The first liquid crystalline phthalocyanine derivative capable of edge-on alignment for solution processed organic thin-film transistors

Article abstract of DOI:10.1039/b822819a

Tetraoctyl-substituted vanadyl phthalocyanine (OVPc4C8) as a new NIR-absorbing discotic liquid crystalline material can form highly ordered thin films with edge-on alignment of the molecules and molecular packing mode identical to that in the phase II of

Full text of DOI:10.1039/b822819a

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The first liquid crystalline phthalocyanine derivative capable of edge-on
alignment for solution processed organic thin-film transistorsw
Shaoqiang Dong, Hongkun Tian, De Song, Zhenhua Yang, Donghang Yan, Yanhou Geng*
and Fosong Wang
Received (in Cambridge, UK) 19th December 2008, Accepted 20th March 2009
First published as an Advance Article on the web 14th April 2009
DOI: 10.1039/b822819a
Tetraoctyl-substituted vanadyl phthalocyanine (OVPc4C8) as a
new NIR-absorbing discotic liquid crystalline material can
form highly ordered thin films with edge-on alignment of the
molecules and molecular packing mode identical to that in the
phase II of OVPc for solution processed OTFTs with mobility
although Langmuir–Blodgett or layer-by-layer deposition
techniques could afford OTFT devices with mTFT up to
0.46 cm2 VÀ1 sÀ1 14–16
Ono recently reported OTFT devices
.
based on soluble Pcs with mTFT up to 6 Â 10À2 cm2 VÀ1 sÀ1
,
but high temperature (350 1C) had to be applied to convert the
precursor to the high mobility form.17 One of the main reasons
leading to this situation may be the difficult edge-on alignment
of disc-like molecules of liquid crystalline (LC) Pcs for formation
of charge transport channels along the source–drain
direction18 and molecular arrangement like that in the high
mobility phase II of OVPc.4 In the current communication, we
report a tetraoctyl-substituted LC MPc, OVPc4C8 (Fig. 1). It
exhibits a rectangular columnar LC mesophase with a
LC isotropic transition temperature (Tc) of 175 1C. Most
importantly, the film comprising edge-on aligned molecules
can be easily prepared by annealing the spin-cast film at a
relative low temperature of 120 1C for preparation of high
performance OTFTs. A mTFT up to 0.017 cm2 VÀ1 sÀ1
along with a low threshold voltage (VT B0 V) has been
demonstrated. To our best knowledge, this is the first Pc
derivative for common solution processed OTFTs with high
field-effect mobility. High solubility, high mobility and strong
NIR-absorption will make this type of materials promising
solution processible organic semiconductors.
up to 0.017 cm2 VÀ1 sÀ1
.
Over the past two decades, considerable interest has been
shown in the use of organic semiconductors as active materials
in optoelectronic devices such as organic thin-film transistors
(OTFTs) and organic solar cells (OSCs).1,2 In order to fully
explore the advantages of organic semiconductors, i.e.,
mechanical flexibility and low-cost for large-area optoelectronic
devices, it is important to develop solution-processible materials
for device fabrication.3
Metal phthalocyanines (MPcs) have become one of the most
important classes of organic semiconductors because of
(1) their easy synthesis and adjustment of physical properties
by selecting the appropriate central metal atom; (2) high
field-effect mobility of vacuum-deposited thin films; (3) intense
absorption in the near-infrared (NIR) region; and (4)
extraordinary thermal and photochemical stability.4 Actually,
MPcs have successfully been used in the fabrication of high
performance optoelectronic devices such as OTFTs5–8 and
OSCs9–11 both by vacuum-deposition. Particularly, OTFTs
with field-effect mobility (mTFT) up to 1.2 cm2 VÀ1 sÀ1
for holes and 0.3 cm2 VÀ1 sÀ1 for electrons have been
demonstrated with vanadyl phthalocyanine (OVPc)7 and
phthalocyanato tin(IV) dichloride (SnCl2Pc),8 respectively.
On the other hand, many soluble phthalocyanines (Pcs) have
been prepared via peripheral substitution, and some of them
are discotic liquid crystals which can form highly ordered
columnar mesophases (e.g., hexagonal and rectangular
mesophases) comprising one-dimensional charge transport
channels.12 Moreover, mobility up to 0.6 cm2 VÀ1 sÀ1 for a
Pc derivative as measured by the pulse-radiolysis time-resolved
microwave conductivity technique has been reported.13
However, soluble Pc-based OTFT devices with reasonably
high mobility (410À3 cm2 VÀ1 sÀ1) have not been demonstrated
yet by common solution process (such as spin-coating),
OVPc4C8 is highly soluble in common organic solvents,
such as chloroform, tetrahydrofuran and toluene. More than
100 mg of OVPc4C8 can be dissolved in 1 mL chloroform.
Alkyl substituents instead of alkoxyl ones were used in order
to exclude the effect of oxygen atoms in the flexible side chains
on alignment behavior of the discotic molecules on the
substrate.19 Randomly arranged alkyl chains can render
OVPc4C8 with high solubility and lower Tc for ease of
preparation of highly ordered film via thermal annealing in
LC state.
State Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Graduate School of Chinese Academy of Sciences, Changchun,
130022, P. R. China. E-mail: yhgeng@ciac.jl.cn;
Fax: +86-431-85685653
w Electronic supplementary information (ESI) available: Complete
synthetic and experimental data. See DOI: 10.1039/b822819a
Fig.
1 Molecular structure of tetraoctyl-substituted vanadyl
phthalocyanine OVPc4C8.
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Fig. 3 Film UV-vis-NIR absorption spectra of OVPc4C8 upon
annealing at different temperature (a) and the thin-film XRD patterns
of OVPc4C8 before and after annealing at 120 1C (b).
Fig. 2 DSC second-heating and cooling traces of OVPc4C8 under
nitrogen with a heating rate of Æ10 1C minÀ1 (a) and powder XRD
patterns of OVPc4C8 at different temperatures (b).
Thermotropic LC properties of OVPc4C8 were studied by
differential scanning calorimetry (DSC), polarizing optical
microscopy (POM) and X-ray diffraction (XRD). As shown
in Fig. 2(a), only one phase transition at 175 1C with a
transition enthalpy value (DH) of 12.3 J gÀ1 corresponding
to Tc was observed upon heating. ‘‘Branched’’ textures typical
for highly ordered columnar mesophases were observed under
POM upon cooling OVPc4C8 from the isotropic state (see
ESIw). Fig. 2(b) shows the XRD patterns of OVPc4C8 at
different temperatures. At 40 1C, the extinction of all the
reflections with h + k a 2n allows us to assign the c2mm
symmetry to the rectangular two-dimensional lattices,20,21
with a quasi-square lattice parameters a = 32.2 A and
b = 31.4 A. A shoulder at 2y = 24.41 suggests an average
p–p stacking distance of about 3.6 A in the columns. The
molecules in the column form slipped p–p stacking. The tilt
angle of the molecules with respect to the columnar axis is
about 541 due to the presence of axial substituents (see ESIw).
At 140 1C, a similar pattern can be observed but with broader
peaks, reduced intensity and disappearance of the 20 and 40
reflection peaks, indicating the lower order of the molecules in
the columns. When OVPc4C8 is heated to an isotropic liquid
at 195 1C, a diffuse small-angle reflection peak appears at
2y = 4.51. This type of reflection was also observed in
hexabenzocoronene derivatives, which was ascribed to the
positional correlation of molecular aggregates possessing
different electron density between the discotic core stacks
and the alkyl chains.22
technique, respectively. As shown in Fig. 3(a), the pristine film
of OVPc4C8 shows a broad absorption band in 500–900 nm.
Upon annealing, the long-wavelength absorption peak
becomes stronger and red-shifted, and meanwhile the two
short-wavelength absorption peaks become weaker, indicative
of a self-organization process of the molecules. With a given
annealing time of 20 min, the spectral change becomes more
pronounced with an increase of the annealing temperature,
and achieves a saturated state at 120 1C. The spectrum is
identical to that of the high mobility phase II of OVPc,
indicative of a similar molecular arrangement.4,23,24 To char-
acterize the orientation of the molecules by thin film XRD,
SiO2/Si substrates were treated with a phenyltrichlorosilane
(PTS) monolayer for spin-coating 40 nm films of OVPc4C8.
As shown in Fig. 3(b), the pristine film of OVPc4C8 only
shows a broad diffraction peak at 2y = 3.961, corresponding
to a d-spacing of 2.23 nm, indicating a low-ordered edge-on
alignment of molecules. After the film is annealed at 120 1C for
20 min, the diffraction peak becomes stronger and sharper
with second- and third-order diffraction peaks at 2y = 7.901
(d-spacing of 1.12 nm) and 11.91 (d-spacing of 0.74 nm),
respectively, indicating highly ordered edge-on alignment
of molecules and the formation of the film with layered
nanostructures. These diffractions are well consistent with
those in powder XRD, revealing that the molecules in the
films should pack in a manner similar to that in the bulk state.
Consistent with absorption spectroscopic observation, further
increasing the annealing temperature to 150 1C afforded
identical XRD patterns.
The self-organization process of OVPc4C8 on quartz and
SiO2/Si substrates upon thermal annealing was monitored by
UV-vis-NIR absorption spectroscopy and the thin-film XRD
Top-contact OTFT devices were fabricated on SiO2/Si
substrate to study charge carrier transport properties of
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This work is supported by National Basic Research
Program of China (973 Project, No. 2009CB623603) of
Chinese Ministry of Science and Technology, NSFC
(Nos. 20521415, 20621401 and 50833004).
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I
In summary, we have designed and synthesized a tetraalkyl-
substituted LC Pc derivative, i.e., OVPc4C8, which is capable
of forming highly ordered thin films by simply annealing the
spin-cast films at 120 1C. A mTFT up to 0.017 cm2 VÀ1 sÀ1
along with a low threshold voltage (VT B0 V) has been
demonstrated with a top-contact OTFT device configuration.
High mobility, strong NIR-absorption and ease of preparation
and processing make this class of materials attractive solution
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This journal is The Royal Society of Chemistry 2009
3088 | Chem. Commun., 2009, 3086–3088