Dithienosilole- and dibenzosilole-thiophene copolymers as semiconductors for organic thin-film transistors

Article abstract of DOI:10.1021/ja062908g

The synthesis and physicochemical properties of a new class of thiophene/arenesilole-containing π-conjugated polymers are reported. Examples of this new polymer class include the following: poly(2,5-bis(3′,3′′-dihexylsilylene-2′,2′′-bithieno)thiophene) (TS6T1), poly(2,5′-bis(3′′,3′″-dihexylsilylene-2′′,2′″-bithieno)bithiophene) (TS6T2), poly(2,5′-bis(2′′,2′″-dioctylsilylene-1′′,1′″-biphenyl)thiophene) (BS8T1), and poly(2,5′-bis(2′′,2′″-dioctylsilylene-1′′,1′″-biphenyl)bithiophene) (BS8T2). Organic field-effect transistors (OFETs) with hole carrier mobilities as high as 0.02-0.06 cm²/V s in air, low turn-on voltages, and current on/off ratios >105-106 are fabricated using solution processing techniques with the above polymers as the active channel layer. OFETs based on this polymer class exhibit excellent ambient operational stability. Copyright

Full text of DOI:10.1021/ja062908g

Published on Web 06/23/2006
Dithienosilole- and Dibenzosilole-Thiophene Copolymers as
Semiconductors for Organic Thin-Film Transistors
Hakan Usta, Gang Lu, Antonio Facchetti,* and Tobin J. Marks*
Department of Chemistry and the Materials Research Center, Northwestern UniVersity, 2145 Sheridan Road,
EVanston, Illinois 60208
Received May 4, 2006; E-mail: a-facchetti@northwestern.edu; t-marks@northwestern.edu
Scheme 1. Synthetic Route to Polymers 1-4
The design and realization of new π-conjugated organic semi-
conductors has been a topic of great research interest for the past
decade.1 The most prominent examples of such materials are
polycondensed benzenoids, polyacetylenes, and (oligo)polyhetero-
aromatics.1,2 Among these, the silole (silacyclopentadiene) moiety
has recently been investigated as a novel conjugated system in
which the σ*-orbital of the silicon-carbon bond effectively interacts
with the π*-orbital of the butadiene fragment, leading to a low-
lying LUMO.3 In particular, since Tamao et al. reported that 2,5-
bis(2-pyridyl)silole derivatives exhibit electron-transporting prop-
erties,4 a number of applications of silole-containing small molecules
and polymers have emerged, including new conducting polymers,5
OLEDs,6 and solar cells.7 Very recently, silole-containing polymers
have also been used to fabricate organic field-effect transistors
(OFETs8).9
Table 1. Physicochemical Properties [Molecular Weight (Mw, KD),
Polydispersity (PD), Solution and Film Optical Absorption Maxima
(λ, nm),a Energy Gaps (Eg, eV)], OFET Charge Carrier Mobilities
(µ, cm2 V-1 s-1),b and Current On/Off Ratios (Ion:Ioff) in Air for
Polymers 1-6
polymer
Mw (PD)
λsol (Eg)
λfilm (Eg)
µh
Ion:Ioff
TS6T1
TS6T2
BS8T1
BS8T2
F8T1
30 (2.9)
41 (3.0)
112 (3.1)
127 (3.7)
17 (2.6)
80 (3.3)
521 (2.0)
544 (1.9)
473 (2.5)
503 (2.3)
427 (2.6)
456 (2.4)
574 (1.8)
545 (1.9)
484 (2.5)
493 (2.3)
440 (2.5)
460 (2.4)
0.02
2 × 105
5 × 105
3 × 104
2 × 106
2 × 105
2 × 105
0.06
6 × 10-5
0.006
9 × 10-5
0.006
F8T2
a λmax at the longest wavelengths. b Calculated in saturation for device
structure n2+-Si/SiO2-HMDS (300 nm)/polymer (20-30 nm)/Au contacts
(50 nm). OFETs tested in ambient conditions.
The above considerations and the possibility that incorporation
of electron-withdrawing silacycles might enhance π-electron overlap
along conjugated polymer backbones, without compromising
environmental stability, prompted us to synthesize new silole-
containing π-conjugated polymers and to investigate their properties
in OFETs. Note that from literature diffraction and computational
studies, silole-containing molecular cores are expected to be planar,
and the bulkiness of the silicon substituents should have little impact
on the ring planarity.10 In this communication, we report the
realization of four new silolearene-thiophene copolymers, 1-4,
and compare their properties with new fluorene-thiophene-based
polymer F8T1 (5) and widely investigated F8T2 (6). It is seen
that the silole-based materials can be synthesized in high yields,
are environmentally stable, and yield solution-processable films
which act as efficient hole transporters in FET devices. Carrier
mobilities as high as ∼0.06 cm2/Vs and current on/off ratios of
∼105-106 for unaligned films are measured in devices functioning
in ambient conditions.
Polymer synthetic details can be found in Supporting Information.
Briefly, polymers 1 and 2 were prepared via Stille coupling
sequences by reacting silole precursor 10 with thiophene reagents
11 and 12, respectively (Scheme 1, top), in refluxing THF using
Pd(PPh3)2Cl2 as the catalyst. The synthesis of polymers 3-6 was
then carried out using a modified Suzuki polycondensation (Scheme
1, bottom for 3 and 4) in a biphasic reaction with Pd[PPh3]4 as the
catalyst. The solvents were toluene and aqueous sodium bicarbonate,
and Aliquat 336 was used as the phase transfer agent to maximize
product molecular weight. All of the polymers were purified by
dissolution in minimal amounts of THF or TCB and precipitation
from large amounts of boiling methanol, cooled to room temper-
ature, stirred overnight, and then collected by centrifugation. The
dissolution-precipitation process was repeated multiple times. The
polymer structures were verified by 1H and 13C NMR spectroscopy
and elemental analysis. Polymer molecular weights were determined
by GPC versus polystyrene, and results are summarized in Table
1. Note that the Mw values of copolymers 3 and 4 are greater than
those of 1 and 2, and we are currently optimizing the synthesis of
1 and 2 following the Suzuki protocol.
The thermal properties of the present polymers were examined
by differential scanning calorimetry (Figure S1). Polymers 2, 3,
and 5 exhibit a single broad endotherm centered at 250-300 °C
without apparent exothermic return peaks. In contrast, polymers 1
and 4 exhibit single reversible endothermic (260 and 330 °C,
respectively) and exothermic (230 and 290 °C, respectively)
transitions, whereas F8T2 (6) behaves as previously reported.11
Thermogravimetric analysis scans of 1-4 (Figure S2) exhibit
negligible weight loss below 400 °C, demonstrating excellent
thermal stability. Optical absorption data for polymers 1-6 in THF
solutions and as thin films are summarized in Table 1, and spectra
are shown in Figure S3.
9
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10.1021/ja062908g CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
F8T2 and polythiophenes. It is likely that OFET performance can
be further improved via optimization of polymer Mw and film
deposition/processing conditions.
Acknowledgment. We thank ONR (N00014-02-1-0909), the
NSF-MRSEC program through the Northwestern Materials Re-
search Center (DMR-0076097), and the NASA Institute for
Nanoelectronics and Computing (NCC 2-3163) for support.
Supporting Information Available: Polymers 1-6 syntheses/
spectroscopic data and FET device fabrication. Figures S1-S8. This
material is available free of charge via the Internet at http://pubs.acs.org.
Figure 1. BS8T2-based FETs. (A) Transfer plots before (dotted line) and
after film annealing (solid line). (B) Output plot of an annealed device. (C)
ON-OFF cycles (0.03 Hz) under ambient conditions for the annealed device
at VDS ) -50 V for different gate biases.
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