High field-effect mobility oligofluorene derivatives with high environmental stability [1]

Full text of DOI:10.1021/ja016525o

9214
J. Am. Chem. Soc. 2001, 123, 9214-9215
In this contribution, we report high-performance OFETs using
two new oligomers, based on fluorene and bithiophene, as the
semiconducting layer. Very high field-effect mobility (greater than
0.1 cm2/Vs) has been achieved with DH-FTTF. More importantly,
these semiconductors exhibit high on/off ratios and no decrease
in the on/off ratios for more than two months in air with exposure
to ambient light. Fluorene-containing polymers have been previ-
ously reported for light-emitting diodes and transistors.6 High
mobility was obtained only when these liquid crystalline (LC)
polymers were annealed in their LC phase aligned with a rubbed
polyimide surface to form a monocrystalline domain.6e In contrast
to the oligofluorenes used for light-emitting diodes, our oligomers
are not substituted at the methylene linkage fusing the two benzene
rings (9-position of fluorene) to facilitate π-π stacking in these
oligomers. The FTTF and DH-FTTF were synthesized using
the Suzuki coupling reaction between 5,5′-dibromo-2,2′-bithiophene
and the corresponding pinacolato boronic ester-substituted fluo-
rene and 2-hexylfluorene.6b The resulting oligomers were purified
by repeated washing with hot chloroform followed by vacuum
sublimation to yield bright yellow powders. The elemental analysis
results matched closely to the calculated values. The UV-vis
absorption spectra of FTTF and DH-FTTF thin films prepared
by vacuum deposition (∼ 50 nm thick) are very similar with the
maximum absorption at ca. 350 nm corresponding to the π-π*
transition.
High Field-Effect Mobility Oligofluorene Derivatives
with High Environmental Stability
Hong Meng,† Zhenan Bao,*,‡ Andrew J. Lovinger,‡
Bo-Cheng Wang,§ and Anthony M. Mujsce‡
Department of Chemistry and Biochemistry
UniVersity of California
Los Angeles, California 90095
Bell Laboratories, Lucent Technologies
600 Mountain AVenue, Murray Hill, New Jersey 07974
Department of Chemistry, Tamkang UniVersity
151 Ying-Chuan Road, Tamsui, Taiwan 251
ReceiVed June 29, 2001
Organic and polymeric semiconductor-based thin-film field-
effect transistors (OFETs) are of great interest for low-cost large-
area memories and driving circuits in display applications.1 In
these organic devices, the charge carrier transport is dominated
by the hopping mechanism at room temperature.2 Therefore,
highly ordered thin films with large interconnected polycrystalline
grains are necessary to obtain high mobilities.3 To date, only a
X-ray diffraction analyses were performed on films vacuum-
deposited onto Si/SiO2 substrates. All films are highly ordered
as evidenced by the fact that sharp diffraction peaks, extending
in some cases to the eighth order, were observed. For unsubstituted
FTTF, the separation (24 Å) between layers calculated from the
primary X-ray peak is almost the same as its extended length,
indicating that the molecules adopt an edge-on orientation similar
to that of other conjugated oligomers.7 This type of orientation
has been found particularly useful in achieving high mobilities
since the π-π stacking direction is in the plane of the current-
flowing direction. The dihexyl-substituted oligomers also appear
to adopt the edge-on orientation but with the chains somewhat
inclined since the layer separation of ca. 34 Å, similar to dihexyl
R-hexathienyl,8 is shorter than the extended molecular length.
FTTF showed a substantial amorphous component for the thin
film deposited with the substrate held at room temperature.
However, this amorphous component disappeared at elevated
substrate temperatures together with increased diffraction peak
intensities, which indicates improved ordering.
All of the oligomers performed as p-type transistors and are
accumulation devices. Figure 1 shows the typical drain-source
current (IDS) characteristics of FETs with FTTF and DH-FTTF
deposited at elevated substrate temperatures with different gate
voltages. It is seen that the drain-source current scales up with
increasing gate voltages (VG). The field-effect mobilities were
calculated using the IDS values in the saturation regions, and the
results for all oligomers obtained at different TD are summarized
in Table 1.9 The mobilities depend strongly on both the deposition
conditions and the device geometry. It has been previously
limited number of organic semiconductors can reach mobilities
similar to those of R-Si transistors.4 Most p-channel organic
semiconductors have been based on thiophene-derivatized oligo-
mers and polymers.5 There are a few other systems relying on
aromatic macrocyclics, such as metallophthalocyanines, and fused
aromatic rings, such as pentacene and tetracene.4a,5 Few other
types of conjugated systems have been explored thus far. In
addition, most of the reported p-channel semiconductors have
relatively low band gaps and high HOMO levels. Therefore, they
tend to be easily photooxidized and cause degradation of the
resulting devices.
† University of California.
‡ Bell Laboratories, Lucent Technologies.
§ Tamkang University.
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10.1021/ja016525o CCC: $20.00 © 2001 American Chemical Society
Published on Web 00/00/0000

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 37, 2001 9215
contact geometry. This problem can be partially or completely
solved by surface treatment of the Au electrodes with thiol
monolayers.10 Such studies with our oligomers are in progress.
The best mobility thus far is achieved with DH-FTTF at a
substrate temperature of 140 °C, which is on the same order of
magnitude as R-Si transistors. This value may be further improved
by surface modification of the dielectric and contact electrodes.4a
The thin film deposited at this temperature was examined by
transmission electron microscope and found to exhibit densely
packed 200-500 nm grains with extensive multilayer terraces
and screw dislocations, which indicated high crystallinity and
three-dimensional order (Figure 2b). Synthesis of other oligo-
fluorene derivatives is underway. Preliminary results indicated
that the mobility of terfluorene is on the order of 10-2 cm2/Vs.
The on/off ratios of the oligomers are quite high (more than
1000 in most cases and more than 105 for FTTF at room
temperature) without any special optimization effort. Careful
purification of the semiconducting materials, surface treatment
of the dielectric surface, and patterning of the semiconducting
layer may lead to improved on/off ratios. The OFETs made from
these two oligomers also show improved oxidative stability. No
decrease in on/off ratio and mobility have been observed even
after the transistors have been exposed to ambient light and air
in a synthetic lab for more than 2 months. Thin films of DH-
FTTF showed no decrease in the peak intensity at the maximum
UV absorption wavelength after being irradiated with a UV lamp
(366 nm, 60 Hz, 0.16 amps) for 48 h. In addition, no carbonyl
formation was observed in the FTIR spectrum for the irradiated
film. Many other high mobility organic semiconductors usually
start to exhibit decreased on/off ratio hours after exposure under
air and ambient light. The improved stability is extremely valuable
for realizing long-lived organic electronic devices.
Figure 1. IDS vs VDS characteristic of (a) FTTF (100 °C) and (b) DH-
FTTF (140 °C) at different gate voltages prepared at elevated substrate
temperatures, with channel length L ) 280 µm and channel width W )
4 mm.
Table 1. Field-Effect Mobilities of FTTF and DH-FTTF
Transistors Prepared at Different Substrate Temperatures (TD) Using
Top-Contact Geometry
FTTF
DH-FTTF
TD
(°C) mobility (cm2/Vs) on/off ratio mobility (cm2/Vs) on/off ratio
25
80
100
120
140
0.02-0.03
0.03
0.07-0.08
0.04-0.05
0.04-0.05
105-106
0.02-0.03
0.04
0.05-0.06
0.04-0.06
0.094-0.11
2 × 10 3
6 × 10 3
2 × 10 3
104-105
104-105
2000
9000-104
8000-104
1000-2000
In summary, high-performance oligofluorene derivatives have
been synthesized and used as the semiconducting materials for
OFETs. These oligomers showed p-channel transistor behavior
with the best mobilities in the same order of magnitude as R-Si
devices. In addition to the improved oxidative stability, DH-FTTF
was also found to be relatively soluble (more than 1 mg/mL) in
warm chlorobenzene. Preliminary results indicate that OFETs can
also be fabricated by casting from solution with mobilities in the
order of 10-3 cm2/Vs. These new high-performance and stable
organic semiconductors will be extremely valuable for the future
commercialization of organic electronic devices.
Figure 2. Transmission electron micrographs of thin films from (a)
FTTF, TD ) 100 °C and (b) DH-FTTF, TD ) 140 °C.
reported that the mobility of most organic semiconductors strongly
depends on the TD.3 Similar trends are found with these fluorene-
containing oligomers.
In general, the field-effect mobility increases with increasing
TD, which is attributed to better ordered thin films and larger grain
sizes at elevated TD. Indeed, X-ray diffraction patterns showed
higher diffraction intensity for thin films of the same thickness
deposited at higher TD. The transmission electron micrographs
also showed sharper grain boundaries and larger crystal sizes for
higher TD thin films. For example, the grain sizes for FTTF and
DH-FTTF were both on the order of 20-50 nm at TD ) 25 °C.
The grain sizes became much bigger at elevated TD, on the order
of 200-500 nm (Figure 2).
Acknowledgment. This paper is dedicated to Professor Fred Wudl
for his election to the Member of the American Academy of Arts and
Sciences as well as for the occasion of his 60th birthday. H.M. thanks
Professor Wudl for his support of an internship at Lucent Technologies.
We thank P. Ho, A. Dodabalapur, H.E. Katz, E.A. Chandross, C. G. Erben
and E. Reichmanis for helpful discussions.
It was found that the mobilities of these oligomeric devices
prepared using the bottom contact geometry (drain and source
electrodes deposited above the dielectric layer and under the
semiconducting layer) always tend to be lower than those using
the top contact geometry (drain and source electrodes deposited
on top of the semiconducting layer). Similar observations have
been reported for pentacene and some n-channel materials.10 This
was attributed to the small charge injection areas and poor wetting
of organic semiconductor vapor to the electrodes in the bottom
Supporting Information Available: (1) Experimental procedures (2)
UV-vis and FT-IR spectra of DHFTTF; (3) X-ray diffraction spectrum
of FTTF (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
JA016525O
(10) (a)Katz, H. E.; Johnson, J.; Lovinger, A. J.; Li, W. J. J. Am. Chem.
Soc. 2000, 122, 7787-7792. (b) Dimitrakopoulos, C. D. Personal communica-
tion.