Ambipolar polymer field-effect transistors based on fluorinated isoindigo: High performance and improved ambient stability

Article abstract of DOI:10.1021/ja310283f

Ambipolar transport behavior in isoindigo-based conjugated polymers is observed for the first time. Fluorination on the isoindigo unit effectively lowers the LUMO level of the polymer and significantly increases the electron mobility from 10^-2

Full text of DOI:10.1021/ja310283f

Communication
pubs.acs.org/JACS
Ambipolar Polymer Field-Effect Transistors Based on Fluorinated
Isoindigo: High Performance and Improved Ambient Stability
Ting Lei,† Jin-Hu Dou,† Zhi-Jun Ma,‡ Cong-Hui Yao,† Chen-Jiang Liu,*,‡ Jie-Yu Wang,† and Jian Pei*,†
†Beijing National Laboratory for Molecular Sciences, the Key Laboratory of Bioorganic Chemistry and Molecular Engineering of
Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
‡Physics and Chemistry Detecting Center, the Key Laboratory of Oil & Gas Fine Chemicals of Ministry of Education, School of
Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, China
S
* Supporting Information
s−1. This represents the first ambipolar transport behavior for
isoindigo-based polymers. To the best of our knowledge, these
mobilities are the highest for an ambipolar FET fabricated under
ambient conditions.13
Recently, we demonstrated that modulating branched alkyl
chains in isoindigo-based polymers significantly increased the
hole mobility.7a The 4-tetradecyloctadecyl group was chosen as
the side chain of the fluorinated polymer. Scheme 1 shows the
synthetic route to polymers PFII2T and PII2T. Commercially
available 3-bromo-2-fluoroaniline was condensed with chloral
hydrate and hydroxylamine hydrochloride to afford compound 1,
catalyzed by sulfuric acid. In presence of conc. sulfuric acid, crude
1 underwent cyclization to provide 6-bromo-7-fluoroisatin (2) in
61% yield. A two-step Wolff-Kishner-Huang reduction was used
ABSTRACT: Ambipolar transport behavior in isoindigo-
based conjugated polymers is observed for the first time.
Fluorination on the isoindigo unit effectively lowers the
LUMO level of the polymer and significantly increases the
electron mobility from 10−2 to 0.43 cm2 V−1 s−1 while
maintaining high hole mobility up to 1.85 cm2 V−1 s−1 for
FET devices fabricated in ambient. Further investigation
indicates that fluorination also affects the interchain
interactions of polymer backbones, thus leading to
different polymer packing in thin films.
mbipolar polymer field-effect transistors (FETs), capable of
Aboth hole and electron transport, exhibit simpler processing to reduce the carbonyl group to methylene, giving 3. Direct
condensation of 2 and 3 in acetic acid afforded 6,6′-dibromo-
7,7′-difluoroisoindigo (4) in 72% yield. Initial attempts to
alkylate 4 with 15-(3-iodopropyl)nonacosane using K2CO3
failed, presumably due to the steric hindrance caused by
fluorines. A stronger base KOH was then used, and the reaction
proceeded smoothly at room temperature to give 5 in 88% yield.
A Stille coupling polymerization between 5 and 5,5′-bis-
(trimethylstannyl)-2,2′-bithiophene gave polymer PFII2T in
high yield. PII2T was prepared as a reference polymer to study
the effect of fluorination. Both polymers were purified by adding
a coordinating ligand to remove residual palladium catalyst, then
Soxhlet extraction to remove oligomers and other impurities.
Molecular weight of both polymers was evaluated by high
temperature gel permeation chromatography (GPC) with 1,2,4-
tricholorobenzene (TCB) as eluent at 140 °C. Both polymers
show comparable Mn (PFII2T, 75.7 kDa; PII2T, 72.8 kDa).
Nonetheless, the PDI of PFII2T (2.58) is larger than that of
PII2I (1.95). Both polymers show excellent thermal stability
with decomposition temperature over 410 °C. No phase
transition was observed for both polymers by differential
scanning calorimetry in the range of room temperature to 330
°C.
requirements in complementary-like circuits and potential
application in organic light-emitting transistors.1−3 Recently,
diketopyrrolopyrole (DPP) based ambipolar polymer FETs have
shown remarkable hole and electron mobilities (over 1 cm2 V−1
s−1).3,4 However, high hole/electron mobilities based on these
devices were achieved under nitrogen. Further investigation on
ambipolar polymers which operate in ambient is very important
for their practical applications,3,4 as well as fundamental
understanding of the carrier transport in conjugated polymers.5
Isoindigo has been used as the acceptor unit to construct low
band gap donor-acceptor copolymers for organic photovoltaics
(OPVs) and p-type FETs.6−8 The highly electron deficient
isoindigo unit endows these copolymers with intriguing
properties, such as broad absorption and high open circuit
voltage in OPVs,6 as well as high hole mobility and good ambient
stability in FETs.7,8 Nevertheless, isoindigo-based polymers have
extremely low electron mobility (the highest one is only 3.7 ×
10−7 cm2 V−1 s−1),9 albeit their low-lying LUMO levels (−3.6 to
−3.8 eV),6−9 thus limiting their application in n-type FETs10 or
as acceptors in OPVs.11
Herein, we introduce fluorine atoms onto the isoindigo units
of the polymer to further lower its LUMO level, intending to
improve the electron mobility of isoindigo-based polymer FETs.
Fluorination of the backbone of polymers can effectively improve
the performance of OPVs, because fluorine is electron-
withdrawing and small, hence capable of modulating the
electronic properties of polymers without deleterious steric
effects.12 To our delight, fluorinated isoindigo-based polymer
FETs fabricated in ambient show electron mobility up to 0.43
cm2 V−1 s−1, while maintaining hole mobility up to 1.85 cm2 V−1
Figure 1a illustrates the absorption spectra of both polymers in
dilute solution and in thin film. Both polymers show typically
dual absorption bands. After introducing fluorine atoms, the
charge transfer absorption band (Band I) of PFII2T red-shifts
obviously, whereas the Band II blue-shifts slightly and the
Received: October 17, 2012
Published: November 29, 2012
© 2012 American Chemical Society
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rotational angle of the fluorinated monomer is decreased from
22.2° to 17.1°, and the calculated F−H distance (2.27 Å) is
significantly shorter than the sum of the F−H van der Waals radii
(2.56 Å)14 (Figure 1b,c). This result indicates that the
introduction of fluorine atoms provides the polymer with a
planar backbone, which is consistent with the absorption
features.
Scheme 1. Synthesis of Fluorinated Isoindigo-Based
Polymers
a
Electrochemical properties of both polymers are explored by
cyclic voltammetry (CV) measurement. Both polymers show
much stronger oxidative peaks than their reductive ones (Figure
2a). After introducing fluorine atoms, the reduction current of
Figure 2. (a) Cyclic voltammograms of both polymers in thin film drop-
casted on a glassy carbon electrode. (b) Calculated molecular orbitals of
the trimmer of PFII2T (B3LYP/6-31G (d)).
PFII2T increased and the doping processes appeared to be more
reversible than those of PII2T. Compared with those of PII2T,
both HOMO and LUMO levels of PFII2T are obviously
decreased. The LUMO level of PFII2T measured by CV reaches
−3.88 eV, 0.18 eV lower than that of PII2T. In our previous
report, changing the donor part of isoindigo-based polymers
hardly affects the LUMO levels of polymers,8a because their
LUMO levels are mostly localized on isoindigo cores (Figure
2b). Herein, fluorination of isoindigo core effectively lowers the
LUMO level. The HOMO level is also lowered after the
fluorination, because the HOMOs of polymers are well
delocalized along the backbone. The CV results are consistent
with the photoelectron spectroscopy (PES) measurement. The
optical bandgaps and PES results give HOMO/LUMO levels of
−5.46/−3.96 eV for PFII2T and −5.31/−3.74 eV for PII2T.
Photophysical and electrochemical properties of both polymers
are in Table S1. Note that the low-lying LUMO level of PFII2T is
lower than several ambient-stable ambipolar polymers13 and
even comparable to some air-stable n-type polymers based on
perylenedicarboximide (PDI) and naphthalenedicarboximide
(NDI).10a Hence, ambient-stable charge transport in PFII2T is
expected.
To test the ambipolar transport capability of our isoindigo-
based polymers, a top-gate/bottom-contact (TG/BC) device
structure was used to fabricate polymer FETs. This device
configuration is preferred for ambient-stable ambipolar or n-type
organic materials, because of its better injection characteristics
and the encapsulation effect of the top dielectric layer.10b,13,15
The semiconducting layer was deposited by spin-coating
polymer solutions (6 mg/mL in DCB) on untreated Au
(Source-Drain)/SiO2/Si substrate. After thermal annealing the
film at optimized temperature (180 °C) for 10 min, a CYTOP
solution was spin-coated on top of the polymer film as the
dielectric layer, and an aluminum layer was thermally evaporated
as the gate electrode (Figure 3a). We fabricated the devices both
in glovebox and in ambient (RH = 50−60%) to compare their
device performance. All devices were tested in ambient on a
probe station. FET devices based on PFII2T fabricated in
glovebox and in ambient clearly showed ambipolar transport
a
Reagents and conditions: (a) NH2OH·HCl, CCl3CH(OH)2, H2SO4
(aq.), 130 °C; (b) conc. H2SO4, 70 °C, 61% for 2 steps; (c) (i)
N2H4·H2O (85%), EtOH, reflux; (ii) t-BuONa, EtOH, reflux, 56%; (d)
2, AcOH/HCl, reflux, 24 h, 72%; (e) 15-(3-iodopropyl)nonacosane,
powder KOH, DMSO/THF (1:1), 20 °C, 88%; (f) 5,5′-bis-
(trimethylstannyl)-2,2′-bithiophene, Pd2dba3, P(o-tol)3, toluene, 110
°C, 48 h, for PFII2T, 99%; for PII2T, 94%.
Figure 1. (a) Normalized UV-vis absorption spectra of both polymers in
CHCl3 (1 × 10−5 M) and in thin film. Molecular models of (b) PFII2T
and (c) PII2T fragments. Alkyl chains were replaced by methyl groups
for simplicity (geometries were optimized at B3LYP/6-31G(d) level).
PFII2T displays smaller dihedral angle (17.1°) and shorter F−H
distance (2.27 Å).
absorption intensity is decreased. Compared with those in
solution, absorption spectra of both polymers in thin film show a
slight red-shift and increased 0−0 vibrational absorption,
indicating that the polymer backbones become more planar in
the solid state. The bandgap of PFII2T in thin film estimated
from the absorption onset is 1.50 eV, 0.07 eV smaller than that of
PII2T. Interestingly, computational analysis of the fragment
structure reveals that the introduction of fluorine atoms does not
increase the dihedral angle of polymer backbones. Instead,
because of fluorine/hydrogen interaction, the phenyl-thienyl
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Figure 3. (a) Schematic diagram of TG/BC OFET device structure. (b) The transfer and (c) output characteristics of PFII2T devices fabricated in
ambient. OFET devices (L = 50 μm, W = 1000 μm) were fabricated with CYTOP around 500 nm thick (capacitance Ci = 3.7 nF cm−2).
Table 1. Summary of Molecular Weight, OFET Device Performance, and GIXD Results of Both Polymers
d
d (Å)
a
b
a
c
polymers
Mn (kDa)
PDI
2.58
μhole (cm2 V−1 s−1
)
VT (V)
log(Ion/Ioff)
μelectron (cm2 V−1 s−1
)
VT (V)
log(Ion/Ioff)
L
π
e
e
PFII2T
75.7
1.25 (1.13)
−40
−38
−35
−14
5−6
6−7
6−7
5−6
0.51(0.49)
+32
+30
+25
+36
5−6
5−6
5−6
3−4
29.0
3.55
f
f
1.85 (1.41)
0.43 (0.38)
e
e
PII2T
72.8
1.95
0.76 (0.66)
0.07 (0.06)
28.8
3.56
f
f
1.27 (1.44)
0.01 (0.004)
a
Measured in ambient conditions (RH = 50−60%). Maximum values of hole or electron mobilities and the average values are in parentheses (over 50
b
c
d
devices). Evaluated at VD = −20 V. Evaluated at VD = +20 V. Lamellar (L) and π−π stacking (π) distances determined by GIXD experiments.
e
f
Devices fabricated in glovebox. Device fabricated in ambient conditions.
characteristics. For devices fabricated in glovebox, hole mobilities
up to 1.25 cm2 V−1 s−1 and electron mobilities up to 0.51 cm2 V−1
s−1 were observed (Table 1), which is comparable to several high-
performance DPP based polymer FETs.2,4
In addition, all devices fabricated in ambient also exhibit
ambipolar properties. The hole mobility is further improved to
1.85 cm2 V−1 s−1 with a little decrease in electron mobility. The
highest electron mobility is up to 0.43 cm2 V−1 s−1, and the
average mobility is 0.38 cm2 V−1 s−1 (Figure 3). These mobilities
are the highest reported to date for an ambipolar FETs fabricated
in ambient.13 The reference nonfluorinated polymer PII2T
shows comparable hole mobilities but decreased electron
mobilities. Electron mobilities up to 0.07 cm2 V−1 s−1 are
observed for devices fabricated in glovebox, but decreased
sharply to 10−3−10−2 cm2 V−1 s−1 for devices fabricated in
ambient. Hence, fluorination can substantially increase the
ambient stability of the device.
The thin film microstructures and morphologies of both
polymers were investigated by grazing incident X-ray diffraction
(GIXD) and tapping-mode atomic force microscopy (AFM). In
the 2D-GIXD, both polymers appear to have dual textures in thin
films (Figure 4a,b). Films of both polymers display strong
lamellar textures with five out-of-plane (Qz) peaks as denoted by
(h00) Bragg diffraction. The (010) diffraction corresponding to
the π-π stacking distances is observed in both out-of-plane (Qz)
and in-plane (Qxy) diffraction. These results indicate that the
polymers have both edge-on and face-on packings in thin films.16
The measured lamellar and π-stacking distances are almost the
same for both polymers (Table 1) because of their similar
backbones and identical alkyl chains. However, the ratios of the
observed edge-on to face-on packing in film are significantly
different for the two polymers. Fluorinated polymer PFII2T
displays much stronger (h00) diffraction and weaker out-of-
plane (010) diffraction. In addition, the in-plane (200)
diffraction is only observed for PII2T. These results suggest
that PFII2T tends to form the edge-on packing, whereas PII2T
Figure 4. 2D-GIXD patterns of (a) PFII2T and (b) PII2T films.
Tapping-mode AFM height images of (c) PFII2T and (d) PII2T films.
Films were spin-coated from the DCB solutions of the polymers (6 mg/
mL) and annealed at 180 °C.
tends to form the face-on packing. Scherrer analysis of the (010)
diffraction shows that the crystalline coherence length of PFII2T
is 62.8 Å, considerably larger than that of PII2T (43.4 Å). As
illustrated in Figure 4c,d, the AFM images of both polymer films
show fibrillar intercalating networks. Crystalline zones similar as
other high-performance polymer FET materials17 are formed,
presumably due to strong intermolecular π-π interactions. In
particular, compared with the PII2T film, the PFII2T film has
stronger crystalline tendency with larger root-mean-square
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In conclusion, we have demonstrated for the first time that the
molecular engineering of the isoindigo core in isoindigo-based
donor-acceptor conjugated polymers can dramatically improve
the FET performance using these polymers as active layer.
Fluorinated isoindigo-based polymer PFII2T shows lowered
bandgaps and HOMO/LUMO levels. Ambipolar FETs based on
PFII2T can be fabricated and tested in ambient by solution-
process, and the electron mobility increases from 10−2 to 0.43
cm2 V−1 s−1 with high hole mobility up to 1.85 cm2 V−1 s−1.
Through fluorination, isoindigo-based polymer PFII2T main-
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starts to possess high electron mobilities. GIXD and AFM results
also indicate that the introduction of fluorine in electron-
deficient isoindigo cores leads to different interchain interactions
and stronger crystalline tendency. A combination of the
molecular engineering strategies toward isoindigo core (devel-
oped herein), donor units (developed previously by us)8a and
side chains (developed previously by Bao et al. and us)7 may
further improve the performance of organic electronics based on
these polymers.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details and characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
jianpei@pku.edu.cn; pxylcj@126.com
■
Notes
The authors declare no competing financial interest.
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Organic and Molecular Electronics; Kluwer Academic Publishers: New
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ACKNOWLEDGMENTS
■
This work was supported by the Major State Basic Research
Development Program (Nos. 2009CB623601 and
2013CB933501) from the Ministry of Science and Technology,
and National Natural Science Foundation of China. The authors
thank beamline BL14B1 (Shanghai Synchrotron Radiation
Facility) for providing the beam time.
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