A novel air-stable n-type organic semiconductor: 4,4′-bis[(6, 6′-diphenyl)-2,2-difluoro-1,3,2-dioxaborine] and its application in organic ambipolar field-effect transistors

Article abstract of DOI:10.1039/b608840f

Novel air-stable n-type organic field-effect transistors based on 4,4′-bis[(6,6′-diphenyl)-2,2-difluoro-1,3,2-dioxaborine] (DOB) have been fabricated. The devices exhibit a filed-effect mobility of 1 × 10^-4 cm² V^-1 s^-1, an on/off ratio of 10⁴ and a threshold voltage of 8.6 V at room temperature under ambient conditions. Moreover, ambipolar organic field-effect transistors based on DOB and copper phthalocyanine (CuPc) have been fabricated. Two device structures were adopted to investigate their transport properties. When devices were constructed with DOB as the first layer and CuPc as the second layer, they showed typical ambipolar transport properties. However, when the two layers were exchanged, the devices only showed p-channel transport properties. It is probable that CuPc, a bad electron transport material, blocks the electron transport to the DOB layer, leading to the disappearance of electron enhancement. The Royal Society of Chemistry.

Full text of DOI:10.1039/b608840f

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
A novel air-stable n-type organic semiconductor: 4,49-bis[(6,69-diphenyl)-
2,2-difluoro-1,3,2-dioxaborine] and its application in organic ambipolar
field-effect transistors
Yanming Sun,a Dirk Rohde,b Yunqi Liu,*a Lijun Wan,*b Ying Wang,a Weiping Wu,a Chongan Di,a Gui Yua
and Daoben Zhu*a
Received 21st June 2006, Accepted 7th September 2006
First published as an Advance Article on the web 3rd October 2006
DOI: 10.1039/b608840f
Novel air-stable n-type organic field-effect transistors based on 4,49-bis[(6,69-diphenyl)-2,2-
difluoro-1,3,2-dioxaborine] (DOB) have been fabricated. The devices exhibit a filed-effect
mobility of 1 6 1024 cm2 V21 s21, an on/off ratio of 104 and a threshold voltage of 8.6 V at room
temperature under ambient conditions. Moreover, ambipolar organic field-effect transistors based
on DOB and copper phthalocyanine (CuPc) have been fabricated. Two device structures were
adopted to investigate their transport properties. When devices were constructed with DOB as the
first layer and CuPc as the second layer, they showed typical ambipolar transport properties.
However, when the two layers were exchanged, the devices only showed p-channel transport
properties. It is probable that CuPc, a bad electron transport material, blocks the electron
transport to the DOB layer, leading to the disappearance of electron enhancement.
and several research groups have reported work in this
field.12–21 Due to the intrinsic properties of organic semi-
Introduction
Organic field-effect transistors (OFETs), whose characteristics
conductors, most OFETs only show unipolar properties, and
are modulated by an electrical field, are probably the most
it is normally difficult to obtain ambipolar OFETs charac-
prominent constituent of modern microelectronics.1 They have
teristics (showing both n- and p-channel properties) in a single
attracted increasing interest especially in the fields of flexible
layer device structure. Currently, ambipolar devices can be
and low-cost electronic applications, such as low-end display
fabricated by several methods, such as employing a blend of
two materials,12–14 a bilayer organic heterostructure15,16 and a
driving circuits, pliable electronic paper, radio-frequency
identification tags and smart cards.2–9 Among the organic
low-function metal, to balance the injection of holes and
electrons in one layer.19 While on the one hand the carrier
semiconductors, most materials tend to transport holes better
than electrons, thus there are more p-type materials, such as
mobilities of these devices are quite low, especially for
the linear acenes and oligothiophenes. For example, for
n-channel transport, on the other hand most of them are
rubrene, a tetraphenyl derivative of tetracene, a field-effect
unstable under ambient conditions. Despite this, air-stable
mobility of 15 cm2 V21 s21 has been obtained for a single
ambipolar OFETs based on blends of poly(benzobisimidazo-
crystal.10 Taking pentacene as the benchmark material in
benzophenanthroline) and copper phthalocyanine (CuPc),14
organic semiconductors, its mobilities above 1 cm2 V21 s21
and hexadecafluorophthalocyaninatocopper (F16CuPc) and
CuPc have been reported recently.20,21 Nevertheless, there
have been reported, which are close to the criterion for
amorphous silicon (a-Si:H) devices.11 However, on the whole,
are few examples of ambipolar OFETs that are stable in air.
the number of n-type materials is still limited and their
This is mainly due to the scarcity of air-stable n-type organic
performance has not up to now been satisfactory. Due to this
semiconductors, most of them being sensitive to O2 and H2O.
and the fact that high performance n-type semiconductors are
In this contribution, we have reported an air-stable compound,
important components of p–n junction diodes, ambipolar
4,49-bis[(6,69-diphenyl)-2,2-difluoro-1,3,2-dioxaborine] (DOB).
transistors, and complementary circuits, the development of
Furthermore, ambipolar organic field-effect transistors based
n-type materials is urgently required.
on DOB and CuPc have been fabricated and characterized.
Ambipolar OFETs are considered to be crucial for the
development of complementary circuits with a high degree of
Results and discussion
robustness, low power dissipation and a good noise margin.
The research on these has lately been gaining much attention
DOB was synthesised by modified methods according to
Scheme 1.22,23 1,6-Dihydroxy-1,6-diphenyl-1,5-diene-3,4-dione
was obtained under Claisen condensation conditions from
acetophenone and oxalic acid diethyl ester in a ratio of two to
one. For the complexation, the 1,6-dihydroxy-1,6-diphenyl-
1,5-diene-3,4-dione was dissolved in acetic acid. To a boiling
solution, an excess of boron trifluoride etherate was added
aKey Laboratory of Organic Solids, Institute of Chemistry, Chinese
Academy of Sciences, Beijing, 100080, People’s Republic of China.
E-mail: liuyq@mail.iccas.ac.cn
bKey Laboratory of Molecular Nanostructure and Nanotechnology,
Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100080,
People’s Republic of China. E-mail: wanlijun@mail.iccas.ac.cn
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Scheme 1 Synthesis of 4,49-bis[(6,69-diphenyl)-2,2-difluoro-1,3,2-dioxaborine] (DOB).
drop-wise. The product was crystallized from the hot solution
and then isolated by filtration. Finally, the product was
recrystallized from acetic acid.
Due to its high electron affinity,24 the dioxaborine is easy to
reduce. From cyclic voltammetric (CV) measurements in aceto-
nitrile, it shows two reversible reduction waves at 0.06 V and
20.44 V versus SCE. Therefore, the LUMO level of DOB
(about 4.4 eV) can be estimated from its CV results;25,26
a
value similar to that of F16CuPc.27 The latter is an excellent
air-stable n-type compound which is well known to us. By
comparison, the HOMO level of CuPc is 5.3 eV,19 which
matches well with the work function of Au (5.1 eV), favoring
hole injection from the electrode to CuPc. Due to the
mismatch in energy levels for DOB, an injection barrier of
0.7 eV for electron injection from Au to DOB is formed. We
note that this energy barrier can be overcome by applying a
perpendicular gate field according to ref. 12.
In order to investigate the electrical characteristics of DOB,
we fabricated a single-layer FET device with a top-contact
configuration. A highly n-doped Si wafer covered with
thermally oxidized SiO2 (470 nm) was used as the gate
electrode. A 50 nm thick DOB thin film was vacuum-deposited
at a pressure of 5 6 1024 Pa. Fig. 1 shows the plots of the
drain current (ID) versus drain-source voltage (VD) and
the drain current versus gate voltage (VG) characteristics for
the devices. From the output curves, good linear regions and
saturation regions can be seen. Only n-channel activity is
observed for the device, indicating electrons are the major
charge carriers in the conducting channel. According to the
equation below, the mobility in the saturation regime can be
calculated.
Fig. 1 The electrical characteristics of single-layer DOB FET devices.
(a) Drain current versus drain-source voltage as a function of gate
voltages. (b) The transfer curve at a drain-source voltage of 100 V and
the square root of the positive value of the current as a function of the
gate voltages.
ID = (W/2L)Cim(VG 2 VT)2
where m is the field-effect mobility, L and W are channel length
and width, respectively, Ci is the insulator capacitance per unit area
(10 nF cm22), and VT is the extrapolated threshold voltage. From
the equation, a mobility of 1 6 1024 cm2 V21 s21, an on/off ratio
about 104, and a threshold voltage of 8.6 V can be obtained with a
gate dielectric treated with octadecyltrichlorosilane (OTS) at room
temperature. Furthermore, after standing for one week under
ambient conditions, the devices show good stability without
substantial change in field-effect performance. The mobility only
varied from 1 6 1024 cm2 V21 s21 to 0.79 6 1024 cm2 V21
s21and the on/off ratio was almost unchanged. The introduction
of the strongly electron-withdrawing fluorine atoms may lower the
LUMO level and enhance the stability of the DOB molecules.
The thin films of DOB were investigated by X-ray
diffraction in reflection mode. Highly crystalline films were
observed, as seen in Fig. 2. The intensity of the first peak is
very strong, as is the second peak, indicating that the films
possess well-ordered, layered microstructures. From the XRD
Fig. 2 X-Ray diffraction patterns acquired for an evaporated thin
film of DOB.
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Fig. 3 AFM images of a DOB thin film evaporated on SiO2 surfaces
with OTS treatment at room temperature, with different dimensions
for (a) and (b).
data of the thin films, the interplanar d spacings can be
calculated. According to the results of the first-layer line, the
monolayer thickness is 0.8 nm. The length of the molecule is
about 1.6 nm, calculated using the PM3 method. This indicates
the molecule has a tilt angle of about 60u from the normal to
the substrate. The big tilt angle is not propitious to the
transport of charge carriers and as a result, the mobility is
moderate. Fig. 3 shows the AFM topographic images of DOB
thin films deposited on SiO2/Si with surfaces treated with OTS
at room temperature. Fig. 3a and Fig. 3b are the images of
DOB with dimensions of 2 mm 6 2 mm and 1 mm 6 1 mm,
respectively. Small crystal grains with an average diameter of
about 200 nm were observed in thin films, and the grains
showed terrace-like layered structures. The height of the two
layers is approximately 0.8 nm, which corresponds well to
the d spacing from the XRD results of thin films of DOB
described above.
Fig. 4 Schematic diagram of heterojunction OFETs and the chemical
structure of CuPc and DOB.
Employing DOB and CuPc as the n-type and p-type
materials, respectively, we have fabricated ambipolar FETs.
Two types of device structures were adopted to investigate the
ambipolar properties. The schematic diagrams of heterojunc-
tion OFETs are given in Fig. 4. DOB thin films 30 nm thick
and CuPc thin films 20 nm thick were vacuum-deposited under
a base pressure of 5 6 1024 Pa. Fig. 5 shows the ID – VD and
ID – VG plots of the device with structure (a) (Fig. 4a). Only
p-channel FET properties have been found from the output
curve. A mobility of 0.033 cm2 V21 s21, an on/off ratio of
about 105, and a threshold voltage VT of 21.2 V can be
obtained at a VD of 260 V. The mobility is among the best
displayed for polycrystalline CuPc. Normally, the mobilities of
this material are less than or similar to 1022 cm2 V21 s21 28–31
.
The high performance can be attributed to the extra
conductive channel at the interface of the heterojunction,
which results from the dipolar interface.32 However, for the
device with structure (b) (Fig. 4b), typical ambipolar
FET properties were found. According to the transfer
curve in Fig. 6, the electron and hole mobilities are 2.5 6
1026 cm2 V21 s21 and 2.2 6 1024 cm2 V21 s21, respectively.
Compared with the single-layer FET devices, the ambipolar
mobilities are lower by about one or two orders of magnitude.
This result indicates that the concentrations of charge
carriers decrease during transport, probably due to the
disordered interface effect of the two opposite polar com-
pounds. Further study to optimize the device fabrication
conditions is underway.
Fig. 5 The electrical characteristics of two-layer FET devices with
structure (a) (Fig. 4, DOB thin films on top of CuPc thin films). (a) The
output curve at different gate voltages. (b) The transfer curve at a
drain-source voltage of 260 V and the square root of the positive value
of the current as a function of the gate voltages.
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Experimental
Characterization
Elemental analysis (EA) was carried out using a Carlo Erba
model 1160 elemental analyzer. 1H NMR spectra were
recorded on a Bruker DMX 400 spectrometer. Chemical shifts
were reported as d values (ppm) relative to internal tetra-
methylsilane. X-Ray diffraction of thin films were performed
in the reflection mode at 40 kV and 200 mA with CuKa
radiation using a 2 kW Rigaku X-ray diffractometer. AFM
images of the thin films were obtained by using a Digital
Instruments Nanoscope IIIa in tapping mode under ambient
conditions. Cyclic voltammetry measurements were recorded
on an Autolab instrument PGSTAT 20. The measurements
were performed under N2 in acetonitrile containing Bu4NPF6
Fig. 6 The transfer characteristics of ambipolar OFETs.
as the supporting electrolyte with a scan rate of 0.1 V s21
,
using a stationary platinum working electrode, a platinum
counter-electrode, and a halogenated silver wire as the
reference electrode. Standard redox potentials have been
estimated using ferrocene–ferrocenium in acetonitrile as the
reference redox system.
As is common in ambipolar charge carrier transport, when a
low bias was applied to the gate electrode, unusual charac-
teristics were seen in the output curve. For p-channel
operation, at a low negative VG and a large negative VD, ID
increased abruptly due to electron enhancement at the drain
electrode. At the same time, holes accumulated at the source
electrode and a pn-junction was formed with the two charge
carriers, which is responsible for the increased ID current at
low gate voltage.12 As the VG increased, the electron enhance-
ment decreased gradually until the appearance of holes and
the output curve became normal. For n-channel operation,
an accumulation layer of holes was formed near the drain
electrode, and an accumulation layer of electrons was formed
near the source electrode. With increasingly positive VG, ID
decreased until the output became normal.
Synthesis of 4,49-bis[(6,69-diphenyl)-2,2-difluoro-1,3,2-
dioxaborine]
A solution of 5 g (0.04 mol) acetophenone and 3 g (0.02 mol)
oxalic acid diethyl ester in 300 ml toluene was stirred and
heated for 8 hours after the addition of 1 g sodium hydride.
After cooling to room temperature, the solution was added
to a mixture of water, ice and acetic acid. The organic layer
was separated, washed with water, dried over Mg2SO4 and
concentrated. The product was isolated by filtration (2.7 g),
and recrystallized from acetic acid to give 1,6-dihydroxy-1,6-
diphenyl-1,5-diene-3,4-dione as a yellow crystalline solid. The
yield of the crude product was 50%, mp 187–189 uC; 1H NMR
(400 MHz, acetone-d6) dH 8.14 (d, 4H), 7.71 (t, 2H), 7.61 (m,
6H); EA: Calc. for C18H14O4 [%]: C 73.4, H 4.7; found [%]: C
73.5, H 4.7.
In the above paragraph, we have mentioned that when CuPc
thin film is the first layer, it is only a p-channel FET action.
The reason why it only shows p-channel performance is
discussed below. For structure (a), when a positive bias was
applied to the gate, electrons should be induced to the interface
between DOB and CuPc. However, because CuPc is a bad
electron transport material, it blocks the transport of electrons,
even though VG increases to +100 V. This caused a leakage of
electrons and led to the absence of n-channel conduction.
Therefore, we only observe p-channel action.
To a boiling solution of 1.7 g (0.0057 mol) 1,6-dihydroxy-
1,6-diphenyl-1,5-diene-3,4-dione in 140 ml acetic acid, 1 ml
boron trifluoride etherate adduct was added. After addition,
the solution was boiled for a further two minutes. The product
crystallized from the hot solution, was isolated by filtration
(2.2 g), and was recrystallized from acetic acid followed
by sublimation to give bis(6,69-diphenyl-2,29-difluoro-1,3,2-
dioxaborine) as a yellow crystalline solid. The yield of the
crude product was 98%, mp 225 uC with decomposition,
labs (toluene) 414 nm, lfluoro (toluene) 556 nm; 1H NMR
(400 MHz, CDCl3) dH 8.3 (d, 4H), 7.87 (t, 2H), 7.68 (m, 6H);
EA: Calc. for C18H12O4B2F4 [%]: C 55.4, H 3.1, found [%]: C
55.2, H 3.1.
Conclusions
In conclusion, we have fabricated novel air-stable FET devices
based on DOB. The devices exhibit a field-effect mobility of
1 6 1024 cm2 V21 s21 and an on/off ratio of 104 at room
temperature. Furthermore, employing DOB and CuPc as
the n-type and p-type materials respectively, we have also
fabricated ambipolar FETs. Two device structures were
adopted to investigate the ambipolar properties. The devices
with DOB as the first layer and CuPc as the second layer show
ambipolar transport properties. However, when CuPc is the
first layer and DOB is the second layer, the devices only show
p-channel transport properties, which is probably due to the
fact that CuPc is a bad electron transport material, resulting in
the leakage of electrons and causing the disappearance of
electron enhancement.
Device fabrication
The devices were fabricated in a top-contact configuration.
The substrate is highly n-doped Si with a 470 nm layer of
thermally oxidized SiO2, which served as the gate electrode.
Gold was deposited through a shadow mask as the source and
drain electrode. The channel length L = 0.05 mm and the
channel width W = 3 mm. The characteristics of the OFETs
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12 E. J. Meijer, D. M. De Leeuw, S. Setayesh, E. Van Veenendaal,
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13 M. Shkunov, R. Simms, M. Heeney, S. Tierney and I. McCulloch,
Adv. Mater., 2005, 17, 2608.
were obtained at room temperature in air using a Hewlett–
Packard (HP) 4140B semiconductor parameter analyzer at
different gate voltages.
14 A. Babel, J. D. Wind and S. A. Jenekhe, Adv. Funct. Mater., 2004,
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Acknowledgements
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The authors gratefully acknowledge financial support from the
National Natural Science Foundation of China (NNSFC), the
Major State Basic Research Development Program and
the Chinese Academy of Sciences.
16 E. Kuwahara, Y. Kubozono, T. Hosokawa, T. Nagano,
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