α,ω-distyryl oligothiophenes: High mobility semiconductors for environmentally stable organic thin film transistors

Article abstract of DOI:10.1021/ja054358c

Critical to the development of organic electronics is the design and synthesis of new organic semiconductors with improved electrical performance and enhanced environmental stability. We present in this communication the synthesis of a series of simple ol

Full text of DOI:10.1021/ja054358c

Published on Web 11/05/2005
r,ω-Distyryl Oligothiophenes: High Mobility Semiconductors for
Environmentally Stable Organic Thin Film Transistors
Christine Videlot-Ackermann,*,† Jo¨rg Ackermann,† Hugues Brisset,† Koji Kawamura,‡
Noriyuki Yoshimoto,‡ Pascal Raynal,† Ahmed El Kassmi,† and Fre´de´ric Fages†
Laboratoire des Mate´riaux Mole´culaires et des Biomate´riaux, GCOM2 UMR CNRS 6114, UniVersite´ de la
Me´diterrane´e, Faculte´ des Sciences de Luminy, Case 901, 13288 Marseille Cedex 9, France, and Graduate School
of Engineering, Iwate UniVersity, 4-3-5 Ueda, Morioka 020-8551, Japan
Received July 1, 2005; E-mail: videlot@luminy.univ-mrs.fr
Scheme 1
The urgent needs in novel information technologies are giving
considerable impetus to the field of organic electronics.1 Clearly,
organic semiconducting materials offer themselves as unique
alternatives to amorphous silicon in low-cost applications, where
easy processability is desirable, such as the fabrication of large area,
flexible devices, and circuits via liquid deposition techniques.2
Organic thin film transistors (OTFTs) are essential for the develop-
ment of organic electronic devices.1,2a High-performance electron-
or hole-transporting materials that display both high charge carrier
mobilities and processing versatility have been reported.3-5 Critical
to the exploitation of OTFTs in commercial applications is the
improvement of the device’s environmental stability. Very recently,
several groups reported significant enhancements of OTFTs’
stability under ambient conditions in the case of semiconducting
small molecules,4 oligomers,5,6 and polymers.7 Exceptional stability
features were described for acene-thiophene semiconductors.5
The oligothiophene skeleton is among the most versatile and
effective molecular scaffold on which to base the rational design
of organic semiconductors.8 However, oligothiophene derivatives
are generally recognized to offer only poor air stability, and the
fact is that their OTFTs show rapid performance degradation.
In this communication, we report a new series of structurally
simple and readily available oligothiophenes end-capped with styryl
units, DS-nT (n ) 2-4), that were used as the semiconducting
layer in OTFTs (Scheme 1).9 The devices not only show very high
field-effect mobility (up to 0.1 cm2/Vs) and on/off ratios (up to
105) but also are found to be exceptionally long-lived (more than
1 year of storage) and stable toward continuous operation, under
atmospheric conditions. To our knowledge, compound DS-4T is
the first example of a long-chain oligothiophene derivative that
allows one to fabricate highly stable OTFTs with a life of more
than 17 months. High mobility is usually obtained for long
thiophene oligomers, but this usually is at the expense of the
device’s stability owing to the lower oxidation potential of such
structures. Remarkably, this is not the case with DS-4T, as this
compound, containing the tetrathiophene core, indeed shows the
highest stability along with the best electrical characteristics within
the series investigated.
cording to Scheme S1 and were characterized by H NMR, mass
spectrometry, and elemental analysis.
1
UV-vis/fluorescence data point to a more extended electron
conjugation pathway within the distyryl derivatives, as indicated
by the substantial red-shift in absorption and emission maxima (λabs
/
λem (nm): DS-2T 425/477; DS-3T 445/506; DS-4T 455/524)
relative to the parent oligothiophenes, such as 4T (λabs/λem ) 393/
451 nm). Accordingly, optical gap values were found to be reduced
(Eg (eV): DS-2T 2.6; DS-3T 2.45; DS-4T 2.37; 4T 2.8). Cyclic
voltammetry measurements of DS-nT show a set of separate
reversible one-electron oxidations (E1/2(1)/E1/2(2) (V): DS-2T 1.03;
DS-3T 0.95/1.17; DS-4T 0.85/1.08). The first oxidation of DS-4T
occurs at a considerably less positive potential than that recorded
for 4T (E1/2(1) ) 1.18 V).
OTFT devices were fabricated using the top contact geometry
(Figure S1). Each entry in Tables 1 and S1 lists the OTFT data as
a range obtained in air for 10 independent devices. Transistor
responses were obtained only for positive bias, which is indicative
that DS-nT compounds are p-type semiconductors (Figure 1, left).
The field effect mobilities calculated in the saturation regime are
shown to increase with the substrate deposition temperature, due
to the formation of better ordered thin films at elevated Tsub. The
highest value of the hole mobility (0.1 cm2/Vs) was obtained for
the longest oligomer (DS-4T) at 110 °C. Increasing the substrate
temperature to 140 °C led to a decrease of the device performance,
as observed for other organic semiconductors.5 The mobility and
on/off ratios of OTFTs based on a silicon oxide insulator layer
coated with hexamethyldisilazane (HDMS) were not significantly
improved relative to the unmodified samples (Table S2). However
HDMS treatment was observed to decrease the threshold voltages,
Vt, and also their range of variation.14
The morphology of vapor-deposited thin films of DS-4T grown
on SiO2/Si was investigated using AFM. Picture in Figure 1 (right)
shows the formation of islands whose size increases with the
substrate deposition temperature. At Tsub ) 110 °C, a terrace-and-
step morphology is clearly observed, an average value of 2.9 nm
being determined for the step height. A DS-4T thin film deposited
on SiO2/Si at room temperature was investigated by X-ray diffrac-
tion in the reflection mode. Sharp and strong reflections up to the
9th order confirm the high degree of lamellar ordering even at low
deposition temperature (Figure S2). The d-spacing obtained from
the peak at 2θ ) 3.15° is 2.802 nm, which is in agreement with
the AFM results.
Styryl end-substitution was selected because (i) the introduction
of double bonds in oligothiophenes was shown to produce planar
structures with reduced overall aromatic character,10 (ii) thiophene
backbones containing phenylene units were observed to give rise
to efficient OTFTs,11 and (iii) the benzene moieties represent
versatile termini for further incorporation of solubilizing or self-
assembling groups.12 Compounds DS-nT were synthesized13 ac-
† Universite´ de la Me´diterrane´e.
‡ Iwate University.
9
16346
J. AM. CHEM. SOC. 2005, 127, 16346-16347
10.1021/ja054358c CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Table 1. OTFT Data of the Semiconductors DS-nT Deposited at
Different Substrate Temperatures on Silicon Oxide Layers
no decrease, of the on/off ratio was noticed upon storage. Moreover,
the Vt value determined for each independent device did not show
significant change over time.
material
Tsub
(
°
C)
µ
(cm2/Vs)
on/off
Vt (V)
1.6-1.9 × 105
4-8
0-19
0.1-3
4-13
3-13
12-20
5-20
9-22
A DS-4T OTFT device that had been stored over 17 months
has been subjected to a continuous sweeping of gate-source
voltages (-40 V < VGS < +40 V) under a constant drain-source
voltage (VDS ) -40 V) (Figure 2, right). After a first series of
1000 double scans, a small increase of IDS was observed, which
almost overlapped the initial value after a rest period of 10 min.
Then a series of 3000 double scans was further applied, which led
to a similar shift-recovery for IDS and to no degradation of
performances (µ, Vt, and on/off ratio). These results show that the
17 months old devices are stable under continuous operation
conditions. A possible reason for the reversible shift observed for
IDS in these experiments could be an increase of the device
temperature during operation or a reversible charging effect at the
semiconductor-dielectric interface.
DS-2T
30
80
30
80
30
80
110
140
0.002-0.006
0.01-0.02
0.001-0.008
0.01-0.02
0.02-0.04
0.03-0.06
0.08-0.1
2-2.3 × 105
DS-3T
DS-4T
2.2-2.8 × 104
2.2-3.3 × 104
1.2-1.8 × 103
1.9-2.5 × 103
0.8-1.2 × 103
1.5-2 × 103
0.05-0.07
In conclusion, distyryl oligothiophenes represent a novel class
of OTFT semiconductors that combine good electrical performances
and exceptional stabilities. The synthesis of soluble and n-type
analogous derivatives is currently underway for applications in
flexible electronics.
Figure 1. (Left) Plot of IDS versus VDS at various gate voltages in a DS-
4T-based OTFT device fabricated at Tsub ) 110 °C. (Right) AFM picture
(scale ) 2.8 µm × 2.8 µm) of a DS-4T thin film evaporated on SiO2/Si at
Tsub ) 110 °C.
Acknowledgment. We thank David Rondeau (Universite´ An-
gers, France) for his help in mass spectrometry measurements.
Supporting Information Available: DS-nT syntheses, details of
experimental procedures in solution, OTFT device fabrication, X-ray
diffraction procedure; Scheme S1; Figures S1, S2; Tables S1, S2. This
material is available free of charge via the Internet at http://pubs.acs.org.
References
(1) For a recent account on organic electronics, see the special issue of: Chem.
Mater. 2004, 16 (23).
(2) (a) Dimitrakopoulos, C. D.; Malefant, P. R. L. AdV. Mater. 2002, 14, 99.
(b) Katz, H. E. Chem. Mater. 2004, 16, 4748 (special issue). (c) Ling, M.
M.; Bao, Z. Chem. Mater. 2004, 16, 4824 (special issue). (d) Murph, A.
R.; Fre´chet, J. M. J.; Chang, P.; Lee, J.; Subramanian, V. J. Am. Chem.
Soc. 2004, 126, 1596. (e) Sirringhaus, H.; Kawase, T.; Friend, R. H.;
Shimoda, T.; Inbasekaran, M.; Wu, W.; Woo, E. P. Science 2000, 290,
2123. (f) Payne, M. M.; Parkin, S. R.; Anthony, J. E.; Kuo, C.-C.; Jackson,
T. N. J. Am. Chem. Soc. 2005, 127, 4986.
(3) (a) Yoon, M.-H.; DiBennedetto, S. A.; Fachetti, A.; Marks, T. J. J. Am.
Chem. Soc. 2005, 127, 1348. (b) Mas-Torrent, M.; Durkut, M.; Hadley,
P.; Ribas, X.; Rovira, C. J. Am. Chem. Soc. 2004, 126, 984.
(4) Jones, B. A.; Ahrens, M. J.; Yoon, M.-H.; Fachetti, A.; Marks, T. J.;
Wasielewski, M. R. Angew. Chem., Int. Ed. 2004, 43, 6363.
(5) (a) Meng, H.; Sun, F.; Goldfinger, M. B.; Jaycox, G. D.; Li, Z.; Marshall,
W. J.; Blackman, G. S. J. Am. Chem. Soc. 2005, 127, 2406. (b) Merlo, J.
A.; Newman, C. R.; Gerlach, C. P.; Kelley, T. W.; Muyres, D. V.; Fritz,
S. E.; Toney, M. F.; Frisbie, C. D. J. Am. Chem. Soc. 2005, 127, 3997.
(6) (a) Meng, H.; Bao, Z.; Lovinger, A. J.; Wang, B.-C.; Mujsce, A. M. J.
Am. Chem. Soc. 2001, 123, 9214. (b) Meng, H.; Zheng, J.; Lovinger, A.
J.; Wang, B.-C.; Van Patten P. G.; Bao, Z. Chem. Mater. 2003, 15, 1778.
(7) Heeney, M.; Bailey, C.; Genevicius, K.; Shkunov, M.; Sparrowe, D.;
Tierney, S.; McCulloch, I. J. Am. Chem. Soc. 2005, 127, 1078.
(8) Handbook of Oligo- and Polythiophenes; Fichou, D., Ed.; Wiley-VCH:
Weinheim, Germany, 1999.
(9) Compounds DS-3T and DS-4T are novel. For a very preliminary report
on DS-2T, see ref 13.
(10) Roncali, J. Acc. Chem. Res. 2000, 33, 147.
(11) Mushrush, M.; Fachetti, A.; Lefenfeld, M.; Katz, H. E.; Marks, T. J. J.
Am. Chem. Soc. 2003, 125, 9414.
(12) An, B.-K.; Lee, D.-S.; Lee, J.-S.; Park, Y.-S.; Song, H.-S.; Park, S. Y. J.
Am. Chem. Soc. 2004, 126, 10232.
(13) Videlot, C.; Ackermann, J.; El Kassmi, A.; Raynal, P. Thin Solid Films
2002, 403-404, 380.
(14) Gojanc, T. C.; Le´vesque, I.; D’Iorio, M. Appl. Phys. Lett. 2004, 84, 930.
Figure 2. Characteristics of DS-4T OTFT devices on SiO2/Si. (Left) Plots
of hole mobilities (µ) versus storage days for OTFTs fabricated at different
substrate deposition temperatures (Tsub). (Right) Plots of IDS versus VGS at
constant VDS ) -40 V at Tsub ) 80 °C after 17 months of storage.
OTFT devices were stored over 17 months in the dark under
ambient conditions, and device performances were measured
periodically. Figure 2 (left) illustrates the stability of the hole
mobility values obtained for DS-4T films deposited at three
temperatures onto unmodified SiO2/Si substrates. In contrast, the
mobility value determined under the same conditions for the
octithiophene semiconductor (8T, Tsub ) 150 °C) showed a decrease
of about 70% of its initial value (0.073 cm2/Vs) after 100 days. In
the case of the shorter derivatives, DS-2T and DS-3T, the mobility
values remained constant over 100 days, but a decrease of about
20-50% of the initial device performance appeared on a shelf-life
time test of 10 months. Strikingly, the highest device stability was
obtained for DS-4T, the compound with the highest HOMO energy
level within the DS-nT series, which is in contrast with previous
observations.6 Although this behavior still remains unclear, an
increase of doping level over time for the more oxidizable DS-4T
oligomer can be ruled out at this stage as no change, and especially
JA054358C
9
J. AM. CHEM. SOC. VOL. 127, NO. 47, 2005 16347