DERIVES CONJUGUES DE POLYCARBAZOLE UTILISES DANS DES DIODES ELECTROLUMINESCENTES ORGANIQUES

CONJUGATED POLYCARBAZOLE DERIVATIVES IN ORGANIC LIGHT EMITTING DIODES

Abrégé français

Une diode électroluminescente organique (DELO) renferme, comme matière active, un dérivé conjugué de poly(N-alkyl-2,7-carbazole) correspondant à la formule (voir la formule ci-dessus), où n est un nombre entier entre environ 3 et environ 100, et R est un substituant alkyle linéaire ou ramifié comptant 1 à 22 atomes de carbone, ou un substituant poly(éthylèneoxy), cyano, aryle, amide ou benzoyle. Ces DELO possèdent des propriétés d'électroluminescence améliorées, surtout dans les régions du bleu et du rouge du spectre électromagnétique.

Abrégé anglais

1. An Organic Light Emitting Diode (OLED) includes as active material a
conjugated poly(N-alkyl-2,7-carbazole) derivative described by the formula:
(see above formula)
wherein n is an integer of about 3 to about 100, and R represents a
substituant that
is a linear or branched alkyl group containing 1 to 22 carbon atoms, or
poly(ethyleneoxy), cyano, aryl, amide or benzoyl. Such OLEDs have improved
light
emitting properties, especially in the blue and red regions of the spectrum.

Revendications

We Claim:
1. An Organic Light Emitting Diode (OLED) including as active material a
conjugated poly(N-alkyl-2,7-carbazole) derivative described by the formula:
<IMG>
wherein n is an integer of about 3 to about 100, and R represents a
substituant that
is a linear or branched alkyl group containing 1 to 22 carbon atoms, or
poly(ethyleneoxy), cyano, aryl, amide or benzoyl.
2. An OLED as claimed in claim 1, wherein said poly(N-alkyl-2,7-carbazole)
is (poly(N-ethythexyl)-2,7-carbazole) (PEHC).
3. An OLED as claimed in claim 2, wherein said poly(N-alkyl-2,7-carbazole)
is mixed with a second active material.
4. An OLED as claimed in claim 3, wherein said second active material is
(N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine) (TPD).
5. An OLED as claimed in claim 1, wherein said poly(N-alkyl-2,7-carbazole)
is POC (poly(N-octyl-2,7-carbazole) (POC).
6. An OLED as claimed in claim 1, wherein said derivatives are co-polymers
formed with co-monomers selected from the group consisting of: ethylene,
acetylene, C6-C22 mononuclear / polynuclear aromatic, C2-C10
mononuclear /polynuclear heterocyclic groups and tertiary arylamines.
7. An OLED as claimed in claim 6, wherein said co-monomers are
mononuclear/polynuclear aromatic groups selected from the group consisting
of: fluorene, anthracene, phenylene, naphthylene, acenaphthene, phenanthrene,
fluoranthene, pyrene, perylene, rubrene, chrysene, tetracene and pentacene.
8. An OLED as claimed in claim 6, wherein said mononuclear/polynuclear
heterocyclic internal groups are selected from the group consisting of: 5-
member
heterocycles; 6-member heterocycles; benzo-fused ring systems such as
benzoxazole, benzothiazole, benzimidazole, quinoline, isoquinoline, cinnoline,
12

quinazoline, quinoxaline, phthalazine, benzothiadiazole, and benzotriazines;
and
polynuclear fused condensed ring systems.
9. An OLED as claimed in claim 8, wherein mononuclear / polynuclear
heterocyclic internal groups are selected from the group consisting of: furan,
thiophene, pyrrole, oxazole, isooxazole, oxadiazoles, thiazole, isothiazole,
imidazole, thiadiazole, and pyrazoles.
10. An OLED as claimed in claim 8, wherein mononuclear / polynuclear
heterocyclic internal groups are selected from the group consisting of:
pyridine,
pyridazine, pyrimidine, pyrazine, triazines, and tetrazene.
11. An OLED as claimed in claim 8, wherein said mononuclear / polynuclear
heterocyclic internal groups are selected from the group consisting of:
benzoxazole, benzothiazole, benzimidazole, quinoline, isoquinoline, cinnoline,
quinazoline, quinoxaline, phthalazine, benzothiadiazole, and benzotriazines.
12. An OLED as claimed in claim 8, wherein said mononuclear / polynuclear
heterocyclic internal groups are selected from the group consisting of:
phenazine,
phenanthridine, acridine, and diphenylene oxide.
13. An OLED as claimed in claim 6, wherein said co-monomers are tertiary
arylamine groups selected from the group consisting of: triphenylamine, N,N'-
diphenylbenzidine, N,N'-diphenyl-1,4-phenylenediamine, and
diphenylnaphthylamine, olefinic, arylamino, aromatic and heterocyclic aromatic
groups containing up to 30 carbons, substituted optionally with one or more
substituents.
14. An OLED as claimed in claim 13, wherein said substituents are selected
from the group consisting of: C1-C20 hydrocarbyl radicals, C1-C20 (thio)alkoxy
radicals, C1-C20 (thio)aryloxy radicals, cyano, fluoro, chloro, C1-C20
alkoxycarbonyl, C1-C2O aryoxylcarbonyl, poly(alkyleneoxy) and
alkyl(aryl)sulfonyl radicals.
15. An OLED as claimed in claim 14, wherein said substituent is selected from
the group consisting of: alkyl, alkoxy, poly(alkyleneoxy), and cyano.
16. An Organic Light Emitting Diode (OLED) comprising:
a hole transport layer;
an electron transport layer; and
wherein at least one of said hole transport layer and said electron transport
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layer comprises either alone or in combination as active material a conjugated
poly(N-alkyl-2,7-carbazole) derivative described by the formula:
<IMG>
wherein n is an integer of about 3 to about 100, and R represents a
substituant
that is a linear or branched alkyl group containing 1 to 22 carbon atoms, or
poly(ethyleneoxy), cyano, aryl, amide or benzoyl.
17. An OLED as claimed in claim 16, further comprising a carrier promotion
layer adjacent at least one of said electron transport layer and said hole
transport
layer.
18. An OLED as claimed in claim 17, wherein said carrier promotion layer is
LiF.
19. An OLED as claimed in claim 18, wherein said hole transport layer is a
blend of PEHC (poly(N-ethylhexyl)-2,7-carbazole) and TPD (N,N'-diphenyl-
N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine) and said electron
transport
layer is Bu-PBD (2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole).
20. An OLED as claimed in claim 19, wherein said hole transport layer is POC
(poly(N-octyl-2,7-carbazole)) and said electron transport layer is Bu-PBD (2-
(4-
biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole).
21. An OLED as claimed in claim 19, wherein said hole transport layer is a
blend of P(OCDOF) (poly(N-octyl-2,7-carbazole-alt-9,9'-dioctyl-2,7-fluorene)
and
TPD (N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine) ans
said electron transport layer is Bu-PBD (2-(4-biphenylyl)-5-(4-tert-
butylphenyl)-
1,3,4-oxadiazole).
14

Description

CA 02344084 2001-04-12
Conjugated Polycarbazole Derivatives in Organic Light Emitting
Diodes.
BACKGROUND OF THE INVENTION
Field of the invention
The present invention relates to the field of optics, and more particularly
Organic
Light Emitting Diodes (OLEDs).
Description of the related art
Organic materials show promise for electronic and opto-electronic
applications.
Low-cost of synthesis, processing and room temperature fabrication are among
the economical advantages. OLEDs are of particular promise for displays as
they
can be tuned to any color, organic diodes are very efficient and the visual
properties of the diodes are excellent. Liquid crystal displays are currently
overwhelmingly dominating the laptop computer market, but the technology has
limitations such as low efficiency, poor vision angle, and speed and
temperature
range limitations.
It has long been felt that a technically viable emissive display technology
could
compete with LCDs, and OLEDs are today considered in the best position to do
just that. OLEDs are also of great interest for other markets, such as
lighting.
Intense research is going on at the chemistry laboratory level to find
materials with
high luminous quantum efficiency, good color purity and stability for the
application to OLED displays. While some materials meet or exceed some of the
requirements for commercial displays, none meets them all. Tang and VanSlyke,
from Eastman Kodak, discovered small molecules that yield very efficient and
stable green diodes [US patent 4356429, Oct. 26,1982; C.W.Tang and
S.A.VanSlyke,
Appl. Phys. Lett. 51, 913 (1987)]. Figure 1 shows a typical OLED diode as
described
by Tang and VanSlyke: it is composed of TPD (a triphenyl diamine derivative),
a
hole transport layer, and A1Q3 (a chelated Aluminum hydroxyquinoline), an
electron transport and emitter layer.
When a sufficiently high positive voltage is applied between the anode and
cathode, holes are injected in the hole transport layer from an anode,
electrons are
injected in the electron transport layer from a cathode. Holes and electrons
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CA 02344084 2001-04-12
recombine at the interface between the two organic materials and create an
exciton
that will release energy as light or heat. Light is emitted through the anode,
which
can be Indium Tin Oxide, a transparent degenerate semiconductor with a
relatively
high work function that favors hole injection. The cathode is usually a low
work
function metal like Ca, Mg or Al. TPD and A1Q3 are now commercially available.
However, blue and red emitters in the same class of materials usually have
shorter
lifetimes and lower luminous efficiencies.
The glass transition temperature of small molecules is usually quite low (as
low as
60 C for TPD), so displays made of small molecules have difficulty meeting
some
of the temperature requirements. On the contrary, polymers usually have high
glass transition temperatures. It is also desirable to find a family of
materials that
will emit in the three primary colors to permit the creation of full-color
displays:
this is easier with polymers than small molecules. Finally, small molecules
have to
be evaporated to form thin films, whereas polymers can be easily soluble and
can
be spun-on or reel-coated. This means that polymers are potentially lower cost
to
process and easier to coat on large surface for large display applications.
Conjugated polymers are considered a very important class of electroactive and
photoactive materials because they possess a highly delocalized pi-electron
system,
and transport charges efficiently [Heeger, McDiarmid and Shirakawa, 2000 Nobel
Prize, http:/ /www.nobelprizes.com/]. Scientists from Cambridge University,
UK,
discovered that PPVs (poly(p-phenyenevinylene)s) can be a very efficient green
emitter [US patent 5247190, priority date April 20,1989; J.H.Burroughes et al,
Nature, v347,539 (1990)].
Figure 2 shows a typical diode as described by Burroughes et al. A 70nm PPV is
sandwiched between an ITO conductive transparent layer and an aluminum
cathode. The thin PPV layer transports electrons, holes and is also the green
emitter.
When a sufficiently high positive voltage is applied between the anode and
cathode, holes injected from the anode and electrons injected from the cathode
recombine radiatively to produce green light. Many other compounds have been
studied since and incorporated in diodes producing all colors [see for review
R.H.
Friend et al, Nature, v397, 121 (1999)]. Green, however, is still the only
color with
sufficient lifetime, so there is still the need for a family of polymers that
will emit in
all colors of the spectrum, and optimize their use in OLEDs.
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CA 02344084 2010-06-23
SUMMARY OF THE INVENTION
According to the present invention there is provided an Organic Light Emitting
Diode (OLED) comprising as its active material a conjugated poly(N-alkyl-2,7-
carbazole) derivative described by the formula:
N n
wherein n is an integer of about 3 to about 100, and R represents a
substituant that
is a linear or branched alkyl group containing 1 to 22 carbon atoms,. or
poly(ethyleneoxy), cyano, aryl, amide or benzoyl.
Polycarbazoles are a well-known class of polymers. Two of the present
inventors
have discovered how to synthesise highly conjugated poly(N-alkyl-2,7-
carbazole)
and have filed a patent application entitled "Conjugated Polycarbazole
Derivatives
and Process for the Synthesis thereof" on their method of synthesis. In the
present invention the conjugated polycarbazole derivatives are used as the
active material in OLEDs either alone or mixed with another material.
Carbazole monomers can also be alternated with other monomers to form
copolymers according to the general formula:
Y Y n
Where Z is any type of comonomer, x is an integer between 1 and 100, y is an
integer between 0 and 100, and n is an integer of about 3 to about 100.
The conjugated polycarbazole.derivatives comprise repeating or alternating
units
of the above formula. For example, the comonomers can be selected from
ethylene, acetylene, C6- C,2 mononuclear/polynuclear aromatic, C2 C10
mononuclear/polynuclear heterocyclic groups and tertiary arylamines.
Examples of mononuclear / polynuclear aromatic groups are: fluorene,
3

CA 02344084 2001-04-12
anthracene, phenylene, naphthylene, acenaphthene, phenanthrene, fluoranthene,
pyrene, perylene, rubrene, chrysene, tetracene and pentacene.
Examples of mononuclear / polynuclear heterocyclic internal groups include 5-
member heterocycles such as furan, thiophene, pyrrole, oxazole, isooxazole,
oxadiazoles, thiazole, isothiazole, imidazole, thiadiazole, and pyrazoles; 6-
member heterocycles such as pyridine, pyridazine, pyrimidine, pyrazine,
triazines, and tetrazenes; benzo-fused ring systems such as benzoxazole,
benzothiazole, benzimidazole, quinoline, isoquinoline, cinnoline, quinazoline,
quinoxaline, phthalazine, benzothiadiazole, and benzotriazines; polynuclear
fused condensed ring systems such as phenazine, phenanthridine, acridine, and
diphenylene oxide. Examples of tertiary arylamine groups include
triphenylamine, N,N'-diphenylbenzidine, N,N'-diphenyl-l,4-phenylenediamine,
and diphenylnaphthylamine. Olefinic, arylamino, aromatic and heterocyclic
aromatic groups containing up to 30 carbons are useful for the present
purpose.They may be substituted optionally with one or more substituents.
Examples of substituents include C, -C2() hydrocarbyl radicals, C, -C20
(thio)alkoxy
radicals, C, -C20 (thio)aryloxy radicals, cyano, fluoro, chloro, C, -C20
alkoxycarbonyl, C, -C20 aryoxylcarbonyl, poly(alkyleneoxy) and
alkyl(aryl)sulfonyl radicals. Such substituents may be selected to improve the
solubility or processing characteristics of the polymer. In such cases, the
substituent is preferably selected from alkyl, alkoxy, poly(alkyleneoxy), and
cyano.
OLEDs can consist of many different layers with different roles. They are
sandwiched between two electrodes, metallic or semiconductive, to provide
injection of electrodes and holes. There is a layer between those two
electrodes
where recombination of those charges takes place and produces the emission of
light. That layer is referred to as the emitter. There might be layers on
either or
both sides, performing the specific task of transporting one of the charges,
electrons or holes. The emitter layer itself might play the role of transport
layer
for one or both charges. There might be a layer or several layers between the
emitter and either or both of the transport layers, or between the emitter and
the
electrodes, to confine excitons and improve the quantum efficiency of the
device.
There might be a layer or several layers between the organic layers and either
or
both of the electrodes to promote injection into the device.
4

CA 02344084 2001-04-12
The polycarbazole compounds can be used in OLEDs as hole transport layers,
electron transport layers, emitters or any combination of those roles. They
can be
used either pure or blended or doped with other hole or electron transport
materials. They can also be used in multilayer arrangements to promote
confinement or as an alternative to doping.
BRIEF DESCRIPTON OF THE DRAWINGS
The invention will now be described in more detail, by way of example only,
with reference to the accompanying drawings, in which:-
Figure 1 shows a small-molecule light-emitting diode according to prior art;
Figure 2 shows a polymer light-emitting diode according to prior art;
Figure 3 shows a first embodiment of the invention where an OLED is formed
using a PEHC polycarbazole thin film as the emitter;
Figure 4 shows the photoluminescence spectrum of PEHC in the solid state
(curve 1) and the electroluminescent spectrum of the diode described in Figure
3
(curve 2);
Figure 5 shows the electroluminescent spectrum (curve 3) of another OLED with
TPD alone as the hole transport layer;
Figure 6 shows a second embodiment of the invention where an OLED is formed
using a POC polycarbazole thin film as the emitter;
Figure 7 shows the photoluminescence spectrum of POC in the solid state (curve
4) and the EL spectrum of the diode described in Figure 5 (curve 5);
Figure 8 shows a first embodiment of the invention where an OLED is formed
using a P(OCDOF) copolymer thin film as the emitter. The diode consists of an
Indium Tin Oxide transparent conductive anode on a glass substrate, a thin LiF
layer to promote hole injection, a blend of P(OCDOF) and TPD as the hole
transport layer and emitter, Bu-PBD as the electron transport layer, a thin
LiF
layer to promote electron injection and an Al cathode; and
Figure 9 shows the photoluminescence spectrum of P(OCDOF) in the solid state
(curve 6) and the electroluminescent spectrum of the diode described in Figure
3
(curve 7).
5

CA 02344084 2001-04-12
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Conjugated poly(N-alkyl-2,7-carbazole) derivatives can be synthesized by the
following route:
Scheme 1
B(OH)2 CI Pd(PPh3)4
Benzene/K2C03 aq.
+ reflux, 2h CI CI
02N / Y = 93 %
CI Br NO2 (1)
P(OEt)3
reflux, 5 h
Y=60%
K2CO3, R-Br
CI \ N\ / CI DMF, 80oC CI' \ N\ / CI
24 h H
R Y=86% (2)
(3)
6

CA 02344084 2001-04-12
Scheme 2
1) NaNO2, H2SO4
- AcOH
O2N NO2 O2N NO2
~ ~ -
2) NaN3
NH2 N3
2 (4)
A, -N2
Y=66%
SnCl2
AcOH/HCl (5:1)
H2N NH2 O2N NO2
Y=78%
H (6) H
(5)
1) HC1(a9.), NaNO2
2) Khay.), 24 It, r. t.
K2CO3, R-Br
DMF, 80 C
H 24 h
(7) Global yield for steps R
4and5=38% (8)
7

CA 02344084 2001-04-12
Scheme 3
PPh3, Zn, NiC12
2,2'-bipyridine
DMAc, 80 C, 3 d n
Y=73and78% N
R R
X= C1,Br,I
O R R O
Pd(PPh3)4
K2C03 (a9.)
THF, 3 d
reflux
Y=75%
~ N
R n
R
C) (8) + Me3Sn-~\ S SnMe
3
OS\
PdC12(PPh3)2
THE
reflux, 3 d
Y=87%
S
C n
QN:o 8

CA 02344084 2001-04-12
The first prior art device shown in Figure 1 consists of an ITO transparent
anode
on a glass substrate, a TPD hole transport layer, an AIQ3 electron transport
and
emitter layer, and an Al cathode. When a sufficient positive voltage is
applied
between the anode and the cathode, holes are injected from the anode,
electrons
from the cathode and they recombine radiatively in the A1Q3 emissive layer,
producing light which is seen through the transparent anode and hole transport
layers.
The second prior device shown in Figure 2 consists of an ITO transparent anode
on a glass substrate, a thin PPV polymeric layer and an Al cathode. When a
sufficiently high positive voltage is applied between the anode and cathode,
holes injected from the anode and electrons injected from the cathode
recombine
radiatively to produce green light.
In the embodiment of the invention, shown in Figure 3, the diode consists of
an
Indium Tin Oxide transparent conductive anode on a glass substrate, a first
thin
LiF layer to promote hole injection, a blend of PEHC (poly(N-ethylhexyl)-2,7-
carbazole) and TPD (N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-
diamine) as the hole transport layer and emitter, PBD (2-(4-biphenylyl)-5-(4-
tert-
butylphenyl)-1,3,4-oxadiazole) as the electron transport layer, a second thin
LiF
layer to promote electron injection and an Al cathode.
FIRST EXAMPLE
In a first embodiment of the invention, shown in Figure 3, an OLED was
fabricated consisting of an Indium Tin Oxide transparent conductive anode on a
glass substrate, a thin LiF layer to promote hole injection, a blend of PEHC
(poly(N-ethylhexyl)-2,7-carbazole) and TPD (N,N'-diphenyl-N,N'-bis(3-
methylphenyl)-1,1'-biphenyl-4,4'-diamine) as the hole transport layer and
emitter,
Bu-PBD (2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole) as the
electron
transport layer, a thin LiF layer to promote electron injection, and an Al
cathode.
The blend of PEHC and TPD is solubilized in chloroform:toluene in a ratio of
9.8:0.2, and spin-coated. PBD, LiF and Al layers were thermally evaporated.
The diode produces the electroluminescence spectrum (curve 2) shown in Figure
4, compared with the photoluminescence spectrum (curve 1) of PEHC in the solid
state.
Both spectra seem to show a maximum of emission at similar wavelengths, i.e.
in
the blue range, with two peaks around 437 nm and 453 run. The
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CA 02344084 2001-04-12
electroluminescence is thus originating from the polymer itself. No emission
of
an excimer is observed (no emission peak at higher wavelength). The relative
intensity of the peaks are somewhat different in PL and EL, and this may be
due
to a difference in morphology of both films. TPD or PBD do not seem to affect
the
color of emission, which remains blue.
A comparison with the electroluminescent spectrum obtained from an OLED
with TPD only as the hole transport layer (curve 3) is shown in Figure 5. The
OLED used to produce the curve in Figure 5 consisted of an Indium Tin Oxide
transparent conductive anode on a glass substrate, a thin LiF layer to promote
hole injection, TPD as the hole transport layer, PBD as the electron transport
layer
and emitter, a thin LiF layer to promote electron injection and an Al cathode.
The
comparison shows that the electroluminescence spectrum obtained in the device
of Figure 3 is due to the incorporation of PEHC.
SECOND EXAMPLE
In a second embodiment of the invention shown in Figure 6, an OLED was
fabricated consisting of an Indium Tin Oxide transparent conductive anode on a
glass substrate, a first thin LiF layer to promote hole injection, a POC
(poly(N-
octyl-2,7-carbazole)) for the hole transport layer, Bu-PBD (2-(4-biphenylyl)-5-
(4-
tert-butylphenyl)-1,3,4-oxadiazole) as the electron transport layer, a second
thin
LiF layer to promote electron injection, and an Al cathode.
Figure 7 shows that the resulting diode emits blue light with emission peaks
around 423 and 447 nm (shoulder), with another weak shoulder around 480 nm,
for photoluminescence (curve 4) and electroluminescence (curve 5). The peaks
are slightly shifted towards higher wavelengths for the electroluminescence,
but
no excimer is observed. This small shift and broadening of the peaks could be
due to the cavity effect when an excess of electrons is injected on the
molecule, or
to an increased radiative decay from longer conjugated segments having a
smaller gap.[See, for example, A.Donat-Bouillud et al.Chem. Mater., 12, 1931
(2000)and ref. within- P.E.Burrows, J.Appl. Phys. Lett., (1996), 79, 79911.
A comparison with the electroluminescent spectrum obtained from an OLED
with TPD only as the hole transport layer (curve 3) in Figure 5 shows that the
electroluminescence spectrum obtained in the device of Figure 6 is due to the
incorporation of POC.

CA 02344084 2001-04-12
The invention thus permits the production of efficient OLEDs that may be
effective in the red and blue regions of the spectrum.
THIRD EXAMPLE
In a third embodiment of the invention, an OLED was fabricated consisting of
an Indium
Tin Oxide transparent conductive anode on a glass substrate, a thin LiF layer
to promote
hole injection, a blend of P(OCDOF) (poly(N-octyl-2,7-carbazole-alt-9,9'-
dioctyl-2,7-
fluorene)and TPD (N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-
diamine)
as the hole transport layer and emitter, Bu-PBD (2-(4-biphenylyl)-5-(4-tert-
butylphenyl)-
1,3,4-oxidiazole) as the electron transport layer, a thin LiF layer to promote
electron
injection, and an Al cathode. See Figure 8. The blend of P(OCDOF) and TPD is
solubilized in chloroform: toluene in a ration of 9.8:0.2, and spin-coated.
PBD, LiF and Al
layers are thermally evaporated.
The diode produces the electroluminescence spectrum (curve 7) shown in Figure
9,
compared with the photoluminescence spectrum (curve 6) of P(OCDOF) in the
solid
state.
Both spectra seem to show a maximum of emission at similar wavelengths, i.e.
in the blue
range, with two peaks around 423 nm and 447 nm. The electroluminescence is
thus
originating from the polymer itself. No emission of an excimer is observed (no
emission
peak at higher wavelength). The relative intensity of the peaks are different
in PL and EL,
and this may be due to a difference in morphology of both films. TPD or PBD do
not
seem to affect the color of emission, which remains blue.
A Comparison with the electroluminescent spectrum obtained from an OLED with
TPD
only as the hole transport layer (curve 3) in Figure 5 shows that the
electroluminescence
spectrum obtained in the device of Figure 8 is due to the incorporation of
P(OCDOF).
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