Organic field-effect transistors from solution-deposited functionalized acenes with mobilities as high as 1 cm2/V·s

Article abstract of DOI:10.1021/ja042353u

We present the device parameters for organic field-effect transistors fabricated from solution-deposited films of functionalized pentacene and anthradithiophenes. These materials are easily prepared in one or two steps from commercially available starting materials and are purified by simple recrystallization. For a solution-deposited film of functionalized pentacene, hole mobility of 0.17 cm²/V·s was measured. The functionalized anthradithiophenes showed behavior strongly dependent on the substituents, with hole mobilities as high as 1.0 cm²/V·s. Copyright

Full text of DOI:10.1021/ja042353u

Published on Web 03/19/2005
Organic Field-Effect Transistors from Solution-Deposited Functionalized
Acenes with Mobilities as High as 1 cm2/V‚s
Marcia M. Payne,† Sean R. Parkin,† John E. Anthony,*,† Chung-Chen Kuo,‡ and
Thomas N. Jackson¶,‡
Department of Chemistry, UniVersity of Kentucky, Lexington, Kentucky 40506-0055, and
Department of Electrical Engineering, Penn State UniVersity, UniVersity Park, PennsylVania 16802
Received December 20, 2004; E-mail: anthony@uky.edu (J.E.A); tnj1@psu.edu (T.N.J.)
Organic field-effect transistors (OFETs) are a cornerstone in the
development of organic electronic devices,1 with a simple device
structure allowing detailed studies of the effects of crystalline order
and interface modifications on device performance. Along with the
basic science that can be extracted from OFET devices, there are
many potential applications for OFETs, including control elements
for displays, RFID tags, and sensors.2 High-performance OFETs typi-
cally require vacuum deposition of the organic layer.3 Realization of
organic electronics’ potential for simple processing requires the abil-
ity to form devices by solution deposition methods, preferably using
simple, inexpensive, easily purified materials. Several research pro-
grams have addressed this issue by preparing soluble precursor mole-
cules that can be deposited by solution methods, with the desired
semiconductor generated by thermal annealing of the film.4 Such
methods have yielded OFETs with mobilities as high as 0.9 cm2/V‚s,
although a 200 °C annealing step is required. An alternative approach
uses molecules with small, soluble aromatic surfaces where deposi-
tion involves crystallization of the molecules on the substrate. While
this approach yields devices with good hole mobility (1 cm2/V‚s),
it relies on the serendipitous growth of crystals across the source
and drain electrodes and is not amenable to large-area production.5
Figure 1. Crystalline order of compounds 1-4 (top-bottom), as viewed
along the short-axis of the acene. Alkyl groups on front-most silicon atoms
omitted for clarity.
deposited films (∼0.01 cm2/V‚s).10c As with pentacene,6 placing
substituents on the central aromatic ring of the anthradithiophenes
We recently reported syntheses and properties of functionalized
2-4 both improves the solubility and tunes the intermolecular order
to favor π-stacking in the solid state. Unlike similar pentacene
derivatives, small changes in the size of the substituent have a
dramatic effect on the solid-state order of these molecules, as shown
in Figure 1. Triisopropylsilyl derivative 2 packs in a 1-D slipped-
stack arrangement, with an average distance of 3.46 Å between
π-faces. Triethylsilyl derivative 3 adopts a 2-D π-stacking arrange-
ment similar to that of compound 1, except for a decrease in π-face
separation (from 3.43 Å for 1 to ∼3.25 Å for 3), and increased
short-axis displacement (see Table 1). Trimethylsilyl derivative 4
shows a molecular order dominated by edge-to-face interactions,
leading to a herringbone arrangement with no π-stacking. For
molecules 1-3, the distance of lateral (short-axis) slip is given in
Table 1 (values for all nearest neighbors are given for materials
exhibiting 2-D π-stacking), along with the total π-overlap area for
these derivatives. As with the materials prepared in the original
anthradithiophene studies,10 compounds 2-4 were prepared as
inseparable mixtures of syn- and anti-isomers.
pentacene (1)6 and anthradithiophene (2)7 derivatives designed to
exhibit enhanced π-stacking interactions. The materials are easily
prepared in one or two high-yielding steps from commercial starting
materials, and derivative 1 showed a hole mobility of 0.4 cm2/V‚s
in an OFET prepared by vacuum deposition.8 The high solubility
of these functionalized materials (>100 mg/mL in chloroform) led
us to investigate the fabrication of OFETs by solution deposition.
Silylethynylated pentacenes typically adopt either a 1-D “slipped-
stack” or 2-D “bricklayer” arrangement in the solid state.9 The only
derivatives that have so far produced acceptable performance in
OFETs have been those that have adopted the latter arrangement,
exemplified by pentacene derivative 1. The crystal order of this
material is represented at the top of Figure 1.
Anthradithiophenes functionalized on the thiophene ring were
reported to yield good hole mobility from vapor-deposited films
(on the order of 0.1 cm2/V‚s)10 and moderate mobility from solution-
* Correspondence author for molecule synthesis and characterization.
¶ Correspondence author for device fabrication and characterization.
† University of Kentucky.
The substrate for the field-effect transistors consisted of a heavily
doped Si wafer with a thermally grown oxide layer (370 nm),
‡ Penn State University.
9
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J. AM. CHEM. SOC. 2005, 127, 4986-4987
10.1021/ja042353u CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Device Parameters for Functionalized Pentacene and
Anthradithiophene OFETs (1-4) and Degree of π-Overlap for 1-3
µ
π
-overlap
(Å2)
lateral slip
(Å)
(cm2/V
‚s)
Ion/Ioff
1
2
3
4
0.17
105
103
107
NA
7.73
2.23
1.57
(no overlap)
0.9, 1.7
1.2
2.75, 1.76
(no π-overlap)
<10-4
1.0
NA
serving as gate electrode and dielectric. Gold source and drain
contacts were evaporated to yield devices with a channel length of
22 µm and a channel width of 340 µm. The gold electrodes were
then treated with pentafluorobenzenethiol to improve the electrode
interface.11 A 1-2 wt % solution of the acene in toluene was spread
across the device surface using a plastic blade, and the solvent was
allowed to evaporate. The devices were then heated in air at 90 °C
for 2 min to drive off residual solvent. All measurements were
performed in air at room temperature, and mobilities were calculated
from the saturation currents.
Pentacene 1 yielded devices with hole mobility of 0.17 cm2/V‚s
and an on/off current ratio of 105. These values are similar in
magnitude to those obtained from a vapor-deposited film of the
same material, likely due to the strong templating effect of the silyl
group. The anthradithiophene derivatives present an excellent study
of how subtle changes in functionalization lead to significant
differences in device performance. Triisopropylsilyl derivative 2
formed uniform thin films from solution, but they were amorphous:
the best mobility measured was <10-4 cm2/V‚s. In contrast,
triethylsilyl compound 3 formed uniform films of excellent quality,
yielding hole mobility of 1.0 cm2/V‚s with an excellent on/off
current ratio (107). The performance of this material is likely due
to the close π-stacked interactions in the crystal.10b Trimethylsilyl
derivative 4 formed needlelike crystals on the substrate, leading to
poor coverage and no transistor action. Clearly, the ability to form
high-quality films is essential to device performance, and the
strength of π-stacking interactions in the solid may influence the
uniformity of the solution-cast film.
Apparent in the ID-VDS plots for both 1 and 3 (Figure 2) is that
the performance of these devices is strongly contact-limited. High
source contact resistance is common in OFETs and can cause
current compression of ID for small VDS and high VG.12 Charge
injection limited by space charges near the source may also result
in the concave-down nonlinearity of ID for small VDS (as is seen
for 3).13 Addition of the silylethynyl groups to the aromatic core
increases the oxidation potential (and thus the HOMO energy level)
of these molecules by >300 mV versus the parent hydrocarbons.
While this increase may play a role in the improved oxidative
stability of the molecules, it also hampers hole injection into the
film.
Figure 2. Electrical characterization for OFETs of functionalized pentacene
1 (top) and anthradithiophene 3 (bottom). Left: plot of drain current (ID)
versus drain-source voltage (VDS). Right: plot of ID and ID versus gate-
1/2
source voltage (VGS).
development of new materials for organic electronics. We are
currently investigating modifications of 1 and 3 to improve the
efficiency of charge injection.
Acknowledgment. This research was supported by the Office
of Naval Research and DARPA (J.E.A.), and the National Science
Foundation (T.N.J.).
Supporting Information Available: Experimental details and cif
files. This material is available free of charge via the Internet at http://
pubs.acs.org.
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