Linearly polarised organic light-emitting diodes (OLEDs): Synthesis and characterisation of a novel hole-transporting photoalignment copolymer

Article abstract of DOI:10.1039/b506063j

We describe the synthesis and characterisation of a novel hole transporting photoalignment copolymer for linearly polarised emission. The copolymer has a coumarin side-chain which undergoes (2 + 2) cycloaddition on irradiation with ultraviolet light. A fluorene side-chain, whose ionisation potential is well matched to the work-function of indium tin oxide, provides hole transporting properties. Polarised green electroluminescence was obtained by spin coating a novel polymerisable and light-emitting liquid crystal onto the photoaligned copolymer. A polarisation ratio value of 13 : 1 and a polarised irradiance of 200 cd m^-2 was obtained at 9 V. Polarised red emission is also described in a guest-host configuration. The spatial patterning of the polarisation direction is also shown. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b506063j

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Linearly polarised organic light-emitting diodes (OLEDs): synthesis and
characterisation of a novel hole-transporting photoalignment copolymer{
Matthew P. Aldred,a Panos Vlachos,{a Adam E. A. Contoret,b Simon R. Farrar,b W. Chung-Tsoi,b
Bassam Mansoor,b Kai L. Woon,b Robert Hudson,a Stephen M. Kelly*a and Mary O’Neill*b
Received 29th April 2005, Accepted 16th June 2005
First published as an Advance Article on the web 30th June 2005
DOI: 10.1039/b506063j
We describe the synthesis and characterisation of a novel hole transporting photoalignment
copolymer for linearly polarised emission. The copolymer has a coumarin side-chain which
undergoes (2 + 2) cycloaddition on irradiation with ultraviolet light. A fluorene side-chain, whose
ionisation potential is well matched to the work-function of indium tin oxide, provides hole
transporting properties. Polarised green electroluminescence was obtained by spin coating a novel
polymerisable and light-emitting liquid crystal onto the photoaligned copolymer. A polarisation
ratio value of 13 : 1 and a polarised irradiance of 200 cd m22 was obtained at 9 V. Polarised red
emission is also described in a guest–host configuration. The spatial patterning of the polarisation
direction is also shown.
of the aromatic main-chain polymer backbone or rigid-rod
Introduction
oligomer and consequently also the orientation of the electro-
Polarised electroluminescence has several potential uses in
luminescent chromophores incorporated within them. Due to
electro-optic applications. A major application is to provide
the intrinsically high order parameter of the liquid crystal
a backlight for liquid crystal displays in order to increase
director obtained using these materials on rubbed alignment
their low contrast and brightness by eliminating one of the
surfaces, polarisation ratios achieved using main chain
polarisers and also to reduce their size, weight and power
polymers are high (.25 : 1). However, the high viscosity of
consumption by replacing the lamp. Organic light-emitting
main chain conjugated polymers means that long annealing
diodes (OLEDs) with polarised emission have a range of
times at high temperatures are required in order to achieve the
additional applications; they give better legibility in bright
desired alignment. Even higher polarisation ratios (up to 33 : 1)
sunlight and the spatial patterning of the polarisation direction
have been obtained using related liquid crystalline fluorene
of emission can form the basis of a 3-dimensional display.1–3
The first reported OLED with polarised emission utilised
a macroscopically aligned film of poly(3-(4-octylphenyl)-2,29-
bithiophene) (PTOPT) oriented on a substrate surface of
polyethylene.4 However, a low polarisation ratio of 2.4 was
obtained. Much more efficient polarised EL was exhibited by
aligned liquid crystalline, main-chain polymers incorporating
a rigid polymer backbone of conjugated aromatic rings,
especially poly(9,9-dioctylfluorene).5 Efficient polarised emis-
sion was also achieved more recently using the corresponding
conjugated oligomers with a lower molecular weight.6,7 The
rigid-rod polymers or oligomers are first aligned in the liquid
crystalline state using a mechanically-rubbed surface, such as
PPV,1,8 doped-polyimide1,5 or PEDOT,6,7 as the alignment
layer and then quenched into the glassy state. The formation of
a liquid crystalline glass freezes in the macroscopic alignment
oligomers on rubbed PEDOT.6,7 This may be attributable
to the lower viscosity of the oligomers compared to that of
the analogous polymers, which facilitates a higher degree of
molecular reorientation within the liquid crystalline state.
However, annealing times at high temperatures of over one
hour are still required.
The thin layers present in OLEDs, compared to those in
LCDs, means mechanical rubbing can cause substrate and
electrode damage, as well as generating dust and static that can
act as traps or even lead to device breakdown. Therefore, the
use of non-contact photoalignment layers to align the EL
chromophores in the LC state appears much more attractive
from a commercial production point of view. Photoalignment
also offers the important added potential of photolithographic
pixel patterning.2,3 Photoalignment of electroluminescent
materials was first reported using a photopolymerisable
nematic liquid crystal, a so-called reactive mesogen (RM),
which was subsequently photopolymerised to form an insoluble
EL film as a crosslinked polymer network.9 Photoalignment
has also been applied to liquid crystalline polyfluorenes more
recently using photoaddressable polymers (PAPs).10 The PAPs
used for the polyfluorene alignment are methacrylate comb
polymers containing mesogenic and non-mesogenic azoben-
zene side-chains. Liquid crystalline PAPs were obtained
by increasing the concentration of mesogenic side-chains.
Good alignment was only observed using the LC PAPs. EL
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
{ Electronic supplementary information (ESI) available: experimental;
current–voltage and luminance–voltage plots; movie showing changes
in the PL image as the polariser is rotated. See http://dx.doi.org/
10.1039/b506063j
{ ZLX Techno Ltd, 3 Trinity Lane, Beverley, UK HU17 0DY.
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polarisation ratios of 14 at a brightness of up to 200 cd m22
and efficiency of 0.3 cd A21 were demonstrated. Improved
efficiencies (0.66 cd A21) have been achieved using ultrathin
PAP layers (5 nm, compared to 20 nm previously).11
A
mainchain photoalignment hole-transporting polymer was
reported recently.12 However, the LC alignment was poor and
polarised EL was not demonstrated.
The alignment of small molecules, rather than that of
oligomers or polymers, in the nematic phase appears to be a
more attractive approach, as spontaneous alignment can be
achieved, i.e., the long thermal annealing process used to
induce alignment of oligomers and polymers is not required.
This approach also requires the stabilisation of the macro-
scopic alignment induced either by formation of the ‘‘glassy
state’’ or fixation by photochemical crosslinking. For
example, even though polarised EL was obtained at 70 uC
Fig. 1 2 + 2 Cycloaddition for a model coumarin molecule. P
indicates the polarisation direction of the incident ultraviolet beam.
nematic liquid crystal is subsequently deposited and aligns
homogeneously with its director oriented parallel and perpen-
dicular to P for weak and strong polarised UV illumination
respectively.15,16 Spectroscopic analysis shows that the com-
petition between two independent LC–surface interactions
determines the resultant director orientation.16 Parallel LC
alignment is determined by a short-range steric interaction
with the dimerised cyclobutane side-chains. The LC alignment
direction switches from parallel to perpendicular above a
critical fluence and subsequently the LC alignment is deter-
mined by a dispersive LC interaction with the intact side-
chains in the bulk of the photoalignment layer. The alignment
direction of luminescent reactive mesogens remains parallel to
the polarisation direction of the UV beam onto the photo-
alignment layer irrespective of its fluence.9 This implies that
alignment is governed by the short-range interaction of the LC
with the dimerised coumarin photoproduct rather than with
the unreacted coumarin.
in the smectic
liquid crystalline 2-(4-hexyloxyphenyl)-5-(4-hexyloxybiphenyl-
49-yl)oxadiazole (HOBP-OXD) on rubbed polyimide,13
X
phase from
a
low-molecular-mass
a
low polarisation order parameter (0.32), poor brightness
(0.8 cd m22) and low quantum efficiency (2.8 6 1025%) were
observed. This material crystallises at lower temperatures. In
contrast, the alignment of a photocrosslinkable glass-forming
nematic liquid crystal by photoalignment on a coumarin photo-
alignment layer occurs at room temperature.9 A polarised
emission ratio of 11 : 1 and brightness of 60 cd m22 were
obtained. Photoalignment of small molecules in the nematic
state is clearly an attractive approach to fabricating OLEDs
with polarised emission. Unfortunately, due to the non-
conducting nature of the coumarin photoalignment material,
doping with a charge-transporting triarylamine was neces-
sary.9 However, triarylamines are non-planar and relatively
high concentrations (50%) of the hole-transport dopant led to
a reduction in the alignment quality of the liquid crystalline
glass. Phase separation at high dopant concentrations is also a
potential problem. Therefore, the synthesis of hybrid hole-
transporting photoalignment layers with rod-like or lath-like
shape would appear to be a promising technological approach
to address these problems. Therefore, in this paper we report
the synthesis and characterisation of a novel conducting
photoalignment polymer with rod-like hole-transport side-
chains. We also demonstrate the alignment of a novel light-
emitting and photopolymerisable liquid crystal and the
subsequent generation of linearly polarised electrolumines-
cence from the corresponding crosslinked polymer network.
We propose a coumarin based copolymer to provide both
hole-transporting and photoalignment properties. The ionisa-
tion potential of the material should be well matched to that of
the indium tin oxide (ITO) anode (#4.8 eV) to minimise the
energy barrier for injection of holes. The film should provide
planar and uniaxial planar liquid crystal alignment without
and following polarised UV irradiation respectively. The
coumarin groups must be relatively mobile so that they can
easily orient into the conformation required for dimerisation.
Hence main-chain copolymers incorporating the hole-
transporting and coumarin units were not considered because
of their high viscosity. A side-chain copolymer is more likely
to give efficient crosslinking, since the coumarin and hole-
transporting side-chains can be attached via flexible aliphatic
spacers to a polymethacrylate backbone. Another design
consideration is that 2 + 2 cycloaddition is only possible
between closely spaced neighbours so that a high fraction of
coumarin side-chains is required. This affects hole transport,
which involves a series of intermolecular hops of charge
carriers between the hole-transporting side-chains. The hop-
ping probability depends on the overlap of the wave-functions
of the aromatic molecular cores and so has an inverse
exponential dependence on the core separation. We use a
linear hole-transporting moiety to reduce the intermolecular
hopping distances and to minimise disruption with the
Design considerations
Coumarin side-chain polymers have been extensively used as
photoalignment materials for nematic LCs for liquid crystal
displays. On irradiation with linearly polarised light, the
probability of a photochemical reaction is proportional to
cos2w where w is the azimuthal angle between the polarisation
direction P of the incident beam and the transition dipole
moment of the coumarin side-chain. As first shown by Schadt
et al.,14 the coumarin side-chains are depleted anisotropically
by a 2 + 2 cycloaddition as illustrated in Fig. 1 for a model
side-chain polymer with a coumarin group at the end of
the side-chain attached to a polymethacrylate backbone. The
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photoalignment process. Indeed, the low fraction of hole-
transporting side-chains may be an advantage for our class of
light-emitting liquid crystals, whose injection and transporting
properties are much better for holes than for electrons. The
photoalignment copolymer reduces the hole-current and so
improves the balance of positive and negative charge in the
device improving its efficiency.
in the next step. 1,4-Dibromobenzene 4 was monoalkylated by
reaction with the Grignard reagent formed from hexylbromide
and magnesium,20 to produce 1-bromo-4-hexylbenzene (5). A
Stille coupling17 of 2-(tributylstannyl)thiophene and the aryl
bromide 5 afforded the thiophene 6, which was converted to
the corresponding tributylstannyl derivative 7 in the usual
way.18 The asymmetrical di-alkylated fluorene 9 was synthe-
sised in a one step alkylation procedure from commercially
available 2-bromo-7-iodofluorene 8.21 Selective reaction at the
iodo-group of the fluorene 9 with the octyloxy-substituted
tributylstannyl derivative 322 in a Stille aryl–aryl coupling
reaction gave the aryl bromide 10. A second Stille reaction at
the bromo-group of the fluorene 10 using the hexyl-substituted
tributylstannyl derivative 7 yielded the asymmetrical 2,7-
disubstituted fluorene 11. Removal of the octyloxy-chain of
the ether 11 using boron tribromide afforded the asymmetrical
phenol 12, which was alkylated using 6-bromohexyl metha-
crylate derivative, in a Williamson ether reaction, to afford
monomer 1.
Synthesis
Monomer 1 was synthesised as shown in Scheme 1. First a
Stille coupling17 of 2-(tributylstannyl)thiophene and 1-bromo-
4-octyloxybenzene 1 afforded the thiophene 2, see Scheme 1.
The tributylstannyl derivative 3 was synthesised by reaction of
compound 2 with n-BuLi in THF at 278 uC followed by
the addition of tri-n-butyltin chloride.18 The tributylstannyl
derivative 3 was found to be sensitive to acidic conditions
and undergo destannylation on contact with silica gel.19
Fortunately the crude product was pure enough to be used
Monomer 2 was synthesised according to a modified
pathway shown in Scheme 2.14,16 Williamson-ether reaction
of the commercially available p-hydroxybenzoic acid methyl-
ester 13 with commercially available 1-bromohexanol afforded
the ester 14, which was saponified to produce the carboxylic
acid 15.23 Esterification of the hydroxyl group of compound 15
using methacroyl chloride produced the polymerisable benzoic
acid derivative 16,24 which was esterified with 7-hydroxy-
coumarin 17 using DCC and DMAP to produce the coumarin
monomer 2.16
Copolymerisation of monomer 1 and monomer 2 was carried
out by a thermally induced radical polymerisation, using
Scheme 1 Reagents and conditions: (i) 2-(tributylstannyl)thiophene,
Pd(PPh3)4, DMF, 90 uC, (ii) a) n-BuLi, THF, 278 uC, b) Sn(C4H9)3Cl,
(iii) C6H13MgBr, PdCl2(dppf), diethyl ether, reflux, (iv) tributyl-
ammonium bromide, 50% NaOH(aq), 1-bromopropane, toluene,
reflux, (v) Pd(PPh3)4, DMF, 90 uC, (vi) a) BBr3, CH2Cl2, 0 uC, b)
H2O (ice), (vii) a) K2CO3, butanone, reflux, b) 2,6-di-tert-butyl-4-
methylphenol (BHT), RBr.
Scheme 2 Reagents and conditions: (i) 1-bromohexanol, K2CO3,
butanone, (ii) a) KOH, EtOH, H2O, b) H3O+, (iii) methacryloyl
chloride, DMA, BHT, dioxane, 60 uC, (iv) DCC, DMAP, BHT,
CH2Cl2, room temperature.
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Fig. 3 Absorption spectrum of copolymer 1 before and after irradia-
tion with ultraviolet light of wavelength 325 nm and fluence 10 J cm22
.
The spectral region above 350 nm is magnified for clarity (64).
Scheme 3 Reagents and conditions: (i) AIBN, THF.
AIBN as initiator, see Scheme 3.25 to produce copolymer 1
1
(x 5 1; y 5 4). The H NMR showed no presence of the two
olefinic hydrogens at dH 5.55/5.56 and dH 6.11, confirming
that no monomer was present in copolymer 1. The chemical
structures of the light-emitting reactive mesogens RM 1 and
RM 2 used to generate polarised electroluminescence are
shown in Fig. 2.
Results and discussion
Fig. 4 Cyclic voltammogram of copolymer 1. The potential, E, is
We first show that 2 + 2 cycloaddition occurs on irradiation of
copolymer 1 with ultraviolet light from a HeCd laser at 325 nm.
Fig. 3 shows the absorbance spectrum of a thin film before and
after irradiation using a fluence of 10 J cm22. By comparison
with the absorption spectra of the monomers, the peaks at
224 nm and 276 nm can be unambiguously assigned to the
coumarin side-chain. The intensity of both these peaks is
reduced on irradiation with ultraviolet light indicating 2 + 2
cycloaddition.16 The weaker and broader peak at 400 nm is
assigned to the fluorene side-chain. The absorbance spectrum
above 350 nm is magnified by a factor of 4 in Fig. 3 to show
that the absorbance of the fluorene side-chain is unaffected
by irradiation.
measured with respect to that of the saturated calomel electrode.
The ionisation potentials (IP) of the compounds were
measured electrochemically by cyclic voltammetry using a
computer-controlled scanning potentiostat (Solartron 1285).
The cyclic voltammetry scan for copolymer 1 is shown in Fig. 4.
The potential, E, is plotted with respect to that of the saturated
calomel electrode. The onset potential for oxidation, Eox is
clearly defined by a step change in current and is obtained
from the intersection of the two tangents at the current
discontinuity. IP is obtained using the empirical relationship
proposed by Bredas et al.: IP 5 [Eox + 4.4] eV.26 Copolymer 1
Fig. 2 Structures of the green light-emitting reactive mesogen RM 1 (upper; Cr–N 5 52 uC; N–I 5 143 uC; Tg 5 0 uC) and red light-emitting
reactive mesogen RM 2 (lower; Cr–N 5 142 uC; N–I 5 123 uC; Tg , 15 uC).
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has an IP value of 5 eV, which is well matched to the work-
function of an ITO/PEDOT anode for the injection of holes.
The oxidation process is electrochemically and chemically
reversible showing that copolymer 1 is stable to oxidation and
as such it is suitable as a hole injection material. The IP of the
light-emitting RM 1 is also found to be 5.0 eV so that there is
no barrier for injection of holes from the copolymer. The hole
mobility of copolymer 1 was too dispersive for measurement.
However, the hole mobility of a series of reactive mesogens
containing the same aromatic core as monomer 1 varies from
2 6 1025 cm2 V21 s21 to 1 6 1023 cm2 V21 s21 depending
on the chromophore separation. As discussed above, we expect
the hole mobility of copolymer 1 to be even lower because of
the low fraction of the hole-transporting component so that a
thin layer was used in polarised EL devices.
(perpendicular) to the alignment direction, of 13 : 1 is
obtained. The polarisation ratio is maximum for the zero
phonon EL peak indicating that it is slightly reduced by
vibronic coupling. Typical current–voltage and luminance–
voltage plots are shown as electronic supplementary informa-
tion (ESI).{ The device has a luminance of 300 cd m22 at 10 V
and a maximum efficiency of 1.6 cd A21. This is an
improvement on previous data for a polarised OLED using
photoalignment.11 The low threshold voltage, 4.2 V at 2 cd m22
,
and the insertion of copolymer 1 between the anode and light-
emitting layer of the OLED confirm that the photoalignment
polymer is an effective hole transporting material. The
alignment direction of copolymer 1 was spatially patterned
by exposing adjacent squares with orthogonally polarised
light. Photoluminescence (PL) from RM 1 deposited on top
appears uniform except when viewed through a polariser.
Then the orthogonally aligned areas appear green and black
when the polarisation direction of the polariser is parallel to
the director of one of these regions.27 A movie, deposited as
electronic supplementary information (ESI),{ shows changes
in the PL image as the polariser is rotated.
Having shown that the photoalignment copolymer 1 exhibits
(2 + 2) cycloaddition and hole-injecting properties, we use it to
produce a polarised OLED. The photoalignment and light-
emitting organic layers were sandwiched between a PEDOT
hole injection layer on ITO coated glass and a commercially
available hole-blocking layer (TAZ). A LiF/Al cathode was
used for electron injection. A thin film of the green RM 1 was
deposited by spin-casting onto a thin film of copolymer 1,
which was previously irradiated with polarised UV light. The
liquid crystal RM 1 retains a metastable supercooled nematic
phase at room temperature on heating above the melting point
and subsequent cooling. Hence, uniform alignment of the
chromophore was obtained by annealing the sample at 110 uC
and slowly cooling to room temperature. The alignment
direction of the chromophore was parallel to the polarisation
direction of the UV beam. The subsequent room temperature
photopolymerisation of RM 1 gives a thermally stable and
robust polymer network without any loss of alignment. The
photopolymerisation process is discussed in more detail
elsewhere.16,22 Fig. 5 shows the polarised electroluminescence
spectrum from the device. A peak polarisation intensity ratio
Photoalignment can also be used to align electroluminescent
materials in a guest–host configuration. The red-emitting RM 2
crystallises rather than retaining a supercooled nematic phase
on cooling to room temperature. Hence, polarised red PL was
obtained by energy transfer in a guest–host blend prepared by
doping the red guest RM 2 in the green host RM 1 with a ratio
of 1 : 3 by weight. The absorbance spectrum of RM 2 overlaps
the PL spectrum of RM 1 so that efficient Fo¨rster energy
transfer is possible. Fig. 6 shows polarised PL from a film of
the guest–host combination aligned using the photoaligned
copolymer 1. The emission is red with no residual green
component from RM 1. The polarisation ratio, PLpara/PLperp
,
is wavelength dependent as indicated in Fig. 6. The maximum
value of 7 : 1 is smaller than that obtained for a red-emitting
oligomer reported recently, probably because of the lower
order parameter of compound RM 2.27
ELpara/ELperp
, where para (perp) refer to light parallel
Fig. 5 Polarised electroluminescence (EL) spectrum from an OLED.
The solid and dashed lines show the EL spectrum of the photo-
chemically crosslinked green RM 1 on a thin crosslinked film of the
photoaligned copolymer 1 parallel and perpendicular to the alignment
direction. The dots show the wavelength dependence of the EL
polarisation intensity ratio, ELpara/ELperp, where para (perp) refer to
light parallel (perpendicular) to the alignment direction.
Fig. 6 The solid line shows the parallel component of the photo-
luminescence (PL) from a photochemically crosslinked guest host
mixture of RM 2 : RM 1 in ratio 1 : 3 by weight on a thin crosslinked
film of the photoaligned copolymer 1. The dots show the wavelength
dependence of the PL polarisation intensity ratio, PLpara/PLperp, where
para (perp) refer to light parallel (perpendicular) to the alignment
direction.
3212 | J. Mater. Chem., 2005, 15, 3208–3213
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9 A. E. A. Contoret, S. R. Farrar, P. O. Jackson, S. M. Khan,
L. May, M. O’Neill, J. E. Nicholls, S. M. Kelly and G. J. Richards,
Adv. Mater., 2000, 12, 971.
10 D. Sainova, A. Zen, H. G. Nothofer, U. Asawapirom, U. Scherf,
R. Hagen, T. Bieringer, S. Kostromine and D. Neher, Adv. Funct.
Mater., 2002, 12, 49.
Conclusions
A
hole-transporting/coumarin photoalignment copolymer
has been synthesised that consists of monomer 1 (rod-like
hole-transporting material) and monomer
2 (coumarin-
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12 J. Lee, J.-I. Lee, S.-J. Sung, H. Y. Chu, J.-K. Park and H.-K. Shim,
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13 H. Tokuhisa, M. Era and T. Tsutsui, Appl. Phys. Lett., 1998, 72,
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14 M. Schadt, H. Seiberle and A. Schuster, Nature, 1996, 351, 212;
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photoalignment material) in a 1 : 4 ratio. Only a small
proportion of the hole-transporting material was incorporated
into the copolymer to preserve the alignment properties of
the coumarin moiety. The green chromophore RM 1 was
aligned on this hybrid hole-transporting photoalignment
layer for the production of linearly polarised light, in which
a peak polarisation intensity ratio of 13 : 1 and polarised
electroluminescent irradiance of 200 cd m22 was obtained at
9 V. Optimisation of this new class of patternable hole-
transporting/coumarin photoalignment polymer will almost
certainly lead to much higher values for the polarisation ratio.
15 M. Obi, S. Morino and K. Ichimura, Chem. Mater., 1999, 11,
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16 P. O. Jackson, M. O’Neill, W. L. Duffy, P. Hindmarsh, S. M. Kelly
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17 J. K. Stille, Angew. Chem., Int. Ed. Eng., 1986, 25, 508.
18 P. Vlachos, Ph.D. Thesis, University of Hull, 2003.
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22 M. P. Aldred, A. J. Eastwood, S. M. Kelly, P. Vlachos,
A. E. A. Contoret, S. R. Farrar, B. Mansoor, M. O’Neill and
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Acknowledgements
The European Union (Photoledd), EPSRC and ZLX Techno
Ltd are thanked for financial support.
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