An ultra-low bandgap diketopyrrolopyrrole (DPP)-based polymer with balanced ambipolar charge transport for organic field-effect transistors

Article abstract of DOI:10.1039/c6ra14327j

A new A₁-A₂ type polymer pDTDPP-TTF containing diketopyrrolopyrrole (DPP) and 2-ethylhexyl-3-fluorothieno[3,4-b]thiophene-2-carboxylate (TTF) as the main electron-accepting building blocks has been synthesized via Suzuki cross-coupling polycondensation. The introduction of TTF as an additional electron-deficient unit significantly lowers the LUMO energy level of this DPP containing polymer to -4.37 eV which is beneficial for efficient electron injection and transport. The pDTDPP-TTF polymer shows an ultra narrow optical bandgap of 1.06 eV with a broad absorption spectra covering the visible to near infrared region of up to 1300 nm. Solution-processed OFET devices with pDTDPP-TTF as the active semiconducting layer have been fabricated with the configurations of both bottom-gate bottom-contact (BG-BC) and top-gate bottom-contact (TG-BC). The BG-BC devices with SiO₂ as a dielectric exhibit ambipolar behavior with a hole and electron mobility of 1.85 × 10^-4 and 3.55 × 10^-5 cm² V^-1 s^-1, respectively. Improved OFET performance has been achieved in TG-BC devices by using PMMA as the dielectric; balanced ambipolar charge transport is obtained with a hole and electron mobility of 6.17 × 10^-3 cm² V^-1 s^-1 and 3.13 × 10^-3 cm² V^-1 s^-1, respectively. To the best of our knowledge, our work is among the very few examples of ultra-low bandgap semiconducting polymers with balanced ambipolar charge transport properties.

Full text of DOI:10.1039/c6ra14327j

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PAPER
An ultra-low bandgap diketopyrrolopyrrole (DPP)-
based polymer with balanced ambipolar charge
transport for organic field-effect transistors†
Cite this: RSC Adv., 2016, 6, 78720
Tao Jiang,a Zhongyuan Xue,a Michael Ford,b Jessica Shaw,ac Xudong Cao,a
a
d
ae
*
*
*
Youtian Tao, Yuanyuan Hu and Wei Huang
A new A1–A2 type polymer pDTDPP–TTF containing diketopyrrolopyrrole (DPP) and 2-ethylhexyl-3-
fluorothieno[3,4-b]thiophene-2-carboxylate (TTF) as the main electron-accepting building blocks has
been synthesized via Suzuki cross-coupling polycondensation. The introduction of TTF as an additional
electron-deficient unit significantly lowers the LUMO energy level of this DPP containing polymer to
ꢀ4.37 eV which is beneficial for efficient electron injection and transport. The pDTDPP–TTF polymer
shows an ultra narrow optical bandgap of 1.06 eV with a broad absorption spectra covering the visible to
near infrared region of up to 1300 nm. Solution-processed OFET devices with pDTDPP–TTF as the
active semiconducting layer have been fabricated with the configurations of both bottom-gate bottom-
contact (BG–BC) and top-gate bottom-contact (TG–BC). The BG–BC devices with SiO2 as a dielectric
exhibit ambipolar behavior with a hole and electron mobility of 1.85 ꢁ 10ꢀ4 and 3.55 ꢁ 10ꢀ5 cm2 Vꢀ1 sꢀ1
,
respectively. Improved OFET performance has been achieved in TG–BC devices by using PMMA as the
Received 2nd June 2016
Accepted 15th August 2016
dielectric; balanced ambipolar charge transport is obtained with a hole and electron mobility of
6.17 ꢁ 10ꢀ3 cm2 Vꢀ1 sꢀ1 and 3.13 ꢁ 10ꢀ3 cm2 Vꢀ1
s
ꢀ1, respectively. To the best of our knowledge, our
DOI: 10.1039/c6ra14327j
work is among the very few examples of ultra-low bandgap semiconducting polymers with balanced
ambipolar charge transport properties.
www.rsc.org/advances
In addition, ambipolar polymer semiconductors, in which both
hole and electron charge carriers are demonstrated in a single
1. Introduction
active layer, have attracted considerable attention for their
application in complementary metal–oxide semiconductor
(CMOS) digital integrated circuits11,12 and light-emitting eld-
effect transistors (LEFETs).13,14 Although several polymer semi-
conducting materials exhibited highly efficient ambipolar
Over the past few decades, organic eld effect transistors
(OFETs) based on semiconducting polymers have been exten-
sively studied and developed for potential applications in ex-
ible displays,1–4 radio-frequency identication (RFID) tags5 and
sensors6,7 etc., due to advantages such as low cost, large area,
light weight and mechanical exibility. Since the rst report on
polymer based OFET devices was published in 1986,8 rapid
progress with impressive charge-carrier mobilities (the typical
gure of merit) has recently been demonstrated in this eld.9,10
charge transport performance with mobilities in excess of
1 cm2 Vꢀ1 sꢀ1
,
the balance of ambipolar transport prop-
10,15–17
erties remains an important issue to solve.
In general, polymer semiconductors exhibit pronounced
ambipolar charge transport behavior when a favorable energetic
alignment is achieved such that both hole and electron trans-
port is permitted. This is typically observed in polymer mate-
rials that possess a HOMO (highest occupied molecular orbital)
energy level higher than ꢂꢀ5.5 eV (relative to the vacuum level)
and a LUMO (lowest unoccupied molecular orbital) energy level
lower than ꢂꢀ4.0 eV.17 Therefore, only relatively low bandgap,
at least below 1.5 eV will allow for potential ambipolar trans-
port. Presently, a general approach to achieve ambipolar
conjugated polymer semiconductors is to design the polymer
framework by involving alternating electron donor (D) and
electron acceptor (A) building blocks.18,19 Highly efficient charge
transport capability is oen observed for these D–A type poly-
meric materials primarily due to the presence of efficient orbital
aKey Lab for Flexible Electronics and Institute of Advanced Materials, Jiangsu National
Synergistic Innovation Center for Advanced Materials (SICAM), Nanjing Tech
University, 30 South Puzhu Road, Nanjing, 211816, P. R. China. E-mail: iamyttao@
njtech.edu.cn; iamwhuang@njtech.edu.cn
bMaterials Department, University of California, Santa Barbara, CA, USA
cDepartment of Chemistry and Centre for Plastic Electronics, Imperial College London,
Exhibition Road, South Kensington, London SW7 2AZ, UK
dCavendish Laboratory, University of Cambridge, JJ Thomson Avenue, CB3 0HE, UK.
E-mail: yuanyuan.hu01@gmail.com
eKey Laboratory for Organic Electronics and Information Displays, Institute of
Advanced Materials, Nanjing University of Posts and Telecommunications,
Wenyuan Road, Nanjing 210046, P. R. China
9
† Electronic supplementary information (ESI) available: Preparation of DPP
monomer and Fig. S1. See DOI: 10.1039/c6ra14327j
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overlap along the polymer backbone, which can facilitate processed OFET devices with both of the BG–BC and TG–BC
intramolecular charge transport.20 Among electron acceptor congurations.
building blocks, diketopyrrolopyrrole (DPP) has gained vast
interest in the past few years for the construction of high charge
2. Results and discussion
2.1 Monomers and polymer synthesis
mobility OFET devices,16,17,20–23 primarily due to its planar,
conjugated, bicyclical structure and remarkable aggregating
properties, which leads to strong p–p interactions with polar
carbonyl groups capable of hydrogen bonding.24–27 Currently,
low bandgap DPP-based polymers have been widely studied in
ambipolar OFET applications, several of which display some of
the highest charge mobilities recorded to date.15,23 For instance,
pTDPPSe–SiC5 (ref. 23) with an optical bandgap of 1.25 eV
exhibits the highest electrical performance in optimized
conditions with high hole and electron mobilities of 8.84 and
4.34 cm2 Vꢀ1 sꢀ1, respectively. However, most DPP-based D–A
type semiconductors exhibit a signicantly higher hole- than
electron-transport performance.28–32 And as such DPP-based
D–A type polymer semiconductors with balanced hole and
electron charge transport mobilities are rarely reported. This is
mainly due to difficulties for reaching a LUMO energy level
below ꢀ4.0 eV which is necessary for facile injection and effi-
cient transport of electrons in DPP-based D–A type
semiconductors.22,33
In an effort to lower the LUMO energy level of DPP polymers,
incorporation of another electron-decient unit to the polymer
backbone would be desirable.34,35 Herein, we designed and
synthesized a new ultra low bandgap A1–A2 type polymer
pDTDPP–TTF via a Suzuki cross-coupling polycondensation
reaction between two electron-decient units: 3,6-di(2-(4,4,5,5-
tetramethyl-[1,3,2]dioxaborolan-2-yl)thien-5-yl)-2,5-di(2-decylte-
tradecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione (DTDPP) and 2-ethyl-
hexyl-4,6-dibromo-3-uorothieno[3,4-b]thiophene-2-carboxylate
(TTF) (Scheme 1). The polymer exhibited an ultra-narrow
bandgap of 1.06 eV and showed a broad and strong absorp-
tion spectrum extending to ꢂ1300 nm. The strategy of molec-
ular design on A1–A2 type DPP polymer successfully enables the
balanced ambipolar behavior of pDTDPP–TTF in solution-
As shown in Scheme 1, pDTDPP–TTF was feasibly synthesized
by conventional Suzuki cross-coupling polycondensation
between 3,6-di(2-(4,4,5,5-tetramethyl-[1,3,2]dioxaborolan-2-yl)
thien-5-yl)-2,5-di(2-decyltetradecyl)-pyrrolo[3,4-c]pyrrole-1,4-
dione (DTDPP) and 2-ethylhexyl-4,6-dibromo-3-uorothieno
[3,4-b]thiophene-2-carboxylate (TTF), synthetic details can be
found in the experimental section. Aer removing the impuri-
ties and low-molecular-weight oligomers of the resultant poly-
mer by Sohxlet extraction (methanol, (6 h) acetone (6 h), and
hexane (6 h)), the residue was extracted with chlorobenzene to
afford the target polymer with moderate number-average
molecular weights (Mn ¼ 16 kg molꢀ1) and reasonable poly-
dispersity indexes (PDIs ¼ 2.2), as listed in Table 1. The polymer
structure was characterized by a combination of 1H NMR
spectroscopy (as shown in ESI of Fig. S1†) as well as elemental
analysis which was in agreement with the predicted values. The
polymer pDTDDP–TTF showed reasonable solubility in
common organic halogenated solvents, such as chloroform,
chlorobenzene and 1,2-dichlorobenzene, likely due to the long
alkyl branched side chains on the DPP and TTF repeat units.
2.2 Thermal stability
The thermal properties of pDTDPP–TTF were examined by
differential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA) measurements. No clear thermal transitions
ꢃ
within the range of 25 to 300 C were detected from the DSC
curves for the polymer. From the TGA curves in Fig. 1, the
polymer pDTDPP–TTF exhibited good thermal stability with
ꢃ
a decomposition temperature (5% weight loss) of 379 C.
2.3 Optical properties
The UV-vis absorption spectra of pDTDPP–TTF in dilute chlo-
roform solution and thin lms are shown in Fig. 2 and the
corresponding absorption properties are summarized in Table
1. In solution, pDTDPP–TTF exhibited a strong, broad and
featureless absorption band spanning the visible to near
infrared range (600 to 1100 nm), arising from p–p* transitions
in the polymer backbone.
Moreover, at the maximum absorption wavelength of 830
nm, pDTDPP–TTF exhibited a very high molar extinction coef-
cient of 3 ¼ 3.24 ꢁ 104 Mꢀ1 cmꢀ1. In comparison, the thin lm
UV-vis absorption spectra of pDTDPP–TTF was red-shied by
approximately 110 nm. We attribute this shi to the longer
wavelength region upon solidication to aggregation and solid
state packing effects. The optical bandgap (Eogpt) of pDTDPP–
TTF was estimated to be ꢂ1.06 eV from the absorption onset in
the solid state. The ultra-low bandgap of pDTDPP–TTF is
comparable or even lower than traditional single crystal silicon
(1.11 eV), due to the introduction of two electron-decient units
in the polymer backbone. To the best of our knowledge, the Eg is
Scheme 1 Synthetic route for the copolymer pDTDPP–TTF. Reagents
and conditions: (i) LDA, THF, ꢀ25 ꢃC under N2, 2 h, 72%; (ii) Pd2(dba)3/
P(o-tol)3, K3PO4, Aliquat 336, toluene/H2O, 95 ꢃC for 48 h, 50%.
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Table 1 Characteristics of the copolymer pDTDPP–TTF
HOMOd (eV)
LUMOe (eV)
c
Polymer
Mn/Mw (kDa)
labs,maxa (nm)
lonsetb (nm)
Eg (eV)
pDTDPP–TTF
16/36
830 (940)
1166
1.06
ꢀ5.43
ꢀ4.37
a
Measured in dilute CHCl3 solution and thin neat lm (parentheses). b Measured at lm state. c Eg was calculated from the onsets of the UV-vis
d
e
spectra in neat lm, Eg ¼ 1240/lonset
.
HOMO was measured by cyclic voltammetry. Calculated from the equation LUMO ¼ HOMO + Eg.
casted from chloroform solution onto a Si wafer. As shown in
Fig. 3(b), the GIWAXS pattern is fairly featureless compared to
other high mobility polymer semiconductors.23,35 Only a single
ꢀ1
˚
order preferentially in-plane alkyl stacking peak at q ¼ 0.29 A
(d ¼ 2.2 nm) and a broad p–p stacking peak in the out-of-plane
direction at q ¼ 1.71 Aꢀ1 (d ¼ 0.37 nm) were observed.
˚
2.5 Electrochemical properties
The electrochemical properties of pDTDPP–TTF were investi-
gated by cyclic voltammetry (CV) in acetonitrile (MeCN)
Fig. 1 TGA curve of the copolymer pDTDPP–TTF measured within the
range from 25 to 600 ꢃC at a heating rate of 20 ꢃC minꢀ1 under
a nitrogen atmosphere.
Fig. 3 (a) Topography (left) and phase (right) AFM images. Scale 5 mm
ꢁ 5 mm (up) and (b) GIWAXS Pattern of pDTDPP–TTF thin film
(bottom).
Fig.
2 UV-vis absorption spectra of pDTDPP–TTF in neat film
(normalized, black) and in 10ꢀ5 M of CHCl3 solution with molar
absorptivity (3) (red).
among the lowest values ever reported for potential ambipolar
semiconducting polymers in the literature.30,31,35,36
2.4 Film morphology
Since lm morphology and crystallinity are known to be key
parameters for device performance, polymer thin lms were
analyzed using atomic force microscopy (AFM) and GIWAXS
measurements. As depicted in the AFM images shown in
Fig. 3(a), the root mean-square surface roughness (RMS) of
pDTDPP–TTF thin lms is 0.60 nm for 5 mm ꢁ 5 mm scan area.
Fig.
4
Cyclic voltammetry of pDTDPP–TTF film in 0.1 mol Lꢀ1
XRD analysis was performed on pDTDPP–TTF thin lms spin- n-Bu4NPF6 of acetonitrile solution, with a scan rate of 100 mV sꢀ1
.
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solution. As shown in Fig. 4, the polymer exhibited quasi- The LUMO energy level can be obtained from the difference
reversible oxidation and undetectable reduction behavior. The between optical bandgap (Eg) and HOMO using the equation
onset of the oxidation potential (Eonset) of pDTDPP–TTF occured LUMO ¼ (HOMO + Eg), which was determined to be ꢀ4.37 eV.
at 1.06 eV, from which the HOMO energy level is calculated to be
ꢀ5.43 eV based on the equation of HOMO ¼ ꢀ(4.4 + Eonset).37
2.6 Density functional theory calculation
Fig. 5 shows the electron-state-density distribution of the
highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) of geometrically opti-
mized structures determined by density functional theory
(DFT)38 at the B3LYP39 hybrid functional level using a 6-31G*
basis set. In order to simplify the calculations and minimize the
computational cost, the polymer model was simplied by
dimeric systems and the long alkyl substituted side chains in
the polymer molecular skeleton were replaced by methyl
groups. As shown in Fig. 5, two dihedral angles were evaluated,
where the rst between the DPP unit and the adjacent
Fig. 5 DFT calculations on the optimized geometries and molecular
orbitals for a DTDPP–TTF dimer (B3LYP/6-31G*).
Fig. 6 Typical transfer and output plot of pDTDPP–TTF OFETs in (a) BG–BC and (b) TG–BC device structures. The devices were annealed at
180 ꢃC. For the BG–BC devices the solvent for pDTDPP–TTF is chloroform while for the TG–BC devices the solvent is 1,2-dichlorobenzene.
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Table 2 Summary of field-effect mobilities for the copolymer over 4 devices
Mobility (cm2 Vꢀ1 sꢀ1
Hole
)
Threshold voltage (V)
Hole
Devices
Electron
Electron
On/off ratio
(BG–BC)
(TG–BC)
(1.85 ꢄ 0.15) ꢁ 10ꢀ4
(6.17 ꢄ 0.59) ꢁ 10ꢀ3
(3.55 ꢄ 0.34) ꢁ 10ꢀ5
(3.13 ꢄ 0.50) ꢁ 10ꢀ3
ꢀ7.4 ꢄ 3.2
ꢀ9.0 ꢄ 0.5
24.9 ꢄ 9.1
23.2 ꢄ 3.7
(7.8 ꢄ 1.2) ꢁ 103
(1.5 ꢄ 0.7) ꢁ 103
thiophene ring (q1), and the second between thiophene and the may yield higher performance.41 In the PMMA-based OFETs,
adjacent TTF unit (q2). The q1 and q2 values obtained from the balanced ambipolar transport with signicantly enhanced
dimmers of pDTDPP–TTF were calculated to be 0.17ꢃ and 6.02ꢃ, performance was obtained. The extracted hole and electron
respectively. From the side view of the optimized geometry, mobilities was measured to be 6.17 ꢁ 10ꢀ3 and 3.13 ꢁ 10ꢀ3 cm2
a relatively planar backbone structure is predicted for pDTDPP– Vꢀ1 sꢀ1, respectively, which was over one order of magnitude
TTF, which is in agreement with other dithienyl-DPP containing higher than the values in the BG–BC devices. It is expected that
copolymers reported in the literature.10,15,21 It is implied that further enhanced mobility is allowed to achieve by optimization
pDTDPP–TTF may exhibit strong p–p stacking due to the of the HOMO/LUMO energy levels alignment with the work
planarity of the polymer backbone.
function of electrodes.
Both the HOMO and LUMO energy levels were mainly delo-
calized through the entire molecular skeleton. The calculated
HOMO and LUMO energy levels for the dimer of pDTDPP–TTF
were ꢀ4.82 and ꢀ3.20 eV, respectively. It should be noted that
slight differences between the absolute values of the computa-
tionally derived and experimentally determined results is
acceptable to exist and can be attributed to the inherent limi-
tations of DFT to accurately predict the behavior of organic
semiconducting polymer as well as the intrinsic discrepancy in
the dimeric and polymeric molecular skeleton.40 However, both
the DFT and CV methods revealed similar trends in the energy
levels suggesting the low bandgap nature and the potential
ambipolar charge transport behaviour of pDTDPP–TTF
polymer.
3. Conclusions
In conclusion, we have designed and synthesized a new ultra-
low bandgap polymer material pDTDPP–TTF with ambipolar
charge transport properties via a Suzuki cross-coupling poly-
condensation reaction. Molecular design strategy on the intro-
duction of A1–A2 type monomer has been successfully
conducted to signicantly lower the LUMO level of DPP based
polymer to as low as ꢀ4.37 eV, which is potentially suitable for
efficient electron injection and transport. The polymer exhibi-
ted a broad and featureless absorption spectrum ranging from
visible to up to 1300 nm, with a narrow bandgap of ꢂ1.06 eV.
Ambipolar transport behaviour has been achieved in solution-
processed OFET devices based on pDTDPP–TTF active layers
in both the BG–BC and TG–BC congurations. Relatively
balanced ambipolar charge transport with a hole and electron
mobility of 6.17 ꢁ 10ꢀ3 cm2 Vꢀ1 sꢀ1 and 3.13 ꢁ 10ꢀ3 cm2 Vꢀ1
sꢀ1 respectively has been obtained when PMMA was used as
dielectric. The results indicate the potential of A1–A2 type
copolymers as active materials for polymer solar cells, near
infrared photodetectors and/or other organic electronic devices.
And the synthetic route on the introduction of an additional
acceptor unit to construct A1–A2 type polymer provides molec-
ular design strategy to lower the LUMO energy levels for
balanced ambipolar ultra-low bandgap semiconducting
polymers.
2.7 Field-effect transistor characteristics
The appropriate HOMO and LUMO energy level alignment of
pDTDPP–TTF encouraged us to investigate the charge transport
properties of the polymer material. Therefore, OFET devices with
a bottom-gate, bottom-contact (BG–BC) conguration were
initially fabricated. Gold (Au) source-drain electrodes were
prepared by photolithography on the SiO2/Si++ substrates. Then
the substrates were passivated with DTS before the polymer lms
were deposited onto the substrates by spin-coating from a chlo-
roform-based solution. Fig. 6(a) shows the representative trans-
fer and output characteristics of BG–BC OFET devices based on
ꢃ
spin-coated lms of pDTDPP–TTF aer annealing at 180 C. It
is seen that pDTDPP–TTF exhibited typical V-shaped ambipolar
behavior. The device performance data are summarized in Table
2. Though the BG–BC devices with SiO2 as dielectric demon-
strated ambipolar transport, the mobility for both holes and
4. Experimental
4.1 Materials
electrons was rather low. The average hole and electron mobil- All of the reagents were obtained from commercial suppliers
ities were measured to be 1.85 ꢁ 10ꢀ4 and 3.55 ꢁ 10ꢀ5 cm2 Vꢀ1 and used without further purication, unless stated otherwise.
sꢀ1, respectively. To further improve the OFET performance, top- THF and toluene were dried over Na/benzophenoneketyl and
gate bottom-contact (TG–BC) devices using PMMA as dielectric freshly distilled prior to use. 3,6-Dithien-2-yl-2,5-di(2-decylte-
were then fabricated, since PMMA is generally known as a good tradecanyl)-pyrrolo[3,4-c]pyrrole-1,4-dione (M1) was prepared
dielectric for efficient electron transport. 1,2-Dichlorobenzene according to established literature procedures,42,43 and the
was selected as the solvent because the high-boiling point solvent monomer TTF (M2) was purchased from Suna Tech Inc.
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by shadow-mask evaporation. Devices were annealed in glove-
box for 10 min at a temperature of 180 C before the electrical
test.
4.2 Measurements and characterization
ꢃ
1H NMR spectra was measured on a Bruker DRX-400 spec-
trometer in CDCl3 using tetramethylsilane as the internal
reference. Mass spectra were measured on Bruker autoex
matrix-assisted laser desorption/ionization time-of-ight
(MALDI–TOF). Elemental analysis of carbon, hydrogen and
nitrogen was performed on a Vario EL III microanalyzer. UV-
vis absorption spectra were recorded on a Shimadzu UV-2500
recording spectrophotometer. Differential scanning calorim-
etry (DSC) was performed on a NETZSCH DSC 200 PC unit at
a heating rate of 10 ꢃC minꢀ1 from 25 to 300 ꢃC, under argon.
Thermogravimetric analysis (TGA) was undertaken with
a NETZSCH STA 449C instrument. The thermal stability of the
samples under a nitrogen atmosphere was determined by
4.4 Synthesis of pDTDPP–TTF
To a 50 mL Schlenk tube, M1 (327 mg, 0.3 mmol), 3-uo-
rothieno[3,4-b]thiophene-2-carboxylate (TTF) M2 (155 mg, 0.3
mmol), Pd2(dba)3 (9 mg, 0.01 mmol), P(o-tol)3 (12 mg, 0.04
mmol) and 2 drops Aliquant 336 were added. The tube was then
charged with Ar through a freeze–pump–thaw cycles for three
times. Then K3PO4 (425 mg, 2 mmol) in toluene (5 mL) with
demineralized water (1 mL) were added under an argon atmo-
sphere. The mixture was stirred for 48 h at 95 ꢃC. Aer cooling
down to room temperature, the highly viscous black gel was
precipitated to 200 mL of methanol containing 20 mL of
distilled water and then stirred for 3 h. The resulting polymer
was collected by ltration and washed with methanol (100 mL)
and acetone (100 mL). The crude product was further puried
by Soxhlet extraction with methanol (6 h), acetone (6 h), hexane
(6 h), and nally CB (6 h). A free-standing lm was obtained
aer removing CB solvent (245.8 mg, 0.21 mmol, 70% yield).
1H NMR (CDCl3, 400 MHz), d (ppm): 9.17 (br, 2H), 7.26 (br,
2H), 4.35 (br, 6H), 1.9 (br, 3H), 1.44–1.02 (br, 72H), 0.81 (br,
18H); anal. calcd: C, 70.72; H, 8.86; N, 2.39% found: C, 70.60; H,
8.66; N, 2.35%. GPC: Mn ¼ 16 kDa, Mw ¼ 36 kDa, PDI ¼ 2.2.
ꢀ1
ꢃ
measuring their weight loss at a heating rate of 20 C min
from 25 to 600 ꢃC. The electrochemical cyclic voltammetry was
conducted on a CHI voltammetric analyzer, in a 0.1 mol Lꢀ1
acetonitrile
solution
of
tetrabutylammonium
hexa-
uorophosphate (n-Bu4NPF6) at a potential scan rate of
100 mV sꢀ1, along with an Ag/AgCl reference electrode and
a platinum wire counter electrode. The polymer sample was
coated on the platinum sheet of working electrode. The
reference electrode was checked versus ferrocene as internal
standard as recommended by IUPAC. Atomic force microscopy
(AFM) was conducted on SPA300HV in tapping mode using an
SPI3800 controller, Seiko Instruments Industry, Co., Ltd.
GIWAXS measurements were done in a helium environment at
the Advanced Light Source beamline 7.3.3.44 Sample-to-
detector distance was 300 mm, calibrated by an AgB stan-
dard, and data was collected at an incident angle of 0.14ꢃ. The
organic eld effect transistor (OFET) device characteristics
were measured using an Agilent 4155B semiconductor
parameter analyzer. The mobility was determined in the
Acknowledgements
The authors thank the National Natural Science Foundation of
China (21304047), Natural Science Foundation of Jiangshu
Province (BK20130919, 13KJB430017), Research Fund for the
Doctoral Program of Higher Education (20133221120015) and
Postgraduate Innovation Foundation of Jiangsu Province (2014,
KYZZ_0226) for nancial support. We acknowledge the
Advanced Light Source, which is supported by the Director,
Office of Science, Office of Basic Energy Sciences, of the U.S.
Department of Energy under Contract No. DE-AC02-
05CH11231.
saturation regime by using the equation IDS ¼ (mWCi/2L)(VG
ꢀ
VT)2, where IDS is the drain-source current, m the eld-effect
mobility, W the channel width, L the channel length, Ci the
capacitance per unit area of the gate dielectric layer and VT the
threshold voltage.
4.3 Device fabrication and measurements
Bottom-gate, bottom-contact (BG–BC) devices were fabricated
on SiO2/Si++ substrates. The source/drain electrodes were
patterned by standard photolithography. The substrates were
then passivated with DTS by immersing them into the solution
(DTS : toluene ¼ 1 : 1000). Top-gate, bottom-contact (TG–BC)
devices were fabricated on glass substrates. Gold source and
drain electrodes (20 nm) were patterned by standard photoli-
thography with a channel length L ¼ 20 mm and width W ¼ 1
mm. All the spin-coating processes were done in nitrogen lled
glovebox. The polymer was spin-coated from a chloroform
solution or 1,2-dichlorobenzene (5 mg mLꢀ1) solution at 1000
rpm. For TG–BC devices, a 500 nm thick PMMA was spin-coated
on the polymer lm and annealed at 80 ꢃC for 30 min. The
PMMA was dissolved in an orthogonal solvent of n-butyl acetate
which prevents dissolving the underneath semiconducting
layers. Finally, a 20 nm thick gold gate electrode was deposited
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