Regioirregular ambipolar naphthalenediimide-based alternating polymers: Synthesis, characterization, and application in field-effect transistors

Article abstract of DOI:10.1002/pola.28747

In this work, we report the synthesis, characterization, and application of two regioirregular naphthalenediimide (NDI)-based alternating conjugated polymers, namely P1 and P2, in which nitrile-substituted moiety, 2,3-bis(thiophen-2-yl)acrylonitrile and N

Full text of DOI:10.1002/pola.28747

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
Regioirregular Ambipolar Naphthalenediimide-Based Alternating
Polymers: Synthesis, Characterization, and Application in Field-Effect
Transistors
Zuzhang Lin,1† Xiaotong Liu,2† Weifeng Zhang,2 Congyuan Wei,2 Jianyao Huang,2
2,3
Zhihui Chen,2 Liping Wang,1 Gui Yu
1School of Material Science and Engineering, University of Science and Technology Beijing, 100083, People’s Republic of China
2Organic Solids Laboratory, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese
Academy of Sciences, Beijing, 100190, People’s Republic of China
3University of Chinese Academy of Sciences, Beijing, 100049, People’s Republic of China
Correspondence to: W. Zhang (E-mail: zhangwf@iccas.ac.cn) or L. Wang (E-mail: lpwang@mater.ustb.edu.cn) or G. Yu (E-mail:
yugui@iccas.ac.cn)
Received 3 June 2017; accepted 28 July 2017; published online 00 Month 2017
DOI: 10.1002/pola.28747
ABSTRACT: In this work, we report the synthesis, characteriza-
tion, and application of two regioirregular naphthalenediimide
(NDI)-based alternating conjugated polymers, namely P1 and
P2, in which nitrile-substituted moiety, 2,3-bis(thiophen-2-yl)a-
crylonitrile and NDI moiety act as donor and acceptor unit,
respectively. The two regioirregular polymers possess low-
lying LUMO energy levels of 23.92 eV for P1 and 23.96 eV for
P2. Both polymers possess typical dual-band UV2Vis2NIR
absorption profiles of NDI-based polymers, and show broad-
ened and red-shifted absorption spectra in the solid state com-
pared with those in solutions. Field-effect transistor devices
with top-gate bottom-contact configuration were used to
evaluate the polymers’ semiconducting properties. The two
polymers exhibited promising and air-stable ambipolar charge
transport characteristics. Thin film microstructure investiga-
tions (AFM and 2D-GIXRD) suggest both polymers formed con-
tinuous and smooth thin films, and adopted predominantly
C
V
face-on molecular packing in the solid state.
2017 Wiley
Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 00,
000–000
KEYWORDS: ambipolar carrier transport; asymmetric units; field-
effect transistors; naphthalenediimide; polymer semiconductors
D or A unit of polymeric chain.3 Nowadays, the state-of-the-
art p-type polymer semiconductors exhibit high hole mobili-
ties that are comparable with those of vacuum-deposited
small molecule-based FETs4,5 and organic single-crystal FETs.5
Nevertheless, the progress in ambipolar6,7 and n-type8,9 poly-
mer semiconductors still falls behind compared to the p-type
counterparts, regardless of mobility values or the device-
stability.
INTRODUCTION In recent decades, conjugated polymers have
been considerably investigated and widely used in optoelec-
tronic devices such as organic field-effect transistors (OFETs)
and organic photovoltaic cells due to the inherent advan-
tages of low fabrication cost, lightweight, flexibility, and solu-
tion processability.1,2 As far as OFETs are concerned, great
progress had been made in the exploration of high-
performance polymer semiconductors for the optoelectronic
devices in past years, especially after polymer structures
containing the donor (D) and acceptor (A) units were
applied. The D2A structures endow advantageous electron
push2pull effect, which tends to induce close interactions
and ordered molecular packing that are both beneficial to
charge transport. Moreover, the D–A structures open up a ver-
satile approach to developing new polymer semiconductors
for researchers: the semiconducting properties of conjugated
polymers can be easily tuned by modifying or changing of the
The key issue confronted of developing ambipolar and n-
type polymer semiconductors is how to obtain high hole or
electron mobilities and air-stable ambipolar charge transport
characteristics. It is a common view that LUMO energy levels
of polymer semiconductors in range of 24.0 6 0.2 eV would
decrease electron injection energy barrier and improve
device-stability in air.10,11 To acquire high-performance ambi-
polar and n-type polymer semiconductors, two approaches
†The authors Z. Z. Lin and X. T. Liu contributed equally to this work.
Additional Supporting Information may be found in the online version of this article.
C
V
2017 Wiley Periodicals, Inc.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2017, 00, 000–000
1

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
are currently adopted: one is designing new electron-
deficient p-conjugated building blocks as A units for D2A
alternating polymers.12,13 However, to be frankly, designing
new electron-deficient unit is relatively difficult. It could be
witnessed by the fact that most of high-performance poly-
mers are synthesized from a handful of lactam or diimide-
based building blocks,14 such as diketopyrrolopyrrole
(DPP),151,4,5,8-naphthalenediimide (NDI),16 isoindigo,17 and
benzobisthiadiazole,18 and 3,7-bis[(E)-2-oxoindolin-3-ylidene]-
3,7-dihydrobenzo[1,2-b:4,5-b]difuran-2,6-dione,19 though many
electron-deficient building blocks were exploited. The other
one, introducing strong electron-withdrawing heteroatoms or
heteroatom-substituted groups into acceptor or donor units,
involves the loading of fluorine,6 chlorine,20 carbonyl oxygen,
pyridine-nitrogen atoms or CN groups and their derived p-con-
jugated systems onto polymeric conjugated backbones.21–23 It
is important to point out that some heteroatoms and
heteroatom-substituted groups can not only lower the LUMO
energy levels but also induce other intramolecular noncovalent
interactions, which could effectively promote the planarity of
polymeric conjugated backbone and endow polymers with
enhanced self-assembly and ordered molecular packing in the
solid state.24,25
film microstructures, indicating that both polymers formed uni-
form thin films, and adopted predominantly face-on molecular
packing in the solid state. Our work demonstrates the great
potential of regioirregular conjugated polymers in solution-
processed FET devices.32
EXPERIMENTAL
Materials
The chemical reagents such as n-butyl lithium and trimethyl-
tin chloride were obtained from Acros, Innochem, or other
commercial resources and used without further purification.
The catalyst and ligand, Pd2(dba)3 and P(o-tol)3, were pur-
chased from Sigma-Aldrich Chemicals. All air-sensitive or
water-sensitive reactions were carried out using the Schlenk
technique under inert atmosphere. (E)-2,3-bis(5-bromothio-
phen-2-yl)acrylonitrile (1), (E)-2,3-bis(5-(trimethylstannyl)th-
iophen-2-yl)acrylonitrile (2) were synthesized according to
the reported procedures.30 2,6-Dibromo-1,4,5,8-naphthalene-
tetracarboxylic dianhydride (3) and 2,6-dibromo-1,4,5,8-
naphthalenediimide (4a and 4b) were synthesized according
to the reported procedures.33
Measurements
1H NMR and 13C NMR spectra were recorded using a
Bruker-300 NMR spectrometer at room temperature. Gel
permeation chromatography (GPC) analyses were carried out
on a Polymer Laboratories PL 220 system using a refractive
index detector at 150 8C. The eluent was 1,2,4-trichloroben-
zene with polystyrenes as standards. High-resolution mass
spectrometry was tested on a Bruker APEXII or 9.4T Solarix
FT-ICR mass spectrometers. Elemental analyses were
obtained on a CARLO ERBA 1106 elemental analyzer. The
UV2Vis2NIR absorption spectra of all polymers were
recorded on a Hitachi U-3010 spectrophotometer. Thermog-
ravimetric analysis (TGA) measurements were performed on
a Perkin2Elmer series 7 thermal analysis system under N2
at a heating rate of 10 8C min21. Cyclic voltammetry (CV)
curves measurements were collected on a CHI660c electro-
chemistry workstation using 0.1 M n-Bu4NPF6, platinum
stick electrode, Ag/AgCl electrode, and platinum wire as sup-
porting electrolyte, working electrode, reference electrode,
and counter electrode, respectively. Thin film morphology of
polymer was analyzed in air using a Digital Instruments
Nanoscope V atomic force microscope operated in tapping
mode. The two-dimensional grazing incidence X-ray diffrac-
tion data were recorded by illuminating drop-coated polymer
thin films at a constant incidence angle of 0.28.
Among several factors that affect the charge transport prop-
erties of polymer semiconductors, the regularity/symmetry
of polymeric conjugated backbones is what the researchers
have been pursuing in organic electronics. The recognition is
originated from the facts that most of high-performance poly-
mer semiconductors are constructed on regular and symmet-
rical electron-efficient building blocks as aforementioned, and
electron-donating building blocks such as 2,2’-bithiophene,26
benzo[1,2-b:4,5-b]dithiophene,27 thieno[3,2-b]thiophene,28 (E)-
1,2-di(thiophen-2-yl)ethene,29 and (E)-1,2-di(selenophen-2-
yl)ethene, etc. However, recent studies revealed that the
regioirregular polymers constructed from asymmetrical donor
or acceptor building blocks, or mixed, can also afford high
charge transport properties.30,31 The seemingly contradictory
results aroused our interest to further develop new ambipolar
or n-type regioirregular conjugated polymers for FET devices.
Herein, we took strong electron-withdrawing NDI moiety as
acceptor units and nitrile-substituted dithiophenethene moi-
ety, 2,3-bis(thiophen-2-yl)acrylonitrile (CNTVT), as donor
units, to give two regioirregular conjugated polymers, namely
P1 and P2. The two polymers possess low-lying LUMO energy
levels of 23.92 eV for P1 and 23.96 eV for P2. Both poly-
mers exhibit typical dual-band UV2Vis2NIR absorption pro-
files of NDI-based polymers, and show broadened and red-
shifted absorption spectra in the solid state compared to
those in solutions, suggesting that the planarity of their conju-
gated backbones was enhanced in the solid state. We
employed top-gate bottom-contact configuration to evaluate
the polymers’ semiconducting properties and found that the
two polymers exhibited promising and air-stable ambipolar
charge transport characteristics with the highest electron and
hole mobilities of 0.048 and 0.0064 cm2 V21 s21, respectively.
Atomic force microscopy (AFM) and two dimensional grazing
incidence X-ray diffraction (2D-GIXRD) were used to study thin
FET Device Fabrication and Measurements
Top-gate/bottom-contact (TG/BC) FET devices were fabri-
cated to investigate the semiconducting properties of the
regioirregular NDI-based alternating polymers. The FET devi-
ces were fabricated on bare glass substrates, which were
fully cleaned by using ultrasonication in acetone, deionized
water, and ethanol. Gold was deposited on the surface of
glass substrates as source/drain electrodes through a metal
shadow mask (L/W 5 80 lm/8800 lm). After the substrates
were washed with ethanol,
a polymer thin film was
2
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2017, 00, 000–000

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
fabricated by spin-coating a polymer solution (10 mg mL21
)
139.17, 128.38, 127.75, 125.30, 124.15, 45.38, 36.52, 31.93,
31.56, 30.03, 29.60, 29.30, 26.34, 22.68, 14.13. HR-MS
(MALDI-TOF) m/z: Calcd for C67H95N3O4S2: 982.4792. Found:
982.4797.
in o-dichlorobenzene at 3000 rpm for 60 s in a N2 glovebox.
The resulting polymer film was annealed in nitrogen at 120
8C for 5 min. Polymethylmethacrylate (PMMA; Mw 5 1000
kDa) dissolved in anhydrous n-butyl acetate (65 mg/mL)
was heated to 80 8C and stirred for 3 h. The PMMA solution
was spin-coated to form a dielectric film on the semiconduc-
tor layer with a thickness of about 850 nm. A 100 nm alumi-
num was then evaporated as gate electrode on top of the
PMMA gate dielectric layer dried at 80 8C for 30 min. A
Keithley 4200 SCS semiconductor parameter analyzer was
used to demonstrate FET characteristics under nitrogen
atmosphere. The mobilities were extracted according to the
equation,
4b (2.69 g, 41.5%). 1H NMR (300 MHz, CDCl3, d): 8.97 (s,
1H), 4.14–4.12 (d, 2H), 1.97 (s, 1H), 1.30–1.23 (m, 40H),
0.88–0.81 (m, 6H). 13C NMR (75 MHz, CDCl3, d): 161.11,
161.00, 139.15, 128.37, 127.74, 125.28, 124.08, 45.45, 36.47,
31.84, 31.56, 29.61, 29.27, 26.27, 22.70, 14.05. HR-MS
(MALDI-TOF) m/z: Calcd for C75H111N3O4S2: 1094.6044.
Found: 1094.6051.
Synthesis of Polymers P1 and P2
To a Schlenk tube were added NDI-based monomer, 4a or
4b (0.2 mmol), tin monomer, 3 (108.6 mg, 0.2 mmol), and
6.00 mL of chlorobenzene under argon atmosphere. Then,
Pd2(dba)3 (6 mg) and P(o-tol)3 (16.2 mg) were added the
reaction tube in one portion. After the tube was charged
with argon through a freeze-pump-thaw cycle for three
times, the reaction mixture was stirred for 4 d at 110 8C
under argon atmosphere. The resulting mixture was cooled
to room temperature and poured into 200 mL of methanol
containing 5 mL of HCl (a.q.), then stirred for two hours. The
dark precipitate was filtered and purified by Soxhlet extrac-
tion using methanol, acetone, hexane, and chloroform as elu-
ent sequentially. After removal of chloroform solvent, the
solid residue was collected and dried under vacuum to
afford the desired alternating polymer.
ꢀ
ꢁ
W
2
I
DS5
CilðVG2VTÞ
2L
where IDS is the drain current, W and L are the channel
width and length, respectively. Ci is the capacitance per unit
area of the gate insulator layer. VG and VT are representative
the gate voltage and the threshold voltage, respectively.
Synthesis of (E)-2,3-Bis(5-(trimethylstannyl)thiophen-2-
Yl)acrylonitrile (2)
To a Schlenk tube were added (E)-2,3-bis(5-bromothiophen-
2-yl)acrylonitrile, 1 (1.87 g, 5.0 mmol) and 60 mL of anhy-
drous THF in nitrogen atmosphere. After the reaction mix-
ture was cooled to 278 8C, 2.5 M n-butyl lithium (5.2 mL,
10.5 mmol) was added dropwise during a period of time of
30 min at 278 8C. Then, trimethyltin chloride (10.6 mL in
1 M hexane solution, 10.6 mmol) was added in one portion.
The resulting mixture was stirred for 10 min and then
warmed to room temperature. After stirring overnight, the
reaction mixture was poured to 100 mL of dichloromethane
and extracted with H2O. After the removal of organic solvent,
the residue was recrystallized from MeOH to obtain the
desired monomer as a yellow solid (1.7 g, 62.6%). 1H NMR
(300 MHz, CDCl3, d): 7.66–7.65 (d, 1H), 7.56 (s, 1H), 7.43–
7.42 (d, 1H), 7.22–7.21 (d, 1H), 7.15–7.13 (d, 1H), 0.53–0.32
(m, 18H).
1
P1 (172.53 mg, 82.74%). H NMR (300 MHz, CDCl3, d): 8.76
(m, 2H), 7.71 (s, 1H), 7.59 (d, J 5 7.5 Hz, 2H), 7.47 (d, J 5 2.9
Hz, 2H), 4.10 (m, 4H), 2.11–1.87 (m, 2H). GPC: Mn 5 42.0
kDa, Mw 5 94.4 kDa, PDI 5 2.24. ELEM ANAL: Found for
C65H91N3O4S2: C 74.26, H 8.56, N 4.05. Calculated for
C65H91N3O4S2: C 74.88, H 8.80, and N 4.03.
1
P2 (182.58 mg, 78.91%). H NMR (300 MHz, CDCl3, d): 8.80
(s, 2H), 7.71 (s, 1H), 7.59 (d, J 5 7.4 Hz, 2H), 7.47 (s, 2H),
7.33 (s, 2H), 4.09 (s, 4H), 2.07–1.86 (m, 2H), 1.25 (s, 80H),
0.86 (m, 12H). GPC: Mn 5 31.4 kDa, Mw 5 66.6 kDa,
PDI 5 2.12. ELEM ANAL: Found for C73H107N3O4S2: C 75.38, H
9.07, N 3.68. Calculated for C73H107N3O4S2: C 75.93, H 9.34,
and N 3.64.
Synthesis of 4,9-Dibromo-2,7-dialkylbenzo[lmn][3,8]
phenanthroline-1,3,6,8(2H,7H)-tetraones (4a and 4b)
To a 100 mL of round bottom flask were added 2,6-dibromo-
1,4,5,8-naphthalenetetracarboxylic dianhydride, 3 (2.2 g, 5.0
mmol), 2-octyldodecylamine or 2-decyltetradecylamine (15.0
mmol), and propionic acid (30 mL). The mixture was heated to
140 8C. After the suspension became clear, the reaction was
stopped and cooled to room temperature. The resulting mix-
ture was poured into H2O (200 mL) and extracted with chloro-
form (100 mL 3 3), washed with water, and dried over
anhydrous MgSO4. The residue was purified by column chro-
matography on silica gel with a mixture of chloroform/petro-
leum ether as eluent to give the desired monomer as yellow
solid.
RESULTS AND DISCUSSION
Synthesis and Thermal Properties
The synthetic routes of monomers and NDI-based copoly-
mers are shown in Scheme 1. The starting material, 1
reacted with n-butyl lithium, quenched with trimethyltin
chloride at 278 8C, and then purified by recrystallization
from cooled MeOH to give the pure tin monomer, 2 as a yel-
low solid. The NDI-based monomers, 4a and 4b, were syn-
thesized from naphthalic anhydride, 3 according to the
reported procedures.32 The polymerization reactions pro-
ceeded under the Stille-coupling condition, in which tin
monomer and NDI-based monomer reacted in the presence
of Pd2(dba)3 and P(o-tol)3. Previous studies indicated that
the molecular weight (the degree of polymerization) of
1
4a (1.82 g, 37%). H NMR (300 MHz, CDCl3, d): 8.99 (s, 1H),
4.15–4.13 (d, 2H), 1.99 (s, 1H), 1.31–1.23 (m, 32H), 0.89–
0.83 (m, 6H). 13C NMR (75 MHz, CDCl3, d): 161.14, 161.03,
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2017, 00, 000–000
3

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
SCHEME 1 The synthetic routes of NDI-based polymers, P1 and P2.
polymer semiconductors obviously affected the self-
aggregation and crystallinity, thereby influencing charge-
hopping between adjacent polymer chains. The polymer semi-
conductors with high molecular weight tend to enhance the
intermolecular interactions, thus maximizing their potentials
in semiconducting behaviors. Therefore, the polymerization
reaction of 4a and 2 were optimized at different reaction
times, suggesting that the reaction time was extended to 4 d
to give P1 with high molecular weight. The extended reaction
time was attributed to the low reactivity of tin monomer,
originated from the introduction of cyano groups. The result-
ing two polymers were characterized by ambient temperature
1H NMR (25 8C). The NMR spectra of synthetic intermediates
and copolymers are shown in Supporting Information. The
number-average molecular weight (Mn) and the polydispersity
indexes (PDIs) were found to be 42.02 kDa and 2.24 for P1,
31.41 kDa and 2.12 for P2, respectively (Table 1). The number-
average molecular weights of P1 and P2 are higher than those
of unsubstituted nitriles polymers.34 The GPC chromatograms
of the P1 and P2 are shown in Supporting Information Figure
S1. It is worth noting that both polymer materials have good
solubility in common organic solvents including chlorobenzene,
dichlorobenzene, chloroform, and tetrahydrofuran. The thermal
properties of the two alternating polymers were evaluated by
TGA techniques, as shown in Supporting Information Figure S2.
Both polymers have high thermal stability with the decomposi-
tion temperatures (Tdec) with 5% weight less of 418 8C for P1
and 427 8C for P2.
Optical Properties
The UV2Vis2NIR absorption spectra of the two alternating
polymers in dilute chloroform and as thin films are shown in
Figure 1. The corresponding data are listed in Table 1. P1
and P2 exhibit similar absorption profiles in chloroform
solution, with absorption maxima at 604 and 586 nm,
respectively. Compared with polymers without CN groups,
TABLE 1 Molecular Weight, Thermal, Optical, and Electrochemical Properties of the NDI-based Copolymers
Mn (kDa)a
Mw (kDa)a
PDIa
Tdec (8C)
kmax (nm)
Egopt (eV)e
E
(V)f
red
onset
ELUMO (eV)f
EHOMO (eV)g
b
copolymer
P1
P2
42.02
31.41
94.37
66.61
2.24
2.12
418
427
604c/642d
586c/644d
1.61
1.61
20.48
20.44
23.92
23.96
25.53
25.57
a
d
Determined by GPC at 150 8C.
Measured by TGA.
In chloroform solution.
In thin film.
b
c
e
f
Calculated from the absorption edges (Egopt51240=k
eV).
edge
Determined by CV. ELUMO 5 –(4.40 1 Eorendset) eV. EHOMO5 ELUMO–Egopt
.
4
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2017, 00, 000–000

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
FIGURE 1 UV2Vis2NIR absorption spectra of P1 and P2 in (a) chloroform solutions and (b) thin films. [Color figure can be viewed
at wileyonlinelibrary.com]
absorption band in the range of 360–470 nm, corresponding
to p 2 p* transitions, and another low-energy band in the
range of 480–750 nm, which is assigned as charge-transfer
character in D and A units along their polymeric conjugated
backbones. Such effect can promote charge transport. Com-
pared with those in solutions, their absorption spectra in
thin films become broader and have slight red-shifts, which
are attributed to the enhanced backbone planarity in the
solid state and strong intermolecular interaction in film. On
the basis of the onset absorption of the thin films, the optical
bandgaps of P1 and P2 are both estimated to be 1.61 eV.
Electrochemical Properties
The electrochemical properties of both polymers were charac-
FIGURE 2 CV traces of the NDI-based polymers P1 and P2.
terized by CV. The CV traces of P1 and P2 are presented in
Figure 2, and the corresponding data are summarized in Table
1. As shown in Figure 2, both polymers exhibit strong reduc-
tive currents and negligible oxidative currents, suggesting the
materials are easily reduced than oxidized, and indicating that
the introduction of the nitrile group on apparently lowered
[Color figure can be viewed at wileyonlinelibrary.com]
P1 and P2 showed blue-shift absorption peaks.34 Both poly-
mers P1 and P2 possess typical dual-band UV–Vis–NIR
absorption profiles of NDI-based polymers: the high-energy
FIGURE 3 Frontier orbital distributions of the NDI-based copolymers and their unsubstituted counterparts (in trimers). [Color
figure can be viewed at wileyonlinelibrary.com]
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2017, 00, 000–000
5

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
FIGURE 4 Typical output and transfer characteristics of (a–d) P1- and (e–h) P2-based TGBC FET devices fabricated at the annealing
temperature of 120 8C. [Color figure can be viewed at wileyonlinelibrary.com]
the oxidation susceptibility of both polymers compared to
their respective ones without CN groups.34 According to
reduction potentials of 20.48 and 20.44 eV vs. Ag/AgCl, the
LUMO energy levels of P1 and P2 are estimated to be 23.92
and 23.96 eV, respectively, calculated from the equation of
localized on the TVT subunit, whereas the LUMO is predomi-
nantly on the NDI subunit. Such localization of orbitals dem-
onstrated the significant charge transfer between the TVT and
NDI units in the HOMO–LUMO transition. The NDI-TVT and
NDI-CNTVT trimers show similar frontier molecular orbital
distributions for HOMOs and LUMOs. This result indicates
that introducing the CN groups into NDI-TVT trimer does not
affect frontier molecular orbital distributions.
E
LUMO 5 – (4.40 1 Eorendset) eV. The HOMO energy levels of P1
and P2 are estimated to be 25.53 and 25.57 eV by the equa-
tion EHOMO5 ELUMO–Egopt, respectively.
Theoretical Modeling
FET Performance
To gain theoretical insight into the electronic structures of
the two regioirregular polymers P1 and P2, density func-
tional theory (DFT) calculation was performed at the B3LYP/
6–31G(d) level. To simplify the calculation, we replaced the
alkyl side chains with methyl groups; and to well understand
the influence of nitrile group, DFT calculation of unsubsti-
tuted counterpart, NDI-TVT, was also performed. To achieve
a more objective theoretical modeling result the two regioirre-
gular polymers, nitrile groups were regionirregularly placed
in the conjugated backbone of the their trimer. Figure 3
shows the optimized geometries and orbital distributions of
NDI-TVT and NDI-CNTVT trimers. The two trimmers show
planar conjugated structures. The frontier orbitals of unsubsti-
tuted NDI-TVT trimer demonstrate that the HOMO is mainly
TG/BC device configuration was applied to fabricate FET
devices based on the two NDI-based copolymers. The bare
glass substrates were fully cleaned, and then modified with
OTS monolayer, before the polymer active layer was fabri-
cated by spin-coating a polymer solutions (10 mg/mL in o-
dichlorobenzene). Then, a PMMA solution and an aluminum
layer were deposited successively and adopted as the dielec-
tric layer and the gate electrode, respectively. The evaluation
of P1- and P2-based FET devices was conducted under
ambient conditions. Both polymers exhibited ambipolar
charge transport behaviors. The typical transfer (IDS2VGS
)
and output (IDS2VDS) curves are presented in Figure 4. The
corresponding device parameters are listed in Table 2.
Before fabricating TGBC devices, devices with bottom-gate/
TABLE 2 The Device Performance of P1- and P2-Based FET Device with TGBC Configuration
For Electrona
For Holea
Polymer
Vth (V)
l (lmax) (cm2 V21 s21
)
Ion/Ioff
Vth (V)
l (lmax) (cm2 V21 s21
)
Ion/Ioff
P1
P2
21.0–44.6
27.7–35.6
4.06 3 1022 (4.82 3 1022
3.25 3 1022 (3.68 3 1022
)
)
103–104
103–104
268 to 256
270 to 256
4.29 3 1023 (6.41 3 1023
3.39 3 1023 (3.72 3 1023
)
)
102–103
102–103
a
The average hole and electron mobilities were obtained from over 15 devices.
6
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2017, 00, 000–000

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
bottom-contact configuration were fabricated firstly to opti-
mize their respective annealing temperatures. Different
annealing temperatures including 80, 120, 160, and 200 8C
were tried and the temperature-dependent mobilities of both
copolymers are shown in Supporting Information Figure S3.
Both polymers exhibited their respective highest mobilities
at annealing temperature of 120 8C, implying that the thin
films of P1 and P2 self-assembled to form their respective
optimal molecular packing at the temperature. Thus, we
adopted the temperature of 120 8C as annealing temperature
in fabricating TGBC devices. P1-based FET devices showed
the maximum hole and electron mobilities of 6.41 3 1023
and 4.82 3 1022 cm2 V21 s21, respectively, while P2-based
devices exhibited the maximum hole and electron mobilities
of 3.68 3 1023 and 3.72 3 1022 cm2 V21 s21, respectively.
Over 15 devices were measured and demonstrated that the
average hole/electron mobilities are 4.29 3 1023/4.06 3
1022 cm2 V21 s21 for P1 and 3.39 3 1023/3.25 3 1022
cm2 V21 s21 for P2.
FIGURE 5 AFM topography images (3 3 3 lm2) of the polymer
thin films: the as-spun film (a), the annealed thin films at 120
8C (b), for P1; and the as-spun film (c), the annealed thin films
at 120 8C (d), for P2. [Color figure can be viewed at wileyonline-
library.com]
Thin Film Microstructure Analysis
The thin film microstructures and surface morphologies of
both polymers were investigated by tapping-mode AFM and
2D-GIXRD. Figure 5 shows AFM height images of as-spun
thin films and the thin films annealed at 120 8C. All thin
films exhibit granular intercalating networks. The grain sizes
FIGURE 6 2D-GIXRD patterns of the NDI-based polymers, (a) as-spun and (b) annealed polymer thin films at 120 8C for P1. (c) as-
spun and (d) annealed polymer thin films at 120 8C for P2. [Color figure can be viewed at wileyonlinelibrary.com]
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2017, 00, 000–000
7

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
ACKNOWLEDGMENTS
grow larger with deeper boundaries being observed upon
heating. The AFM images reveal that root-mean-square
(RMS) roughness of thin films increases with increasing the
annealing temperatures. RMS values for P1 and P2 thin films
are 0.426 and 0.434 nm at the room temperature, and 0.571
and 0.449 nm at the annealing temperature of 120 8C.
Because a deep grain boundary is not beneficial for intergran-
ular charge transport, the resulting negative contribution to
carrier mobility may lead to a partial cancellation of the
improvement in grain sizes. The compromise between grain
size and boundary interprets why the best charge transport
property was achieved at the annealing temperature of 120 8C.
The work was financially supported by the National Natural
Science Foundation of China (Grants 51473021, 21673258 and
21474116), the National Key Research and Development Pro-
gram of China (2016YFB0401100), and the Strategic Priority
Research Program of the Chinese Academy of Sciences
(XDB12030100). The high temperature GPC measurements are
supported by Open Research Fund of State Key Laboratory of
Polymer Physics and Chemistry, Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences. The GIXRD analyses
were performed at the BL14B1 Station of Shanghai Synchro-
tron Radiation Facility (SSRF), 23A1 Station of National Syn-
chrotron Radiation Research Centre (NSRRC, Taiwan), and
1W1A station of Beijing Synchrotron Radiation Facility (BSRF).
The authors are very grateful to the assistance of scientists
from the stations during the experiments.
Figure 6 depicts GIXRD patterns of the as-spun and annealed
P1 and P2 films. From the diffraction patterns, we clearly
observed that both polymers formed lamellar molecular pack-
ing thin films, and adopted predominantly face-on molecular
packing mode in the solid state, judged from strong (010) dif-
fraction peaks appearing in out-of-plane orientation, whereas
no (010) diffraction peaks were found in the in-plane orienta-
tion. The as-spun P1 thin film displays strong (100) and weak
(200) diffraction peaks in the out-of-plane direction, and
(100), (001), (200), (300) diffraction peaks in the in-plane
direction. In its annealed thin film, the (200) and (300) dif-
fraction peaks in the in-plane direction become stronger, indi-
cating that crystallinity of polymers of the films are improved
and molecular packing become more ordered. Both polymers
exhibited distinctly (001) diffraction peak distance of 15.5 Å,
indicating the length of a co-repeat unit.35 Based on the locat-
ing positions of (100) diffraction peaks in as-spun thin films,
the d-d distances of polymers were estimated to be 22.77 Å
for P1 and 25.07 Å for P2. After annealing, d-spacing distan-
ces of P1 and P2 thin films increase to 23.03 and 25.23 Å,
respectively. On the basis of the positions of (010) diffraction
peaks in annealed thin films, the p 2 p stacking distances of
P1 and P2 were 3.82 and 3.84 Å, respectively.
REFERENCES AND NOTES
1 R. S. Ashraf, I. Meager, M. Nikolka, M. Kirkus, M. Planells, B.
C. Schroeder, S. Holliday, M. Hurhangee, C. B. Nielsen, H.
Sirringhaus, I. McCulloch, J. Am. Chem. Soc. 2015, 137, 1314.
2 J. Cao, L. Qian, L. Ding, Polym. Chem. 2016, 7, 1027.
3 T. Erdmann, S. Fabiano, B. Milian, Medina, D. Hanifi, Z.
Chen, M. Berggren, J. Gierschner, A. Salleo, A. Kiriy, B. Voit, A.
Facchetti, Adv. Mater. 2016, 28, 9169.
4 J. H. Kim, S. Han, H. Jeong, H. Jang, S. Baek, J. Hu, M. Lee,
B. Choi, H. S. Lee, ACS Appl. Mater. Interfaces 2017, 9, 9910.
5 H. Jiang, H. Zhao, K. K. Zhang, X. Chen, C. Kloc, W. Hu, Adv.
Mater. 2011, 23, 5075.
6 W. Zhang, Z. Mao, Z. Chen, J. Huang, C. Wei, D. Gao, Z. Lin,
H. Li, L. Wang, G. Yu, Polym. Chem. 2017, 8, 879.
7 H. Usta, C. Risko, Z. Wang, H. Huang, M. K. Deliomeroglu, A.
Zhukhovitskiy, A. Facchetti, T. J. Marks, J. Am. Chem. Soc.
2009, 131, 5586.
8 Y.-J. Hwang, N. M. Murari, S. A. Jenekhe, Polym. Chem.
2013, 4, 3187.
CONCLUSIONS
9 J. Rivnay, M. F. Toney, Y. Zheng, I. V. Kauvar, Z. Chen, V.
Wagner, A. Facchetti, A. Salleo, Adv. Mater. 2010, 22, 4359.
In this contribution, we synthesized two regioirregular
naphthalenediimide-based alternating polymers, namely P1
and P2, containing nitrile-substituted moiety, 2,3-bis(thio-
phen-2-yl)acrylonitrile. The optical, electrochemical, and
thermal properties were characterized. The two regiorregu-
lar polymers showed typical dual-band UV–Vis–NIR absorp-
tion profiles of NDI-based polymers. Both polymers possess
exhibited excellent thermal stability with high decomposition
temperatures (5% weight less) up to 427 8C. Both polymers
possessed low-lying LUMO energy levels of down to 23.96
eV, which is beneficial for electron-injection and the air-
stability for FET devices. The two polymers exhibited prom-
ising, balanced, and air-stable ambipolar charge transport
characteristics with the highest electron and hole mobilities
of 0.048 and 0.0064 cm2 V21 s21, respectively. Thin film
microstructure investigations indicated that both polymers
formed continuous and smooth thin films with mainly face-
on molecular packing oriented to glass substrate. Such
observations imply higher charge mobilities might be
obtained by systematical optimization such as more complex
solution-processed techniques.
10 R. Chen, R. Q. Lu, K. Shi, F. Wu, H. X. Fang, Z. X. Niu, X. Y.
Yan, M. Luo, X. C. Wang, C. Y. Yang, X. Y. Wang, B. Xu, H.
Xia, J. Pei, X. Y. Cao, Chem Commun. 2015, 51, 13768.
11 H. Zhou, L. Yang, S. C. Price, K. J. Knight, W. You, Angew.
Chem. 2010, 49, 7992.
12 H. Usta, A. Facchetti, T. J. Marks, Org. Lett. 2008, 10, 1385.
13 R. Stalder, J. Mei, K. R. Graham, L. A. Estrada, J. R.
Reynolds, Chem. Mater. 2014, 26, 664.
14 Y. Cao, Z.-H. Guo, Z.-Y. Chen, J.-S. Yuan, J.-H. Dou, Y.-Q.
Zheng, J.-Y. Wang, J. Pei, Polym. Chem. 2014, 5, 5369.
15 S. Cho, J. Lee, M. H. Tong, J. H. Seo, C. Yang, Adv. Funct.
Mater. 2011, 21, 1910.
16 Z. Fei, Y. Han, J. Martin, F. H. Scholes, M. Al, Hashimi, S. Y.
AlQaradawi, N. Stingelin, T. D. Anthopoulos, M. Heeney, Mac-
romolecules. 2016, 49, 6384.
17 R. S. Ashraf, A. J. Kronemeijer, D. I. James, H. Sirringhaus,
I. McCulloch, Chem. Commun. 2012, 48, 3939.
18 J. D. Yuen, R. Kumar, D. Zakhidov, J. Seifter, B. Lim, A. J.
Heeger, F. Wudl, Adv. Mater. 2011, 23, 3780.
19 Y. Cao, J. S. Yuan, X. Zhou, X. Y. Wang, F. D. Zhuang, J. Y.
Wang, J. Pei, Chem. Commun. 2015, 51, 10514.
8
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2017, 00, 000–000

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
20 M. L. Tang, J. H. Oh, A. D. Reichardt, Z. Bao, J. Am. Chem.
Soc. 2009, 131, 3733.
28 X. Liu, J. Huang, J. Xu, D. Gao, W. Zhang, K. Shi, G. Yu,
RSC Adv. 2016, 6, 35394.
21 M. Zhu, H. Luo, L. Wang, G. Yu, Y. Liu, Acta Chim. Sinica
2012, 70, 1599.
29 Y. Gao, X. Zhang, H. Tian, J. Zhang, D. Yan, Y. Geng, F.
Wang, Adv. Mater. 2015, 27, 6753.
22 S. Chai, S. H. Wen, J. D. Huang, K. L. Han, J. Comput.
Chem. 2011, 32, 3218.
30 Y. Ji, C. Xiao, Q. Wang, J. Zhang, C. Li, Y. Wu, Z. Wei, X.
Zhan, W. Hu, Z. Wang, R. A. Janssen, W. Li, Adv Mater. 2016,
28, 943.
23 J. M. Park, S. K. Park, W. S. Yoon, J. H. Kim, D. W. Kim, T.
L. Choi, S. Y. Park, Macromolecules. 2016, 49, 2985.
31 H. J. Yun, S. J. Kang, Y. Xu, S. O. Kim, Y. H. Kim, Y. Y. Noh,
S. K. Kwon, Adv. Mater. 2014, 26, 7300.
24 H. G. Kim, B. Kang, H. Ko, J. Lee, J. Shin, K. Cho, Chem.
Mater. 2015, 27, 829.
32 Z. Chen, D. Gao, J. Huang, Z. Mao, W. Zhang, G. Yu, ACS
Appl. Mater. Interfaces 2016, 8, 34725.
25 H. Huang, Z. Chen, R. Ponce Ortiz, C. Newman, H. Usta, S.
Lou, J. Youn, Y. Y. Noh, K. J. Baeg, L. X. Chen, A. Facchetti, T.
J. Marks, J. Am. Chem. Soc. 2012, 134, 10966.
33 Z. Chen, Y. Zheng, H. Yan, A. Facchetti, J. Am. Chem. Soc.
2008, 131, 8.
26 Y. Li, P. Sonar, S. P. Singh, W. Zeng, M. S. Soh, J. Mater.
Chem. 2011, 21, 10829.
34 H. Chen, Y. Guo, Z. Mao, G. Yu, J. Huang, Y. Zhao, Y. Liu,
Chem. Mater. 2013, 25, 3589.
27 M. A. M. Leenen, F. Cucinotta, W. Pisula, J. Steiger, R.
Anselmann, H. Thiem, L. De Cola, Polymer. 2010, 51, 3099.
35 B. Kang, R. Kim, S. B. Lee, S. K. Kwon, Y. H. Kim, K. Cho, J.
Am. Chem. Soc. 2016, 138, 3679.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2017, 00, 000–000
9