Large area liquid crystal monodomain field-effect transistors

Article abstract of DOI:10.1021/ja055337l

Butyl, hexyl, and decyl derivatives of the liquid-crystalline organic semiconductor 5,5″-bis(5-alkyl-2-thienylethynyl)-2,2′:5′, 2″-terthiophene were synthesized and studied with respect to their structural, optical, and electrical properties. By means of an optimized thermal annealing scheme the hexyl and decyl compounds could be processed into self-assembled monodomain films of up to 150 mm in diameter. These were investigated with X-ray diffractometry, which revealed a clearly single-crystalline monoclinic morphology with lamellae parallel to the substrate. Within the lamellae the molecules were found to arrange with a tilt of about 50° with the rubbing direction of the polyimide alignment layer. The resulting, close side-to-side packing was confirmed by measurements of the UV/vis absorption, which showed a dichroic ratio of 19 and indicated H-aggregation. AFM analyses revealed self-affinity in the surface roughness of the monodomain. The compounds showed bipolar charge transport in TOF measurements, with hole mobilities reaching up to 0.02 cm²/Vs and maximum electron mobilities around 0.002 cm²/Vs. The hexyl derivative was processed into large-area monodomain top-gate field-effect transistors, which were stable for months and showed anisotropic hole mobilities of up to 0.02 cm²/Vs. Compared to multidomain bottom-gate transistors the monodomain formation allowed for a mobility increase by 1 order of magnitude.

Full text of DOI:10.1021/ja055337l

Published on Web 01/26/2006
Large Area Liquid Crystal Monodomain Field-Effect
Transistors
Albert J. J. M. van Breemen,*,† Peter T. Herwig,† Ceciel H. T. Chlon,†
Jo¨rgen Sweelssen,† Herman F. M. Schoo,† Sepas Setayesh,‡ Willie M. Hardeman,‡
Christian A. Martin,‡ Dago M. de Leeuw,‡ Josue´ J. P. Valeton,§
Cees W. M. Bastiaansen,§ Dirk J. Broer,§,‡ Andreea R. Popa-Merticaru,| and
Stefan C. J. Meskers|
Contribution from TNO Science and Industry, De Rondom 1, P. O. Box 6235, 5600 HE
EindhoVen, The Netherlands, Philips Research Laboratories, Professor Holstlaan 4, 5656 AA,
EindhoVen, The Netherlands, Department of Polymer Technology, Faculty of Chemistry and
Chemical Engineering, EindhoVen UniVersity of Technology, P. O Box 513, 5600 MB
EindhoVen, The Netherlands, and Laboratory of Macromolecular and Organic Chemistry,
EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands
Received August 29, 2005; E-mail: albert.vanbreemen@tno.nl
Abstract: Butyl, hexyl, and decyl derivatives of the liquid-crystalline organic semiconductor 5,5′′-bis(5-
alkyl-2-thienylethynyl)-2,2′:5′,2′′-terthiophene were synthesized and studied with respect to their structural,
optical, and electrical properties. By means of an optimized thermal annealing scheme the hexyl and decyl
compounds could be processed into self-assembled monodomain films of up to 150 mm in diameter. These
were investigated with X-ray diffractometry, which revealed a clearly single-crystalline monoclinic morphology
with lamellae parallel to the substrate. Within the lamellae the molecules were found to arrange with a tilt
of about 50° with the rubbing direction of the polyimide alignment layer. The resulting, close side-to-side
packing was confirmed by measurements of the UV/vis absorption, which showed a dichroic ratio of 19
and indicated H-aggregation. AFM analyses revealed self-affinity in the surface roughness of the
monodomain. The compounds showed bipolar charge transport in TOF measurements, with hole mobilities
reaching up to 0.02 cm2/Vs and maximum electron mobilities around 0.002 cm2/Vs. The hexyl derivative
was processed into large-area monodomain top-gate field-effect transistors, which were stable for months
and showed anisotropic hole mobilities of up to 0.02 cm2/Vs. Compared to multidomain bottom-gate
transistors the monodomain formation allowed for a mobility increase by 1 order of magnitude.
1. Introduction
molecules, such as oligoacene and thiophene derivatives, exhibit
mobilities of up to 15 cm2/Vs, but their production relies on
technologically unfavorable vapor growth routes.3-6 Semicon-
ducting polymers, in contrast, are typically solution processable
but exhibit instabilities under the influence of water and oxygen.
Even though the deposition of tetrathiafulvalene derivatives
(TTF) in single crystals and modified doctor blading of
functionalized anthradithiophenes have recently yielded solution-
processed OFETs with promising mobilities in the order of 1
cm2/Vs, the suitability of these processes for large-area deposi-
tion has not been demonstrated.7-9
Over the last years the field of organic electronics has seen
enormous progress. The performance of the best organic field-
effect transistors (OFETs) is already exceeding that of amor-
phous silicon devices, and the first integrated organic circuits
such as drivers for low-end displays and low-cost memory
devices for disposable smart labels and transponders have been
demonstrated.1,2
However, organic electronics are still far from exploiting their
market potential. The key to their commercial success is the
availability of a high-mobility organic semiconductor that is
solution-processable and stable under standard environmental
conditions. OFETs from single crystals of small conjugated
(3) Lin, Y.-Y.; Gundlach, D. J.; Nelson, S. F.; Jackson, T. N. IEEE Trans.
Electron DeVices 1997, 44, 1325.
(4) Sundar, V. C.; Zaumseil, J.; Podzorov, V.; Menard, E.; Willett, R. L.;
Someya, T.; Gershenson, M. E.; Rogers, J. A. Science 2004, 303, 1644.
(5) Meng, H.; Sun, F.; Goldfinger, M. B.; Jaycox, G. D.; Li, A.; Marshall, W.
J.; Blackman, G. S. J. Am. Chem. Soc. 2005, 127, 2406.
(6) Mello, 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.
(7) Mas-Torrent, M.; Hadley, P.; Bromley, S. T.; Ribas, X.; Tarre´s, J.; Mas,
M.; Molins, E.; Veciana, J.; Rovira, C. J. Am. Chem. Soc. 2004, 126, 8546.
(8) Mas-Torrent, M.; Hadley, P.; Bromley, S. T.; Crivillers, N.; Veciana, J.;
Rovira, C. Appl. Phys. Lett. 2005, 86, 012220.
(9) Payne, M. M.; Parkin, S. R.; Anthony, J. E.; Kuo, C.-C.; Jackson, T. N. J.
Am. Chem. Soc. 2005, 127, 4986.
† TNO Science and Industry.
‡ Philips Research Laboratories.
§ Faculty of Chemistry and Chemical Engineering, Eindhoven University
of Technology.
| Laboratory of Macromolecular and Organic Chemistry, Eindhoven
University of Technology.
(1) Huitema, H. E. A.; Gelinck, G. H.; van der Putten, J. B. P. H.; Kuijk, K.
E.; Hart, C. M.; Cantatore, E.; Herwig, P. T.; van Breemen, A. J. J. M.; de
Leeuw, D. M. Nature 2001, 414, 599.
(2) Gelinck, G. H.; et al. Nat. Mater. 2004, 3, 106.
9
2336
J. AM. CHEM. SOC. 2006, 128, 2336-2345
10.1021/ja055337l CCC: $33.50 © 2006 American Chemical Society

Large-Area Liquid Crystal Monodomain Field-Effect Transistors
A R T I C L E S
Scheme 1. Synthesis of Liquid-Crystalline Semiconductors 6
In this paper, we report on a successful route toward large-
area semiconducting films with a highly ordered morphologys
the controlled self-organization of solution-processable liquid-
crystalline organic semiconductors. The potential of liquid
crystals (LCs) in organic electronics has only recently been
recognized.10 The first report of a time-of-flight (TOF) mobility
of 0.1 cm2/Vs in discotic liquid crystals was published in 1994.11
Since then, numerous systematic studies have been carried out.
In a calamitic dioctylterthiophene (8-TTP-8), Funahashi and
Hanna achieved ambipolar TOF mobilities of 0.01 cm2/Vs.12
In general, highest mobilities can only be expected for ordered
monodomains with a large orbital overlap of neighboring
molecules. In optoelectronics, the alignment of LCs is often
achieved with the help of rubbed layers of polyimide (PI) or
modified poly(phenylenevinylene) (PPV)[10]. This concept has
also been applied in the production of the first LC polymer thin-
film transistor (TFT) in 2000.13 A thin film of poly-9,9-dioctyl-
fluorene-alt-bithiophene (F8T2) was deposited on a layer of
rubbed PI. A glass formed from the nematic phase then exhibited
mobilities of up to 0.02 cm2/Vs. Recently, quenching of another
LC oligofluorene semiconductor in the nematic phase allowed
for the fabrication of top-gate transistors with mobilities on the
order of 0.01 cm2/Vs.14 However, alignment processes that can
be carried out in thermal quasi-equilibrium and that lead to large
monodomains have not yet been demonstrated for LC organic
semiconductors.
The processability of LC semiconductors into large-area
ordered films depends strongly on their liquid-crystalline
mesophases. In general, a direct transition from the isotropic to
a highly ordered smectic or crystalline phase results in non-
aligned multidomain structures, even in the presence of align-
ment layers.15 Hence, our synthesis aimed at an organic liquid-
crystalline semiconductor with a tailored, more diversified phase
behavior. The formation of an aligned, defect-free monodomain
was expected to be facilitated when a sequence of phases with
increasing order and small formation enthalpies is present upon
cooling from the isotropic phase. A typical sequence of phases
which might enable the formation of monolithic structures could
be: from the isotropic phase to the nematic phase and then to
low ordered and highly ordered smectic phases and finally to a
crystalline phase. Here, the gradual increase in order and the
slow transitions between the phases were expected to suppress
the formation of a multidomain structure while going through
a phase transition.
important if plastic substrates such as poly(ethylene tereftalate)
are considered. The transition to the nematic phase for 6a is at
a relatively low temperature of 101 °C, making this class of
materials potentially very interesting for self-organized mon-
odomain FETs.
In this work, we report on the design and characterization of
soluble liquid-crystalline semiconductors (Scheme 1). By means
of optimized thermal processing, these semiconductors self-
organize in monodomain thin films over large areas of up to
150 mm in diameter, which can be used as active layers in field-
effect transistors. The formation of monodomains is ac-
companied by a 10-fold improvement of the charge carrier
mobility if compared to devices based on multidomain thin
films.
2. Experimental Section
General. All reagents and chemicals were obtained from commercial
sources and used without further purification. All reactions were
performed under an inert atmosphere of nitrogen. 1H NMR spectra were
recorded on a Varian 400 MHz spectrometer in deuterated chloroform
with tetramethylsilane as an internal reference. The 13C NMR spectra
were recorded at 100 MHz on the same spectrometer in deuterated
chloroform. Chemical shifts were defined relative to the 13C resonance
shift of chloroform (77.0 ppm). The absorption of the organic
semiconductors in solution and in thin films was determined with a
Perkin-Elmer Lambda 950 UV/vis spectrometer. The phase transition
temperatures were determined with a TA Instruments Q1000 differential
scanning calorimeter (DSC). The heating and cooling rates were kept
constant at 10 °C/min. The reported temperatures are the onset
temperatures. A Veeco multimode scanning probe microscope was used
to investigate the surface structure of organic semiconductor films. For
the measurements a stiff cantilever was used (40 N/m, 325 kHz). All
images were recorded with a Q-factor of 1000 and a scanning resolution
of 512 pixels × 512 pixels. Fourier analysis was carried out ex situ
with image analysis software from Nanoscope. TOF mobilities were
measured using 3.5 eV photon energies as obtained for a tripled Nd:
YAG laser with 5 ns pulse width. TOF signals were recorded using a
liquid crystal cell with 10 µm spacing between the electrodes. Bottom
and top electrodes consisted of tin-doped indium oxide, covered with
a thin, rubbed polyimide orientation layer. Rubbing direction of the
bottom and top contact were parallel. The cells were filled in an inert
atmosphere utilizing the capillary force acting on the liquid-crystalline
material in its nematic phase. Temperature-dependent current transients
Liquid-crystalline semiconductors with the above-mentioned
phase behavior are rare. A nice example, 5,5′′-bis(5-butyl-2-
thienylethynyl)-2,2′:5′,2′′-terthiophene (6a in Scheme 1), was
published by Ikeda.16 This material, developed to enhance the
optical-field-induced reorientation of LCs, shows upon cooling
from the isotropic phase a broad nematic phase, a smectic B
phase, and a crystalline phase. Since a nematic phase is preferred
for alignment, the corresponding transition temperature is
(10) O’Neill, M.; Kelly, S. M. AdV. Mater. 2003, 15, 1135.
(11) Adam, D.; Schuhmacher, P.; Simmerer, J.; Haussling, L.; Siemensmeyer,
K.; Etzbach, K. H.; Ringsdorf, H.; Haarer, D. Nature 1994, 371, 141.
(12) Funahashi, M.; Hanna, J. Appl. Phys. Lett. 2000, 76, 2574.
(13) Sirringhaus, H.; Wilson, R. J.; Friend, R. H.; Inbasekaran, M.; Wu, W.;
Woo, E. P.; Grell, M.; Bradley, D. D. C. Appl. Phys. Lett. 2000, 77, 406.
(14) Yasuda, T.; Fujita, K.; Tsutsui, T.; Geng, Y.; Culligan, S. W.; Chen, S. H.
Chem. Mater. 2005, 17, 264.
(15) Amundson, K. R.; Katz, H. E.; Lovinger, A. J. Thin Solid Films 2003,
426, 140.
(16) Zhang, H.; Shiino, S.; Shishido, A.; Kanazawa, A.; Tsutsumi, O.; Shiono,
T.; Ikeda, T. AdV. Mater. 2000, 12, 1336.
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A R T I C L E S
van Breemen et al.
Table 1. Phase Behavior of 6a-c and Their Mixtures (K: crystalline, Sm: smectic, N: nematic, I: isotropic)
were measured by first heating the cells to the isotropic phase followed
by slow (<10 deg/min) cooling to the temperature of interest. Processing
and electrical characterization of the semiconductors was performed
in ambient conditions. Bottom-gate transistors were prepared by spin-
coating a 2 wt % solution of the semiconductor in toluene at a speed
of 2000 rpm for 30 s on OFET test substrates. Heavily doped silicon
was used as the gate electrode, with a hexamethyldisilazane (HMDS)-
treated 200 nm thick layer of thermally oxidized SiO2 as the gate
insulating layer. Gold was thermally evaporated and patterned to form
interdigitated source and drain contacts. Monodomain top-gate transis-
tors were fabricated on ion-free glass substrates. Before the deposition
of the semiconductor, an alignment layer was applied according to the
following scheme:
The mixture was again degassed and heated for 15 min at 40 °C.
Subsequently, CuI (100 mg, 0.52 mmol) was added, and the mixture
was heated at reflux for 18 h. The solution was allowed to cool to
room temperature, CH2Cl2 (100 mL) was added, and the precipitate
was filtered off over Celite. The filtrate was concentrated in vacuo,
purified by column chromatography on silica using hexane-CH2Cl2
followed by recrystallization from hexane.
5,5′′-Bis(5-butyl-2-thienylethynyl)-2,2′:5′,2′′-terthiophene, 6a. This
product was prepared according to the general procedure and obtained
as an orange-red powder (3.1 g, 72%). Thermal behavior: see Table
1. 1H NMR (CDCl3, δ): 0.89 (t, 6H, CH3), 1.2-1.4 (m, 12H, -CH2-
), 1.67 (m, 4H, -CH2-), 2.80 (t, 4H, ar‚CH2-), 6.68 (d, 2H, ar‚H),
7.04-7.16 (m, 8H, ar‚H). 13C NMR (CDCl3, δ): 14.00, 22.23, 30.00,
33.69, 85.42, 88.30, 119.88, 122.14, 123.63, 124.31, 124.83, 132.34,
132.66, 136.02, 138.27, 149.08. Anal. Calcd for C28H28S5: C, 67.09;
H, 4.93; S, 27.98. Found: C, 67.13; H, 4.98; S, 27.89.
(1) 1 mL of PI (AL 1051 from JSR, Japan) was dissolved in 3 mL
of γ-butyrolacton,
(2) the substrate was fully covered with the PI-solution and spin-
coated for 20 s at 3000 rpm.
(3) the PI was cured on a hot plate at 90°C for 1 h and then at 180
°C for 3 h.
5,5′′-Bis(5-hexyl-2-thienylethynyl)-2,2′:5′,2′′-terthiophene 6b. This
product was prepared according to the general procedure and obtained
as orange fluffy crystals (3.8 g, 80%). Thermal behavior: see Table 1.
1H NMR (CDCl3, δ): 0.89 (t, 6H, CH3), 1.3-1.5 (m, 4H, -CH2-
CH3), 1.66 (m, 4H, -CH2-), 2.81 (t, 4H, ar‚CH2-), 6.68 (d, 2H, ar‚
H), 7.05-7.16 (m, 8H, ar‚H). 13C NMR (CDCl3, δ): 14.05, 22.54,
28.69, 30.22, 31.47, 31.51, 85.40, 88.27, 119.87, 122.14, 123.61, 124.33,
124.83, 132.34, 132.68, 136.01, 138.29, 149.07. Anal. Calcd for
C44H52S5: C, 68.74; H, 5.77; S, 25.49. Found: C, 68.67; H, 5.72; S,
25.61.
(4) the layer was manually rubbed on a velvet cloth.
Source and drain electrodes were lithographically defined after the
evaporation of a gold layer. The rubbed PI was then used as an
alignment layer for the mesogen materials. A mesogen film was first
applied by spin-coating and subsequently treated thermally. The
optimized process for the monodomain formation of 6b was the
following:
5,5′′-Bis(5-decyl-2-thienylethynyl)-2,2′:5′,2′′-terthiophene 6c. This
product was prepared according to the general procedure and obtained
as a red-orange solid (4.3 g, 77%). Thermal behavior: see Table 1. 1H
NMR (CDCl3, δ): 0.88 (t, 6H, CH3), 1.1-1.4 (m, 28H, -CH2-), 1.67
(m, 4H, -CH2-), 2.79 (t, 4H, ar‚CH2-), 6.69 (d, 2H, ar‚H), 7.04-
7.16 (m, 8H, ar‚H). 13C NMR (CDCl3, δ): 14.10, 22.67, 29.03, 29.31
(2 C-atoms), 29.53, 29.57, 30.23, 31.52, 31.88, 85.39, 88.28, 119.88,
122.18, 123.63, 124.34, 124.85, 132.35, 132.68, 136.04, 138.29, 149.11.
Anal. Calcd for C44H52S5: C, 71.30; H, 7.07; S, 21.63. Found: C, 71.21;
H, 6.97; S, 21.82.
(1) 10 mg of the semiconductor were dissolved in 1 mL toluene.
(2) the substrate with the rubbed polyimide was fully covered with
the freshly prepared semiconductor solution.
(3) a thin film was obtained by spin-coating for 25 s at 1200 rpm.
(4) the substrate was placed into a hot stage that was continuously
purged with dry nitrogen.
(5) the substrate was heated to 145 °C and then cooled to 30 °C at
a rate of 5 °C/min.
After thermal treatment, an insulating layer of a fluorinated polymer
(Teflon AF1600, DuPont) was spin-coated from FC75 (Acros), and
the gold top-gate electrode was evaporated through a shadow mask.
Single transistors were characterized at 40 °C in darkness, both in
ambient atmosphere and in a vacuum using a HP4156B semiconductor
parameter analyzer. Mobility statistics were obtained in ambient
atmosphere and darkness.
General Procedure 5,5′′-Bis(5-alkyl-2-thienylethynyl)-2,2′:5′,2′′-
terthiophenes, 6. To a mixture of 2-alkyl-5-trimethylsilylethynyl
thiophene (19 mmol) in dry THF (50 mL) was added under N2 TBAF
on silica (21.6 g, 24 mmol). After 5 min the mixture was filtered over
Celite and concentrated in vacuo. This material was immediately used
in the next step.
3. Results and Discussion
Synthesis. The new liquid-crystalline semiconductors 6b and
6c were targeted to study in detail the phase behavior,
monodomain formation, and electrical transport. These materials
can be conveniently synthesized according to the procedure
outlined in Scheme 1. Thus, alkylation of thiophene by means
of lithiation and quenching with an alkylbromide gave the
monoalkylthiophenes 2 in 75% yield. These were subsequently
brominated with N-bromosuccinimide to afford intermediates
3 in 90% yield, which were converted to 4 using trimethylsi-
lylacetylene in a Sonogashira17 coupling in 80% yield. Depro-
tection with tetrabutylammoniumfluoride on silica afforded the
To a degassed solution of dibromoterthiophene (3.05 g, 7.5 mmol)
and 2-alkyl-5-ethynyl thiophene 5 (19 mmol) in diisopropylamine (60
mL) and THF (15 mL) was added Pd(PPh3)4 (260 mg, 0.22 mmol).
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Large-Area Liquid Crystal Monodomain Field-Effect Transistors
A R T I C L E S
Figure 1. DSC traces (cooling run) of mesogens 6a-c and 1:1 mixture of
6b and 6c.
acetylenes 5 in nearly quantitative yield, which were directly
coupled with dibromoterthiophene, yielding the bis(5-alkyl-2-
thienylethynyl)-2,2′:5′,2′′-terthiophenes 6 in 80% yield. The
overall yield of this synthesis route was typically 40-45%.
Phase Behavior and Identification. The DSC traces and
phase transition temperatures of the calamitic compounds 6a-c
and the 1:1 mixture of 6b and 6c are collected in Figure 1 and
Table 1, respectively.
The determination and assignment of the mesophases were
carried out using differential scanning calorimetry (DSC) and
optical microscopy. All the compounds exhibit a nematic liquid-
crystalline phase at an elevated temperature. The clearing
temperature, the transition from the nematic to the isotropic
phase, decreases with increasing length of the alkyl side chain
of the compounds. Only the compounds with the longest alkyl
chains, hexyl derivative 6b and decyl derivative 6c, give rise
to a smectic C phase. The transition of the nematic to SmC
phase shows the typical bars under polarizing optical micros-
copy.18 As for the butyl-substituted derivative, 6a, the SmB
phase of 6b, 6c, and their 1:1 mixture display a mosaic texture.
Upon further cooling, molecules 6b and 6c show a highly
ordered SmG phase, which is missing for 6a and for the 1:1
mixture of 6b and 6c. Only the compounds 6a and 6b exhibit
a crystalline phase.
Monodomain Formation. To study the ability of mon-
odomain formation, thin films of the compounds were made
by spin-coating a toluene solution on rubbed polyimide.
Annealing the substrate to the nematic phase of the compound
(145 °C) and gradual cooling (5 °C/min) to 30 °C, resulted in
the formation of a monodomain in the size of the substrate.
Using this process we were able to produce monodomains of
up to 150 mm in diameter (see Figure 2).
Figure 3 presents the result of polarized light microscopy on
an as-spun and an annealed film of 6b. During spin-coating on
rubbed polimide, the mesogen crystallized in the form of
needles, which can be observed in Figure 3a. Upon heating to
the nematic phase, the mesogen self-organizes into a mon-
odomain. This monodomain is maintained upon cooling and
passing through the SmC-, SmB-, and SmG phases. At about
45 °C, crystallization starts at one side of the substrate. There
appears to be a cooperative effect, as the solid monodomain
grows from one side of the substrate to the other side. Under
crossed polarizers, this process can be observed as a gradual
Figure 2. Photograph of a 150-mm monodomain of 6b illuminated from
the back. The lower left of the wafer is placed between two crossed linear
polarizers. The enhanced transmission through the monodomain illustrates
the dichroism and uniformity of the ordered film.
darkening of the film. During this crystal growth, no grain
boundaries are formed, and hence the monodomain is main-
tained. Figure 3b-d depicts such a monodomain for different
rotation angles versus the polarization plane of the illumination.
While the uniform brightness indicates the absence of domain
boundaries in the films, the optical anisotropy gives an indication
of the order and the molecular alignment, which will be
discussed in more detail in the next section.
Although all compounds studied here display, upon cooling
from the isotropic phase, a sequence of phases with increasing
order, only defect-free monodomains at room temperature could
be made from 6b, 6c, and the 1:1 mixture of 6b and 6c. These
(mixtures of) compounds all show a slow transition to either
the crystalline phase (6b), the SmI phase (6c), or the SmF phase
(1:1 mixture of 6b and 6c). This is in contrast to the sharp peak
observed in DSC for the crystallization of 6a at about 50 °C
(see Figure 1). During this transition a lot of defects are
introduced in the monodomain. Apparently, the slow transition
found in 6b, 6c, and the 1:1 mixture of 6b and 6c is essential
for the preservation of defect-free monodomains when cooling
to room temperature. Furthermore, the layer thickness was, in
general, found to be essential for monodomain formation, a fact
which must be attributed to the anchoring strength of the rubbed
PI. Its influence on the long-range order of the mesogen is only
maintained for the critical thickness,19 beyond which the
monodomain order is lost and the material self-organizes in
multiple domains. In practice this means that for layer thick-
nesses below 150 nm, monodomains can be formed in a
reproducible way.
Polarized UV Absorption. The UV/vis absorption spectrum
of 6b in toluene is presented as an inset in Figure 4. It shows
two distinct absorption bands, one at a wavelength of 424 nm
(2.93 eV) and another at 311 nm (3.99 eV). The long wavelength
(17) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 16, 4467.
(18) Demus, D.; Richter, L. Textures of Liquid Crystals; Verlag Chemie:
Weinheim, 1978.
(19) Collings, P. J.; Patel, J. S. Handbook of Liquid Crystal Research; Oxford
University Press: New York, 1997.
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A R T I C L E S
van Breemen et al.
Figure 3. Light transmission micrographs of two films of compound 6b (polarizer in the horizontal, analyzer in the vertical direction): (a) untreated,
as-spun film; (b) monodomain with the rubbing direction of the PI layer parallel to the analyzer; (c) rubbing direction at 22.5° with respect to the analyzer;
(d) rubbing direction at an angle of 45° with respect to the analyzer.
maximum and a simultaneous red-shift of the onset. These
changes are often observed for oligothiophenes upon the
formation of aggregates21 or microcrystals, and these features
have also been observed in crystalline quinquethiophene films.22
The changes in the spectra can be rationalized in terms of
formation of H-aggregates. Here the close proximity and side-
to-side arrangement of the oligothiophene chromophores gives
rise to relatively strong interactions between the transition dipole
moments of the chromophoric units. This results in excited states
which are delocalized over a significant number of molecules
with the transition from the ground state and the lowest-energy
delocalized states being dipole-forbidden in nature.
For polarization angles >0° the overall absorbance of the
monodomain is reduced and reaches a minimum at an angle of
Figure 4. Absorbance of a monodomain of 6b on a glass/rubbed PI
substrate as a function of probe beam polarization. Film thickness: 50 nm.
(Inset) Molar absorption coefficient in toluene as a function of wavelength.
90°. At 357 nm, the dichroic ratio, i.e. the ratio of maximum to
minimum absorption, is as large as 19. Since the maximum
transition dipole moment in oligothiophenes can be expected
to be aligned along their backbone,22 this result suggests that
the 6b molecules are aligned or tilted in the direction of the
rubbing. By rocking the sample around an axis perpendicular
to the rubbing, a further increase in absorption was observed
for 0° degrees polarization angle. This indicates that the
molecules are not oriented perfectly perpendicular to the
substrate but that all molecules in the film are tilted in the
direction of the rubbing.
band is a dipole-allowed, π-π* absorption band of the
conjugated π-electron system of the core of the mesogens. This
band corresponds to the transition from the ground state to the
lowest excited singlet state of the molecule. The position of
this band matches sufficiently well with the long wavelength
absorption band of alkyl-substituted quinquethiophene in chlo-
roform, which has been reported at 420 nm.20 The second
absorption band at 311 nm corresponds to a transition to a
higher-lying singlet state.
It is well-known that, in first approximation, the dipole
strength of an absorption band is conserved when comparing
isolated and interacting molecules.23 This allows us to obtain
an estimate of the tilt angle of molecules by evaluating the area
under the absorption curve of the monodomain and comparing
The absorption spectra of monodomain thin films of 6b differ
significantly from those of the isolated molecules in solution.
A plot of the monodomain absorbance at different polarization
angles with respect to the PI rubbing direction is presented in
Figure 4. For an angle of 0°, the overall film absorbance is at
a maximum. Here, the spectrum displays a single peak at 357
nm and two shoulders at 431 and 469 nm. Comparison with
the absorption in solution reveals both a blue-shift of the
(21) Schenning, A. P. H. J.; Kilbinger, A. F. M.; Biscarini, F.; Cavallini, M.;
Cooper, H. J.; Derrick, P. J.; Feast, W. J.; Lazzaroni, R.; Leclere, Ph.;
McDonell, L. A.; Meijer, E. W.; Meskers, S. C. J. J. Am. Chem. Soc. 2002,
124, 1269.
(22) Kanemitsu, Y.; Shimizu, N.; Suzuki, K.; Shiraishi, Y.; Kuroda, M. Phys.
ReV. B 1996, 54, 2198.
(23) See, e.g.: Harada, N.; Nakanishi, K. Circular Dichroic Spectroscopy:
Exciton Coupling in Organic Stereochemistry; University Science Books:
Mill Valley, CA; Oxford University Press: Oxford, UK, 1983.
(20) Barbarella, G.; Favaretto, L.; Zambianchi, M.; Pudova, O.; Arbizzani, C.;
Bongini, A.; Mastragostino, M. AdV. Mater. 1998, 10, 551.
9
2340 J. AM. CHEM. SOC. VOL. 128, NO. 7, 2006

Large-Area Liquid Crystal Monodomain Field-Effect Transistors
A R T I C L E S
Figure 5. Wide angle (WAXS) specular Θ/2Θ scan of a monodomain of
6b. (Inset) Typical X-ray reflection scan.
this with isotropically averaged dipole strength of the isolated
molecules as observed in toluene solution. For compound 6b
in toluene we obtain a dipole strength of 48 D2 and hence a
transition dipole moment of 7 D when including a correction
for the Lorentz local field in the solvent (n ) 1.5) cage around
the molecule.24 For the thin monodomain film we find that the
projection of the transition dipole moment in the plain of the
film along the rubbing direction amounts to 4.6 D. This implies
an angle between the transition dipole moment of the molecule
and the in-plane rubbing direction of approximately 50°.
Microstructure of Monodomains. The microscopic order
in the monodomains was investigated with X-ray diffraction.
A wide angle (WAXS) specular Θ/2Θ scan is presented in
Figure 5. The diffraction peak at 5.93° can be assigned to the
200 reflection. Peaks at larger angles are due to higher-order
reflections, up to the 10th order. The observed order in one
dimension of this highly layered structure corresponds to at least
a liquid-crystalline phase, but possibly a crystalline phase. There
are no other lattice planes parallel to the substrate. The
interplanar spacing amounts to 30.1 Å. The length of the
molecule as calculated with ChemDraw3D is 37 Å. When we
assume that the packing of the molecules (or mesogens) is
comparable to that of compound 6a,25 we can estimate that the
molecules are tilted with respect to the polyimide alignment
layer under an angle of about 54°, a value that is supported by
the tilt calculated from the absorption measurements. A detailed
analysis of the in-plane order using pole figure measurements
will be published elsewhere. The small-angle reflections are
shown in the insert in Figure 5. The insert shows the 100, 200,
and 300 diffraction peaks and the total internal reflection and a
number of interference, or Kiessig,26 fringes. Interpretation of
the fringes, i.e. determination of polyimide- and mesogen 6b
layer thickness and roughness, was not attempted.
Figure 6. (Upper image) Atomic force micrograph of a monodomain of
6b. The polyimide rubbing direction is parallel to the horizontal axis. The
topography shows elongated fibrils perpendicular to the rubbing direction
and lamellae parallel to the rubbing direction. (Lower figure) Typical height
profile observed at small scan lengths. The profile shows discrete steps
whose height corresponds to the interplanar spacing of 30 Å.
fibrils perpendicular to the rubbing direction, and an underlying
structure of lamellae parallel to the rubbing direction. Height
profiles measured at small scan lengths show discrete steps. A
typical example is given in the lower part of Figure 6. The height
of the steps corresponds to the interplanar spacing of 30 Å as
obtained from WAXS measurements. Hence, the monodomain
grows as a stack of discrete monolayers. The origin of the fibrils
is not yet known; they might be due to changes in the orientation
during crystal growth.
The height profiles were analyzed from the power spectral
density (PSD) as a function of the spatial frequency, f.27 The
PSD is calculated by using eq 1, taking scan lengths, L, from
80 to 2 µm both parallel and perpendicular to the polyimide
rubbing direction. The calculated PSD is presented in Figure 7.
PSD(f) ) [ Lh(x) exp(i2πfx) dx]2
1
L
∫
0
Topography of Monodomains. The surface roughness of
the monodomains was analyzed with atomic force microscopy
(AFM) in noncontact tapping mode using scan lengths, L, from
80 to 2 µm. A micrograph of 20 µm scan length is presented in
Figure 6. The rubbing direction of the polyimide corresponds
with the horizontal direction. The micrographs show elevated
with
(γ - d)
R )
(1)
2
Both horizontal and vertical PSD scans show that for each scan
length the height fluctuations are spatially correlated. The top
graph shows uncorrelated behavior or a white spectrum at low
(24) Michl, J.; Thulstrup, E. W. Spectroscopy with Polarized Light; VCH: New
York, 1986.
(25) Zhang, H.; Shiino, S.; Kanazawa, A.; Tsutsumi, O.; Shiono, T.; Ikeda, T.;
Nagase, Y. Synth. Met. 2002, 126, 11.
(27) Biscarini, F.; Samori, P.; Greco, O.; Zamboni, R.; Phys. ReV. Lett. 2002,
78, 2389.
(26) Kiessig, H. Ann. Phys. 1931, 10, 769.
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J. AM. CHEM. SOC. VOL. 128, NO. 7, 2006 2341

A R T I C L E S
van Breemen et al.
Figure 8. Time-of-flight photocurrent transients for 6b with 10 µm
electrode spacing, 3.5 eV photon energy, and T ) 100 °C, transport due to
holes. Electric field strength 4 × 104 V/cm.
Figure 7. Power spectral density (PSD) of the monodomain topography
from AFM images of different scan lengths, L, from 80 µm through 2 µm.
Top and bottom figures correspond respectively to the scan direction parallel
and perpendicular to the rubbing direction. (Inset in top figure) Estimated
correlation length ê versus the scan length L. The solid line is a power law
fit.
frequencies. However, this plateau spans only a few points,
suggesting that the apparent breakdown of correlations is partly
due to the finite AFM image size.28 This interpretation is
supported by the inset showing an increase of the estimated
correlation length (viz. the inverse of the breakdown frequency
separating the power law decay from the apparent plateau) with
the scan length L. A power law fit yielded ê ≈ L0.38, while a
linear dependence would be expected for the ideal case. We
note that the scans perpendicular to the rubbing direction do
not show a plateau related to the image size. Hence, the origin
of the loss of correlation at low spatial frequencies for the
parallel scan direction remains unclear. It might also be due to
the fibrils (see Figure 6). These protrusions run perpendicular
to the rubbing direction and, therefore, the height profiles on
both sides are not correlated. This is supported by the values
obtained for the coherence length.27 The coherence length in
the rubbing direction is about 2-5 µm, comparable to the
distance between the fibrils. The coherence length perpendicular
to the rubbing direction is estimated to be larger by an order of
magnitude.
Figure 9. (Lower part) Time-of-flight mobility for 6b as a function of
temperature. (9) Holes. (O) Electrons. Electric field 4 × 104 V/cm, path
length: 10 µm. (Upper part) DSC trace for 6b indicating the various phase
transitions.
directions a value for R of about 0.6-0.7 is obtained. This might
indicate that the growth mechanism of the monodomains is
dominated by adsorption of molecules on kink sites similar to
those observed for MBE-grown films.27,31
Electrical Transport Measurements. To evaluate the mobil-
ity of charge carriers in the bulk of the mesogenic material we
have used conventional time-of-flight (TOF) mobility measure-
ments. Figure 8 shows the TOF photocurrent transients for 6b
for positive charge carriers in the SmB phase. After excitation
at t ) 0, the arrival time of the holes at the electrode opposite
to the side of illumination is clearly marked by a kink in the
photocurrent at t ) 1.1 µs.
For compound 6b, slow charge carrier transport (µ ) 10-5
cm2/Vs) is observed in the nematic phase of the material (Figure
9). Such low mobilities at high temperatures agree with the
behavior of other mesogens and may be interpreted in terms of
transport of ionized impurities in the low-viscosity phases.32
Upon lowering the temperature for 6b, almost nondispersive
transport and high carrier mobilities are observed in the SmB
phase. Here, the hole mobility amounts to 2 × 10-2 cm2/Vs
and is almost independent of the applied field and temperature
and shows a negligible variation with the intensity of the light
pulse. The hole mobility found is slightly higher than that
reported for alkyl-functionalized terthiophenes.12 Mobilities for
electrons (1.2 × 10-3 cm2/Vs, T ) 80°C, electric field strength
Figure 7 shows that the PSD scales over more than 5 decades
with spatial frequency. This indicates that height fluctuations
are spatially correlated. The topography of the monodomains
shows anisotropic or self-affine scaling.27 Similar scaling
behavior has been observed for, e.g. plasma-deposited polymer
films29 and thin films of oligothiophenes.27,28,30 The power law
exponent is related to the static scaling parameter R via eq 1,
where the scan dimension d is equal to unity. For both scanning
(28) Melucci, M.; Gazzano, M.; Barbarella, G.; Cavallini, M.; Biscarini, F.;
Maccagnani, P.; Ostoja, P. J. Am. Chem. Soc. 2003, 125, 10266.
(29) Collins, G. W.; Letts, S. A.; Fearon, E. M.; McEachern, R. L.; Bernat, T.
P. Phys. ReV. Lett. 1994, 73, 708.
(30) Biscarini, F.; Zamboni, R.; Samor´ı, P.; Ostoja, P.; Taliani, C. Phys. ReV. B
1995, 52, 14868.
(31) Lai, Z.-W.; Das Sarma, S. Phys. ReV. Lett. 1991, 66, 2348.
9
2342 J. AM. CHEM. SOC. VOL. 128, NO. 7, 2006

Large-Area Liquid Crystal Monodomain Field-Effect Transistors
A R T I C L E S
E ) 3 × 104 V/cm) are almost 1 order of magnitude lower
than those for holes (1.7 × 10-2 cm2/Vs, T ) 80°C, E ) 3 ×
104 V/cm) and increase with temperature (see Figure 9). Similar
differences between electron and hole transport have been
observed in other smectic organic semiconductors, where the
differences were attributed to different transport mechanisms.33
In the latter case, electron transport was found to be limited by
shallow trap levels. Upon further lowering the temperature below
the phase transition between the SmB and SmG phase, the
current transient becomes strongly dispersive, which we attribute
to the scattering effects of grain boundaries. Optical microscopy
showed that for these thick films (10 µm) domains of limited
spatial extent are formed, in contrast to thin films which can
form large monodomains.
Mobilities for compound 6c are almost the same as for 6b
(µhole ) 1.9 × 10-2 cm2/Vs, µelectron ) 2.4 × 10-3 cm2/Vs, T )
100° C, E ) 3 × 104 V/cm). Interestingly, for a mixture of 6b
and 6c (1:1 by weight), mobilities are comparable to those of
the pure compounds (µhole ) 1.1 × 10-2 cm2/Vs, µelectron ) 1.4
× 10-3 cm2/Vs, T ) 80°C, E ) 3 × 104 V/cm) Apparently,
the disorder introduced by mixing the two different mesogens
does not affect the charge carrier transport. This may be related
to the fact that the difference in chemical structure between the
two mesogens is located in the side chains far away from the
central π-conjugated moiety.
To investigate the electrical performance of thin films from
the LC semiconductor 6b and to compare it to previous results
on mesogen devices, thin-film transistors in the bottom- and
top-gate architecture were prepared. Spin-coating of 6b on
bottom-gate substrates led to the formation of multiple domains
with sizes on the order of several micrometers. Similar results
have been reported for other mesogens deposited without
alignment layers. Arrangement of mesogens in such multido-
mains usually prevents full exploitation of their semiconducting
properties in thin-film transistors.34
A detrimental influence of the domain boundaries could also
be observed in our samples. The transfer characteristics of a
representative bottom-gate transistor from 6b in a vacuum are
given in Figure 10. Above a gate bias of approximately 0 V,
the depletion of the semiconductor layer leads to a drain current
on the order of 100 pA. The remaining current can be attributed
to parasitic conduction from the gate probe, which, in this setup,
cannot be shielded from the semiconductor film. For a gate bias
below 0 V, the drain current increases rapidly. Microscopically,
this effect corresponds to the build-up of an accumulation
channel at the interface to the insulator. As Vg is decreased
further, the current increases to reach a maximum value of 3
µA in the saturated regime, leading to an on/off ratio of more
than 104. However, the counterclockwise hysteresis in the
transfer characteristics is rather large and might be attributed
to the presence of trapping sites at domain boundaries or to
states at the semiconductor-dielectric interface.35
Figure 10. Transfer characteristics of a transistor with interdigitated
electrodes, prepared from 6b in the bottom-gate architecture. The channel
width was 10 mm, the length was 10 µm. The device was measured in a
vacuum.
regime, was calculated according to
∂Id
VdWCi′ ∂Vg
L
µlin
)
(2)
For maximum hole accumulation at Vg ) -40V it is around
5 × 10-4 cm2/Vs. This value is much lower than the mobility
in the saturated regime, which is given by
∂2Id
L
µsat
)
(3)
2
WCi′ ∂V
g
and which amounts to approximately 0.004 cm2/Vs. The
difference can be attributed to an injection limitation in the
device, which will be addressed later. In the following, the
saturated mobility will be used for discussion. For the device
presented in Figure 10 it is on the order of the values reported
for other mesogen multidomain TFTs by McCulloch and co-
workers,34 but far from the mobility achieved in the TOF
measurements. Hence, the device geometry was adapted to
improve the order in the semiconductor films.
In the resulting top-gate architecture the Au source and drain
contacts were located on top of a PI layer, rubbing of which
enabled the alignment of the LC semiconductor. In these
devices, spin-coating and annealing of a film of 6b (see
Experimental Section) was followed by spin-coating the gate
dielectric and evaporation of the gate electrode. Control samples
in which the PI was not rubbed exhibited a performance similar
to the bottom-gate devices presented earlier, with saturated
mobilities reaching 0.004 cm2/Vs and trapping-dominated
hysteresis. Full exploitation of the electrical properties of 6b
was only possible in top-gate devices with a rubbed PI layer.
Here, the alignment of the LC semiconductor led to mon-
odomain TFTs with a significant improvement in mobility and
a decrease in hysteresis.
Figure 10 also displays the hole mobilities of the transistor.
The mobility for a drain bias of Vd ) -5 V, i.e., in the linear
Figure 11 presents the transfer characteristics of a typical
monodomain top-gate transistor with interdigitated electrodes.
The device exhibits a large on/off ratio of approximately 106
in the saturated regime. To a large extent, this high performance
is due to the significantly small off current of 10 pA. A further
benefit of the top-gate architecture is illustrated by the extremely
small hysteresis in the displayed transfer characteristics. The
(32) Funahashi, M.; Hanna, J. Chem. Phys. Lett. 2004, 397, 319.
(33) Shiyanovskaya, I.; Singer, K. D.; Twieg, R. J.; Sukhomlinova, L.; Gettwert,
V. Phys. ReV. E 2002, 65, 041715.
(34) McCulloch, I.; Zhang, W.; Heeney, M.; Bailey, C.; Giles, M.; Graham,
D.; Shkunov, M.; Sparrowe, D.; Tierney, S. J. Mater. Chem. 2003, 13,
2436.
(35) Zilker, S. J.; Detcheverry, C.; Cantatore, E.; de Leeuw, D. M. Appl. Phys.
Lett. 2001, 79, 1124.
9
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A R T I C L E S
van Breemen et al.
Figure 13. Statistical results of the saturated mobilities of devices of 6b.
The squares give the resulting mean mobility as a function of channel length,
and the bars mark the standard deviation.
Figure 11. Transfer characteristics of a monodomain top-gate TFT with
its channel perpendicular to the rubbing direction of the alignment layer.
The channel width was 20 mm, the length was 20 µm. The device was
measured in a vacuum.
gate transistors from R,R′-bisdecyl-sexithiophene, hole mobilities
on the order of 0.1 cm2/Vs and on/off ratios of 105 have been
reported.37 Evaporation of R,R′-bishexyl-sexithiophene, in con-
trast, has led to comparable mobilities, but much lower on/off
ratios.38
Within the experimental error, the mobility in monodomain
transistors was found to be independent of channel width and
considerably consistent throughout the sample. Figure 13
displays the saturated mobility in TFTs as a function of channel
length. In the devices based on 6b the saturated mobility in
channels perpendicular to the rubbing direction of the polyimide
is by a factor of 1.2-3.3 larger than that parallel to the rubbing.
Electrical anisotropy has also been reported in the LC
copolymer transistors fabricated by Sirringhaus and co-work-
ers.13 There, the mobility values in the direction of the
conjugated backbone were by a factor of 10 larger than those
in the direction perpendicular to the molecule chains. However,
faster intramolecular charge transport processes cannot account
for the results in the case of 6b. Both parallel and perpendicular
to the rubbing direction, hole transport in the 6b monodomain
is mainly governed by intermolecular charge transfer. Possible
differences in the electrical properties of the bulk can merely
be induced by differing π-orbital overlaps. These could either
be due to the tilt of the molecules revealed by optical analyses,
or due the rotational asymmetry of the backbone of 6b. Another
origin of the mobility anisotropy in the field-effect transistors
might be found in the increased surface roughness of the 6b
monodomain in the direction of the rubbing. The extent of the
individual contributions of surface and bulk properties is not
accessible without further information about the microstructure
of the material.
Figure 12. Output characteristics of the same monodomain top-gate TFT
as displayed in Figure 11. The device was measured in a vacuum.
absence of domain boundaries reduced the number of trapping
sites in the device significantly.
This also led to an increase in mobilities compared to the
bottom-gate TFTs. In the linear regime the top-gate device
displayed in Figure 11 exhibits a mobility around 0.01 cm2/Vs,
which is slightly smaller than the value in saturation. As
discussed earlier, this difference can be attributed to an injection
limitation in the device, which might be due to a mismatch of
the HOMO level of 6b and the work function of the source and
drain electrodes. It is further illustrated by the output charac-
teristics of the device (see Figure 12), which display a
superlinear current onset.
The saturated mobility in the monodomain TFT is as large
as 0.02 cm2/Vs and hence by almost an order of magnitude
larger than that in multidomain devices. This also led to an
increase in the saturated mobility compared to that of the
bottom-gate TFT, which is as large as 0.02 cm2/Vs in the
monodomain devices. It is in good agreement with the hole
mobility obtained from the TOF experiments, which often
represents an upper limit to the transport properties in FETs.36
The result exceeds the values reported for other reactive and
nonreactive mesogens by far34 and is on the order of the highest
value reported for LC TFTs.13,14 Furthermore, it is close to that
of evaporated alkyl-substituted oligothiophenes. For bottom-
Overall, the saturated mobility displayed in Figure 13 seems
to be largest for the smallest channel length. The output
characteristics of devices with several channel lengths revealed
short-channel effects, a phenomenon which has already been
observed in other organic TFTs.39 As a consequence of the large
longitudinal electric field in the channel, the ideal equation
employed for the mobility analysis does not hold, and the
saturated mobility might be overestimated.
(37) Halik, M.; Klauk, H.; Zschieschang, U.; Schmid, G.; Radlik, W.; Pono-
marenko, S.; Kirchmeyer, S.; Weber, W. J. Appl. Phys. 2003, 93, 2977.
(38) Deman, A.-L.; Tardya, J.; Nicolas, Y.; Blanchard, P.; Roncali, J. Synth.
Met. 2004, 146, 365.
(39) Chabinyc, M. L.; Lu, J.-P.; Street, R. A.; Wu, Y.; Liu, P.; Ong, B. S. J.
Appl. Phys. 2004, 96, 2063.
(36) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99.
9
2344 J. AM. CHEM. SOC. VOL. 128, NO. 7, 2006

Large-Area Liquid Crystal Monodomain Field-Effect Transistors
A R T I C L E S
parallel to the substrate. Within these lamellae, the molecules
are tilted in the rubbing direction of the underlying PI layer at
an angle of roughly 40-50 degrees versus the plane normal.
The semiconducting properties of the compounds were
studied by time-of-flight and transistor measurements. The hexyl
compound showed the best bipolar transport properties, with
maximum hole and electron TOF mobilities of 0.02 and 0.002
cm2/Vs, respectively. This compound was further processed into
top-gate monodomain field-effect transistors. Only this geometry
allows the full exploitation of the self-organization of the liquid-
crystalline semiconductor. Hole mobilities in this architecture
reached up to 0.02 cm2/Vs and were by 1 order of magnitude
larger than those in multidomain bottom-gate devices. They were
found to be stable for at least 3 months in ambient conditions.
The mobility in monodomain transistors was anisotropic by a
factor of about 2, with enhanced transport in the direction
perpendicular to the rubbing of the PI alignment layer. A
possible origin of this anisotropy can be found in both the bulk
packing and in the surface properties of the monodomain, which
showed the smoothest features in the direction perpendicular
to the rubbing direction.
Figure 14. Transfer characteristics of 6b as a function of time. Devices
were stored at room temperature in ambient conditions.
To investigate the stability of the compound 6b, some devices
were again measured after prolonged storage in ambient
conditions (see Figure 14). The devices were found to be stable,
with mobility changes of less than 10% after three months in
air and darkness. This good stability might be attributed to the
absence of domain boundaries which, in the case of pentacene,
have shown to be sensistive to degradation by water.40 Fur-
thermore, the position of the HOMO level is important for the
stability of organic semiconductors against oxidative doping.34
For 6b a value of 5.7 eV vs vacuum was measured, which is
apparently large enough to prevent degradation of the TFT
characteristics on the time scale studied.
In terms of phase behavior, the studied 5,5′′-bis(5-hexyl-2-
thienylethynyl)-2,2′:5′,2′′-terthiophene must be regarded as an
ideal model compound for self-organizing liquid-crystalline
organic semiconductors. On the other hand, the monodomain
transistor mobility was similar to the transport data in TOF
experiments and might therefore mark an intrinsic performance
limit of this material. We anticipate, however, that this study
paves the way for further improvement of the performance of
solution-processable organic semiconductors, using other semi-
conducting cores that exhibit denser packing and thus enhanced
charge transport in monodomain field-effect transistors.
4. Conclusion
Butyl, hexyl, and decyl derivatives of the solution-processable
compound 5,5′′-bis(5-alkyl-2-thienylethynyl)-2,2′:5′,2′′-ter-
thiophene were synthesized to target self-organized semiconduc-
tor films for the application in large-area organic electronic
devices. The phase behavior of the molecules with hexyl and
decyl spacer groups was found to be ideal for the formation of
highly ordered monodomains on an alignment layer of rubbed
PI. Due to the slowly increasing order of their smectic
mesophases and due to a stable crystalline phase at room
temperature it was possible to process these compounds and
their mixtures into monodomains of up to 150 mm in diameter.
The order in these domains was confirmed by X-ray diffraction
measurements, AFM studies and absorption spectroscopy, which
suggest a side-to-side arrangement of the molecules in lamellae
Acknowledgment. We thank Dr. H. Wondergem of Philips
Research for measuring X-ray spectra, Dr. K. Genevicius, Dr.
M. Shkunov and Dr. I. McCulloch from Merck Chemicals for
assistance with the TOF measurements and generous supply of
liquid crystal cells, Prof. P. Strohriegl from the University of
Bayreuth for valuable discussions, and the Dutch Ministry of
Economic Affairs for supporting this work under the BTS
framework program “StableFET”.
Supporting Information Available: Complete ref 2; synthesis
and analytical data of compounds 2-4. This material is available
free of charge via the Internet at http://pubs.acs.org.
(40) Qiu, Y.; Hu, Y.; Dong, G.; Wang, L.; Xie, J.; Ma, Y. Appl. Phys. Lett.
2003, 83, 1644.
JA055337L
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