A bis(2-oxoindolin-3-ylidene)-benzodifuran-dione containing copolymer for high-mobility ambipolar transistors

Article abstract of DOI:10.1039/c3cc48695h

A bis(2-oxoindolin-3-ylidene)-benzodifuran-dione (BIBDF)-based low band gap polymer (PBIBDF-BT), containing a solubilizing alkyl chain bithiophene unit as a donor, has been synthesized. The polymer with a low-lying LUMO/HOMO energy level (-4.03/-5.55 eV) exhibits efficient ambipolar charge transport. The electron and hole mobilities are as high as 1.08 and 0.30 cm² V ^-1 s^-1, respectively. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3cc48695h

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A bis(2-oxoindolin-3-ylidene)-benzodifuran-dione
containing copolymer for high-mobility ambipolar
transistors†
Cite this: Chem. Commun., 2014,
0, 3180
5
Received 14th November 2013,
Accepted 16th January 2014
a
ab
b
b
a
Guobing Zhang,* Peng Li, Longxiang Tang, Jingxuan Ma, Xianghua Wang,
a
c
c
a
Hongbo Lu, Boseok Kang, Kilwon Cho and Longzhen Qiu*
DOI: 10.1039/c3cc48695h
www.rsc.org/chemcomm
A bis(2-oxoindolin-3-ylidene)-benzodifuran-dione (BIBDF)-based low gap (less than 1.8 eV) material with a deep LUMO energy level
band gap polymer (PBIBDF-BT), containing a solubilizing alkyl chain (Bꢀ4.0 eV) to reduce the injection barrier for both charge
7
bithiophene unit as a donor, has been synthesized. The polymer with a carriers and to achieve efficient ambipolar transport. Recently,
low-lying LUMO/HOMO energy level (ꢀ4.03/ꢀ5.55 eV) exhibits efficient donor–acceptor (D–A) low band gap copolymers have exhibited
8
ambipolar charge transport. The electron and hole mobilities are as very promising performances in OTFTs applications.
2
ꢀ1 ꢀ1
high as 1.08 and 0.30 cm
V
s
, respectively.
D–A polymer-based devices are reported to have hole mobilities
2
ꢀ1 ꢀ1
as high as about 10 cm V
s
and possess electron mobilities
. A D–A polymer constructed with alternate
electron-rich and deficient units could facilitate the formation of a
2
ꢀ1 ꢀ1 9
Organic thin-film transistors (OTFTs) based on conjugated over 1 cm V
s
1
polymers have undergone significant progress in recent years.
Most of the OTFTs to date show only unipolar behavior (either low band gap as well as a low HOMO energy level and result in high
holes or electrons). Ambipolar OTFTs showing both n- and p-type air stability. Moreover, the addition of strong electron-withdrawing
channel performance in one device are very important for applica- groups in the acceptor unit can reduce the LUMO energy level
tions in large-area high-performance complementary circuits at a which has been confirmed to be beneficial to the stability of charge
2
10
0
lower cost. The energy levels of the highest occupied molecular transport. (3E,7E)-3,7-bis(2-oxoindolin-3-ylidene)benzo-[1,2-b:4,5-b ]-
orbitals (HOMOs) and the lowest unoccupied molecular orbitals difuran-2,6(3H,7H)-dione (BIBDF) has a large fused aromatic ring
3
(LUMOs) are the key factors for the ambipolar semiconductors.
with a symmetric and planar structure, which would increase the
The semiconductors are required to have relatively low HOMO p–p overlap and intermolecular interaction. The lactone groups in
energy levels, down to ꢀ5.0 eV for hole transport. And for stable the central core coupled with the lactam groups make the BIBDF
11
electron transport, the LUMO energy levels as low as ꢀ4.0 eV unit strongly electron-deficient. If the BIBDF is introduced into
4
are required. So the energy gap of semiconductors should be the D–A polymer backbone as an acceptor unit, the polymer would
narrow enough to allow charge injection of both carriers. In fact, have lower LUMO and HOMO energy levels, thereby be suitable for
most of the common organic small molecules and polymers both hole and electron injection and transport.
show band gaps of above 2 eV, which result in high injection
The insoluble D–A polymer based on bithiophene and BIBDF
11a
barriers for either electrons or holes when the charges are units was reported by Li’s group. To resolve the insolubility of this
5
injected from a metal electrode with a given work function.
polymer, increasing the length of branched chains and/or introducing
The development of high performance ambipolar materials has solubilizing alkyl chains in the dithiophene unit are proved to
6
lagged far behind that of p-channel materials. To resolve this be effective approaches. During the preparation of this paper, Pei
problem, one of the effective ways is to synthesize a low band et al. reported a side-chain substituted BIBDF-based polymer with
12
a large branch group showing high mobility. Here, we indepen-
dently synthesized a conjugated polymer (PBIBDF-BT, Scheme 1) by
a
Key Lab of Special Display Technology, Ministry of Education,
National Engineering Lab of Special Display Technology, State Key Lab of
Advanced Display Technology, Academy of Opto-Electronic Technology, Hefei
University of Technology, Hefei, 230009, China. E-mail: gbzhang@hfut.edu.cn,
lzqiu@ustc.edu; Fax: +86 (0)551 62902572
b
c
Department of Polymer Science and Engineering, School of Chemistry and
Chemical Engineering, Hefei University of Technology, Hefei, 230009, China
Department of Chemical Engineering, Pohang University of Science and
Technology, Pohang, 790-784, Korea
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3cc48695h
Scheme 1 The synthetic route to a polymer.
3
180 | Chem. Commun., 2014, 50, 3180--3183
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level of PBIBDF-BT can rival the reported PDI (Bꢀ3.9 eV) and
1
c,14
NDI (Bꢀ4.0 eV) based n-type polymers.
For OTFT applications,
a deep LUMO energy level is beneficial to facilitate charge injection
and stable electron transport and a low-lying HOMO energy level is
beneficial to achieve stable hole transport and essential to resist air
oxidation and thus increase device stability.
The charge transport properties of PBIBDF-BT were investi-
gated with organic thin film transistors (OTFTs). PBIBDF-BT
showed typical ambipolar field-effect charge transport. The
preliminary device performances of PBIBDF-BT are listed in
Fig. 1 UV-vis NIR absorption spectra of PBIBDF-BT in chloroform and in
a thin film.
2
ꢀ1 ꢀ1
Table S1 (ESI†). An electron mobility of 0.28 cm V
s
was
measured for the OTFT devices without thermal annealing. In
this work, the thermal annealing was performed as an effective
method to optimize the device performance because thermal
annealing had the ability of increasing the molecular packing
order of polymer film, which could improve the charge
mobility of the semiconducting polymer. The polymer films
introducing the solubilizing alkyl chain grafted bithiophene unit as
the donor and the BIBDF unit as the acceptor. The OTFT devices
based on PBIBDF-BT as a semiconductor yield mobility as high as
9a
2
ꢀ1 ꢀ1
2
ꢀ1 ꢀ1
1.08 cm V
s
for electrons and 0.3 cm V
s
for holes. And
after three months air exposure, the mobility of the annealed devices
were subsequently annealed at different temperatures (100 1C,
2
ꢀ1 ꢀ1
2
ꢀ1 ꢀ1
still maintained at 0.73 cm V
electrons and holes, respectively.
s
and 0.035 cm V
s
for
150 1C, 175 1C and 200 1C) in glovebox under nitrogen ambient.
Fig. S1 (ESI†) shows the electron and hole mobilities of PBIBDF-BT-
based OTFT devices thermally annealed at temperatures ranging
from 100 to 200 1C. The corresponding device performances
annealed at these temperatures are summarized in Table S1 (ESI†).
The average field-effect electron mobility was increased from
Synthesis of monomers is shown in Scheme S1 (ESI†),
copolymer PBIBDF-BT was synthesized by the Stille coupling reac-
tion. The copolymer showed good solubility and the number-average
molecular weight was 38.6 kDa with a polydispersity index of 1.52.
The UV-vis-NIR absorption spectra of PBIBDF-BT in dilute chloro-
form solution and as thin film cast from chloroform are shown in
Fig. 1. In chloroform solution, PBIBDF-BT had a maximal absorption
2
ꢀ1 ꢀ1
0
.27 to 0.68 cm V
s
and correspondingly, the hole mobility
2
ꢀ1 ꢀ1
was increased from 0.06 to 0.24 cm
V
s . The threshold
voltage of the n-channel operation ranges from 24.7 V to 31.0 V,
abs
abs
peak (lmax) at about 811 nm and an absorption band-edge (lonset) at
while that of the p-channel mode is in the range from ꢀ21.8 V to
about 1039 nm, respectively. As a thin film, PBIBDF-BT had an
ꢀ
46.1 V. Device performance is optimized by thermal annealing
at 175 1C. The typical ambipolar output and transfer curves of
abs
abs
absorption l at 823 nm and a l
at about 1047 nm. Compared
max
onset
abs
to the solution, the lmax of polymer film was red-shifted about
the OTFTs based on PBIBDF-BT after thermal annealing at
12 nm, which indicated the improvement of the molecule ordering
1
1
75 1C, are shown in Fig. 3. The highest electron mobility is
due to the strong p–p* interchain interactions in the solid thin
2
ꢀ1 ꢀ1
2
ꢀ1 ꢀ1
.08 cm V
s
and the average mobility is 0.68 cm V
s .
1
3
opt
films. The optical band-gap (E
g
) of PBIBDF-BT is 1.18 eV as
estimated from the absorption edge (B1047 nm) of the polymer
ox
thin film. The onset oxidation potential (Eonset) and onset
red
onset
reduction potential (E
) of the polymer were located at
0
.84 V and ꢀ0.68 V (Fig. 2), respectively. The HOMO and LUMO
ox
energy levels of PBIBDF-BT were calculated from the Eonset to
be ꢀ5.55 eV and ꢀ4.03 eV, respectively. The very low HOMO–
LUMO energy level can be attributed to the electron-deficient
lactone and lactam groups on the BIBDF unit which remarkably
lowered the HOMO–LUMO energy level. The low LUMO energy
Fig. 3 Output (a, b) and typical transfer characteristics (c, d) of OTFTs
devices based on PBIBDF-BT at an optimized annealing temperature
of 175 1C.
Fig. 2 Cyclic voltammograms of PBIBDF-BT thin film.
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Meanwhile, the maximum hole mobility of 0.30 cm V
Communication
2
ꢀ1 ꢀ1
s
and attributed to the p–p interchain stacking. The calculated p–p
2
ꢀ1 ꢀ1
an average mobility of 0.24 cm V
s
were obtained. Therefore, stacking distance of PBIBDF-BT was 3.55 Å. The p–p stacking
the thermal annealing is confirmed to have a substantial influ- distance of PBIBDF-BT was even lower than those of reported
ence on charge transport. The PBIBDF-BT-based OTFT devices high performance DPP-based polymers, PDVT-10 (3.66 Å) and
9
a
exhibited excellent ambipolar performance. Moreover, all the PDVT-8 (3.78 Å). Such a small p–p distance between polymer
output curves of electron and hole transport showed negligible backbones suggested the strong intermolecular interaction,
hysteresis. After three months air exposure, the mobility of the which accounted for the p–p interchain stacking and the strong
2
ꢀ1 ꢀ1
annealed devices was still maintained at 0.73 cm V
s
and D–A interaction. Charge transport of the conjugated polymer is highly
2
ꢀ1 ꢀ1
0.035 cm V
s
for electrons and holes (Table S1 and Fig. S2, dependent on the intermolecular overlap integral. PBIBDF-BT,
ESI†), respectively. The PBIBDF-BT-based devices exhibited having a small p–p stacking distance that is beneficial to
good environmental stability which is in agreement with the improve the intermolecular overlap integral, is expected to
low LUMO and HOMO energy levels.
show a high charge carrier mobility.
Film morphology and crystallinity played an important role
In conclusion, the soluble low band gap D–A polymer based on
in OTFT device performance. The thin film morphologies and bithiophene as donor and bis(2-oxoindolin-3-ylidene)-benzodifuran-
microstructures of PBIBDF-BT were investigated using atomic force dione as acceptor was synthesized and characterized. A certain
microscopy (AFM) in the tapping-mode and grazing incident X-ray amount of large side chains are required to ensure the solubility
scattering (GIXD). Fig. 4 and Fig. S3 (ESI†) show that the AFM phase of the BIBDF-based polymer. Preliminary investigations on thin film
images of the polymer films annealed at different temperatures. transistor devices with polymer PBIBDF-BT were also carried out.
Dense nanofibrillar structures were observed over the entire area in The polymer PBIBDF-BT with a deep LUMO–HOMO energy level, a
PBIBDF-BT samples. The high mobility can be ascribed to the highly ordered lamellar structure and a small p–p stacking distance
nanofibers that form an interconnected polymer chain network showed excellent field-effect electron and hole transport. The BIBDF
and function as a highly efficient pathway for charge carrier trans- unit with its strong electron-deficient nature and a perfect
port throughout the polymer film. The as-cast PBIBDF-BT films planar p-conjugated structure may be a useful new monomer
exhibited much smaller fibrillar domains as compared to the films for the construction of low band gap conjugated polymers and
annealed at 175 1C, implying enhanced charge transport in the be applied in the high performance organic thin film transistor.
annealed films. Further increase of the annealing temperature to
The authors thank 3C beamlines (the Pohang Accelerator
200 1C resulted in obvious dewetting in the film, which was Laboratory in Korea) for providing the beam time. This work
responsible for the decline of field effect performance.
was supported by National Nature Science Foundation of China
The molecular ordering is one of the key factors determining (NSFC Grant No. 21204017, 51203039, 61107014, 51103034,
the device performance, and GIXD patterns of the PBIBDF-BT 21174036), Program for New Century Excellent Talents in Uni-
film without and with annealing at 175 1C, were also investi- versity (NCET-12-0839) and the Major State Basic Research Devel-
gated. As shown in Fig. 4 and Fig. S4 (ESI†), the diffraction opment Program of China (2012CB723406).
peaks of the annealed film were obviously enhanced, which is
consistent with the AFM results. The film of PBIBDF-BT showed
a strong out-of-plane diffraction peak (100) and also exhibited
well defined (200), (300), and (400) diffraction peaks along the
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