High-performance, stable organic thin-film field-effect transistors based on bis-5′-alkylthiophen-2′-yl-2,6-anthracene semiconductors

Article abstract of DOI:10.1021/ja043189d

The development of new organic semiconductors with improved electrical performance and enhanced environmental stability is the focus of considerable research activity. This communication presents the design, synthesis, and device stability data for novel bis-5′-alkylthiophen-2′yl-2,6-anthracene organic semiconductors. When incorporated into thin-film field-effect transistors, mobilities as high as 0.5 cm²/Vs and on/off current ratios greater than 107 are observed. We have investigated device stability in terms of both shelf life and operating lifetime. Devices incorporating the reported semiconductors display an average field-effect mobility of 0.4 cm²/Vs for DHTAnt and an on/off current ratio of 10⁶ even after 15 months of storage. Furthermore, there is no decrease in performance during continuous operation of the devices over several thousand cycles. Copyright

Full text of DOI:10.1021/ja043189d

Published on Web 02/03/2005
High-Performance, Stable Organic Thin-Film Field-Effect Transistors Based
on Bis-5′-alkylthiophen-2′-yl-2,6-anthracene Semiconductors
Hong Meng,* Fangping Sun, Marc B. Goldfinger, Gary D. Jaycox, Zhigang Li, Will J. Marshall, and
Gregory S. Blackman
Central Research and DeVelopment, Experimental Station, E. I. DuPont Company,
Wilmington, Delaware 19880-0328
Received November 11, 2004; E-mail: hong.meng@usa.dupont.com
The fabrication of flexible circuitry using modified printing
technologies is emerging as a low cost product alternative to
amorphous Si technology.1 The key component of such circuitry
is the organic thin-film transistor (OTFT). Although the field-effect
mobility of OTFTs is still lower than those of the inorganic thin-
film transistors, the advantages of easy manufacturing and process-
ing make them suitable for selected commercial applications, such
as electrophoretic displays.2
A critical technical challenge in this area of research is to improve
the environmental stability of OTFTs.3 OTFTs have been fabricated
incorporating semiconductors based on a wide variety of organic
structure classes.4 The most promising performance has been
observed for oligothiophene derivatives and linearly fused poly-
cyclic aromatic compounds such as pentacene and its derivatives,5,6
yet the stability of these semiconductor types is limited.3b Recent
developments by several groups have resulted in improved stability,
primarily through the judicious choice of conjugated units and side
chains. However, the improved stability comes at the cost of lower
field-effect mobilities and higher substrate deposition temperatures,
the latter precluding application to plastic substrates.6a,b Still needed
are environmentally stable organic semiconductors with high
mobility and on/off current ratios that can be deposited at low
substrate temperatures.
Figure 1. Crystal packing view of the unit cell of the semiconductors
DTAnt: (Pccn, a ) 34.243(7) Å, b ) 7.5118(15) Å, c ) 6.0908(12) Å,
V ) 1566.7(5) Å3, Z ) 4) and DHTAnt: (P21, a ) 31.075(6) Å, b )
7.3950(15) Å, c ) 5.8660(12) Å, â ) 93.97(3)°, V ) 1344.8(5) Å3,
Z ) 4).
In this communication, we report two novel semiconductors
based on thiophene-anthracene oligomers that show field-effect
mobilities as high as 0.50 cm2/Vs and on/off ratios greater than
107. Most importantly, we have found that the stability of OTFTs
fabricated with these materials is significantly improved relative
to those measured for pentacene-based devices.3b In addition, the
high mobility of such semiconductors can be achieved at relatively
low substrate deposition temperatures.
The semiconductors were synthesized using a Suzuki coupling
reaction between pinacolato boronic ester-substituted 2,6-anthracene
and the corresponding 2-bromothiophenes. UV-vis spectra of these
semiconductors as deposited thin films showed a maximum
wavelength absorption peak at 445 nm. This corresponds to an
energy gap of 2.8 eV, significantly higher than that of pentacene
(2.2 eV), possibly explaining the greater stability of these materials
relative to that of pentacene.7 Both DTAnt and DHTAnt are soluble
in hot toluene or xylene, which allows these semiconductors to be
purified by a combination of recrystallization and gradient sublima-
tion. The compounds were characterized by H NMR, 13C NMR,
high-resolution mass spectrometry, elemental analysis, and X-ray
crystallography.
Figure 1 shows the molecular and crystallographic packing
structures of the semiconductors DTAnt and DHTAnt. Both
molecules display a nearly flat, symmetric molecular geometry,
which facilitates tight molecular packing. The molecules pack in
the herringbone geometry, similar to pentacene.7 The XRD reflec-
tion patterns of the thin films of these materials suggest the same
packing motifs as those observed in the single-crystal data and will
be described in detail elsewhere.
OTFT devices were fabricated in a “top contact” geometry.8
Semiconductors were evaporated onto a SiO2 (200 nm)/n-doped
Si substrate. Gold was evaporated as the source/drain electrodes.
Table 1 lists the semiconductor OTFT device data deposited over
a range of substrate temperatures. All OTFTs showed very well-
defined linear- and saturation-regime output characteristics (see
Supporting Information). Interestingly, the mobility of thiophene-
anthrance DTAnt was 1 order of magnitude lower than that of the
hexyl-substituted DHTAnt. This is likely due to the self-assembly
properties of the alkyl side chain, which leads to more ordered films,
especially at high substrate temperatures.9 This is in agreement with
differential scanning calorimetry (DSC) measurements, which
indicate that DHTAnt has high-temperature liquid crystalline
mesophases, while DTAnt shows only a melt transition with no
mesophase observed. Further increasing the substrate temperatures
for DHTAnt decreases the device performance. A similar trend
has been observed in pentacene and oligothiophenes.5
1
As corroborated by AFM measurements, the low mobility of
the devices under high substrate deposition temperatures is due to
9
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J. AM. CHEM. SOC. 2005, 127, 2406-2407
10.1021/ja043189d 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 (Average of Eight Individual Devices) of the
Semiconductors under Different Substrate Deposition
Temperatures
We also fabricated OTFTs using these semiconductors on plastic
substrates with organic dielectric materials as insulators. Their
performance was the same as that with the silicon wafer substrates,
and these results will be reported elsewhere.
In conclusion, we have synthesized two novel organic semicon-
ductors based on thiophene-anthracene molecules, which show
remarkably high stability and very good electrical characteristics,
rendering these materials excellent OTFT semiconductors for
flexible electronics applications.
SC material
Tsub
(
°C)
µ
(cm2/Vs)
on/off
Vt (V)
SubTS (V/d)
DTAnt
23
60
80
90
100
20
60
80
100
120
120/80
0.037 ( 0.004 9.15 × 105 -13.6
2.40
1.32
1.75
2.27
1.94
2.56
2.17
2.56
2.46
2.77
1.87
0.048 ( 0.015 7.36 × 106
0.063 ( 0.006 8.76 × 105
0.035 ( 0.001 3.82 × 105
0.025 ( 0.001 2.92 × 105
-5.4
-6.3
-3.4
-3.0
DHTAnt
0.13 ( 0.016
0.31 ( 0.014
0.34 ( 0.011
0.48 ( 0.096
0.21 ( 0.023
0.50 ( 0.045
2.0 × 106 -21.8
2.5 × 107 -12.5
6.6 × 108 -13.6
5.5 × 107 -10.7
2.6 × 107 -11.4
2.8 × 107 -10.4
Acknowledgment. We thank our colleagues I. Malajovich, R.
J. Chesterfield, J. S. Meth, F. Gao, G. Nunes, L. K. Johnson, H. E.
Simmons, C. R. Fincher, G. B. Blanchet, P. F. Carcia, K. G. Sharp,
and K. L. Adams for useful discussions. We are grateful to K. D.
Dobbs for molecular modeling; R. S. McLean and D. J. Brill for
AFM; E. Y. Hung and C. M. Ruley for MS; S. A. Hill for NMR;
D. J. Redmond and R. V. Davidson for XRD; and R. A. Twaddell,
M. Y. Keating, and S. Ahooraei for TGA and DSC tests. H.M.
thanks Prof. Bao and Prof. Perepichka for valuable discussions.
Supporting Information Available: Details of experimental
procedures and additional data or spectra, information of OTFT device
fabrication (PDF), and X-ray crystal structure (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
References
(1) (a) Horowitz, G. J. Mater. Res. 2004, 19, 1946. (b) Kelley, T. W.; Baude,
P. F.; Gerlach, C.; Ender, D. E.; Muyres, D.; Haase, M. A.; Vogel, D. E.;
Theiss, S. D. Chem. Mater. 2004, 16, 4413. (c) Dimitrakopoulos, C. D.;
Malenfant, P. R. L. AdV. Mater. 2002, 14, 99. (d) Bao, Z. AdV. Mater.
2000, 12, 227.
(2) (a) Baude, P. F.; Ender, D. A.; Haase, M. A.; Kelley, T. W.; Muyres, D.
V.; Theiss, S. D. Appl. Phys. Lett. 2003, 82, 3964. (b) Blanchet, G. B.;
Loo, Y.; Rogers, J. A.; Gao, F.; Fincher, C. R. Appl. Phys. Lett. 2003,
82, 463. (c) Gelinck, G. H.; Huitema, H. E. A.; van Veenendaal, E.;
Cantatore, E.; Schrijnemakers, L.; van der Putten, J. B. P. H.; Geuns, T.
C. T.; Beenhakkers, M.; Giesbers, J. B.; Huisman, B.; Meijer, E. J.; Benito,
E. M.; Touwslager, F. J.; Marsman, A. W.; van Rens, B. J. E.; de Leeuw,
D. M. Nat. Mater. 2004, 3, 106.
(3) (a) Qiu, Y.; Hu, Y.; Dong, G.; Wang, L.; Xie, J.; Ma, Y. Appl. Phys.
Lett. 2003, 83, 1644. (b) Pannemann, Ch.; Diekmann, T.; Hilleringmann,
U. J. Mater. Res. 2004, 19, 1999.
(4) (a) Gorjanc, T. C.; Levesque, I.; D’Iorio, M. Appl. Phys. Lett. 2004, 84,
930. (b) Katz, H. E. Chem. Mater. 2004, 16, 4748.
(5) (a) Facchetti, A.; Mushrush, M.; Yoon, M.; Hutchison, G. R.; Ratner, M.
A.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 13859. (b) Murphy, A. R.;
Frechet, J. M. J.; Chang, P.; Lee, J.; Subramanian, V. J. Am. Chem. Soc.
2004, 126, 1596. (c) Mas-Torrent, M.; Durkut, M.; Hadley, P.; Ribas, X.;
Rovira, C. J. Am. Chem. Soc. 2004, 126, 984. (d) Lin, Y.-Y.; Gundlach,
D. J.; Nelson, S. F.; Jackson, T. N. IEEE Trans. Electron DeVices 1997,
44, 1325. (e) Sakamoto, Y.; Suzuki, T.; Kobayashi, M.; Gao, Y.; Fukai,
Y.; Inoue, Y.; Sato, F.; Tokito, S. J. Am. Chem. Soc. 2004, 126, 8138. (f)
Meng, H.; Bendikov, M.; Mitchell, G.; Helgeson, R.; Wudl, F.; Bao, Z.;
Siegrist, T.; Kloc, C.; Chen, C. AdV. Mater. 2003, 15, 1090. (g) Sheraw,
C. D.; Jackson, T. N.; Eaton, D. L.; Anthony, J. E. AdV. Mater. 2003, 15,
2009.
(6) (a) Ito, K.; Suzuki, T.; Sakamoto, Y.; Kubota, D.; Inoue, Y.; Sato, F.;
Tokito, S. Angew. Chem., Int. Ed. 2003, 42, 1159. (b) Meng, H.; Zheng,
J.; Lovinger, A. J.; Wang, B.; Van Patten, P. G.; Bao, Z. Chem. Mater.
2003, 15, 1778. (c) Miao, Q.; Nguyen, T.; Someya, T.; Blanchet, G. B.;
Nuckolls, C. J. Am. Chem. Soc. 2003, 125, 10284. (d) Mushrush, M.;
Facchetti, A.; Lefenfeld, M.; Katz, H. E.; Marks, T. J. J. Am. Chem. Soc.
2003, 125, 9414. (e) Laquindanum, J. G.; Katz, H. E.; Lovinger, A. J. J.
Am. Chem. Soc. 1998, 120, 664.
Figure 2. Characteristics of DHTAnt OFET devices (L ) 60 µm, W )
600 µm) fabricated at Tsub 80 °C. (Left) Plot of IDS vs VGS at various gate
voltages: Shelf life time test. (Right) Plots of IDS vs VGS at constant VDS
)
-40 V: Operation-lifetime test. (NB: Current data are absolute values.)
voids or cracks formed under elevated substrate temperature. The
origin of the cracks is likely due to different thermal expansion
coefficients between the semiconductor and the substrate. To further
investigate these morphology issues, we fabricated FETs using a
multiple-deposition method.10 By first evaporating to a 20-nm-
thickness film under a high substrate deposition temperature (120
°C), we were able to cover the majority of the surface, maintaining
the large grain size of the semiconductor. The cracks and voids
were then filled during a second 20-nm deposition at a lower
substrate deposition temperature (80 °C). By depositing the
semiconductor using this two-stage process, we were able to
increase the device mobility to 0.50 cm2/Vs with an on/off current
ratio of 107.
To test the device stability, we performed both shelf life tests
and continuous operation tests under ambient conditions. In the
shelf life test, the device mobility and on/off ratios were measured
periodically over a span of 15 months. Figure 2 (left) shows the IV
characteristics of the device before and after 15 months of storage,
over which time the performance of the device was largely
unchanged. The mobility varied from 0.35 to 0.42 cm2/Vs, and the
on/off ratio varied from 1.1 × 106 to 4.7 × 107. In the operation-
lifetime tests, the device was subjected to continuous operation
under a constant drain-source voltage VDS of -40 V and a
continuous sweeping of gate-source voltages VGS between +40
and -40 V. Figure 2 (right) plots the characteristics of IDS versus
(7) (a) Cornil, J.; Calbert, J. Ph.; Bredas, J. L. J. Am. Chem. Soc. 2001, 123,
1250. (b) Fritz, S. E.; Martin, S. M.; Frisbie, C. D.; Ward, M. D.; Toney,
M. F. J. Am. Chem. Soc. 2004, 126, 4084
(8) Horowitz, G. AdV. Mater. 1998, 10, 365.
(9) Garnier, F.; Yassar, A.; Hajlaoui, R.; Horowitz, G.; Deloffre, F.; Servet,
VGS during the first gate voltage scan (double scan done in forward
B.; Ries, S.; Alnot, P. J. Am. Chem. Soc. 1993, 115, 8716.
and reverse bias) between +40 and -40 V and after approximately
2960 double scans, showing little drop in performance. In all cases
there is no obvious hysteresis present in the IV curves. DTAnt-
based OTFT exhibited similar stability as DHTAnt.
(10) (a) Lin, Y. Y.; Gundlach, D. J.; Nelson, S. F.; Jackson, T. N. IEEE Electron
DeVice Lett. 1997, 18, 606. (b) Horowitz, G.; Hajlaoui, M. E. AdV. Mater.
2000, 12, 1046.
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