A BDOPV-based donor-acceptor polymer for high-performance n-type and oxygen-doped ambipolar field-effect transistors

Article abstract of DOI:10.1002/adma.201302278

An electron-deficient building block BDOPV is developed to construct a new donor-acceptor conjugated polymer BDOPV-2T for high-performance n-type and oxygen-doped ambipolar polymer field-effect transistors. A high electron mobility up to 1.74 cm² V^-1 s^-1 is demonstrated under ambient conditions. Furthermore, the oxygen-doping effect and possible mechanism are discussed.

Full text of DOI:10.1002/adma.201302278

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A BDOPV-Based Donor–Acceptor Polymer for High-
Performance n-Type and Oxygen-Doped Ambipolar Field-
Effect Transistors
Ting Lei, Jin-Hu Dou, Xiao-Yu Cao,* Jie-Yu Wang,* and Jian Pei*
Due to their solution processability and mechanical robust-
ness, semiconducting polymers have been widely used for
high-throughput production of low-cost, light-weight, and
flexible devices.[ A powerful strategy to achieve high carrier
mobilities in semiconducting polymers is the development of
protons, maintaining good planarity and shape-persistency of
BDOPV, which may facilitate the overlap of intermolecular
frontier orbitals of the BDOPV segments in polymers. Indeed,
a second-generation polymer benzodifurandione-based poly(p-
phenylene vinylene) (BDPPV) based on BDOPV provided
1]
2
−1 −1
an alternating conjugated polymer containing electron-rich
remarkably high electron mobilities up to 1.1 cm V
s
under
donor) and electron-deficient (acceptor) units.[ Because of
2]
ambient conditions.
[13]
(
their successful application in high-performance organic field-
effect transistors (OFETs) and organic photovoltaics (OPVs),
these donor–acceptor polymers were named third-generation
polymers by Heeger.[ Recently, electron-deficient fused aro-
Previously, we significantly increased hole mobilities of II-
[
14]
based polymers by using longer branched alkyl chains
or
centrosymmetric donors (versus axisymmetric donors).[
15]
3]
Herein, we incorporate all the strategies by using the BDOPV
unit as the acceptor, a long branched 4-octadecyldocosyl group
as the side chain, and a centrosymmetric 2,2′-bithiophene unit
as the donor to synthesize donor–acceptor polymer BDOPV-2T,
[4]
matic rings such as diketopyrrolopyrrole (DPP), benzothia-
diazole (BT),[ isoindigo (II), naphthalene diimide (NDI),
5]
[6]
[7]
[
8]
and benzobisthiadiazole (BBT) have been used as acceptors
to construct donor–acceptor polymers, leading to the signifi-
cant development of polymer FETs. High hole mobilities over
[16]
a typical third-generation polymer (Scheme 1).
To our
delight, BDOPV-2T exhibits n-type transport behavior with
2
−1
−1
2
−1 −1
1
0 cm
V
s
under ambient conditions and electron
high electron mobilities up to 1.74 cm V
s
under ambient
2
−1 −1
mobilities over 1 cm V
s
in inert atmosphere have been
conditions. Upon oxygen exposure, this polymer displays inter-
esting ambipolar transporting behavior, which maintains high
achieved.[ Nevertheless, very few n-type polymers are known
9]
[10]
2
−1 −1
to operate under ambient conditions,
and they generally
electron mobilities up to 1.45 cm V s , along with signifi-
2
−1 −1
2
−1 −1
exhibit electron mobilities lower than 1 cm V s . Moreover,
oxygen (even short-time exposure) can significantly degrade the
device performance of these n-type FETs.[
cantly increased hole mobilities up to 0.47 cm V s . To our
knowledge, these electron mobilities are among the highest in
polymer FETs, and our work represents the first polymer exhib-
11]
2
−1 −1
Two methods are generally used to improve electron
mobility and device stability in polymer FETs: 1) lower the
LUMO level to stabilize electron transport and 2) enlarge the
intermolecular overlaps of LUMOs to facilitate electron hop-
ping.[ Using both methods, we recently developed a new
electron-deficient building block benzodifurandione-based
oligo(p-phenylene vinylene) (BDOPV), regarded as a deriva-
tive of oligo(p-phenylene vinylene) (Scheme 1).[ In BDOPV,
four carbonyl groups were introduced to give a low LUMO level
of –4.24 eV. Furthermore, these carbonyl groups formed four
intramolecular hydrogen bonds with the neighboring phenyl
iting electron mobilities over 1 cm V
s
for devices fabri-
cated under ambient conditions.
As illustrated in Scheme 1, BDOPV was synthesized from
2,2′-(2,5-dihydroxy-1,4-phenylene)diacetic acid (1) in two steps
12]
[13]
with moderate yield. Note that different functional groups can
be readily introduced on compound 3 to give other BDOPVs,
thus providing a facile route to modulate the electronic prop-
erties of BDOPV. A Stille coupling polymerization between
BDOPV and 5,5′-bis(trimethylstannyl)-2,2′-bithiophene gave
polymer BDOPV-2T in 94% yield after standard purification.
The molecular weight of BDOPV-2T was evaluated by high-
temperature gel permeation chromatography at 140 °C using
13]
1
,2,4-tricholorobenzene as the eluent. The polymer displayed a
T. Lei, J.-H. Dou, Dr. J.-Y. Wang, Prof. J. 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
high molecular weight with an M
n
of 77.2 kDa and a polydis-
persity index (PDI) of 3.00, and excellent thermal stability with
a decomposition temperature over 390 °C (Figures S1 and S2,
Supporting Information).
The absorption spectra of polymer BDOPV-2T in dilute
solution, thin film, and annealed film are shown in Figure 1a.
BDOPV-2T showed typical dual-band absorption, where Band I
is a typical charge transfer absorption from the thiophene unit
to the BDOPV core, because computational results showed that
its HOMO was well delocalized along the polymer chain; in
contrast, its LUMO was mostly localized on the BDOPV cores
(Figure 1b). The absorption maximum of BDOPV-2T in a film
Beijing, 100871, China
E-mail: jieyuwang@pku.edu.cn; jianpei@pku.edu.cn
Prof. X.-Y. Cao
College of Chemistry and Chemical Engineering
Xiamen University
Xiamen, 361005, China
E-mail: xcao@xmu.edu.cn
DOI: 10.1002/adma.201302278
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Scheme 1. Design and synthesis of polymer BDOPV-2T.
only showed a minimal red-shift in comparison to that in the
solution, and a slight increase of the 0–0 vibrational peak was
observed. Annealing the film led to a further increase of the
for 5 min, a CYTOP solution was spin-coated as the dielec-
tric layer, and an aluminum layer was thermally evaporated as
the gate electrode. Except for the aluminum layer which was
deposited under high vacuum, we fabricated the devices either
in a glovebox or under ambient conditions. Several annealing
temperatures were performed, and annealing at 200 °C gave
the best device performance (Table S2). All devices were tested
under ambient conditions (relative humidity, RH = 40–50%).
For devices fabricated in a glovebox, BDOPV-2T displayed
clearly n-type transport characteristics (Figure 2a and 2b), with
0
–0 peak. These results indicate that the polymer backbone in
the film becomes more planar and annealing is helpful for a
further adjustment of the polymer backbone. The bandgap
of BDOPV-2T is calculated to be 1.31 eV using the onset of
its thin-film absorption spectrum. The cyclic voltammetry
(
CV) measurement of BDOPV-2T gave HOMO/LUMO levels
of –5.72/–4.15 eV, consistent with the HOMO/LUMO levels
–5.66/–4.35 eV) estimated from photoelectron spectroscopy
2
−1 −1
(
high electron mobilities up to 1.74 cm V
s
and an average
mobility of 1.42 cm2
V
−1
s , which is the highest electron
−1
and the optical bandgap (Table S1 and Figure S3). Note that this
LUMO level is significantly lower than those of many previ-
due to the strong
electron-withdrawing ability of the BDOPV moiety.
mobility for polymer FETs tested under ambient conditions.
For devices fabricated under ambient conditions, the hole
mobilities of BDOPV-2T significantly increased. The highest
ously reported donor–acceptor polymers,[
2a,b]
2
−1 −1
The low-lying LUMO level of polymer BDOPV-2T indicates
that it is perhaps suitable for ambient-stable electron transport.
To achieve this goal, a top-gate/bottom-contact (TG/BC) device
configuration was used to fabricate polymer FETs, because
this device configuration has better injection characteristics
and encapsulation effects.[ The semiconducting layer was
hole mobility of 0.47 cm
V
s
and an average mobility
were obtained. In contrast, the highest
electron mobility of BDOPV-2T only slightly decreased to
of 0.20 cm2
V
−1
s
−1
2
−1 −1
2
−1 −1
1.45 cm V
s
(average: 1.20 cm V s ) (Figure 2c and 2d).
Note that the transfer characteristics of electron transport showed
negligible hysteresis even at a low drain voltage (VD = + 20 V),
presumably due to the much lowered LUMO level of BDOPV-2T.
Moreover, devices fabricated under ambient conditions dis-
played a larger threshold voltage in n-channel operation,
17]
−
1
deposited by spin-coating the polymer solution (3 mg mL
++
in 1,2-dichlorobenzene (DCB)) on a patterned n Si/SiO /Au
2
(source–drain) substrate. After thermally annealing the film
Figure 1. a) Normalized absorption spectra of BDOPV-2T in CHCl (1 × 10−5M), thin film, and annealed film (200 °C for 5 min). b) Calculated molecular
3
orbitals of the trimer of BDOPV-2T (B3LYP/6-311G(d,p)).
6
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Figure 2. a) Transfer and b) output characteristics of a BDOPV-2T device fabricated in a glovebox and tested under ambient conditions. c) Transfer and
d) output characteristics of a BDOPV-2T device both fabricated and tested under ambient conditions. Device configuration: TG/BC, L = 5 μm, W = 100 μm,
−2
C = 3.7 nF cm . e) The voltage transfer characteristic and gain of a complementary-like inverter fabricated with BDOPV-2T under ambient conditions.
i
suggesting some kinds of shallow traps were formed in the
film. To further explore the application of this ambipolar FET,
we also fabricated complementary-like inverters based on
polymer BDOPV-2T under ambient conditions. The inverter
consisted of two connected TG/BC ambipolar transistors with
three occupied molecular orbitals apparently increased after
exposing to oxygen, leading to the inactivation of hole traps and
[
21]
the reduction of the hole injection barrier.
To understand
the oxygen doping on a molecular level, we performed density
functional theory (DFT) calculations to model the interaction
between oxygen and BDOPV-2T (Figure 3). We found that the
oxygen and BDOPV-2T formed a stable complex through van
der Waals interactions. The complex exhibited a lower LUMO
level and the LUMO clearly delocalized onto the oxygen. These
results are in agreement with the device performance that some
a common gate as input (V ) and a common drain as output
IN
(VOUT). Obviously inverting functionality was observed for the
inverter based on BDOPV-2T, and the highest gain value up to
4
9 was obtained (Figure 2e). This gain value is also among the
[18]
highest in ambipolar FETs.
Time-dependent decay of the polymer devices was measured
by exposing the devices to air in a desiccator or under ambient
conditions (RH = 40–50%). Devices under both conditions did
not show any obvious difference, suggesting that water has neg-
ligible effects on the device performance. For devices fabricated
in a glovebox, we observed a gradual decay of electron mobili-
ties and an increase of hole mobilities (Figure S4). After one
2
day exposure, the electron mobilities dropped to around 1 cm
−
1
−1
−2
V
s
and the hole mobilities increased from 10 to around
.1 cm V s . The device performance is comparable to those
2
−1 −1
0
of the devices fabricated under ambient conditions. Prolonged
storage in air led to a further decrease of electron mobilities.
2
−1 −1
After 15 days, an electron mobility of 0.35 cm V
s
and a
hole mobility of 0.15 cm2
V
−1
s
−1
were obtained. Thus, we
propose that the significantly increased hole mobilities for
BDOPV-2T devices fabricated under ambient conditions were
largely attributed to the oxygen doping in the polymer film.
Oxygen doping has been widely investigated in inorganic
materials, which has a strong influence on their electrical
characteristics.[ Recently, a p-type small molecule, picene,
was reported to have a much higher hole mobility in air than
in vacuum.[ In this system, the energy levels of the highest
19]
Figure 3. Calculated energy levels and frontier orbitals for the monomer
and for the monomer–oxygen complex. Calculations were performed at
the M06–2X/6–311G(d,p) level.
20]
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For the ambient-fabricated devices, the
hole mobility of BDOPV-2T significantly
increased to 0.47 cm2
V
−1
s
−1
after oxygen
doping, whereas a high electron mobility
2
−1 −1
up to 1.45 cm V
s
was still maintained.
Furthermore, inverters with high gain values
up to 49 are also demonstrated based on the
oxygen-doped devices, suggesting the poten-
tial application of the polymer in complex
logic circuits. Our results represent the first
polymer that exhibits electron mobility over
2
−1 −1
1
cm V
s
for devices both fabricated and
tested under ambient conditions, thus sug-
gesting promising application of BDOPV-
based donor–acceptor polymers in semicon-
Figure 4. a) 2D-GIXD pattern and b) AFM height image of polymer BDOPV-2T film fabricated ducting materials that require both high elec-
under the same conditions as the FET device (200 °C annealing).
tron mobility and good ambient stability.
shallow traps were formed in the polymer film. Furthermore,
oxygen also caused the increase of the HOMO level, which
would reduce the hole injection barrier from the Au electrode,
and therefore increased the hole mobility. Very recently, Sir-
ringhaus et al. found that the HOMO level of polymers slightly
increased upon interacting with molecular oxygen,[ which
resulted in hole accumulation in the electrical characteristics.
Their results are consistent with our findings and support our
hypothesis that the significantly increased hole mobilities were
caused by oxygen doping of the polymer in the solid state.
To investigate the film microstructure and morphology,
grazing incident X-ray diffraction (GIXD) and tapping-mode
atomic force microscopy (AFM) were used (Figure 4). The
film of BDOPV-2T showed a strong out-of-plane diffraction
peak (100) at 2θ of 2.48°, corresponding to a d-spacing of
Experimental Section
Synthesis of BDOPV-2T: BDOPV (50 mg, 0.0289 mmol),
,5′-bis(trimethylstannyl)-2,2′-bithiophene (14.2 mg, 0.0289 mmol),
5
Pd (dba) (0.5 mg, 2 mol %), P(o-tol) (0.7 mg, 8 mol %), and 10 mL
2
3
3
of toluene were added to a Schlenk tube. The tube was charged with
nitrogen through a freeze–pump–thaw cycle three times. The mixture
was stirred for 24 h at 120 °C. N,N′-Diethylphenylazothioformamide
(10 mg) was then added and the mixture was stirred for 1 h to remove
any residual catalyst before being precipitated into methanol (200 mL).
The precipitate was filtered through a nylon filter and purified via Soxhlet
extraction for 8 h with acetone, 12 h with hexane, and finally collected
with chloroform. The chloroform solution was then concentrated by
evaporation and precipitated into methanol (200 mL) and filtered off to
afford a dark solid (47 mg, yield 94%).
22]
FET and inverter fabrication and characterization: TG/BC FET devices
and complementary inverters were fabricated using
+
+
n -Si/SiO2
(300 nm) substrates. The gold source and drain bottom electrodes (with
2
8.6 Å (λ = 1.24 Å) (Figure 4a). Another four orders of diffrac-
Ti as the adhesion layer) were patterned by photolithography on the SiO2
surface. The substrates were subjected to cleaning using ultrasonication
in acetone, detergent, deionized water (twice), and isopropyl alcohol.
The cleaned substrates were dried under vacuum at 80 °C. All the above
processes were performed under ambient conditions. The substrates
were transferred into a glovebox (for n-type) or directly used under
ambient conditions (for ambipolar). A thin film of the polymer was
deposited on the treated substrates by spin-coating at 2000 rpm for 60 s
tion peaks attributed to (h00) diffractions were also observed,
suggesting that a good lamellar edge-on packing occurred
in the film. Interestingly, the lamellar distance of 28.6 Å is
shorter than the molecular modeled length of the alkyl chains,
suggesting that the long alkyl chains were not fully extended
in the film. An in-plane (010) peak was also observed, which
was attributed to the π–π stacking distance. The measured π–π
stacking distance was 3.55 Å, slightly smaller than those of pol-
ymers with similar branched alkyl chains.[ The AFM height
image of a BDOPV-2T fi lm showed a crystalline fiber-like inter-
calating network with a relatively high root-mean-square (RMS)
deviation of 6.19 nm, presumably due to the strong interchain
interactions. These results indicate that BDOPV-2T has a strong
tendency to crystallize and undergo ordered interchain packing
in a film, which may largely contribute to its high charge car-
rier mobility.[
−1
using a polymer solution (3 mg mL in DCB), optionally followed by
thermal annealing at 140, 160, 180, 200, or 220 °C for 5 min. After the
deposition of the polymer thin film, a CYTOP solution (CTL809M:CT-
solv180 = 3:1) was spin-coated onto the semiconducting layer at
2000 rpm for 60 s, resulting in a 500 nm thick dielectric layer. The CYTOP
layer was then baked at 100 °C for 30 min in a glovebox or under ambient
conditions. Gate electrodes comprising a layer of Al (50 nm) were then
evaporated through a shadow mask onto the dielectric layer by thermal
14]
−
4
evaporation under high vacuum (10 Pa) for both n-type and ambipolar
devices. The evaluations of the FETs for both n-type and ambipolar
devices and inverters were carried out under ambient conditions
(RH = 40–50%) on a probe stage using a Keithley 4200 SCS as parameter
analyzer. The carrier mobility (μ) was calculated from the data in the
15]
In summary, a novel low-bandgap donor–acceptor polymer
BDOPV-2T has been developed for high-performance n-type
and oxygen-doped ambipolar polymer FETs. The giant aromatic
BDOPV unit, long branched alkyl chains, centrosymmetric
donors, and low-lying LUMO level of the polymer work syner-
2
saturated regime according to the equation ISD = (W/2L)C μ(VG – V ) ,
i
T
where ISD is the drain current in the saturated regime. W and L are the
semiconductor channel width and length, respectively. C (C = 3.7 nF)
i
i
is the capacitance per unit area of the gate dielectric layer. VG and VT
2
−1 −1
gistically to give high electron mobilities up to 1.74 cm V
s
are the gate voltage and threshold voltage, respectively. VG – V of the
T
under ambient conditions, which is among the highest electron
device was determined from the relationship between the square root of
ISD and VG at the saturated regime.
[23]
mobilities in both small-molecule and polymer-based FETs.
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Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
[
Acknowledgements
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.
Received: May 19, 2013
Revised: July 9, 2013
Published online: August 23, 2013
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