Charge-transport in crystalline organic semiconductors with liquid crystalline order

Article abstract of DOI:10.1039/b501875g

We report the design and synthesis of a liquid crystalline material exhibiting highly ordered smectic phases and high charge carrier-mobility; by a process known as "paramorphosis" highly ordered smectic phases can be transferred to the amorphous crystall

Full text of DOI:10.1039/b501875g

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Charge-transport in crystalline organic semiconductors with liquid
crystalline order{
Panos Vlachos,a Bassam Mansoor,b Matthew P. Aldred,a Mary O’Neill*b and Stephen M. Kelly*a
Received (in Cambridge, UK) 4th February 2005, Accepted 13th April 2005
First published as an Advance Article on the web 29th April 2005
DOI: 10.1039/b501875g
The aim of this paper is to demonstrate that the process of
paramorphosis can be applied to the amorphous solid state of
organic semiconductors formed on cooling from highly ordered
smectic phases to resolve this problem. Paramorphosis often
occurs when a smectic phase formed on cooling from another
smectic phase exhibits the macroscopic organisation and therefore
the optical texture of the smectic phase above it rather than its own
more thermodynamically stable structure and textures as shown by
optical microscopy. We designed and synthesised compound 1{
according to Scheme 1. The aromatic core is asymmetrical and
non-linear in order to increase the viscosity. Long and branched
terminal chains are present to further increase the viscosity and
lower the melting point. The presence of a chiral centre should not
to affect the charge carrier mobility in calamitic LCs.13 The
heterocyclic rings containing the electron-rich sulfur atom were
chosen to contribute to a low ionisation potential and also induce
a smectic phase. The aromatic core of compound 1 exhibits a flat
molecular shape with very little inter-annular twisting due to the
presence of fused heterocyclic rings and the thiophene ring. This
shape facilitates a high degree of overlap of the molecular wave
functions for an efficient hopping mechanism of charge transport.
The DSC trace shown in Fig. 1 shows that on heating crystals of
compound 1 they melt first (Cryst–SmE 5 75 uC) to form the
We report the design and synthesis of a liquid crystalline
material exhibiting highly ordered smectic phases and high
charge carrier-mobility; by a process known as ‘‘paramor-
phosis’’ highly ordered smectic phases can be transferred to the
amorphous crystalline state on crystallisation without the
formation of significant crystal grain boundaries and deep
traps.
Conjugated polymers and oligomers have been investigated as
organic semiconductors in thin film devices such as organic field
effect transistors (OFETs)1–6 and organic light-emitting diodes
(OLEDs)7–9 for almost two decades as an alternative to organic
single crystals such as pentacene. More recently smectic liquid
crystals have been shown to be a promising alternative approach
to organic semiconductors.10–13 Smectic liquid crystalline phases
are self-assembling and self-annealing supramolecular assemblies
that facilitate the formation of multidomain films with no physical
grain boundaries between the small domains (.1 mm). The thin
film then effectively resembles one large monodomain with no
defects just like a single crystal. The layer structure present in all
smectic liquid crystalline phases induces a high overlap integral of
the characteristic long thin aromatic cores to facilitate hopping of
charge carriers between them. The degree of order within different
types of smectic phases has been shown to strongly influence the
charge carrier mobility. The hole mobility is higher in the more
ordered smectic E phase (1 6 1022 cm2 V21 s21) with
orthorhombic symmetry than in the less-ordered smectic B
phase (1.6 6 1023 cm2 V21 s21) with hexagonal symmetry,
which in turn is higher than that in the smectic A phase (2.5 6
1024 cm2 V21 s21) with no internal order within or between the
layers.10–13 The smectic E phase can be considered as a soft or
plastic crystal with limited, cooperative rotational motion about
the molecular long axis. This allows greater overlap of the
wavefunctions of the molecular orbitals and facilitates charge
carrier hopping.12 However, the smectic phases of the organic
semiconductors reported so far exist above room temperature and
on cooling crystallisation occurs with the formation of large defects
and crystal grain boundaries. These act as traps and lead to a
significant reduction in carrier mobility and the total charge
collected.14,15 This severely limits the applicability of smectic liquid
crystals in practical room temperature devices.
{ Electronic supplementary information (ESI) available: Experimental
procedures and characterization data for the compounds shown in
Scheme 1. See http://www.rsc.org/suppdata/cc/b5/b501875g/
*m.oneill@hull.ac.uk (Mary O’Neill)
Scheme 1 Synthetic route to benzothiazole 1.{
Chem. Commun., 2005, 2921–2923 | 2921
s.m.kelly@hull.ac.uk (Stephen M. Kelly)
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Fig. 1 Differential scanning calorimetry (DSC) scan for compound 1.
Fig. 3 Photocurrent transients for (a) holes and (b) electrons at 25 uC.
The arrows indicate the transit times of carriers across the cell.
smectic E phase and then a narrow smectic A phase (SmE–SmA 5
143 uC) before clearing to form the isotropic liquid (SmA–I 5
145 uC). The smectic A phase is reformed on cooling the isotropic
liquid (I–SmA 5 145u) followed by the formation of the smectic E
phase (SmA–SmE 5 143 uC). Hence the presence of two
overlapping peaks. On further cooling a monotropic (unidentified)
ordered smectic 3 phase is observed (SmE–Sm3 5 60 uC).
Compound 1 then crystallizes to form an amorphous solid (Sm3–
cryst y 40 uC) with the same melting point as the original crystal
as shown by a second heating cycle. A glass transition temperature
is not observed. This behaviour is reproducible through many
heating and cooling cycles. The optical textures of the liquid
crystalline phases of compound 1 viewed between crossed
polarisers on cooling is shown in Fig. 2. The focal conic texture
of the narrow smectic A phase (A) is transferred to the smectic E
phase (B), then the ordered smectic 3 phase (C) and finally the
amorphous crystalline state (D). These micrographs confirm that
paramorphosis occurs on cooling. The short-range spatial order
present in the small domains of the smectic phase as well as the
overall macroscopic structure of the smectic film is transferred to
the crystalline state of compound 1 with no formation of crystal
grain boundaries, see Fig. 2(d). The dark disclination lines are
domain walls representing changes in the director orientation
and not physical defects such as those present in crystals as grain
boundaries.
Fig. 3 shows the photocurrent [I(t)] transients for electrons and
holes at 25 uC. The transients are somewhat dispersive and the
photocurrent for holes is much greater than that for electrons.
Fig. 4 shows the electron and hole mobilities as a function of
temperature on cooling from the isotropic phase. The cell thickness
was 2.3 mm and V 5 12 V.§ For both carrier types, there is a
discontinuity in the mobility values at the transition from the liquid
to the smectic A phase. The temperature range of the smectic A
phase is very narrow (2 uC) and a high degree of order is expected
in the smectic A phase close to the transition to the smectic E
phase due to pretransitional effects. This is evidenced by the
overlapping peaks in the DSC trace, see Fig. 1. Therefore, the
mobility is comparable in both phases due to a high induced order
in the smectic A phase. A further discontinuity is observed for
electrons at the smectic E to smectic 3 transition but, in this case,
the mobility increases with increasing order. The hole mobility is
approximately temperature independent in the smectic E phase
and decreases sharply on approaching the smectic 3 phase. This is
a commonly observed phenomenon for the mobility in smectic
liquid crystals and is usually regarded as a characteristic feature of
electronic transport in ordered organic materials.10,11 It increases
Fig. 2 Photomicrographs of compound 1 on cooling: (A) smectic A
Fig. 4 Mobility of compound 1 as a function of temperature on cooling
phase, (B) smectic E phase, (C) smectic 3 phase, (D) crystal phase.
from the isotropic phase.
2922 | Chem. Commun., 2005, 2921–2923
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tetrakis(triphenylphosphine)palladium(0) (0.23 g, 2.03 6 1024 mol) in
DMF (75 cm3) was heated at 90 uC for 24 h. The mixture was allowed to
cool to RT and the solution was treated with a saturated potassium
fluoride solution (100 cm3) to destroy the tin side products. DCM (2 6
200 cm3) was added and the combined organic layers were washed with
brine (4 6 200 cm3), dried (MgSO4), filtered and concentrated under
reduced pressure. The crude product was purified by gravity column
chromatography [silica gel, ethyl acetate–hexane, 10%:90%] to yield a white
solid (1.31 g, 58%), which was further purified by preparative HPLC and
recrystallisation from a DCM–ethanol solvent mixture. Purity: 99.9% (GC/
HPLC). 1H NMR (CDCl3) dH: 0.89 (3H, t), 0.96 (3H, d), 1.16–1.40 (10H,
m), 1.48 (2H, qnt), 1.56 (2H, qnt), 1.62 (3H, s), 1.69 (3H, s), 1.70–1.76 (1H,
m), 1.82 (2H, qnt), 1.94–2.07 (2H, m), 2.76–2.88 (2H, m), 4.03 (2H, t), 5.11
(1H, t), 6.76 (1H, d, J 5 3.6), 6.98 (2H, d, J 5 8.7), 7.18 (1H, d, J 5 3.6),
7.67 (1H, dd, J 5 8.7, 1.7), 7.97 (2H, d, J 5 8.7), 8.00–8.02 (2H, m). IR
with decreasing temperature in the smectic 3 phase. For both
carriers there is no change in transport characteristics on entering
the amorphous crystalline phase from the soft crystalline smectic 3
phase. Similar trends in the carrier mobility vs. temperature are
observed for both electrons and holes on heating but with a
substantial degree of hysteresis; the discontinuities in the mobility
values, which occur on entering the smectic E phase, are observed
at 75 uC in accordance with the DSC data. These results confirm
that paramorphosis results in the retention of high carrier mobi-
lities in the crystal at room temperature and is in stark contrast to
previous results on smectic organic semiconductors.10–13
The integral I(t)dt can be used as a measure of the total collected
charge when the dielectric relaxation time is longer than the transit
time.16 The total collected charge of both electrons and holes does
not change on crystallisation from the smectic 3 phase showing
that there is no change in the quantum efficiency of photogenera-
tion and no new bulk traps are formed by defects or grain
boundaries. The charge generated by electrons is almost a factor
of seven lower than that observed for holes, which implies that
the compound has substantially more deep traps for electrons
than holes.
n
max/cm21: 3045, 2919, 2851, 1605, 1520, 1465, 1301, 1261, 1222, 1175,
1045, 998, 834, 817, 694, 561. MS (m/z): 559 (M+, 100%), 474, 434, 322, 293.
§ Device fabrication: cells of thickness 1.5–10 mm were constructed by gluing
two glass slides with patterned InSnO electrodes and separated with
spacers. The cell thickness was measured using white-light transmission
prior to filling. The cells were filled with compound 1 in its isotropic phase
by vacuum-assisted capillary action and then sealed. Time-of-flight
measurements were made using a conventional set-up.17 The samples were
excited with UV light from a nitrogen laser of wavelength 337 nm and a
pulse width of 5 ns. The laser fluence was kept sufficiently low to avoid
space-charge distortion of the applied electric field.
In conclusion it appears apparent that the high degree of short-
range order present in highly ordered smectic phases can be
transferred by paramorphosis to the amorphous crystalline state
on crystallisation without the formation of significant crystal grain
boundaries and many deep traps. An ideal order for the
manifestation of this phenomenon on cooling is a relatively fluid
and disordered smectic A phase first for macroscopic alignment
followed by the formation of ordered smectic phases with
orthorhombic and hexagonal symmetry followed by the formation
an amorphous solid with only short-range order and no crystal
grain boundaries. These findings suggest that the high mobility of
existing organic semiconductors could be further increased by
modifying their chemical structure to induce the formation of
highly ordered smectic phases.
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Panos Vlachos,a Bassam Mansoor,b Matthew P. Aldred,a
Mary O’Neill*b and Stephen M. Kelly*a
aDepartment of Chemistry, University of Hull, Hull, UK HU6 7RX.
E-mail: s.m.kelly@hull.ac.uk; Fax: +44 01482 466410;
Tel: +44 01482 465464
bDepartment of Physics, University of Hull, Hull, UK HU6 7RX.
E-mail: m.oneill@hull.ac.uk; Fax: +44 01482 465606;
Tel: +44 01482 465501
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Notes and references
{ 2-{5-[(S)-3,7-Dimethyloct-6-enyl]thiophen-2-yl}-5-(4-octyl-
oxyphenyl)benzothiazole (compound 1):
A mixture of
6-bromo-2-(4-octyloxyphenyl)benzothiazole14 (1.7 g, 0.0041 mol) 5-tributyl-
stannyl-2-[(S)-3,7-dimethyloct-6-enyl]thiophene (2.29 g, 0.0045 mol) and
17 M. Funahashi and J.-I. Hanna, Appl. Phys. Lett., 1996, 35, L703.
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 2921–2923 | 2923