High hole mobility of 1,2-bis[4′-(diphenylamino)biphenyl-4-yl]-1,2- diphenylethene in field effect transistor

Article abstract of DOI:10.1039/c1cc12011e

Triphenylamine-functionalized tetraphenylethene shows aggregation-induced emission feature with unity solid-state fluorescence efficiency. Its amorphous film can function in a p-type FET device with field effect mobility up to 2.6 × 10^-3 cm

Full text of DOI:10.1039/c1cc12011e

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COMMUNICATION
High hole mobility of 1,2-bis[40-(diphenylamino)biphenyl-4-yl]-
1,2-diphenylethene in field effect transistorw
Zujin Zhao,ab Zhefeng Li,c Jacky W. Y. Lam,a Jose-Luis Maldonado,d
Gabriel Ramos-Ortiz,d Yang Liu,a Wangzhang Yuan,a Jianbin Xu,e
Qian Miao*c and Ben Zhong Tang*a
Received 8th April 2011, Accepted 5th May 2011
DOI: 10.1039/c1cc12011e
Triphenylamine-functionalized tetraphenylethene shows aggregation-
induced emission feature with unity solid-state fluorescence
efficiency. Its amorphous film can function in a p-type FET
device with field effect mobility up to 2.6 ꢀ 10ꢁ3 cm2/Vs.
mobility of 2.3 ꢀ 10ꢁ3 cm2/Vs and a current on/off ratio of 105.
However, their linear analogue showed only inferior performances
with low mobilities (10ꢁ4–10ꢁ5 cm2/Vs).7 Recently, we succeeded
in creating a new AIE luminogen (2TPATPE or p-BTPATPE)
by capping the TPE core with TPA peripheries. OLED utilizing
p-BTPATPE as light-emitting and hole-transporting material
is constructed, which exhibits maximum luminance and
efficiencies of 33 700 cd/m2, 13.0 cd/A, 11.0 lm/W, and 4.4%.4d
Such good results are attributable to its efficient solid-state PL
and high hole mobility and stimulate us to investigate its FET
application in this work.
Design and synthesis of novel organic molecules for optoelectronic
devices, such as organic light-emitting diodes (OLEDs), field
effect transistors (FETs), and solar cells, have been a critical
research topic for several decades. In 2001, we discovered that
a series of propeller-like luminogens exhibit a novel pheno-
menon of aggregation-induced emission (AIE).1 An example
of such luminogens is represented by tetraphenylethene (TPE),
which is non-emissive in solution but emits intensely by aggregate
formation due to the restriction of intramolecular rotation.2,3
Since then, many AIE luminogens with different structures
and emission colors have been designed and prepared. By
melting AIE units with conventional chromophores at molecular
level, new AIE molecules with efficient solid-state photo-
luminescence (PL) and useful functional properties are generated,
from which OLEDs with outstanding performances have been
fabricated.4
p-BTPATPE was synthesized by Suzuki coupling of 1 with 2
catalyzed by Pd(PPh3)4 in basic medium followed by McMurry
reaction of the resultant product 3 in the presence of TiCl4 and Zn
dust (Scheme 1). Detailed procedures and characterization
data are given in the Electronic Supplementary Information.
Unlike the recent reported synthetic route, which was based
on Suzuki coupling of 2 with 1,2-bis(4-bromophenyl)-1,2-
diphenylethene,4d the present one can generate the desirable
product in a higher isolation yield.
p-BTPATPE shows an absorption maximum at 357 nm
with a molar absorptivity of 46 500 Mꢁ1 cmꢁ1 in dilute THF
solution (Fig. 1A). Its PL spectrum in THF (10 mM) exhibits
only noisy signals. The absolute fluorescence quantum yield
(FF) measured by a calibrated integrating sphere is as low as
1.8%, corroborating that p-BTPATPE is a genuinely weak
emitter when molecularly dissolved in solution. Addition of
poor solvents such as water into its THF solution has aggregated
its molecules and enhanced its emission (Fig. 2A). Clearly, the
emission of p-BTPATPE is induced by aggregate formation,
Triphenylamine (TPA) and its derivatives have been widely
studied as active materials for OLEDs and solar cells due to
their good PL and hole-transporting properties.5 However,
TPA-based materials have rarely been utilized to fabricate
FETs, perhaps due to their amorphous nature in the solid
state, which is unfavorable for efficient charge transport.5
Zhu et al.6 synthesized TPA-based macrocycles, whose FET
devices constructed from their amorphous films exhibited a
a Department of Chemistry, The Hong Kong University of Science &
Technology, Clear Water Bay, Kowloon, Hong Kong, China.
E-mail: tangbenz@ust.hk
b College of Material, Chemistry and Chemical Engineering,
Hangzhou Normal University, Hangzhou 310036, China
c Department of Chemistry, The Chinese University of Hong Kong,
Shatin New Territories, Hong Kong, China.
E-mail: miaoqian@cuhk.edu.hk
´
Centro de Investigaciones en Optica, A.P. 1-948, 37000 Leon, Gto.,
d
´
´
Mexico
e Department of Electronic Engineering, The Chinese University of
Hong Kong, Shatin New Territories, Hong Kong, China
w Electronic supplementary information (ESI) available: Experimental
section. See DOI: 10.1039/c1cc12011e
Scheme 1 Synthetic route to TPA-functionalized TPE.
c
6924 Chem. Commun., 2011, 47, 6924–6926
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Thus, the introduction of C60 into the p-BTPATPE-doped
PS film is to facilitate exciton dissociation and hence increase
the efficiency for charge carrier generation owing to its
strong electron-accepting capability.10 As anticipated, the
PS/p-BTPATPE/C60 composite exhibits a higher photocurrent,
which gives a better plot from which the charge mobility can
be measured. As shown in Fig. 3, the transient photocurrent
for holes increases as the bias voltage becomes higher. The detected
photocurrent is dispersive. Since the charge transportation in
amorphous films fabricated from organic materials occurs
via a hopping process, the dispersive behavior can be explained
by the presence of traps such as oxygen and water that may
arise during the film preparation.11 The transient time determined
from the logarithmic plot is 4.3 ms under an applied electric
field of 60 V/mm. The hole mobility is estimated to be
5.2 ꢀ 10ꢁ4 cm2/Vs, which is higher than that of N,N0-diphenyl-
N,N0-bis(3-methylphenyl)-1,10-biphenyl-4,40-diamine, a well-
known hole-transporting material, under the same measurement
conditions.10a
Fig. 1 (A) Absorption spectrum of p-BTPATPE in dilute THF
solution (10 mM) and (B) PL spectrum of thin film of p-BTPATPE.
in other words, it is AIE-active.4d The thin film of p-BTPATPE
also emits intensely at 509 nm (Fig. 1B). The associated
FF value is 100%, which is more than 2-fold higher than that
of unsubstituted TPE (49.2%), presumably due to the electronic
communication between the TPA and TPE units.
Cyclic voltammetry analysis of p-BTPATPE detects two
peaks at 1.00 and 1.41 V associated with the oxidation of its
TPA unit (Fig. 2B). The process is reversible and two corres-
ponding peaks are observed at 0.85 and 1.30 V in the backward
scan. The HOMO energy level of p-BTPATPE can be determined
from its onset oxidation potential (Eonsetox) based on the value
of ꢁ4.8 eV for ferrocene as internal standard with respect to
The field effect transistors were fabricated by depositing
60 nm-thick p-BTPATPE films onto silicon wafers. Before
deposition, the silicon surface was treated with a self-assembled
monolayer of octadecyltrichlorosilane (OTS) by thermal
evaporation under high vacuum. A layer of gold was then
applied on the top of the organic layer through a shadow mask
to form the top-contact source and the drain electrodes.
Doped silicon and 300 nm-thick silicon layer were used as
gate electrode and dielectrics, respectively. The p-BTPATPE
film is amorphous as suggested by the absence of sharp
reflections from its X-ray diffractogram. It is also confirmed
by its smooth morphology as observed under the atomic force
microscope (Fig. 4A). Transistors of p-BTPATPE with varied
channel length (50, 100 and 150 mm) and width (1 and 2 mm)
are tested in air under ambient conditions. Results show that
p-BTPATPE works as a p-type semiconductor, and the thin
film deposited on the OTS-treated silicon substrate at 80 1C
gives the best device performance. Fig. 4B shows the transfer
I–V curve of p-BTPATPE obtained under optimized conditions,
from which a field-effect mobility of 2.6 ꢀ 10ꢁ3 cm2/Vs
is obtained in the saturation regime using the equation:
IDS = (mWCi/2L)(VG ꢁ VT)2, where Ci is the specific capacitance
(11 nF/cm2) for the 300 nm-thick OTS-treated silicon wafer.12,13
ox
zero vacuum level.8 The Eonset of p-BTPATPE is found at
0.75 eV, from which a HOMO value of ꢁ5.15 eV is derived.
The HOMO energy level is close to the workfunction of
metallic gold (ꢁ5.1 eV), suggesting that p-BTPATPE possesses
good hole-transporting property, which is advantageous
for the fabrication of OLEDs and FETs.9 The band gap (Eg)
of p-BTPATPE can be determined from its onset absorption
wavelength and is equal to 2.9 eV. Its LUMO energy level is
thus calculated to be ꢁ 2.25 eV.
We used the time-of-flight (TOF) transient photocurrent
technique to investigate the hole-transporting property of
p-BTPATPE. p-BTPATPE-doped poly(styrene) (PS) films
(10 mm) with or without C60 were prepared and sandwiched
between two ITO-coated glass slides for the measurement.
Photogeneration and charge transport could be detected in
presence or absence of fullerene C60. The efficient PL emission
of p-BTPATPE in the solid state suggests that the probability
for exciton generation is high in its photophysical process.
Fig. 2 (A) PL spectra of p-BTPATPE in THF/water mixtures with
different water fractions (fw). Insert: photos of p-BTPATPE in pure
THF and THF/water mixture (fw = 99.5%) taken under the illumination
of a UV lamp. (B) Cyclic voltammogram of p-BTPATPE measured in
dichloromethane containing 0.1 M Bu4NPF6. Scan rate: 100 mV/s.
Fig. 3 Transient photocurrent for hole transport in p-BTPATPE/
PS/C60 composite (50/48.5/1.5 wt%) under an applied electric field of
60 V/mm. Inset: logarithmic plot of the photocurrent versus time.
c
This journal is The Royal Society of Chemistry 2011
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1/2
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OTS-treated silicon substrate. (B) Drain current (ꢁIDS) and (ꢁIDS
)
versus gate voltage (VGS) at a drain voltage (VDS) of ꢁ50 V in a FET
device of p-BTPATPE with a channel length (L) of 100 mm and a
channel width (W) of 1 mm.
On/off ratio of the drain current obtained between 0 and ꢁ50 V
gate bias from the transfer I–V curve is larger than 2 ꢀ 104. The
fact that the field effect mobility of p-BTPATPE is higher than the
hole drift mobility determined through the TOF measurement
can be attributed to different films used in the two measurements.
The mobility measured from a p-BTPATPE-doped PS film in the
TOF experiment is affected by many parameters such as the PS
matrix and the dopant concentration. It is also limited by the
efficiency of charge separation at the interface of p-BTPATPE
and fullerene. The measured field effect mobility of p-BTPATPE
is high for amorphous organic semiconductor films,14 and among
the reported values for TPA-based organic semiconductors.6,7,15
In summary, we synthesized an AIE luminogen p-BTPATPE by
attaching TPA peripheries to a TPE core. Whereas p-BTPATPE is
weakly emissive in solution, it is induced to emit efficiently in film.
p-BTPATPE possesses good hole-transporting property, as
revealed by its high HOMO energy level (ꢁ5.15 eV) and hole
mobility (5.2 ꢀ 10ꢁ4 cm2/Vs) measured by the TOF technique. A
p-type FET device utilizing amorphous film of p-BTPATPE
is fabricated, which shows high field effect mobility up to
2.6 ꢀ 10ꢁ3 cm2/Vs. Combining this with its efficient PL and
electroluminescence, p-BTPATPE is anticipated to find an array
of applications in organic electronics and optics.
The work reported in this paper was partially supported by
the Research Grants Council of Hong Kong (603509, 601608,
CUHK2/CRF/08, and HKUST2/CRF/10), the Innovation and
Technology Committee of Hong Kong (AoE/P-03/08), and the
National Natural Science Foundation of China (20634020).
Z.J.Z. acknowledges the financial support from the Initial
Funding of Hangzhou Normal University (HSQK0085) and
Natural Science Foundation of Zhejiang Province (Y4110331).
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Notes and references
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c
6926 Chem. Commun., 2011, 47, 6924–6926
This journal is The Royal Society of Chemistry 2011