Intrinsic charge carrier mobilities at InsulatorSemiconductor interfaces probed by microwave-based techniques: Studies with liquid crystalline organic semiconductors

Article abstract of DOI:10.1246/cl.150593

The intrinsic, local-scale hole and electron mobilities at the interface between perylenediimide (PDI)-based liquid crystalline organic semiconductors and insulating polymers were evaluated by field-induced time-resolved microwave conductivity (FITRMC). T

Full text of DOI:10.1246/cl.150593

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Received: June 18, 2015 | Accepted: July 16, 2015 | Web Released: July 25, 2015
CL-150593
Intrinsic Charge Carrier Mobilities at Insulator-Semiconductor Interfaces
Probed by Microwave-based Techniques:
Studies with Liquid Crystalline Organic Semiconductors
Tsuneaki Sakurai,*1,2 Yusuke Tsutsui,1,2 Wookjin Choi,1,2 and Shu Seki*1,2
1Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871
2Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510
(E-mail: sakurai-t@moleng.kyoto-u.ac.jp, seki@moleng.kyoto-u.ac.jp)
The intrinsic, local-scale hole and electron mobilities at the
interface between perylenediimide (PDI)-based liquid crystalline
organic semiconductors and insulating polymers were evaluated
by field-induced time-resolved microwave conductivity (FI-
TRMC). The PDI liquid crystals having alkyl and semi-
fluoroalkyl tails showed ambipolar hole and electron mobilities
of 0.2 and 0.3 cm2 V¹1 s¹1, respectively. In contrast, the PDI
having triethylene glycol and semifluoroalkyl tails showed no
significant conductivity, suggesting an important role of side
chain structures on the charge transport properties. The FI-
TRMC technique demonstrated here serves as a powerful tool to
screen out the interfacial structure of liquid crystalline semi-
conductor materials.
Figure 1. Molecular structures of perylenediimide (PDI) deriva-
tives PDIC12/C12, PDIF/C12, and PDIF/TEG
.
Macroscopic orientation control of liquid crystalline (LC)
materials has been of particular interest since its discovery1 on
substrate surfaces, and a variety of “commanding” methods of
the orientation have been developed and reported to date.2 The
commanding approach was at first realized by simple rubbing of
the substrate surfaces, and subsequently demonstrated by an use
of self-assembled monolayer,3 macromolecules,4 photoaligned
molecular surfaces,5 and finally by free surfaces.2 The successful
commanding techniques are strongly suggestive of a crucial role
of the interface structure between LCs and substrates not only in
the macroscopic orientation of the materials but also the short-
range order of LCs at the interfaces.
To address the interfacial charge carrier mobility, we
developed a novel system referred to as field-induced time-
resolved microwave conductivity (FI-TRMC).12-15 In the FI-
TRMC technique, simple metal-insulator-semiconductor (MIS)
devices are prepared and set into a microwave cavity. Because of
the presence of metal electrodes, effective power of the reflected
microwave is decreased, giving smaller dynamic range than that
of FP-TRMC. However, upon gate bias being applied to MIS
devices, we can independently monitor the time dependence of
both dielectric loss and injected charge amounts at the interface.
These two kinetic traces are overlapped, indicating the accumu-
lated mobile charges solely rely on the observed dielectric loss
phenomena. Namely, complete in situ observation is achieved
in the FI-TRMC system, leading to the comprehensive analysis
of nanometer-scale charge carrier motions at the interface.
Furthermore, the polarity of the applied gate bias controls the
polarity of the accumulated charge carrier species, enabling the
separate evaluation of hole and electron mobilities. In the present
work, to confirm the availability of the FI-TRMC technique, we
tried to measure spin-coated films of LC semiconductors based
on perylenediimide (PDI) derivatives (Figure 1). As a result, we
successfully obtained the microwave response as well as charge
amount profiles for LC organic semiconductors. Of further
interest, obvious effects of side chain structures on the mobility
of distinct negative and positive charge carriers as well as
density of interfacial carrier traps were clearly demonstrated,
proposing the importance of side chains of LC semiconductors
on the intrinsic charge carrier transporting phenomena.
Since the first discovery of discotic LC materials,6 LC-
substrate interactions have become more significant in control-
ling the orientation because of the relatively weak motivation to
produce fundamental 1D columnar structures via π-π stacking
interactions in comparison with LCs with covalently extended
rod-like mesogens. Moreover, the fundamental π-π stacking
interactions preserve the electronic communication among the
π-conjugated cores, leading to the semiconducting nature of the
LCs materials.7-9 Considering that many kinds of π-conjugated
motifs, designed for the application in organic electronics,
have been continuously reported over the recent decades, quick
screening of their conductive property is strongly needed. The
pulse-radiolysis (PR-) or flash-photolysis (FP-) time-resolved
microwave conductivity (TRMC) methods10,11 have served as
a rapid and accurate measurement technique for the screening
process because it only requires a small amount of sample and
can accept a variety of sample morphologies including crystals,
powders, films, and so on. However, in these classical TRMC
methods, charge carriers are generated in bulk. Therefore, it is
difficult to access the charge carrier transport property at the
insulator-semiconductor interfaces.
PDI-based molecules having immiscible side chain
pairs16,17 were synthesized as described in the Supporting
Information. PDIF/C12 has taper-shaped semifluoroalkyl chains
at one imide position while the other imide position carries
Chem. Lett. 2015, 44, 1401–1403 | doi:10.1246/cl.150593
© 2015 The Chemical Society of Japan | 1401

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Table 1. Phase behaviors with transition temperatures (°C)a
aTransition enthalpies (kJ mol¹1) are given in parentheses. Symbols G,
Colh, Colr, Colx, and IL denote glassy, hexagonal columnar liquid
crystalline, rectangular columnar liquid crystalline, unidentified
columnar, and isotropic liquid phases, respectively.
Figure 3. Injected charge amount profiles of PDIF/C12-based MIS
device under applied (a) positive (electron injection) and (b) negative
(hole injection) bias. Kinetic traces of changes in reflected microwave
power under application of (c) positive and (d) negative bias to
PDIF/C12-based MIS device and (e) positive and (f) negative bias to
PDIF/TEG-based MIS device.
fringent textures appeared under the crossed polarized condition
(Figures 2c and 2d), which indicates that LC columns mostly
align parallel to the substrate in the as-cast thin films.
Figure 2. Electronic absorption spectra of PDIF/C12 (red) and
PDIF/TEG (blue) in (a) thin films and (b) CHCl3 solutions at
4 © 10¹5 M. Optical (left) and polarized (right) micrographs of spin-
We performed the FI-TRMC technique to evaluate the local-
scale charge carrier mobility at insulator/semiconductor inter-
faces. Analogous to the previous report, a poly(methyl meth-
acrylate) (PMMA) thin film, selected as an insulating polymer,
was fabricated by spin-coating onto a SiO2-Au-quartz substrate
from a toluene solution.12-15 Then the semiconducting LCs were
spin-coated on the substrate from a THF solution, followed by
the deposition of a Au electrode. When the MIS device (Au-
SiO2-PMMA-PDIF/C12-Au) was set in the cavity and subjected
to positive gate bias voltages, electrons were injected into the
interface between PDIF/C12 and PMMA (Figure 3a). Accord-
ingly, we observed the response of the reflected microwave
changes (Figure 3c), indicating that the injected electrons con-
tribute to the local-scale conductivity at the interface. By using
the empirical equation between ¦Pr and ¦N®,13 we evaluated
the local-scale electron mobility (®e) of 0.3 cm2 V¹1 s¹1 from the
¦N-¦N® plot (Figure 4a). A similar process was carried out for
the negative bias condition (Figures 3b and 3d), affording the
coated films of (c) PDIF/C12 and (d) PDIF/TEG
.
taper-shaped dodecyl chains (Figure 1). PDIF/TEG is an ana-
logue of PDIF/C12 decorated with semifluoroalkyl/triethylene
glycol chains (Figure 1). PDIC12/C1218,19 was also synthesized as
a reference material exhibiting a typical hexagonal columnar LC
phase. A summary of the phase transition profiles is shown
in Table 1, where the temperature and enthalpy changes were
determined based on differential scanning calorimetry while
phase structures were characterized by X-ray diffraction analysis
(Figure S1).20 PDIF/C12 exhibited two rectangular columnar
phases with a alkyl/semifluoroalkyl side-chain-segregated pack-
ing structure. PDIF/TEG displayed not a typical LC but a soft
crystal. Its mesophase structure of ColX was unidentified but
revealed as a lamellar columnar-based structure. Both PDI
molecules form lower-symmetry, structurally higher-ordered
assemblies compared to the conventional hexagonal columnar
phases. Spin-coated thin films of PDIF/C12 and PDIF/TEG
showed electronic absorption spectra (Figure 2a) characteristic
of π-stacked PDI chromophores.21 In CHCl3 solution, PDI
chromophores were molecularly dispersed (Figure 2b). These
spectra indicated the side-chain structures essentially did not
affect electronic states of PDI moiety. Both spin-coated films
showed polydomain textures in optical microscopy and bire-
¹1
estimated hole mobility (®h) of 0.2 cm2 V¹1 s for PDIF/C12
(Figure 4a). Meanwhile, the obtained ®e and ®h values of
¹1
PDIC12/C12 were 0.1 and 0.2 cm2 V¹1 s (Figures 4b, S2, and
S3). The evaluated mobilities were larger for PDIF/C12, which
might be correlated with the higher structural ordering in its
mesophase at room temperature. The slope of both electron and
hole signals in Figures 4a and 4b shows a clear single intersect
at ¦N = 0. The intersection of the ¦N-¦N® plot has been
1402 | Chem. Lett. 2015, 44, 1401–1403 | doi:10.1246/cl.150593
© 2015 The Chemical Society of Japan

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This work was partly supported by a Grant-in-Aid for
Scientific Research (Nos. 26620104 and 26102001) from the
Japan Society for the Promotion of Science (JSPS) and partially
supported by Shorai Foundation for Science and Technology.
Supporting Information is available electronically on J-STAGE.
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Þ
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Chem. Lett. 2015, 44, 1401–1403 | doi:10.1246/cl.150593
© 2015 The Chemical Society of Japan | 1403