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FIELD OF THE INVENTION
This invention relates to organic electroluminescent devices. More
specifically, this invention relates to devices which emit light from a
current conducting organic layer.
BACKGROUND OF THE INVENTION
While organic electroluminescent devices have been known for about two
decades, their performance limitations have represented a barrier to many
desirable applications. (For brevity EL, the common acronym for
electroluminescent, is sometimes substituted.)
Representative of earlier organic EL devices are Gurnee et al U.S. Pat. No.
3,172,862, issued Mar. 9, 1965, filed Sept. 9, 1960; Gurnee U.S. Pat. No.
3,173,050, issued Mar. 9, 1965; Dresner, "Double Injection
Electroluminescence in Anthracene", RCA Review, Vol. 30, pp. 322-334,
1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The
organic emitting material was formed of a conjugated organic host material
and a conjugated organic activating agent having condensed benzene rings.
Naphthalene, anthracene, phenanthrene, pyrene, benzopyrene, chrysene,
picene, carbazole, fluorene, biphenyl, terphenyls, quarterphenyls,
triphenylene oxide, dihalobiphenyl, trans-stilbene, and
1,4-diphenylbutadiene were offered as examples of organic host materials.
Anthracene, tetracene, and pentacene were named as examples of activating
agents. The organic emitting material was present as a single layer having
thicknesses above 1 .mu.m.
The most recent discoveries in the art of organic EL device construction
have resulted from EL device constructions with the organic luminescent
medium consisting of two extremely thin layers (<1.0 .mu.m in combined
thickness) separating the anode and cathode, one specifically chosen to
inject and transport holes and the other specifically chosen to inject and
transport electron and also acting as the organic luminescent zone of the
device. The extremely thin organic luminescent medium offers reduced
resistance, permitting higher current densities for a given level of
electrical biasing. Since light emission is directly related to current
density through the organic luminescent medium, the thin layers coupled
with increased charge injection and transport efficiences have allowed
acceptable light emission levels (e.g., brightness levels capable of being
visually detected in ambient light) to be achieved for the first time with
low applied voltages in ranges compatible with integrated circuit drivers,
such as field effect transistors.
For example, Tang U.S. Pat. No. 4,356,429 discloses an EL device formed of
an organic luminescent medium consisting of a hole injecting and
transporting layer containing a porphyrinic compound and an electron
injecting and transporting layer also acting as the luminescent zone of
the device. In Example 1 an EL device is disclosed formed of a conductive
glass transparent anode, a 1000 Angstrom hole injecting and transporting
layer of copper phthalocyanine, a 1000 Angstrom electron injecting and
transporting layer of tetraphenylbutadiene in poly(styrene) also acting as
the luminescent zone of the device, and a silver cathode. The EL device
emitted blue light when biased at 20 volts at an average current density
in the 30 to 40 mA/cm.sup.2 range. The brightness of the device was 5
cd/m.sup.2.
A further improvement in such organic EL devices is taught by Van Slyke et
al U.S. Pat. No. 4,539,507. Van Slyke et al realized a dramatic
improvement in light emission by substituting for the hole injecting and
transporting porphyrinic compound of Tang an aromatic tertiary amine
layer. Referring to Example 1, onto a transparent conductive glass anode
were vaccum vapor deposited successive 750 Angstrom hole injecting and
transporting 1,1-bis(4-di-p-tolylaminophenyl)cyclohexane and electron
injecting and transporting
4,4'-bis(5,7-di-t-pentyl-2-benzoxazolyl)-stilbene layers, the latter also
providing the luminescent zone of the device. Indium was employed as the
cathode. The EL device emitted blue-green light (520 nm peak). The maximum
brightness achieved 340 cd/m.sup.2 at a current density of about 140
mA/cm.sup.2 when the applied voltage was 22 volts. The maximum power
conversion efficiency was about 1.4.times.10.sup.-3 watt/watt, and the
maximum EL quantum effeiciency was about 1.2.times.10.sup.-2
photon/electron when driven at 20 volts. Note particularly that Example 1
of Van Slyke et al produced a maximum brightness of 340 cd/m.sup.2 when
the EL device was driven at 22 volts while Example 1 of Tang produced only
5 cd/m.sup. 2 when that EL device was driven at 20 volts.
The organic EL devices have been constructed of a variety of cathode
materials. Early investigations employed alkali metals, since these are
the lowest work function metals. Other cathode materials taught by the art
have been higher work function (4 eV or greater) metals, including
combinations of these metals, such as brass, conductive metal oxides
(e.g., indium tin oxide), and single low work function (<4 eV) metals.
Gurnee et al and Gurnee, cited above, disclosed electrodes formed of
chrome, brass, copper, and conductive glass. Dresner U.S. Pat. No.
3,710,167 employed a tunnel injection cathode consisting of aluminum or
degenerate N.sup.+ silicon with a layer of the corresponding aluminum or
silicon oxide of less 10 Angstroms in thickness. Tang, cited above,
teaches useful cathodes to be formed from single metals with a low work
function, such as indium, silver, tin, and aluminum while Van Slyke et al,
cited above, discloses a variety of single metal cathodes, such as indium,
silver, tin, lead, magnesium, manganese, and aluminum.
Tang et al U.S. Ser. No. 13,530, concurrently filed and commonly assigned,
titled ELECTROLUMINESCENT DEVICE WITH IMPROVED CATHODE, discloses an EL
device comprised of a cathode formed of a plurality of metals other than
alkali metals, at least one of which has a work function of less than 4
eV.
SUMMARY OF THE INVENTION
Although recent preformance inprovements in organic EL devices have
suggested a potential for widespread use, most practical applications
require limited voltage input or light output variance over an extended
period of time. While the aromatic tertiary amine layers employed by Van
Slyke et al, cited above, have resulted in highly attractive initial light
outputs in organic EL devices, the limited stability of devices containing
these layers has remained a deterrent to widespread use. Device
degradation result in obtaining progressively lower current densities when
a constant voltage is applied. Lower current densities in turn result in
lower levels of light output. With a constant applied voltage, practical
EL device use terminates when light emission levels drop below acceptable
levels--e.g., readily visually detectable emission levels in ambient
lighting. If the applied voltage is progressively increased to hold light
emission levels constant, the field across the EL device is
correspondingly increased. Eventually a voltage level is required that
cannot be conveniently supplied by the EL device driving circuitry or
which produces a field gradient (volts/cm) exceeding the dielectric
breakdown strength of the layers separating the electrodes, resulting in a
catastrophic failure of the EL device.
It has been discovered quite surprisingly that stability and sustained
operating performance of the organic EL devices of Van Slyke et al, cited
above, can be markedly improved by forming the hole injecting and
transporting zone of the organic luminescent medium of two distinct
layers, one specifically chosen to interface with the anode and inject
holes and one specifically chosen to interface with and transport holes to
the electron injecting and transporting organic layer. In this respect the
organic EL devices of this invention differ from those previously known to
the art in forming the organic luminescent medium of a minimum of three
distinct layers of differing composition, each tailored to perform a
specific role in charge handling and luminescence.
In one aspect this invention is directed to an electroluminescent device
comprising in sequence, an anode, an organic hole injecting and
transporting zone, an organic electron injecting and transporting zone,
and a cathode, characterized in that the organic hole injecting and
transporting zone is comprised of a layer in contact with anode containing
a hole injecting porphyrinic compound and a layer containing a hole
transporting aromatic tertiary amine interposed between the hole injecting
layer and the electron injecting and transporting zone.
When organic EL devices according to this invention are constructed with
cathodes formed of a plurality of metals other than alkali metals, at
least one of the metals having a work function of less than 4 eV, as
taught by Tang et al, cited above, further advantages are realized.
Therefore, in another aspect this invention is directed to an
electroluminescent device comprising in sequence, an anode, an organic
hole injecting and transporting zone, an organic electron injecting and
transporting zone, and a cathode, characterized in that (1) the organic
hole injecting and transporting zone is comprised of a layer in contact
with the anode containing a hole injecting porphyrinic compound and a
layer containing a hole transporting aromatic tertiary amine interposed
between the hole injecting layer and the electron injecting and
transporting zone and (2) the cathode is comprised of a layer consisting
of a plurality of metals other than alkali metals, at least one of the
metals having a work function of less than 4 eV.
In addition to the stability advantages of the organic luminescent medium
discussed above, it has been further discovered quite unexpectedly that
the combination of a low work function metal and at least one other metal
in the cathode of an organic EL device results in improving the stability
of the cathode and consequently the stability of the device. It has been
observed that the initial performance advantages of low work function
metals other than alkali metals as cathode materials are only slightly
diminished when combined with more stable, higher work function metals
while marked extensions of EL device lifetimes are realized with even
small amounts of a second metal being present. Further, the advantages in
extended lifetimes can be realized even when the cathode metals are each
low work function metals other than alkali metals. Additionally, the use
of combinations of metals in forming the cathodes of the organic EL
devices of this invention has resulted in unexpected advantages in
fabrication, such as improved acceptance by the electron transporting
organic layer during vacuum vapor deposition of the cathode.
Another unexpected advantage realized with the cathode metal combination of
this invention is that low work function metals can be employed to prepare
cathodes which are light transmissive and at the same time exhibit low
levels of sheet resistance. Thus, the option is afforded of organic EL
device constructions in which the anode need not perform the function of
light transmission, thereby affording new use opportunities for organic EL
devices.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other advantages of this invention can be better appreciated by
reference to the following detailed description considered in conjunction
with the drawings, in which
FIGS. 1, 2, and 3 are schematic diagrams of EL devices;
FIGS. 4 and 5 are micrographs of conventional and inventive cathodes,
respectively.
The drawings are necessarily of a schematic nature, since the thickness of
the individual layers are too thin and thickness differences of the
various device elements too great to permit depiction to scale or to
permit convenient proportionate scaling.
DESCRIPTION OF PREFERRED EMBODIMENTS
An electroluminescent or EL device 100 according to the invention is
schematically illustrated in FIG. 1. Anode 102 is separated from cathode
104 by an organic luminescent medium 106, which, as shown, consists of
three superimposed layers. Layer 108 located on the anode forms a hole
injecting zone of the organic luminescent medium. Located above the hole
injecting layer is layer 110, which forms a hole transporting zone of the
organic luminescent medium. Interposed between the hole transporting layer
and the cathode is layer 112, which forms an electron injecting and
transporting zone of the organic luminescent medium. The anode and the
cathode are connected to an external power source 114 by conductors 116
and 118, respectively. The power source can be a continuous direct current
or alternating current voltage source or an intermittent current voltage
source. Any convenient conventional power source, including any desired
switching circuitry, can be employed which is capable of positively
biasing the anode with respect to the cathode. Either the anode or cathode
can be at ground potential.
The EL device can be viewed as a diode which is forward biased when the
anode is at a higher potential than the cathode. Under these conditions
injection of holes (positive charger carriers) occurs into the lower
organic layer, as schematically shown at 120, while electrons are injected
into the upper organic layer, as schematically shown at 122, into the
luminescent medium. The injected holes and electrons each migrate toward
the oppositely charged electrode, as shown by the arrows 124 and 126,
respectively. This results in hole-electron recombination. When a
migrating electron drops from its conduction potential to a valence band
in filling a hole, energy is released as light. Hence the organic
luminescent medium forms between the electrodes a luminescence zone
receiving mobile charge carriers from each electrode. Depending upon the
choice of alternative constructions, the released light can be emitted
from the organic luminescent material through one or more edges 128 of the
organic luminescent medium separating the electrodes, through the anode,
through the cathode, or through any combination of the foregoing.
Reverse biasing of the electrodes reverses the direction of mobile charge
migration, depletes the luminescent medium of mobile charge carriers, and
terminates light emission. The most common mode of operating organic EL
devices is to employ a forward biasing d.c. power source and to rely on
external current interruption or modulation to regulate light emission.
Since the organic luninescent medium is quite thin, it is usually preferred
to emit light through one of the two electrodes. This is achieved by
forming the electrodes as a translucent or transparent coating, either on
the organic luminescent medium or on a separate translucent or transparent
support. The thickness of the coating is determined by balancing light
transmission (or extinction) and electrical conductance (or resistance). A
practical balance in forming a light transmissive metallic electrode is
typically for the conductive coating to be in the thickness range of from
about 50 to 250 Angstroms. Where the electrode is not intended to transmit
light or is formed of a transparent material, such as a transparent
conductive metal oxide, any greater thickness found convenient in
fabrication can also be employed.
Organic EL device 200 shown in FIG. 2 is illustrative of one preferred
embodiment of the invention. Because of the historical development of
organic EL devices it is customary to employ a transparent anode. This is
achieved by providing a transparent insulative support 202 onto which is
deposited a conductive light transmissive relatively high work function
metal or metal oxide layer to form anode 204. The organic luminescent
medium 206 and therefore each of its layers 208, 210, and 212 correspond
to the medium 106 and its layers 108, 110, and 112, respectively, and
require no further description. With preferred choices of materials,
described below, forming the organic luminescent medium the layer 212 is
the zone in which luminescence occurs. The cathode 214 is conveniently
formed by deposition on the upper layer of the organic luminescent medium.
Organic EL device 300, shown in FIG. 3, is illustrative of another
preferred embodiment of the invention. Contrary to the historical pattern
of organic EL device development, light emission from the device 300 is
through the light transmissive (e.g., transparent or substantially
transparent) cathode 314. While the anode of the device 300 can be formed
identically as the device 200, thereby permitting light emission through
both anode and cathode, in the preferred form shown the device 300 employs
an opaque charge conducting element forming the anode 302, such as a
relatively high work function metallic substrate. The organic luminescent
medium 306 and therefore each of its layers 308, 310, and 312 correspond
to the medium 106 and layers 108, 110, and 112, respectively, and require
no further description. The significant difference between devices 200 and
300 is that the latter employs a thin, light transmissive (e.g.,
transparent or substantially transparent) cathode in place of the opaque
cathode customarily included in organic EL devices and, in most instances,
employs an opaque anode instead of the light transmissive anode normally
employed.
Viewing organic EL devices 200 and 300 together, it is apparent that the
present invention offers the option of mounting the devices on either a
positive or negative polarity opaque substrate.
The organic luminescent medium of the EL devices of this invention contains
a minimum of three separate organic layers, at least one layer forming the
electron injecting and transporting zone of the device, and at least two
layers forming the hole injecting and transporting zone, one layer of the
latter zone providing a hole injecting zone and the remaining layer
providing a hole transporting zone.
A layer containing a porphyrinic compound forms the hole injecting zone of
the organic EL device. A porphyrinic compound is any compound, natural or
synthetic, which is derived from or includes a porphyrin structure,
including porphine itself. Any of the porphyrinic compounds disclosed by
Adler U.S. Pat. No. 3,935,031 or Tang U.S. Pat. No. 4,356,429, the
disclosures of which are here incorporated by reference, can be employed.
Preferred porphyrinic compounds are those of structural formula (I):
##STR1##
wherein
Q is--N.dbd. or --C(R).dbd.;
M is a metal, metal oxide, or metal halide;
R is hydrogen, alkyl, aralkyl, aryl, or alkaryl, and
T.sup.1 and T.sup.2 represent hydrogen or together complete a unsaturated 6
membered ring, which can include substituents, such as alkyl or halogen.
Preferred 6 membered rings are those formed of carbon, sulfur, and
nitrogen ring atoms. Preferred alkyl moieties contain from about 1 to 6
carbon atoms while phenyl constitutes a preferred aryl moiety.
In an alternative preferred form the porphyrinic compounds differ from
those of structural formula (I) by substitution of two hydrogen for the
metal atom, as indicated by formula (II):
##STR2##
Highly preferred examples of useful porphyrinic compounds are metal free
phthalocyanines and metal containing phthalocyanines. While the
porphyrinic compounds in general and the phthalocyanines in particular can
contain any metal, the metal preferably has a positive valence of two or
higher. Exemplary preferred metals are cobalt, magnesium, zinc, palladium,
nickel, and, particularly, copper, lead, and platinum.
Illustrative of useful porphyrinic compounds are the following:
PC-1: Porphine
PC-2: 1,10,15,20-Tetraphenyl-21H,23H-porphine copper (II)
PC-3: 1,10,15,20-Tetraphenyl-21H,23H-porphine zinc (II)
PC-4: 5,10,15,20-Tetrakis(pentafluorophenyl)-21H,23H-porphine
PC-5: Silicon phthalocyanine oxide
PC-6: Aluminum phthalocyanine chloride
PC-7: Phthalocyanine (metal free)
PC-8: Dilithium phthalocyanine
PC-9: Copper tetramethylphthalocyanine
PC-10: Copper phthalocyanine
PC-11: Chromium phthalocyanine fluoride
PC-12: Zinc phthalocyanine
PC-13: Lead phthalocyanine
PC-14: Titanium phthalocyanine oxide
PC-15: Magnesium phthalocyanine
PC-16: Copper octamethylphthalocyanine
The hole transporting layer of the organic EL device contains at least one
hole transporting aromatic tertiary amine, where the latter is understood
to be a compound containing at least one trivalent nitrogen atom that is
bonded only to carbon atoms, at least one of which is a member of an
aromatic ring. In one form the aromatic tertiary amine can be an
arylamine, such as a monoarylamine, diarylamine, triarylamine, or a
polymeric arylamine. Exemplary monomeric triarylamines are illustrated by
Klupfel et al U.S. Pat. No. 3,180,730. Other suitable triarylamines
substituted with vinly or vinylene radicals and/or containing at least one
active hydrogen containing group are disclosed by Brantley et al U.S. Pat.
Nos. 3,567,450 and 3,658,520.
A preferred class of aromatic tertiary amines are those which include at
least two aromatic tertiary amine moieties. Such compounds include those
represented by structural formula (III):
##STR3##
wherein
Q.sup.1 and Q.sup.2 are independently aromatic tertiary amine moieties and
G is a linking group such an arylene, cycloalkylene, or alkylene group of a
carbon to carbon bond.
A particularly preferred class of class of triarylamines satisfying
structural formula (III) and containing two triarylamine moieties are
those satisfying structural formula (IV):
##STR4##
where
R.sup.1 and R.sup.2 each independently represents a hydrogen atom, an aryl
group, or an alkyl group or R.sup.1 and R.sup.2 together represent the
atoms completing a cycloalkyl group and
R.sup.3 and R.sup.4 each independently represents an aryl group which is in
turn substituted with a diaryl substituted amino group, as indicated by
structural formula (V):
##STR5##
wherein R.sup.5 and R.sup.6 are independently selected aryl groups.
Another preferred class of aromatic tertiary amines are tetraaryldiamines.
Preferred tetraaryldiamines include two diarylamino groups, such as
indicated by formula (V), linked through an arylene group. Preferred
tetraaryldiamines include those represented by formula (VI).
##STR6##
wherein
Are is an arylene group,
n is an integer of from 1 to 4, and
Ar, R.sup.7, R.sup.8, and R.sup.9 are independently selected aryl groups.
The various alkyl, alkylene, aryl, and arylene moieties of the foregoing
structural formulae (III), (IV), (V), can each in turn be substituted.
Typical substituents including alkyl groups, alkoxy groups, aryl groups,
aryloxy groups, and halogen such as fluoride, chloride, and bromide. The
various alkyl and alkylene moieties typically contain from about 1 to 6
carbon atoms. The cycloalkyl moieties can contain from 3 to about 10
carbon atoms, but typically contain five, six, or seven ring carbon
atoms--e.g., cyclopentyl, cyclohexyl, and cycloheptyl ring structures. The
aryl and arylene moieties are preferably phenyl and phenylene moieties.
While the entire hole transporting layer of the organic electroluminesce
medium can be formed of a single aromatic tertiary amine, it is a further
recognition of this invention that increased stability can be realized by
employing a combination of aromatic tertiary amines. Specifically, as
demonstrated in the examples below, it has been observed that employing a
triarylamine, such as a triarylamine satisfying formula (IV), in
combination with a tetraaryldiamine, such as indicated by formula (VI),
can be advantageous. When a triarylamine is employed in combination with a
tetraaryldiamine, the latter is positioned as a layer interposed between
the triarylamine and the electron injecting and transporting layer.
Representative useful aromatic tertiary amines are disclosed by Berwick et
al U.S. Pat. No. 4,175,960 and Van Slyke et al U.S. Pat. No. 4,539,507,
here incorporated by reference. Berwick et al in addition discloses as
useful hole transporting compounds N substituted carbazoles, which can be
viewed as ring bridged variants of the diaryl and triarylamines disclosed
above.
Illustrative of useful aromatic tertiary amines are the following:
ATA-1: 1,1-Bis(4-di-p-tolylaminophenyl)cyclohexane
ATA-2: 1,1-Bis(4-di-p-tolylaminophenyl)-4-phenylcyclohexane
ATA-3: 4,4'-Bis(diphenylamino)quadriphenyl
ATA-4: Bis(4-dimethylamino-2-methylphenyl)phenylmethane
ATA-5: N,N,N-Tri(p-tolyl)amine
ATA-6: 4-(di-p-tolylamino)-4'-[4(di-p-tolylamino)styryl]stilbene
ATA-7: N,N,N',N'-Tetra-p-tolyl-4,4'-diaminobiphenyl
ATA-8: N,N,N',N'-Tetraphenyl-4,4'-diaminobiphenyl
ATA-9: N-Phenylcarbazole
ATA-10: Poly(N-vinylcarbazole)
Any conventional electron injecting and transporting compound or compounds
can be employed in forming the layer of the organic luminescent medium
adjacent the cathode. This layer can be formed by historically taught
luminescent materials, such as anthracene, naphthalene, phenanthrene,
pyrene, chrysene, and perylene and other fused ring luminescent materials
containing up to about 8 fused rings as illustrated by Gurnee et al U.S.
Pat. No. 3,172,862, Gurnee U.S. Pat. No. 3,173,050, Dresner, "Double
Injection Electroluminescence in Anthracene", RCA Review, Vol. 30, pp.
322-334, 1969; and Dresner U.S. Pat. No. 3,710,167, cited above. Although
such fused ring luminescent materials do not lend themselves to forming
thin (<1 .mu.m) films and therefore do not lend themselves to achieving
the highest attainable EL device performance levels, organic EL devices
incorporating such luminescent materials when constructed according to the
invention show inprovements in performance and stability over otherwise
comparable prior art EL devices.
Among electron transporting compounds useful in forming thin films are the
butadienes, such as 1,4-diphenylbutadiene and tetraphenylbutadiene;
coumarins; and stilbenes, such as trans-stilbene, disclosed by Tang U.S.
Pat. No. 4,356,429, cited above.
Still other thin film forming electron transporting compounds which can be
used to form the layer adjacent the cathode are optical brighteners,
particularly those disclosed by Van Slyke et al U.S. Pat. No. 4,539,507,
cited above and here incorporated by reference. Useful optical brighteners
include those satisfying structural formulae (VII) and (VIII):
##STR7##
wherein
R.sup.1, R.sup.2, R.sup.3, and R.sup.4 are individually hydrogen; saturated
aliphatic of from 1 to 10 carbon atoms, for example, propyl, t-butyl,
heptyl, and the like; aryl of from 6 to 10 carbon atoms, for example,
phenyl and naphthyl; or halo such as chloro, fluoro, and the like; or
R.sup.1 and R.sup.2 or R.sup.3 and R.sup.4 taken together comprise the
atoms necessary to complete a fused aromatic ring optionally bearing at
least one saturated aliphatic of from 1 to 10 carbon atoms, such as
methyl, ethyl, propyl and the like;
R.sup.5 is a saturated aliphatic of from 1 to 20 carbon atoms, such as
methyl, ethyl, n-eicosyl, and the like; aryl of from 6 to 10 carbon atoms,
for example, phenyl and naphthyl; carboxyl; hydrogen; cyano; or halo, for
example, chloro, fluoro and the like; provided that in formula (VIII) at
least two of R.sup.3, R.sup.4 and R.sup.5 are saturated aliphatic of from
3 to 10 carbon atoms, e.g., propyl, butyl, heptyl and the like;
Z is --O--, --NH--, or --S--; and
Y is
##STR8##
wherein
m is an integer of from 0 to 4;
n is arylene of from 6 to 10 carbon atoms, for example phenylene and
naphthylene; and
Z' and Z" are individually N or CH.
As used herein "aliphatic" includes substituted aliphatic as well as
unsubstituted aliphatic. The substituents in the case of substituted
aliphatic include alkyl of from 1 to 5 carbon atoms, for example, methyl,
ethyl, propyl and the like; aryl of from 6 to 10 carbon atoms, for example
phenyl and naphthyl; halo, such as chloro, fluoro and the like; nitro; and
alkoxy having 1 to 5 carbon atoms, for example, methoxy, ethoxy, propoxy,
and the like.
Still other optical brighteners that are contemplated to be useful are
listed in Vol. 5 of Chemistry of Synthetic Dyes, 1971, pages 618-637 and
640. Those that are not already thin-film-forming can be rendered so by
attaching an aliphatic moiety to one or both end rings.
Particularly preferred thin film forming materials for use in forming the
electron injecting and transporting layers of the organic EL devices of
this inventions are metal chelated oxinoid compounds, including chelates
of oxine itself (also commonly referred to as 8-quinolinol or
8-hydroxyquinoline). Such compounds exhibit both high levels of
performance and are readily fabricated in the form of thin films.
Exemplary of contemplated oxinoid compounds are those satisfying
structural formula (IX):
##STR9##
wherein
Me represents a metal;
n is an integer of from 1 to 3; and
Z independently in each occurrence represents the atoms completing a
nucleus having at least two fused aromatic rings.
From the foregoing it is apparent that the metal can be monovalent,
divalent, or trivalent metal. The metal can, for example, be an alkali
metal, such as lithium, sodium, or potassium; an alkaline earth metal,
such as magnesium or calcium; or an earth metal, such as boron or
aluminum. Generally any monovalent, divalent, or trivalent metal known to
be a useful chelating metal can be employed.
Z completes a heterocyclic nucleus containing at least two fused aromatic
rings, at one of which is an azole or azine ring. Additional rings,
including both aliphatic and aromatic rings, can be fused with the two
required rings, if required. To avoid adding molecular bulk without
improving on function the number of ring atoms is preferably maintained at
18 or less.
Illustrative of useful chelated oxinoid compounds are the following:
CO-1: Aluminum trisoxine [a.k.a., tris(8-quinolinol)aluminum]
CO-2: Magnesium bisoxine [a.k.a., bis(8-quinolinol)magnesium]
CO-3: Bis[benzo{f}-8-quinolinol]zinc
CO-4: Bis(2-methyl-8-quinolinolato)aluminum oxide
CO-5: Indium trisoxine [a.k.a., tris(8-quinolinol)indium]
CO-6: Aluminum tris(5-methyloxine) [a.k.a.,
tris(5-methyl-8-quinolinol)aluminum
CO-7: Lithium oxine (a.k.a., 8-quinolinol lithium]
CO-8: Gallium tris(5-chlorooxine) [a.k.a,
tris(5-chloro-8-quinolinol)gallium]
CO-9: Calcium bis(5-chlorooxine) [a.k.a, bis(5-chloro-8-quinolinol)calcium]
CO-10: Poly[zinc (II)-bis(8-hydroxy-5-quinolinyl)methane]
CO-11: Dilithium epindolidione
In the organic EL devices of the invention it is possible to maintain a
current density compatible with efficient light emission while employing a
relatively low voltage across the electrodes by limiting the total
thickness of the organic luminescent medium to less than 1 .mu.m (10,000
Angstroms). At a thickness of less than 1 .mu.m an applied voltage of 20
volts results in a field potential of greater than 2.times.10.sup.5
volts/cm, which is compatible with efficient light emission. An order of
magnitude reduction (to 0.1 .mu.m or 1000 Angstroms) in thickness of the
organic luminescent medium, allowing further reductions in applied voltage
and/or increase in the field potential and hence current density, are well
within device construction capabilities.
One function which the organic luminescent medium performs is to provide a
dielectric barrier to prevent shorting of the electrodes on electrical
biasing of the EL device. Even a single pin hole extending through the
organic luminescent medium will allow shorting to occur. Unlike
conventional EL devices employing a single highly crystalline luminescent
material, such as anthracene, for example, the EL devices of this
invention are capable of fabrication at very low overall organic
luminescent medium thicknesses without shorting. One reason is that the
presence of three superimposed layers greatly reduces the chance of pin
holes in the layers being aligned to provide a continuous conduction path
between the electrodes. This in itself permits one or even two of the
layers of the organic luminescent medium to be formed of materials which
are not ideally suited for film formation on coating while still achieving
acceptable EL device performance and reliability.
The preferred materials for forming the organic luminescent medium are each
capable of fabrication in the form of a thin film--that is, capable of
being fabricated as a continuous layer having a thickness of less than 0.5
.mu.m or 5000 Angstroms.
Wnen one or more of the layers of the organic luminescent medium are
solvent coated, a film forming polymeric binder can be conveniently
codeposited with the active material to assure a continuous layer free of
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