Light-emitting organic field-effect transistors based on highly luminescent single crystals of thiophene/phenylene co-oligomers

Article abstract of DOI:10.1039/c4tc00164h

A series of thiophene/phenylene co-oligomers containing π-conjugated anthracene, naphthalene, and biphenyl central cores have been developed as new organic laser active materials. Light-emitting organic field-effect transistors (LE-OFETs) based on 2,6-bis(5-phenylthiophen-2-yl)anthracene (BPTA), 2,6-bis(5-phenylthiophen-2-yl)naphthalene (BPTN), and 2,6-bis(5-phenylthiophen- 2-yl)-1,1′-biphenyl (BPTB) single crystals were fabricated. A clear laser oscillation was observed for BPTN and BPTB single crystals. Especially for BPTB, a low amplified spontaneous emission threshold of 1.8 ± 0.2 μJ cm^-2 was achieved. In addition, all of the devices showed ambipolar transport characteristics, in which both electron and hole carriers are transported in a single FET device, and electroluminescence was clearly observed. An obvious difference in the light emission direction was observed for LE-OFET devices, which is attributed to the difference in transition dipole moment arrangement of individual molecules within the crystal. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4tc00164h

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Materials Chemistry C
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Light-emitting organic field-effect transistors
based on highly luminescent single crystals of
Cite this: J. Mater. Chem. C, 2014, 2,
thiophene/phenylene co-oligomers†
abde
4918
bd
Takahiro Komori,ab Hajime Nakanotani,bc Takuma Yasuda
*
*
and Chihaya Adachi
A series of thiophene/phenylene co-oligomers containing p-conjugated anthracene, naphthalene, and
biphenyl central cores have been developed as new organic laser active materials. Light-emitting
organic field-effect transistors (LE-OFETs) based on 2,6-bis(5-phenylthiophen-2-yl)anthracene (BPTA),
2,6-bis(5-phenylthiophen-2-yl)naphthalene (BPTN), and 2,6-bis(5-phenylthiophen-2-yl)-1,10-biphenyl
(BPTB) single crystals were fabricated. A clear laser oscillation was observed for BPTN and BPTB single
crystals. Especially for BPTB, a low amplified spontaneous emission threshold of 1.8 ꢀ 0.2 mJ cmꢁ2 was
achieved. In addition, all of the devices showed ambipolar transport characteristics, in which both
electron and hole carriers are transported in a single FET device, and electroluminescence was clearly
observed. An obvious difference in the light emission direction was observed for LE-OFET devices,
which is attributed to the difference in transition dipole moment arrangement of individual molecules
within the crystal.
Received 24th January 2014
Accepted 12th April 2014
DOI: 10.1039/c4tc00164h
www.rsc.org/MaterialsC
rubrene single-crystal OFET shows excellent p-channel tran-
Introduction
sistor characteristics with a hole mobility (m) as high as 40
cm2 V sꢁ1 in the four-terminal measurement, and 18 cm2
V
Organic semiconductors have shown great potential as laser
active materials because of their attractive features, such as
high optical gain, broad emission spectra, and good proc-
essability. The realization of organic injection laser is a
challenging research subject in the eld of organic semi-
conductor devices, and various approaches have been
attempted.1–4 In particular, light-emitting organic eld-effect
transistors (LE-OFETs) are suitable for laser devices in terms
of their efficient electroluminescence (EL) properties and high
current capabilities.5 LE-OFETs have been extensively investi-
gated to achieve superior transistor characteristics, and the
basic properties for carrier transport and light emission have
been claried. High performance ambipolar LE-OFETs
strongly require high carrier mobility and luminescence effi-
ciency for their active materials. It has been reported that a
sꢁ1 in the two-terminal measurement.6 Despite its high
mobility, signicant reduction in the photoluminescence (PL)
quantum yield (FPL) is generally seen in such p-type organic
semiconductor systems, due to detrimental concentration
quenching.
In this context, we focus on thiophene/phenylene co-oligo-
mers (TPCOs) as a promising active material for LE-OFETs,
which can offer both high luminescence and charge transport
capability. With the recent seminal report by Hotta and
coworkers,7–9 TPCOs are of great interest as laser active mate-
rials. Stimulated emission behavior, including amplied
spontaneous emission (ASE),10–15 is observed in molecular
single crystals. In particular, LE-OFETs based on a single
crystal of 5,500-bis(1,10-biphenyl-4-yl)-2,20:50,200-terthiophene
(BP3T) have shown a high hole mobility of over 1.0 cm2 V sꢁ1
and an electron mobility of over 0.24 cm2 V sꢁ1 and a threshold
aDepartment of Automotive Science, Graduate School of Integrated Frontier Sciences,
Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan. E-mail: yasuda@
cstf.kyushu-u.ac.jp
bCenter for Organic Photonics and Electronics Research (OPERA), Kyushu University,
744 Motooka, Nishi, Fukuoka 819-0395, Japan. E-mail: adachi@cstf.kyushu-u.ac.jp
cInnovative Organic Device Laboratory, Institute of Systems, Information Technologies
and Nano-Technologies (ISIT), 744 Motooka, Nishi, Fukuoka 819-0395, Japan
dInternational Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu
University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan
for stimulated emission as low as 8.0 mJ cmꢁ2 5,16 In addition to
.
the high carrier transport ability, suitably well-aligned transi-
tion dipole moments in BP3T single crystals can promote a
high optical gain. These excellent photonic and electronic
functions prompted us to investigate the laser and LE-OFET
properties of new TPCO derivatives (Fig. 1). In this study, we
designed and synthesized a series of TPCOs incorporating
p-conjugated anthracene, naphthalene, and biphenyl cores
(BPTA, BPTN, and BPTB), and studied their ASE and LE-OFET
properties.
ePRESTO, Japan Science and Technology Agency (JST), Chiyoda-ku, Tokyo 102-0076,
Japan
† Electronic supplementary information (ESI) available: Synthesis details and
results of TD-DFT calculations. See DOI: 10.1039/c4tc00164h
4918 | J. Mater. Chem. C, 2014, 2, 4918–4921
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Table 1 Photophysical properties of BPTA, BPTN, and BPTB single
crystalsa
FPL
sPL
(ns)
lPL
lASE
Eth
Compound
(%)
(nm)
(nm)
(mJ cmꢁ2
)
BPTA
BPTN
BPTB
31
56
87
2.28
1.76
1.97
567
521
512
—
518
514
—
9 ꢀ 1
1.8 ꢀ 0.2
Fig. 1 Chemical structures of thiophene/phenylene co-oligomers.
a
Abbreviations: FPL ¼ PL quantum yield; sPL ¼ PL emission lifetime;
lPL ¼ PL emission peak at the 0–1 transition; Eth ¼ ASE threshold.
Results and discussion
Single crystals of BPTA, BPTN, and BPTB were grown by means
of physical vapor transport with a continuous ow of pure
argon gas. The sizes of individual single crystals range from
approximately 100 to 500 mm with the thickness of 200–300
nm. Fig. 2 shows the PL and ASE spectra of BPTA, BPTN, and
BPTB single crystals. Table 1 summarizes the photophysical
characteristics of each material. For the anthracene-based
BPTA single crystals, yellow-green PL with a FPL of 31% was
observed. In the naphthalene-based BPTN single crystals,
emerald-green PL with a FPL of 56% was observed. For the
biphenyl-based BPTB single crystals, a slight blue shi of the
PL peaks was observed compared with that of BPTN, giving rise
to a high FPL of 87%. In addition, it is noteworthy that BPTN
and BPTB display lasing activity in crystalline states. The ASE
spectra of BPTN and BPTB appeared at 518 nm and 514 nm,
respectively, suggesting that the 0–1 transition is the highest
gain for ASE. Fig. 3 presents the pumping energy dependence
of the peak full-width at half maximum (FWHM) of the ASE
spectra for the BPTN and BPTB single crystals. The ASE
threshold (Eth) for these single crystals was determined to be 9
ꢀ 1 and 1.8 ꢀ 0.2 mJ cmꢁ2, respectively. Upon UV irradiation,
intense uorescence emission was detected only from the
crystal edges in the BPTN and BPTB single crystals, whereas in
BPTA, both the crystal surface and edges were highly lumi-
nescent (Fig. 4). This result indicates that optical connement
inside the single crystals is attained for BPTN and BPTB, and
this conned propagation propensity is similar to the case of a
single crystal of 1,4-bis(5-phenylthiophene-2-yl)benzene (AC5)
reported previously.17
Fig. 3 Emission peak intensity and FWHM of PL spectra of (a) BPTN
and (b) BPTB single crystals as a function of incident photon density
(with 337 nm excitation).
Fig. 4 Fluorescence micrographs of (a) BPTA, (b) BPTN, and (c) BPTB
single crystals taken under UV irradiation (365 nm).
We also observed similar light propagation behavior in
emission from LE-OFETs based on the BPTA, BPTN, and BPTB
single crystals. The LE-OFETs were fabricated using the
following procedures (Fig. 5). A highly doped silicon wafer with
a 500 nm SiO2 layer was coated with a 30 nm thick poly(methyl
methacrylate) (PMMA; Sigma-Aldrich Co., average Mw ꢂ 930 00)
layer to remove electron traps on the surface of an SiO2 layer.
The substrates were heated at 70 ꢃC for 8 h, followed by
Fig.
2 Photoluminescence and amplified spontaneous emission
spectra of BPTA, BPTN, and BPTB single crystals.
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Fig. 5 Schematic representation of an LE-OFET based on an organic
single crystal.
annealing at 110 ꢃC for 1 h.18 The BPTA, BPTN, and BPTB single
crystals were laminated onto the PMMA-coated SiO2–Si
substrates in a globe box. Molybdenum oxide (MoOx) and
calcium were used as the source and drain electrodes, respec-
tively, to reduce the injection barrier for both the hole and
electron carriers.19
Electrical measurements of the devices were performed
under vacuum (<1.0 ꢄ 10ꢁ3 Pa) in the dark using a semicon-
ducting parameter analyzer (Agilent Co., E5273A). Fig. 6 shows
the gate voltage (VG) dependence of current (ID) between the
drain and source electrodes, and the photographs of light
emission from each device. All of the LE-OFET devices showed
ambipolar transport characteristics, in which electron and hole
carriers are transported within the single device. The hole and
electron mobilities were mh ¼ 1.4 ꢄ 10ꢁ1 cm2 V sꢁ1 and me ¼ 1.9
ꢄ 10ꢁ1 cm2 V sꢁ1 for BPTA; mh ¼ 2.6 ꢄ 10ꢁ2 cm2 V sꢁ1 and me ¼
4.6 ꢄ 10ꢁ2 cm2 V sꢁ1 for BPTN; mh ¼ 3.6 ꢄ 10ꢁ3 cm2 V sꢁ1 and me
¼ 1.6 ꢄ 10ꢁ2 cm2 V sꢁ1 for BPTB. Furthermore, bright EL was
clearly observed from the single-crystal devices. Yellow emis-
sion was observed from both the top surface and side edges of
the BPTA single crystal, whereas blue and green emissions were
detected only from the side edges of the BPTN- and BPTB-based
LE-OFETs, similar to the case of the ongoing optical excitation
experiments (Fig. 2). It is thus inferred that the difference in
light propagation and emission patterns of the devices should
be attributed to the variation of molecular orientation in single
crystals.
Fig. 6 Plots of ID–VG transfer characteristics of the devices based on
(a) BPTA, (b) BPTN, and (c) BPTB single crystals during n-channel
operation at VD ¼ 100 V (blue circles), and p-channel operation at VD
¼
ꢁ100 V (red circles). The insets represent optical photomicrographs
demonstrating EL emission from the devices. The channel width (W)
and length (L) are W/L ¼ 459/87, 240/92, and 129/71 mm for BPTA-,
BPTN-, and BPTB-based devices, respectively.
dependent density functional theory (TD-DFT) computations
To clarify the molecular orientation in crystals, X-ray (see ESI†). In general, light emission of organic molecules
diffraction (XRD) measurements were performed.20 Fig. 7 occurs along the direction perpendicular to the transition
depicts the out-of-plane XRD patterns of BPTA, BPTN, and BPTB dipole moment. For instance, in an LE-OFET based on a tetra-
single crystals. Predominant peaks assignable to the (00l) cene single crystal the transition dipole moment is parallel to
reections are obtained. For the BPTA single crystals, the rst the substrate, and hence light emission from the crystal surface
order diffraction peak with a d-spacing of 2.62 nm coincides (i.e., the direction perpendicular to the transition dipole
well with the molecular length of BPTA (2.59 nm), indicating moment) can be observed.21 In our study, the calculated tran-
that the molecules are arranged with their long axis perpen- sition dipole moments for BPTN and BPTB are parallel to the
dicular to the substrate to form a lamellar structure. Likewise, longitudinal molecular axes. The angle between the dipole
the XRD patterns of BPTN and BPTB single crystals exhibit a set moment and the molecular long axis is 1.5ꢃ and 0.1ꢃ for BPTN
of (00l) reections. The molecular lengths of these compounds and BPTB, respectively. Therefore, the EL emission from the
also agree with the d-spacing of the respective (001) peak, BPTN- and BPTB-based LE-OFETs was detected predominantly
indicative of perpendicular molecular orientation to the from the crystal side edges, as a result of effectively waveguided
substrate surface in single crystals.
emission parallel to the device substrate. In contrast, for BPTA
To assess the light emission direction, we calculated the the inclination angle of the transition dipole moment is calcu-
transition dipole moment for individual molecules by time- lated to be 11.5ꢃ with respect to the molecular long axis,
4920 | J. Mater. Chem. C, 2014, 2, 4918–4921
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(FIRST). T.Y. is grateful for a Grant-in-Aid for PRESTO (no.
10111) from JST, for Young Scientists (A) (no. 25708032), and for
Kyushu Univ, P&P. C.A. and T.Y. gratefully acknowledge the
support of the International Institute for Carbon Neutral Energy
Research (WPI-I2CNER), sponsored by MEXT, Japan.
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presumably because the local electric transition in the anthra-
cene core is polarized perpendicular to the molecular long axis.
Thus, the difference in light propagation and emission patterns
should be ascribed to the inclination of the transition dipole
moment to the longitudinal axis of the luminescent molecules,
which can also inuence their ASE activity in single crystals.
Conclusions
In this study, we have developed TPCO derivatives (BPTA, BPTN,
and BPTB) by incorporating anthracene, naphthalene, and
biphenyl cores as laser active materials, and fabricated LE-OFET
devices based on the BPTA, BPTN, and BPTB single crystals. A
clear laser oscillation is observed for BPTN and BPTB. A
signicantly low ASE threshold (1.8 ꢀ 0.2 mJ cmꢁ2) is obtained
for the BPTB single crystals. In addition, EL along with ambi-
polar transport can be induced in the LE-OFETs based on the
BPTA, BPTN, and BPTB single crystals. It has been found that
the direction of the transition dipole moment of the molecules
signicantly affects the light emission pattern as well as the ASE
characteristics in single-crystalline states. To attain high optical
gain and lower the ASE threshold, the control of molecular
orientation and transition dipole moment arrangement should
be crucial for single-crystal organic materials.
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Acknowledgements
This work was supported in part by the Funding Program for 21 T. Takahashi, T. Takenobu, J. Takeya and Y. Iwasa, Adv.
World-Leading Innovative R&D on Science and Technology
Funct. Mater., 2007, 17, 1623.
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