U.S. patent application number 13/805285 was filed with the patent office on 2013-07-25 for organic light-emitting composition comprising anthranthene derivates and device and method using the same.
This patent application is currently assigned to SUMITOMO CHEMICAL COMPANY LIMITED. The applicant listed for this patent is Ruth Pegington, Jonathan Pillow, Martina Pintani. Invention is credited to Ruth Pegington, Jonathan Pillow, Martina Pintani.
Application Number | 20130187145 13/805285 |
Document ID | / |
Family ID | 44511799 |
Filed Date | 2013-07-25 |
United States Patent
Application |
20130187145 |
Kind Code |
A1 |
Pegington; Ruth ; et
al. |
July 25, 2013 |
ORGANIC LIGHT-EMITTING COMPOSITION COMPRISING ANTHRANTHENE
DERIVATES AND DEVICE AND METHOD USING THE SAME
Abstract
Composition comprising a fluorescent light-emitting material and
a triplet-accepting unit comprising an optionally substituted
compound of formula (I): The composition may be used in an organic
light-emitting device; the optionally substituted compound of
formula (I) may be blended with or attached to the fluorescent
light emitting material; and the composition may be deposited by
solution deposition. ##STR00001##
Inventors: |
Pegington; Ruth;
(Cambridgeshire, GB) ; Pintani; Martina;
(Cambridgeshire, GB) ; Pillow; Jonathan;
(Cambridgeshire, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Pegington; Ruth
Pintani; Martina
Pillow; Jonathan |
Cambridgeshire
Cambridgeshire
Cambridgeshire |
|
GB
GB
GB |
|
|
Assignee: |
SUMITOMO CHEMICAL COMPANY
LIMITED
TOKYO
JP
CAMBRIDGE DISPLAY TECHNOLOGY LIMITED
CAMBRIDGESHIRE
GB
|
Family ID: |
44511799 |
Appl. No.: |
13/805285 |
Filed: |
June 24, 2011 |
PCT Filed: |
June 24, 2011 |
PCT NO: |
PCT/GB11/00949 |
371 Date: |
April 8, 2013 |
Current U.S.
Class: |
257/40 ;
252/301.16; 252/301.35; 438/46; 525/540; 570/183; 585/26 |
Current CPC
Class: |
C07C 15/52 20130101;
C08L 2205/02 20130101; H01L 51/0002 20130101; H01L 51/0052
20130101; C08G 2261/1414 20130101; H05B 33/14 20130101; H01L
51/0058 20130101; C07C 13/567 20130101; H01L 51/0039 20130101; C07C
2603/18 20170501; C08G 2261/15 20130101; C08G 2261/228 20130101;
H01L 51/5016 20130101; C08G 2261/312 20130101; C09K 2211/1433
20130101; C08L 65/00 20130101; C08G 2261/5222 20130101; H01L 51/005
20130101; C07C 15/18 20130101; C08G 2261/3245 20130101; C08G 61/122
20130101; C08G 2261/226 20130101; C08G 2261/148 20130101; C08G
2261/3162 20130101; C09K 2211/1416 20130101; H01L 51/0043 20130101;
H01L 51/5012 20130101; C08G 61/12 20130101; C08L 65/00 20130101;
C09K 11/06 20130101; C08L 65/00 20130101; H01L 51/0032 20130101;
C08G 2261/3422 20130101; C08G 2261/95 20130101; C08G 61/02
20130101; H01L 51/56 20130101; C09K 2211/1425 20130101 |
Class at
Publication: |
257/40 ; 438/46;
252/301.16; 252/301.35; 525/540; 570/183; 585/26 |
International
Class: |
H01L 51/00 20060101
H01L051/00; H01L 51/50 20060101 H01L051/50 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 25, 2010 |
GB |
1010741.5 |
Jun 25, 2010 |
GB |
1010742.3 |
Jun 25, 2010 |
GB |
1010743.1 |
Jun 25, 2010 |
GB |
1010745.6 |
Jan 31, 2011 |
GB |
1101642.5 |
Claims
1. A composition comprising a fluorescent light-emitting material
and a trip et-accepting unit comprising an optionally substituted
compound of formula (I): ##STR00018##
2. A composition according to claim 1 wherein the fluorescent
light-emitting material is a polymer comprising fluorescent
light-emitting repeat units.
3. A composition according to claim 1 wherein the compound of
formula (I) is substituted with one or more substituents selected
from the group consisting of: alkyl wherein one or more
non-adjacent C atoms may be replaced with O, S, substituted N,
C.dbd.O and --COO-- and wherein one or more H atoms may be replaced
with F; or aryl, heteroaryl, arylalkyl or heteroarylalkyl, each of
which may optionally be substituted with halogen, cyano, or alkyl
wherein one or more non-adjacent C atoms may be replaced with O, S,
substituted N, C.dbd.O and --COO--.
4. A composition according to claim 1 wherein the compound of
formula (I) comprises formula (Ia): ##STR00019## wherein each
R.sup.3 is independently selected from H and a substituent selected
from the group consisting of alkyl wherein one or more non-adjacent
C atoms may be replaced with O, S, substituted N, C.dbd.O and
--COO-- and wherein one or more H atoms may be replaced with or
aryl, heteroaryl, arylalkyl or heteroarylalkyl, each of which may
optionally be substituted with halogen, cyano, or alkyl wherein one
or more non-adjacent C atoms may be replaced with O, S, substituted
N, C.dbd.O and --COO--, and at least one R.sup.3 is not H.
5. A composition according to claim 1 wherein a substituent R.sup.3
that is not H is present in the 2- and/or 8- positions of the
compound of formula (I).
6. A composition according to claim 1 where the composition
comprises a mixture of the light-emitting material and the compound
of formula (I).
7. A composition according to of claim 1 wherein the
triplet-accepting unit is chemically bound to the fluorescent
light-emitting material or, if present, to another component of the
light-emitting composition.
8. A composition according to claim 2 wherein the triplet-accepting
unit is bound in the main chain of the polymer or bound as a side
group or end group of the polymer.
9. A composition according to claim 8 wherein the triplet accepting
unit is bound in the main chain of the polymer through its 2- and
8-positions or is bound as a side-group or end-group of the polymer
through its 2- or 8- position.
10. A composition according to claim 1 wherein the light-emitting
material comprises an amine.
11. A composition according to claim 2 wherein the light-emitting
material is a polymer comprising amine repeat units.
12. A composition according to claim 1 wherein the
triplet-accepting unit is present in an amount of at least 0.1 mol
% relative to the light-emitting material.
13. A solution comprising a solvent and a composition according to
claim 1.
14. An organic light-emitting device comprising an anode, a cathode
and a light-emitting layer between the anode and cathode, wherein
the light-emitting layer comprises a composition according to claim
1.
15. A method of forming an organic light-emitting device according
to claim 14 comprising the steps of depositing a solution
comprising a solvent and a composition comprising a fluorescent
light-emitting material and a triplet-accepting unit comprising an
optionally substituted compound of formula (I): ##STR00020## and
evaporating the solvent.
16. Use of an optionally substituted unit of formula (I) for
acceptance of triplet excitons generated by a light-emitting
material in a composition comprising the triplet-accepting unit and
the light-emitting material: ##STR00021##
17. Use according to claim 16 wherein the composition comprises a
physical mixture of the light-emitting material and a compound of
formula (I).
18. Use according to claim 16 wherein the unit of formula (I) is
chemically bound to the fluorescent light-emitting material.
19. Use according to claim 18 wherein the light-emitting material
is a polymer and the unit of formula (I) is bound in the main chain
of the polymer or bound as a side group or end group of the
polymer.
20. Use according to claim 16 wherein the unit of formula (I)
quenches triplet excitons generated by the light-emitting
material.
21. Use according to claim 16 wherein the unit of formula (I)
mediates triplet-triplet annihilation of triplet excitons
transferred from the light emitting material to the
triplet-accepting unit.
22. A light-emitting composition comprising a polymer and an
optionally substituted light emitting unit of formula (I):
##STR00022##
23. A light-emitting composition according to claim 22 wherein the
composition comprises a blend of the polymer and a compound of
formula (I).
24. A composition according to claim 22 wherein the unit of formula
(I) is bound in the main chain of the polymer or bound as a side
group or end group of the polymer.
25. A compound of formula (I): ##STR00023## wherein the compound is
substituted with at least one solubilising group.
26. A compound according to claim 25 wherein the solubilising group
is selected from alkyl and arylalkyl.
Description
[0001] This application claims priority from UK patent application
no. 1010741.5 filed on Jun. 25, 2010, UK patent application no.
1010742.3 filed on Jun. 25, 2010, UK patent application no.
1010745.6 filed on Jun. 25, 2010, UK patent application no.
1010743.1 filed on Jun. 25, 2010 and UK patent application no.
1101642.5 filed on Jan. 31, 2011. The contents of each
aforementioned priority-forming application are incorporated herein
in their entirety by reference.
SUMMARY OF THE INVENTION
[0002] This invention relates to organic light emitting
compositions, organic light-emitting devices comprising the same,
and methods of making said devices.
BACKGROUND OF THE INVENTION
[0003] Electronic devices comprising active organic materials are
attracting increasing attention for use in devices such as organic
light emitting diodes, organic photovoltaic devices, organic
photosensors, organic transistors and memory array devices. Devices
comprising organic materials offer benefits such as low weight, low
power consumption and flexibility. Moreover, use of soluble organic
materials allows use of solution processing in device manufacture,
for example inkjet printing or spin-coating.
[0004] A typical organic light-emissive device ("OLED") is
fabricated on a glass or plastic substrate coated with a
transparent anode such as indium-tin-oxide ("ITO"). A layer of a
thin film of at least one electroluminescent organic material is
provided over the first electrode. Finally, a cathode is provided
over the layer of electroluminescent organic material. Charge
transporting, charge injecting or charge blocking layers may be
provided between the anode and the electroluminekent layer and/or
between the cathode and the electroluminescent layer.
[0005] In operation, holes are injected into the device through the
anode and electrons are injected into the device through the
cathode. The holes and electrons combine in the organic
electroluminescent layer to form an excitons which then undergo
radiative decay to give light.
[0006] In WO90/13148 the organic light-emissive material is a
conjugated polymer such as poly(phenylenevinylene). In U.S. Pat.
No. 4,539,507 the organic light-emissive material is of the class
known as small molecule materials, such as
tris-(8-hydroxyquinoline) aluminium ("Alq.sub.3"). These materials
electroluminesce by radiative decay of singlet excitons
(fluorescence) however spin statistics dictate that up to 75% of
excitons are triplet excitons which undergo non-radiative decay,
i.e. quantum efficiency may be as low as 25% for fluorescent
OLEDs--see, for example, Chem. Phys. Lett., 1993, 210, 61, Nature
(London), 2001, 409, 494, Synth. Met., 2002, 125, 55 and references
therein.
[0007] It has been postulated that the presence of triplet
excitons, which may have relatively long-lived triplet excited
states, can be detrimental to OLED lifetime as a result of
triplet-triplet or triplet-singlet interactions ("lifetime" as used
herein in the context of OLED lifetime means the length of time
taken for the luminance of the OLED at constant current to fall by
50% from an initial luminance value, and "lifetime" as used herein
in the context of lifetime of a triplet excited state means the
half-life of a triplet exciton). US 2007/145886 discloses an OLED
comprising a triplet-quenching material to prevent or reduce
triplet-triplet or triplet-singlet interactions.
[0008] OLEDs comprising light-emitting anthanthrenes, including
blue light-emiting anthanthrenes, are disclosed in U.S. Pat. No.
7,135,243, US 2006/141287 and Shah et al, J Phys Chem A 2005, 109,
7677-7681.
[0009] OLEDs have great potential for display and lighting
applications. However, there remains a need to improve performance
of these devices.
SUMMARY OF THE INVENTION
[0010] In a first aspect, the invention provides a composition
comprising a fluorescent light-emitting material and a
triplet-accepting unit comprising an optionally substituted
compound of formula (I):
##STR00002##
[0011] Optionally, the fluorescent light-emitting material is a
polymer comprising fluorescent light-emitting repeat units.
[0012] Optionally, the compound of formula (I) is substituted with
one or more substituents selected from the group consisting of:
[0013] alkyl wherein one or more non-adjacent C atoms may be
replaced with O, S, substituted N, C.dbd.O and --COO-- and wherein
one or more H atoms may be replaced with F; or [0014] aryl,
heteroaryl, arylalkyl or heteroarylalkyl, each of which may
optionally be substituted with halogen, cyano, or alkyl wherein one
or more non-adjacent C atoms may be replaced with O, S, substituted
N, C.dbd.O and --COO--.
[0015] Optionally, the compound of formula (I) comprises formula
(Ia):
##STR00003##
wherein each R.sup.3 is independently selected from H and a
substituent selected from the group of claim 3, and at least one
R.sup.3 is not H.
[0016] Optionally, a substituent R.sup.3 that is not H is present
in the 2 and/or 8- positions of the compound of formula (I).
[0017] Optionally, the composition comprises a physical mixture of
the light-emitting material and the compound of formula (I).
[0018] Optionally, the triplet-accepting unit is chemically bound
to the fluorescent light-emitting material.
[0019] Optionally, the triplet-accepting unit is bound in the main
chain of the polymer or bound as a side group or end group of the
polymer.
[0020] Optionally, the triplet accepting unit is bound in the main
chain of the polymer through its l- and 8-positions or is bound as
a side-group or end-group of the polymer through its 2- or
8-position.
[0021] Optionally, the light-emitting material comprises an
amine.
[0022] Optionally, the light-emitting material is a polymer
comprising amine repeat units.
[0023] Optionally, the triplet-accepting unit is present in an
amount of at least 0.1 mol % relative to the light-emitting
material.
[0024] In a second aspect the invention provides a solution
comprising a solvent and a composition according to any preceding
claim.
[0025] In a third aspect the invention provides an organic
light-emitting device comprising an anode, a cathode and a
light-emitting layer between the anode and cathode, wherein the
light-emitting layer comprises a composition according to the first
aspect.
[0026] In a fourth aspect the invention provides a method of
forming an organic light-emitting device according to the third
aspect comprising the steps of depositing the solution according to
the second aspect and evaporating the solvent.
[0027] In a fifth aspect the invention provides use of an
optionally substituted unit of formula (I) for acceptance of
triplet excitons generated by a light-emitting material in a
composition comprising the triplet-accepting unit and the
light-emitting material:
##STR00004##
[0028] Optionally according to the fifth aspect the composition
comprises a physical mixture of the light-emitting material and a
compound of formula (I).
[0029] Optionally according to the fifth aspect the unit of formula
(I) is chemically bound to the fluorescent light-emitting
material.
[0030] Optionally according to the fifth aspect the light-emitting
material is a polymer and the unit of formula (I) is bound in the
main chain of the polymer or bound as a side group or end group of
the polymer.
[0031] Optionally according to the fifth aspect the unit of formula
(I) quenches triplet excitons generated by the light-emitting
material.
[0032] Optionally according to the fifth aspect the unit of formula
(I) mediates triplet-triplet annihilation of triplet excitons
transferred from the light emitting polymer to the
triplet-accepting unit.
[0033] In a sixth aspect the invention provides a light-emitting
composition comprising a polymer and an optionally substituted
light emitting unit of formula (I):
##STR00005##
[0034] Optionally, the composition of the sixth aspect comprises a
blend of the polymer and a compound of formula (I).
[0035] Optionally according to the sixth aspect the unit of formula
(I) is bound in the main chain of the polymer or bound as a side
group or end group of the polymer.
[0036] In a seventh aspect the invention provides a compound of
formula (I):
##STR00006##
wherein the compound is substituted with at least one solubilising
group. Optionally, the solubilising group is selected from alkyl
and arylalkyl.
[0037] It will be appreciated that the invention in its first
aspect relates to a composition wherein the triplet-accepting unit
of formula (I) emits substantially no light. The excited singlet
state energy level (S.sub.1) of the light-emitting material is no
higher than, and preferably lower than, the corresponding energy
level of triplet-accepting unit in order to prevent any substantial
transfer of singlet excitons from the S.sub.1 energy level of the
light-emitting material to the S.sub.1 level of the
triplet-accepting material.
[0038] The opposite is the case in the second aspect of the
invention, wherein the unit of formula (I) is the light-emitting
material.
[0039] "Aryl" and "heteroaryl" as used herein includes both fused
and unfused aryl and heteroaryl groups respectively.
[0040] "Triplet accepting unit" as used herein means a unit capable
of receiving triplet excitons from the light emitting unit. In
order to function efficiently, the triplet accepting unit has a,
triplet excited state energy level T1 that is lower in energy than
that of the light-emitting unit, preferably lower by at least kT to
prevent back-transfer of triplet excitons from the triplet
accepting unit to the light-emitting unit.
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] FIG. 1 is a schematic illustration of triplet quenching by a
compound of formula (I);
[0042] FIG. 2 is a schematic illustration of a first
triplet-triplet annihilation mechanism involving a by a compound of
formula (I);
[0043] FIG. 3 illustrates a second triplet-triplet annihilation
mechanism involving a compound of formula (I);
[0044] FIG. 4 illustrates an organic light-emitting device
according to an embodiment of the invention;
[0045] FIG. 5 illustrates the electroluminescent spectrum of an
exemplary OLED of the invention compared to the electroluminescent
spectrum of a comparative device;
[0046] FIG. 6 illustrates the lifetime of 4 exemplary OLEDs of the
invention compared to the electroluminescent spectrum of a
comparative device;
[0047] FIG. 7 illustrates the T.sub.90 lifetime (i.e. time for
decay to 90% of initial luminescence) of 4 exemplary OLEDs of the
invention compared to the electroluminescent spectrum of a
comparative device;
[0048] FIG. 8 illustrates current density vs. voltage for an
exemplary device and a comparative device; and
[0049] FIG. 9 illustrates external quantum efficiency vs. voltage
for an exemplary device and a comparative device.
DETAILED DESCRIPTION OF THE INVENTION
[0050] The invention is described in detail hereinafter with
respect to (A) compositions wherein the compound of formula (I) is
a triplet-accepting material, and (B) compositions wherein the
compound of formula (I) is a light-emitting material.
A. Anthanthrene as a Triplet-Accepting Material.
[0051] The present inventors have identified a number of pathways
by which triplet excitons may be caused to undergo decay in order
to reduce or eliminate decay by pathways that cause a drop in
device lifetime. Some of these pathways allow for radiative exciton
decay by delayed fluorescence that can provide for better device
efficiency as compared to non-radiative decay pathways.
[0052] Without wishing to be bound by any theory, the mechanisms of
triplet quenching and delayed fluorescence believed to occur are
described below.
Triplet Quenching
[0053] FIG. 1 illustrates a first energy transfer mechanism for an
exemplary OLED. For the avoidance of any doubt energy level
diagrams herein, including FIG. 1, are not drawn to any scale. FIG.
1 illustrates energy transfer for an OLED provided with a light
emitting material having a singlet excited state energy level
S.sub.1E and a singlet ground state energy level S.sub.0E. Singlet
excitons having energy S.sub.1E decay by emission of fluorescent
light h.nu., illustrated by the solid arrow between S.sub.1E and
S.sub.0E in FIG. 1. Triplet-triplet exciton interactions or
triplet-singlet exciton interactions may create "super-excited"
states on the light-emitting material. Without wishing to be bound
by any theory, it is believed that formation of these highly
energetic "super-excited" states on the light emitting material may
be detrimental to operational lifetime of the material. However, by
providing a triplet accepting unit having an excited triplet state
energy level T.sub.1A that is lower than T.sub.1E, it is possible
for triplet excitons to be transferred for non-radiative quenching
to the triplet accepting unit, the alternative of radiative decay
from T.sub.1E to S.sub.0E, illustrated by a dotted line in FIG. 1,
being a spin-forbidden process. S.sub.1 and T.sub.1 levels can be
measured from the fluorescence and phosphorescence spectra
respectively.
[0054] The triplet accepting unit has a singlet excited state
energy level S.sub.1A that is higher than the singlet excited state
energy level S.sub.1E in order to substantially or completely
prevent transfer of singlet excitons from S.sub.1E to S.sub.1A.
Preferably, S.sub.1A is at least kT higher in energy than S.sub.1E
in order to prevent any substantial back-transfer of excitons.
Likewise, T.sub.1E is preferably at least kT higher in energy than
T.sub.1A.
Triplet-Triplet Annihilation
[0055] FIG. 2 illustrates a second energy transfer mechanism for an
exemplary OLED.
[0056] According to this embodiment, triplet-triplet annihilation
(TTA), caused by an interaction between two triplet-accepting
units, results in a triplet-triplet annihilated singlet exciton
having an energy of up to 2.times.T.sub.1A, wherein T.sub.1A
represents the triplet excited state energy level of the
triplet-accepting material. This singlet exciton, formed on a first
of the two triplet-accepting units, has energy level S.sub.nA that
is higher in energy than S.sub.1A and S.sub.1E and so it may
transfer to S.sub.1A and then to S.sub.1E from which light h.nu.
may be emitted as delayed fluorescence. The triplet exciton on the
second of the two triplet-accepting units may decay to the ground
state T.sub.0A.
[0057] Initially, the triplet exciton formed at T.sub.1E is
transferred to T.sub.1A. By providing a triplet-accepting material
having energy level T.sub.1A that is lower than T.sub.1E, rapid
transfer of excitons from T.sub.1E to T.sub.1A may occur. This
transfer is relatively rapid compared to the rate of decay of
triplet excitons from T.sub.1E to S.sub.OE, illustrated by a dotted
arrow in FIG. 1, which is a spin-forbidden process. The energy gap
between T.sub.1E and T.sub.1A is preferably greater than kT in
order to avoid back-transfer of excitons from T.sub.1A to T.sub.1E.
Likewise, the energy gap between S.sub.1A and S.sub.1E is
preferably greater than kT in order to avoid back-transfer of
excitons from S.sub.1E to S.sub.1A.
[0058] A pathway for decay of the triplet exciton on T.sub.1A in
competition with triplet-triplet annihilation is the non-radiative
(quenching) pathway to S.sub.0A described above with reference to
FIG. 1. A number of measures may be taken to maximise the
probability of TTA rather than decay to S.sub.0A, in
particular:
[0059] i) The triplet-absorbing material may be selected such that
triplet excitons on T.sub.1A have a relatively long lifetime
.tau..sub.TA. A relatively long lifetime not only means that the
rate of decay to S.sub.OA is relatively slow but also that the
likelihood of TTA is relatively high.
[0060] ii) The concentration of triplet-absorbing material in the
light-emitting layer may be relatively high, for example greater
than 1 mol %, for example in the range of 1-10 mol %.
[0061] iii) Two or more triplet-accepting materials may be provided
in close proximity, for example as described below with reference
to units of formula (II).
[0062] Each of these measures may be used alone or in
combination.
[0063] FIG. 3 illustrates a third energy transfer mechanism for an
exemplary OLED.
[0064] In this case, triplet-triplet annihilation occurs between
the triplet exciton of energy T.sub.1A located on the triplet
accepting triplet-accepting unit and the triplet exciton of energy
T.sub.1E located on the light-emitting material. It will be
appreciated that this results in a triplet-triplet annihilated
singlet exciton (TTAS) having an energy of up to T.sub.1E T.sub.1A.
This singlet exciton's energy level of S.sub.nA is higher in than
that of S.sub.1E and so it may transfer its energy to S.sub.1A and
from there to S.sub.1E from which light h.nu. may be emitted as
delayed fluorescence. Without wishing to be bound by any theory, it
is believed that avoiding formation of super-excited states on the
light-emitting polymer formed during OLED driving may improve
device lifetime. Moreover, by utilising TTA to produce stable
delayed fluorescence it is possible to improve efficiency as
compared to a device in which all triplet excitons are quenched (as
illustrated in FIG. 1) or as compared to a device in which there is
no triplet accepting unit wherein intensity of delayed fluorescence
may drop sharply following initial OLED driving.
[0065] It will be appreciated that it is possible for two or all
three of the triplet-quenching mechanism and the two TTA mechanisms
described above to occur within the same device, and that the
amount of delayed fluorescence from each of the TTA two mechanisms
will depend on factors such as the concentration of light emitting
material, the concentration of triplet accepting units, the rate
constants of competing processes and the excited state lifetime of
triplet excitons on the light emitting unit and the triplet
accepting unit. Measures described above with reference to FIG. 2
may be employed to increase the probability of TTA.
[0066] The rate constant for transfer of triplet excitons from the
light-emitting material to the triplet-accepting material may be
selected so as to be greater than the rate constant for quenching
of triplet excitons.
Triplet-Accepting Material
[0067] The triplet-accepting anthanthrene may be blended with the
light-emitting material, and the light-emitting composition may
comprise one or more further components, for example one or more
charge transporting materials, in particular one or more hole
transporting or electron transporting materials. Alternatively, the
triplet-accepting anthanthrene may be bound to the light-emitting
material, or to any of the aforementioned other components, where
present.
[0068] In the case where the anthanthrene is blended with the
light-emitting material, the anthanthrene is optionally substituted
with solubilising groups. Anthanthrene compounds may have formula
(Ia):
##STR00007##
wherein each R.sup.3 is independently selected from H or a
substituent, and at least one R.sup.3 is not H. Preferred
substituents R.sup.3 are alkyl and optionally substituted aryl or
heteroaryl, in particular optionally substituted phenyl.
[0069] In the case where R.sup.3 is alkyl, one or more H atoms may
be replaced by F and one or more non-adjacent C atoms may be
replaced with O, S, substituted N, C.dbd.O and --COO--.
Particularly preferred alkyl groups are n-butyl, t-butyl, n-hexyl
and n-octyl.
[0070] Substitution of R.sup.3 in the 2- and/or 8- positions of the
anthanthrene ring is particularly preferred.
[0071] In the case where R.sup.3 is aryl or heteroaryl, preferred
substituents include alkyl and alkoxy groups. Exemplary compounds
of formula (Ia) include the following:
##STR00008##
wherein Ak is alkyl, in particular branched or straight chain
C.sub.1-10 alkyl.
[0072] In the case where the light-emitting material is a polymer,
the anthanthrene may be provided in the form of repeat units in the
main chain of the polymer. These repeat units may correspond to
compounds of formula (Ia) wherein two R.sup.3 groups are replaced
with a bond to an adjacent repeat unit. Linking through the 2-
and/or 8- positions allows for linkage to adjacent aromatic repeat
units without significant change in the HOMO-LUMO gap of the
material due to the relatively low electron density at these
positions as compared to the 4- and/or 10- positions, resulting in
little or no conjugation to adjacent aromatic repeat units.
[0073] Exemplary repeat units include the following:
##STR00009##
wherein * denotes the linking points for linking the repeat unit
into the polymer chain, and Ak is as described above.
[0074] The anthanthrene group may be bound into the main chain of a
light-emitting polymer by polymerising a monomer comprising the
repeat unit illustrated above substituted with a leaving group
capable of participating in a metal-catalysed cross-coupling
reaction. Exemplary leaving groups include halogen and boronic acid
or ester groups for use in Suzuki or Yamamoto polymerisation
reactions. These reactions are described in more detail below.
[0075] Alternatively, or additionally, in the case where the
light-emitting material is a polymer the anthanthrene may be
provided in the form of polymer end groups or side-groups pendant
from the polymer main chain. In this case, exemplary side-groups or
end-groups include the following optionally substituted repeat
units:
##STR00010##
[0076] As described above, linkage through the 2- or 8- positions
results in little or no conjugation to adjacent aromatic
groups.
[0077] The anthanthrene side-group or end-group may be formed by
reacting an anthanthrene compound substituted with a suitable
leaving group capable of participating in a metal-catalysed
cross-coupling reaction, such as a halogen or boronic acid or
ester, with a leaving group on the polymer.
[0078] Alternatively, an anthanthrene side-group may be
incorporated into a light-emitting polymer by providing it as a
substituent of a monomer as illustrated below:
##STR00011##
wherein PG represents a polymerisable group such as a leaving group
as described above, or a polymerisable double bond.
[0079] In order to increase the probability of TTA and delayed
fluorescence as described above, a plurality of triplet-accepting
anthanthrene units may be provided in close proximity. For example,
two anthanthrene groups may be provided in an optionally
substituted compound having the formula (II):
Anthanthrene-Spacer-Anthanthrene (II)
wherein "Anthanthrene" represents a compound of formula (I) and
"Spacer" is a conjugated or non-conjugated spacer group. The spacer
group separates the two triplet-accepting anthanthrene groups, and
preferably separates their electronic characteristics (for example
the HOMO and LUMO). Depending on the precise nature of the
conjugation and orbital overlap, Sp could optionally comprise one
or more arylene or heteroarylene groups such as substituted phenyl,
biphenyl or fluorene. Alternatively, Sp could optionally comprise a
non-conjugated linking group such as alkyl, or another molecular
link that does not provide a conjugation path between the
anthanthrenes.
[0080] The unit of formula (II) may be a separate compound
physically mixed with the light-emitting material or it may be
bound to the light-emitting material. In the case where the
light-emitting material is a polymer, the unit of formula (II) may
be bound as a main-chain repeat unit, a side group or an end-group
as described above.
[0081] Alternatively or additionally, the triplet-accepting unit
may be an oligomer or polymer, or a component of an oligomer or
polymer, comprising a repeat structure of formula (IIb):
(Anthanthrene-Spacer).sub.m (IIb)
wherein m is at least 2. This oligomer or polymer may be mixed with
the light-emitting material or may be provided within the polymer
backbone.
[0082] Although binding of the triplet-accepting unit to the
light-emitting polymer is described above, it will be appreciated
that the triplet-accepting unit may be bound to any other component
of the composition, where present, in the same way.
[0083] The concentration of the triplet-accepting unit of formula
(I) is optionally at least 0.1 mol % or at least 1 mol %, for
example in the range of 1-10 mol %. A higher concentration of the
triplet-accepting material increases the probability of TTA.
[0084] In order to increase the probability of TTA, the lifetime of
excited state triplets residing on the triplet accepting material
is optionally at least 1 microsecond. The lifetime of a triplet
exciton is its half-life, which may be measured by flash photolysis
to measure monomolecular triplet lifetime as described in Handbook
of Photochemistry, 2.sup.nd Edition, Steven L Murov, Ian Carmichael
and Gordon L Hug and references therein, the contents of which are
incorporated herein by reference.
[0085] Anthanthrene has a T.sub.1 level of 850 nm and a S.sub.1
level of 430 nm. By comparison, anthracene has a T.sub.1 level of
670 nm and a S.sub.1 level of 375 nm. The lower energy T.sub.1
level of anthanthrene means that it can be used as a triplet
absorber with a wider variety of light-emitting materials than
anthracene.
[0086] It will be appreciated that, unlike phosphorescent dopants,
the triplet-accepting material does not provide an energetically
favourable pathway for absorbed triplets to undergo radiative
decay, and as a result substantially none of the energy of the
triplet exciton absorbed by the triplet-accepting material is lost
from the triplet-accepting material in the form of light emission
from the triplet-accepting material.
[0087] The dynamics of singlet and triplet excitons may be studied
using time resolved electroluminescence as well as quasi-continuous
wave (quasi-cw) and time resolved excited state absorption. The
density of triplet excitons on a light-emitting material, for
example on the polymer backbone of a conjugated light-emitting
polymer, may be measured using quasi-cw excited state
absorption.
[0088] The excited state absorption techniques have been described
elsewhere (King, S., Rothe, C. & Monkman, A. Triplet build in
and decay of isolated polyspirobifluorene chains in dilute
solution. J. Chem. Phys. 121, 10803-10808 (2004), and Dhoot, A. S.,
Ginger, D. S., Beljonne, D., Shuai, Z. & Greenham, N. C.
Triplet formation and decay in conjugated polymer devices. Chemical
Physics Letters 360, 195-201 (2002)).
[0089] For example, the triplet state of polyfluorenes has been
well characterised with these techniques with a strong excited
state absorption feature peaking at 780 nm attributed to the
triplet state (King, S., Rothe, C. & Monkman, A. Triplet build
in and decay of isolated polyspirobifluorene chains in dilute
solution. J. Chem. Phys. 121, 10803-10808 (2004) and Rothe, C.,
King, S. M., Dias, F. & Monkman, A. P. Triplet exciton state
and related phenomena in the beta-phase of
poly(9,9-dioctyl)fluorene. Physical Review B 70, (2004)).
[0090] Accordingly, probes of triplet population of a polyfluorene
may be performed at 780 nm, and the skilled person will understand
how to modify this probe for other light-emitting materials based
on the excited state absorption features of those materials.
[0091] FIG. 4 illustrates the structure of an OLED according to an
embodiment of the invention. The OLED comprises a transparent glass
or plastic substrate 1, an anode 2, a cathode 4 and a
light-emitting layer 3 provided between anode 2 and the cathode 4.
Further layers may be located between anode 2 and the cathode, such
as charge transporting, charge injecting or charge blocking
layers.
Light Emitting Material
[0092] Suitable light-emitting materials for use in layer 3 include
small molecule, polymeric and dendrimeric materials, and
compositions thereof. Suitable light-emitting polymers for use in
layer 3 include poly(arylene vinylenes) and polyarylenes such as:
polyfluorenes, particularly 2,7-linked 9,9 dialkyl polyfluorenes or
2,7-linked 9,9 diaryl polyfluorenes; polyspirofluorenes,
particularly 2,7-linked poly-9,9-spirofluorene;
polyindenofluorenes, particularly 2,7-linked polyindenofluorenes;
polyphenylenes, particularly alkyl or alkoxy substituted
poly-1,4-phenylene. Such polymers as disclosed in, for example,
Adv. Mater. 2000 12(23) 1737-1750 and references therein.
[0093] A suitable light-emitting polymer may be a light-emitting
homopolymer comprising light-emitting repeat units, or it may be a
copolymer comprising light-emitting repeat units and further repeat
units such as hole transporting and/or electron transporting repeat
units as disclosed in, for example, WO 00/55927. Each repeat unit
may be provided in a main chain or side chain of the polymer.
[0094] Polymers for use as light-emitting materials in devices
according to the present invention preferably comprise a repeat
unit selected from arylene repeat units as disclosed in, for
example, Adv. Mater. 2000 12(23) 1737-1750 and references therein.
Exemplary first repeat units include: 1,4-phenylene repeat units as
disclosed in J. Appl. Phys. 1996, 79, 934; fluorene repeat units as
disclosed in EP 0842208; indenofluorene repeat units as disclosed
in, for example, Macromolecules 2000, 33(6), 2016-2020; and
spirofluorene repeat units as disclosed in, for example EP 0707020.
Each of these repeat units is optionally substituted. Examples of
substituents include solubilising groups such as C.sub.1-20 alkyl
or alkoxy; electron withdrawing groups such as fluorine, nitro or
cyano; and substituents for increasing glass transition temperature
(Tg) of the polymer.
[0095] Particularly preferred polymers comprise optionally
substituted, 2,7-linked fluorenes, most preferably repeat units of
formula IV:
##STR00012##
wherein R.sup.1 and R.sup.2 are independently H or a substituent
and wherein R.sup.1 and R.sup.2 may be linked to form a ring.
R.sup.1 and R.sup.2 are preferably selected from the group
consisting of hydrogen; optionally substituted alkyl wherein one or
more non-adjacent C atoms may be replaced with O, S, substituted N,
C.dbd.O and --COO--; optionally substituted aryl or heteroaryl; and
optionally substituted arylalkyl or heteroarylalkyl. More
preferably, at least one of R.sup.1 and R.sup.2 comprises an
optionally substituted C.sub.4-C.sub.20 alkyl or aryl group.
[0096] In the case where R.sup.1 or R.sup.2 is aryl or heteroaryl,
preferred optional substituents include alkyl groups wherein one or
more non-adjacent C atoms may be replaced with O, S, substituted N,
C.dbd.O and --COO--.
[0097] Optional substituents for the fluorene unit, other than
substituents R.sup.1 and R.sup.2, are preferably selected from the
group consisting of alkyl wherein one or more non-adjacent C atoms
may be replaced with O, S, substituted N, C.dbd.O and --COO--,
optionally substituted aryl, optionally substituted heteroaryl,
alkoxy, alkylthio, fluorine, cyano and arylalkyl.
[0098] Preferably, the polymer comprises an arylene repeat unit as
described above and an arylamine repeat unit, in particular a
repeat unit V:
##STR00013##
wherein Ar.sup.1 and Ar.sup.2 are optionally substituted aryl or
heteroaryl groups, n is greater than or equal to 1, preferably 1 or
2, and R is H or a substituent, preferably a substituent. R is
preferably alkyl or aryl or heteroaryl, most preferably aryl or
heteroaryl, in particular phenyl. Any of the aryl or heteroaryl
groups in the unit of formula 1, including the case where R is aryl
or heteroaryl, may be substituted. Preferred substituents are
selected from alkyl wherein one or more non-adjacent C atoms may be
replaced with O, S, substituted N, C.dbd.O and --COO--, optionally
substituted aryl, optionally substituted heteroaryl, alkoxy,
alkylthio, fluorine, cyano and arylalkyl. Preferred substituents
include alkyl and alkoxy groups. Any of the aryl or heteroaryl
groups in the repeat unit of Formula 1 may be linked by a direct
bond or a divalent linking atom or group. Preferred divalent
linking atoms and groups include O, S; substituted N; and
substituted C.
[0099] Particularly preferred units satisfying Formula 1 include
units of Formulae 1-3:
##STR00014##
wherein Ar.sup.1 and Ar.sup.2 are as defined above; and Ar.sup.3 is
optionally substituted aryl or heteroaryl. Where present, preferred
substituents for Ar.sup.3 include alkyl and alkoxy groups.
[0100] The arylamine repeat units are preferably present in an
amount up to 30 mol %, preferably up to 20 mol %. These percentages
apply to the total number of arylamine units present in the polymer
in the case where more than one type of repeat unit of formula (V)
is used.
[0101] The polymer may comprise heteroarylene repeat units for
charge transport or emission.
[0102] Binding the anthanthrene to the light-emitting material may
result in more efficient triplet acceptance as compared to mixing
of the anthanthrene with the light-emitting material because this
binding may provide intramolecular triplet acceptance pathways
unavailable to a corresponding mixed system. In the case where a
light-emitting polymer is used, the anthanthrene may be bound to a
light-emitting repeat unit of the polymer and/or to any other
repeat unit of the polymer that may be present, for example an
electron transporting repeat unit and/or a hole transporting repeat
unit.
[0103] Moreover, binding may be beneficial for processing reasons.
For example, if the anthanthrene has low solubility then binding it
to a soluble light-emitting material, in particular a
light-emitting polymer, allows the anthanthrene unit to be carried
in solution by the light-emitting material, enabling device
fabrication using solution processing techniques. Finally, binding
the anthanthrene unit to the light-emitting material may prevent
phase separation effects in solution-processed devices that may be
detrimental to device performance.
[0104] Where the light-emitting material is a conjugated polymer
comprising light-emitting repeat units and further repeat units,
for example light-emitting amine repeat units of formula (V) and
fluorene repeat units of formula (IV), conjugation of the
anthanthrene unit into the polymer main chain (for example by
conjugation with fluorene repeat units) may reduce the T.sub.1
energy level of the anthanthrene unit, thus increasing the
energetic favourability of triplet exciton transfer from the
emitter unit to the anthanthrene unit. This reduction in T.sub.1
energy level of the anthanthrene unit may also enable use of the
anthanthrene unit with light-emitting materials with T.sub.1 levels
that are too low for use with an anthanthrene unit that is not
conjugated in this way.
[0105] Preferred methods for preparation of conjugated
light-emitting polymers comprise a "metal insertion" wherein the
metal atom of a metal complex catalyst is inserted between an aryl
or heteroaryl group and a leaving group of a monomer. Exemplary
metal insertion methods are Suzuki polymerisation as described in,
for example, WO 00/53656 and Yamamoto polymerisation as described
in, for example, T. Yamamoto, "Electrically Conducting And
Thermally Stable .pi.-Conjugated Poly(arylene)s Prepared by
Organometallic Processes", Progress in Polymer Science 1993, 17,
1153-1205. In the case of Yamamoto polymerisation, a nickel complex
catalyst is used; in the case of Suzuki polymerisation, a palladium
complex catalyst is used.
[0106] For example, in the synthesis of a linear polymer by
Yamamoto polymerisation, a monomer having two reactive halogen
groups is used. Similarly, according to the method of Suzuki
polymerisation, at least one reactive group is a boron derivative
group such as a boronic acid or boronic ester and the other
reactive group is a halogen. Preferred halogens are chlorine,
bromine and iodine, most preferably bromine.
[0107] It will therefore be appreciated that repeat units
illustrated throughout this application may be derived from a
monomer carrying suitable leaving groups. Likewise, an end group or
side group may be bound to the polymer by reaction of a suitable
leaving group.
[0108] Suzuki polymerisation may be used to prepare regioregular,
block and random copolymers. In particular, homopolymers or random
copolymers may be prepared when one reactive group is a halogen and
the other reactive group is a boron derivative group.
Alternatively, block or regioregular, in particular AB, copolymers
may be prepared when both reactive groups of a first monomer are
boron and both reactive groups of a second monomer are halogen.
[0109] As alternatives to halides, other leaving groups capable of
participating in metal insertion include groups include tosylate,
mesylate and triflate.
[0110] Light-emitting layer 3 may consist of the light-emitting
polymer and the triplet accepting unit alone, alone or may comprise
these materials in combination with one or more further materials.
In particular, the light-emitting polymer may be blended with hole
and/or electron transporting materials or alternatively may be
covalently bound to hole and/or electron transporting materials as
disclosed in for example, WO 99/48160.
[0111] Light-emitting copolymers may comprise a light-emitting
region and at least one of a hole transporting region and an
electron transporting region as disclosed in, for example, WO
00/55927 and U.S. Pat. No. 6,353,083. If only one of a hole
transporting region and electron transporting region is provided
then the electroluminescent region may also provide the other of
hole transport and electron transport functionality--for example,
an amine unit as described above may provide both hole transport
and light-emission functionality. A light-emitting copolymer
comprising light-emitting repeat units and one or both of a hole
transporting repeat units and electron transporting repeat units
may provide said units in a polymer main-chain, as per U.S. Pat.
No. 6,353,083, or in polymer side-groups pendant from the polymer
backbone.
[0112] The light-emitting material may emit light of any colour
provided that its S.sub.1 and T.sub.1 energy levels relative to the
anthanthrene unit are as described above, however the
light-emitting material is preferably a blue light-emitting
material, in particular a material having photoluminescent light
emission with a peak wavelength in the range of from 400 to 500 nm,
preferably 430 to 500 nm.
[0113] Light-emitting layer layer 3 may be patterned or
unpatterned. A device comprising an unpatterned layer may be used
an illumination source, for example. A white light emitting device
is particularly suitable for this purpose. A device comprising a
patterned layer may be, for example, an active matrix display or a
passive matrix display. In the case of an active matrix display, a
patterned electroluminescent layer is typically used in combination
with a patterned anode layer and an unpatterned cathode. In the
case of a passive matrix display, the anode layer is formed of
parallel stripes of anode material, and parallel stripes of
electroluminescent material and cathode material arranged
perpendicular to the anode material wherein the stripes of
electroluminescent material and cathode material are typically
separated by stripes of insulating material ("cathode separators")
formed by photolithography.
B. Anthanthrene as a Light-Emitting Material
[0114] In the case where an anthanthrene compound is used as a
light-emitting material, the S.sub.1 level of the anthanthrene
compound is lower than that of the host material from which it
receives singlet excitons.
[0115] Preferably, the unit of formula (I) emits blue light.
[0116] The unit of formula (I) may be a compound that is physically
mixed with, and not chemically bound to, its polymer host material.
Alternatively, the unit of formula (I) may be chemically bound to
its host material, in which case the unit of formula (I) may be
provided as a repeat unit in the polymer main chain or bound to the
polymer as a side group or end group. Suitable light-emitting
compounds, repeat units, side groups and end groups of formula (I)
are as described above with respect to triplet-accepting materials.
Surprisingly, the present inventors have found that incorporation
of an anthanthrene unit into a conjugated polymer, in particular by
incorporation of the unit into a conjugated polymer main chain,
does not result in significant red shifting of the colour of
emission of the resulting polymer, in particular when the unit is
linked through its 2- and 8- positions. In particular, this
red-shift is typically no more than 10 nm or even 5 nm.
[0117] Colour shifting of an anthanthrene following substitution
with solubilising substituents may be avoided by selection of the
position of solubilising substituents.
[0118] A suitable host material in this case includes fluorene
homopolymer or a copolymer comprising fluorene units and one or
more co-repeat units having an S.sub.1 level higher than that of
the anthanthrene emitter.
[0119] Exemplary materials, processes and device architectures of
the OLED are described in more detail below. It will be appreciated
that these materials, processes and device architectures are
applicable to any OLED comprising a unit of formula (I), regardless
of whether that unit is functioning as an emitter unit or a
substantially non-emissive triplet-accepting unit.
Hole Injection Layers
[0120] A conductive hole injection layer, which may be formed from
a conductive organic or inorganic material, may be provided between
the anode 2 and the electroluminescent layer 3 to assist hole
injection from the anode into the layer or layers of semiconducting
polymer. Examples of doped organic hole injection materials include
optionally substituted, doped poly(ethylene dioxythiophene) (PEDT),
in particular PEDT doped with a charge-balancing polyacid such as
polystyrene sulfonate (PSS) as disclosed in EP 0901176 and EP
0947123, polyacrylic acid or a fluorinated sulfonic acid, for
example Nafion.RTM.; polyaniline as disclosed in U.S. Pat. No.
5,723,873 and U.S. Pat. No. 5,798,170; and optionally substituted
polythiophene or poly(thienothiophene). Examples of conductive
inorganic materials include transition metal oxides such as VOx
MoOx and RuOx as disclosed in Journal of Physics D: Applied Physics
(1996), 29(11), 2750-2753.
Charge Transporting Layers
[0121] A hole transporting layer may be provided between the anode
and the electroluminescent layer. Likewise, an electron
transporting layer may be provided between the cathode and the
electroluminescent layer.
[0122] Similarly, an electron blocking layer may be provided
between the anode and the electroluminescent layer and a hole
blocking layer may be provided between the cathode and the
electroluminescent layer. Transporting and blocking layers may be
used in combination. Depending on its HOMO and LUMO levels, a
single layer may both transport one of holes and electrons and
block the other of holes and electrons.
[0123] If present, a hole transporting layer located between anode
2 and electroluminescent layer 3 preferably has a HOMO level of
less than or equal to 5.5 eV, more preferably around 4.8-5.5 eV.
HOMO levels may be measured by cyclic voltammetry, for example.
[0124] If present, an electron transporting layer located between
electroluminescent layer 3 and cathode 4 preferably has a LUMO
level of around 3-3.5 eV. For example, a layer of a silicon
monoxide or silicon dioxide or other thin dielectric layer having
thickness in the range of 0.2-2 nm is provided between
electroluminescent layer 3 and layer 4.
[0125] Polymers for use as charge transporting materials may
comprise arylene units, such as fluorene units of formula (IV) and
other units described above.
[0126] A hole-transporting polymer may comprise arylamine repeat
units, in particular repeat units of formula (V), such as repeat
units of formulae 1-3, described above. This polymer may be a
homopolymer or it may be a copolymer comprising arylene repeat
units in an amount up to 95 mol %, preferably up to 70 mol %. These
percentages apply to the total number of arylamine units present in
the polymer in the case where more than one type of repeat unit of
formula (V) is used.
[0127] Charge transporting units may be provided in a polymer
main-chain or polymer side-chain.
Cathode
[0128] Cathode 4 is selected from materials that have a
workfunction allowing injection of electrons into the
electroluminescent layer. Other factors influence the selection of
the cathode such as the possibility of adverse interactions between
the cathode and the electroluminescent material. The cathode may
consist of a single material such as a layer of aluminium.
Alternatively, it may comprise a plurality of metals, for example a
bilayer of a low workfunction material and a high workfunction
material such as calcium and aluminium as disclosed in WO 98/10621;
elemental barium as disclosed in WO 98/57381, Appl. Phys. Lett.
2002, 81(4), 634 and WO 02/84759; or a thin layer of metal
compound, in particular an oxide or fluoride of an alkali or alkali
earth metal, to assist electron injection, for example lithium
fluoride as disclosed in WO 00/48258; barium fluoride as disclosed
in Appl. Phys. Lett. 2001, 79(5), 2001; and barium oxide. In order
to provide efficient injection of electrons into the device, the
cathode preferably has a workfunction of less than 3.5 eV, more
preferably less than 3.2 eV, most preferably less than 3 eV. Work
functions of metals can be found in, for example, Michaelson, J.
Appl. Phys. 48(11), 4729, 1977.
[0129] The cathode may be opaque or transparent. Transparent
cathodes are particularly advantageous for active matrix devices
because emission through a transparent anode in such devices is at
least partially blocked by drive circuitry located underneath the
emissive pixels. A transparent cathode will comprises a layer of an
electron injecting material that is sufficiently thin to be
transparent. Typically, the lateral conductivity of this layer will
be low as a result of its thinness. In this case, the layer of
electron injecting material is used in combination with a thicker
layer of transparent conducting material such as indium tin
oxide.
[0130] It will be appreciated that a transparent cathode device
need not have a transparent anode (unless, of course, a fully
transparent device is desired), and so the transparent anode used
for bottom-emitting devices may be replaced or supplemented with a
layer of reflective material such as a layer of aluminium. Examples
of transparent cathode devices are disclosed in, for example, GB
2348316.
Encapsulation
[0131] Optical devices tend to be sensitive to moisture and oxygen.
Accordingly, the substrate preferably has good barrier properties
for prevention of ingress of moisture and oxygen into the device.
The substrate is commonly glass, however alternative substrates may
be used, in particular where flexibility of the device is
desirable. For example, the substrate may comprise a plastic as in
U.S. Pat. No. 6,268,695 which discloses a substrate of alternating
plastic and barrier layers or a laminate of thin glass and plastic
as disclosed in EP 0949850.
[0132] The device is preferably encapsulated with an encapsulant
(not shown) to preventingress of moisture and oxygen. Suitable
encapsulants include a sheet of glass, films having suitable
barrier properties such as silicon dioxide, silicon monoxide,
silicon nitride or alternating stacks of polymer and dielectric as
disclosed in, for example, WO 01/81649 or an airtight container as
disclosed in, for example, WO 01/19142. In the case of a
transparent cathode device, a transparent encapsulating layer such
as silicon monoxide or silicon dioxide may be deposited to micron
levels of thickness, although in one preferred embodiment the
thickness of such a layer is in the range of 20-300 nm. A getter
material for absorption of any atmospheric moisture and/or oxygen
that may permeate through the substrate or encapsulant may be
disposed between the substrate and the encapsulant.
Solution Processing
[0133] Light-emitting layer 3 may be deposited by any process,
including vacuum evaporation and deposition from a solution in a
solvent. In the case where the light emitting layer comprises a
polyarylene, such as a polyfluorene, suitable solvents for solution
deposition include mono- or poly-alkylbenzenes such as toluene and
xylene. Particularly preferred solution deposition techniques
including printing and coating techniques, preferably spin-coating
and inkjet printing.
[0134] Spin-coating is particularly suitable for devices wherein
patterning of the electroluminescent material is unnecessary--for
example for lighting applications or simple monochrome segmented
displays.
[0135] Inkjet printing is particularly suitable for high
information content displays, in particular full colour displays. A
device may be inkjet printed by providing a patterned layer over
the first electrode and defining wells for printing of one colour
(in the case of a monochrome device) or multiple colours (in the
case of a multicolour, in particular full colour device). The
patterned layer is typically a layer of photoresist that is
patterned to define wells as described in, for example, EP
0880303.
[0136] As an alternative to wells, the ink may be printed into
channels defined within a patterned layer. In particular, the
photoresist may be patterned to form channels which, unlike wells,
extend over a plurality of pixels and which may be closed or open
at the channel ends.
[0137] Other solution deposition techniques include dip-coating,
roll printing, nozzle printing, and screen printing.
[0138] If multiple layers of an OLED are formed by solution
processing then the skilled person will be aware of techniques to
prevent intermixing of adjacent layers, for example by crosslinking
of one layer before deposition of a subsequent layer or selection
of materials for adjacent layers such that the material from which
the first of these layers is formed is not soluble in the solvent
used to deposit the second layer.
Material Examples 1 and 2
[0139] Anthanthrene compounds 1 and 2 were prepared according to
the following synthetic method:
##STR00015##
[0140] This synthesis illustrates substitution at the 2 and 8
positions. Analogous substitution may be provided at the 6,12
and/or 4,10- positions as illustrated below.
##STR00016##
Material Example 3
[0141] Anthanthrene compound 3 was prepared according to the
following synthetic method, starting from commercially available
anthanthrene:
##STR00017##
Device Example
[0142] A device having the following structure was formed:
ITO/HIL/HTL/EL/MF/Al
[0143] wherein ITO represents an indium-tin oxide anode; HIL is a
hole-injection layer formed from Plextronics Inc; HTL is a hole
transport layer of a polymer comprising fluorene repeat units of
formula (IV) and amine repeat units of formula (V); EL is an
electroluminescent layer comprising fluorene repeat units of
formula (IV) and amine repeat units of formula (V) blended with 1
mol % of anthranthene compound 3; MF is a metal fluoride; and the
bilayer of MF/Al forms a cathode for the device.
[0144] As shown in FIG. 5 this device has an electroluminescent
spectrum that is virtually identical to that of a comparative
device wherein no anthanthrene compound is present, indicating that
the S.sub.1 energy level of the anthanthrene compound is higher
than that of the light-emitting unit of the polymer.
[0145] As shown in FIG. 6, the T.sub.50 lifetime (that is, the time
taken for the device brightness to fall to 50% of its original
brightness at constant current) is approximately the same as that
of the comparative device, however the decay curve is considerably
flatter. In particular, as shown in more detail in FIG. 7, the
T.sub.90 lifetime is much longer for the exemplary device as
compared to the comparative device.
[0146] FIGS. 8 and 9 illustrate that there is little change in
current density vs. voltage or external quantum efficiency vs.
voltage in the exemplary device as compared to the comparative
device.
[0147] Although the present invention has been described in terms
of specific exemplary embodiments, it will be appreciated that
various modifications, alterations and/or combinations of features
disclosed herein will be apparent to those skilled in the art
without departing from the scope of the invention as set forth in
the following claims.
* * * * *