE-100, PSIA) operating in the tapping mode
with a silicon cantilever was used to characterize the surface
morphologies of the samples. The film samples were fabri-
cated by spin coating (2000 rpm) on silicon wafers followed
ꢃ
by drying at 80 C under vacuum (solvent: chloroform, con-
centration: 10 mg mLꢀ1).
UV-Vis Absorption Spectroscopy
To study absorption behaviors, films of the two polymers
were fabricated on glass substrates as follows. A chloroform
solution (1 wt %) of each polymer was filtered through an
acrodisc syringe filter (Millipore 0.45 lm, Millipore, Billerica,
MA) and subsequently spin cast on glass. The films were
1H NMR (300 MHz, CDCl3): d (ppm) 8.40 (broad, 4H), 7.61
(broad, 14H), 3.77 (m, 4H), 2.68 (broad, 4H), 1.88 (m, 4H),
1.18 (broad, 85H), 0.88 (m, 33H). Anal. Calcd. for
ꢃ
dried at 80 C for 2 h under vacuum.
Absorption spectra of the samples as films and as solutions
(chloroform, conc. 1 ꢂ 10ꢀ5 mol Lꢀ1) were obtained using a
UV–Vis spectrometer (HP 8453, photodiode array type) in
the wavelength range 190–1100 nm.
C
102H132N2O2S2: C, 82.65; H, 8.98; N, 1.89; S, 4.33; found: C,
82.69; H, 8.83; N, 1.62; S, 4.32.
Instruments
The microwave polymerization reactions were performed
R
using a DiscoverV CEM instrument, operating with continu-
OTFT Fabrication
ous irradiation power at 50 W. All reactions were performed
in 10-mL intelligent microwave vials sealed with septa,
Bottom-gate top-contact device geometry was used for char-
acterization of TFT performance. Spin-coated films (thickness
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SCHEME 1 Synthesis of the D–A amorphous conjugated polymers.
ꢁ 60–65 nm) of 4 and (thickness ꢁ 50 nm) of 7 were pre-
pared on a heavily n-doped SiO2/Si substrate with chloro-
form as a solvent. Surface modification was carried out with
OTS to give hydrophobic dielectric surfaces on SiO2/Si sub-
strate. The OTFTs were fabricated by directly spin coating
the polymer solutions onto the dielectric substrate at 2000
rpm. The fabricated thin films were dried at 80 ꢃC for 2 h
under vacuum.
threshold voltage. The device performance was evaluated in
air using 4200-SCS semiconductor characterization system
under ambient conditions.
RESULTS AND DISCUSSION
Synthesis
Here, we report the facile synthesis of new p-type DPP-con-
taining p-conjugated semiconducting polymers. Scheme 1
illustrates the synthetic routes for the two polymers, 4 and
7. X-shaped anthracene-based crystalline molecules have
exhibited highly promising carrier mobilities in our previous
studies.27–29 Thus, we used their derivatives as monomers
for the polymerization conducted in this study. Monomers 2
and 6 contain ethynyl groups at the 2,6- and 9,10-positions
in the anthracene ring, respectively.
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(VGꢀVTH)2], where
IDS is the a saturation drain current, C is the capacitance of
the silicon oxide dielectric, VG is the gate bias, and VTH is the
4
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FIGURE 1 DSC thermograms of 4 (A) and 7 (B). Inset: expanded thermogram in the range of 127–129 ꢃC. Solid line: heating cycle,
short-dashed line: cooling cycle.
Sonogashira coupling of 3 and the anthracene-based mono-
mers of 2 and 6 with the different reaction sites produces
the conjugated polymers 4 and 7, respectively, under micro-
wave irradiation for 0.5–1.0 h, in yields of 55–69%.
4 and 7 exhibited no distinct crystalline-isotropic transitions
in the range of 25–250 ꢃC but did show almost identical
glass transition temperatures (Tg of 4 ¼ 126 ꢃC, Tg of 7 ¼
ꢃ
128 C) with a very small enthalpy change (sꢀe1e Fig. 1). TGA
ꢃ
measurements at a heating rate of 10 C min under nitro-
The resulting compounds were then purified by silica-gel
column chromatography or, in the case of the two polymers,
by Soxhlet extraction and reprecipitation in methanol. The
identity and purity of the synthesized materials were con-
gen revealed that the polymers had good thermal stabilities
and enhanced onset decomposition temperatures (337–343
ꢃC) (see Fig. 2 and Table 1).
1
firmed by H NMR, LR-MS, and elemental analysis. The num-
UV-Vis Absorption Spectroscopy
ber-average molecular weights (Mn) of 4 and 7 were deter-
mined by GPC using a polystyrene standard and was found
to be 29,078 (Mw/Mn ¼ 4.09) and 42,727 (Mw/Mn ¼ 13.47),
respectively.
The UV–Vis absorption spectra of the solutions in chloroform
(conc. 1 ꢂ 10ꢀ5 mol Lꢀ1) and of the thin films were
obtained (see Fig. 3). The thin films were spin cast onto a
glass substrate and annealed at 120 ꢃC for 10 min. The
introduction of DPP into the polymer backbone resulted in
broad absorption spectra and led to the formation of a dis-
tinct dual absorption bands in both the solutions and the
thin films. Two distinct absorption bands in 4 and 7 were
attributed to p–p* (short-wavelength band) and intramolecu-
lar charge transfer (long-wavelength band) transitions. The
absorption maxima (kambasx) of 4 and 7 in solution were
located at 684 and 756 nm, respectively, and at 697 and 794
The polymers were found to have good self-film-forming
properties and were readily soluble in various organic sol-
vents such as chloroform, MC, and chlorobenzene. In particu-
lar, more than 10 mg of 4 and 7 could be dissolved in 1 mL
of chloroform at room temperature.
Thermal Analysis
The thermal properties of the polymers were characterized
by differential scanning calorimetry (DSC) and TGA. DSC
measurements were performed at a heating (cooling) scan
rate of 10 (ꢀ10) ꢃC minꢀ1 under nitrogen, with the highest
temperature limited to being below the decomposition
temperature.
nm, respectively, in the film states. The difference between
abs
max
k
in solution and that in the thin film was relatively small
(ca. 13–38 nm) compared to that of crystalline polymers
such as regioregular P3HT (Dk ¼ 100 nm),31 but it was simi-
lar to that of amorphous regiorandom P3HT and other
FIGURE 2 TGA thermograms of 4 (A) and 7 (B).
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TABLE 1 Measured and Calculated Parameters of the Two Polymers
Energy Level
(V)d EEgC (V)d HOMO (eV) LUMO (eV)
.
.
Polymer Tg (ꢃC) Td (ꢃC)
k
(nm)a
k
(nm)a
Eogpt (eV)b
E
ox
(V)d
E
red
abs
max
abs
max
onset
onset
4
126
337
433, 628, 684 436, 646, 697b 1.63b
437, 651, 705c 1.60c
1.16
1.10
ꢀ0.97
ꢀ0.71
2.13
ꢀ5.52d
ꢀ5.46d
ꢀ3.39d
ꢀ3.89e
ꢀ3.65d
ꢀ4.02e
7
128
343
719, 756
728, 794b
736, 810c
1.44b
1.41c
1.81
a
b
c
d
e
Measured in chloroform with a concentration of 10ꢀ5 M.
Films were spin-coated from chloroform solution.
Thermally annealed film (T ¼ 120 ꢃC for 10 min).
The values were obtained cyclic voltammograms.
HOMO (eV) – Eogpt (eV).
reported amorphous polymers.25a More importantly, only a
slight change after thermal annealing was observed in the
film samples of the two molecules (Dk of 4 ¼ 8 nm, Dk of 7
¼ 6 nm), implying that although intermolecular interaction
was affected slightly by thermal annealing, the degree of
ordering in the film state was still poor due to the amor-
phous morphology.32
showed quasi-reversible oxidation peaks. Unfortunately, the
oxidation and reduction behaviors were irreversible; we
used onset potential to determine the electrochemical
HOMO, LUMO, and bandgap energies. We also 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 7 in the film states show that their
lowest oxidative waves are at þ1.16 and þ1.10 V, respec-
tively. As shown in Table 1, 4 and 7 have HOMO levels of
ꢀ5.52 and ꢀ5.46 eV, respectively. The relatively deeper
HOMO level of 4 should endow the material with better
environmental stability.
Electron-State-Density Distribution Obtained by Density
Functional Theory Calculation
Figure 4 shows the electron-state-density distribution of the
highest occupied molecular orbital (HOMO) and the lowest
occupied molecular orbital (LUMO) of geometrically opti-
mized structures (DFT B3LYP/6-31G) of the two polymers,
using the Spartan program (’06). More efficient p-electron
delocalization could be observed in the HOMO and LUMO
states of the repeating group along the 9,10-position of the
anthracene ring. The coefficients in the HOMO and LUMO
orbitals of the repeating unit in 7 [Fig. 4(B)] are mainly
located on the anthracene unit along 9,10-substituted posi-
tions more effectively. Because of more extended conjugated
structure along the DPP units, 7 shows a smaller calculated
bandgap energy of 2.02 eV than does 4 (EDg FT ¼ 2.06 eV).
XRD Study of Two Polymers
The microstructures of the polymers were investigated by
wide-angle X-ray scattering using the thin films prepared by
spin-coating chloroform solutions onto OTS-modified SiO2/Si
substrates. The as-spun films of polymers 4 (A) and 7 (B)
exhibited no crystallinity in the diffractogram, indicating that
the sample had an amorphous structure (see Fig. 6). In addi-
tion, thermal annealing did not induce any change in the dif-
fraction pattern. Thus, the two polymers were proven to be
perfectly amorphous in nature and are expected to be insen-
sitive to thermal conditions when preparing and operating
the TFT device.
Electrochemical Analysis
Cyclic voltammograms (CV) of the film samples were
recorded and the redox potentials relative to an internal fer-
rocene reference (Fc/Fcþ) were obtained (Figure 5). These
CV scans in the range from ꢀ2.0 to þ2.0 V (vs. Ag/Agþ)
FIGURE 3 Optical absorption spectra of 4 (A) and 7 (B). Solid line: solution, short-dashed line: as-spun film, dash-dot-dot line:
thermally annealed film.
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FIGURE 4 Molecular simulation results of the simplified repeating unit in two polymers, showing HOMO and LUMO distribution.
(A) Polymer 4 and (B) polymer 7.
FIGURE 5 CV of 4 (A) and 7 (B) in film states.
FIGURE 6 XRD patterns of two polymers, 4 (A) and 7 (B) thin films on OTS-treated SiO2/Si substrate. Solid line (a): as-spun film,
short-dashed line (b): annealed film at 120 ꢃC for 10 min.
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FIGURE 7 Output (a) and transfer curves (b) of TFT device fabricated with pristine 4 (A) and 7 (B) film. OTS-SiO2/Si gate insulator;
the device performances were measured in air. VDS ¼ ꢀ120 V. Inset of Figure 7A-a, TFT device configuration.
Properties of OTFTs Made From the Polymers
devices of as-spun film of amorphous semiconducting
polymers.
Bottom-gate top-contact OTFT devices were fabricated using
gold source and drain electrodes that were thermally evapo-
rated using the conventional method. A p-doped polycrystal-
line silicon was used as the gate electrode, with an OTS-
treated SiO2 surface layer used as the gate dielectric insulator.
A 50–65 nm thin film of the semiconductor was deposited by
spin coating a 1 wt % solution of the polymers in chloroform.
After spin coating, the polymer solution onto the insulator sur-
face, gold was deposited via thermal evaporation.
After annealing, the TFT devices on the two polymers showed
only a slight change in TFT mobility. This behavior is attrib-
uted to the amorphous structures of the two polymers, in con-
trast to the behavior of TFT devices based on general semi-
crystalline polymers (see Table 2). Briefly, amorphous
polymers have significant advantages over polycrystalline
films because the amorphous structure obviates the need for a
thermal annealing process during device fabrication.
The output characteristics showed very good saturation
behaviors and clear saturation currents that were quadratic
to the gate bias (see Fig. 7). The saturated field-effect mobili-
ties, lFET can be calculated from the amplification character-
istics, using the classical equations describing FETs. The mo-
bility values obtained by measuring the different devices are
listed in Table 2.
Atomic Force Microscopy
Finally, we analyzed the topography of the semiconducting
layer on the OTS-treated SiO2 layer. The two polymer films
showed a compact structure on the surface with the amor-
phous structure, without annealing treatment (Figure 8).
A smoother surface (roughness <0.8 nm) for the polymer 7
Figure 7 shows typical drain current (IDS) versus source-
drain voltage (VDS) plots at various gate voltages in accu-
mulation mode for TFTs based on polymers 4 and 7. When
using an OTS-treated SiO2 insulator, the transistor devices
made of 4 and 7 provided a field-effect mobility of 0.015–
film indicated highly connected and compact structures
TABLE 2 TFT Characteristics: l (cm2 V21 s21) is the Carrier
Mobility, Ion/off is the On/Off Current Ratio, and VTH (V) is the
Threshold Voltage
0.12 cm2 Vꢀ1 ꢀ1, together with a high current on/off ratio
s
W/L
Ion off
/
VTH
l(cm2
(>105) and threshold voltage (VTH: ꢀ20ꢀ32 V). The devices
made of unannealed polymer 4 exhibited a field-effect mo-
ꢀ1
Polymer
(lm/lm)
VDS (V)
(V)
(V)
V
sꢀ1
)
bility of around ꢁ 0.015 cm2 Vꢀ1
s
ꢀ1. However, the unan-
nealed films of polymer 7 exhibited a higher mobility of
4a
4b
7a
7b
a
1500/100
1500/100
1500/100
1500/100
ꢀ120
ꢀ120
ꢀ120
ꢀ120
105–106
105
105–106
105–106
ꢀ32
ꢀ20
ꢀ30
ꢀ21
0.015
0.026
0.10
around ꢁ 0.1 cm2 Vꢀ1
s
ꢀ1, indicating that the low energy
bandgap (EEgC ¼ 1.81 eV; Eogpt ¼ 1.44 eV) and donor–
acceptor (D–A) interaction of polymer 7 facilitates carrier
transport more effectively. Note that carrier mobilities
greater than 0.1 cm2 Vꢀ1 sꢀ1 have rarely been achieved in
0.12
Unannealed films.
b
Thermally annealed film at 120 ꢃC for 10 min.
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FIGURE 8 AFM height images of 4 (A) and 7 (B) pristine film on OTS-treated SiO2/Si substrate (10 ꢂ10 lm2).
compared with those in the polymer 4 film (roughness <4.8
nm). This could explain why the device based on 7 demon-
strated the higher mobility. The two thermally annealed films
showed quite similar surface topographic images, even after
thermal annealing.
(NRF20100020209). D.H. Choi particularly thanks for the
support by 21st century Frontier Research Program
(F0004011201134100200051196F0004000201134, 2011).
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