High-performance organic semiconductors: Asymmetric linear acenes containing sulphur

Article abstract of DOI:10.1021/ja066824j

Two new linear acenes with fused thiophene units have been synthesized. These acenes have conjugation lengths between anthracene and pentacene. Thin films of these linear molecules were characterized by ultraviolet spectroscopy, X-ray diffraction, atomic force microscopy (AFM), and field-effect transistor measurements. Submonolayer AFM studies show growth that greatly resembles pentacene, while thin-film growth is dendritic. Mobilites as high as 0.47 cm² V^-1 s^-1 have been found for the tetraceno[2,3-b]thiophene and are as high as 0.15 cm² V^-1 s^-1 for anthra[2,3-b]thiophene. Copyright

Full text of DOI:10.1021/ja066824j

Published on Web 11/16/2006
High-Performance Organic Semiconductors: Asymmetric Linear Acenes
Containing Sulphur
Ming L. Tang,† Toshihiro Okamoto,‡ and Zhenan Bao*,‡
Department of Chemistry and Department of Chemical Engineering, Stanford UniVersity, California 94305
Received September 21, 2006; E-mail: zbao@stanford.edu
Scheme 1. Synthesis of Compounds 1 and 2a
Organic semiconductors show great promise in various low-cost,
large-area, electronic applications. Currently, the best performing
molecule as the active material in p-type organic transistors is
pentacene,1-3 with high mobilities over 1.0 cm2 V-1 s-1 and decent
on-off ratios. However, pentacene is subject to rapid degradation
in ambient conditions, presumably to form the transannular en-
doperoxide4 or dimeric Diels-Alder adducts.5,6 As such, much
attention in the field has been focused on designing new organic
semiconductor candidates.7,8 The ideal candidate should be suitable
for solution processing, stable in ambient conditions, and possess
a (i) Anthraquinone for 1, naphthoquinone for 2, tetrabutylammonium
fluoride in THF, DMF; (ii) Al, HgCl2, CBr4, cyclohexanol.
high-charge mobility. Among the few examples of pentacene
derivatives are Anthony’s triisopropylsilyl(TIPS)-ethynyl substituted
pentacene9 and anthradithiophene derivatives. In addition to confer-
ring solubility on the parent molecule, the TIPS group enhances
packing by introducing π-stacking into the crystal structure,10 as
opposed to the previous herringbone packing. This creates a more
densely packed crystal structure, thus optimizing charge transport.
However, the synthesis of the anthradithiophene11 parent molecule
and its substituted version creates two isomers that are not easily
separable. This may contribute to the disorder in the thin film.
In this work, we present two asymmetric linear pentacene and
tetracene derivatives containing fused thiophene units. Compound
1, tetraceno[2,3-b]thiophene, is a reddish purple compound with a
conjugation length between tetracene and pentacene, while com-
pound 2, anthra[2,3-b]thiophene, is a bright yellow molecule with
a conjugation length between tetracene and anthracene.
Scheme 1 shows the synthesis of 1 and 2. The quinone precursor
of 1, tetraceno[2,3-b]thiophene-5,12-dione was made in one step
from a known compound, 2-(trimethylsilylmethyl)-3-(trimethylam-
monium)-thiophene iodide12,13 by a Diels-Alder reaction in 60%
yield. This precursor quinone was reduced to 1 in 87% yield.6 In
this study, 2 was synthesized in a manner analogous to 1 using
naphthoquinone instead of anthraquinone, but anthra[2,3-b]-
thiophene has been made before by condensation reactions.14 Both
molecules were purified by three-zone sublimation three times and
characterized by 1H NMR, mass spectrometry, elemental analysis,
and cyclic voltammetry. Thin-films were characterized by atomic
force microscopy (AFM), X-ray diffraction (XRD), and transistor
measurements.
Table 1. Oxidation Potentials of 1 and 2 vs SCE Reference by
Comparison to Ferrocene and Their Optical Bandgaps from Thin
Film, Determined at an Onset Absorption Peak
compound
EONSET (V)
E1 2 (V)
/
IP (eV)
optical EG (eV)
2
0.86
5.30
5.40
5.17
5.14
2.51
2.25
1.96
1.77
tetracene
1
pentacene
0.64
0.63
0.73
0.70
Table 2. OTFT Data of Tetraceno[2,3-b]thiophene, Averaged over
at Least Four Devices, Channel Length L ) 50 µm, W/L ) 20 (W:
Channel Width)
PVP
SiO2
OTS
temp (°C)
µ (cm2/Vs) 0.049 ( 0.0007 0.051 ( 0.001 0.10 ( 0.01 room temp
on/off
VT (V)
131
13
14
44
5.8 × 105
-14
µ (cm2/Vs) 0.054 ( 0.0004 0.062 ( 0.002 0.31 ( 0.11 60
on/off
VT (V)
58
20
26
40
1.3× 106
7
[2,3-b]thiophene decays to zero in 60 min (see Figures S-3 and
S-4, Supporting Information).
Thin films of 1 and 2 were evaporated on SiO2/Si, octadecyl-
trichlorosilane (OTS) treated SiO2/Si, and a spin-coated polyvi-
nylphenol (PVP) mixture with a copolymer3 cured at 200 °C on Si
to make organic thin film transistors (OTFTs) with top-contact
geometry.17 The average mobility of 1 is in the range of 0.04-
0.31 cm2 V-1 s-1, and varies with the surface treatment of the device
and substrate temperature, as shown in Table 2. The best perfor-
mance of 1 is on OTS/SiO2/Si at 60 °C with a mobility of 0.47
cm2 V-1 s-1, while the mobility on SiO2/Si and PVP/Si is an order
of magnitude less. Under similar device fabrication conditions,
purified pentacene in our hands gave a mobility of 0.5 cm2 V-1
The optical bandgaps of 1 and 2 and their electrochemical
HOMO levels are listed in Table 1 and compared with tetracene15,16
and pentacene. Structure 1 has a slightly higher oxidation potential
than pentacene. Its electrochemical stability is also monitored in
solution, where its long wavelength absorption peak outlives that
of pentacene. Performing UV-vis absorption measurements in a
dilute solution of o-dichlorobenzene with similar initial optical
densities (OD), we observed that the long-wavelength OD of
pentacene decays to nothing in 10 min, while that of tetraceno-
s-1
.
For 2, the mobility ranges from 1 × 10-5 to 0.15 cm2 V-1 s-1
,
with no transistor behavior observed on PVP/Si. The mobility is
high for OTS/SiO2/Si, consistently more than 0.1 cm2 V-1 s-1
.
Interestingly, on SiO2/Si, 40 nm films have a mobility of 1 × 10-5
† Department of Chemistry.
‡ Department of Chemical Engineering.
cm2 V-1 s-1, while 120 nm films show an improved performance,
9
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J. AM. CHEM. SOC. 2006, 128, 16002-16003
10.1021/ja066824j CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
diffraction observed. In the diffractograms of both 1 and 2, the
intensity of the diffracted peaks on different surfaces and substrate
temperatures corresponded with mobility. For instance, for 1, peak
intensities are higher for the films grown on OTS/SiO2/Si compared
to the film grown on SiO2/Si or PVP dielectric surface at both room
temperature and at 60 °C (see Supporting Information).
To conclude, we have synthesized two new asymmetric linear
acenes that show high charge mobility and stability. These new
conjugated cores can be further functionalized to make a variety
of small molecule and polymer based OTFTs, which may aid in
the understanding of thin film packing and mobility.
Acknowledgment. The authors thank Mark E. Roberts and
Jason J. Locklin for helpful lab assistance. M.L.T. acknowledges
financial support from a Kodak Graduate Fellowship. Z.B. ac-
knowledges financial support from the Air Force Office of Scientific
Research Grant FA9550-06-1-0126 and NSF Center for Polymer
and Macromolecular Assemblies.
Figure 1. Characteristic transfer curves of 1 and 2 on OTS/SiO2/Si at
60 °C and room temperature, respectively (L ) 50 µm, W/L ) 20), and the
corresponding AFM.
with a mobility of 0.006 cm2 V-1 s-1. This may be due to the grains
being more connected at the interface with the dielectric for thicker
films. For both 1 and 2, the on/off ratio is on the order of 106 for
the OTS treated surface but only about 102 for the other surfaces.
To probe the morphology of the molecules in an evaporated thin
film, we performed AFM scans. AFM images of submonolayer
films of 1 on bare SiO2/Si and OTS/SiO2/Si reveal grains as large
as 1 µm that greatly resemble that of pentacene.18,19 AFM images
of 50 nm films of 1 grown at room temperature on device wafers
show a dendritic growth characteristic of pentacene. Dendritic
growth is especially apparent on OTS/SiO2/Si and PVP/Si (see
Figure S-10). From room temperature to a substrate temperature
of 60 °C, 50 nm films of 1 show larger grains on all substrates,
corresponding well with the increase in the thin film mobility (see
Figure S-11). At 80 °C, 3D growth dominates on SiO2/Si and OTS/
SiO2/Si, resulting no working devices. AFM studies of the sub-
monolayer growth of 2 were difficult to perform because of the
high roughness of the films. Perhaps this is related to the low
molecular weight of 2, which encouraged desorption at room
temperature. However, the morphology of 40 nm films of 2 at room
temperature showed grains as large as 3 µm on SiO2/Si and OTS/
SiO2/Si, with 3D growth occurring much more prominently than
for 1. This is not unexpected, considering tetracene forms towers
in thin film and does not give evaporated films with good grain
connectivity and thus poor mobility.20 The 40 nm films of 2 at
room temperature on PVP/Si showed disconnected grains with high
walls at the boundary of the grains, commensurate with no mobility
measured on this surface. Substrate-molecule interactions seem
to be less favored than molecule-molecule interactions on the PVP/
Si surface. At 40 °C, nothing seems absorbed on SiO2/Si, while
the large disconnected grains exceeding 10 µm on OTS/SiO2/Si
account for nonworking devices made at this temperature (see
Figure S-12).
Supporting Information Available: Details of experimental
procedures, UV-vis, cyclic voltammagrams, table content of OTFT
device data and X-ray OOP d-spacings, X-ray diffraction graphs, AFM
images. This material is available free of charge via the Internet at
http://pubs.acs.org.
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