Organic thin film transistors based on 2,3-dimethylpentacene and 2,3-dimethyltetracene

Article abstract of DOI:10.1002/jccs.201200383

2,3-Dimethylpentacene (DMP) and 2,3-dimethyltetracene (DMT) were synthesized, characterized and employed as the channel material in the fabrication of thin-film transistors. The two methyl groups increase the chemical stability of the compounds versus the

Full text of DOI:10.1002/jccs.201200383

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
Organic Thin Film Transistors Based on 2,3-Dimethylpentacene and
2,3-Dimethyltetracene
Famil Valiyev,a Chi-Wei Huang,b Hwo-Shuenn Sheuc and Yu-Tai Taoa,*
aInstitute of Chemistry, Academia Sinica, Taipei, Taiwan, Republic of China
bDepartment of Chemistry, National Central University, Chung-Li, Taiwan, Republic of China
cNational Synchrotron Radiation Research Center, Hsinchu 300, Taiwan, Republic of China
(Received: Jul. 9, 2012; Accepted: Jul. 11, 2012; Published Online: ??; DOI: 10.1002/jccs.201200383)
2,3-Dimethylpentacene (DMP) and 2,3-dimethyltetracene (DMT) were synthesized, characterized and
employed as the channel material in the fabrication of thin-film transistors. The two methyl groups in-
crease the chemical stability of the compounds versus the pristine acene analogues. The crystals maintain
herringbone-like molecular packing, whereas the weak dipole associated with the unsymmetrical mole-
cule induces an anti-parallel alignment among the neighbors. This structural motif favors layered film
growth on SiO2/Si surface. Thin film transistors prepared on SiO2/Si and n-nonyltrichlorosilane-modified
SiO2/Si at different substrate temperatures were compared. DMP-based transistors prepared on rubbed
n-nonyltrichlorosilane-modified SiO2/Si substrate gave the highest field-effect mobility of 0.46 cm2/Vs,
whereas DMT-based transistor gave a mobility of 0.028 cm2/Vs.
Keywords: 2,3-Dimethylpentacene; 2,3-Dimethyltetracene; Organic thin film transistors;
Monolayer.
INTRODUCTION
less will change the crystal packing and thus the electronic
coupling between the molecules, which is believed to be
crucial to the charge transport between the molecules.8,9
Pentacene is a well-known and benchmark example that at-
tracts much attention for its high mobility and on/off ratio.
A reproducible mobility of > 1 cm2/Vs can be achieved in
thin film configurations in various laboratories.10,11,12 A lot
of efforts have been devoted to modifying the pentacene
framework. For example, 2,9-dimethylpentacene and
2,3,9,10-tetramethylpentacene have been shown to give
thin film transistors with high mobility. The introduction of
small electron-donating methyl groups presumably in-
creases the p-type behavior of these molecules. Yet substi-
tution at these symmetric positions does not change the
non-polar nature of these molecule, so that very similar
packing as pentacene itself was obtained.13,14 The replace-
ment of one end ring with a thiophene unit,15,16 which
breaks the symmetry of the molecule and imparts a dipole
to the molecule, may enhance the intermolecular packing
without major change in the molecular arrangement in the
crystal. Indeed similar herringbone packing was obtained
and reasonably good field-effect mobility was achieved
with this compound as the channel material. Another exam-
ple is the substitution of the two hydrogen atoms on the
central ring with two bulky triisopropylsilylethynyl groups,
Organic semiconductors are of scientific and techno-
logical interest because of their electrical properties and
potential use in low-cost, large-area electronic applications
such as in organic light-emitting diodes, organic field-ef-
fect transistors (OFETs), and organic photovoltaics.1 Dif-
ferent properties are desired in different applications
though. The most important material properties for appli-
cations in OFETs are high charge mobility, large on/off ra-
tio, low threshold voltage, besides stability, and process-
ability. In recent years, much progress has been made in de-
veloping oligothiophenes and acene derivatives,2-5 among
others, as promising p-type organic semiconductors in
OFETs.
While impressive increase in charge mobility has
been achieved, mainly through improved device fabrica-
tion techniques including optimization in film deposition
parameters, dielectric surface modifications and etc. to in-
fluence the film morphology and crystallinity.6,7 A quest
for more materials is always needed to push the intrinsic
molecular property to a higher level.
One strategy in molecular design is to modify a prom-
ising material by introducing various substituents at differ-
ent positions and seek for property enhancement as well as
performance improvement. These modifications neverthe-
* Corresponding author. Tel: +886-2-27898580; E-mail: ytt@gate.sinica.edu.tw
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1

Article
Valiyev et al.
which discourage the herringbone packing. Instead, a
shifted p-p stacking of the molecules resulting in a brick
wall-like structure in crystal was observed. Enhanced elec-
tronic coupling and good mobility were obtained.17 These
efforts have been reviewed recently.18
water. The cleaned silicon wafers were soaked into 1%
n-nonyltrichlorosilane (NTS) solution in dry toluene for 3
minutes to generate the NTS-modified silicon oxide sur-
face (NTS-SiO2/Si). The substrate surface turned from to-
tally hydrophilic to highly hydrophobic (with a water con-
tact angle ~102º-104º), as a result of the adsorption of a
self-assembled monolayer (SAM). Rubbing of the NTS-
SiO2/Si substrates with a rubbing machine (machine type:
LY-E, with the roller kept at 140 mm height scale and ro-
tated at 600 rpm) for two minutes in the same direction
served to generate a smoother surface.20 These samples are
designated as r-NTS-SiO2/Si. Thin films of DMP and DMT
were thermally evaporated under a vacuum of 1.33 × 10-3
Pa and deposited on the bare SiO2/Si, NTS-SiO2/Si, or
r-NTS-SiO2/Si surfaces at different substrate temperatures
in the same batch. The film thickness was monitored with a
quartz crystal microbalance and the deposition rate was
controlled at 0.1~0.3 Å/sec.
In this work, we report the synthesis and characteriza-
tion of 2,3-dimethylpentacene (DMP, 1) and 2,3-dimethyl-
tetracene (DMT, 2) and the fabrication of transistor devices
with these materials. We figured that by substituting hydro-
gen atoms at 2,3-positions for methyl groups is expected to
introduce a small dipole moment (e.g. calculated to be 0.76
Debye for 2,3-dimethylpentacene), which may enhance the
intermolecular packing through anti-parallel disposition
without major change in the molecular arrangement in the
crystal. The closer packing may improve the electronic
coupling and thus charge transport. It is noted that single
crystals of DMP had been used to fabricate FET devices to
explore the effect dielectric surface modification.19 Here,
thin film transistors were prepared on SiO2/Si surfaces. It
was shown that depending on the surface treatment of the
dielectric layer, bare SiO2, n-nonyltrichlorosilane (NTS)
monolayer-covered SiO2, or NTS monolayer pre-rubbed
before material deposition, the mobility varies signifi-
cantly. A high mobility of 0.46 cm2/Vs was obtained for
DMP and 0.028 cm2/Vs was observed for DMT on SiO2
covered with a rubbed monolayer of n-nonyltrichloro-
silane.
The powder X-ray diffraction pattern of DMP was re-
corded at the BL01C2 beamline of National Synchrotron
Radiation Research Center in Taiwan. The structure was
determined from the powder diffraction data with the pro-
gram DASH.21 Powder diffraction data were indexed with
Dicvol and Treor. The structure factors of the powder dif-
fraction patterns were extracted with Pawley’s method, and
simulated annealing was employed to determine the crystal
structure. The final refinement with the Rietveld method
was performed with the GSAS program.
Atomic force microscopic analyses were carried out
by using a Digital Instruments Multi-mode 3 atomic force
microscope. Images were captured by tapping mode with a
silicon tip at 300 kHz frequency. The powder X-ray diffrac-
tion was carried out by using the Philips X’Pert diffract-
ometer with X’Celerator detector. The Cu Ka radiation at a
power of 45 kV and 40 mA with a scanning step size of
0.008° was used.
2
1
EXPERIMENTAL SECTION
All starting materials were purchased from Aldrich,
TCI and Across. Tetrahydrofuran (THF) was distilled from
1
sodium benzophenone ketyl. H-Nuclear magnetic reso-
nance (NMR) spectra were recorded with Bruker AMX400
spectrometer, with proton chemical shifts (d) reported in
parts per million (ppm) relative to the methine singlet at
7.24 ppm for the residual CHCl3 in the deuteriochloroform.
Electron Impact (EI)-mass spectra were obtained on a
JMS-700 double focusing mass spectrometer (JEOL, To-
kyo, Japan). Elemental analyses were carried out on FlashEA
1112 Series CHNS-O Analyzer.
FET devices were fabricated on silicon wafers with
the 300 nm-thick SiO2 surface as the gate dielectric and the
underlying highly n-doped silicon as the gate electrode.
The 50 nm-thick gold source and drain electrodes were de-
posited on the organic films (top contact) through a shadow
mask. The channel length (L) and width (W) are 50 mm and
500 mm respectively. The electrical characteristics of the
devices were measured in air using a computer-controlled
Agilent 4156C Semiconductor Parameter Analyzer. The
field-effect mobility of the OFET device was calculated
from the saturation region according to equation 1:
The silicon wafers, with 300 nm thermally grown ox-
ide layer, were cleaned by Piranha solution (70% H2SO4:
30% H2O2) and rinsed thoroughly with ultra high purity
2
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Organic Thin Film Transistors Based on Acene Derivatives
Ids,sat = (WCi/2L) msat(Vgs-Vth)2
(1)
ature before 100 mL of water was added. The solid was fil-
tered off, washed with water, ethanol, EA and hexane, to
gave 1.95 g (68% yield) of 8,9-dimethyltetracene-5,12-
dione as an orange-yellow solid. MS (EI, 70 eV) 286.1
(M+, 100%). 1H NMR (400 MHz, CDCl3) d = 8.71 (s, 2H);
8.374-8.350 (dd, J = 3.36 Hz, J = 3.32 Hz, 2H); 7.82 (s,
2H); 7.799-7.776 (dd, J = 3.28 Hz, J = 3.32 Hz, 2H); 2.471
(s, 6H).
where Ci is the capacitance per unit area of the SiO2 insula-
tor, Vgs is the gate voltage and Vth is the threshold voltage.
1,2-Bis(bromomethyl)-4,5-dimethylbenzene (3)
4,5-Dimethyl-1,2-phenylenedimethanol (7.5 g, 0.045
mol) was added to 200 mL concentrated hydrobromic acid
(48%). The mixture was refluxed for 4 h., and then cooled
to room temperature, diluted with water, extracted with di-
chloromethane. Drying and evaporation of solvent gave 2
as a white crystal (13.2 g, 98% yield). M.p. 118-120 °C.
1H-NMR (400 MHz, CDCl3) d = 7.12 (s, 2H), 4.61 (s, 4H),
2.23 (s, 6H).
2,3-Dimethyltetracene (2)
LiAlH4 (0.95 g, 25 mmol) was added to a suspension
of 1.72 g (6 mmol) of 8,9-dimethyltetracene-5,12-dione in
100 mL of dry THF at 0 °C and the reaction mixture was
stirred for 3 h. at room temperature. Hydrochloric acid (6
M, 50 mL) was added slowly under cooling with ice. The
mixture was refluxed for 30 min. The residue was filtered,
washed with water, dried under vacuum and again treated
with 0.95 g (25 mmol) LiAlH4 and reacted with the same
procedure. The solid was filtered and washed with water,
ethanol, EA, and hexane. The crude product was purified
by vacuum sublimation at 230-250 °C (1.33 ´ 10-3 Pa), to
give 1.54 g., 85% yield of 2,3-dimethyltetracene as an or-
ange crystal. M.p. > 350 °C (dec.), MS (EI, 70 eV) 256.13
(M+, 100%), 1H-NMR (400 MHz, CDCl3) d = 8.592 (s,
2H); 8.502 (s, 2H); 7.964-7.331 (dd, J = 3.44 Hz, J = 3.20
Hz, 2H); 7.727 (s, 2H); 7.356-7.331 (dd, J = 3.12 Hz, J =
3.16 Hz, 2H); 2.454 (s, 6H). Anal. calc. C 93.71%, H
6.29%, Found: C 93.58%, H 6.38%.
2,3-Dimethylpentacene-6,13-dione (4)
Dibromide 3 (2.92 g, 10.0 mmol), 1,4-anthraquinone
(2.08 g, 10.0 mmol), and KI (16.6 g, 100 mmol) were dis-
solved in 50 mL of dry dimethylformamide (DMF) and
stirred at 110 °C for two days. The reaction mixture was
cooled to room temperature before 100 mL of water was
added. The solid was filtered off, washed with water, etha-
nol, acetone, ethyl acetate (EA) and hexane, to gave 2.18 g
(65% yield) of 2,3-dimethylpentacene-6,13-dione as a yel-
low solid. MS (EI, 70 eV) 336.1 (M+, 100%).1H-NMR (400
MHz, CDCl3) d = 8.921 (s, 2H); 8.814 (s, 2H); 8.121-8.097
(dd, J = 3.12 Hz, J = 3.16 Hz, 2H); 7.852 (s, 2H); 7.697-
7.673 (dd, J = 3.08 Hz, J = 3.12 Hz, 2H); 2.492 (s, 6H).
2,3-Dimethylpentacene (1)
LiAlH4 (0.95 g, 25 mmol) was added to a suspension
of 2.1 g (6 mmol) of 2,3-dimethylpentacene-6,13-dione in
100 mL of dry THF at 0 °C and the reaction mixture was
stirred for 3 h. at room temperature. Hydrochloric acid (6
M, 50 mL) was added slowly under cooling with ice. The
mixture was then refluxed for 30 min. The residue was fil-
tered, washed with water, dried under vacuum and again
treated with 0.95 g (25 mmol) LiAlH4 and reacted with the
same procedure. The solid was filtered and washed with
water, ethanol, EA, dichloromethane, and hexane. The
crude product was purified by vacuum sublimation at 310-
320 °C (1.33 ´ 10-3 Pa), to give 1.44 g., 75% yield of 2,3-di-
methylpentacene as a dark blue crystal. M.p. > 350 °C
(dec.), MS (EI, 70 eV) 306.14 (M+, 100%). Anal. calc. C
94.08%, H 5.92%, Found: C 94.04%, H 5.72%.
RESULTS AND DISCUSSION
Scheme I shows the synthetic route of DMP and
DMT. 1,2-Bis(bromomethyl)-4,5-dimethyl-benzene (3)
was obtained by the reaction of (4,5-dimethyl-1,2-phenyl-
ene)dimethanol22 with concentrated hydrobromic acid in
98% yield. A cyclization reaction of the dibromide 3 with
commercially available anthracene-1,4-dione or naphtha-
lene-1,4-dione yielded the quinones 4 or 5 in 65-70% yield.
Reduction of diketones 4 and 5 with LiAlH4 resulted16,22 in
DMP and DMT in 75-85% yield, respectively. The com-
pounds were purified through chain sublimation. In con-
trast to previous method23 of preparing substituted penta-
cene derivatives which requires ten steps, the method here
involves three steps from the starting bis(phenylene)di-
methanol to final products. Pure product was obtained
without resorting to column chromatography.
8,9-Dimethyltetracene-5,12-dione (5)
Dibromide 4 (2.92 g, 10.0 mmol), 1,4-naphthaquin-
one (1.58 g, 10.0 mmol), and KI (16.6 g, 100 mmol), were
dissolved in 50 mL of dry DMF and stirred at 110 °C for
two days. The reaction mixture was cooled to room temper-
DMP is a dark blue solid, slightly soluble in hot chlo-
roform, dichloromethane, chlorobenzene and 1,2-dichloro-
benzene. While it is stable in the solid state and can be vac-
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Article
Valiyev et al.
the same period of time. A new peak at 257 nm grew at the
expense of the long-wavelength absorption (Figure 2b).
Thus the introduction of two methyl groups improves the
chemical stability of the pentacene framework. Similar
measurements were made for DMT and tetracene (Figure
3). For tetracene, the intensity of the long-wavelength ab-
sorption at 276 nm decayed by 8% with a new peak grew at
236 nm after 2 h, but virtually no change in the intensity
over 3 h duration for DMT.
Scheme I Synthesis of DMP and DMT
Reagents and conditions: (a) HBr, reflux, 4 h., 98%; (b) Anthra-
cene-1,4-dione, KI, DMF, 48 h., 65%; (c) Naphthalene-1,4-dione,
KI, DMF, 48 h., 68%; (d) LiAlH4, THF, r.t., 3 h, HCl (20%), re-
flux., 30 min., 75% (1), 85% (2).
Crystal structure
Attempts to grow thick enough single crystals of
DMP and DMT for x-ray structure determination were un-
successful. The crystal of DMP was nevertheless ap-
proached by power x-ray diffraction with synchrotron radi-
ation. It was indexed as triclinic crystal system, with the
space group P-1. The crystal data are shown in Table 1, to-
gether with that of pentacene’s. Figure 4 shows the crystal
packing along the long molecular axis and projection from
top. The crystal packing looked down from the top of mole-
cule (Fig. 4b) clearly shows zigzag arrangement, thus in a
herringbone fashion similar to that of pentacene. Thus the
two methyl groups at the end do not alter the packing motif
much. Within the same layer (Fig. 4d), the unsymmetrical
molecules are anti-parallel to the nearest neighbors, pre-
sumably to minimize the electrostatic repulsion of the weak
dipoles associated with the molecules.
uum-sublimed, its solutions are unstable and bleached with
time in the ambient. Yet it is more stable than similar solu-
tion of pentacene (see below). DMT is an orange solid, sol-
uble in most organic solvents, including dichloromethane,
chloroform, THF and etc. and its solutions are stable for
more than 1 day.
Thermal analysis of compounds 1 and 2 were per-
formed under nitrogen. The weight loss of DMP began at
335 °C, whereas DMT started at 267 °C (Figure 1). These
temperatures are lower than that of corresponding acene
derivatives. Thus the introduction of two methyl group
does not improve the thermal stability.
The chemical stability of DMP and DMT was also
compared with that of pentacene and tetracene by monitor-
ing the time-dependence of UV-absorption spectra in solu-
tion. Figure 2a shows the UV absorption of pentacene in di-
lute dichloromethane. The absorption at 303 nm decayed
nearly completely in 40 min. In the mean time, a new ab-
sorption peak at 252 nm grew. It is known that pentacene in
solution tends to degrade upon exposure to air in the pres-
ence of light to give the peroxide.24 With DMP of similar
initial optical densities, one observed a 30% decay of the
intensity of the long-wavelength absorption at 305 nm over
Fig. 2. Time-dependent UV-vis spectra for (a) penta-
cene, and (b) DMP in a solution of dichloro-
methane.
Fig. 3. Time-dependent UV-vis spectra for (a) DMT
and (b) tetracene in a solution of dichloro-
methane.
Fig. 1. Thermograms of DMP and DMT.
4
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Organic Thin Film Transistors Based on Acene Derivatives
Table 1. Crystal data of DMP
domained SAM surface, reducing the number of nucleation
sites and favoring larger grains. The orderliness of the
SAM was also suggested to be important and crucial to
larger grain formation in a work using SAM prepared by
Langmuir-Blodgett technique.26 We prepared and com-
pared films deposited on substrates of bare SiO2/Si, the
n-nonyltrichlorosilane-modified surface (NTS-SiO2/Si)
and the NTS-SiO2/Si that has been rubbed (r-NTS-SiO2/Si)
before semiconductor film deposition. Figure 5 shows the
AFM images of 50 nm DMP deposited on these surfaces at
different temperatures. In general, for films prepared on the
same substrate, the grain size increases with increasing
substrate temperature. However, the grains deposited on
SiO2/Si surface at 80 °C appeared less connected than that
deposited on NTS-SiO2/Si substrates. At the same sub-
strate temperature, the grains on r-NTS-SiO2/Si are signifi-
cantly larger than that on unrubbed NTS-SiO2/Si surface,
and the grains on SiO2/Si are the smallest. The larger grains
show clear terrace structure and the section analysis shows
that the layered morphology has similar step heights around
1.67 nm. Similar observations were made for films of DMT
deposited on these surfaces, as shown in Figure 6. Section
analysis of the terraced structure shows a step height of
1.45 nm. However, at a substrate temperature of 80 °C, no
film was obtained, presumably due to the volatility of the
molecules (or a low sticking ability) at this substrate tem-
perature under the vacuum used.
DMP
Pentacene [4]
Space group
Z
Cell dimension a (Å)
b (Å)
P -1
4
P -1
2
5.9611(4)
7.7911(8)
33.4682(28)
92.480(21)
94.078(25)
90.762(7)
6.2660(10)
7.7750(10)
14.5300(10)
76.475(4)
87.682(4)
84.684(4)
c (Å)
a
b
g
Similar packing was observed for tetracenothiophene
molecules.16 Such a packing will favor layered film forma-
tion, as shown in atomic force microscope (AFM) images
below. One of the short axes is shorter than corresponding
axis for pentacene (5.961 Å vs 6.266 Å). The molecules
also tilt less than pentacene in the unit cell. Thus DMP ap-
pears to pack more closer with the introduction of methyl
substituents.
Thin film deposition and structure characterization
The charge transport in an organic film is critically
dependent on the morphology of the film, which is deter-
mined by the deposition conditions, including the vacuum,
substrate temperature, substrate surface treatment and etc.
“Hydrophobation” of silica surface by a SAM has been
shown to be advantageous to the hole mobility of semicon-
ducting films prepared on silica surface.25 Recently we fur-
ther showed that a rubbing treatment of a SAM of alkyl-
silane on silica resulted in much improved grain morphol-
ogy and crystallinity of pentacene film deposited on it.20
The rubbing process is suggested to smoothen the multi-
The films were also examined for their powder X-ray
diffraction. For DMP deposited on NTS-SiO2/Si and SiO2/
Fig. 5. AFM micrographs of DMP films deposited on
(a) NTS-SiO2/Si at r.t.; (b) NTS-SiO2/Si at 55
°C; (c) NTS-SiO2/Si at 80 °C; (d) r-NTS-SiO2/
Si at r.t.; (e) r-NTS-SiO2/Si at 55 °C; (f) r-
NTS-SiO2/Si at 80 °C; (g) SiO2/Si at r.t. (h)
SiO2/Si at 55 °C; (i) SiO2/Si at 80 °C.
Fig. 4. Crystal packing of DMP (a, b) and Pentacene
(c, d) viewed along the cell a axis and along the
long molecular axis.
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Article
Valiyev et al.
Si surfaces at different temperatures. Diffraction peaks at
2q of 5.28°, 10.54° and 15.84° were obtained at all sub-
strate temperatures and are assigned as the diffraction peaks
from the (002), (004), (006) planes respectively (Figure 7).
This indicates that the same crystal phase was obtained at
different deposition conditions. The d-spacing calculated
from (002) peak is 1.67 nm, which is half of the c-axis of
the crystal cell, suggesting molecules were standing near
upright on all surfaces. This is in agreement with the step
heights obtained from the AFM microscopy. This orienta-
tion will facilitate charge transporting in a horizontal de-
vice configuration since this is the direction of p-p stacking
of the herringbone arrangement along the conduction path-
way. The intensity of the diffraction peaks shows a clear
trend: higher with increasing substrate temperature on
NTS-SiO2/Si and r-NTS-SiO2/Si surfaces, and higher on
r-NTS-SiO2/Si than on NTS-SO2/Si surfaces if comparing
at the same deposition temperature. The higher intensity
correlates with improved crystallinity and grain size as
shown by AFM results. Similar observation was noted for
pentacene deposited on rubbed monolayer surface.20 It is
nevertheless noted that the diffraction peak intensities for
films deposited on bare SiO2/Si surface are in general much
lower than that on NTS-covered substrates. This could be
attributed to the lower crystallinity and/or lower thickness.
Films grown on bare SiO2/Si surface at 80 °C also gave an
intensity lower than that grown at 55 °C, a phenomena con-
trary to expectation. An examination of the AFM micro-
graphs suggests that the semiconducting molecules are less
adhering on SiO2/Si surface than on NTS-SiO2/Si surface.
Material loss is more significant at higher substrate temper-
ature. The AFM micrographs above also show that the
grains are less connected than that deposited on silane-cov-
ered surfaces.
Figure 8 shows the x-ray diffraction results for DMT
deposited on various substrates. Again a single crystalline
phase was obtained giving diffraction peaks at 2q of 6.09°,
12.28°, 18.45°,respectively. The d-spacing calculated from
the first peak is 14.5 Å, close to the thickness with a perpen-
dicularly oriented DMT on the surfaces and in agreement
with the section analysis in AFM analysis. Thus similar
packing as in DMP is suggested. Again, the diffraction
peak intensity is much reduced for films deposited on SiO2/
Si surfaces than on NTS-SiO2/Si surfaces. Also at a sub-
strate temperature of 55 °C, the peak intensity already be-
came lower than that deposited at room temperature. In
view of the absence of film formation on surface when the
substrate was kept at 80 °C, the decreased intensity may be
ascribed to partial material loss due to volatility of the
molecules at this temperature.
FET device fabrication and characterization
Characteristics of FET devices prepared from DMP
and DMT are collected in Table 2 and 3 with the representa-
tive curves for DMP provided in Figure 9. For DMP, the
best device performance was obtained for film deposited
on r-NTS-SiO2/Si at a substrate temperature of 80 °C. A
small anisotropy in the mobility was observed: 0.46 cm2/Vs
along the rubbing direction and 0.39 cm2/Vs in direction
Fig. 6. AFM micrographs of films of DMT deposited
on (a) NTS-SiO2/Si at r. t.; (b) NTS-SiO2/Si at
55 °C; (c) r-NTS-SiO2/Si at r.t.; (d) r-NTS-
SiO2/Si at 55 °C; (e) SiO2/Si at r.t. and (f)
SiO2/Si at 55 °C.
Fig. 8. X-ray diffraction of 50 nm thick film of DMT
deposited on (a)NTS-SiO2/Si and r-NTS-SiO2/
Si and (b) SiO2/Si surfaces at different tempera-
tures.
Fig. 7. X-ray diffraction of 50 nm thick film of DMP
deposited on (a) NTS-SiO2/Si and r-NTS-SiO2/
Si, (b) SiO2/Si, surfaces at different tempera-
tures.
6
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© 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J. Chin. Chem. Soc. 2012, 59, 000-000

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Organic Thin Film Transistors Based on Acene Derivatives
Table 2. Field-effect mobility, on/off ratio and threshold voltage for DMP-based transistors fabricated at
different substrate temperature and surfaces
R.T.
55 °C
80 °C
NTS-SiO2
0.042~0.048/103/-10
0.063~0.081/103/-3
0.017~0.032/104/-2
(1.7~1.9)*10-3/102/+39
0.047~0.051/104/-17
0.068~0.087/103/-13
0.054~0.07/104/-2
0.044~0.0537/103/-11
0.37~0.46/103/-2
r-NTS-SiO2(para)
r-NTS-SiO2(perp)
SiO2
0.32~0.39/103/+2
(1.1~1.31)*10-2/102/+36
(4.7~4.84)*10-3/102/+20
Table 3. Field-effect mobility, on/off ratio and threshold voltage
for DMT transistors fabricated at different substrate
temperature and surfaces
off the device. For DMT, the best performance was also ob-
tained on r-NTS-SiO2/Si surface with the substrate temper-
ature at room temperature. A mobility of 0.028 cm2/Vs and
an on/off ratio up to 106 were achieved. Higher deposition
temperatures greatly reduced the mobility.
R.T.
55 °C
NTS-SiO2
(8.5~9.4)*10-3/105/-10 (6.2~7.1)*10-4/105/-25
NTS-SiO2(para) (2.4~2.8)*10-2/106/-17 (1.29~1.42)*10-2/106/-25
NTS-SiO2(perp) (1.46~1.7)*10-2/105/-5
SiO2
(5.2~5.7)*10-4/103/+8
(2~2.3)*10-2/105/-22
*/*/*
CONCLUSION
In conclusion, two acene derivatives, DMP and DMT,
were synthesized and characterized for applications in
field-effect transistor fabrication. The introduction of two
methyl groups at 2,3-positions results in improved chemi-
cal stability of the compounds in the solution. The methyl
substitution also imparts a weak dipole moment to the mol-
ecule which dictates the molecules to pack in anti-parallel
disposition of neighboring molecules without changing the
herringbone-like packing motif. Layered films were ob-
tained upon thermal deposition. Mobility for FET devices
based on these compounds depends critically on the sub-
strate on which they were deposited. Rubbing of the SAM-
covered silica substrate much improved the mobility of
DMP-based devices to a high value of 0.46 cm2/Vs, com-
pared to 0.057 cm2/Vs obtained for the unrubbed counter-
part. For DMT-based devices, a mobility of 0.028 cm2/Vs
was observed on rubbed NTS-SiO2 substrate.
perpendicular to the rubbing direction, with a similar on/off
ratio of 103. The mobility decreased with lower substrate
temperature. This trend also parallels that revealed from
AFM analyses. That is, the larger the grain size, the higher
the mobility. For films deposited on bare SiO2/Si surface,
the highest mobility of 0.013 cm2/Vs was obtained at a sub-
strate temperature of 55 °C. Further increasing substrate
temperature to 80 °C resulted in reduced intensity of dif-
fraction peaks and lower device performance. It is noted
that the threshold voltages shifted to negative values for the
SAM-covered surfaces and are positive on the bare SiO2/Si
surfaces. The on/off ratios for devices prepared on bare
SiO2/Si surfaces are also lower. It is suggested that some
hole carriers are present at zero bias and the device is al-
ready on at zero bias. It takes a large positive gate to turn
ACKNOWLEDGEMENT
The financial support from National Science Council
of Taiwan, the Republic of China is acknowledged.
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