U.S. patent application number 12/439163 was filed with the patent office on 2010-01-21 for compounds for use in opto-electrical devices.
This patent application is currently assigned to CDT OXFORD LIMITED. Invention is credited to Nigel Male, Jonathan Pillwo.
Application Number | 20100013377 12/439163 |
Document ID | / |
Family ID | 37137113 |
Filed Date | 2010-01-21 |
United States Patent
Application |
20100013377 |
Kind Code |
A1 |
Male; Nigel ; et
al. |
January 21, 2010 |
Compounds for use in Opto-Electrical Devices
Abstract
A composition for use in fabricating opto-electrical devices
comprising a solution processable triazine host material and a
phosphorescent moiety.
Inventors: |
Male; Nigel; (Salisbury,
GB) ; Pillwo; Jonathan; (Cambridge, GB) |
Correspondence
Address: |
MARSHALL, GERSTEIN & BORUN LLP
233 SOUTH WACKER DRIVE, 6300 SEARS TOWER
CHICAGO
IL
60606-6357
US
|
Assignee: |
CDT OXFORD LIMITED
Cambridgeshire
GB
SUMATION CO. LIMITED
Tokyo
JP
|
Family ID: |
37137113 |
Appl. No.: |
12/439163 |
Filed: |
August 30, 2007 |
PCT Filed: |
August 30, 2007 |
PCT NO: |
PCT/GB07/03300 |
371 Date: |
July 8, 2009 |
Current U.S.
Class: |
313/503 ;
252/182.12; 427/64; 528/423; 544/180; 544/215 |
Current CPC
Class: |
H01L 51/5004 20130101;
C07F 15/0033 20130101; H01L 51/5024 20130101; H01L 51/0085
20130101; H05B 33/14 20130101; H01L 51/0067 20130101; C09K
2211/1059 20130101; H01L 51/0003 20130101; H01L 51/5016 20130101;
C09K 11/025 20130101; H01L 51/0007 20130101; H01L 51/0035 20130101;
C09K 2211/185 20130101; C09K 11/06 20130101; H01L 51/0043
20130101 |
Class at
Publication: |
313/503 ;
544/180; 544/215; 528/423; 252/182.12; 427/64 |
International
Class: |
H01J 1/62 20060101
H01J001/62; C07D 251/24 20060101 C07D251/24; C08G 73/06 20060101
C08G073/06; C09K 11/00 20060101 C09K011/00; B05D 5/12 20060101
B05D005/12 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 31, 2006 |
GB |
0617167.2 |
Claims
1. A composition for use in fabricating opto-electrical devices
comprising a solution processable triazine host material and a
phosphorescent moiety.
2. A composition according to claim 1 wherein the triazine host
material is a small molecule triazine compound.
3. A composition according to claim 1 wherein the solution
processable triazine host material is a polymer comprising a
triazine repeat unit.
4. A composition according to claim 1, wherein the triazine host
material comprises at least one C.sub.4-C.sub.20 alkyl chain
solubilizing substituent.
5. A composition according to claim 1, wherein the triazine host
material comprises a central triazine ring substituted with at
least one aryl group.
6. A composition according to claim 5, wherein the aryl group is
directly bonded to the central triazine ring.
7. A composition according to claim 6, wherein the aryl group is
directly bonded to the central triazine ring via a carbon atom.
8. A composition according to claim 7, wherein the aryl group is a
phenyl ring.
9. A composition according to claim 4, wherein the triazine host
material is a tris-aryl triazine compound with at least one of the
aryl groups having the at least one solubilizing substituent bonded
thereto.
10. A composition according to claim 9, wherein two or three of the
aryl groups have solubilizing substituents bonded thereto.
11. A composition according to claim 1, wherein the triazine host
material has the formula: ##STR00024## where the R groups are
solubilizing substituents which may be the same or different.
12. A composition according to claim 11, wherein the triazine host
material has one of the following formulas: ##STR00025##
13. A composition according to claim 4, wherein the at least one
solubilizing substituent comprises an amine group which is not
directly bonded to the central triazine ring via a nitrogen
atom.
14. A composition according to claim 3, wherein the triazine repeat
unit is provided in a copolymer with a second repeat unit.
15. A composition according to claim 14, wherein the second repeat
unit is a twisted co-monomer.
16. A composition according to claim 1, wherein the phosphorescent
moiety is provided as a separate chemical entity to the solution
processable triazine host material and is blended with the solution
processable triazine host material.
17. A composition according to claim 1, wherein the phosphorescent
moiety is chemically bound to the solution processable triazine
host material.
18. A composition according to claim 17, wherein the phosphorescent
moiety is provided in the polymer backbone or in a pendent side
chain.
19. A composition according to claim 1, wherein the phosphorescent
moiety is a green emitter.
20. A method of fabricating an opto-electrical device, the method
comprising: depositing from solution, on a substrate comprising a
first electrode for injecting charge carriers of a first polarity,
a composition according to claim 1; and depositing thereover a
second electrode for injecting charge carriers of a second polarity
opposite to the first polarity.
21. An opto-electrical device comprising: a substrate; a first
electrode disposed over the substrate for injecting charge carriers
of a first polarity; a layer disposed over the first electrode
comprising a composition according to claim 1; and a second
electrode disposed thereover for injecting charge carriers of a
second polarity opposite to the first polarity.
Description
FIELD OF INVENTION
[0001] This invention relates to compositions comprising triazine
host compounds for use in fabricating phosphorescent
opto-electrical devices, and methods of fabricating opto-electrical
devices using these compositions.
BACKGROUND OF INVENTION
[0002] One class of opto-electrical devices is that using an
organic material for light emission or detection. The basic
structure of these devices is a light emissive organic layer, for
instance a film of a poly(p-phenylenevinylene) ("PPV") or
polyfluorene, sandwiched between a cathode for injecting negative
charge carriers (electrons) and an anode for injecting positive
charge carriers (holes) into the organic layer. The electrons and
holes combine in the organic layer generating photons. In WO
90/13148 the organic light-emissive material is a polymer. In U.S.
Pat. No. 4,539,507 the organic light-emissive material is of the
class known as small molecule materials, such as
(8-hydroxyquinoline)aluminium ("Alq3"). In a practical device one
of the electrodes is transparent, to allow the photons to escape
the device.
[0003] A typical organic light-emissive device ("OLED") is
fabricated on a glass or plastic substrate coated with a
transparent first electrode such as indium-tin-oxide ("ITO"). A
layer of a thin film of at least one electroluminescent organic
material covers the first electrode. Finally, a cathode covers the
layer of electroluminescent organic material. The cathode is
typically a metal or alloy and may comprise a single layer, such as
aluminium, or a plurality of layers such as calcium and aluminium.
Other layers can be added to the device, for example to improve
charge injection from the electrodes to the electroluminescent
material. For example, a hole injection layer such as poly(ethylene
dioxythiophene)/polystyrene sulfonate (PEDOT-PSS) or polyaniline
may be provided between the anode and the electroluminescent
material. When a voltage is applied between the electrodes from a
power supply one of the electrodes acts as a cathode and the other
as an anode.
[0004] 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 exciton which then undergoes
radiative decay to give light.
[0005] For organic semiconductors, important characteristics are
the binding energies, measured with respect to the vacuum level of
the electronic energy levels, particularly the "highest occupied
molecular orbital" (HOMO) and the "lowest unoccupied molecular
orbital" (LUMO) level. These can be estimated from measurements of
photoemission and particularly measurements of the electrochemical
potentials for oxidation and reduction. It is well understood in
this field that such energies are affected by a number of factors,
such as the local environment near an interface, and the point on
the curve (peak) from which the value is determined. Accordingly,
the use of such values is indicative rather than quantitative.
[0006] The optical and electronic properties of an organic
semiconductor are highly dependent on the energy of the
aforementioned HOMO and LUMO levels. Furthermore, these energy
levels are highly dependent on the chemical structure of the
organic semiconductor. By selecting suitable materials, or
combinations of materials, device performance can be improved.
[0007] For example, one way of improving efficiency of devices is
to provide hole and electron transporting materials. WO 99/48610
discloses blending of hole transporting polymers, electron
transporting polymers and electroluminescent polymers. A 1:1
copolymer of dioctylfluorene and triphenylamine is disclosed as a
hole transporting polymer in this document. The type of charge
transporting material which is most effective will be dependent on
the HOMO and LUMO of the other components in the device.
[0008] Although there has been much improvement in the efficiency
of devices using charge transporting materials, there is always a
desire to develop new charge transporting materials to further
improving efficiency when compared with existing devices.
[0009] WO 02/083760 discloses copolymers for use as charge
transporting materials and fluorescent emissive materials in
organic opto-electrical devices. The co-polymers comprise a first
repeat unit which may be a triazine unit as shown in formula
(a):
##STR00001##
[0010] wherein R'' is selected from hydrogen, branched or linear
C1-C20 alkyl or alkoxy.
[0011] The copolymers comprise a second repeat unit which may be
selected from the group consisting of optionally substituted
phenylenes, fluorenes, heteroaryls and triarylamines.
[0012] Triazines as components of blue electroluminescent devices
are disclosed in WO 2004/077885.
[0013] U.S. Pat. No. 6,821,643 discloses arylated triazines that
are deposited by evaporation to form a blue fluorescent
light-emitting layer or an electron transport layer of an OLED.
[0014] U.S. Pat. No. 6,352,791 discloses arylated triazines that
are deposited by evaporation to form an electron transport layer of
an OLED.
[0015] WO 2005/105950 discloses certain tri-substituted triazines
used as a blue fluorescent light-emitting layer of an OLED.
[0016] EP 1385221 discloses a light-emitting device comprising a
luminescent region comprising an anthracene derivative compound and
a triazine derivative compound.
[0017] Phosphorescent materials are also useful and in some
applications may be preferable to fluorescent materials. One type
of phosphorescent material comprises a host and a phosphorescent
emitter in the host. The emitter may be bonded to the host or
provided as a separate component in a blend.
[0018] Numerous hosts for phosphorescent emitters are described in
the prior art including "small molecule" hosts such as
4,4'-bis(carbazol-9-yl)biphenyl), known as CBP, and
(4,4',4''-tris(carbazol-9-yl)triphenylamine), known as TCTA,
disclosed in Ikai et al. (Appl. Phys. Lett., 79 no. 2, 2001, 156);
and triarylamines such as
tris-4-(N-3-methylphenyl-N-phenyl)phenylamine, known as MTDATA.
Homopolymers are also known as hosts, in particular poly(vinyl
carbazole) disclosed in, for example, Appl. Phys. Lett. 2000,
77(15), 2280; polyfluorenes in Synth. Met. 2001, 116, 379, Phys.
Rev. B 2001, 63, 235206 and Appl. Phys. Lett. 2003, 82(7), 1006;
poly[4-(N-4-vinyl benzyloxyethyl, N-methyla
mino)-N-(2,5-di-tert-butylphenylnapthalimide] in Adv. Mater. 1999,
11(4), 285; and poly(para-phenylenes) in J. Mater. Chem. 2003, 13,
50-55.
[0019] A problem with known host-phosphor systems is that the host
may quench emission from the phosphor. In general, the lower the
triplet energy level of the host (relative to the phosphor) then
the more likely quenching will occur. Polymerisation can exacerbate
this problem by reducing the triplet energy level to below that of
a monomer when forming a host polymer. Accordingly, there is a need
to produce materials with a high triplet energy level for use as
hosts in phosphorescent systems.
[0020] JP 2005071983 discloses tris-carbazolyl substituted
triazines as a host for Irppy, for example:
##STR00002##
[0021] In Chemistry of Materials (2004), 16(7), 1285-1291 a
tris-carbazolyl compound and other arylamino-substituted triazines
are disclosed as hosts for Irppy in OLEDs, for example:
##STR00003##
[0022] WO 2005029923 discloses tris-benzimidazolyl substituted
triazine as a host for the red phosphor, Ir(piq):
##STR00004##
[0023] US2005/0287393 discloses a light emissive composition
comprising a phosphorescent dopant and a host including a carbazole
compound and a small molecule triazine compound. Exemplary triazine
compounds are given as 2,4,6-tris(diarylamino)-1,3,5-triazine,
2,4,6-tris(diphenylamino)-1,3,5-triazine,
2,4,6-tricarbazolo-1,3,5-triazine,
2,4,6-tris(N-phenyl-2-naphthylamino)-1,3,5-triazine,
2,4,6-tris(N-pheyl-1 -naphthyl amino)-1,3,5-triazine, or
2,4,6-trisbiphenyl-1,3,5-triazine.
[0024] All of the above-identified small molecule triazines for use
as hosts in phosphorescent compositions are amino derivative except
for the last example, 2,4,6-trisbiphenyl-1,3,5-triazine, given in
the list of examples in US2005/0287393. The amino derivatives all
have a nitrogen atom directly bonding the aryl substituents to the
central triazine ring. However, the present applicant proposes that
this nitrogen linkage is not completely stable which can reduce the
lifetime of these compositions. Furthermore, none of the
above-identified small molecule compounds are soluble enough to be
readily solution processed and instead these compounds are more
suited to vacuum thermal deposition.
[0025] Host-emitter systems are not limited to phosphorescent
devices. A wide range of fluorescent low molecular weight metal
complexes are known and have been demonstrated in organic light
emitting devices [see, e. g., Macromol. Sym. 125 (1997) 1-48, U.S.
Pat. No. 5,150,006, U.S. Pat. No. 6,083,634 and U.S. Pat. No.
5,432,014].
[0026] As with phosphorescent systems, a problem with known
host-fluorescent emitter systems is that the host may quench
emission from the fluorescent emitter. It is advantageous to
provide a host having a higher LUMO than that of the emitter to
inject electrons into the emitter. It is advantageous to provide a
host having a lower HOMO than that of the emitter to inject holes
into the emitter. Accordingly, there is a need to produce materials
with a large band gap between the HOMO and LUMO for use as hosts in
fluorescent systems.
[0027] Another factor affecting the performance of opto-electonic
devices is morphology of the films which make up the device. For
semiconductive organic materials it is advantageous to have an
amorphous rather than a crystalline film. However, it is desirable
not to have too much disorder in the film in order to achieve a
device with better performance. Accordingly, there is a desire to
produce materials with better film forming characteristics.
[0028] It is an aim of the present invention to solve one or more
of the problems outlined above.
SUMMARY OF THE PRESENT INVENTION
[0029] According to a first aspect of the present invention there
is provided a composition for use in fabricating opto-electrical
devices comprising a solution processable triazine host material
and a phosphorescent moiety.
[0030] In one embodiment, the solution processable triazine host
material is a small molecule triazine compound.
[0031] In another embodiment, the solution processable triazine
host material is a polymer comprising a triazine repeat unit.
[0032] By "small molecule" we mean non-polymeric. The term "small
molecule" is often used in this art to have this meaning as there
are two main families of organic opto-electrical materials:
polymers and small molecules. Oligomers consist of 1 to 10 repeat
units, more preferably, 1 to 5 repeat units.
[0033] Preferably, the triazine host material comprises a
solubilising substituent.
[0034] By "solubilizing substituent" we mean a substituent which
renders the triazine sufficiently soluble in organic solvents that
it can be solution processed to fabricate an opto-electrical device
by, for example, ink-jet printing or spin-coating. Generally,
compounds must be soluble to a concentration above 8 mg ml.sup.-1,
preferably above 10 mg ml.sup.-1, in order for them to be solution
processed in a device manufacturing method. Organic solvents
suitable for opto-electrical device fabrication include xylene,
toluene, chlorobenzene, chloroform or tetrahydrofuran but
preferably toluene or xylene. The prior art compound
2,4,6-tris(biphenyl)-1,3,5-triazine has a solubility <5 mg
ml.sup.-1 in all of these solvents and thus is not a suitable
material for fabricating an OLED device by solution processing.
[0035] It has been found that by providing a solubilising
substituent on a small molecule triazine compound, the compound can
be readily solution processed. As such, the compositions comprising
small molecule triazine compounds of the present invention can be
deposited to form a film by, for example, ink-jet printing or
spin-coating rather than thermal vapour deposition.
[0036] Furthermore, it has been found that solubilising
substituents reduce the crystallinity of a film formed by
depositing triazine compounds when compared with prior art small
molecule triazine compounds resulting in a better film morphology.
In addition, the solubilizing substituent can be used to tune the
physical properties of a film.
[0037] Thus, the present invention provides triazine compounds
which can be solution processed and which produce films having a
good morphology and tunable physical properties for use in
opto-electrical devices. Furthermore, the triazine compounds of the
present invention have both a deep LUMO and a deep HOMO. As such,
the triazine compounds readily accept electrons but do not readily
accept holes in an opto-electrical device. Moreover, these
compounds serve as electron transporting host materials for
emissive materials without quenching emission from the emissive
materials. Preferably, the LUMO level is intermediate in energy
between the fermi level of the electron injecting material in the
cathode and the LUMO level (or triplet level in phosphorescent
emissive materials) of the emissive material or at least not
significantly higher than the fermi level of the electron injecting
material or significantly lower than the LUMO (or triplet level) of
the emissive material as to require a high drive voltage.
Preferably, the HOMO level of the triazine compounds is lower than
the HOMO level of the emissive material. Due to the poor hole
accepting/hole transporting properties of the triazines, it is
preferable to use them in conjunction with a hole transporting
material or with an emissive material which is a good hole
transporting material.
[0038] The LUMO levels of the triazine compounds are preferably
intermediate in energy between the injecting cathode and the
emitter. If a barrier to electron injection is present it is
preferably not more than 0.4 eV above the work function of the
cathode. The HOMO level of the triazine should be such as to
maintain an optical band-gap large enough to maintain a triplet
level energy gap that will not quench the phosphorescence from the
emitter species.
[0039] The electrical characteristics of the triazine compounds of
the present invention are such that in additional to acting as good
host materials, they can also be used as blue emissive materials,
electron transporting materials (either in a separate electron
transporting layer located between the cathode and the emissive
layer of a device or incorporated into the emissive layer of the
device as a blend with an emissive material or bonded to the
emissive material) and/or hole blocking materials (located in a
layer between the emissive layer and the cathode in a device).
Furthermore, the low LUMO of the triazine compounds also allows
cathode materials with higher work-functions to be used. Triazine
compounds with LUMO level of, for example, 2.8 eV will allow the
use of higher work function cathodes where the energy gap is not
greater than 0.4 eV. Thus cathode materials with work functions in
the range 2.6 to 3.2 eV are preferred.
[0040] Preferably, the at least one solubilizing substituent
comprises a C.sub.4-C.sub.20 alkyl chain. It has been found that
alkyl chains within this range render the triazine compound
sufficiently soluble to be readily solution processed and form
films having a good morphology. The length and composition of the
alkyl chain can be tailored to tune the physical properties of the
film according to its specific use.
[0041] Preferably, the central triazine ring is substituted with at
least one aryl group. Most preferably, the aryl group is directly
bonded to the central triazine ring. The aryl group may be a phenyl
ring. Most preferably, the triazine compound is a tris-aryl
triazine with at least one of the aryl groups having a solubilizing
substituent bonded thereto. More preferably, two or three of the
aryl groups have solubilizing substituents bonded thereto. The
solubilizing groups may be the same or different.
[0042] It has been found that tris-aryl triazine compounds having
solubilizing groups thereon form films with a good physical
morphology and good physical properties. In particular, the aryl
groups and the solubilising groups provide a film which is
amorphous but does not have too much disorder. These films have
better opto-electrical properties and produce a device with better
performance.
[0043] Preferably, the at least one solubilizing substituent is
directly bonded to the central triazine ring via a carbon atom. As
previously stated, the prior art triazine compounds which have
substituents bonded to the central triazine ring via a nitrogen
atom may be unstable in an opto-electrical device thus reducing the
lifetime of the device. The nitrogen linking atom may act as a
reactive site for substitution of the substituent during device
operation. In contrast, carbon bonded substituents according to
embodiments of the present invention are more stable in an
opto-electrical device.
[0044] According to embodiments of the present invention, the
triazine structures are simple triaryl substituted compounds
without heteroatom linkages but with alkyl substituents or
preferably aryl substituents having alkyl groups thereon, selected
to allow tuning of the physical properties of the films formed
using these structures, e.g. Tg>150.degree. C., good film
morphology, appropriate HOMO and LUMO levels etc. An example of
such a structure is shown in the formula below:
##STR00005##
[0045] where the R groups are solubilizing substituents. Asymmetric
structures where R on one ring can differ from R on the other two
rings are also possible. Further examples are given in the formulas
below:
##STR00006## ##STR00007## ##STR00008## ##STR00009## ##STR00010##
##STR00011##
[0046] The specific compounds shown above have been device tested
as host materials for green phosphorescent materials and exhibit
superior lifetime performance to known solution processable green
hosts. It is also envisaged that embodiments of the present
invention may be utilized as hosts for red phosphorescent
materials.
[0047] It is believed that the triphenyl triazine moiety that is
common to the above structures is more chemically and
electrochemically inert and is therefore more stable in the device
when compared with standard hosts, for example CBP, optionally
substituted with solubilising groups such as alkyl or alkoxy
groups. The triazines are also advantageous as they have good
electron transport properties, which make them much more suitable
for use in common device structures, as they can be used in
conjunction with an electron-blocking/hole transporting layer
disposed between the light-emissive layer and a hole injecting
layer. There is also no need for a hole-blocking/electron
transporting layer disposed between the light-emissive layer and
the cathode, which is commonly required in phosphorescent
devices.
[0048] One potential problem with the compounds shown above is that
their energy levels might be too deep for some applications.
However, with appropriate substitution (with e.g. amines) optimum
energy levels can be obtained for these applications. It should be
noted that amine substituents should not be directly bonded to the
central triazine ring via a nitrogen atom for the reasons discussed
previously. Alternatively any other functional groups with suitable
electron donating or electron withdrawing groups may be used as
required including but not limited to for example Fluoro-aryl,
perfluoro-aryl, alkyl-aryl or alkoxy-aryl groups.
[0049] It has been found that solution processable triazine
materials are excellent hosts for phosphorescent emissive materials
as their deep LUMO provide enhanced electron-injection/transport
and their wide triplet band-gap reduces quenching. The advantageous
features described in relation to the first aspect of the invention
are also applicable to the second aspect of the invention.
[0050] The triazine units may be provided in a copolymer with, for
example, hole transporting units such as a triaryl amine or a
twisted co-monomer which will maintain the triplet level of the
polymer so as not to quench the emission from the phosphorescent
emitter. For example,
##STR00012##
[0051] Alternatively, or additionally, the co-polymer may comprise
the phosphorescent moiety. The phosphorescent moiety may be
provided in the polymer backbone or in a pendent side chain.
[0052] Preferably the phosphorescent moiety is a green emitter.
Known hosts for green emitters such as solution processable
polyfluorene host materials generally have triplet levels that are
too low to be efficient hosts for green phosphorescent moieties. In
contrast, embodiments of the present invention are efficient host
materials for green phosphorescent moieties. However, it is
envisaged that the solution processable triazines may also be used
as hosts for red phosphorescent moieties.
[0053] An example of a triazine monomer suitable for fabricating
polymer hosts is shown below:
##STR00013##
[0054] As a host material for green phosphorescence, these monomers
can be used to prepare wide band-gap polymers with similarly good
electron injection/transport, but that avoid the need for
polyfluorenes that generally have triplet levels that are too low
to use as a green phosphorescent host. For example:
##STR00014##
[0055] This polymer has the processing properties of a conjugated
polymer with the triplet band-gap and electron injection properties
of the triazine. Other monomers could also be introduced to
fine-tune the charge injection/conduction properties, or to add
luminescent centres.
[0056] According to a third aspect of the present invention there
is provided a method of fabricating an opto-electrical device, the
method comprising the steps: depositing from solution, on a
substrate comprising a first electrode for injecting charge
carriers of a first polarity, a composition according to the first
aspect of the present invention; and depositing thereover a second
electrode for injecting charge carriers of a second polarity
opposite to the first polarity.
[0057] According to a fourth aspect of the present invention there
is provided an opto-electrical device comprising: a substrate; a
first electrode disposed over the substrate for injecting charge
carriers of a first polarity; a layer disposed over the first
electrode comprising a composition according to the first aspect of
the present invention; and a second electrode disposed thereover
for injecting charge carriers of a second polarity opposite to the
first polarity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0058] Embodiments of the present invention will now be described
by way of example only with reference to the accompanying drawing
in which:
[0059] FIG. 1 shows an organic light emitting device according to
an embodiment of the present invention.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0060] The device shown in FIG. 1 comprises a transparent glass or
plastic substrate 1, an anode 2 of indium tin oxide and a cathode
4. An electroluminescent layer 3 is provided between anode 2 and
cathode 4.
[0061] Further layers may be located between anode 2 and cathode 3,
such as charge transporting, charge injecting or charge blocking
layers.
[0062] In particular, it is desirable to provide a conductive hole
injection layer formed of a doped organic material located 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: 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 poly(thienothiophene). Conductive
inorganic materials may also be employed as hole injection layers.
Suitable 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.
[0063] 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.
[0064] 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.
[0065] Electroluminescent layer 3 may consist of the
electroluminescent material alone or may comprise the
electroluminescent material in combination with one or more further
materials. In particular, the electroluminescent material may be
blended with hole and/or electron transporting materials as
disclosed in, for example, WO 99/48160. Alternatively, the
electroluminescent material may be covalently bound to a charge
transporting material.
[0066] 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 disclosed in WO 98/57381, Appl. Phys. Left. 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
disclosed in WO 00/48258; barium fluoride, 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, yet more preferably less than 3 eV, most
preferably less than 2.8 eV.
[0067] 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.
[0068] The device is preferably encapsulated with an encapsulant
(not shown) to prevent ingress of moisture and oxygen. Suitable
encapsulants include a sheet of glass, films having suitable
barrier properties such as 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. 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.
[0069] In a practical device, at least one of the electrodes is
semi-transparent in order that light may be absorbed (in the case
of a photoresponsive device) or emitted (in the case of an OLED).
Where the anode is transparent, it typically comprises indium tin
oxide. Examples of transparent cathodes are disclosed in, for
example, GB 2348316.
[0070] The embodiment of FIG. 1 illustrates a device wherein the
device is formed by firstly forming an anode on a substrate
followed by deposition of an electroluminescent layer and a
cathode. However it will be appreciated that the device of the
invention could also be formed by firstly forming a cathode on a
substrate followed by deposition of an electroluminescent layer and
an anode.
[0071] Semiconductive polymers may be provided for the purpose of
charge transport, emission or as host polymers. A range of repeat
units for such polymers are described in more detail below. These
polymers may be used separately from the triazines of the present
invention (for example as a separate emissive centre in a
multicolour device).
[0072] Alternatively, these repeat units may be used in combination
with the triazines. For example, the solution processable triazine
host material according to the second aspect of the invention may
comprise a triazine repeat unit and one or more further repeat
units selected from those described below. In this case, it will be
appreciated that the polymer formed from such repeat units used in
combination with the triazine repeat units must have a triplet
energy higher than that of the phosphorescent dopant.
[0073] Polymers may comprise a first repeat unit selected from
arylene repeat units, in particular: 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.
[0074] Polymers may comprise optionally substituted, 2,7-linked
fluorenes, for example repeat units of formula:
##STR00015##
[0075] wherein R.sup.1 and R.sup.2 are independently selected from
hydrogen or optionally substituted alkyl, alkoxy, aryl, arylalkyl,
heteroaryl and 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 optionally substituted aryl group.
[0076] Polymers may provide one or more of the functions of hole
transport, electron transport and emission depending on which layer
of the device it is used in and the nature of co-repeat units.
[0077] In particular:
[0078] a homopolymer of fluorene repeat units, such as a
homopolymer of 9,9-dialkylfluoren-2,7-diyl, may be utilised to
provide electron transport.
[0079] a polymer, preferably a copolymer, comprising a triarylamine
repeat unit, in particular a repeat unit selected from formulae
1-6, may be utilised to provide hole transport and/or emission of a
different colour to that of the composition of the invention:
##STR00016##
[0080] wherein X, Y, A, B, C and D are independently selected from
H or a substituent group. More preferably, one or more of X, Y, A,
B, C and D is independently selected from the group consisting of
optionally substituted, branched or linear alkyl, aryl,
perfluoroalkyl, thioalkyl, cyano, alkoxy, heteroaryl, alkylaryl and
arylalkyl groups. Most preferably, X, Y, A and B are C.sub.1-10
alkyl. The aromatic rings in the backbone of the polymer may be
linked by a direct bond or a bridging group or bridging atom, in
particular a bridging heteroatom such as oxygen or sulfur.
[0081] Particularly preferred hole transporting polymers of this
type are copolymers of arylene repeat units, in particular fluorene
repeat units,and a triarylamine repeat unit.
[0082] A copolymer comprising a first repeat unit and heteroarylene
repeat unit may be utilised for charge transport or emission.
Heteroarylene repeat units may be selected from formulae 7-21:
##STR00017##
[0083] wherein R.sub.6 and R7 are the same or different and are
each independently hydrogen or a substituent group, preferably
alkyl, aryl, perfluoroalkyl, thioalkyl, cyano, alkoxy, heteroaryl,
alkylaryl or arylalkyl. For ease of manufacture, R.sub.6 and
R.sub.7 are preferably the same. More preferably, they are the same
and are each a phenyl group.
##STR00018## ##STR00019## ##STR00020##
[0084] Electroluminescent copolymers may comprise an
electroluminescent 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.
[0085] The different regions within such a polymer may be provided
along the polymer backbone, as per U.S. Pat. No. 6,353,083, or as
groups pendent from the polymer backbone as per WO 01/62869.
[0086] Preferred methods for preparation of these polymers 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. These polymerisation
techniques both operate via a "metal insertion" wherein the metal
atom of a metal complex catalyst is inserted between an aryl group
and a leaving group of a monomer. In the case of Yamamoto
polymerisation, a nickel complex catalyst is used; in the case of
Suzuki polymerisation, a palladium complex catalyst is used.
[0087] 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.
[0088] It will therefore be appreciated that repeat units and end
groups comprising aryl groups as illustrated throughout this
application may be derived from a monomer carrying a suitable
leaving group.
[0089] 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.
[0090] As alternatives to halides, other leaving groups capable of
participating in metal insertion include tosylate, mesylate, phenyl
sulfonate and triflate.
[0091] A single polymer or a plurality of polymers may be deposited
from solution to form layer 5. Suitable solvents for polyarylenes
include mono- or poly-alkylbenzenes such as toluene and xylene.
Particularly preferred solution deposition techniques are
spin-coating and inkjet printing.
[0092] 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.
[0093] Inkjet printing is particularly suitable for high
information content displays, in particular full colour displays.
Inkjet printing of OLEDs is described in, for example, EP
0880303.
[0094] If multiple layers of the device 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.
[0095] Preferred phosphorescent metal complexes comprise optionally
substituted complexes of formula (34):
ML.sup.1.sub.qL.sup.2.sub.rL.sup.3.sub.s (34)
[0096] wherein M is a metal; each of L.sup.1, L.sup.2 and L.sup.3
is a coordinating group; q is an integer; r and s are each
independently 0 or an integer; and the sum of (a. q)+(b. r)+(c.s)
is equal to the number of coordination sites available on M,
wherein a is the number of coordination sites on L.sup.1, b is the
number of coordination sites on L.sup.2 and c is the number of
coordination sites on L.sup.3.
[0097] Heavy elements M induce strong spin-orbit coupling to allow
rapid intersystem crossing and emission from triplet states
(phosphorescence). Suitable heavy metals M include:
[0098] lanthanide metals such as cerium, samarium, europium,
terbium, dysprosium, thulium, erbium and neodymium; and
[0099] d-block metals, in particular those in rows 2 and 3 i.e.
elements 39 to 48 and 72 to 80, in particular ruthenium, rhodium,
palladium, rhenium, osmium, iridium, platinum and gold.
[0100] Suitable coordinating groups for the f-block metals include
oxygen or nitrogen donor systems such as carboxylic acids,
1,3-diketonates, hydroxy carboxylic acids, Schiff bases including
acyl phenols and iminoacyl groups. As is known, luminescent
lanthanide metal complexes require sensitizing group(s) which have
the triplet excited energy level higher than the first excited
state of the metal ion. Emission is from an f-f transition of the
metal and so the emission colour is determined by the choice of the
metal. The sharp emission is generally narrow, resulting in a pure
colour emission useful for display applications.
[0101] The d-block metals form organometallic complexes with carbon
or nitrogen donors such as porphyrin or bidentate ligands of
formula (35):
##STR00021##
[0102] wherein Ar.sup.4 and Ar.sup.5 may be the same or different
and are independently selected from optionally substituted aryl or
heteroaryl; X.sup.1 and Y.sup.1 may be the same or different and
are independently selected from carbon or nitrogen; and Ar.sup.4
and Ar.sup.5 may be fused together. Ligands wherein X.sup.1 is
carbon and Y.sup.1 is nitrogen are particularly preferred.
[0103] Examples of bidentate ligands are illustrated below:
##STR00022##
[0104] Each of Ar.sup.4 and Ar.sup.5 may carry one or more
substituents. Particularly preferred substituents include fluorine
or trifluoromethyl which may be used to blue-shift the emission of
the complex as disclosed in WO 02/45466, WO 02/44189, US
2002-117662 and US 2002-182441; alkyl or alkoxy groups as disclosed
in JP 2002-324679; carbazole which may be used to assist hole
transport to the complex when used as an emissive material as
disclosed in WO 02/81448; bromine, chlorine or iodine which can
serve to functionalise the ligand for attachment of further groups
as disclosed in WO 02/68435 and EP 1245659; and dendrons which may
be used to obtain or enhance solution processability of the metal
complex as disclosed in WO 02/66552.
[0105] Other ligands suitable for use with d-block elements include
diketonates, in particular acetylacetonate (acac);
triarylphosphines and pyridine, each of which may be
substituted.
[0106] Main group metal complexes show ligand based, or charge
transfer emission. For these complexes, the emission colour is
determined by the choice of ligand as well as the metal.
[0107] The host material and metal complex may be combined in the
form of a physical blend. Alternatively, the metal complex may be
chemically bound to the host material. In the case of a polymeric
host, the metal complex may be chemically bound as a substituent
attached to the polymer backbone, incorporated as a repeat unit in
the polymer backbone or provided as an end-group of the polymer as
disclosed in, for example, EP 1245659, WO 02/31896, WO 03/18653 and
WO 03/22908.
[0108] Embodiments of the present invention can also be used as
hosts for fluorescent emitters. A wide range of fluorescent low
molecular weight metal complexes may be used with the present
invention. Suitable ligands for di or trivalent metals include:
oxinoids, e. g. with oxygen-nitrogen or oxygen-oxygen donating
atoms, generally a ring nitrogen atom with a substituent oxygen
atom, or a substituent nitrogen atom or oxygen atom with a
substituent oxygen atom such as 8-hydroxyquinolate and
hydroxyquinoxalinol-10-hydroxybenzo (h) quinolinato (II),
benzazoles (III), schiff bases, azoindoles, chromone derivatives,
3-hydroxyflavone, and carboxylic acids such as salicylato amino
carboxylates and ester carboxylates. Optional substituents include
halogen, alkyl, alkoxy, haloalkyl, cyano, amino, amido, sulfonyl,
carbonyl, aryl or heteroaryl on the (hetero) aromatic rings which
may modify the emission colour.
[0109] While this invention has been particularly shown and
described with reference to preferred embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
scope of the invention as defined by the appended claims.
EXAMPLES
2,4,6-tris(4'-bromophenyl)-1,3,5-triazine(1)
[0110] A 1 L three-necked round bottomed flask was fitted with a
magnetic stirrer, a reflux condenser with N.sub.2 inlet bubbler,
and a 500 ml pressure-equalising dropping funnel. The flask was
charged with trifluoromethane sulfonic acid (60 g, 35 ml) that was
stirred at room temperature. In a separate flask
4-bromobenzonitrile (36.4 g, 0.20 mol) was dissolved in anhydrous
CHCl.sub.3 (500 ml) and the solution transferred into the dropping
funnel under N.sub.2 via cannula. The benzonitrile solution was
added dropwise and then the reaction mixture was heated to reflux
at 90-95.degree. C. for 20-24 hours. The reaction mixture was
allowed to cool before cautiously adding to stirred dilute aqueous
ammonia solution (250 ml, 3%) cooled in an ice-bath. The product
precipitated as an off-white solid and was collected by filtration
and washed with H.sub.2O and Et.sub.2O. The product was
recrystallised from refluxing toluene to give pure product (100% by
HPLC). Yield 24.0 g, 66%.
2,4,6-tris(4''-tert-butylbiphenyl)-1,3,5-triazine(2)
[0111] (1) (24.0 g, 44 mmol), 4-.sup.tBu-phenyl(pinacol)boronate
(38.9 g, 150 mmol) and Pd(PPh.sub.3).sub.4 (4.56 g, 3.95 mmol) were
placed in 3 L multineck round bottomed flask fitted with a large
stirrer bar and reflux condenser with a N.sub.2 inlet bubbler. The
apparatus was purged with N.sub.2 and toluene (1.5 L) degassed by
sparging with N.sub.2 was transferred into the reaction vessel via
cannula. The mixture was stirred and Et.sub.4NOH (155 ml, 20 wt %
aq.) was degassed and added. The reaction was heated to reflux at
115.degree. C. for 44 hours. The reaction was monitored by TLC (20%
DCM/hexane) and was stopped when judged to be complete. After the
mixture had cooled to room temperature the organic and aqueous
layers were separated. The organic layer was washed with HCl (600
ml, 10%) and H.sub.2O (2.times.600 ml). The collected organic layer
was dried over MgSO.sub.4, filtered and reduced to dryness in vacuo
to give the crude product. The product was recrystallised twice
from refluxing dichloromethane/methanol to give good purity (99.84%
by HPLC). Yield 13.0 g, 42%
2,4,6-tris(4'-bromo-3'-methylphenyl)-1,3,5-triazine(3)
[0112] From 4-bromo-3-methylbenzonitrile in an analogous method to
(1). The compound was purified by recrystallisation from
dichloromethane/methanol.
2,4,6-tris(4''-tert-butyl-3'-methylbiphenyl)-1,3,5-triazine(4)
[0113] By reaction of (3) in an analogous method to (2). The
compound was purified by repeated recrystallisation from
toluene.
2,4-bis(4'-bromophenyl)-6-phenyl-1,3,5-triazine(5)
[0114] AlCl.sub.3 (8.68 g, 65.1 mmol) and NH.sub.4Cl (10.45 g, 195
mmol) were place in a 250 ml multineck round bottomed flask fitted
with a stirrer bar and reflux condenser with N2 inlet bubbler. The
apparatus was purged with N.sub.2. 4-Bromobenzonitrile (20 g, 110
mmol) and benzoyl chloride (7.94 g, 56 mmol) were added to the
reaction vessel and the flask was heated at 150.degree. C. allowing
the molten contents to stir. The stirred mixture evolved HCl gas
forming a slurry and then resolidified. Heating was continued for
20 hours. The product was obtained by extracting the mixture in
refluxing toluene and recrystallisation from refluxing toluene.
2,4-bis(4 ''-tert-butylbiphenyl)-6-phenyl-1,3,5-triazine(6)
[0115] By reaction of (5) in an analogous method to (2). The
compound was purified by column chromatography (10%
dichloromethane/hexane).
Device Example
[0116] Onto a glass substrate comprising an ITO electrode was
deposited, in sequence, a layer of PEDOT/PSS (available from H C
Starck of Leverkusen, Germany as Baytron P.RTM.); a hole transport
layer comprising a copolymer of fluorene and triarylamine repeat
units; an emissive layer comprising compound (6) as a host material
and the green-emitting iridium-cored dendrimer illustrated below,
and as disclosed in WO 02/066552; and a cathode comprising a layer
of barium oxide (5 nm) and thick capping layer of aluminium. All
layers were deposited by spin-coating from solution.
##STR00023##
Comparative Device Example
[0117] For the purpose of comparison, a device was prepared as per
the Device Example above except that a solution processable version
of the commonly used host material 4,4'-di(N-carbazole)biphenyl
(CBP) was used in place of the solution processable triazine host
material (6). CBP was rendered solution processable by substitution
with alkyl groups.
* * * * *