Pyromellitic diimide-based copolymers for ambipolar field-effect transistors: Synthesis, characterization, and device applications

Article abstract of DOI:10.1002/pola.27259

A pyromellitic diimide building block, 2,6-bis(2-decyltetradecyl)-4,8- di(thiophen-2-yl)pyrrolo[3,4-f]isoindole-1,3,5,7(2H,6H)-tetraone (4), is synthesized. Based on this building block and other electron-rich units such as 2,2′-bithiophene, thieno[3,2-b]

Full text of DOI:10.1002/pola.27259

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Pyromellitic Diimide-Based Copolymers for Ambipolar Field-Effect
Transistors: Synthesis, Characterization, and Device Applications
Jinjun Shao, Jingjing Chang, Gaole Dai, Chunyan Chi
Department of Chemistry, National University of Singapore, Singapore 117543, Singapore
Correspondence to: C. Chi (E-mail: chmcc@nus.edu.sg)
Received 1 March 2014; accepted 19 May 2014; published online 00 Month 2014
DOI: 10.1002/pola.27259
ABSTRACT: A pyromellitic diimide building block, 2,6-bis(2-
decyltetradecyl)24,8-di(thiophen-2-yl)pyrrolo[3,4-f]isoindole-
1,3,5,7(2H,6H)-tetraone (4), is synthesized. Based on this build-
ing block and other electron-rich units such as 2,20-bithiophene,
thieno[3,2-b]thiophene and 4,8-bis(dodecyloxy)benzo[1,2-b:4,5-
b0]dithiophene, three conjugated polymers P1, P2, and P3 are
prepared in good yield via Stille coupling polymerization.
These new copolymers have good solubility in common
organic solvents and exhibit good thermal stability. The opti-
cal, electrochemical, and thermal properties of these polymers
P1–P3 are carefully investigated, and their applications in
solution-processed organic field-effect transistors are also stud-
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Chem. 2014, 00, 000–000
KEYWORDS: ambipolar organic semiconductors; charge trans-
port; conjugated polymers; copolymerization; organic field
effect transistors; pyromellitic diimide; thiophene
Pyromellitic diimide (PMDI) is a promising building block
for organic electronic materials, which has been used to
build up oligmers to fabricate n-type OFETs with electron
mobility up to 0.079 cm2 V21 s21 by vapor deposition tech-
nique.8 Their longer acene analogues, anthracene diimide,
and pentacene diimide have also been synthesized and
showed n-channel FET behavior with electron mobility up to
0.08 cm2 V21 s21 from solution-processed devices.9 How-
ever, the synthesis of these longer analogues is quite compli-
cated. For practical applications, the solution-processable
polymers with easy synthesis are preferred. So far, only few
examples of polymers based on PMDI building block have
been prepared, and studied for OFET applications.10 PMDI–
INTRODUCTION Ambipolar polymer organic field-effect
transistors (OFETs), capable of efficiently transporting
both electrons and holes,1 are of currently great interest
owing to their potential applications in complementary cir-
cuits,2 and in organic light-emitting transistors.3 Over the
past decades, various conjugated polymers have been pre-
pared with the aim of improving the charge-carrier mobil-
ity. Considerable effort has been made to develop p-type
conjugated polymers4; however, conjugated polymer-based
ambipolar OFETs have not been systematically studied.5
Polythiophenes are an important class of conjugated poly-
mers with high charge-carrier mobilities,6 such as regiore-
gular poly(3-hexylthiophene) (P3HT) which is the
benchmark among polymeric electronic materials. How-
ever, its devices have poor stability because the P3HT can
be easily oxidized in air owing to its high-lying highest
occupied molecular orbital (HOMO) energy level. To
improve the device stability of P3HT, some structural mod-
ifications have been performed by introducing electron-
withdrawing groups into the polythiophene backbone, and
alternating donor–acceptor (D–A) structured polymers
have been prepared. For example, a series of polymers
containing thiophene backbones and electron-withdrawing
groups (e.g., imide groups) have been designed and syn-
thesized, and they exhibited ambipolar behavior with good
air stability.7
ethynylene-based homopolymer showed an electron mobility
10(c)
of 2 3 1024 cm2 v21 s21
.
A 3,6 and N,N0-linked PMDI-
based polymer blended with PCBM in 1:9 weight ratio
showed electron mobility of 102321022 cm2 v21 s21 10(d)
PMDI-based oligothiophene-containing D–A polymers
.
(PPMDI-nT) with the different ratios of PMDI/thiophene in
the polymer backbone were reported by Pei’s group, and
these polymers showed hole mobility of 102521023 cm2
v21 s21, and electron mobility of 2–6 3 1024 cm2 v21
s21 10(e)
From the previous study, PMDI has been proved as
.
a strong electron acceptor. Herein, we report a series of
ambipolar-conjugated polymeric semiconductors by introduc-
ing the PMDI-containing unit (5) (Scheme 1) into the
Additional Supporting Information may be found in the online version of this article.
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mix as internal standard or external standard. UV–vis
absorption and fluorescence spectra were recorded on Shi-
madzu UV-1700 and RF-5301 spectrometers in high-
performance liquid chromatography pure solvents. Cyclic vol-
tammetry (CV) was performed on a CHI 620C electrochemi-
cal analyzer with a three-electrode cell in a solution of 0.1M
tetrabutylammonium hexafluorophosphate (Bu4NPF6) dis-
solved in dry DCM at a scan rate of 50 mV s21. A gold elec-
trode with a diameter of 2 mm, a Pt wire, and an Ag/AgCl
electrode were used as the working electrode, the counter
electrode, and the reference electrode, respectively. The
potential was calibrated against the ferrocene/ferrocenium
couple. Thermogravimetric analysis (TGA) was carried out
on a TA instrument 2960 at a heating rate of 10 ꢀC min21
under N2 flow and differential scanning calorimetry (DSC)
was performed on a TA instrument 2920 at a heating/cool-
ing rate of 10 ꢀC min21 under N2 flow. X-ray diffraction
(XRD) measurements were performed on a Bruker-AXS D8
DISCOVER with GADDS Powder X-ray diffractometer, both
with Cu Ka radiation.
SCHEME 1 Synthesis route of monomer 5.
Synthesis
3,6-Dibromobenzene-1,2,4,5-tetracarboxylic acid (2)
To a suspension of 1,4-dibromo-2,3,5,6-tetramethylbenzene 1
(8.760 g, 30 mmol) and celite (40 g) in water (100 mL) and
t-BuOH (100 mL), KMnO4 (40.300 g, 255 mmol, 8.5 eq) was
added in portions within 30 min. The mixture was heated to
95 C for overnight under air. After cooling to 80 C, ethanol
was slowly added, until there were no bubbles coming out.
The reaction mixture was filtered and filtrate was obtained.
The solvent was removed in vacuum, then concentrated HCl
(aq) was added, and ultrasonicated for 30 min; the mixture
polythiophene backbone. Based on this PMDI building block,
three D–A-conjugated copolymers P1, P2, and P3 have been
synthesized via Stille coupling polymerization (Scheme 2).
Their optical and electrochemical properties and thermal
behavior were investigated. The OFET performances of these
polymers by using low-cost solution-processing techniques
were examined in detail.
ꢀ
ꢀ
EXPERIMENTAL
Materials
All reagents were purchased from commercial sources and
used without further purification. Anhydrous dichloromethane
(DCM), chloroform (CHCl3), and N,N-dimethylformaldehyde
(DMF) were distilled from CaH2. Anhydrous toluene and tetra-
hydrofuran (THF) were distilled from sodium/benzophenone
immediately prior to use. 1,4-Dibromo-2,3,5,6-tetramethylben-
zene (1),11 tributyl(thiophen-2-yl)stannane,12 2-decyltetrade-
can-1-amine,13 5,50-bis(trimethylstannyl)22,20-bithiophene (6),13
2,5-bis(trimethylstannyl)thieno[3,2-b]thiophene (7),13 and
(4,8-bis(dodecyloxy)benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl)bis
(trimethylstannane) (8)13 were prepared by following the
reported literature procedures.
General Characterization Methods
All NMR spectra were recorded on Bruker AMX500 at 500-
MHz spectrometers. 1H NMR and 13C NMR spectra were
recorded in CDCl3 and DMSO-d6. All chemical shifts are
quoted in ppm, using the residual solvent peak as a refer-
ence standard. Mass spectrometry (MS) was recorded in
electron ionization (EI) mode and high-resolution mass spec-
trometry was recorded with EI source. Matrix-assisted laser
desorption ionization time-of-flight (MALDI-TOF) analysis
was performed on a Bruker Autoflex III MALDI-TOF instru-
ment by using 1,8,9-trihydroxyanthracene as matrix and Pep-
SCHEME 2 Synthesis route of copolymers P1, P2, and P3.
2
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was filtered, washed with concentrated HCl (aq), dried in
vacuum, and 2 was obtained as a white solid (7.470 g, 60%).
4,8-Bis(5-bromothiophen-2-yl)22,6-bis(2-decyltetradecyl)
pyrrolo[3,4-f]isoindole-1,3,5,7(2H,6H)-tetraone (5)
To a solution of compound 4 (1.053 g, 1.0 mmol) in dry
CHCl3 (50 mL), dropwise of bromine (0.22 mL, 4.2 mmol,
4.2 eq) was added at 0 ꢀC. After stirring at room tempera-
ture for overnight, the reaction mixture was quenched with
50 mL of saturated NaHCO3 solution. The mixture was
washed with NaHCO3 (100 mL 3 1) and brine (100 mL 3
2). The organic phase was dried over anhydrous Na2SO4 and
the organic solvent was removed under reduced pressure.
The crude product was purified by column chromatography
(silica gel, CHCl3:Hex 5 1:2), then precipitated in MeOH to
afford compound 5 as yellow solid (1.172 g, 97%).
13C NMR (125 MHz, DMSO-d6): d ppm 5 165.44, 137.00,
114.72. MALDI-TOF-MS (m/z): calcd. for
409.8273; found 448.0424 [M1K]1.
C10H4Br2O8:
4,8-Dibromo-2,6-bis(2-decyltetradecyl)pyrrolo
[3,4-f]isoindole-1,3,5,7(2H,6H)-tetraone (3)
To a suspension of 3,6-dibromobenzene-1,2,4,5-tetracarbox-
ylic acid 2 (0.412 g, 1.0 mmol) and dry DMF (two drops) in
dry toluene (5 mL), thionyl chloride (0.6 mL, 8 mmol, 8 eq)
ꢀ
was added dropwise. The mixture was heated to 80 C for 2
h under argon until no HCl (g) coming out, and then cooled
to room temperature. The solvent was removed in vacuum,
then C24H49NH2 (0.672 g, 1.9 mmol, 1.9 eq) and anhydrous
acetic acid (10 mL) were added. The reaction mixture was
rapidly heated to 145 ꢀC for 3 h under argon. The mixture
was cooled to room temperature and extracted with CHCl3
(50 mL 3 2). The combined organic phase was washed with
brine (100 mL 3 3) and saturated NaHCO3 (100 mL 3 1),
then dried over anhydrous Na2SO4. The organic solvent was
removed under reduced pressure and the residue was puri-
fied by column chromatography (silica gel, EA:Hex 5 1:10) to
afford compound 3 as white solid (0.607 g, 58%).
1H NMR (500 MHz, CDCl3, 300 K): d ppm 5 7.16 (d, J 5 4.0
Hz, 2H), 7.01 (d, J 5 4.0 Hz, 2H), 3.49 (d, J 5 7.0 Hz, 4H),
1.81 (br, 2H), 1.29–1.23 (m, 80H), 0.89–0.86 (m, 12H); 13C
NMR (125 MHz, CDCl3, 300 K): d ppm 5 164.84, 135.22,
130.97, 130.68, 129.83, 129.66, 115.56, 43.04, 36.81, 31.91,
29.97, 29.68, 29.66, 29.63, 29.55, 29.34, 29.33, 26.16, 22.68,
14.11. MALDI-TOF-MS (m/z): calcd. for C66H102Br2N2O4S2:
1208.5648; found 1209.2799 [M1H]1.
Copolymer P1
A mixture of monomer compound 5 (242 mg, 0.2 mmol), mono-
mer 5,50-bis(trimethylstannyl)22,20-bithiophene (6) (149 mg,
0.2 mmol, 1.0 eq), and catalyst Pd(PPh3)4 (5 mg, 2 mol %) in
anhꢀydrous toluene (5 mL) and dry DMF (1 mL) was heated to
90 C for 36 h under argon. After cooling to room temperature,
the polymer was precipitated in acetone/methanol (1:1) solvent
and filtered. Then the polymer was dissolved in CHCl3, precipi-
tated in MeOH/THF (5:1), and filtered. This procedure was
repeated for five times to give the pure copolymer P1 as a dark
red solid (207 mg, 85%). Gel permeation chromatography (GPC)
1H NMR (500 MHz, CDCl3, 300 K): d ppm 5 3.62 (d, J 5 7.5
Hz, 2H), 1.89 (br, 2H), 1.26 (m, 40H), 0.90–0.87 (m, 6H); 13C
NMR (125 MHz, CDCl3, 300 K): d ppm 5 163.70, 136.30,
114.11, 43.50, 37.07, 31.94, 31.74, 29.93, 29.69, 29.66,
29.62, 29.57, 29.34, 29.32, 26.37, 22.66, 14.00. MALDI-TOF-
MS (m/z): calcd. for
1045.1869 [M1H]1.
C58H98Br2N2O4: 1044.5893; found
2,6-Bis(2-decyltetradecyl)24,8-di(thiophen-2-yl)pyrrolo
[3,4-f]isoindole-1,3,5,7(2H,6H)-tetraone (4)
(THF at 23 ꢀC): Mn 5 28, 012 g mol21, Mw 5 77,689 g mol21
,
and PDI 5 2.77 (against PS standard).
A
mixture of compound 3 (1.047 g, 1.0 mmol), 2-
1H NMR (500 MHz, CDCl3, 323 K): d ppm 5 7.30–7.39 (br,
4H), 7.22–7.21 (br, 4H), 7.19–7.18 (br, 4H), 7.14–7.13 (br,
4H), 3.54 (br, 4H), 1.88 (br, 2H), 1.50–1.26 (m, 80H), 0.90–
0.87 (m, 12H); ELEM. ANAL. Calcd: for (C74H106N2O4S4)n: C,
73.10; H, 8,79; N, 2.30; Found: C, 72.85; H, 8.63; N, 2.18.
tributylstannylthiophene (0.119 g, 3.0 mmol, 3 eq), and cata-
lyst Pd(PPh3)4 (70 mg, 10%) in anhydrous DMF (3 mL) and
toluene (15 mL) was degassed by three freeze–pump–thaw
ꢀ
cycles. The mixture was heated to 110 C under argon over-
night. The mixture was cooled to room temperature and
extracted with CHCl3 (50 mL 3 2). The combined organic
phase was washed with NaHCO3 (50 mL 3 2) and brine
(50 mL 3 1). The organic phase was dried over anhydrous
Na2SO4 and the organic solvent was removed under reduced
pressure. The crude product was purified by column chro-
matography (silica gel, CHCl3:Hex 5 1:2), then precipitated in
MeOH to afford compound 4 as yellow solid (0.727 g, 69%).
Copolymer P2
A mixture of monomer compound 5 (242 mg, 0.3 mmol),
monomer 2,5-bis(trimethylstannyl)thieno[3,2-b]thiophene (7)
(140 mg, 0.3 mmol, 1.0 eq), and catalyst Pd(PPh3)4 (7 mg, 2
mol %) in anhydrous toluene (7.5 mL) and dry DMF
(1.5 mL) was heated to 90 ꢀC for 36 h under argon. After
cooling to room temperature, the polymer was precipitated
in acetone/methanol (1:1) solvent and filtered. Then the
polymer was dissolved in CHCl3, precipitated in MeOH/THF
(5:1), and filtered. This procedure was repeated for five
times to give the pure copolymer P2 as a dark red solid
1H NMR (500 MHz, CDCl3, 300 K): d ppm 5 7.62 (dd,
J1 5 5.05 Hz, J2 5 1.25 Hz, 2H), 7.27–7.26 (m, 2H), 7.23 (dd,
J1 5 5.05 Hz, J2 5 3.80 Hz, 2H), 3.48 (d, J 5 7.5 Hz, 4H),
1.81(br, 2H), 1.25–1.22 (m, 40H), 0.89–0.86 (m, 12H); 13C
NMR (125 MHz, CDCl3, 300 K): d ppm 5 65.04, 135.23,
130.77, 129.95, 129.84, 128.16, 126.89, 12.92, 36.75, 31.90,
31.89, 31.43, 29.96, 29.67, 29.64, 29.61, 29.53, 29.34, 29.32,
26.15, 22.67, 14.09. MALDI-TOF-MS (m/z): calcd. for
(318 mg, 89%). GPC (THF at 23 ꢀC): Mn 5 30,788 g mol21
Mw 5 57,464 g mol21, and PDI 5 1.87 (against PS standard).
,
1H NMR (500 MHz, CDCl3, 323 K): d ppm 5 7.41 (br, 2H),
7.34–7.33 (br, 2H), 7.24–7.23 (br, 2H), 3.55 (br, 4H), 1.88
C
66H104N2O4S2: 1052.7438; found 1053.3062 [M1H]1.
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TABLE 1 Molecular Weights and Decomposition Temperatures
4200 semiconductor parameter analyzer in the N2 glovebox or
in air. For each condition, about 8–10 devices were tested.
of the Polymers
a
b
d
Tapping-mode Atomic Force Microscopy (TM-AFM) was per-
formed on Bruker AXS ICON-PKG AFM. XRD patterns of the
thin film were measured on a Bruker-AXS D8 DISCOVER
with GADDS X-ray diffractometer.
Polymer
Mw
Mn
PDIc
Td
P1
P2
P3
77,689
57,464
48,740
28,012
30,788
30,576
2.77
1.87
1.59
395
432
345
a
RESULTS AND DISCUSSION
Weight-average molecular weight (Mw).
Number-average molecular weight (Mn).
Polydispersity index (Mw/Mn) determined by means of GPC with THF
b
c
Synthesis
as eluent on the basis of polystyrene calibration.
The synthetic routes to the monomer 5 and polymers P1, P2,
and P3 are shown in Schemes 1 and 2. The reported proce-
dures to prepare the PMDI containing building block 4 were
revised by shortening the synthetic steps and simplifying the
purification.10(b) 1,4-Dibromo-2,3,5,6-tetramethylbenzene (1)
was prepared according to the published literature.11 KMnO4
in H2O/t-BuOH mixture was employed to oxidize 1, affording
3,6-dibromobenzene-1,2,4,5-tetracarboxylic acid 2 with 60% of
yield. Compound 2 was treated with SOCl2 in toluene to give
the corresponding acyl chloride. After excess SOCl2 was
removed, 2-decyltetradecan-1-amine (C24H49NH2) in acetic
acid was quickly injected, and refluxed for 3 h to achieve pyro-
mellitic diimide 3 in a good yield of 58%. To get compound 4,
2-thiopheneboronic acid was first applied to react with 3 via
Suzuki coupling; however, the reaction cannot take place, prob-
ably owing to the steric hindrance of the four carbonyl groups
(AC@O) surrounding the two bromine atoms. Later Stille cou-
pling reaction was taken by refluxing tributyl(2-thienyl)stan-
nane13 with 3 in toluene, affording 4 with 69% of yield. The
monomer 5 was obtained in an excellent yield of 97% by bro-
mination of 4 with Br2 in CHCl3. The dibromo-monomer 5
was used to prepare the copolymers by reacting with 5,50-
bis(trimethylstannyl)22,20-bithiophene (6),13 2,5-bis(trimethyl-
stannyl)thieno[3,2-b]thiophene (7),13 and (4,8-bis(dodecyloxy)-
benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl)bis(trimethylstannane)
(8)13 through the Stille coupling reaction to give the polymers
P1, P2, and P3, respectively. These new polymers have good
solubility in common organic solvents such as THF, CHCl3,
and DCM. Their molecular weight (Mn) and polydispersity
index (PDI) were measured by GPC using THF as the solvent
and polystyrene as the standard. The data are summarized in
Table 1. The Mn (PDI) values are 28,012 (2.77), 30,788 (1.87),
and 30,576 (1.59) for P1, P2, and P3, respectively.
d
Temperature at 5% weight loss estimated using TGA under N2.
(br, 2H), 1.48–1.26 (m, 80H), 0.90–0.87 (m, 12H). ELEM. ANAL.
Calcd: for (C72H104N2O4S4)n: C, 72.68; H, 8,81; N, 2.35;
Found: C, 72.43; H, 8.68; N, 2.29.
Copolymer P3
A mixture of monomer compound 5 (242 mg, 0.2 mmol),
monomer (4,8-bis(dodecyloxy)benzo[1,2-b:4,5-b0]dithiophene-
2,6-diyl)bis(tri-methylstannane) (8) (177 mg, 0.2 mmol, 1.0
eq), and catalyst Pd(PPh3)4 (5 mg, 2 mol %) in anhydrous tol-
uene (5 mL), and dry DMF (1 mL) was heated to 90 ꢀC for
36 h under argon. After cooling to room temperature, the
polymer was precipitated in acetone/methanol (1:1) solvent
and filtered. Then the polymer was dissolved in CHCl3, pre-
cipitated in MeOH/THF (5:1), and filtered. This procedure
was repeated for five times to give the pure copolymer P3 as
a dark red solid (290 mg, 90%). GPC (THF at 23 ꢀC):
Mn 5 30,576 g mol21, Mw 5 48,740 g mol21, and PDI 5 1.59
(against PS standard).
1H NMR (500 MHz, CDCl3, 323 K): d ppm 5 7.62 (br, 2H),
7.45 (br, 2H), 7.26 (br, 2H), 4.34 (br, 4H), 3.56 (br, 4H),
1.96–1.89 (br, 6H), 1.61–1.25 (m, 116H), 0.88–0.87 (m, 18H).
ELEM. ANAL. Calcd: for (C100H154N2O6S4)n: C, 74.67; H, 9.65; N,
1.74; Found: C, 74.32; H, 9.47; N, 1.61.
Device Fabrication and Characterization
Bottom-gate, top-contact OFET devices were prepared on the
n1 silicon wafer, and 200-nm thermal SiO2 layer serves as
the gate dielectric. The SiO2/Si substrate was cleaned with
acetone and isopropanol, and then immersed in a piranha
solution for 8 min. The substrate was rinsed with deionized
water and then spin-casted with 3 mM of octadecyltrime-
thoxysilane (OTS) solution in trichloroethylene and placed in
an environment saturated with ammonia vapor for 7 h at
room temperature. Then the substrates were washed with
deionized water and toluene. The semiconductor layer was
deposited on top of the OTS-modified dielectric surface by
spin-coating the solution in chlorꢀoform (0.8 wt %). Then the
thin films were annealed at 200 C or selected temperatures
for 30 min. Subsequently, gold source/drain electrodes were
deposited by thermal evaporation through a metal shadow
mask to create a series of FETs with various channel lengths
(L 5 100 mm) and width (W 5 4 or 1 mm) dimensions.
The FET devices were then characterized using a Keithley SCS-
Photophysical Properties
The UV–vis absorption spectra of P1, P2, and P3 were
recorded in both chloroform solution and spin-coated thin
films. The spectra are shown in Figure 1 and the relevant
data are summarized in Table 2. Broad absorption bands
located in the visible region were observed for both the solu-
tion and the thin films of P1, P2, and P3 (Fig. 1). The
absorption maxima for P1, P2, and P3 in solutions are
located at 456, 453, and 451 nm, respectively. In the thin
films, the absorption spectra showed the absorption maxima
at 474, 474, and 479 nm, with obvious red shifts of 18, 21,
and 28 nm for P1, P2, and P3, respectively, indicating strong
intermolecular interactions between the polymer chains in
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FIGURE 2 Cyclic voltammograms of P1, P2, and P3 in dry DCM
with 0.1 M of Bu4NPF6 as supporting electrolyte, a gold elec-
trode with a diameter of 2 mm, a Pt wire, and an Ag/AgCl elec-
trode were used as the working electrode, the counter
electrode, and the reference electrode, respectively, with a
scan rate at 50 mV s21. The potential was calibrated against
the ferrocene/ferrocenium couple. [Color figure can be viewed
in the online issue, which is available at wileyonlinelibrary.
com.]
Fig. S1 and Table S1). As shown in Figure 2, one quasi-
reversible reduction wave and three quasi-reversible oxida-
tion waves were observed for P1, with the onset reduction
onset
potential (E
) at 21.29 V, and the onset oxidation poten-
red
onset
tial (E
) at 0.22 V. Polymer P2 exhibited one quasi-
reversible reduction wave with E
onset
reversible oxidation with E
red
ox
FIGURE 1 UV–vis absorption spectra in CHCl3 solution (a)
(c 5 1 3 1025 M) and spin-coated thin films (b) for P1, P2, and
P3. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
onset
red
at 21.19 V, and one
at 0.53 V. Polymer P3
onset
red
showed two quasi-reversible reduction waves with E
at
21.20 V, and three quasi-reversible oxidation waves with
onset
ox
E
at 0.34 V. The HOMO and LUMO energy levels were
the solid state. The corresponding optical energy gaps esti-
mated from the lowest-energy absorption edge of the
absorption spectra in solution were calculated to be 1.90,
1.93, and 1.93 eV for P1, P2, and P3, respectively (Table 2).
No fluorescence can be observed for all of polymers, indicat-
ing a charge-transfer nature in the excited state between
donor and acceptor components.
determined according to the following equations: HOMO 5 –
onset
(E
1 4.8) eV, and lowest unoccupied molecular orbital
ox
onset
(LUMO) 5 2(E
1 4.8) eV, respectively.14 The HOMO/
red
LUMO energy levels are calculated to be 25.02/23.51,
25.33/23.61, and 25.14/23.60 eV for P1, P2, and P3,
respectively. For the three polymers, the electron-deficient
block PMDI is the same, and thus the LUMO energy level is
similar. However, the electron-rich block is different, varying
from 2,20-bithiophene (in P1) to thieno[3,2-b]thiophene (in
P2) to benzo[1,2-b:4,5-b0]dithiophene (in P3). 2,20-Bithio-
phene is the most electron-rich block, and hence P1 has the
highest HOMO energy level, whereas thieno[3,2-b]thiophene
Electrochemical Properties
The electrochemical properties of P1, P2, and P3 were
investigated by CV and differential pulse voltammetry in
DCM solution (Fig. 2 and Table 2; Supporting Information
TABLE 2 Summary of the Photophysical Properties and Electrochemical Data of Polymers P1, P2, and P3
max
abs
k
ðnm Þ
emax 3 1025
(M21 cm21
)
E
(V)
E
(V)
LUMO (eV)
HOMO (eV)
EgCV ðeV Þ
Egopt ðeV Þ
onset
re
onset
ox
Solution
Film
P1
P2
P3
456
453
451
474
474
479
0.471
0.414
0.382
21.29
21.19
21.20
0.22
0.53
0.34
23.51
23.61
23.60
25.02
25.33
25.14
1.51
1.72
1.54
1.90
1.93
1.93
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FIGURE 3 TGA curves of polymers P1, P2, and P3. [Color fig-
ure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
(in P2) is the least electron-rich moiety among three donor
blocks, and hence P2 has the lowest HOMO energy level. The
similar trend can also be found in the reported study.13,15
The corresponding electrochemical energy gaps EgCV
(5 LUMO 2 HOMO) are estimated to be 1.51, 1.72, and 1.54
eV for P1, P2, and P3, respectively, which are slightly
smaller than the optical band gaps Egopt
.
Thermal Behavior
Thermal stability is one of the key requirements for the
practical application of organic electronic materials. The
thermal stability of the polymers P1, P2, and P3 was meas-
ured by TGA in N2 at a heating rate of 10 C min (Fig. 3).
The decomposition temperature (Td, correꢀsponding to a 5%
weight loss) locates at 395, 432, and 345 C for P1, P2, and
P3, respectively, demonstrating their sufficiently high ther-
mal stability for the application of OFETs. For P3, two stages
of decomposition were observed. The first stage is caused by
FIGURE 4 DSC curves of P2 (a) and P3 (b).
21
ꢀ
However, no obvious phase transition was observed below
300 C for P1.
ꢀ
Thin Film Field-Effect Transistors
Bottom-gate top-contact OFETs were fabricated to investigate
the charge-transport properties of the PMDI-based copoly-
mers P1–P3. The devices were fabricated by the procedure
reported previously.17 The thin films were deposited by spin
coating a 0.8 wt % chloroform solution onto OTS-modified
SiO2/Si substrates. Prior to patterning the source and
drain Au electrodes, the polymer thin films were thermally
annealed at 200 ꢀC or selective temperatures (for P2) for
0.5 h. The FET devices were then characterized using a
the decomposition of the flexible alkoxy chains (AOC12H25
)
attached to the benzodithiophene. In this case, the two
dodecyl groups are lost. The weight loss is about 20% of the
sample, and it is consistent with the polymer structure. This
phenomenon can also be observed from other polymers con-
taining benzodithiophene units with alkoxy chains.13,16 The
glass transition temperatures of P2 and P3 were observed
at 234 and 260 ꢀC, respectively, from DSC curves (Fig. 4).
TABLE 3 FET Performance for Copolymers P1, P2, and P3 Tested in N2 Condition
h
e
Annealing
Temperatures (ꢀC)
mh(cm2 V21 s21
)
VT (V)
Ion/off
me (cm2 V21 s21
)
VT (V)
Ion/off
P1
P2
P3
As-spun
200
(3.2 6 0.34) 3 1024
(1.1 6 0.12) 3 1023
(7.2 6 0.24) 3 1024
(8.0 6 0.13) 3 1023
(1.0 6 0.46) 3 1026
(4.5 6 0.40) 3 1026
233
230
225
223
228
225
103
103
103
103
102
102
(1.0 6 0.35) 3 1025
(1.6 6 0.23) 3 1025
(3.4 6 0.24) 3 1025
(4.5 6 0.12) 3 1024
(1.3 6 0.36) 3 1025
(4.0 6 0.28) 3 1025
54
50
41
42
45
42
102
102
102
102
102
102
As-spun
200
As-spun
200
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FIGURE 5 Transfer (a) characteristics of OFET devices based on P2 thin films at different thermal annealing temperatures and (b)
the hole and electron mobility as a function of annealing temperatures. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
FIGURE 6 Transfer (a, b) and output (c, d) characteristics of OFET devices based on P2 thin films after thermal annealing at
240 ꢀC. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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TABLE 4 FET Performance of Copolymer P2 Measured on OTS-Modified Si/SiO2 (200 nm) Wafers Under Different Annealing
Temperatures
h
e
Annealing
temperatures (ꢀC)
mh (cm2 V21 s21
)
VT (V)
Ion/off
me (cm2 V21 s21
)
VT (V)
Ion/off
120
160
200
240
280
(2.3 6 0.21) 3 1023
(4.2 6 0.24) 3 1023
(8.0 6 0.13) 3 1023
(2.1 6 0.10) 3 1022
(2.8 6 0.28) 3 1023
225
227
223
215
213
103
103
103
103
103
(1.2 6 0.23) 3 1024
(2.6 6 0.18) 3 1024
(4.5 6 0.15) 3 1024
(5.4 6 0.12) 3 1024
(1.4 6 0.14) 3 1024
41
36
42
36
48
102
102
102
102
102
Keithley SCS-4200 probe station in N2 atmosphere or in air.
The field-effect mobility was calculated from the following
equation in the saturation regime: Ids 5 (W/2L)Cim(Vgs 2 Vth)2,
where W and L are the channel width and length, respectively.
Ci is the capacitance of the insulating SiO2 layer per unit, Vgs
and Vth are the gate voltage and the threshold voltage,
respectively.
tion hole and electron mobility of 8.0 3 1023 (on/off 5 103,
V
V
th 5 223 V) and 4.5 3 1024 cm2 V21 s21 (on/off 5 102,
th 5 42 V), respectively. For P1, the hole/electron mobilities
are 1.1 3 1023/1.6 3 1025 cm2 V21 s21, respectively.
Among these three polymers, P3 exhibited the lowest per-
formance. This could be explained by the increased steric
hindrance between the long alkyl chains at imide position
and the benzodithiophene unit, which could affect the effi-
cient packing in the solid state.
The device performance of P1–P3 is summarized in Table 3
(See transfer and output characteristics in Supporting Infor-
mation Figs. S2–S4). Ambipolar charge-transport characteris-
tics were obserꢀved in all the devices. After thermal
annealing at 200 C, P2 devices revealed an effective satura-
For the highest mobility polymer of this series P2, we also
investigate the device performance as a function of annealing
temperatures. Figures 5 and 6 and Table 4 show and sum-
marize the device performance characteristics of P2 after
different annealing temperatures. Higher annealing tempera-
tures result in higher mobilities, at the optimum temperature
of 240 ꢀC (Figs. 5 and 6), the device exhibited the best per-
formance with hole/electron mobility of 0.021/5.4 3 1024
cm2 V21 s21. Further increase of annealing temperature will
decrease the device performance owing to dewetting prob-
lem of the polymer on the dielectric surface.
Thin Film Morphology and Microstructure
To understand the effect of the polymer structure on the
electrical performance of the copolymers, the thin-film
microstructure and surface morphology were studied by
contact-mode AFM and two-dimensional (2D) XRD. Figure 7
shows the AFM images of the annealed polymer thin film.
P1–P2 thin films are uniform and display microcrystalline
morphology, whereas P3 was amorphous. 2D X-ray patterns
of the annealed thin films of P1–P3 show a series of Bragg
reflections, indicating an ordered packing structure (Fig. 8
and Supporting Information Fig. S5). The primary peaks with
a 2h angle of 4.0, 3.7, and 3.2ꢀ were observed for P1–P3,
respectively, which are correlated to the d-spacing values
(d1) of 22.0, 24.1, and 27.5 Å, respectively. Up to fourth
order diffraction peaks were observed for thin film of P2
and the other d-values of the reflections are approximately
the 1/2, 1/3, and 1/4 of d1 value, indicating that P2 adopts
an ordered lamellar packing structure. P1 and P3 showed
similar packing structure but only lower order (second order
and third order) diffraction peaks are visible, indicating their
lower crystallinity compared to P2. Such difference well
explains the observed best device performance for P2. This
d1 value is correlated to the distance between the polymer
FIGURE 7 AFM images (left: height image; right: phase image)
for copolymer P1 (a), P2 (b), and P3 (c) on OTS-modified sub-
strates and annealing at 200 ꢀC. [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
8
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FIGURE 8 Respective 2D XRD patterns and corresponding diffraction spectra of P1 (a), P2 (b), and P3 (c) thin films after annealing
at 200 ꢀC. They were obtained with the incident X-ray parallel to the thin films. The d-spacing value is labeled for each diffraction
peak. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
FIGURE 9 Tapping-mode AFM images of copolymer P2 thin films on OTS-modified SiO2/Si substrates annealed at different tem-
peratures. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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backbones in the adjacent layers and it is determined by
many factors such as the breadth of backbone, the alkyl
chain interdigitation, and the space filling, and so forth. P3
shows larger d1 value compared to P1 and P2 owing to the
existence of long alkoxy chains on the 4,8-bis(dodecyloxy)-
benzo[1,2-b:4,5-b0]dithiophene unit. The d1 value of P2 is
slightly larger than that of P1, indicating a slight difference
of their space filling. No obvious diffraction peaks correlated
to p–p stacking could be observed.
The annealing temperature is substantial for polymer P2,
which was tracked by AFM and XRD characterization. As
shown in Figure 9, the thin-film crystallinity is greaꢀtly
enhanced when the temperature ranged from 120 to 240 C,
and the crystalline grains size significantly increased com-
pared to the smaller, less well-defined grains at low tempera-
ture. The surface roughness (RMS) of the thin films were
0.50, 0.71, 1.14, 2.58, and 0.94 nm for annealed films at tem-
peratures of 120, 160, 200, 240, and 280 ꢀC, respectively.
However, at 280 ꢀC, the thin film formed rod-like crystals
after recrystallization and isolated islands were formed
owing to dewetting problem (Supporting Information Fig.
S6). To investigate the effect of temperature on the crystillin-
ity of P2 thin films, the 2D XRD were conducted. Figure 10
shows the diffraction peaks of the films annealed at different
temperatures. It was found that the diffraction intensity
increased with annealing temperature, especially at 240 ꢀC,
and the full-width half-maximum (FWHM) of (100) peaks
decreased gradually as the annealing temperature increases.
This suggests that the crystallinity of polymer chains is sig-
nificantly enhanced through thermal treatment.
FIGURE 11 Transfer and output characteristics of OFET with P2
thin films test in ambient conditions after thermal annealing at
240 ꢀC. [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
portation. Even storing in air for 3 months, the charge-
carrier mobility remains 95% of the initial value, indicating
good air stability of these PMDI-based polymers.
CONCLUSIONS
We have readily synthesized a PMDI building block, 2,6-
bis(2-decyltetradecyl)24,8-di(thiophen-2-yl)pyrrolo[3,4-f]iso-
indole-1,3,5,7(2H,6H)-tetraone (4), and three conjugated
polymers P1, P2, and P3 via Stille coupling polymerization.
The optical, electrochemical, and thermal properties of these
polymers P1–P3 were carefully investigated, and their appli-
cations in OFETs were also presented. These new copoly-
mers have good solubility in common organic solvents, and
exhibit good thermal stability. Their OFET devices fabricated
by solution processing technique showed ambipolar behavior
with hole mobility up to 0.021 cm2 V21 s21 and electron
mobility around 5.4 3 1024 cm2 V21 s21 under N2 atmos-
phere. P2 also shows a good air stability under ambient con-
dition. This study provides the insight and useful
information for the further design of PMDI-based materials.
The air stability of p-type OFET is largely determined by the
ionization potential of the semiconductor. Introduction of
imide subunit in the conjugated polymers lower the HOMO
energy level (i.e., increase the ionization potential), which
makes the copolymers more stable in the ambient condi-
tions. P2 (HOMO 5 25.33 eV) showed good air stability
when exposed to air (Fig. 11) whereas in its device, there is
almost no change in charge-carrier mobility for hole trans-
ACKNOWLEDGMENT
The authors acknowledge financial support from NUS start up
grant R-143-000-486-133 and MOE AcRF FRC grants (R-143-
000-510-112 and R-143-000-573-112).
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