Air-stable, solution-processable n-channel and ambipolar semiconductors for thin-film transistors based on the indenofluorenebis(dicyanovinylene) core

Article abstract of DOI:10.1021/ja802266u

We present here the synthesis, characterization, and field-effect performance of a novel n-channel semiconducting molecule TIFDMT and of the corresponding thiophene-based copolymer P-IFDMT4 based on the indenofluorenebis(dicyanovinylene) core. TIFDMT-based field-effect transistors fabricated by spin-coating exhibit high electron mobilities of 0.10-0.16 cm²/V s in air, low turn-on voltages (～0 to +5 V), and high on/off ratios of 10⁷-10⁸. These devices also exhibit excellent air stability over a prolonged time of storage in ambient conditions. P-IFDMT4-based devices exhibit the first example of an air-stable ambipolar polymer processable from solution. Copyright

Full text of DOI:10.1021/ja802266u

Published on Web 06/11/2008
Air-Stable, Solution-Processable n-Channel and Ambipolar Semiconductors for
Thin-Film Transistors Based on the Indenofluorenebis(dicyanovinylene) Core
Hakan Usta, Antonio Facchetti,* and Tobin J. Marks*
Department of Chemistry and the Materials Research Center, Northwestern UniVersity, 2145 Sheridan Road,
EVanston, Illinois, 60208
Received March 27, 2008; E-mail: a-facchetti@northwestern.edu; t-marks@northwestern.edu
The development of solution-processable small-molecule and
polymeric semiconductors for field-effect transistors (FETs) exhibit-
ing high carrier mobility and good ambient stability is crucial to
Scheme 1. Synthetic Routes to TIFDMT and Polymer P-IFDMT4
1
realizing low-cost and mechanically flexible printed electronics.
During the past decade, intense research efforts have yielded a
2
number of air-stable p-channel (hole-transporting) and n-channel
3
(
electron-transporting) semiconductors. Nevertheless, very few
3
solution-processable, air-stable n-channel materials are known.
Furthermore, known examples generally exhibit modest FET
-
5
2
-1 -1 4
performance (µ
e
≈ 0.01-10 cm V
s ) versus the corre-
2
-1 -1
sponding vacuum-deposited films (µ
e
≈ 0.64-0.01 cm V
s ),
likely reflecting microstructural irregularities in the solution-
processed films. Realization of air-stable, solution-processable,
3
n-channel molecules and polymers is important for p-n junctions,
bipolar transistors, organic complementary circuitry (CMOS), and
1
,5
for stimulating fundamental research on OFET charge transport.
Recently, a solution-processable dicyanomethylene-substituted ter-
2
thienoquinoid derivative was reported to exhibit µ
e
≈ 0.16 cm
yield). Intramolecular Friedel-Crafts acylation of 1 is achieved by
SO treatment at 120 °C (91% yield). Pd(PPh Cl -catalyzed
Stille coupling of 2 and 5 in DMF then yields compound 3 (35%
yield) which undergoes reaction with excess malononitrile in the
4
presence pyridine and TiCl to afford TIFDMT in 40% yield.
Polymer building block 4 is prepared in 45% yield by bromination
of compound 3, followed by condensation with malononitrile.
Monomer 4 and TIFDMT are very soluble in common organic
solvents, allowing convenient purification by flash chromatography.
Monomer 4 is copolymerized with 6 via microwave-assisted
-
1
-1
V
s
in air, however it suffers from a low Ion/Ioff ratio (ca.
H
2
4
3
)
2
2
3
4
2
-1 -1
1
0 -10 ) and µ
e
drops to ca. 0.01 cm
V
s
over time in
6
ambient conditions. Importantly, the paucity of air-stable n-channel
polymers has significantly hindered utilizing the superior rheological
7
properties of polymers in printing processes.
2 3 3
Pd (dba) /P(o-Tol) -catalyzed Stille coupling. The resulting polymer
is purified by multiple dissolution/precipitation (60% yield). Final
1
13
products are characterized by H and C NMR, EA, IR, GPC,
and MS.
The above considerations prompted us to pursue new polymer-
izable, electron-deficient architectures and to investigate their
properties as semiconducting polymer building blocks. In this
Communication, we report the synthesis, characterization, and field-
effect response of a novel n-channel semiconducting molecule
TIFDMT and of the corresponding thiophene-based copolymer
P-IFDMT4. In these structures, the highly electron-deficient, ladder-
type indenofluorenebis(dicyanovinylene) skeleton is utilized to
depress LUMO energies, providing ambient stability to the gate
field-induced electron carriers. Furthermore, the donor-acceptor
backbone enhances core rigidity and π-conjugation, affording low
band gap semiconductors. We report here that solution-processed
Thin-film cyclic voltammetry reveals five reversible reductions
red-1
for TIFDMT with the first half-wave potential (E1/2
) at -0.12
V, whereas P-IFDMT4 exhibits six reversible reductions and one
red-1
ox-1
oxidation (E1/2
) -0.29 V and E1/2
) 1.07 V; Supporting
Information, Figure S1). Thin-film optical band gaps are estimated
from the low energy band edges in the optical spectra as 1.52 and
1.36 eV for TIFDMT and P-IFDMT4, respectively (Figure S2).
Consequently, the solid-state HOMO/LUMO energy levels are at
-5.84/-4.32 eV for TIFDMT and -5.51/-4.15 eV for P-
9
IFDMT4. The combined low LUMO energies and small band gaps
are indicative of the highly electron-deficient and π-conjugated
nature of these new structures. These are among the lowest LUMO
energies reported to date for a semiconducting polymer, approaching
those of air-stable n-channel core-cyanated perylene-, anthracene-,
2
-1 -1
TIFDMT FETs exhibit µ
e
) 0.10-0.16 cm V
s
in ambient,
while P-IFDMT4-based devices are interestingly ambipolar, having
-
4
2
-1 -1
electron and hole mobilities of ca. 2 × 10 cm V
s . To the
3
best of our knowledge, this is the first ambipolar OFET polymer
and naphthalene-based small molecule semiconductors.
8
which operates in air.
Top-contact FETs were fabricated by spin-coating TIFDMT or
The syntheses of TIFDMT and P-IFDMT4 are shown in
Scheme 1. Suzuki coupling of 1,4-benzenediboronic acid dipinacol
ester with methyl 5-bromo-2-iodobenzoate yields compound 1 (89%
P-IFDMT4 solutions in CHCl
3
(5.0 mg/mL) on OTS (octadecyl-
trichlorosilane) treated p -Si/SiO (300 nm) substrates. Next, the
semiconductor films (60-65 nm) were annealed at 150 °C for 30
+
2
8
580 9 J. AM. CHEM. SOC. 2008, 130, 8580–8581
10.1021/ja802266u CCC: $40.75  2008 American Chemical Society

C O M M U N I C A T I O N S
textured thin films, and large film grain sizes. Furthermore, the
preferential “edge-on” molecular orientation doubtless favors in-
10
plane source-to-drain (S f D) transport. The excellent air-stability
of TIFDMT and P-IFDMT4 is likely related to the low LUMO
3
energies (-4.15 and -4.32 eV). The unique electronic structure
of the present polymer provides a very small band gap (1.36 eV),
rendering the HOMO level (-5.51 eV) accessible for hole injection
by Au contacts, thus enhancing p-channel operation.
In summary, we report the synthesis and characterization of new
air-stable molecular and polymeric semiconductors based on
indenofluorenebis(dicyanovinylene). Solution-processed FETs ex-
hibit high electron mobility with excellent ambient stability. The
first example of an air-stable, ambipolar polymer (P-IFDMT4) is
reported. Studies are underway to further optimize the polymer
architecture and device-processing characterisitics.
Figure 1. (A) θ-2θ XRD and grazing-incidence XRD (inset) scans for
spin-coated TIFDMT (blue line) and P-IFDMT4 (red line) films; (B)
tapping mode AFM image of a spin-coated TIFDMT film.
Acknowledgment. We thank ONR (N00014-05-1-0766) and
Polyera for support of this research, and the NSF-MRSEC program
through the Northwestern University MRSEC (DMR-0520513) for
providing characterization facilities. We thank Dr. E. Szuromi and
Prof. R. F. Jordan for hospitality with GPC measurements.
Supporting Information Available: Synthetic procedures for 1-6,
device fabrication details; UV-vis/electrochemical/FET/AFM data;
Figures S1-S5. This material is available free of charge via the Internet
at http://pubs.acs.org.
Figure 2. OFET plots of devices measured in air. (A) Output curves as a
function of gate bias for P-IFDMT4-based devices; (B) transfer curve (VSD
)
100 V) for fresh TIFDMT-based device (red line) and after 5 months
storage in air (blue line).
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2
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,10
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2
-1
(
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-1
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e
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(
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4
2
-1 -1
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T
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JA802266U
J. AM. CHEM. SOC. 9 VOL. 130, NO. 27, 2008 8581