CA1285638C

CA1285638C - Electroluminescent device with organic luminescent medium

Info

Publication number: CA1285638C
Application number: CA000557849A
Authority: CA
Inventors: Steven Arland Vanslyke; Ching Wan Tang; Luther Craig Roberts
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1987-02-11
Filing date: 1988-02-01
Publication date: 1991-07-02
Anticipated expiration: 2008-07-02
Also published as: EP0278758B1; DE3879727T2; EP0278758A2; EP0278758A3; US4720432A; DE3879727D1; JP2597377B2; JPS63295695A

Abstract

ELECTROLUMINESCENT DEVICE
WITH ORGANIC LUMINESCENT MEDIUM
Abstract of the Disclosure An electroluminescent device is disclosed comprising in sequence, an anode, an organic hole injecting and transporting zone, an organic electron injecting and transporting zone, and a cathode. The organic hole injecting and transporting zone is comb prised of a layer in contact with the anode contain-ing 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.

Description

g~2~5~

ELECTROLUMINESCENT DEVICE
WITH ORGANIC LUMINESCENT MEDIUM
This in~ention relates to organic electro-lum~nescent devices. More specifically, this inven-5 tion relates to devices which em1t light from & cur-rent conducting organic layer.
While organic electrolumine~cent devices have been known for about two decades, their per~or-mance limitations have represented a barrier to m~ny 10 desirable applications. (For brevity EL, the common acronym ~or electroluminescent, is ~ometlme~ ~ubsti-tuted.) Representative of earlier organic EL devices are Gurnee et al ~.S. Patent 3,172,862, issued Msrch 15 9, 1965, filed Sept. 9, 1960; Gurnee U.S. Patent 3,173,050, issued March 9, 1965; Dresner, "Double Injection Electroluminescence in Anthracene", RCA
Review, Vol. 30, pp. 322--334, 1969; and Dresner U.S.
Patent 3,710,167, issued Jsn. 9, 1973. The organic 20 emitting material was formed of a conJugated organic host material and a conJugated orgsnic activflting agent having condensed benzene rings. Naphthalene, anthracene, phenanthrene, pyrene, benzopyrene, chrys-ene, picene, carbazole, fluorene, biphenyl, terphen-25 yls, quaterphenyls, triphenylene oxide, dihalobi-phenyl, trans-stilbene, and 1,4-diphenylbutadiene were offered as examples of organic host materials.
Anthracene, tetracene, and pentacene were named ~s examples of activating agents. The org~nic emitting 30 material was present as a single layer having thick-nesses above 1 ~m.
The most recent discoveries in ~he ar~ of organic EL device constructlon have resulted from EL
device constructions with the organic luminescent 35 medium consisting of two extremely thin layer~ (<
1.0 ~m in combined thickness) separ~ting the anode and cathode, one specifically chosen to in~ect and ~k o~g transport holes and the other specifically chosen to in~ect Hnd transport electrons and al50 acting as the organic luminescent zone of the device. The extremely thin organic luminescent medium offers 5 reduced resistance, permitting higher current densi-ties for a given level of electrical biasing. Since light emission is directly rel&ted to current density through the orgflnic lumine~cent medium, the thin lay-ers coupled with increased charge inJection and 10 transport e~ficiencies have allowed acceptable light emission levels (e.g., brightness levels capable of being visually detected in ffmbient light) to be achieved for the first time with low applied voltages in ranges compat~ble with integrated circuit drivers, 15 such as field effect transistors.
For example9 Tang U.S. Patent 4,356,429 dis-closes an EL device formed of an organic luminescent medium consisting of a hole in~ecting and transport-ing layer containing a porphyrinic compound and an 20 electron injecting and transporting layer al~o acting as the luminescent zone of the device. In Example l an EL device is disclosed formed of a conductive glass transparent anode, a lO00 Angstrom hole ~n~ect-ing and transporting layer of copper phth~locyanine, 25 a lO00 Angstrom electron injecting and transporting layer of tetraphenylbutadiene in poly(styrene) also acting a~ the luminescent æone o$ the device, snd a silver cathode. The EL device emitted blue light when biRsed at 20 volts ~t an average current densit 30 in the 30 to 40 mA/cm2. The brightness of the device was 5 cd/m2.
A further improvement in such organic EL
devices is taught by Van Slyke et al U.S. P~tent 4,539,507. Van Slyke et al realized a dramatic 35 improvement in light emission by substituting $or the hole injecting and transporting porphyrinic compound of Tang an aromatic tertiary amine layer. Referring ~21~J$~

to Example 1, onto a transparent conductive glass anode were vacuum vapor deposited ~uccessive 750 Angstrom hole in~ectlng and transporting 1,1-bis(4- ;
di-~-tolylaminophenyl)cyclohexane ~nd electron 5 in~ecting and transporting 4,4' - bis(5,7 - di - t-pentyl-2-benzoxazolyl)-stilbene layers, the latter al50 pro-viding the luminescent zone of the device. Indium was employed ~5 the cAthode. The EL device emitted blue-green light (520 nm peak). The m~ximum bright-10 ness achieved 340 cd/m2 at a current density ofabout 140 mA/cm2 when the applied voltAge was 22 voltq. The maximum power conversion efficiency was about 1.4 X 10 3 watt/watt, and the maximum EL quan-tum efficiency was about 1.2 X 10 2 photontelectron l5 when driven at 20 volts. Note pArticul~rly that Example 1 of Van Slyke et Al produced a maximum brightness o~ 340 cd/m2 when the EL device was driven ~t 22 volts while Ex~mple l of Tang produced only 5 cd/m when that EL device w~s driven at 20 20 volts.
The organic EL devices have been constructed of a variety of cathode materials. E~rly investiga-tions employed alkali metals, since these are the lowest work function metals. Other cathode materials 2S taught by the art have been higher work function (4 eV or greater) metals, lncluding combinations of these metals, such as br~ss, conductive metal oxides (e.g., indium tin oxide), and single low work func-tion (< 4 eV) metals. Gurnee et ~1 and Gurnee, 30 cited above, disclosed electrodes ormed of chrome, brass, copper, and conductive gl~ss. Dresner U.S.
Patent 3,710,167 employed a tunnel in~ection c~thode consisting of ~luminum or degenerate N silicon with a layer of the corresponding ~luminum or silicon 35 oxide of less 10 Angstroms in thickness. Tang, cited above, teaches useul cathodes to be formed from sin-gle metals with a low work func~ion, such as indium, silver, tin, ~nd aluminum while V~n Slyke et al, cited above, discloses a Y~riety of single metal cathodes, such as indlum, silver, tin, lead, magne-5 ium, manganese, and aluminum.
Although recent performAnce improvementq in organic EL devices hsve suggested ~ potential for widespread use, most practical applications require limited voltage input or light output variance over an extended period of time. While the aromatic ter-10 tiary amine layers employed by Van Slyke et al, cited above, have resulted in highly attractive initial light outputs in organic EL devices, the limited sta-bility of devices containing these layers has remained a deterrent to widespread use. Device 15 degradation results 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 deYice use terminates 20 when light emission levels drop below acceptable lev-els -e.g., readily visually detectable emission lev-els in ambien~ lighting. If the applied voltage is progressively increased to hold light emission levels constant, the field across the EL device is corre-25 spondingly increased. Eventually a voltage level isrequired th~t cannot be conveniently supplied by the EL device driving circuitry or which produces a field gradient (volts/cm) exceeding the dielectric bre~k-down strength of the layers separating the elec-30 trodes, resulting in a catastrophic failure of the ELdevice.
It is an object of this invention to provide an electroluminescent device comprising in se~uence, an &node, an organic hole inJecting and tr~nsporting 35 zone, an organic electron in~ecting and transporting zone, and a cathode, exhibiting lmproved st~bility and su~t~ined operating performance.

.
~ .

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This ob~ect is achieved by employing ~9 the organic hole inJecting and transporting zone a layer ln contact wlth the anode containing a hole injecting porphyrinic compound and a layer cont~ining a hole 5 transporting aromat~c tertiary amine lnterposed between the hole in~ecting layer ~nd the electron in~ecting and trensporting zone.
It has been discovered quite surprisingly that stability and sustained operating performance of 10 the organic EL devices of Van Slyke et al, cited above, can be markedly improved by forming the hole in~ecting And transporting zone of the organic lumi-nescent medium of two distinct layers, one ~pecifi-cally chosen to interface with the anode and in~ect 15 holes and one specifically chosen to interface with and transport holes to the electron in~ecting 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 20 organic luminescent medium of a minimum==of t~hree_dis-tinct layers of differing composition, each tailored to perform a specific role in charge handling and luminescence.
When organic EL devlces according to this 25 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, further advantages are realized.
Therefore, in another aspect this invention 30 is directed to an electroluminescent device compris-ing in sequence, ~n anode, an organic hole in~ecting and tranQporting zone, an organic electron in~ecting and transporting zone, and a c&thode, characterized in that (l) the organic hole in~ecting and transport-35 ing zone is comprised of a layer in contact with theanode containing a hole in~ecting porphyrinic com-pound and a layer containing a hole transporting aro-matic tertiary amine interposed between the holeinJecting l~yer and the electron in~ecting and trans-porting zone and (2) the cathode is comprised of a layer consisting of ~ plurality of metals other than 5 ~lkali metals, at least one of the metals h~ving a - work function of less than 4 eV.
In addition to the st~bility advantages of the organic luminescent medium discussed above, it has been further discovered quite unexpectedly that 10 the combination of a low work function metal snd at least one other metaï in the cathode of an organic EL
device results in improving the stability of the cathode and consequently the ~tability of the device. It has been observed that the initial per-15 formance advantages of low work function metals otherthan alkali metals as cathode materials are only sl~ghtly diminished when combined with more stable, higher work function metals while marked exten~ions of EL device lifetimes are realized with even small 20 amounts of a second metal being present. Further, the advantages in extended li~etimes can be realized even when the cathode metals are each low work func-tion metals other than alkali metals. Additionally, the use of combinations of metals in forming the 25 cathodes of the organic EL devices of this invention has resulted in unexpected advantages in fabrication, such as improved acceptance by the electron tr~ns-porting organic layer during vacuum vapor deposition of the cathode.
Another unexpected advantage realized with the cathode metal combinations of this invention is that low work function metals can be employed to pre-pare cathodes which are light transmis~ive and at the same time exhibit low levels of sheet resistance.
35 Thus, the option is afforded of organic EL device constructions in which the anode need not perform the ~ ~ 5 ~

function of light transmission, thereby affording new u~e opportunities for organic EL devices.
These ~nd other advantflges of th$s $nvention can be better ~ppreciated by referencs to the ~ollow-5 ing detailed description considered in con~unctionw~th the drawings, in which Figures 1, 2, and 3 are schematic diagr~ms of EL devices;
Figures 4 and 5 are micrograph~ of conven-10 tional and inventive cathodes, respectively.
The drawings are necessarily of a schematicnature, since the thicknesses of the individual 16y -ers are too thin and thickness differences of the various device elements too great to permit depiction 15 to scale or to permit convenient proportionate scal-ing.
An electroluminescent or EL device 100 according to the invention is schematically illus-trated in Figure 1. Anode 102 is separated from 20 cathode 104 by an organic luminescent medium 106, which, as shown, consists of three superimposed lay-ers. Layer 108 located on the anode forms a hole in~ecting zone of the organic luminescent medium.
Located above the hole in~ecting layer is layer 110, 25 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 in~ect~n~ and transporting zone of the organic luminescent medium. The anode 30 and the cathode are connected to an external power source 114 by conductors 116 and 118, respectively.
The power ~ource can be a continuous direct current or alternating current voltage source or an intermit-tent current voltage source~ Any convenient conven-35 tional power source, including any desired ~witchingcircuitry, can be employed which i~ c~pable of posi-tively bia~ing the anode with respect to the cath-ode. Either the anode or cathode can be ~t groundpotential.
The EL device can be viewed as A diode which is forward biased when the anode iR ~t A higher 5 potential than the cathode. Under these conditions injection of holes (positive charger carriers) occurs into the lower org~nic layer, RS schematic~lly shown at 120, while electrons are in~ected into the upper organic layer, as schematically shown at 122, into 10 the luminescent medium. The in~ected holes and elec-trons each migrate toward the oppositely charged electrode, as shown by the arrows 124 and 126~
respectively. This results in hole-electron recombi-nation. When a migrating electron drops from its 15 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 rom each electrode. Depending upon the choice of 20 alternative constructionq, the released light can be emitted from the organic luminescent material through one or more of edges 128 of the organic luminescent medium separating the electrodes, through the anode, through the cathode, or through any combination of 25 the foregoing.
Reverse biasing of the electrodes reverseæ
the direction of mobile charge migration, depletes the luminescent medium of mobile charge c~rriers, and terminates light emission. The most common mode of 30 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 org~nic luminescent medium is 35 quite thin, it is usuall7 preferred to emit light through one of the two electrodes. This is achieved by forming the electrode ~s a translucent or trans-parent coating, either on the organic luminescentmedium or on a separate translucent or transparent support. The thickness of the coating i~ determined by balancing light transmiss~on (or extinction) and 5 electrical conduc~ance (or resistance). A practic~l balsnce in forming a light tr~nsmis~ive metallic electrode is typically for the conductive coating to be in the thickness range of from about 50 to 250 Angstroms. Where the electrode ls not intended to lO transmit light or ls formed of a transparent mate-rial, such as a transparent conductive met&l oxide, any greater thickness found convenlent ln fabrication can also be employed.
Organic EL device 200 shown in Flgure 2 is 15 illustrative of one preferred embodiment of the ~nvention. Because of the historical development of organic EL devices it is customary to employ a trans-parent anode. This is achieved by providing a tr~ns-parent insulative support 202 onto which is deposited 20 a conductive light transmissive relatively high work function metal or metal oxide layer to form anode 204. The organic luminescent medium 206 ~nd there-fore each of its layers 203, 210, and 212 correspond to the medium 106 and its layers 108, 110, and 112, 25 respectively, and require no further description.
With preferred choices of materials, described below, forming the organic luminsscent medium the lsyer 212 is the zone in which luminescence occurs. The cath-ode 214 is conveniently ~ormed by depositlon on the 30 upper layer of the organic luminescent medium.
Organic EL device 300, shown in Figure 3, is illustrative of another preferred embodiment of the invention. Contrary to the historical psttern of organic EL device development, light emission from 35 the device 300 is through the light transmissive (eO4~ transparent or substantially transparent) cathode 314. Whlle the ~node of the device 300 can be formed identically as the device 200, thereby per-mitting light emission through both snode and cath-ode, in the preferred form ~hown the device 300 employs an opaque charge conducting element forming 5 the anode 302, such as a relatively high work func-tion metallic substrate. The organic luminescent medium 306 and therefore each of its layer~ 308, 310, and 312 correspond to the medium 106 and layers 108, 110, and 112, respectively, and require no further 10 description. The significant difference between devices 200 ~nd 300 is that the latter employs a thin, light transmissive (e.g., tr~nsparent or QUb -stantially transparent) cathode in place of the opaque cathode customarily included in organic EL
15 devices and, in most instances, employs an opaque anode instead of the light transmissive anode nor-mally employed.
Viewing organic EL devices 200 and 300 together, it is app~rent that the present invention 20 offers the option of mounting the devices on either a positive or negative polarity opaque sub-Qtrate.
The organic luminescent medium of the EL
devices of this invention conta~ns a minimum o$ three separate organic layers, at least one layer forming 2S the electron in~ecting and transporting zone of the device, ~nd 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 containlng a porphyrinic compound forms the hole in~ecting zone of the organic EL
device. A porphyrinic compound is any compound, natural or syntheti~ 9 which is derived from or includes a porphyrin structure, including porphine 35 itself. Any of the porphyrinic compounds di~closed by Adler U.S. Patent 3,935,031 or Tang U.S. Patent 4,356,429, can be employed.

'~

Preferred porphyrinic compounds are those of structural formul~
(I) 1 2 T\ /T

T~ T

T2/ \ ~ 0 ~\.~\,,~Q/
\.=./
T~ T~
wherein Q ls -N= or -C(R)=;
M is a metal, metal oxide, or metal halide;
R is hydrogen, alkyl, aralkyl, aryl, or alkaryl, and T and T represent hydrogen or toge~her com-plete a unsaturated 6 membered ring, which can include substituents, such as alkyl or halogen. Pre-ferred 6 membered rings are those formed of carbon, sulfur, and nitrogen ring atoms. Preferred alkyl moieties contain from about l to 6 carbon atoms while phenyl constitutes a preferred aryl moiety.
In an alternative preferred form the por phyrinic compounds differ from those of structural : ormula (I) by substitution of two hydrogen for the metal atom, as indicated by formula (II):

.

:
: :
.

(II) \,=./

Tl /Q~ T2 5 ~ [

~ ~, = . /

Highly preferred examples of useful porphyr-inic compounds are metal free phthalocy~nines ~nd metal containlng phthalocyanines. While the porphyr-inic compounds in general and the phthalocyanines in 15 particular can contain any metal, the metal prefera-bly ha~ a positive valence of two or higher. Exem-plary preferred metals ~re cobalt, magnesium, zinc, palladium, nickel, and, particularly, copper, le~d, ~nd platinum.
Illustrati~e of u~e~ul porphyrinic compounds are the ~ollowing:
: PC-l Porphine PC-2 1,10,15?20-Tetraphenyl-~lH,23H- porphine copper (II) PC-3 1,10,15,20-Tetraphenyl-21H,23H-porphine zinc (II) : ; PC-4 5,I0,15,20-Tetr~kis(pentafluorophenyl)-21H,23H-porphine : : PC-5 Sil1con phthalocyanine oxide : 30PC-6 Aluminum phthalocyanine chloride : PC-7 Phthalocyanine (metal free) PC-8 Dilithium phthalocyanine : PC-9 Copper ~etramethylphth~locyanine : : PC-10 Copper phthalocyanine ~: 35PC-ll Chromium phthAlocyanine fluoride PC-12 Zinc phthalocy~nine PC-13 Lead phthalocyanine ' ' ` `' ~- ~' ', `

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PC 14 Tit~nium phthalocyanine oxide PC-15 Magnesium phthalocyanine PC-16 Copper octamethylphthalocyanine The hole transporting layer of the organic 5 EL device contains at least one hole transporting aromatic tertiary amine, where the lstter is under-stood to be a compound containing at least one triva-lent nitrogen atom that is bonded only to carbon fltoms, at least one of which is a member of an aro-10 matic ring. In one form the aromatic tertiary aminecan be an arylamine, such as a monoarylamine, diaryl-amine, triarylamine, or a polymeric arylamine. Exem-plary monomeric triarylamines are illustrated by Klup~el et al U.S. Patent 3,180,730. Other suitable 15 triarylamine~ substituted with vinyl or vinylene radicals and/or containing at least one active hydro-gen containing group are diqclosed by Brantley et al U.S. Patents 3,567,450 and 3,658,520.
A preferred class of aromatic tertiary 20 amines are those which include at least two aromatic tertiary amine moieties. Such compounds include those represented by structural formula (III):
(III) 1 2 Q\ /Q
G
wherein Ql and Q2 are independently aromatic tertiary amine moieties and G is a linking group ~uch an arylene, cycloalkyl-30 ene, or alkylene group or a carbon to carbon bond.
A particularly preferred class of class of triarylamines satisfying ~tructural formula (III) and containing two triarylamine moietles are those satis-fying structural formula (IV~:

(IV) R
Rl- C - R3 l4 5 where Rl and R2 each independently repre~ent~ ~
hydrogen atom~ an aryl group, or an &lkyl group or Rl and R2 together represent the Qtoms completing e cycloalkyl group and R3 and R4 each independently represents an aryl group which is in turn substituted with a diaryl substituted amino group, as indicated by structural formula (V):
(V) 5 _ ~

wherein R5 ~nd R6 are independently selected aryl groups.
Another preferred class of aromatic tertiary 20 amines are tetraaryldiamines. Preferred tetraaryldi-amines include two diarylamino groups, such as indi-cated by formula (~), linked through an arylene group. Preferred tetraaryldiamines include those represented by formula (VI).
~VI) R7 8 \~ - Aren - ~ 9 wherein Are is an arylene group, n is an integer of from 1 to 4, and Ar, R7, R8, and R9 ~re independently selected aryl groups.
The various alkyl, alkylene, aryl, ~nd aryl-ene moieties of the foregoing structural formulae tIII), (IV), (V), and (VI) can each in turn be sub-stituted. Typical substituents including alkyl groups, ~lkoxy groups, aryl groups, aryloxy groups, and halogen ~uch as fluoride, chloride, flnd bromide.
The various alkyl and alkylene moieties typically contain from about l to 6 carbon atoms. The cycloal-5 kyl moieties c~n contain from 3 to about lO c~rbonatoms, but typical1y contain five, six, or ~even 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 ~ingle aromatic tertiary amine, it is a further recognition of this invention that increased sta-bility can be realized by employing a combination of 15 arom~tic tertiary amines. Specificall~, as demon-strated in the examples below, it has been observed that employing a triarylamine, such as a triarylamine sati~fying formula (IV), in combination with a tetra-aryldiaMine, such as indicated by formula (VI), can 20 be advantageous. When a triarylamine is employed in combination with a tetraaryldiamine, the latter is positioned as a layer interposed between the triaryl-amine and the electron in~ecting and transporting layer.
Representative useful aromatic ter~iary amines are disclosed by Berwick et al U.S. Patent 4,175,960 and Van Slyke et al U.S. Patent 4,53~,507.
Berwick et sl in addition discloses as useful hole transporting compounds N sub~tituted carbazoles, 30 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-l l,l-Bis~4-di-p-tolylaminophenyl)cyclo-hexane ATA-2 l,l-Bis(4-di-p-tolylam1nophenyl)-4-phen-ylcyclohexane ~2~ 8 ~TA-3 4,4'-Bis(diphenylamino)quadriphenyl ATA-4 Bis(4-~imethylamino-2-methylphenyl)phen-ylmethane ATA-5 N,N,N-Tri(~-tolyl)amine ATA-6 4-(di-p-tolylamino)-4'-[4~di-~-tolyl-flmino)styr,yl]stilbene ATA-7 N,N,N',N'-Tetra-P-tolyl-4,4'-diaminobi-phenyl ATA-8 N,N,N',N'-Tetraphenyl-4,4'-dlaminobiphe-nyl AT~-9 N-Phenylcarbazole ATA-10 Poly(N-vinylcarbazole) Any conventional electron in;ecting and transporting compound or compounds can be employed in 15 forming the layer of the organic luminescent medium adjacent the cathode. This layer can be formed by hi~torically tflught lumlne cent materials, such as anthracene, naphthalene, phenanthrene, pyrene, chrys-ene, and perylene and other fused ring luminescent 20 materials containing up to about 8 fused rings as illustrated by Gurnee et Hl U.S. Patent 3,172,862, Gurnee U.S. Patent 3,173,050, Dresner, "Double In~ec-~ion Electroluminescence in Anthracene", RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Patent 25 3,710,167, cited above. Although such fused ring luminescent materials do not lend themselves to form-: ing thin (~ 1 ~m) films and therefore do not lend themselves to achieving the highest attainable EL
device performance levels, organic EL devices incor-30 porating such luminescent materials when constructed according to the invention how improvements in per-formance and stability over otherwise comparable prior ~rt EL devices.
Among electron transporting compounds useful :35 in forming thin films are the butadienes, ~uch as 1,4-diphenylbutadiene and tetraphenylbutadiene; cou-.

1~

marins; and stilbenes, such a~ trans-stilbene, dis-closed by Tang U.S. Patent 4,356,429~ cited above.
Still other thin film forming electron transporting compounds whlch can be used to form the 5 layer adjacent the cflthode are optlcal brightener~, particularly those disclosed by Van Slyke et al U.S.
Patent 4,539,507. Useful optical brighteners include those ~atisfying structural formulae (VII) and (~III):
(VII) R~ f~ R3 or 2~ Z \ Z/ ~ ~ ~4 R

(VIII) 3 ~ 5 4 ~ \z/ .~._.~.

wherein Rl, R , R3, and R are individually 20 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 Rl and R2 or R3 and R4 taken 25 together comprise the atoms necessary to complete a fused aromatic ring optionally bearing at least one saturated aliphatic of from l to 10 carbon atoms, such as methyl, ethyl, propyl and the like;
R5 is a saturated aliphatic of from 1 to 30 20 csrbon 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;
CyRnO; or h~lo, for example, chloro, fluoro and the l~kP; provided that in formula (VIII) at least two o~
35 R3, R and RS are ~aturated al~phatic of ~rom 3 to 10 carbon atoms, e.g., propyl, butyl, heptyl and the like;

. ~ . .

Z is -O-, -NH-, or -S-; and Y is --R6tCH=CH~--nR6_, ¦_f ~ - ¦ , -CH=CH-, - m tCH=CH tmR ~CH=CH ~ , or ~
1 0 \S/
wherein m 1s an integer of from 0 to 4;
n is arylene of from 6 to 10 carbon atoms, 15 for example, phenylene and naphthylene; and : Z' and Z" are individually N or CH.
As used herein "aliphatic" includes substituted ali~
phatic flS well as unsubstituted aliphatic. The sub-stituents in the case of substituted aliphatic 20 include alkyl of from 1 to 5 carbon atoms, for exam-ple, methyl, ethyl, propyl and the like; aryl of from 6 to 10 carbon atoms, for example, phenyl and naph-thyl; halo, such as chloro, fluoro and the like;
~: nitro; and alkoxy having 1 to 5 c~rbon ~toms, for 25 example, methoxy, ethoxy, propoxy, and the like.
Still other optical brighteners th~t are contemplated to be useful ~re listed in Vol. 5 of Chemi~trY oE Synthetic DYes, 1971, pages 618-637 ~nd : 640. Those that are not already thin-film-forming : 30 can be rendered so by att~ching an ~liphatic moiety to one:or both end r~ngs.
Particul~rly preferred thin ~ilm ~orming materials for use in forming the electron in~ecting and transporting layers of the organic EL devices of 35 this inventions~are metal chela~ed oxinoid compounds, including chel&tes of oxine itself (~lso commonly -~ referred to as 8-quinolinol or 8-hydroxyquinoline).

:, .
. .

.

Such compounds exhi~it both high levels of perfor-mance and ~re readlly fabrlcated in the form of thin films. Exemplary of cont~mplated oxinold compounds sre those satisEying structural formula (IX):
5 (IX) 10 0 \ / - n e+n 1, ~ ~ N
20 o \ / - n Me n 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 rused aromatic rings.
From the foregoing it is apparent that the metal can be monovalent, divalent, or trivalent metal. The metal c~n, for example, be an alkali metal, such as lithium, sodium, or potassium; an alkaline oarth metal, such as magnesium or calcium;
35 or ~n earth metal, such as boron or aluminum. Gener-ally any ,sonovalent, divalent, or trivalent metal known to be a useful chelating metal can be employed.

, . .

. .

Z completes a heterocyclic nucleus cont~in-ing at least two fused ~rom~tic rings, at one of which is an azole or azine ring. Additional rings, including both aliphatic and aromatic rings, cRn be 5 fu3ed with the two required rings, if required. To ~ avoid adding molecular bulk without improving on function the number of ring atoms is preferably ma~n-tained at 18 or less.
Illustrative of useful chelsted oxinoid 10 compounds are the following:
C0-1 Aluminum trisoxine [~.k.a., tris(8-quinolinol) aluminum]
C0-2 Magnesium bisoxine [a.k.a., bis(8-quinolinol) magnesium]
C0-3 Bis[benzo{f}-8-quinolinol] zinc C0-4 Bis(2-methyl-8-quinolinolato) aluminum oxide C0-5 Indium trisoxine [a.k.a., trls(8-quinolinol) indium3 C0-6 Aluminum tris(5-methyloxine) [a.k.a., tris(5-methyl-8-quinolinol~
aluminum C0-7 Lithium oxine (a.k.a., 8-quinolinol lithium3 ; 25 C0-8 Gallium trisoxine [a.k.a, tris(S-chloro-8 - quinolinol) gallium3 C0-9 Calcium b~s(5-chlorooxine) [2.k.s, bis(5-chloro-8-quinolinol) cal-cium3 C0-10 Poly~zinc (II)- bis(8 -hydroxy-5-quino-linyl)methane]
C0-11 Dilithium epindolidione In the organic EL devices of the invention 35 it is possible to m~intain a current density compati-ble with efficient ligh~ emission while employing ~
relatively low voltage ~cross the electrodes by lim-~2135~ii3~

iting the total thickne~s of the organic luminescent medium to less than l ~m (10,000 Angstroms). At ~
thickness of less than 1 ~m an applied voltage of 20 volts results in a field potential of Æreater than 2 5 X 105 volts/cm, which is compatible with efficient light emission. An order of magnitude reduction ~to 0.1 ~m or lOOO Angstroms) in thickness of the organic luminescent medium, allowing further reduc-tions in applied voltage and/or increase in the field lO potential and hence current density, are well within device construct~on capabilities.
One function wh~ch the organic luminescent medium performs is to provide a dielectric barrier to prevent shorting of the electrodes on electrical 15 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 lumi-nescent material, such as anthracene, for example, 20 the EL devices of this invention are capable of fab-rication 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 25 being ~ligned to provide a continuous conduçtion 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 30 still achieving acceptable EL device performance and reliability.
The preferred materials for forming the organic luminescent medium are each capable of fabri-cation in the form of a thin film--that is, capable 35 of being fabricated as a continuous layer having a thickness of less than 0.5 ~m or 5000 Angstroms.

When one or more of the layers o~ the organic luminescent medium are solvent coatPd, a film forming polymeric binder can be convenlently code-posited with the active material to assure a continu-5 ous layer free of structural defects, such ~s pinholes. If employed, A binder must, of course, it~elf exhibit a high dielectric strength, preferably at least about 2 X 106 volt/cmO Suitable polymers cen be chosen from a wide variety o~ known ~olvent CQSt 10 addition and condensation polymers. Illustrative of suitable addition polymers are polymers and copoly-mers (including terpolymers) of styrene, t-butylsty-rene, N-vinyl carbazole, vinyltoluene, methyl methac-rylate, methyl acrylate, acrylonitrile, and vinyl 15 acetate. Illustrative of suitable condensation poly-mers are polyesters, polycarbonates, polyimides, and polysulfones. To avoid unnecessary dilution of the active material, binders are preferably limited to less than 50 percent by weight, based on the total 20 weight of the material forming the layer.
The preferred active materials forming the organic luminescent medium are each film forming materials and capabIe of vacuum vapor deposition.
Extremely thin defect free continuous layers can be 25 formed by vacuum vapor deposition. Specifically, individual layer thicknesses as low as about 50 Angstroms can be present while still realizing s~tis-factory EL device performance. Employing a vacuum vapor deposited porphorin~c compound as a hole 30 in~ecting layer, a film forming aromatic tertiary amine as a hole transporting layer (which c~n in turn be comprised of a triarylamine layer and a tetrsaryl-diamine layer), ~nd a chelated oxinoid compound as an electron in~ectin~ and transporting layer, individual 35 layer thicknesses in the range of from about 50 to 5000 Angstroms aLe contemplated, with layer thick-nesses in the range of from lO0 to 2000 Angstroms ~2~

being preferred. It ~s generally preferred that the overall thickness of the organic luminescen~ medium be at least ~bout 1000 Angstroms.
The anode and cathode of the organic EL
device can each take any convenient conventional form. Where it is intended to trsnsmit light from the organic EL device through the anode, this can be conveniently achieved by coating a thin conductive layer onto a light transmisslve substrate- e.g., a transparent or substantifllly transparent glass plate or plastic film. In one form the organic EL devlces of this invention can follow the historical practice of including a light transmissive anode formed of tin oxide or indium tin oxide coated on a glass plate, a9 disclosed by Gurnee et al U.S. Pstent 3,172,862, Gurnee U.S. Patent 3,173,050, Dresner, "Double In~ec-tion Electroluminescence in Anthracene;', RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Patent 3,710,167, cited above. While any light transmissive polymeric film can be employed as a substrate, Gillson U.S. Patent 2,733,367 and Swindells U.S. Pat-ent 2,941,104 disclose polyméric films specific~lly selected for this purpose.
As employed herein the term "light tran~mis-~ive" means simply that the layer or element under discusslon transmits greater than 50 percent of the light of at least one wavelength it receive~ and preferably over at least a 100 nm interval. Since both specular (unscattered) and diffused (scattered) emitted light are desirable device outputs, both translucent and transparent or substantially trans-parent materials are useful. In most instances the light transmissive layers or elements of the organic EL device are also colorless or of neutral optic~l density -that i~, exhibiting no markedly higher absorption of light in one wavelength range ~s com-pared to another. However, it is, of course, recog-~:2~S~
-~4-nized that the light transmis3ive electrode supports or sep~r~te superimposed films or elements can be tailored in their ligh~ ~bsorption properties to ACt t as emission trimming filters, if de ired. Such an 5 electrode construction is di~closed, for example, by Fleming U.S. Patent 4,035,686. The light tran~mi~-sive conductive layers of the electrodes, where fab-ricated of thicknesses approximating the wavelengths or multiples of the light wavelengths received can 10 act as interference filters.
Contrary to historical practice, in one pre-ferred form the organic EL devices of this invention emit light through the cathode rather than the anode. This relieves the anode of any requirement lS that it be light transmissive, and it is, in fact, preferably opaque to light in this form of the inven-tion. Opaque anodes can be formed of ~ny metal or combination of metals having a suitably high work function for anode construction. Preferred anode 20 metals have a work function of greater than 4 elec-tron volts (eV). Suitable anode metals can be chosen from among the high (> 4 eV~ work function metals listed below. An opaque anode can be formed of an opflque metal layer on a support or as a separate 25 metal foil or sheet.
The organic EL devices of this invention can employ a c~thode constructed of any met~l, including any high or low work function metal, heretofore tAught to be useful for this purpose. Unexpected 30 f~brication, performance, and stAbility advantages have been realized by forming the cathode of A comhi-nfltion of a low work functlon metAl and ~t least one other metAl. A low work ~unction metal i3 herein defined as a metal having a work function of less 35 than 4 eV. Generally the lower the work function of the metal, the lower the voltage required for elec-tron in~ection into the organic luminescent medium.

~L26i5~

However, alkali metals, the lowest work function met-als, are too reactive to achieve stable EL device performance with simple device constructions ~nd con-struction procedures ~nd ar~ excluded (apart from 5 impurity concentrations) from the preferred cathodes ~ of this invention.
Available low work functlon metal choices for the cathode (other alkali metals) are listed below by periods of the Periodic Table of Elements 10 and categorized into 0.5 eV work function groups.
All work functions provided are taken Sze, Physics of Semiconductor Devices, Wiley, N.Y., 1969, p. 366.
Work Function Period Element By eV Grou~
2 Beryllium 3.5 - 4.0 3 Magnesium 3.5 - 4.0 4 Calcium 2.5 - 3.0 Scandium 3.0 - 3.5 Titanium 3.5 - 4.0 Manganese 3.5 - 4.0 Gallium 3.5 - 4.0 Strontium 2.0 - 2.5 Yttrium 3.0 - 3.5 Indium 3.5 - 4.0 6 Barium ~2.5 Lanthanum 3.0 - 3.5 Cerium 2.5 - 3.0 Praseodymium 2.5 - 3.0 Neodymium 3.0 - 3.5 Promethium 3.0 - 3.5 Samarium 3.0 - 3.5 Europium 2.5 - 3.0 Gadolinium 3.0 - 3.5 Terbium 3.0 - 3.5 Dysprosium 3.0 - 3.5 Holmium 3.Q - 3.5 ~: Erbium 3.0 - 3.5 Thulium 3.0 - 3.5 Ytterbium ~.5 - 3.0 Lutetium 3.0 - 3.5 Hafnium ~3.5 7 Radlum 3.0 - 3.5 Actinium 2.5 3.0 Thorium 3.0 - 3.5 Uranium 3.0 - 3.5 From the foregoing listing it is apparent 10 that the available low work function metals for the mo~t part belong to the Group IIa or alkaline earth group of metals, the Group III group of metals (lncluding the rare earth metals- i.e. yttrium and the lanthanides, but excluding boron and aluminum), 15 and the actinide groups of metals. The aIkaline earth metals, owing to their ready availability, low cost, ease of handling, and minimal adverse environ-mental impact potential, constitute a preferred class of low work function metals for use in the cathodes 20 of EL devices of this invention. Magnesium and cal-cium are particularly preferred. Though signifl-; cantly morP expensive, the included Group III metals, particularly the rare earth metals, possess similar advantag~s ~nd are specifically contemplated as pre-25 ferred low work function met~ls. The low work func-tion metals exhibiting work functions in the range of from 3.0 to 4.0 eV are generally more stable than metals exhibiting lower work functions and are there-fore generally preferred.
A s cond metal included in the construction of the cathode has as one prlmary purpose ~o increase the stability (both stor~ge and operational) of the cathode. It can be chosen from among any metal other than an alkali mst~l. The second mekal can ~tself be .
35 a low work function metal and thus be chosen from the met~ls listed above having a work function o less than 4 eV, with the same preferences above discussed : :

:

.
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, ' 12~

being ~ully applic~ble. To the extent that the ~ec-ond metal exhibits a low work function it c~n, of course, ~upplement the fir~t metal in facilit~ting electron in;ection.
Alternatively, the ~econd metal c~n be cho-sen from any of the various metals having ~ work function greater than 4 eV, which includes the ele-ment~ more resist~nt to oxid~tion and therefore more commonly fabric~ted as metallic elements. To the 10 extent the second metal remains invariant in the org~nic EL device as fabricated, it contributes to the stability of the device.
Available higher work function (4 eV or greater) met~l choices for the cathode are listed 15 below by periods of the Periodic T~ble of Elements and categorized into 0.5 `eV work function groups.
Work Function Period Element BY eV GrouP
2 Boron ~4.5 Carbon 4.5 - 5.0 3 Aluminum 4.0 - 4.5 4 V~nadium 4.0 - 4.5 Chromium 4.5 - 5.0 Iron 4.0 - 4.5 Cobalt 4.0 - 4.5 Nickel ~4.5 Copper 4.0 - 4.5 Zinc 4.0 - 4.5 Germanium 4.5 - 5.0 Arsenic 5.0 - 5.5 Selen~um 4.5 - 5.0 Molybdenum 4.0 ~ 4.5 Technetium 4.0 - 4.5 Ruthenium 4.5 - 5.0 Rhodium 4.5 - 5.0 Palladium 4.5 - 5~0 Silver 4.0 - 4.5 Cadmlum 4.0 - 4.5 Tin 4.0 - 4.5 Antimony 4.0 - ~.5 Tellurium 4.5 - 5.0 6 Tantslum 4.0 - 4.5 Tungsten ~4.5 Rhenlum ~5.0 Osmium 4.5 - 5.0 Irid1um 5.5 - 6.0 lo Platinum 5.5 - 6.0 Gold 4.5 - 5.0 Mercury ~4.5 Lead ~4.0 Bismuth 4.0 - 4.5 Polonium 4.5 - 5.0 From the foregoing llsting of avail~ble met-als having a work function o~ 4 eV or greater attrac-tive higher work function metals for the most p~rt ~re Aecounted for ~luminum, the Group Ib metals ~cop-20 per, s11ver, and gold), the metals in Groups IV, V,and VI, and the Group VIII transition metals, par-ticul~rly the noble metals from this group. Alumi-num, copper, silver, gold, tin, lead, bismuth, tel-lurium, ~nd antimony are particularly preferred 25 higher work function second metals for incorporation in the cathode.
There are several reasons for not restrict-~ ing the choice~of the second metal based on either : its work function or oxidative stability. The second 30 metal is only a minor component of the cathode. One of its primary functions is to stabilize the f~rst, low work functiDn metal, and, surprisingly, it accom-plishes th~s objective independent of its own work function and susceptibility to oxidation.
A xecond valuable function which the second met~l performs is to reduce the sheet resis~ance of the cathode as a function of the thickness of the :: :

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cathode. Since acceptably low sheet resi~tance lev-els (less than 100 ohms per square) can be reallzed at low csthode thicknesses (less than 250 Angstroms), cathodes can b~ formed which exhibit high level~ of 5 light transmission. This permits highly stable, thln, transparent cathodes of acceptably low resis-tance levels and high electron in~ecting eff1ciencies to be achieved for the first time. This in turn per-mits (but does not require~ the organic EL devices of 10 this invention to be constructed with 11ght transmis-sive cathodes and frees the organic EL devices of any necessity of having a light tran~missive anode to achieve light emission through an electrode area.
A third valuable ~unction which the second 15 metal has been observed to perform is to facilitate vacuum vapor deposi~ion of a first metal on~o the organic luminescent medium of the EL device. In vapor deposition less metal is depos~ted on the walls of the vacuum chamber and more metal is deposited on 20 the organic luminescent medium when a second metal is also deposited. The efficacy of the second metal in stabilizing organic EL device, reducing the sheet resistance of thin cathodes, and in improving accep-tance of the first metal by the organic luminescence 25 medium is demonstrated by the examples below.
Only a very small prvportion of a ~econd metal need be present to achieve these advantages.
Only about 0.1 percent of the totfll metal atoms of the cathode need be accounted for by the second metal 30 to achieve R substantial improvement. Where the sec ond metal i~ itself a low work function metal, both the first and second metals are low work function met~ls, and it is immaterial which ls regarded AS the first metal and which is regarded as the second 35 metal. For example, the cathode composition can range from about 0.1 percent of the metal atoms of the cathode b~ing accounted for by one low work func-tion metal to about 0.1 percent of the total metal atoms being accounted for by a second low work func-tion metal. Preferably one of the two metal account for at le~st 1 percent snd optimally at least 2 per-S cent of the total metal present.
Wh~n the second metal is a relatively higher (at least 4.0 eV) work function metal, ~he low work function metal preferably accounts for &reater than 50 percent of the total met~l atoms of the c~thode.
lO This is to avoid reduction in electron inject~on efficiency by the cathode, but it is also predicated on the observation that the benefits of adding a sec-ond metal are essentially realized when the second metal accounts for less than 20 percent of the total 15 metal atoms of the cathode.
Although the foregoing discussion has been in terms of a binary comb~nation of metals forming the cathode, it is, of course, appreciated that com-binations of three, four, or even higher numbers of 20 metals are possible and can be employed, if desired.
The proportions of the first met~l noted above can be accounted for by any convenient combination of low work function metals and the proportions of the sec-ond metal can be accounted for any combination of 25 high and/or low work function metals.
While the second metal or metals can be relied upon to enhance electrical conductivity, their mlnor proportion of the total cathode metal renders it unnecessary ~hat these metals be present in an 30 electricallg conducting form. The second metal or metals can be present as compounds (e.g., lead, tin, or antimony telluride) or in an oxidized form, ~uch as in the form of one or more metal oxides or salts.
Slnce the first, low work ~unction metal or metals 35 account for the ma~or proportion of the cathode metsl content and are relied upon for electron conduction, ~ 31-they are preferably employed in their element~l form, although some oxidation may occur on aging.
The manner ~n which the presence of a second metal physically intervenes to enhance cathode ~t~-5 bllity and light transmission enhancement whilereducing sheet resistance can be appreciated by com-paring Figures 4 and 5. Figure 4 is a micrograph, enlarged to the scale indic~ted, of a vacuum vapor deposited conventional, prior art cathode consisting 10 of magnesium. The thickness of the magnesium coating is 2000 Angstroms. The non-uniformity of the coat-ing, detracting both from its electrical conductivity and its ability to transmit light, is readily appar-ent. Because of its non-uniformlty the coating is 15 also more readily penetrable and therefore more 5US -ceptible to oxidative degradation.
In direct contrast, the cathode of Figure 5 illustrating the invention, also 2000 Angstroms in thickness, is smooth and featureless. This cathode 20 is formed by the vacuum vapor deposition of magnesium and silver, with the magnesium and silver being pres-ent in an atomic ratio of 10:1. That is, the silver atoms are present in a concentration of 9 percent of total metal atoms present. The imperceptibly low 25 granularity of the invention cathode is indicative of a higher and more uniform coverage of the deposition substrate. Identical glass substrates coated first with indium tin oxide and then oxine (CO-l~ were employed in forming the Figures 4 and S coatings.
In depositing the first metal alone onto a substrate or onto the organic luminescent medium, whether from solution or, preferably, from ~he vapor phase, initial, spatially separated deposits of the first metal form nuclei for subsequent deposition.
35 Subsequent deposition leads to the growth of thcse nuclei into microcrystals. The result is ~n uneven and random distribution of microcrystals, leading to a non-uniform cathode. By presenting a second metal during ~t least one of the nucleation and growth stages ~nd, preferably, both, the high degree of sym-metry which ~ single element affords is reduced.
5 Since no two substances ~orm crystal cells of exactly the same habit and size, any second metal reduces the degree of symmetry and at least to some extent acts to retard microcrystal growth. Where the first and second metals have distinctive crystal habits, spa-10 tial symmetry is ~urther reduced and microcrystalgrowth is further retarded. Retarding microcrystal growth favors the ~ormation of additional nucleation sites. In this way the number of deposition ~ites is increased and a more uniform coatlng is achieved.
Depending upon the specific choice of met-als, the second metal, where more compatible with the substrate, can produce a disproportionate number of the nucleation sites, with the first metal then depositing at these nucleation sites. Such R mecha-20 nism may, if fact, account for the observation that, with a second metal present, the efficiency with which the first metal is accepted by a substrate is significantly enhanced. It has been observed, for ~xample, that less deposition of the first metal 25 occurs on vacuum chamber walls when a second met~l is being codeposited.
The first and second metals of the cathode are intimately intermingled, being codeposited. That is, the deposition of neither the first nor second 30 metals is completed before at least a portion of the remaining metal ~s deposited. Simultan~ous deposi-tion of the first and second metals is generally pre-ferred. Alternatively, successive incremental depo-sitions of the first and second metals can be under-3S taken, which at their limlt may approximate concur-rent deposition.

~2~

While not required, the cathode, once formed can be given post treatments. For example, the cath-ode may be heated within the stability limlts of the substrate in a reducing ~tmosphere. Other action on 5 the cathode can be undertaken ag A conYentionally attendflnt feature of lead bonding or device encapsu-lation.
ExamPles The invention and its advantages are further 10 illustrated by the specific examples which follow.
The term "atomic percent" indicates the percentage of a particular metal present, based on the total number of metal atoms present. In other words, it is analo-gous to mole percent, but is based on atoms rather 15 than molecules. The term "cell" as employed in the examples denotes an organic EL device.
Example l Three LaYer~ anic Luminescence Medium An EL device containing a three layer organic luminescent medium satisfying the require-20 mentq of the invention was constructed in the follow-ing manner:
a) A transparent anode of indium tin oxide coated glass was polished with 0.05 ~m alumina abra-sive for a few minutes, followed by ultrasonic clean-25 ing in a l:l (volume) mixture of isopropyl alooholand distilled water. It was rinsed with isopropyl alcohol and then immersed in toluene vapor for about 5 minutes.
b) A hole in~ecting PC-lO (350 A) layer was 30 deposited on the anode by vacuum deposition. PC - lO
was evaporated from a quartz boat using a tungsten filament.
c) A hole tr~nsporting ATA-l ~350 A) layer was then depo~ited on top of the PC-lO layer. ATA-l was 35 also evaporated from a quartz boat using a tungsten filament.

-3~-d) An electron in~ecting and transporting CO-1 (600 A) layer was then depoqited on top of the ATA-l layer. CO-1 was also evaporated from a quartz boat using a tungsten filament.
e) On top of the CO-l layer was depo ited a 2000 A cathode formed of a 10:1 atomic ratio of Mg and Ag.
When a positive voltage was connected to the anode ~nd the cathode was connected to ground, elec-lO troluminescence was visible through the tr~nsparentanode. Operating the EL device for 500 hours at a constant current denslty of 5 mA/cm2, only a modest voltage increase of from 6 to 7.2 volts was required to maintain the light output in the range of from 15 0.08 mW/cm2, initial output, to 0.05 mW/cm2, final output. This demonstrated a sustained high level of performance for the EL device.
ExamPle 2 Two LaYer Control An EL device was constructed identically to 20 that of Example 1, except for omitting the PC-10 layer.
While this EL device required a similar ini-tial voltage as the EL device of Example 1 to achleve a current density of 5 mA/cm2 and therefore a simi-~5 lar initial light output, an &ttempt to operate theEL device at a constant current density resulted in EL device failure a~ter only 160 hours of operation.
Whereas an initial applied po~ential of 6.5 volts produced an initial light output of 0.1 mW/cm2, 30 after 160 hours of operation, a potential of 20 volts was required to achieve a light output of 0.05 mW/cm .
ExamPle 3 Metal Free PorPhyrinic ComPound An El device was constructed identically to 35 that of Exa~ple 1, except for substituting PC - 7, metal free phthalocyanine, for PC-10, copper phthalo-cyanine. When tested under the same conditions as ~L2~

reported in Example 1, identical results were obtained. This demonstrated that a central metal atom is not required in a porphyrinic compound.
ExamPles 4-9 Other Porphyrinic Compound~
Five additional EL devices were constructed as described in Ex~mple 1, ~xcept that the porphyr-inic compound and ATA-l layer~ were 375 Angstroms in thickness and a different porphyrinic compound was incorporated in esch device. Initial efficiencies 10 and applied voltages when oper~ted &t a 5 mA~cm2 current density are listed ln Table I.
Table 1 PorphYrin EfficiencY (W/W)Volta~e PC-ll 2.2 X 10 10.5 15PC-12 4.3 X 1~ 6.2 PC-13 4.8 X 10 3 5.2 PC-14 3.9 X 10 3 5.8 PC-15 2.4 X lO 3 6.6 PC-16 3.~ X 10 3 7.4 20 The EL devices exhibited performance characteristics over extended periods of operation comp~rable to those of the EL device of Example 1.
ExamPle 10 Hi~her Current Densities Devices as described in Examples 1 and 2 25 were again tested, but with the maintained current density being increased to 20 mA/cm2.
In testing the EL device of the invention, ~ corresponding to that of Example 1, light intensity ; declined from an initial level of 0.45 mW/cm2 to 30 0.06 mW/cm2 at the end of 500 hours, with the ini-tial and final applied potent~als being 7 and 11 volts, respectively.
In testing the control EL~ corresponding to th~t of Example 2, catastrophic cell failure occurred 35 after only 17 hours of operation. Again, the supe-rior stability of the EL device of the invention was clearly demonstrated.

:
, .

Example 11-13 Varied Hole TransPortin~ LaYers EL deviceq each containing a three layer organic luminescent medium satisfylng the require-ments o~ the invention were constructed in the fol-5 lowing m~nner:
- a) A transparent ~node of indium tin oxidP
coated gla~s was polished with 0O05 ~m alumina ~bra-sive for a few minutes, ~ollowed by ultrasonic clean-ing in ~ 1:1 (volume) mixture ~f isopropyl alcohol lO and distilled water. It was rinsed with isopropyl alcohol and blown dry with nitrogen.
b) A hole injecting PC-10 (375 A) layer was deposited on the anode by vacuum deposition. PC-10 was ev~porated from a quartz boat using a tungsten 15 filamentO
c) A hole transporting (375 A) l~yer was then deposited on top of the PC-10 layer. The hole trans-porting material, an aromatic tertiary amine identi-fied in T~ble II below, was also evaporated from a 20 ~uartz boat using a tungsten filament.
d) An electron in~ecting and transporting CO-l (600 A) layer was then deposited on top of the hole tr~nsporting layer. CO-1 w~s also ev~porated from a quartz boat using a tungsten fil~ment.
e) On top of the CO-l layer was deposited a 2000 R cathode formed of a 10:1 ~tomic ratio of Mg and A~. `
Table II
Li~ht Output mW/cm2 30 Cell ATA 050 100 500 1000 2000 (hrs.) Ex. 11 1 1.15 0.25 0.1 <0.1 Ex. 12 7 0.8 0.6 0.5 0.3 0.2 0.12 Ex. 13 8 0.5 0.35 0.3 0.22 0.17 The EL devices of Examples ll and 12 were 35 driven ~t a current density of 40 mA/cm2 while the EL device of Example 13 was driven at a current den-sity of 20 mA/cm2. These high current densities .

were chosen to accelerate testing. Light outputs at these elevated current density levels were well in excess of that required to produce light o$ adequate brightness for di~play applications. All of the 5 devices demonstrated acceptable stability level~.
The results further demonstrate the superiority of tetraaryldiamines of the type called for by formula (~I).
ExamPles 14 and 15 MultiPle Hole Transporting Layer~
An EL device representing Example 14 WAS
prepared in the following manner:
R) A transparent anode of indium tin oxide coated glass was polished with 0.05 ~m alumina abra-sive for a few minutes, followed by ultrasonic clean-15 ing in a 1:1 (volume) mixture of isopropyl alcoholand distilled water. It was rinsed with isopropyl alcohol and blown dry with nitrogen.
b) A hole injecting PC-10 (375 A) layer was deposited on the anode by vacuum deposition. PC-10 20 was evaporated from a quartz boat using a tungsten filament.
c) A triarylamine (ATA-l) first hole transport-ing (185 A) layer was then deposited on top of the PC-10 layer. ATA-l was also evaporated from a quartz 25 boat using a tungsten filament.
d) A tetraarylamine (ATA-7) second hole trans-porting (185 A) layer was then deposited on top of the ATA-l lflyer. ATA-7 was also evaporated from a quartz boat using a tungsten filament.
e) An electron injecting and transporting CO-1 (600 R) layer was then deposited on top of the hole tran~porting layer. CO-l was also evaporated from a quartz boat using a tungsten filament.
f) On top o$ the CO-1 layer was depo~ited a 35 2000 A cathode formed of a 10:1 atomic ratio of Mg ~nd Ag.

An EL device representing Example 15 was constructed identically to that of Example 14, except that th~ order of depositlon of the hole tran~porting t l~yers c) and d) was reversed.
In testing both EL devices were electric~lly biased to m~intain a current density of 40 mA/cm .
The results are summarized in Table III.
T~ble III
Li~ht OUtPut mW/cm 10 Cell ATA 0 50 100 500 (hrs.) Ex. 14 1/7 0.8 0.5 0.5 0.45 Ex. 15 7/1 1.15 0.25 0.1 <0.1 Both EL devices exhibited satisfactory sta-bility. The 40 mA/cm current density level was 15 much higher than required to obtain adequate bright-ness levels. High current density levels were chosen to exaggerate variances in light output levels ~nd to predict light variances to be expected on operating the EL devices at lower current densities over much 20 longer time periods.
By comparing the performance o~ the two EL
devices, it is apparent that a substantial improve-ment in performance can be realized by locating the tetraaryldiamine hole in~ecting layer in contact with 2S the electron in~ecting layer. By comparing Examples 11 and 12 in Table II and with Example 14 in Table III it is app~rent th~t when both a tetr~aryldiamine - and a triarylamine hole transporting layer are pres-ent in ~ ~ingle EL device with the tetraaryldiamine 30 hole in~ecting layer in contact with the electron in~ecting layer performance is realized that is supe-rior to that obtained when elther of the two ~mlne l~yers 1-Q omitted.
The invention has been described in detail 35 with particul~r reference to preferred embodiments thereof, but it will be understood that v~riations 156~

and modifications can be efI'ected within the 3pirit and scope of the invention.

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Claims

1. 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 said organic hole injecting and transporting zone is comprised of a layer in contact with said anode contain-ing a hole injecting porphyrinic compound and a layer containing a hole transporting aro-matic tertiary amine interposed between said hole injecting layer and said electron injecting and transporting zone.

2. An electroluminescent device according to claim 1 in which said cathode contains a metal having a work function of less than 4 eV other than an alkali metal and is light transmissive.

3. An electroluminescent device according to claim 1 in which said hole transporting layer is comprised of a tetraphenyldiamine layer contacting said elec-tron injecting and transporting zone and a triarylamine layer contacting said hole inject-ing layer.

4. An electroluminescent device according to claim 1 in which said aromatic tertiary amine is a diphenylamine.

5. An electroluminescent device according to claim 4 in which said diphenylamine is a carbazole.

6. An electroluminescent device according to claim 1 in which said aromatic tertiary amine is a triphenylamine.

7. An electroluminescent device according to claim 6 in which said triphenylamine is N,N,N-tri-phenylamine.

8. An electroluminescent device according to claim 1 in which said aromatic tertiary amine sat-isfies the structural formula:

wherein Q1 and Q2 are independently aromatic tertiary amine moieties and G is a linking group chosen from the class con-sisting of phenylene, cycloalkylene having 5 to 7 ring carbon atoms, or alkylene having from 1 to 6 carbon atoms, or a carbon to carbon bond.

9. An electroluminescent device according to claim 8 in which said aromatic tertiary amine is a triphenylamine satisfying the structural formula:

where R1 and R2 each independently represents a hydrogen atom, a phenyl group, or an alkyl group of from 1 to 6 carbon atoms or R1 and R2 together represent the atoms completing a cycloalkyl group containing from 5 to 7 ring carbon atoms and R3 and R4 each independently represents a phenyl group which is in turn substituted with a diphenylamino group, as indicated by the structural formula:

wherein R5 and R6 are independently selected phenyl groups.

10. An electroluminescent device according to claim 1 in which said aromatic tertiary amine is tetraphenyldiamine.

11. An electroluminescent device according to claim 10 in which said tetraphenyldiamine satis-fies the formula:
(VI) wherein Are is a phenylene group, n is an integer of from l to 4, and Ar, R7, R8, and R9 are independently selected phenyl groups.

12. An electroluminescent device according to claim 1 in which said porphorinic compound is a metal containing porphorinic compound which satisfies the structural formula:

wherein Q is -N= or -C(R)=;
M is a metal, metal oxide, or metal halide;
R is hydrogen, alkyl, phenalkyl, phenyl, or alkylphenyl, each alkyl moiety containing from l to 6 carbon atoms, and T1 and T2 represent hydrogen or together com-plete a unsaturated 6 membered ring containing ring atoms chosen from the group consisting of carbon, nitrogen, and sulfur atoms.

13. An electroluminescent device according to claim 1 in which said porphorinic compound is a metal free porphorinic compound which satisfies the structural formula:

wherein Q is -N= or -C(R)=;
R is hydrogen, alkyl, phenalkyl, phenyl, or alkylphenyl, each alkyl moiety containing from 1 to 6 carbon atoms, and T1 and T2 represent hydrogen or together com-plete a unsaturated 6 membered ring containing ring atoms chosen from the group consisting of carbon, nitrogen, and sulfur atoms.

14. An electroluminescent device according to claim 1 in which said electron injecting and transporting zone is comprised of a stilbene or che-lated oxinoid compound.

15. An electroluminescent device according to claim 14 in which said chelated oxinoid compound is represented by the formula:

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.

16. An electroluminescent device comprising in sequence an anode, a hole injecting layer comprised of a porphyrinic compound, a hole transporting layer comprised of an aro-matic tertiary amine, an electron injecting and transporting layer com-prised of a chelated oxinoid compound, and a cathode comprised of a layer consisting essen-tially of a plurality of metals other than alkali metals, at least one of said metals having work func-tion greater than 4 eV.

17. An electroluminescent device according to claim 16 in which said anode is opaque and said cathode is light transmissive.

18. An electroluminescent device according to claim 16 in which said metal having a work func-tion of less than 4 eV includes at least one alkaline earth metal, rare earth metal, or group III metal.

19. An electroluminescent device according to claim 16 in which said cathode includes at least one metal having a work function greater than 4 eV.

20. An electroluminescent device comprising in sequence an opaque anode, a hole injecting layer comprised of a phthalocya-nine, a first hole transporting layer comprised of a tetraphenyldiamine, a second hole transporting layer comprised of a triphenylamine, an electron injecting and transporting layer com-prised of a chelated oxinoid compound, and a light transmissive cathode comprised of a layer consisting essentially of a plurality of metals other than alkali metals, at least one of said metals having work function greater than 4 eV and being chosen from the group consisting of magnesium, a rare earth metal, or indium, and at least one other of said metals being chosen from the group consisting of aluminum, copper, sil-ver, gold, tin, lead, bismuth, tellurium, indium, and antimony.