Perfluoropentacene: High-performance p-n junctions and complementary circuits with pentacene

Article abstract of DOI:10.1021/ja0476258

We report the synthesis and characterization of perfluoropentacene as an n-type semiconductor for organic field-effect transistors (OFETs). Perfluoropentacene is a planar and crystalline material that adopts a herringbone structure as observed for pentacene. OFETs with perfluoropentacene were constructed using top-contact geometry, and an electron mobility of 0.11 cm² V^-1 s^-1 was observed. Bipolar OFETs with perfluoropentacene and pentacene function at both negative and positive gate voltages. The improved p-n junctions are probably due to the similar d-spacings of both acenes. Complementary inverter circuits were fabricated, and the transfer characteristics exhibit a sharp inversion of the output signal with a high-voltage gain. Copyright

Full text of DOI:10.1021/ja0476258

Published on Web 06/15/2004
Perfluoropentacene: High-Performance p-n Junctions and Complementary
Circuits with Pentacene
Youichi Sakamoto,† Toshiyasu Suzuki,*,† Masafumi Kobayashi,‡ Yuan Gao,‡ Yasushi Fukai,‡
Youji Inoue,§ Fumio Sato,§ and Shizuo Tokito§
Institute for Molecular Science, Myodaiji, Okazaki 444-8787, Japan, New Products DeVelopment DiVision, Kanto
Denka Kogyo Co., Ltd., 1-2-1 Marunouchi, Chiyoda-ku, Tokyo 100-0005, Japan, and NHK Science and Technical
Research Laboratories, Kinuta, Setagaya-ku, Tokyo 157-8510, Japan
Received April 24, 2004; E-mail: toshy@ims.ac.jp
Chart 1. Structures of Pentacene and Perfluoropentacene
Organic field-effect transistors (OFETs) are of great interest
because of applications for plastic electronics such as electronic
papers and flexible displays.1 Pentacene (C22H14) is a planar aro-
matic hydrocarbon (Chart 1) and has been widely studied as a p-type
semiconductor for OFETs. Of all the OFET materials reported so
far, the highest field-effect mobility has been recorded in pentacene
(0.3-0.7 cm2 V-1 s-1 on SiO2/Si substrates,2 1.5 cm2 V-1 s-1 on
chemically modified SiO2/Si substrates,3 and 3 cm2 V-1 s-1 on
polymer gate dielectrics4). When one considers producing bipolar
transistors5,6 and complementary circuits7,8 with pentacene, the n-
type organic semiconductor should have similar physical and elec-
trical properties except for the type of carriers. Previously, we
demonstrated that aromatic perfluorocarbons such as perfluoro-p-
sexiphenyl (C36F26) were efficient n-type semiconductors for the
electron-transport layer of organic light-emitting diodes.9 Because
fluorine is the most electronegative of all the elements and relatively
small (hydrogen < fluorine < carbon), perfluorination is an effective
way to convert a p-type organic semiconductor to an n-type one
without changing the molecular size greatly. We have designed
perfluoropentacene (C22F14) as a potential n-type semiconductor for
OFETs (Chart 1). Perfluoropentacene is expected to be a planar
and crystalline material with high electron affinity. We report herein
the synthesis, characterization, and X-ray structure of perfluoro-
pentacene.10,11 OFETs with this perfluorocarbon have been fabri-
cated, and bipolar OFETs and complementary circuits with penta-
cene are also presented.
Perfluoropentacene has been synthesized as shown in Scheme
1. The Friedel-Crafts reaction of tetrafluorophthalic anhydride (1)
with hydroquinone in the presence of aluminum chloride and so-
dium chloride at 200 °C gave anthraquinone 2 in 71% yield.12 Re-
duction of 2 with tin yielded 2,3-dihydro-1,4-anthracenedione 3 in
95%.12 Again, the Friedel-Crafts reaction of 1 with 3 at 280 °C
provided 6,13-pentacenedione 4 in 85% yield. Fluorination of 4
with sulfur tetrafluoride in hydrogen fluoride10b at 150 °C afforded
perfluoro-(5,6,7,12,13,14-hexahydropentacene) (5) in 40% yield.
Defluorination of 5 with zinc10b at 280 °C gave perfluoropentacene
in 65% yield.
Scheme 1. Synthesis of Perfluoropentacene
from the onset of absorption are 2.07 eV for pentacene and 1.95
eV for perfluoropentacene. The emission maxima in 1,2-dichlo-
robenzene exhibit a red shift from pentacene (589 nm) to perfluo-
ropentacene (652 nm).14
The electrochemical measurements on pentacene and perfluo-
ropentacene were performed in 1,2-dichlorobenzene. The differential
pulse voltammogram (DPV) of pentacene shows a reduction peak
at -1.87 V and an oxidation peak at 0.22 V (versus the ferrocene/
ferrocenium couple). As expected, the reduction and oxidation peaks
of perfluoropentacene (-1.13 and 0.79 V, respectively) shift
positively relative to pentacene.14 The reduction potential of
perfluoropentacene is almost the same as that of C60 (-1.14 V by
DPV under the same conditions), which is known as an excellent
n-type semiconductor for FETs.15 The HOMO-LUMO gaps
obtained from the redox peaks are 2.09 eV for pentacene and 1.92
eV for perfluoropentacene. Density functional theory (DFT)
calculations are in agreement with the experimental results: The
HOMO-LUMO gaps calculated at the B3LYP/6-31G(d) level are
2.21 eV for pentacene and 2.02 eV for perfluoropentacene.14
Single crystals of perfluoropentacene were successfully grown
by slow sublimation at 260 °C under a flow of 250 Pa of argon. A
dark blue plate was used for X-ray crystallography.16 The structure
of perfluoropentacene is planar as observed for pentacene (Figure
1). The C-C bond distances range from 1.35-1.45 Å and are quite
similar to those of pentacene (1.34-1.46 Å).17 The C-F bond
distances (1.34-1.35 Å) are typical of aromatic organofluorine
compounds. As shown in Figures 2a and 2b, pentacene and
perfluoropentacene adopt herringbone structures with the angles
Perfluoropentacene was purified by train sublimation13 and used
for characterization. It is a dark blue crystalline solid and slightly
soluble in hot 1,2-dichlorobenzene. Its structure was determined
by mass spectrometry, elemental analysis, and X-ray crystallo-
graphy.14 The UV-vis absorption spectra of pentacene and per-
fluoropentacene were measured in 1,2-dichlorobenzene. Both acenes
show five major peaks in the range of 400 to 700 nm, but their
shapes are quite different.14 The HOMO-LUMO gaps obtained
† Institute for Molecular Science.
‡ Kanto Denka Kogyo Co., Ltd.
§ NHK Science and Technical Research Laboratories.
9
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10.1021/ja0476258 CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Figure 1. ORTEP drawing of the molecular structure of perfluoropentacene.
Figure 4. (a) Structure of a perfluoropentacene/pentacene bipolar OFET.
(b) Drain current (ID) versus gate voltage (VG) characteristics at drain
voltages VD ) -40 and 40 V for a perfluoropentacene/pentacene bipolar
OFET. The field-effect mobilities are calculated to be 0.024 and 0.035 cm2
V-1 s-1 for the n- and p-channel operations, respectively.
(50 nm) were deposited on the organic layer through a shadow
mask. The channel length (L) and width (W) are 100 and 1000
µm, respectively. The electrical measurements for the devices were
carried out at room temperature in a vacuum chamber (10-5 Pa)
without exposure to air. The n-channel FET activity was observed
at the substrate temperatures between 25 and 60 °C. The electron
mobilities calculated in the saturation regime range from 0.043 to
0.049 cm2 V-1 s-1, and the on/off ratios are 104-105. The FET
performances are improved when SiO2 is treated with octadecyl-
trichlorosilane (OTS). Figure 3b shows the drain current (ID) versus
drain voltage (VD) characteristics for a perfluoropentacene OFET
on an OTS-SiO2/Si substrate at various gate voltages (VG). Figure
Figure 2. Molecular packing diagrams of (a) pentacene (ref 17) and (b)
perfluoropentacene.
1/2
3c indicates the ID and ID versus VG plots at VD ) 40 V for the
same device. The highest mobility of 0.11 cm2 V-1 s-1 was ob-
served at Tsub ) 50 °C (on/off ratio ) 105). For comparison, a
pentacene OFET with the same channel length and width was
fabricated on OTS-SiO2 at Tsub ) 50 °C. The hole mobility is
0.45 cm2 V-1 s-1, and the on/off ratio is 106. Because the electron
mobility of perfluoropentacene is comparable to the hole mobility
of pentacene, perfluoropentacene is a good candidate for bipolar
OFETs and complementary circuits with pentacene. Although N,N′-
dioctyl-3,4,9,10-perylene tetracarboxylic diimide recorded the high-
est electron mobility of all the n-channel materials (0.6 cm2 V-1
s-1),18 its bipolar OFETs and complementary circuits have not been
reported.
Bipolar OFETs with pentacene and perfluoropentacene were
fabricated using top-contact geometry (Figure 4a). Perfluoropen-
tacene (10 nm) was evaporated on an OTS-SiO2/Si substrate at
Tsub ) 50 °C. Then, the second layer of pentacene (35 nm) was
formed on the perfluoropentacene layer at the same substrate
temperature. Gold source and drain contacts (50 nm) were deposited
on the pentacene layer through a shadow mask (L ) 75 µm; W )
1000 µm). The drain current versus gate voltage characteristics
(Figure 4b) indicate that the OFET functions at both negative and
positive gate voltages with rather narrow off regions (VG ) 0 V
for VD ) 40 V and VG ) -11 V for VD ) -40 V). When the gate
is biased positively relative to the off region, the device works as
a normal n-type perfluoropentacene OFET. The field-effect mobility
is calculated to be 0.024 cm2 V-1 s-1 for the n-channel operation.
When the gate is biased negatively relative to the off region, an
accumulation layer of holes is formed in the pentacene layer, not
in the perfluoropentacene layer. In this case, the device functions
as a p-type pentacene OFET. The field-effect mobility is 0.035 cm2
V-1 s-1 for the p-channel operation. When perfluoropentacene is
deposited on the first layer of pentacene, the device also works
well as a bipolar OFET.14 The field-effect mobilities for the p- and
Figure 3. (a) Structure of a perfluoropentacene OFET. (b) Drain current
(ID) versus drain voltage (VD) characteristics as a function of gate voltage
(VG) for a perfluoropentacene OFET on OTS-modified SiO2 (Tsub ) 50
°C). (c) ID and ID1/2 versus VG plots at VD ) 40 V for the same device. The
field-effect mobility calculated in the saturation regime is 0.11 cm2 V-1
s-1
.
of 51.9 and 91.2°, respectively. In pentacene, the intermolecular
C-C contacts are longer than 3.64 Å. Interestingly, short C-C
contacts (3.22-3.25 Å) less than the sum of van der Waals radii
(3.40 Å) were observed in perfluoropentacene.14 The interplanar
distance within the stack is 3.21 Å, which is shorter than the layer
separation of graphite (3.35 Å). This is probably because of the
electrostatic interaction between electropositive pentacene moieties
and electronegative fluorine atoms.
OFETs with perfluoropentacene were constructed on SiO2/Si
substrates using top-contact geometry (Figure 3a).14 The gate is an
n-type Si wafer, and the gate insulator is a 200 nm thick layer of
thermally grown SiO2. The thin film of perfluoropentacene (35 nm)
was formed on SiO2 by high-vacuum evaporation (5 × 10-5 Pa) at
different substrate temperatures. Gold source and drain contacts
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C O M M U N I C A T I O N S
tacene precursors is possible by adaptation of the procedures for
soluble pentacene precursors.24,25 The p-n junctions between
aromatic and perfluoroaromatic compounds may be useful for other
applications such as organic photovoltaic cells.26
Acknowledgment. This work was supported by Japan Society
for the Promotion of Science (Grant-in-Aid for Scientific Research
B14340226 and Grant-in-Aid for Young Scientists B15750161).
We thank Prof. H. Kawaguchi and Dr. T. Matsuo for help in X-ray
structural determination.
Supporting Information Available: Experimental details, absorp-
tion and emission spectra, DPVs, DFT calculations, molecular packing
diagrams, ID versus VG characteristics for a pentacene/perfluoropenta-
cene bipolar OFET, and X-ray diffractograms (PDF); X-ray crystal-
lographic data in CIF format. This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 5. (a) Inverter circuit configuration. (b) Transfer characteristics of
a pentacene/perfluoropentacene complementary inverter with a 100 V
supply.
n-channel operations are 0.52 and 0.022 cm2 V-1 s-1, respectively.
Thin films of perfluoropentacene were deposited on OTS-SiO2/
Si substrates by vacuum evaporation at different substrate temper-
atures (Tsub ) 25, 50, and 70 °C). The X-ray diffraction at room
temperature shows reflections up to the third order.14 The d-spacing
calculated from the first reflection peak is 15.8 Å, which is close
to the d-spacing observed in the single crystal (15.5 Å). In the case
of pentacene, the d-spacing of the thin-film phase (15.4 Å)19 is
much larger than that of the single crystal (14.1 Å).17
Pentacene and perfluoropentacene have similar molecular shapes
and sizes, and their thin-film d-spacings are almost identical (15.4
and 15.8 Å, respectively). This may lead to the continuous crystal
growth at the interface as if a single material is used.14 The well-
ordered second layer can be formed, and good electrical contacts
between the two layers are expected. The stacking interactions are
generally found in cocrystals of aromatic and perfluoroaromatic
compounds such as benzene and perfluorobenzene.20 There should
be such an interaction between pentacene and perfluoropentacene
at the interface, which could be another reason for the improved
p-n junction. The first bipolar OFET consisted of R-sexithiophene
(p-type) and C60 (n-type).5 These compounds have completely
different d-spacings, which causes inferior quality of the second
layer. Ambipolar transport in OFETs with a single organic
semiconductor has been reported, but the mobilities were less than
10-4 cm2 V-1 s-1 for both channel types.21
A p-channel pentacene OFET (L ) 200 µm; W ) 1000 µm)
and an n-channel perfluoropentacene OFET (L ) 100 µm; W )
1000 µm) were constructed on the same OTS-SiO2/Si substrate
using shadow masks. These OFETs were connected in an inverter
circuit configuration (Figure 5a). The transfer characteristics with
a 100 V supply exhibit a sharp inversion of the output signal with
a high voltage gain of 57 (Figure 5b). Organic complementary
circuits reported previously showed lower gains, probably because
of low electron mobilities of n-type semiconductors such as copper
hexadecafluorophthalocyanine.8,22
In conclusion, perfluoropentacene is the n-type organic semi-
conductor of choice for constructing high-performance p-n junc-
tions and complementary circuits with pentacene. Organic integrated
circuits23 based on pentacene/perfluoropentacene complementary
inverters would exhibit higher operating frequencies. For the
solution-processed OFETs, the synthesis of soluble perfluoropen-
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