Naphthodithiophene-diketopyrrolopyrrole-based Donor-Acceptor Alternating Conjugated Polymers for Organic Thin Film Transistors
Tae Wan Lee, Dae Hee Lee, Jicheol Shin, Min Ju Cho and 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)
((Additional Supporting Information may be found in the online version of this article.))
ABSTRACT : Novel naphtho[1,2-b:5,6-b′]dithiophene (NDT) and diketopyrrolopyrrole (DPP)containing donor-acceptor conjugated polymers (PNDTDPPs) with different branched side chains
were synthesized via Pd(0)-catalyzed Stille coupling reaction. Octyldodecyl (OD) and
dodecylhexadecyl (DH) groups were tethered to the DPP units as the side chains. The soluble fraction
of PNDTDPP-OD polymer in chloroform has much lower molecular weight (MW) than that of
PNDTDPP-DH polymer. PNDTDPP-DH polymer bearing relatively longer DH side chains exhibited
much better charge-transport behavior than PNDTDPP-OD polymer with shorter OD side-chains. The
thermally annealed PNDTDPP-DH polymer thin films exhibited an outstanding charge carrier mobility
of 1.32 cm2 V−1 s−1 (Ion/Ioff  108) measured under ambient conditions, which is almost six times
higher than that of thermally annealed PNDTDPP-OD polymer thin films.
KEYWORDS: naphtodithiophene, diketopyrrolopyrrole, semiconductor, copolymer, organic thin film
transistor
INTRODUCTION
The semiconducting -conjugated polymers
have gained importance in organic electronics
and optoelectronic devices because of their
high charge transport and great mechanical
properties including the feasibility of solution
processing for fabricating large scale and
flexible transistors and solar cells.1–11 Among
various types of semiconducting -conjugated
polymers, the polymers bearing donor (D) and
acceptor (A) moieties exhibited high
performances in thin-film transistors (TFTs). The
TFT devices made of D-A type copolymers were
found to exhibit high charge carrier mobility
because the use of D-A molecular framework in
the -conjugated polymer backbone is a core
methodology to promote strong intermolecular
interactions (stacking) and to decrease
bandgap energies.6–11 Therefore, the D-A
conjugated polymers have been recognized as
promising materials both for organic field effect
transistors (OFETs)12 and organic photovoltaic
devices (OPVs).13 The incorporation of extended heteroaromatic rings into the
polymer backbone is also beneficial for
enhancing its crystallinity and intermolecular
interactions.14 Therefore, the use of such extended heteroarenes, as either D or A unit,
for constructing alternating -conjugated
polymers is expected to afford highperformance TFT devices.
Recently, the diketopyrrolopyrrole (DPP)-based
conjugated polymers such as PDBT-co-TT,
PDQT, PDVT-10, P(DPP-alt-DTBSe), pDPPT2TTOD, P(DPP-ANT), PBBT12DPP, and PSeDPP have
been reported to exhibit outstanding charge
carrier
mobilities.6,7,10,15–17
The
desired
properties of the DPP-based copolymers are
mainly governed by the choice of an
appropriate donor unit and its corresponding
electronic properties. In particular, the solidstate morphology of the resultant copolymers
1
will be strongly influenced by the structure of
the repeating units in the polymer backbone
and the intermolecular interactions between
the polymer chains, which in turn may also be
controlled by the donor units. Commonly, DPPbased copolymers showing high carrier mobility
consist of longer donating units such as
bithiophene,
terthiophene,
and
fused
thiophenes. Although many DPP-based
copolymers have been synthesized by
introducing donating units, it is useful to
explore new structure of donating units for
obtaining much higher carrier mobility.
When designing novel D-A -conjugated
polymers,
naphtho[1,2-b:5,6-b]dithiophene
(NDT) unit could be considered as a promising
donor moiety, imposing the molecular
coplanarity
and
strong
intermolecular
interaction between the polymer chains.18,19,20
The NDT unit consists of a naphthalene core
and two fused thiophene rings, thereby
providing a rigid planar structure. Takimiya et
al. developed a scalable synthetic route for this
NDT building block.18 Using the newly
developed synthetic procedure, they prepared a
series of novel semiconducting polymers
composed of NDT and alkyl substituted
bithiophene moieties for their applications in
TFT devices.19 The maximum charge carrier
mobility of the NDT-containing -conjugated
polymers was estimated to be 0.77 cm2 V1 s1
and a very high current on/off ratio of 107 was
achieved. However, the use of NDT (D) and alkyl
substituted thiophene (D) derivatives was still
limited compared to D-A type copolymers to
obtain higher TFT performance.
In this study, novel D-A alternating conjugated polymers bearing NDT and DPP
units were successfully synthesized. As
mentioned above, the NDT units could improve
the planarity of polymer chain resulting in an
efficient -electron overlap for enhancing
intermolecular interactions between the
polymer chains, which in turn improved their
charge transport properties in the solid state.
2
However, the soluble fraction of PNDTDPP-OD
polymer containing octyldodecyl (OD) side
chain groups in chloroform exhibited relatively
low molecular weight (MW) than that of
PNDTDPP-DH. The insoluble part with high
molecular weight was filtered when carrying
out GPC measurement. A large portion of
insoluble PNDTDPP-OD is due to the fact that
NDT unit could be inducing the high crystallinity
and dense crystallite packing.21 In order to
overcome this problem, the dodecylhexadecyl
(DH) group as a longer branched alkyl chain was
introduced into the DPP units. The resulting
copolymer, PNDTDPP-DH showed higher
solubility in common solvents without having
insoluble residue after Soxhlet extraction with
chloroform and higher MW, which could
facilitate the solution processing for fabricating
the TFT devices. The effects of side chains and
MWs in these copolymers were evaluated on
the basis of corresponding TFT performances
and solid-state morphology of the polymers,
respectively. Additionally, the physical and
electrochemical properties of the polymers
were precisely analyzed and bottomgate/bottom-contact (BGBC) TFT devices were
fabricated by using the two polymers. TFT
device based on PNDTDPP-DH polymer with a
higher MW exhibited much higher charge
carrier mobility than the TFT device based on
PNDTDPP-OD polymer, and the maximum
charge carrier mobility of thermally annealed
PNDTDPP-DH thin films was estimated to be
1.32 cm2 V1 s1 and a high current on/off ratio
(Ion/Ioff 108) was achieved.
EXPERIMENTAL
Synthesis
All the reagents were purchased from SigmaAldrich, TCI, and Acros companies and used
without further purification, unless stated
otherwise. The reagent-grade solvents used in
this study were freshly dried using standard
methods. Compounds 5 and 6 were synthesized
by
following
the
modified
literature
method.6,18,19(a)
2-Dodecylhexadecyl iodide (2)
2-Dodecylhexadecanol (34.0 g, 82.8 mmol),
imidazole (6.76 g, 99.3 mmol), and
triphenylphosphine (26.1 g, 99.3 mmol) were
dissolved in dichloromethane (250 mL) and
cooled to 0 C under continuous stirring. Next,
iodine (25.2 g, 99.3 mmol) was added and the
resulting mixture was stirred for another 15 min
at 0 C. The reaction mixture was kept stirring at
room temperature for overnight. The reaction
mixture
was
quenched
with
sodium
metabisulfite, and the organic phase was
washed with water and brine several times,
then dried over sodium sulfate and evaporated.
The crude product was purified using column
chromatography on silica (eluent: hexane) to
obtain compound 2 as a colorless oil (38.9 g, 90
%). 1H NMR (400 MHz, CDCl3, , ppm): 3.27 (d, J
= 4.8 Hz, 2H), 1.26 (m, 48H), 1.12 (m, 1H), 0.88
(t, J = 6.4 Hz, 6H). 13C NMR (100 MHz, CDCl3, ,
ppm): 38.74, 34.40, 31.93, 29.69, 29.36, 26.51,
22.70, 16.88, 14.17. Anal. Calcd for C30H61I: C,
65.67; H, 11.21. Found: C, 65.52; H, 11.09.
2,5-Di(2-dodecylhexadecyl)-3,6-bis(thiophenyl)-1,4-diketopyrrolo[3,4-c]pyrrole
(3)
2-Dodecylhexadecyl iodide (22.0 g, 40.0 mmol)
was added to a mixture of 3,6-bis-(thiophenyl)1,4-diketopyrrolo[3,4-c]pyrrole (2) (3.0 g, 10.0
mmol), potassium carbonate (6.22 g, 45.0
mmol), and 18-crown-6 (30 mg) in dry DMF
(80 mL) at 120 C. After 18 h reaction, the
mixture was cooled, poured into ice-water (200
mL), and then extracted with chloroform (3 
100 mL). The combined extracts were washed
with water and brine and then dried over
sodium sulfate. The solvent was then removed
under reduced pressure and the crude product
was purified using column chromatography on
silica (eluent: MC:hexane = 1:1) to obtain
compound 3 as a deep red solid (3.4 g, 30 %). 1H
NMR (400 MHz, CDCl3, , ppm): 8.87 (d, J = 4.0
Hz, 2H), 7.62 (d, J = 4.8 Hz, 2H), 7.27 (t, 2H),
4.03 (d, J = 8.0 Hz, 4H), 1.90 (br, 2H), 1.21–1.28
(m, 96H), 0.88 (t, 12H). 13C NMR (100 MHz,
CDCl3, , ppm): 161.74, 140.41, 135.24, 130.34,
129.84, 128.31,107.94, 46.21, 37.75, 31.94,
31.20, 30.02, 29.67, 29.36, 26.21, 22.69, 14.08.
Anal. Calcd for C74H128N2O2S2: C, 77.83; H, 11.30;
N, 2.45; S, 5.62. Found: C, 77.70; H, 11.27; N,
2.56; S, 5.68.
2,5-Di(2-dodecylhexadecyl)-3,6-bis-(5bromothiophenyl)-1,4-diketopyrrolo[3,4c]pyrrole (4)
To a stirred solution of compound 3 (1.14 g, 1.0
mmol) in chloroform (40 mL) under argon
atmosphere, N-bromosuccinimide (NBS, 0.39 g,
2.2 mmol) was added in small portions. The
reaction mixture was protected from light and
stirred at room temperature for overnight. The
reaction mixture was then poured into
methanol (300 mL). The resulting precipitates
were filtered and washed with hot distilled
water and methanol. The crude product was
further purified by column chromatography on
silica (eluent: MC:hexane = 2:3) to obtain
compound 4 as a dark purple solid (0.85 g, 65
%). 1H NMR (400 MHz, CDCl3, , ppm): 8.63 (d, J
= 4.0 Hz, 2H), 7.22 (d, J = 4.0 Hz, 2H), 3.93 (d, J =
8.0 Hz, 4H), 1.87 (br, 2H), 1.21–1.25 (m, 96H),
0.88 (t, 12H). 13C NMR (100 MHz, CDCl3, ,
ppm): 161.37, 139.37, 135.33, 131.46, 131.15,
118.93, 108.00, 46.33, 37.75, 31.93, 31.19,
29.98, 29.71, 29.37, 26.17, 22.69, 14.15. Anal.
Calcd for C74H126Br2N2O2S2: C, 68.38; H, 9.77; N,
2.16; S, 4.93. Found: C, 68.12; H, 9.52; N, 2.33;
S, 5.05.
Synthesis of PNDTDPP-OD
DPP-OD, 5 (360 mg, 0.35 mmol) and 2,7bis(trimethylstannyl)naphtho[1,2-b:5,6b]dithiophene, 6 (200 mg, 0.35 mmol) were
dissolved into 20 mL of toluene in a Schlenk
flask under argon. The solution was flushed
with argon for 10 min, then Pd(PPh3)4 (40 mg,
10 mol%) was added into the Schlenk flask. The
solution was flushed with argon again for
another 10 min. The oil bath was gradually
heated to 90 C, and the reaction mixture was
stirred for 18 h under argon atmosphere. Next,
the mixture was cooled down to room
temperature and the polymer was precipitated
in 200 mL methanol/water (9:1 v/v). The crude
3
polymer obtained was collected by filtration
and then successively purified by Soxhlet
extraction with methanol, acetone, hexane, and
chloroform. The polymer was obtained as a
dark green-purple solid (100 mg, 26 %). (Mn =
6,715, Mw = 10,380, polydispersity index (PDI) =
1.54). Anal. Calcd for C76H110N2O2S4: C, 75.32; H,
9.15; N, 2.31; S, 10.58. Found: C, 75.41; H, 9.27;
N, 2.25; S, 10.65.
Synthesis of PNDTDPP-DH
DPP-DH, 4 (150 mg, 0.12 mmol) and 2,7bis(trimethylstannyl)naphtho[1,2-b:5,6b]dithiophene, 6 (68.3 mg, 0.12 mmol) were
dissolved into 15 mL of toluene in a Schlenk
flask under argon. The solution was purged with
argon for 10 min, then Pd(PPh3)4 (14 mg, 10
mol%) was added into the reaction mixture. The
solution was flushed with argon again for
another 10 min. The oil bath was gradually
heated to 90 C and the reaction mixture was
stirred for 72 h under argon atmosphere. Next,
the mixture was cooled down to room
temperature and the polymer was precipitated
using 200 mL of methanol/water (9:1 v/v). The
crude polymer was collected by filtration and
then successively purified by Soxhlet extraction
with methanol, acetone, hexane, and
chloroform. The polymer was obtained as a
dark green-purple solid (114 mg, 72 %). (Mn =
30,775, Mw = 65,518, PDI = 2.13). Anal. Calcd for
C88H134N2O2S4: C, 76.57; H, 9.79; N, 2.03; S, 9.29.
Found: C, 76.10; H, 9.58; N, 2.10; S, 9.45.
Instruments
The 1H NMR spectra were recorded using a
Varian Mercury NMR 400 MHz spectrometer
(Varian, Palo Alto, CA, USA) in deuterated
chloroform purchased from Cambridge Isotope
Laboratories, Inc. (Andover, MA, USA). The 13C
NMR spectra were recorded using a Varian
Inova-500 spectrometer. The elemental
analyses were performed using an EA1112
(Thermo Electron Corp., West Chester, PA, USA)
elemental analyzer. The MWs of the polymers
were determined using gel permeation
chromatography (GPC, Waters GPC, Waters 515
pump, Waters 410 RI, 2 Shodex LF-804) using
4
polystyrene as the standard and chloroform as
the eluent (T = 35 C) at the Korean Polymer
Testing and Research Institute, Seoul, Korea.
The thermal properties were studied under a
nitrogen atmosphere on a Mettler differential
scanning calorimetry (DSC) 821e instrument
(Mettler, Greifensee, Switzerland). Thermal
gravimetric analysis (TGA) was conducted on a
Mettler TGA50 (temperature rate 10 C min1
under nitrogen). The redox properties of the
two polymers were studied using cyclic
voltammetry (Model: EA161 eDAQ). The
polymer thin films were coated on a Pt plate
using
chloroform
as
the
solvent.
Tetrabutylammonium
hexafluorophosphate
(Bu4NPF6, 0.10 M) in freshly dried acetonitrile
was employed for the analysis of the thin film
samples. Ag/AgCl and Pt wire (0.5 mm in
diameter) electrodes were used as the
reference electrode and the counter electrode,
respectively. The scan rate was 20 mV s1.
The X-ray diffraction (XRD) measurements of
the thin film samples were performed using a
Rigaku D/Max Ultima 3 (Cu Kα:  = 1.54 Å) at
Nanochemistry Laboratory in Korea University.
The measurements were performed in a
scanning interval of 2 between 2 and 45. The
thin film samples were fabricated by spincasting the polymer solutions on noctyltrichlorosilane (OTS) treated SiO2 wafer,
followed by drying at 50 C under vacuum
(solvent: chloroform, concentration: 10 mg
mL1).
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 thin film samples. The thin
film samples were fabricated by spin-coating
(1500 rpm) on silicon wafers followed by drying
at 50 C under vacuum (solvent: chloroform,
concentration: 4 mg mL1).
Ultraviolet-Visible (UV-Vis) Absorption
Spectroscopy
In order to study the UV-visible absorption
behavior, the thin film samples 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 m, Millipore,
Billerica, MA, USA) and subsequently spincasted on glass. The thin films were dried at 50

C for 2 h under vacuum. The absorption
spectra of the samples as thin films and as
solutions (chloroform, conc. 1  105 mol L1)
were obtained using a UV-Vis absorption
spectrometer (HP 8453, photodiode array type)
in the wavelength range 190–1100 nm.
Organic thin film transistors (OTFT) Fabrication
For characterization of TFT performances, the
BGBC device geometry was employed. The
OTFT devices were fabricated using n-doped
Si/SiO2 (300 nm) substrates, where n-doped Si
and SiO2 were used as the gate electrode and
gate dielectric, respectively. The substrate was
cleaned with acetone, cleaning agent, deionized
water, and isopropanol in an ultrasonic bath.
The cleaned substrates were dried under
vacuum at 120 C for 1 h, and then treated with
UV/ozone for 20 min. Before the deposition of
electrodes, the OTS treatment was performed
on the SiO2 gate dielectrics to form an OTS selfassembled monolayer. The source and drain
electrodes were prepared using thermal
evaporation of gold (70 nm) through a shadow
mask with a channel width of 1500 m and a
length of 100 m. Finally, the polymer layer was
deposited on the OTS-treated substrates by
spin-coating a solution (solvent: chloroform,
conc.: 4 mg mL1) in ambient conditions at 1500
rpm for 40 s. For annealing the TFTs, the
samples were further placed on a hotplate in air
for 10 min. Field-effect current-voltage
characteristics of the devices were determined
in air using a Keithley 4200 SCS semiconductor
parameter analyzer. The field-effect mobility
upon saturation () is calculated from the
equation: IDS = (W/2L)Ci(VG  VTH)2, where W/L
is the channel width/length, Ci is the gate
insulator capacitance per unit area, and VG and
VTH are the gate voltage and threshold voltage,
respectively.
RESULTS AND DISCUSSION
Synthesis
New p-type DPP and NDT-containing
semiconducting -conjugated polymers were
synthesized by a facile route. SCHEME 1
illustrates the synthetic routes used to generate
the two alternating copolymers, PNDTDPP-OD
and PNDTDPP-DH. Because the electrondeficient DPP monomer unit was employed as
the building block, the electron-rich NDT donor
units were combined to increase the highest
occupied energy molecular orbital (HOMO)
level and to fabricate new D-A alternating conjugated polymers for TFT applications. The
monomer 6, (2,7-bis(trimethylstannyl) naphtho
[1,2-b:5,6-b]dithiophene) was synthesized
according to the literature.18,19(a)
In order to ensure good solubility of the
resultant polymers, novel long branched alkyl
chains were introduced using alkylation with 1iodo-2-dodecylhexadecane, (2). Bromination of
the resulting compound 3 with NBS afforded
monomer 4. The copolymers were obtained
through Stille-coupling polymerization using
Pd(PPh3)4 as the catalyst. All the polymers were
purified using successive Soxhlet extraction with
methanol, acetone, hexane, and chloroform.
Dark solids were obtained in both cases. The
number-average MWs (Mn) of the soluble
polymers PNDTDPP-OD and PNDTDPP-DH in
chloroform were determined using GPC and a
polystyrene standard, and were found to be
6,715 (PDI = 1.54) and 30,775 (PDI = 2.13),
respectively. PNDTDPP-OD polymers with OD
side-chains were precipitated during the
polymerization in the reaction system because
of its limited solubility, thereby resulting in a
low MW. Meanwhile, PNDTDPP-DH polymers
with
DH-branched
side-chains
showed
improved solubility and did not precipitate
before completing the polymerization, leading
to much higher MWs for the polymer. The two
polymers are soluble in chloroform, and
5
1
2
3
4
+
6
PNDTDPP-DH (R1=C12H25 R2=C14H29)
PNDTDPP-OD (R1=C8H17, R2=C10H21)
4 (R1=C12H25, R2=C14H29)
5 (R1=C8H17, R2=C10H21)
SCHEME 1 Synthetic procedure for NDT-based polymers.
transitions in 25–250 C range were observed
(Fig. 1). The TGA measurements performed at a
heating rate of 10 C min1 under nitrogen show
that PNDTDPP-OD and PNDTDPP-DH polymers
possess good thermal stabilities and high onset
decomposition temperatures (311–322 C) (Fig.
2 and Table 1).
PNDTDPP-DH polymers with longer side-chains
show much better solubility, which can help the
processing of the solid thin films via spin coating
process.
Thermal Analysis of PNDTDPP-OD and
PNDTDPP-DH
The thermal properties of PNDTDPP-OD and
PNDTDPP-DH polymers were characterized
using DSC and thermogravimetric analysis (TGA).
The DSC measurements were performed at a
heating (cooling) scan rate of 10 (10) C min1
under nitrogen, with the highest temperature
kept below the decomposition temperature. No
distinct
crystalline-isotropic
and
glass
A
Heat Flow
2
1.5
Cooling
1
0
Heating
-1
B
1.0
Heat Flow
3
Optical Properties of PNDTDPP-OD and
PNDTDPP-DH
The absorption spectra of the two polymers
were measured both in solution and thin films
(Fig. 3), and dual absorption bands in the
solution state at 732 (673) and 729 (669) nm for
PNDTDPP-OD and PNDTDPP-DH polymers,
Cooling
0.5
0.0
Heating
-0.5
-1.0
-2
-1.5
-3
50
100
150
Temperature (oC)
200
50
100
150
200
Temperature (oC)
FIGURE 1 DSC thermograms of PNDTDPP-OD (A) and PNDTDPP-DH (B).
6
110
100
90
80
70
B
100
Weight (%)
Weight (%)
110
A
90
80
70
60
60
50
50
100
200
300
400
100
500
200
300
400
500
o
o
Temperature ( C)
Temperature ( C)
FIGURE 2 TGA thermograms of PNDTDPP-OD (A) and PNDTDPP-DH (B).
respectively, were observed. No significant
effect of the side chains on the absorption
bands of the polymers was observed. The
absorption bands at around 425 nm can be
assigned to the π−π* transition, while the
absorption bands at longer wavelengths of 732
and 729 nm for PNDTDPP-OD and PNDTDPP-DH,
respectively, are originated from the
intramolecular charge transfer (ICT) between D
and A moieties.4(a),21(b),22 The absorption spectra
of thin films did not show any significant change
and almost identical absorption peaks
compared to those in the solution spectra were
observed. The absorption spectrum of
PNDTDPP-OD polymer showed that in the thin
films, the intensity of the 0–0 vibrational
transition at 734 nm relatively increases
compared to that of the solution state,
Electrochemical Properties of PNDTDPP-OD
and PNDTDPP-DH
The electrochemical properties of PNDTDPP-OD
and PNDTDPP-DH thin films were examined
0.20
A
PNDTDPP-OD
PNDTDPP-DH
0.6
Absorbance (a.u.)
Absorbance (a.u.)
0.8
suggesting that the interchain packing between
the polymer chains was enhanced.23 In the case
of PNDTDPP-DH polymer, the absorbance at
729 nm did not show any significant difference
in the solution and thin film states, which
indicates that strong interchain interactions
exists even in the solution state (Fig. 3B). The
results of absorption spectral analysis indicate
that the polymers might possess high
crystallinity in the solid states. The optical band
gaps (Egopt) of thin films made from the two
polymers were identically calculated to be 1.38
eV using their absorption edges at 898-899 nm.
0.4
0.2
0.0
B
PNDTDPP-OD
PNDTDPP-DH
0.15
0.10
0.05
0.00
400
600
800
Wavelength (nm)
1000
400
600
800
1000
Wavelength (nm)
FIGURE 3 A: Optical absorption spectra of PNDTDPP-OD and PNDTDPP-DH polymers in chloroform
solution. B: Optical absorption spectra of PNDTDPP-OD and PNDTDPP-DH polymers in films.
7
0.002
0.0006
A
B
Current (A)
Current (A)
0.0004
0.001
0.000
0.0002
0.0000
-0.0002
-0.001
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
-0.0004
-2.0
2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
Potential (V vs Ag/AgCl)
Potential (V vs Ag/AgCl)
FIGURE 4 Cyclic voltammograms of polymer thin films. A: PNDTDPP-OD, B: PNDTDPP-DH.
with cyclic voltammetry (CV) in 0.1 M Bu4NPF4
solution in anhydrous acetonitrile, using
ferrocene (with a HOMO energy level, EHOMO =
4.8 eV)24 as an external standard. For all the
polymers, a reproducible oxidation process was
observed (Fig. 4). The HOMO energy levels of
PNDTDPP-OD and PNDTDPP-DH polymers were
calculated to be 5.30 and 5.34 eV,
respectively, by using their onset oxidation
potentials, (Table 1). On the other hand, the
reduction processes of the two polymers
showed weak and irreversible current-voltage
loops, indicating that these polymers are
unstable during the reduction processes under
the experimental conditions. Therefore the
lowest unoccupied molecular orbital (LUMO)
energy levels of PNDTDPP-OD and PNDTDPP-DH
polymers were calculated to be 3.92 and 3.96
eV, respectively, by using their HOMO levels
and optical band gaps. The fact that very similar
HOMO and LUMO levels are observed for these
two polymers indicate that the energy levels are
mainly determined by their -conjugated
backbone, while the variation in the side chains
slightly influence the energy levels.
X-ray diffractions of PNDTDPP-OD and
PNDTDPP-DH
The microstructures of PNDTDPP-OD and
PNDTDPP-DH polymers were investigated using
wide-angle X-ray scattering of the thin films
prepared by spin-coating of chloroform
solutions onto OTS-modified SiO2/Si substrates
via annealing. The highly resolved diffraction
peaks in the XRD patterns presented in Fig. 5(A)
is indicative of the crystalline nature of
PNDTDPP-OD polymer, and the variations in the
peak intensity with the annealing temperature
indicate the growth of polycrystallites in the
spin-coated thin film on the OTS-modified
TABLE 1 Measured and Calculated Parameters of the Two Polymers
Absorbance (nm)
Mn
(kDa)
a
PDI
Td
(oC)
Solution
Film
cut-off
(nm)
Egopt a
(eV)
Eoxonset
(V)
Energy Level (eV)
HOMO b
LUMO c
PNDTDPP-OD
6.7
1.54
311
673, 732
662, 734
898
1.38
1.00
-5.30
-3.92
PNDTDPP-DH
30.8
2.13
322
669, 729
665, 738
899
1.38
1.04
-5.34
-3.96
Energy band gap was estimated from the onset wavelength of the optical absorption in film. b Calculated from
the oxidation potentials. c Calculated from the HOMO energy levels and Egopt.
8
Intensity
4000
10
3
10
2
10
1
A
6000
10
20
30
2 (deg)
3000
2000
(vii)
(vi)
(v)
(iv)
(iii)
(ii)
(i)
1000
0
5
10
15
20
deg
25
10
4
10
3
10
2
10
1
B
8000
Intensity
5000
4
30
Intensity
Intensity
6000
10
10
4000
20
30
2 (deg)
(vii)
(vi)
(v)
(iv)
(iii)
(ii)
(i)
2000
0
5
10
15
20
deg
25
30
FIGURE 5 X-ray diffraction patterns of the thin films of PNDTDPP-OD polymer (A) and PNDTDPP-DH
polymer (B) on OTS-treated SiO2/Si substrate. (i) As-spun film, and thin films thermally annealed at (ii)
120 C, (iii) 150 C, (iv) 180 C, (v) 200 C, (vi) 230 C, and (vii) 250 C. Inset: XRD pattern of thermally
annealed film at 250 C, Logarithmic y-axis scale is displayed.
SiO2/Si substrate. The high crystallinity of the
as-spun thin film of PNDTDPP-OD polymer is
plausibly attributed to an ordered lamellar
structure in the thin film. As shown in the
diffractograms of the thin films, the order of
diffraction gradually increased with the
annealing temperature. In the thin film
thermally annealed at 250 C, distinct fourthorder diffraction peaks resulting from the
interlayer stacking behavior were observed.
Four highly resolved diffraction peaks in the
thermally annealed thin film at 250 C were
observed at 2 = 4.86 (d(100) = 18.2 Å), 9.34
(d(200) = 9.47 Å), 13.9 (d(300) = 6.37 Å), and
18.52 (d(400) = 4.79 Å). In the case of PNDTDPPDH, the as-spun thin film sample also exhibited
highly crystalline behavior (Fig. 5(B)). In the
case of the thin film thermally annealed at 250

C, the (100) diffraction peak became relatively
more intense, and well-resolved fourth-order
diffraction peaks were observed. The (100)
diffraction intensity further increased with
increasing
annealing
temperature.
The
diffraction peaks were clearly observed at 2 =
3.94 (d(100) = 22.4 Å), 7.70 (d(200) = 11.4 Å),
11.54 (d(300) = 7.67 Å) and 15.28 (d(400) = 5.80
Å), which indicate that the polymer chains form
a lamellar crystalline arrangement in the thin
films. The longer lamella spacing (22.4 Å) is
due to longer branched alkyl chains in
PNDTDPP-DH polymer. The  stacking
distance d(010) of PNDTDPP-DH was determined
to be 3.65 Å (224.4. The short interlayer
distance observed implies the existence of
interchain interactions induced by  stacking
via planar molecular units and the
intermolecular interactions between the donor
and acceptor moieties. This phenomenon is
quite promising for realizing enhanced charge
carrier mobility in OTFT devices as a result of
more facile charge transport via hopping
through  stacked molecular planes.
In addition, the diffractograms of two
polymer films annealed at 250 C were
compared; The full width at half maximum
(FWHM) of (100) peak in PNDTDPP-DH is found
0.30 which is much smaller than that of
PNDTDPP-OD (FWHM = 0.57). This indicates
that the distribution of inter-lamella spacing
(d(100)) of PNDTDPP-DH is more narrow and the
crystallite packing structure is more ordered
9
A
-5
10
0.0025
-6
10
VG=
0V
-20 V
0.0020
-7
10
-40 V
-7
0.0015
10
1/2
-9
0.0010
0.0005
-11
1/2
-10
10
(A )
10
-3.0x10
IDS (A)
-8
(-IDS)
(-IDS) (A)
B
0.0
-60 V
-7
-6.0x10
-80 V
-7
-9.0x10
-100 V
10
-60
-40
-20
0
-120 V
-6
0.0000
-12
10
-1.2x10
0
20
-20
-40
VG (V)
C
0.004
-7
0.003
1/2
0.002
-9
0.001
-10
0.000
10
10
-20
0
20
VG (V)
1/2
-8
10
(A )
(-IDS) (A)
10
0V
-20 V
-40 V
-6
(-IDS)
-6
10
VG=
D
0.00
0.005
-1.50x10
IDS (A)
-5
-40
-80 -100 -120 -140
VDS (V)
10
-60
-60
-60 V
-6
-3.00x10
-80 V
-6
-4.50x10
-100 V
-120 V
-6
-6.00x10
0
-20
-40
-60
-80 -100 -120 -140
VDS (V)
FIGURE 6 Transfer (A and C) and output curves (B and D) of TFT devices fabricated with a pristine film of
PNDTDPP-OD. A, B: pristine thin film, C, D: thin film thermally annealed at 250 C for 10 min. OTS-SiO2/Si
gate insulator; the device performances were measured in air; VDS = -120 V.
than PNDTDPP-OD. Therefore, PNDTDPP-DH
might exhibit better charge transport property
than PNDTDPP-OD.
Performance of TFTs made of PNDTDPP-OD
and PNDTDPP-DH
In order to investigate the effect of MW and
side chain bulkiness on the charge transport of
the -extended NDT-based copolymers, BGBC
TFT devices were fabricated using gold source
and drain electrodes, by thermally evaporating
through a shadow mask. An n-doped
polycrystalline silicon substrate was used as the
gate electrode, and an OTS-treated SiO2 surface
layer was used as the gate dielectric insulator.
The gold was deposited via thermal evaporation
onto the insulator surface. Next, the thin film of
the semiconductor was deposited using spin
10
coating with a 0.4 wt% solution of the polymers
in chloroform.
The output characteristics showed very good
saturation behavior and clear saturation
currents that were quadratic to the gate bias
(Figs. 6 and 7). The saturated field-effect
mobilities (µFET) can be calculated from the
amplification characteristics by using the
classical equations describing field-effect
transistors. The mobility values obtained from
measurements using the various devices are
listed in Table 2. The transfer and output curve
(Fig. 6) shows typical drain current (IDS) versus
gate voltage (VG) and drain current (IDS) versus
source-drain voltage (VDS) plots at various gate
voltages in the accumulation mode for TFTs
made of PNDTDPP-OD polymer. In the case of
the OTS-treated SiO2 insulator, the transistor
A
-5
10
0.0025
-6
10
VG=
0V
-20 V
0.0020
-7
10
-40 V
-7
0.0015
10
1/2
-9
0.0010
0.0005
-11
1/2
-10
10
(A )
10
-3.0x10
IDS (A)
-8
(-IDS)
(-IDS) (A)
B
0.0
-60 V
-7
-6.0x10
-80 V
-7
-9.0x10
-100 V
10
-60
-40
-20
0
-120 V
-6
0.0000
-12
10
-1.2x10
0
20
-20
-40
VG (V)
C
0.004
-7
0.003
1/2
0.002
-9
0.001
-10
0.000
10
10
-20
0
-40 V
-1.50x10
1/2
-8
10
(A )
(-IDS) (A)
10
0V
-20 V
-6
(-IDS)
-6
10
VG=
D
0.00
0.005
IDS (A)
-5
-40
-80 -100 -120 -140
VDS (V)
10
-60
-60
-60 V
-6
-3.00x10
-80 V
-6
-4.50x10
-100 V
-120 V
-6
-6.00x10
0
20
-20
VG (V)
-40
-60
-80 -100 -120 -140
VDS (V)
FIGURE 7 Transfer (A and C) and output curves (B and D) of TFT devices fabricated with a pristine film of
PNDTDPP-DH. A, B: pristine thin film, C, D: thin film thermally annealed at 250 C for 10 min. OTS-SiO2/Si
gate insulator; the device performances were measured in air; VDS = -120 V.
devices fabricated with the unannealed film of
PNDTDPP-OD polymer exhibited a field-effect
mobility of 0.03 cm2 V1 s1, along with a high
current on/off ratio (>106–7) and threshold
voltage (VTH = 3 V). The devices made of
thermally annealed PNDTDPP-OD (Tannealing = 250

C for 10 min) exhibited a field-effect mobility
of 0.20 cm2 V1 s1.
In comparison, relatively higher-MW PNDTDPPDH polymer exhibited much better device
performance than PNDTDPP-OD polymer.
Recently, it has often been suggested that MW
may play an important role in improving charge
carrier mobilities.25-27 The results obtained in
this study is well consistent with the earlier
published results. The TFTs fabricated with the
unannealed film of PNDTDPP-DH polymer
exhibited a mobility of 0.06 cm2 V1 s1 with a
high current on/off ratio (>106–7) and low
threshold voltage (VTH = 0 V), and a significant
improvement in the mobility of 1.32 cm2 V1 s1
(Ion/Ioff = 107–8, VTH = 2 V) was obtained in the
TFTs made of the thermally annealed film of
TABLE 2 Device performances of TFTs made
from two polymers.
PNDTDPP-OD
a
Tannealing
(oC)
Mobility, 
(cm2 V-1 s-1)
Ion/Ioff
VTH
(V)
Unannealed
0.033 (0.021)a
106-7
-3
(0.095)a
106-7
-7
PNDTDPP-OD
200
0.11
PNDTDPP-OD
250
0.20 (0.15)a
105-6
-5
PNDTDPP-DH
Unannealed
0.063 (0.047)a
106-7
0
PNDTDPP-DH
200
1.08 (0.94)a
106-7
3
PNDTDPP-DH
250
1.32 (1.12)a
107-8
2
Average mobilities in the parentheses.
11
A
B
C
D
Figure 8 The AFM height images of the thin films PNDTDPP-OD and PNDTDPP-DH polymers on OTStreated SiO2/Si substrate (10 × 10 m). A and B: pristine thin films of PNDTDPP-OD and PNDTDPP-DH
polymers; C and D: thin films of PNDTDPP-OD and PNDTDPP-DH polymers thermally annealed at 250 C.
PNDTDPP-DH polymer (Fig. 7). The TFT device
fabricated with the thermally annealed film of
PNDTDPP-DH polymer exhibited a mobility that
is almost six times higher than that of the lowMW PNDTDPP-OD polymer.
Surface morphologies observed by atomic
force microscope
In order to verify the origin of the higher
performance of the OTFT fabricated with the
thermally annealed PNDTDPP-DH polymers, the
topography of the semiconducting layer
12
deposited on the OTS-treated SiO2 substrate
was analyzed (Fig. 8). The surface morphology
of PNDTDPP-OD thin films displayed larger
roughness and larger crystallites than that of
PNDTDPP-DH thin films. It also exhibits larger
crystallites with lower surface connectivity
compared to those in PNDTDPP-DH thin films.
The larger voids between the crystallites may
act as charge-trapping sites to retard the
charge-carrier transport by building an energy
barrier.28 However, the surface (roughness <10
nm) of PNDTDPP-DH thin films was
characterized as densely connected crystallites
structures relative to that of the PNDTDPP-OD
thin films (roughness <23 nm). The PNDTDPPDH polymer thin films exhibited smaller surface
roughness and best connectivity between the
fibrous crystallites, and therefore, these
properties can facilitate charge transport,
contributing to the superior mobility of TFTs
fabricated from this polymer. The AFM
topography images of the thermally annealed
PNDTDPP-DH thin films showed highly fibrous
morphology, which is most probably
responsible for the efficient charge transport in
the active channel of the TFT devices.
CONCLUSIONS
We successfully synthesized novel NDT- and
DPP-based copolymers where the MW and the
size of the alkyl chains were varied in order to
investigate their effects on the charge carrier
mobility in OTFT devices. PNDTDPP-DH with
longer branched alkyl chains showed improved
solubility in organic solvents, leading to higher
MW obtained from the identical polymerization
process. PNDTDPP-DH polymer in TFTs showed
much better hole mobility than PNDTDPP-OD
polymer, indicating pronounced effect of the
size of alkyl side chains. According to XRD
results, PNDTDPP-DH polymer exhibited more
uniform lamella structures in their crystallites. It
might be attributed to the fact that the thin film
surface of the PNDTDPP-DH polymer exhibited
highly dense fibrous crystalline network, known
to facilitate charge-carrier transport in TFTs.
Therefore, the TFTs prepared from PNDTDPPDH polymer exhibited the highest hole mobility
of 1.32 cm2 V1 s1 with a high current on/off
ratio (Ion/Ioff 108).
ACKNOWLEDGEMENTS
The authors acknowledge the financial support
from the Basic Science Research Program
through the NRF funded by the Ministry of
Education
(NRF2012R1A2A1A01008797,
20110026418 and NRF20100020209).
REFERENCES AND NOTES
1 I. Osaka, M. Shimawaki, H. Mori, I. Doi, E.
Miyazaki, T. Koganezawa, K. Takimiya, J. Am.
Chem. Soc. 2012, 134, 3498-3507.
2 J. Mei, D. H. Kim, A. L. Ayzner, M. F. Toney, Z.
Bao, J. Am. Chem. Soc. 2011, 133, 20130-20133.
3 B. Tieke, A. R. Rabindranath, K. Zhang, Y. Zhu,
Beilstein J. Org. Chem. 2010, 6, 830-845.
4 (a) R. Stalder, J. Mai, J. R. Reynolds,
Macromolecules 2010, 43, 8348-8352; (b) T. Lei,
Y. Cao, Y. Fan, C.-J. Liu, S.-C. Yuan, J. Pei, J. Am.
Chem. Soc. 2011, 133, 6099-6101; (c) E. Wang, Z.
Ma, Z. Zhang, K. Vandewal, P. Henriksson, O.
Inganas, F. Zhang, M. R. Andersson, J. Am. Chem.
Soc. 2011, 133, 14244-14247
5 K. Cao, Z. Wu, S. Li, B. Sun, G. Zhang, Q. Zhang,
J. Polym. Sci. Part A: Polym. Chem. 2013, 51, 94100.
6 Y. Li, S. P. Singh, P. Sonor, Adv. Mater. 2010,
22, 4862-4866.
7 Y. Li, P. Sonar, S. P. Singh, M. S. Soh, M. van
Meurs, J. Tan, J. Am. Chem. Soc. 2011, 133,
2198-2204.
8 H. Bronstein, Z. Chen, R. S. Ashraf, W. Zhang, J.
Du, J. R. Durrant, P. S. Tuladhar, K. Song, S. E.
Watkins, Y. Geerts, M. M. Wienk, R. A. J.
Janssen, T. Anthopoulos, H. Sirringhaus, M.
Heeney, I. McCulloch, J. Am. Chem. Soc. 2011,
133, 3272-3275.
9 H. N. Tsao, D. M. Cho, I. Park, M. R. Hansen, A.
Mavrinskiy, D. Y. Yoon, R. Graf, W. Pisula, H. W.
Spiess, K. Müllen, J. Am. Chem. Soc. 2011, 133,
2605-2612.
10 H. Chen, Y. Guo, G. Yu, Y. Zhao, J. Zhang, D.
Gao, H. Liu, Y. Liu, Adv. Mater. 2012, 24, 46184622.
11 W. Zhang, J. Smith, S. E. Watkins, R. Gysel, M.
McGehee, A. Salleo, J. Kirkpatrick, S. Ashraf, T.
Anthopoulos, M. Heeney, I. McCulloch, J. Am.
Chem. Soc. 2010, 132, 11437-11439.
12 (a) I. Osaka, G. Sauvé, R. Zhang, T.
Kowalewski, R. D. McCullough, Adv. Mater.
2007, 19, 4160-4165; (b) M. Zhang, H. N. Tsao,
W. Pisula, C. Yang, A. K. Mishra, K. Müllen, J. Am.
Chem. Soc. 2007, 129, 3472-3473.
13 (a) D. Mühlbacher, M. Scharber, M. Morana,
Z. Zhu, D. Waller, R. Gaudiana, C. Brabec, Adv.
13
Mater. 2006, 18, 2884−2889; (b) N. Blouin, A.
Michaud, D. Gendron, S. Wakim, E. Blair, R.
Neagu-Plesu, M. Belletête, G. Durocher, Y. Tao,
M. Leclerc, J. Am. Chem. Soc. 2008, 130, 732742.
14 (a) H. Pan, Y. Li, Y. Wu, P. Liu, B. S. Ong, S.
Zhu, G. Xu, J. Am. Chem. Soc. 2007, 129, 41124113; (b) J. Li, F. Qin, C. M. Li, Q. Bao, M. B.
Chan-Park, W. Zhang, J. Qin, B. S. Ong, Chem.
Mater. 2008, 20, 2057-2059; (c) H. H. Fong, V. A.
Pozdin, A. Amassian, G. G. Malliaras, D.-M.
Smilgies, M. He, S. Gasper, F. Zhang, M.
Sorensen, J. Am. Chem. Soc. 2008, 130, 1320213203.
15 (a) J. S. Ha, K. H. Kim, D. H. Choi, J. Am. Chem.
Soc. 2011, 133, 10364-10367; (b) J. S. Lee, S. K.
Son, S. Song, H. Kim, D. R. Lee, K. Kim, M. J. Ko,
D. H. Choi, B. S. Kim, J. H. Cho, Chem. Mater.
2012, 24, 1316-1323; (c) D. H. Lee, J. Shin, M. J.
Cho, D. H. Choi, Chem. Commun. 2013, 49,
3896-3898.
16 (a) J. D. Yuen, J. Fan, J. Seifter, B. Lim, R.
Hufschmid, A. J. Heeger, F. Wudl, J. Am. Chem.
Soc. 2011, 133, 20799–20807; (b) H. W. Lin, W.
Y. Lee, W. C. Chen, J. Mater. Chem. 2012, 22,
2120-2128.
17 M. Shahid, T. McCarthy-Ward, J. Labram, S.
Rossbauer, E. B. Domingo, S. E. Watkins, N.
Stingelin, T. D. Anthopoulos, M. Heeney, Chem.
Sci. 2012, 3, 181-185.
18 S. Shinamura, E. Miyazaki, K. Takimiya, J. Org.
Chem. 2010, 75, 1228-1234.
19 (a) I. Osaka, T. Abe, S. Shinamura, E. Miyazaki,
K. Takimiya, J. Am. Chem. Soc. 2010, 132, 50005001; (b) I. Osaka, T. Abe, S. Shinamura, K.
Takimiya, J. Am. Chem. Soc. 2011, 133, 68526860.
20 (a) S. Shi, P. Jiang, S. Yu, L. Wang, X. Wang, M.
Wang, H. Wang, Y. Li, X. Li, J. Mater. Chem. A
14
2013, 1, 1540-1543; (b) S. Loser, H. Miyauchi, J.
W. Hennek, J. Smith, C. Huang, A. Facchetti, T. J.
Marks, Chem. Commun. 2012, 48, 8511-8513; (c)
I. Osaka, T. Abe, M. Shimawaki, T. Koganezawa,
K. Takimiya, ACS Macro Lett. 2012, 1, 437-440;
(d) P. Dutta, W. Yang, S. H. Eom, W.-H. Lee, I. N.
Kang, S.-H. Lee, Chem. Commun. 2012, 48, 573575; (e) P. Dutta, H. Park, W.-H. Lee, K. Kim, I. N.
Kang, S.-H. Lee, Polym. Chem. 2012, 3, 601-604.
21 (a) R. Rieger, D. Beckmann, W. Pisula, M.
Kastler, K. Müllen, Macromolecules 2010, 43,
6264-6267; (b) Y. Deng, Y. Chen, X. Zhang, H.
Tian, C. Bao, D. Yan, Y. Geng, F. Wang,
Macromolecules 2012, 45, 8621-8627.
22 (a) P. M. Beaujuge, C. M. Amb, J. R. Reynolds,
Accounts Chem. Res. 2010, 43, 1396-1407; (b)
W. S. Yoon, S. K. Park, I. Cho, J.-A. Oh, J. H. Kim,
S. Y. Park, Adv. Funct. Mater. 2013, 23, 35193524.
23 T. Lei, J.-H. Dou, J. Pei, Adv. Mater. 2012, 24,
6457-6461.
24 B. W. D’Andrade, S. Datta, S. R. Forrest, P.
Djurovich, E. Polikarpov, M. E. Thompson, Org.
Electron. 2005, 6, 11-20.
25 A. Zen, J. Pflaum, S. Hirschmann, W. Zhuang,
F. Jaiser, U. Asawapitom, J. P. Rabe, U. Scherf, D.
Neher, Adv. Funct. Mater. 2004, 14, 757-764.
26 M. Tong, S. Cho, J. T. Rogers, K. Schmidt, B. B.
Y. Hsu, D. Moses, R. C. Coffin, E. J. Kramer, G. C.
Bazan, A. J. Heeger, Adv. Funct. Mater. 2010, 20,
3959-3965.
27 W. Li, C. Roelofs, M. M. Wienk, R. A. J.
Janssen, J. Am. Chem. Soc. 2012, 134, 1378713795.
28 (a) R. J. Kline, M. D. McGehee, E. N.
Kadinikova, J. Liu, J. M. Fréchet, Adv. Mater.
2003. 15. 1519-1522; (b) H. N. Tsao, K. Müllen,
Chem. Soc. Rev. 2010, 39, 2372-2386.
GRAPHICAL ABSTRACT
AUTHOR NAMES
Tae Wan Lee, Dae Hee Lee, Jicheol Shin, Min Ju Cho and Dong Hoon Choi*
TITLE
Naphthodithiophene-diketopyrrolopyrrole-based Donor-Acceptor Alternating Conjugated Polymers for Organic Thin Film Transistors
TEXT ((up to 75 words, not the same as the abstract text, present tense, no personal pronouns, written
for a non-specialist, see recent issue for examples))
Novel donor-acceptor conjugated polymers with different branched side chains are synthesized via
Pd(0)-catalyzed Stille coupling reaction. PNDTDPP-DH polymer bearing relatively longer
dodecylhexadecyl side chains exhibits much better charge-transport behavior than PNDTDPP-OD
polymer with shorter octyldodecyl side-chains. The thermally annealed PNDTDPP-DH polymer thin films
exhibit an outstanding charge carrier mobility of 1.32 cm2 V−1 s−1 (Ion/Ioff  108) measured under
ambient conditions.
GRAPHICAL ABSTRACT FIGURE ((Please provide a square image to be produced at 50 mm wide by 50
mm high. Please avoid graphs and other figures with fine detail due to the relatively small size of this
image.))
PNDTDPP-DH (R1=C12H25 R2=C14H29)
 = 1.32 cm2 V-1 s-1
22.4 Å
3.65 Å
15