High-mobility anthracene-based X-shaped conjugated molecules for thin film transistors

Article abstract of DOI:10.1039/b911780f

New anthracene-based X-shaped conjugated molecules 1 and 2 for use as highly soluble p-type organic semiconductors were developed and exhibited large field-effect mobilities of 0.04 cm² V^-1 s^-1 (I_on/I_off

Full text of DOI:10.1039/b911780f

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High-mobility anthracene-based X-shaped conjugated molecules
for thin film transistorsw
Ki Hwa Jung,a Suk Young Bae,a Kyung Hwan Kim,a Min Ju Cho,a Kwangyeol Lee,a
Zee Hwan Kim,a Dong Hoon Choi,*a Dong Hoon Lee,b Dae Sung Chungb
and Chan Eon Park*b
Received (in Cambridge, UK) 16th June 2009, Accepted 14th July 2009
First published as an Advance Article on the web 28th July 2009
DOI: 10.1039/b911780f
New anthracene-based X-shaped conjugated molecules 1 and 2
for use as highly soluble p-type organic semiconductors
were developed and exhibited large field-effect mobilities of
0.04 cm2 VÀ1 sÀ1 (Ion/Ioff = 2.8 Â 105) and 0.24 cm2 VÀ1 sÀ1
(Ion/Ioff = 5.4 Â 106) for devices of 1 and 2, respectively.
conjugated molecules decorated by aliphatic end-groups,
which are shown in structures 1 and 2. The role of the aliphatic
end-groups is two-fold: (1) to increase the solubility of the
compounds, and (2) to induce lateral molecule-to-molecule
recognition within a single molecular layer. We find that the
additional side aliphatic group enhances not only the solubility
but also the degree of self-organization (i.e., better molecular
ordering). The proposed structure 2 would satisfy the above-
mentioned three requirements for highly efficient OTFTs.
Two X-shaped molecules, 1 and 2 (see Fig. 1A), were
synthesized with a yield of 71–87% by the Pd-catalyzed
Sonogashira coupling reaction of 2,6-dibromo-9,10-bis-
(phenylethynyl)anthracene and 2,6-dibromo-9,10-bis((4-hexyl-
phenyl)ethynyl)anthracene with 2-ethynyl-5-hexyl thiophene.
The detailed synthetic procedure and the structure
characterization are described in ESIw.
p-Type organic semiconductors, namely derivatives of pentacene,
rubrene, anthracene or thiophene, have attracted great
attention for their application in high-performance organic
thin film transistors (OTFTs).1–5 Recent developments in this
field have led to very high hole mobilities of the order of
1 cm2 VÀ1 À1. The deposition methods for thin film of these
s
molecules under vacuum or an inert atmosphere, however,
suffer from severe limitation in large-scale device fabrication
due to the complexity of the process. In this regard, the
alternative solution-based device fabrication approach using
soluble materials6–11 seems to be particularly promising,
provided the following conditions are met: (1) the organic
semiconducting molecules should be easily soluble in typical
organic solvents at room temperature, (2) the spin-cast films
should have high quality, and (3) the device should show high
mobility without any additional annealing process. These
conditions are found to be extremely difficult to satisfy.
Highly soluble, high mobility low molar mass molecules
such as [1]benzotheno[3,2-b]benzothiophene derivatives,
selenophene-containing heteroacenes and silylethynylated
polyacenes are not capable of achieving large-area devices
since their molecular weights are not sufficient for fabricating a
uniform film, with favorable interfacial contact and coverage
on the substrate.6–9 Although some soluble anthracene-based
semiconducting molecules have been reported already, they all
exhibited poor OTFT device performances.10–12 Herein we
report high-performance OTFT fabrication (hole mobility =
0.24 cm2 VÀ1
s
À1) solely based on a solution process.
The OTFTs are based on new anthracene-based X-shaped
a Department of Chemistry, Advanced Materials Chemistry Research
Center, Korea University, Seoul 136-701, South Korea.
E-mail: dhchoi8803@korea.ac.kr; Fax: +82-2-924-3141;
Tel: +82-2-3290-3140
b Organic Electronics Laboratory, Department of Chemical
Engineering, Pohang University of Science and Technology, Pohang,
790-784, South Korea. E-mail: cep@postech.ac.kr;
Fig. 1 (A) Structures of anthracene-based conjugated molecules 1
and 2. Inset: AFM topography of an as-spun film. (B) GI-XRD
patterns of the spin-coated thin film of 2. Out-of-plane (B) and
in-plane (C) diffraction intensities as a function of the scattering angle.
Inset: expected schematic crystallographic diagram of (a) the as-spun
film, and (b) the annealed film at 130 1C for 20 min.
Fax: +82-2-924-3141; Tel: +82-2-3290-3140
w Electronic supplementary information (ESI) available: Synthetic
details, instrumentation, OTFT device fabrication, thermal analysis,
CV data, tables, UV-Vis spectral changes with temperature, AFM
images, XRD data. See DOI: 10.1039/b911780f
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5290 | Chem. Commun., 2009, 5290–5292

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The molecules 1 and 2 were found to have good film
forming properties and are highly soluble in various organic
solvents such as chloroform, xylene, methylene chloride, and
chlorobenzene at room temperature. The compounds have a
solubility of 45 wt%, which is comparable to the most
soluble organic molecule, 6,13-bis(triisopropyl silylethynyl)
pentacene, typically employed in OTFTs.8
DSC measurement was performed at a heating (cooling)
scan rate of 10 (À10) 1C minÀ1 under nitrogen with the highest
temperature limited to below the decomposition temperature.
1 and 2 exhibit distinct crystalline–isotropic transitions in the
range of 25–250 1C (Tm of 1 = 153 1C, Tm of 2 = 164 1C).
TGA measurements at a heating rate of 10 1C minÀ1 under
nitrogen revealed that the molecules had good thermal stabilities
and have enhanced onset decomposition temperatures
(B409–426 1C) (see Table 1 and ESIw).
Fig. 2 UV-vis absorption spectra of 1 and 2. Film spectra (solid lines)
and solution spectrum (dashed line). The spectral change of the film
was monitored with temperature. The film sample was stored at each
temperature for 10 min. l2A À l1A = 26 nm; l2B À l1B = 59 nm, a
larger shift of lmax in the film of 2 indicates the larger degree of
intermolecular interaction.
Fig. 1A compares the AFM topographic images of 1 and 2.
As can be seen, 1 shows a less compact structure on the
surface with larger crystallites. A high coverage of the
substrate is achieved simply by spin coating of 2. It is thought
that the highly packed crystalline molecules on the
substrate can aid carrier transport owing to good connectivity.
Fig. 1B and 1C show diffractograms of a spun-cast sample
directions, and hence help to enhance the carrier transport
properties in TFT devices.
Fig. 2 shows the UV-VIS spectral variation of films of 1 and
2 with varying temperature, together with the solution spectra
of 1 and 2 (see Table 1). In both 1 and 2 (Fig. 2A and 2B), we
observe a significant red shift of absorption spectra upon film
formation, which indicates strong intermolecular interactions
amongst the molecules 1 and 2 in the films. The spectra of film
of 1 (Fig. 2A) drastically change with increasing temperature,
which shows an increase in molecular order with increased
temperature. On the other hand, the spectrum of the as-spun
film of 2 (Fig. 2B) does not significantly change with
temperature. This, together with the GI-XRD result of 2,
proves that the as-spun film prepared at room temperature
already has a fully organized molecular structure.
of
2 obtained by synchrotron grazing–incidence X-ray
diffraction (GI-XRD) measurements, which clearly indicate
the periodic/uniform molecular structures both in the in-plane
and out-of-plane directions. More specifically, the out-of-
plane XRD pattern of an as-spun 2 film on a Si/SiO2 substrate
shows fifth-order reflections at 4.25, 8.50, 12.85, 17.05,
and 21.051 indicating that the film has a highly crystalline
order along the out-of-plane direction. In Fig. 1C, clear
diffractions are observed in the in-plane XRD measurement,
also indicating high crystalline order. Detailed analysis
of the diffractograms shown in Fig. 1B and 1C leads us
to conclude that the molecules of 2 are oriented perpendicular
to the substrate in an edge-on manner (see inset of Fig. 1C).
Obviously, the presence of four peripheral hexyl moieties in
2 further strengthens the bilateral interlayer interactions
as compared with 1, thereby driving the formation of inter-
connecting microscopic grains.
In addition, the molecules of
1 also exhibited highly
anisotropic XRD patterns in the out-of-plane and in-plane
modes, indicating the edge-on orientation of the as-spun film
(see ESIw).
Because 1 and 2 possess very high solubility, thin-films were
easily fabricated on an octyltrichlorosilane (OTS)-treated SiO2
surface using a simple spin-coating method using a 41.0 wt%
chloroform solution (4000 rpm, 30 s). The source and drain
electrodes were prepared on the semiconductor surface via
thermal evaporation. Bottom-gate, top-contact TFT devices
of 1 and 2 were fabricated under ambient conditions without
thermal annealing, and the mobilities were obtained from the
source (S)–drain (D) current–voltage curves (IDS vs. VDS) in
well-resolved saturation regions. We obtain large field-effect
mobilities of 0.04 cm2 VÀ1 sÀ1 (Ion/Ioff = 2.8 Â 105) and
0.24 cm2 VÀ1 sÀ1 (Ion/Ioff = 5.4 Â 106) for devices of 1 and 2,
respectively (see Fig. 3 and Table 2). Note that mobilities
In most of the semiconducting molecules for devices,
thermal annealing is usually required in order to achieve high
TFT device performance. As we will show below, the molecule
2 shows high device performance without additional annealing
processes. We find that the diffraction patterns of samples
before and after the annealing process are nearly identical
(compare traces (a) and (b) in Fig. 1B), indicating the
structural stability of the as-spun film.
We hypothesize that the four peripheral hexyl groups in 2
cause the tight arrangement of the molecules along the lateral
Table 1 Measured and calculated parameters of 1 and 2
Film labsmax/nm
Energy level
Solution
em/nm
Before annealing After annealing
445, 478 510 484, 517
HOMO/eV LUMOc/eV
Eoxonset/eV
Solution labsmax/nm
l
l
cut off/nm Egopt/eV
1
2
414, 439 460, 491
417, 442 465, 492
512, 546 545a, 555b 2.27a, 2.23b 0.81
424, 452 503, 544 425, 466 506, 551 515, 551 575a, 580b 2.15a, 2.14b 0.81
À5.21
À5.21
À2.93a, À2.97b
À3.05a, À3.06b
a
b
c
opt
Before annealing. After annealing. HOMO (eV) À Eg (eV).
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molecular arrangement, and high solubility, can be fully
utilized for fabricating improved OTFT devices. Improved
design of related structures for optimized device performance
is being carried out currently.
This research work was supported by 21st century Frontier
Research Program (F0004091-0000-00). Synchrotron X-ray
experiments at PLS were supported by the Ministry of Science
and Technology and Pohang Steel Company.
Notes and references
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(C and D). Left: plot of drain current (IDS) versus drain–source
1/2
voltage (VDS). Right: plot of IDS and IDS versus gate voltage (Vg),
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V
DS = 80 V. The performances were measured at ambient conditions.
Table 2 OTFT device performances
Moleculea
W/L ratio
Ion/Ioff
Vth/V
m
TFT/cm2 VÀ1 sÀ1
1
2
10
15
2.8 Â 105
À11.8
À18.0
0.040
0.240
5.4 Â 106
a
Sample: as-spun film prepared at ambient conditions.
larger than 0.1 cm2 VÀ1 sÀ1 have rarely been achieved in
devices of as-spun films of organic semiconducting molecules.
The significantly larger mobility of 2 as compared with 1 is
attributed to the high degree of microstructural order of the
grains as well as the 2-dimensional molecular arrangement
within the grain associated with the molecular structure of the
peripheral hexyl groups.
In summary, we have developed new anthracene-based
X-shaped conjugated molecules 1 and 2 for use as highly
soluble p-type organic semiconductors. The as-spun film of 2
was intrinsically highly crystalline and the molecules showed
an edge-on orientation with regard to the substrate. Remark-
ably, the device bearing the as-spun film of 2 showed greater
enhancement of carrier mobility, which indicates that the
uniform lamella and p-stacking of the 2-dimensional
interactive molecule, 2, facilitates the carrier transport
phenomenon. Promising environmental stabilities of the new
molecules were observed in this work and a study on the
long-term stability of the device performance is in progress.
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p-channel OTFT devices reported thus far.
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¨
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Our study unambiguously demonstrates that the
anthracene-based X-shaped molecules with a high degree of
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5292 | Chem. Commun., 2009, 5290–5292