Hexathienoacene: Synthesis, characterization, and thin-film transistors

Article abstract of DOI:10.1002/asia.201000001

A new linear six thiophene-fused system of hexathienoacene (HTA) has been synthesized, which has also been compared with pentathienoacene (PTA) in terms of optical, electrochemical, and charge-transfer properties of organic semiconductors. The highest mobility of 0.06 cm² V^-1 s^-1 with a current on/off ratio of 105 on n-octadecyltrichlorosilane (OTS)-treated SiO₂/Si substrates has been achieved.

Full text of DOI:10.1002/asia.201000001

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DOI: 10.1002/asia.201000001
Hexathienoacene: Synthesis, Characterization, and Thin-Film Transistors
Ying Liu,[a, b] Xiangnan Sun,[a, b] Chong-an Di,[a] Yunqi Liu,*[a] Chunyan Du,[a, b] Kun Lu,[a]
Shanghui Ye,[a, b] and Gui Yu[a]
During the past ten years, organic thin-film transistors
(OTFTs) based on organic p-conjugated molecules have at-
tracted much attention because of many advantages in con-
trast to conventional silicon-based transistors, for instance,
low fabrication cost, large-area manufacturing, and flexibili-
ty.[1] Linearly fused acenes and heteroacenes have attracted
considerable attention as the most viable materials for
OTFTs.[2] Pentacene, a member of the acene series of linear
aromatic systems, is the current benchmark semiconductor
for OTFTs. However, pentacene is unstable and degrades
rapidly under ambient conditions, because it is subject to
photoinduced decomposition in the 6- and 15-positions
owing to p-electron localization.[3] Hexacene, as higher poly-
acenes, was found to be extremely unstable in solution, pre-
sumably because of its higher reactivity toward dimerization
and oxidation.[4] The only characterization data reported to
date are a combustion analysis[5] and a UV/Vis spectrum,[6]
which has limited its proper study for applications. Oligo-
thienoacenes (nTAs: fully fused oligothiophenes) are recog-
nized as the thiophene analogues of acenes. The advantages
of these molecules have been attributed to their unique
structural features: All the sulfur atoms are positioned at
the molecular periphery, facilitating multiple short intermo-
lecular S···S contacts which increase the effective dimension-
ality of the electronic structure, with the result that they
may possess enhanced transport properties.
The charge carrier mobility in organic materials generally
increases with the extension of conjugation. For example,
hole mobilities of 0.12 and 3.0 cm2 VÀ1 sÀ1 have been report-
ed for thin films of tetracene[7] and pentacene,[8] respectively.
As for oligothienoacenes, our group has fabricated penta-
thienoacene (PTA) as active semiconducting layers for
OTFTs with good mobility, which indicates that materials
based on thiophene-fused systems are a promising class of
organic materials for their use in OTFTs.[9] Further exten-
sion of the conjugation results in hexathienoacene (HTA, a
six-thiophene-fused system, Scheme 1) or other higher fused
Scheme 1. Chemical structures of pentacene, pentathienoacene, hexacene,
and hexathienoacene.
oligothiophenes;[10] probably because of the lack of conven-
ient synthetic routes, devices utilizing these oligothienoa-
cenes have not been studied. Herein we report the synthesis,
characterization, and performance of a fully fused oligothio-
phene HTA as a new p-type organic semiconductor and also
compare the properties to those of PTA. Although the yield
of HTA (6.0%) was a little lower, as shown in Scheme 2,
the compounds bis(3-thienyl) disulfide (1)[11] and 3,6-dibro-
mothienothiophene (2)[12] can be operated in a large scale to
[a] Y. Liu, X. Sun, Dr. C.-a. Di, Prof. Y. Liu, C. Du, Dr. K. Lu, S. Ye,
Prof. G. Yu
Key Laboratory of Organic Solids
Institute of Chemistry
Chinese Academy of Sciences
Beijing 100190 (P.R. China)
Fax : (+86)10-62559373
synthesize
6-bis-(thiophen-3-ylsulfanyl)-thienoACHTUGNTERN[UNG 3,2-b]thio-
E-mail: liuyq@iccas.ac.cn
phene (3) in high yield. In this paper, on the basis of our re-
search interests in fused thiophenes for OTFTs,[9,13] we tried
to investigate the effects of increasing extension of the con-
jugation from PTA to HTA on the optical, electrochemical
properties, and charge-transfer properties of organic semi-
[b] Y. Liu, X. Sun, C. Du, S. Ye
Graduate School of Chinese Academy of Sciences
Beijing 100039 (P.R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201000001.
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Figure 1. Normalized UV/Vis absorption spectra and emission spectra in
THF solution.
Table 1), which indicates that introduction of a thiophene
unit leads to extended conjugation and decreased HOMO–
LUMO gap. A HOMO–LUMO gap of 3.10 eV was estimat-
ed from the absorption edge of 400 nm in dilute THF solu-
tion, which is close in energy to that of PTA (3.20 eV). For
the vacuum-deposited thin-film absorption spectrum of
HTA, the maximum absorption appears at 352 nm, slightly
blue-shifted relative to the solution phase value, indicating
that H aggregates were formed in the solid state.[15] On fur-
ther performing UV/Vis absorption measurements for a
HTA thin film deposited on quartz with similar initial opti-
cal densities (OD; Figure 2), no change of the intensity of
absorption was observed for five months, which proved the
high photostability of HTA.
Scheme 2. Synthetic routes of HTA.
conductors by comparison with the density functional theory
(DFT) study by Jiang and co-workers.[14]
HTA was synthesized as shown in Scheme 2. Compounds
1[11] and 2[12] were prepared according to the literature pro-
cedures. Compound 2 was lithiated with n-butyllithium
(nBuLi) in diethyl ether, and then reacted with 1 to give 3
as a white solid in 85% yield. Oxidative ring closure of 3 to
obtain HTA is a vital procedure. The solution of 3 in diethyl
ether lithiated with nBuLi at À108C was added to a vigo-
rously stirred suspension of anhydrous copper chloride
(CuCl2) at 08C in diethyl ether, which was employed to
afford the final product HTA in the yield of 6.0%. After pu-
rification by thermal gradient zone purification processes,
the product is a yellow solid and slightly soluble in hot tolu-
ene or chlorobenzene. Its chemical structure was determined
by high-resolution mass spectrometry (HRMS) and elemen-
tal analysis. The thermal property was measured by thermal
gravimetric analysis (TGA) performed under nitrogen with
the onset of weight loss at well over 3008C, demonstrating a
high thermal stability.
The optical and electrochemical data of PTA and HTA
are summarized in Table 1. The UV/Vis absorption spectros-
copy data for HTA were measured in THF solution with
many absorption peaks at 312, 324, 360, and 380 nm and the
emission peak at 416 nm (Figure 1). As expected, the maxi-
mum intensity wavelength of HTA is red-shifted compared
to that of PTA in both absorption and emission (listed in
Figure 2. The photostability of HTA thin film deposited on quartz.
Further insight into the electronic properties of HTA was
gained by cyclic voltammetry (CV) investigation at room
temperature. We used a conventional three-electrode cell
with a platinum wire counterelectrode, an Ag/AgCl refer-
ence electrode, and indium tin
oxide (ITO)-coated glass work-
ing electrode in 0.1m Bu4NPF6/
CH3CN solution at room tem-
Table 1. Optical and electrochemical properties of PTA and HTA.
[c]
Cmpd
labs[a] [nm]
lem[a] [nm]
labs[b] [nm]
Eg [eV]
EHOMO[d] [eV]
mTFT [cm2 VÀ1 sÀ1
]
perature. The CV measure-
ments were acquired by means
of an electrochemical method
described in the literature.[16]
The CV plots displayed three-
stage oxidation processes, and
PTA[e]
HTA
358
380
344
416
342
352
3.20
3.10
3.77[f]
3.53[f]
À5.33
À5.06
À5.30[f]
2.2ꢁ10À3
4.3ꢁ10À3
0.55[f]
0.48[f]
À5.20[f]
[a] Measurements performed in a dilute THF solution (ca. 10À5 m). [b] Measurements performed on vacuum-
deposited film. [c] Estimated from the onset of absorption in a dilute solution (Eg =1240/lonset eV). [d] Calcu-
lated using the empirical equation: EHOMO =À
ACHTUNGRTENN(NUG 4.40+Eox) eV. [e] All the data of PTA are cited in reference
[9]. [f] Density functional theory (DFT) calculations; see reference [14].
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Y. Liu et al.
COMMUNICATION
reduction potentials below zero voltage could not be ob-
served, as shown in Figure 3. However, this is an irreversible
process, as the film degrades at high potential and dissolves
flowing direction, and in that case the charge carriers would
transport easily with high mobilities.[18] The higher-order
peaks became weaker until disappearance with increasing
substrate temperatures (Tsub), which could be further con-
firmed by the following OTFT study. AFM images showed
that the morphology of thin films strongly depended on the
temperature of deposition. For the film on OTS/SiO2/Si or
PS/SiO2/Si, at low temperature, large and consecutive grains
were obtained, corresponding well with the high mobility.
However, when the temperature of the substrate was in-
creased further, the grains became small and the morpholo-
gies of the films became worse, which may be related to the
drop in mobility.
OTFT devices were fabricated in a top-contact configura-
tion with thin films of HTA as active organic semiconductor
layers deposited onto SiO2/Si, OTS/SiO2/Si, or PS/SiO2/Si.
All the devices showed typical p-channel transistor charac-
teristics under ambient conditions. The typical output and
Figure 3. Cyclic voltammogram of HTA in 0.1m Bu4NPF6 acetonitrile so-
lution.
transfer characteristics of HTA on OTS/SiO2/Si at Tsub
=
208C are shown in Figure 4. The main parameters of device
in solution. The first oxidation potential of HTA is 0.85 V
versus Ag/AgCl, which is lower than that of PTA (1.12 V).
The ionization potentials (EHOMO) of p-conjugated systems
can be estimated by using the equation EHOMO
(4.40+Eox) eV. The Eox value is the onset potential for oxi-
=
ÀACHTUNGTRENNUNG
dation for HTA of 0.66 V, corresponding to the estimated
EHOMO of À5.06 eV. The HOMO value is close to the DFT-
calculated value of À5.20 eV and higher than that of PTA
(À5.33 eV) reported by us. In line with the DFT calculation
of fused thiophenes, the HOMO energy increases while the
energy gap decreases along with the increase of one thio-
phene unit. HTA has low-lying HOMO energy levels and
large band gaps, which indicates that it is oxidatively more
stable than pentacene (EHOMO =À4.60 eV, Eg =2.21 eV).[17]
Moreover, the HOMO level of HTA of À5.06 eV matches
well with the work function of Au electrode (À5.10 eV),
which results in less traps and reduced contact resistance
and hence enhancement of the hole injection between the
electrode and semiconductor layer in OTFTs.
To better understand the relationship between the film
morphology and the mobility, thin films of HTA on different
substrate surfaces were investigated by thin-film X-ray dif-
fraction (XRD) and atomic force microscopy (AFM). The
XRD profile of a vacuum-evaporated thin film of about
50 nm thickness deposited onto bare, n-octadecyltrichlorosi-
lane (OTS)-treated and polystyrene (PS)-treated SiO2/Si
substrates at different Tsub values. In all films, the first-order
reflection is very intense at 2q=5.988 (d spacing 1.476 nm)
with the second-order diffraction peaks at 2q=11.948
(d spacing 0.740 nm). The first of these are coincident with
the length of the molecules of HTA based on the extended
molecular lengths optimized by ChemDraw3D and calculat-
ed by Material Studio 3.0,[16a] and the others are its second
orders, indicating well-ordered, layered microstructures. It
suggests that the molecules of HTA are standing near up-
right on the substrate along their long axis, with the p–p
stacking direction parallel to the substrate with the current
Figure 4. The output (a) and transfer (b) characteristics of OTFT devices
based on HTA deposited on OTS-modified SiO2/Si substrate at Tsub
208C.
=
performance, that is, mobility (mTFT), threshold voltage (VT),
Ion/off at different Tsub values are summarized (see Table S1
in the Supporting Information). For data analysis, it is clear
that the OTFT performance strongly depends on both di-
electric surface treatment and substrate temperature value.
Film morphology variations with Tsub may play an impor-
tant role in mobility. The semiconductors based on HTA on
untreated SiO2/Si exhibit a mobility of 0.0022 cm2 VÀ1 sÀ1 at
a substrate temperature of room temperature, which shows
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Hexathienoacene: Synthesis, Characterization, and Thin-Film Transistors
Synthesis of hexathienoacene (HTA): To a stirred solution of sulfide 3
(2 g, 5.5 mmol) in anhydrous diethyl ether (100 mL) was added a 2.5m so-
lution of nBuLi in hexane (10 mL, 25 mmol) under a nitrogen atmos-
phere at À108C. The mixture was stirred for 2 h and then added to a vig-
orously-stirred suspension of anhydrous CuCl2 (3.38 g, 25 mmol) at 08C
in diethyl ether (30 mL). The mixture was allowed to warm and was
stirred at room temperature for further 12 h. After removal of diethyl
ether under reduced pressure, the residual was washed with dilute acid
(5% HCl), water, methanol, then with acetone three times to remove the
starting material. Further purification was carried out by sublimation
three times to give HTA (120 mg, 6%) as yellow solids. HRMS
(MALDI): m/z calcd for [C14H4S6]: 363.8637; found: 363.8639. Elemental
analysis (%) calcd for C14H4S6: C 46.12, H 1.11, S 52.77; found: C 46.04,
H 1.39, S 52.65.
a
bit lower charge mobility than that of PTA
(0.0043 cm2 VÀ1 sÀ1). The experimental results are in good ac-
cordance with the calculated mobility trend by DFT calcula-
tions (hole transfer mobility: HTA 0.48 cm2 VÀ1 sÀ1, PTA
0.55 cm2 VÀ1 sÀ1) whereby PTA displayed quite good OTFT
performance as p-type semiconductors of oligothienoacenes.
For the same device geometry of semiconductor evaporated
onto a SiO2/Si substrate fabricated by our group, the grain
sizes for PTA were on the order of 0.8ꢁ2.5 mm2 at Tsub
=
808C with the best mobility of 0.045 cm2 VÀ1 sÀ1 while the
grain sizes for HTA were much smaller at Tsub =1008C, on
the order of 0.2ꢁ0.2 mm2, with its best mobility of
0.0059 cm2 VÀ1 sÀ1. This difference of an order of magnitude
in mobility is due to the difference in grain sizes observed
by AFM images of the thin films when Tsub is changed, with
smaller grains and poorer surface coverage noted for HTA
than for PTA. It is demonstrated that the performances of
field effect transistors are not enhanced largely by using
HTA as semiconductor. Research interest has primarily fo-
cused on further extension of the conjugation, as in HTA,
resulting in low mobility relative to PTA. Modification of
the substrate with a self-assembled monolayer of OTS or PS
was found to greatly improve the device performance rela-
tive to untreated SiO2/Si substrates. The best performance
of mobility (0.06 cm2 VÀ1 sÀ1) and an on/off ratio of 7ꢁ105
were obtained at Tsub =208C on OTS-treated SiO2/Si. A mo-
bility of 0.03 cm2 VÀ1 sÀ1 and an on/off ratio of 2ꢁ103 were
obtained at Tsub =208C on PS-treated SiO2/Si.
Acknowledgements
The present research was financially supported by the National Natural
Science Foundation of China (20825208, 60736004, 60911130231,
20721061, 20973184, 60901050), the National Major State Basic Research
Development Program (2006CB806203, 2006CB932103, 2009CB623603),
and the Chinese Academy of Sciences.
Keywords: acenes
·
conjugation
·
density functional
calculations · sulfur · transistors
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AHCTUNGTRENNUNG
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Received: January 1, 2010
Published online: May 17, 2010
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