Improved blue emitting internal junction organic electroluminescent device

Abstract: An internal junction organic electroluminescent device (200) is disclosed comprised of, in sequence, an anode (202), an organic hole injecting and transporting zone (210), an organic electron injecting and transporting zone (212), and a cathode (216). The organic electron injecting and transporting zone (212) is comprised of an electron injecting layer (222) in contact with the cathode (216) and, interposed between the electron injecting layer (222) and the organic hole injecting and transporting zone (210), a blue emitting luminescent layer comprised of an aluminum chelate containing a phenolato ligand and two R s -8-quinolinolato ligands, where R s substituents are chosen to block the attachment of more than two substituted 8-quinolinolato ligands to the aluminum atom. The presence of the phenolato ligand shifts device emission to the blue region of the spectrum and increases emission efficiency. Device emission is shifted to even shorter blue wavelengths and increased operating stability can be realized by the incorporation of a pentacarbocyclic aromatic fluorescent dye.

Description: The invention relates to internal junction organic electroluminescent devices. More specifically, the invention relates to organic electroluminescent devices of the type in which an organic medium contains an internal junction formed at the interface of an electron injecting and transporting zone in contact with a cathode and a hole injecting and transporting zone in contact with an anode. Electroluminescent devices (hereinafter also referred to as EL devices) contain spaced electrodes separated by an electroluminescent medium that emits light in response to the application of an electrical potential difference across the electrodes. Through intensive investigations and a series of recent inventions organic electroluminescent devices of improved characteristics, both in terms of fabrication feasibility and operating performance have been developed. In current preferred forms organic EL devices are comprised of an anode, an organic hole injecting and transporting zone in contact with the anode, an electron injecting and transporting zone forming a junction with the organic hole injecting and transporting zone, and a cathode in contact with the electron injecting and transporting zone. When an electrical potential is placed across the electrodes, holes and electrons are injected into the organic zones from the anode and cathode, respectively. Light emission results from hole-electron recombination within the device. Perry and others U.S. Patent 4,950,950 discloses organic EL devices in which the hole injecting and transporting zone is comprised of (a) a layer in contact with the anode containing a hole injecting porphyrinic compound and (b) a layer containing a hole transporting silazane interposed between the hole injecting layer and the electron injecting and transporting zone. The metal oxinoid charge accepting compounds are those disclosed to form the electron injecting and transporting zone in Tang and others U.S. Patent 4,769,292. Aluminum oxinate is set out in the Examples. Kushi and others, "The Crystal and Molecular Structure of Bis(2-methyl-8-quinolinolato)aluminum(III)-µ-oxo-bis(2-methyl-8-quinolinolato)aluminum(III)", J. Amer. Chem. Soc., 92(1), pp.91-96 (1970), discloses the preparation of the title compound. It is another object of this invention to provide a blue emitting organic EL device which exhibits both a high level of efficiency and a high level of stability as compared to conventional blue emitting organic EL devices. It is a further object to provide a blue emitting organic EL device that is shifted in its emission to shorter blue wavelengths. In one aspect, this invention is directed to an internal junction organic electroluminescent device comprised of, in sequence, an anode, an organic hole injecting and transporting zone, an organic electron injecting and transporting zone, and a cathode. The organic electroluminescent device is characterized in that the organic electron injecting and transporting zone is comprised of an electron injecting layer in contact with the cathode and, interposed between the electron injecting layer and the organic hole injecting and transporting zone, a blue emitting luminescent layer comprised of a charge accepting compound of the formula: (II) (R S -Q)₂-Al-O-L where Q in each occurrence represents a substituted 8-quinolinolato ring nucleus, R S represents an 8-quinolinolato ring substituent chosen to block sterically the attachment of more than two substituted 8-quinolinolato ring nuclei to any one aluminum atom, O-L is phenolato ligand, and L is a hydrocarbon of from 6 to 24 carbon atoms comprised of a phenyl moiety. Figure 1 is a 1931 C.I.E. chromaticity diagram with color regions of specific interest delineated. Figure 2 is a schematic diagram of a conventional green emitting organic EL device. Figure 3 is a schematic diagram of a blue emitting organic EL device satisfying the requirements of the invention. The present invention is directed to a blue emitting organic EL device that exceeds the performance efficiencies of conventional green emitting organic EL devices. The terms "blue emitting" and "green emitting" are easy enough to understand and identify in most instances; but since there is a continuous spectrum of hues ranging from pure blue to pure green, a quantitative basis is required for precise delineation. This quantitative basis is provided by the 1931 C.I.E. chromaticity diagram shown in Figure 1. The 1931 C.I.E. chromaticity diagram is a widely accepted approach for quantifying hue within the visible spectrum. A full explanation of the 1931 C.I.E. chromaticity diagram is provided by Wyszecki and Stiles, Color Science, Concepts and Methods: Quantitative Data and Formulae , 2nd Ed., Chapter 3, Colorimetry, Wiley, 1982, pp. 117-143, and more succinct explanation is provided by James, The Theory of the Photographic Process , 4th Ed., Macmillan, 1977, Chapter 19, II, B. Colorimetry, pp. 563-565. Referring to Figure 1, fully saturated monochromatic hues ranging from 380 to 770 nm form a curve defining the saturation boundaries of the visible spectrum. Hues that lie within the curve boundary are to some extent desaturated, meaning that they tend more toward white. The x and y axes are employed as descriptors for precisely locating each visible hue. As herein employed the term "blue emitting" refers to the area of the diagram defined by the points extending from 430 to 499.2 nm to D, C, B and A and back to 430 nm. The area extending from 460 to 480 nm to C to B and back to 460 nm is perceived by the eye as being blue. The area extending from 430 to 460 nm to B to A and back to 430 nm is perceived by the eye as being bluish purple. The area extending from 480 to 499.2 nm to D to C and back to 480 nm is perceived by the eye as being greenish blue or bluish green. The area to right of points A, B, C and D are excluded, since the hue is so desaturated that the visual perception is primarily that of white. As herein employed the term "green emitting" refers to the area of the diagram defined by the points extending from 499.2 to 576 nm to E and D and back to 499.2 nm. It is in this area of the spectrum that conventional green emitting organic EL devices emit. To the right of the boundary defined by 499.2 and D the observed hue is green while the left of the boundary defined by 576 and E the observed hue is greenish yellow. A conventional green emitting organic EL device 100 is shown in Figure 2. An anode 102 of the device is shown constructed of a transparent support 104 and a thin transparent conductive layer 106. Overlying and in contact with the anode is an organic medium 108 formed by a hole injecting and transporting zone 110 in contact with the anode and an electron injecting and transporting zone 112 forming a junction 114 with the zone 110. The electron injecting and transporting zone is in contact with a cathode 116. In operation, when the cathode 116 is electrically biased to a negative potential with respect to the anode 102 holes are injected into the organic hole injecting and transporting zone 110 at its interface with the anode and transported across this zone to the junction 114. Concurrently electrons are injected into the electron injecting and transporting zone 112 at its interface with the cathode 116, and the injected electrons are transported toward the junction 114. Recombination of the holes and electrons occurs within the electron injecting and transporting zone adjacent the junction 114 resulting in electroluminescence within the electron injecting and transporting zone. The hue of the luminescence is determined by the composition of the electron injecting and transporting zone. The light emitted can leave the organic EL device in any direction--that is,, through the edges of the organic medium, the cathode and/or the anode. In the construction shown, which is most common, principal emission occurs through the transparent anode. While the electron injecting and transporting zone 112 of the conventional organic EL device 100 can take any of the varied forms disclosed in U.S. Patents 4,539,507, 4,769,292, 4,720,432, 4,885,211, and 4,950,950, best performance is realized when the zone 112 employs a metal oxinoid charge accepting compound. Of the various metal oxinoids, the most highly preferred are the tris-chelates of aluminum. These chelates are formed by reacting three 8-hydroxyquinoline moieties with a single aluminum atom. The specific examples of such aluminum compounds provided in U.S. Patents 4,539,507, 4,769,292, 4,720,432, 4,885,211, and 4,950,950 are aluminum trisoxine [a.k.a., tris(8-quinolinol) aluminum] and aluminum tris(5-methyloxine) [a.k.a. tris(5-methyl-8-quinolinol) aluminum]. These aluminum trisoxines are green emitting. The present invention is directed to the discovery of an organic EL device construction that improves on the efficiencies of conventional green emitting aluminum trisoxines used to form an electron injecting and transporting zone, but produces a blue emitting organic EL device. A preferred blue emitting organic EL device 200 satisfying the requirements of the invention is shown in Figure 3. The anode 202 is in its preferred form constructed of a transparent support 204 and a conductive layer 206 similarly as conventional anode 102 described above. The cathode 216 can also be identical to conventional cathode 116. An organic medium 208 contacting each of the anode and cathode and extending therebetween consists of a hole injecting and transporting zone 210 and an electron injecting and transporting zone 212. A junction 214 is formed at the interface of the zones 210 and 212. The hole injecting and transporting zone 210 can take any convenient conventional form and can, if desired, be formed of a single material, similarly as corresponding zone 110. In the preferred construction shown the hole injecting and transporting zone consists of a hole injecting layer 218 in contact with the anode and a contiguous hole transporting layer 220 interposed between the hole injecting layer and the electron injecting and transporting zone. Unitary and two layer hole injecting and transporting zones are illustrated by U.S. Patents 4,539,507, 4,769,292, 4,720,432, 4,885,211, and 4,950,950. A particularly preferred hole transporting layer 220 contains a hole transporting aromatic tertiary amine comprised of at least two tertiary amine moieties and includes attached to a tertiary amine nitrogen atom an aromatic moiety containing at least two fused aromatic rings. The electron injecting and transporting zone 212 is formed of an electron injecting layer 222, which is in contact with the cathode, and a contiguous electron transporting layer 224 that is interposed between layer 222 and the hole injecting and transporting zone 210. The electron transporting layer forms a junction 214 with the hole injecting and transporting zone 210. The electron transporting layer is comprised of a mixed ligand aluminum chelate, specifically a bis(R S -8-quinolinolato)(phenolato)aluminum(III) chelate serving as a charge accepting compound, where R S is a ring substituent of the 8-quinolinolato ring nucleus chosen to block the attachment of more than two 8-quinolinolato ligands to the aluminum atom. These compounds can be represented by the formula: (II) (R S -Q)₂-Al-O-L where Q in each occurrence represents a substituted 8-quinolinolato ligand, R S represents an 8-quinolinolato ring substituent chosen to block sterically the attachment of more than two substituted 8-quinolinolato ligands to the aluminum atom, O-L is phenolato ligand, and L is a hydrocarbon of from 6 to 24 carbon atoms comprised of a phenyl moiety. The advantage of employing an aluminum chelate with two substituted 8-quinolinolato ligands and a phenolato ligand is that all of the desirable physical properties of tris(8-quinolinolato)aluminum(III) chelates, the preferred green emitting luminophors of organic EL devices, are retained while emission is shifted to the blue region of the spectrum. More specifically, the combination of two substituted 8-quinolinolato ligands and a phenolato ligand produces an aluminum chelate that can be deposited from the vapor phase to form the electron transporting layer of the organic EL device. Vapor phase deposition is the preferred approach to construction of the organic layer sequence of organic EL devices. Vapor phase deposition allows extremely thin layers of well controlled thickness and uniformity to be deposited. No solvents or other extraneous materials need be brought into contact with the deposition substrate, the hole injecting and transporting zone, that would dissolve, contaminate or degrade the performance of this substrate zone. Vapor phase deposition has the further advantage of allowing the rate of deposition to be controlled and of allowing greater freedom and flexibility in device construction. The presence of the phenolato ligand is responsible for shifting emissions to the blue portion of the spectrum. As employed herein the term "phenolato ligand" is employed in its art recognized usage to mean a ligand bonded to the aluminum atom by the deprotonated hydroxyl group of a phenol. In its simplest form the phenolato ligand can be provided by deprononation of hydroxybenzene. Organic EL device performance has demonstrated that peak emission at a shorter wavelength than 500 nm and acceptable device stability (retention of at least a half of initial luminescent intensity for more than 50 hours) can be realized. In an effort to improve performance, substituted phenols were next investigated. It was observed that methoxy and dimethoxy substituted phenolato ligands exhibited relatively weak luminescent intensities. Since methoxy substituents are electron donating, phenols were also investigated with strongly electron withdrawing substituents, such as halo, cyano and α-haloalkyl substituents. Aluminum chelates with these ligands, though luminophors, did not undergo successful vapor phase conversions. From further investigations, illustrated by the Examples below, it has been determined that the preferred phenolato ligands for the aluminum chelates of formula II are derived from HO-L phenols, where L is a hydrocarbon of from 6 to 24 carbon atoms comprised of a phenyl moiety. This includes not only hydroxybenzene, but a variety of hydrocarbon substituted hydroxybenzenes, hydroxynaphthalenes and other fused ring hydrocarbons. Since monomethyl substitution of the phenyl moiety shorten emission wavelengths, it is preferred that the phenolato ligand contain at least 7 carbon atoms. Generally there is little advantage to be gained by employing phenolato ligands with very large numbers of carbon atoms. However, investigations of phenolato ligands with 18 aromatic ring carbon atoms have revealed high levels of stability. Thus, the phenoloato ligands preferably contain from 7 to 18 total carbon atoms. Aliphatic substituents of the phenyl moiety of phenolato ligand are contemplated to contain from 1 to 12 carbon atoms each. Alkyl phenyl moiety substituents of from 1 to 3 carbon atoms are specifically preferred, with the best overall characteristics having been observed to be produced with methyl substituents. Aromatic hydrocarbon substituents of the phenyl moiety are preferably phenyl or naphthyl rings. Phenyl, diphenyl and triphenyl substitution of the phenyl moiety have all been observed to produce highly desirable organic EL device characteristics. Phenolato ligands derived from α or β naphthols have been observed to produce aluminum chelates of exceptional levels of stability. A limited degree of emission shifting to shorter wavelengths is also realized, similar to that exhibited by hydroxybenzene derived phenolato ligands. By employing naphtholato ligand containing aluminum chelates in combination with blue emitting fluorescent dyes, described below, highly desirable device constructions are possible. From comparisons of ortho , meta and para substituted homologues of the various phenolato ligands it has been determined that little, if any, difference in performance is attributable to the position on the phenyl moiety ring occupied by the hydrocarbon substituent. In a preferred form the aluminum chelates satisfy the following formula: where Q and R S are as defined above and L¹, L², L³, L⁴ and L⁵ collectively contain 12 or fewer carbon atoms and each independently represent hydrogen or hydrocarbon groups of from 1 to 12 carbon atoms, with the proviso that L¹ and L² together or L² and L³ together can form a fused benzo ring. Although either or both of the 8-quinolinolato rings can contain substituents other than the steric blocking substituent, further substitution of the rings is not required. It is appreciated further that more than one substituent per ring can contribute to steric blocking. The various steric blocking substituent possibilities are most easily visualized by reference to the following formula: where L can take any form described above and R² to R⁷ represent substitutional possibilities at each of ring positions 2 to 7 inclusive of the 8-quinolinolato rings. Substituents at the 4, 5 and 6 ring positions are not favorably located to hinder sterically the bonding of three 8-quinolinolato nuclei to a single aluminum atom. While it is contemplated that large substituents at the 3 or 7 ring positions could provide sufficient steric hindrance, the incorporation of bulky substituents substantially increases molecular weight without enhancing molecular performance and therefore detracts from overall performance. On the other hand, the 2 ring position is suited to provide steric hindrance, and even a very small substituent (for example, a methyl group) in one of these ring positions provides an effective steric blocking substituent. For synthetic convenience it is specifically preferred that steric blocking substituents be located in the 2 ring positions. As employed herein the term "steric blocking" is employed to indicate that the R S -Q ligand is incapable of competing for inclusion as the third ligand of the aluminum atom. Although the phenolato ligand is primarily relied upon to obtain blue emission, it has been observed that substituents to the 8-quinolinolato rings can also perform useful hue shifting functions. The quinoline ring consists of fused benzo and pyrido rings. When the pyrido ring component of the quinoline ring is substituted with one or more electron donating substituents the effect is to shift the hue of emission away from the axis 499.2-D in Figure 1 and toward the axis 480-C. That is, emission is shifted away from the green region of the spectrum and toward a more primary blue emission. Electron donating substituents at the ortho and para positions of the pyrido ring (that is, the 2 and 4 positions of the quinoline ring) particularly influence the hue of emission, while the meta position on the pyrido ring (the 3 position on the quinoline ring) has a comparatively small influence on the hue of emission. It is, in fact, recognized that an electron accepting substituent could, if desired, be located at the 3 ring position while retaining a blue emission characteristic. Although steric hindrance is entirely independent of electron donating or accepting properties and, thus, R² can in theory take the form of either an electron donating or accepting group, it is preferred to choose R² from among electron donating groups. By adding a second electron donating group R⁴ a further shift in hue away from the green portion of the spectrum is achieved. R³, when present, can take any synthetically convenient form, but is preferably also electron donating. By contrast electron accepting substituents of the benzo ring component of the quinoline nucleus shift the hue of emission away from axis 499.2-D and toward axis 480-C in Figure 1. Thus, any or all of substituents at the 5, 6 and 7 quinoline ring positions, when present, are preferably electron accepting. It is well within the skill of the art to determine whether a particular substituent is electron donating or electron accepting. The electron donating or accepting properties of several hundred of the most common substituents, reflecting all common classes of substituents have been determined, quantified and published. The most common quantification of electron donating and accepting properties is in terms of Hammett σ values. Substituents with negative Hammett σ values are electron donating while those with positive Hammett σ values are electron accepting. Hydrogen has a Hammett σ value of zero, while other substituents have Hammett σ values that increase positively or negatively in direct relation to their electron accepting or donating characteristics. Lange's Handbook of Chemistry, 12th Ed., McGraw Hill, 1979, Table 3-12, pp. 3-134 to 3-138, here incorporated by reference, lists Hammett σ values for a large number of commonly encountered substituents. Hammett σ values are assigned based on phenyl ring substitution, but they provide a workable guide for qualitatively selecting electron donating and accepting substituents for the quinoline ring. Taking all factors together, steric blocking, synthetic convenience, and electron donating or accepting properties, R² is preferably an amino, oxy or hydrocarbon substituent. Adequate steric hindrance is provided when R² is methyl and is the sole 8-quinolinolato ring substituent (that is, each of R³, R⁴, R⁵, R⁶ and R⁷ is hydrogen). Thus, any amino, oxy or hydrocarbon substituent having at least 1 carbon atom falls within the perview of preferred substituents. Preferably no more than 10 carbon atoms are present in any one hydrocarbon moiety and optimally no more than 6 carbon atoms. Thus, R² preferably takes the form of -R′, -OR′ or -N(R″)R′, where R′ is a hydrocarbon of from 1 to 10 carbon atoms and R″ is R′ or hydrogen. Preferably R² contains 10 or fewer carbon atoms and optimally 6 or fewer carbon atoms. R³ and R⁴ for the reasons set forth above can take a broader range of forms than R², but are specifically contemplated to be selected from among the same group of preferred substituents as R². Since 3 and 4 ring position substitution is not required, R³ and R⁴ can additionally be hydrogen. Since 5, 6 or 7 ring position substitution is not required, R⁵, R⁶ and R⁷ can represent hydrogen. In preferred forms R⁵, R⁶ and R⁷ can be selected from synthetically convenient electron accepting substituents, such as cyano, halogen, and α-haloalkyl, α-haloalkoxy, amido, sulfonyl, carbonyl, carbonyloxy and oxycarbonyl substituents containing up to 10 carbon atoms, most preferably 6 or fewer carbon atoms. The following constitute specific examples of preferred mixed ligand aluminum chelates satisfying the requirements of the invention: PC-1 Bis(2-methyl-8-quinolinolato)(phenolato)aluminum(III) PC-2 Bis(2-methyl-8-quinolinolato)(ortho-cresolato)aluminum(III) PC-3 Bis(2-methyl-8-quinolinolato)(meta-cresolato)aluminum(III) PC-4 Bis(2-methyl-8-quinolinolato)(para-cresolato)aluminum(III) PC-5 Bis(2-methyl-8-quinolinolato)(ortho-phenylphenolato)aluminum(III) PC-6 Bis(2-methyl-8-quinolinolato)(meta-phenylphenolato)aluminum(III) PC-7 Bis(2-methyl-8-quinolinolato)(para-phenylphenolato)aluminum(III) PC-8 Bis(2-methyl-8-quinolinolato)(2,3-dimethylphenolato)aluminum(III) PC-9 Bis(2-methyl-8-quinolinolato)(2,6-dimethylphenolato)aluminum(III) PC-10 Bis(2-methyl-8-quinolinolato) (3,4-dimethylphenolato)aluminum(III) PC-11 Bis(2-methyl-8-quinolinolato)(3,5-dimethylphenolato)aluminum(III) PC-12 Bis(2-methyl-8-quinolinolato)(3,5-di-tert-butylphenolato)aluminum(III) PC-13 Bis(2-methyl-8-quinolinolato)(2,6-diphenylphenolato)aluminum(III) PC-14 Bis(2-methyl-8-quinolinolato)(2,4,6-triphenylphenolato)aluminum(III) PC-15 Bis(2-methyl-8-quinolinolato)(2,3,6-trimethylphenolato)aluminum(III) PC-16 Bis(2-methyl-8-quinolinolato)(2,3,5, 6-tetramethylphenolato)aluminum(III) PC-17 Bis(2-methyl-8-quinolinolato)(1-naphtholato)aluminum(III) PC-18 Bis(2-methyl-8-quinolinolato)(2-naphtholato)aluminum(III) PC-19 Bis(2,4-dimethyl-8-quinolinolato)(ortho-phenylphenolato)aluminum(III) PC-20 Bis(2,4-dimethyl-8-quinolinolato)(para-phenylphenolato)aluminum(III) PC-21 Bis(2,4-dimethyl-8-quinolinolato)(meta-phenylphenolato)aluminum(III) PC-22 Bis(2,4-dimethyl-8-quinolinolato)(3,5-dimethylphenolato)aluminum(III) PC-23 Bis(2,4-dimethyl-8-quinolinolato)(3,5-di- tert -butylphenolato)aluminum(III) PC-24 Bis(2-methyl-4-ethyl-8-quinolinolato)(para-cresolato)aluminum(III) PC-25 Bis(2-methyl-4-methoxy-8-quinolinolato)( para -phenylphenylato)aluminum(III) PC-26 Bis(2-methyl-5-cyano-8-quinolinolato)( ortho -cresolato)aluminum(III) PC-27 Bis(2-methyl-6-trifluoromethyl-8-quinolinolato)(2-naphtholato)aluminum(III) If organic EL device 200 is modified by omitting the electron transporting layer 224, it has desirable performance properties; however, the organic EL device is green emitting, not blue emitting. If the electron injecting layer 222 is omitted so that layer 224 forms the electron injecting and transporting zone in its entirety, a blue emitting organic EL device is formed, but its operating efficiency is markedly reduced. It has been discovered that blue emitting characteristics and increased operating efficiencies are imparted to organic EL devices when blue emitting formula II materials are used to form the electron transporting layer 224 in combination with conventional electron injecting and transporting materials used to form the electron injecting layer 222. By employing the formula II material to form the interface 214 with the hole injecting and transporting zone and keeping the electron injecting layer 224 out of direct contact with the hole injecting and transporting zone the hue of emission from the organic EL device is controlled by the formula II material lying along the junction. If the material of formula II comes in direct contact with the cathode 216, the operating efficiency of the organic EL device is markedly decreased. On the other hand, when a conventional material, such as a formula I material, is employed to form the electron injecting layer 222 and a formula II material is employed to form the electron transporting layer 224, the operating efficiency is surprisingly and markedly more efficient than when a conventional green emitting material is employed to form the entire electron injecting and transporting zone. It is specifically contemplated to incorporate in the electron transporting layer 224 a fluorescent dye following the teachings of Tang and others U.S. Patent 4,769,292. It has been discovered quite unexpectedly that by employing a fluorescent dye having a chromophoric unit containing at least 5 fused carbocyclic aromatic rings (hereinafter referred to as a pentacarbocyclic aromatic fluorescent dye) increased stability of organic EL device operation is achieved and a shift to shorter wavelengths of blue emission can be realized. In one preferred form of the invention the organic EL device is a first category construction in which the electron transporting layer 224 contains a formula II compound as a host and at least one pentacarbocylic aromatic fluorescent dye. Since it is the potential gradient maintained across the organic medium 208 that is responsible for electroluminescence, constructing the organic EL device with the thinnest possible organic medium allows electroluminescence to be achieved with a minimum potential difference between the anode and cathode of the device. Therefore, the smallest practical thickness of the organic medium is preferred. Typically, the thickness of the organic medium is less than 1 µm, preferably less than 5000 Å. The minimum thickness of the organic medium 208 is determined by the minimum thicknesses of the component zones and layers. To avoid quenching of luminescence the cathode 216 should be separated from the junction 214 by a distance of at least 300 Å--that is, the electron injecting and transporting zone 212 preferably has a thickness of at least 300 Å. The only remaining constraint on construction dimensions are the minimum layer thicknesses required to assure continuous layers. Each of the layers 218, 220, 222 and 224 has a minimum thickness of at least 20 Å and preferably at least 50 Å. Although the hole injecting and transporting zone 210 can therefore be quite thin, it is preferred that this zone also have a thickness of at least 300 Å. Among compounds other than the oxines of formula I useful in forming thin films suitable for constructing the electron injecting layer 222 within the preferred thickness ranges are the butadienes, such as 1,4-diphenylbutadiene and tetraphenylbutadiene; coumarins; and stilbenes, such as trans-stilbene, disclosed by Tang U.S. Patent 4,356,429. 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. In a preferred form of the invention a porphyrinic compound forms the hole injecting layer 218 of the organic EL device 200. A porphyrinic compound is any compound, natural or synthetic, which is derived from or includes the porphyrin structure. Any of the porphyrinic compounds disclosed by Adler U.S. Patent 3,935,031 or Tang U.S. Patent 4,356,429 can be employed. The hole transporting layer 220 of the organic EL device 200 preferably 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 and others U.S. Patent 3,180,730. Other suitable triarylamines substituted with vinyl or vinylene radicals and/or containing at least one active hydrogen containing group are disclosed by Brantley and others U.S. Patents 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 (VIII): wherein Q¹ and Q² are independently aromatic tertiary amine moieties and G is a linking group such an arylene, cycloalkylene, or alkylene group or a carbon to carbon bond. A particularly preferred class of triarylamines satisfying structural formula (VIII) and containing two triarylamine moieties are those satisfying structural formula (IX): where R¹ and R² each independently represents a hydrogen atom, an aryl group or alkyl group or R¹ and R² together represent the atoms completing a 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 (X): wherein R⁵ and R⁶ 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 (IX), linked through an arylene group. Preferred tetraaryldiamines include those represented by formula (XI). wherein Are is an arylene group, n is an integer of from 1 to 4, and Ar, R⁷, R⁸, and R⁹ are independently selected aryl groups. The various alkyl, alkylene, aryl, and arylene moieties of the foregoing structural formulae (VIII), (IX), (X), and (XI) 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 5 carbon atoms. The cycloalkyl moieties can contain from 3 to about 10 carbon atoms, but typically contain five, six, or seven ring carbon atoms--for example, cyclopentyl, cyclohexyl, and cycloheptyl ring structures. The aryl and arylene moieties are preferably phenyl and Representative useful aromatic tertiary amines are disclosed by Berwick and others U.S. Patent 4,175,960 and Van Slyke and others U.S. Patent 4,539,507. Berwick and others 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. The anode and cathode of the internal junction organic EL device can each take any convenient conventional form, such as any of the various forms disclosed by Tang and others U.S. Patent 4,885,211. Examples The invention and its advantages can be better appreciated by the following specific examples. Examples 1-23 Blue Emitting Organic EL Devices A series of organic EL devices satisfying the requirements of the invention were constructed in the following manner: (a) An indium tin oxide (ITO) coated glass substrate was ultrasonically cleaned in a commercial detergent, rinsed in deionized water, degreased in toluene vapor, and exposed to a strong oxidizing agent. (b) A hole injecting layer of copper phthalocyanine (CuPc) having a thickness of 375 Å was deposited over the ITO on the substrate by vacuum evaporation from a tantalum boat. (c) Onto the CuPc layer was deposited a 375 Å hole transporting layer of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl, also vacuum evaporated from a tantalum boat. (d) A blue emitting electron transporting layer (300 Å) was deposited onto the hole transporting layer. This compound was also vacuum evaporated from a tantalum boat. (e) Over the electron transporting layer was deposited a 300 Å electron injecting layer of aluminum trisoxine, again by vacuum evaporation from a tantalum boat. (f) A 2000 Å cathode of a 10:1 atomic ratio of Mg to Ag was formed by vacuum deposition onto the aluminum trisoxine layer to complete the organic EL device. In operation, the peak intensity wavelength of emission and the chromicity of emission in terms of the Figure 1 C.I.E. x and y coordinates were recorded. This information is summarized below in Table I. The luminescence of all of the aluminum chelates containing a phenolato ligand were measured as a powder and found to be blue emitting. When the aluminum chelates were incorporated in an organic EL device, the device was also blue emitting, although the emission peak was shifted to some extent to longer wavelengths as compared to the powder. The organic EL device with a C.I.E. chromaticity index nearest the green portion of the spectrum was that containing the aluminum chelate PC-17, shown as E-17 in Figure 1. The organic EL device with a C.I.E. chromaticity index at the shortest wavelength position was that containing the aluminum chelate PC-23, shown as E-23 in Figure 1. E-17 in Figure 1 lies within the blue-green portion of the spectrum defined by points D-C-480-499.2. E-23 in Figure 1 lies within the pure blue portion of the spectrum defined by points C-B-460-480. All of the remaining aluminum chelates containing a phenolato ligand were located within these same blue emitting regions of the spectrum at points intermediate the PC-17 and PC-23 extremes. The operation of the organic EL devices in terms of efficiency (measured in watts of emission per ampere of current), initial light output (initial intensity in milliwatts per cm²) and stability (measured as the number of hours required for initial light output to decline to one half its original intensity when driven at a constant current of 20 mA/cm²) is summarized in Table II. Each of the organic EL devices were considered acceptable in terms of both initial efficiency and light output. Being able to maintain at least half of initial light output after 50 hours was taken as a measure of minimum acceptable stability. From Table II certain performance characteristics were correlated with the phenolato ligand of the aluminum chelate. Ligands derived from unsubstituted and methyl substituted hydroxybenzene ligands (PC-1-4) demonstrated acceptable chromaticitiy, maximum emission wavelengths, efficiencies and initial light output while exceeding minimum stability requirements. The methyl substituent produced a significant hypsochromic shift in emission as compared to the unsubstituted hydroxybenzene ligand. The ring position of the methyl substituent had little influence on any of the performance characteristics. When methyl substituents were replaced with phenyl substituents, very high levels of stability were realized while maintaining essentially similar initial performance characteristics (PC-5,6,7,13). The 2-naphthol ligand (PC-18) produced performance characteristics similar to those of the phenyl substituted hydroxyphenyl ligands, while the 1-naphthol ligand showed reduced efficiency and initial light output, but increased stability. Taking both performance and chromaticity into account it can be seen that best overall performance was achieved with methyl or phenyl substituted hydroxybenzene phenolato ligands. Methyl substituents are considered representative of lower alkyl (1, 2 or 3 carbons atom) substituents while the phenyl substituents are considered representative of phenyl, biphenyl and naphthyl substituent group performance. Comparative Examples 24-29 Green Emitting Organic EL Devices Organic EL devices were constructed similarly as in Examples 1-23, except that the phenolato ligand containing aluminum chelate was replaced by one of the following aluminum chelates: C-24 Tris(8-quinolinolato)aluminum(III) C-25 Tris(4-methyl-8-quinolinolato)aluminum(III) C-26 Tris(5-methyl-8-quinolinolato)aluminum(III) C-27 Tris(3,4-dimethyl-8-quinolinolato)aluminum(III) C-28 Tris(4,6-dimethyl-8-quinolinolato)aluminum(III) C-29 Tris(4,5-dimethyl-8-quinolinolato)aluminum(III) The organic EL device containing C-24 exhibited an efficiency of 0.024 W/A and an initial light out of 0.48 mW/cm², indicating a performance characteristic similar to those of the Examples 1-23; however, the stability of C-24 was markedly superior to the compounds in Examples 1-23. None of the control compounds were suitable for replacing any one of PC-1 through P-23, since in all instances the organic EL devices containing C-24 to C-29 were clearly green emitting. This result is shown below in Table III. The most favorable chromaticity position measured was that of C-25. This point is shown in Figure 1. It lies in the green portion of the spectrum. Comparative Examples 30-32 Chelates with Strongly Electron Withdrawing Ring Substituents The following compounds were prepared with the intention that they be substituted for one of aluminum chelate compounds PC-1 to P-23 in Example 1: C-30 Bis(2-methyl-8-quinolinolato)(4-chlorophenolato)aluminum(III) C-31 Bis(2-methyl-8-quinolinolato)(4-cyanophenolato)aluminum(III) C-32 Bis(2-methyl-8-quinolinolato)(4-trifluoromethylphenolato)aluminum(III) Comparisons of elemental analyses of initial materials and vacuum vapor deposited materials revealed that a significant degree of decomposition had occurred in coverting these compounds to the vapor phase and back to a solid. Organic EL devices were constructed similarly as in Examples 1-23, except that the phenolato ligand containing aluminum chelate was replaced using C-30 or C-31 as a starting material for vacuum vapor deposition. The organic EL device constructed starting with C-30 exhibited a peak wavelength of emission of 493 nm and an efficiency of 0.022 W/A. The organic EL device constructed starting with C-31 exhibited a peak wavelength of emission of 532 nm and an efficiency of 0.018 W/A. Taking both peak emission wavelength and efficiency into account, C-30 and C-31 produced inferior blue emitting organic EL devices as compared with PC-1 to PC-23. Comparative Examples 33-34 Chelates with Strongly Electron Donating Ring Substituents Organic EL devices were constructed similarly as in Examples 1-23, except that the phenolato ligand containing aluminum chelate was replaced by one of the following aluminum chelates: C-33 Bis(2-methyl-8-quinolinolato)(4-methoxyphenolato)aluminum(III) C-34 Bis(2-methyl-8-quinolinolato)(3,5-dimethoxyphenolato)aluminum(III) The organic EL device constructed starting with C-33 exhibited a peak wavelength of emission of 490 nm and an efficiency of 0.008 W/A--that is, luminescence was objectionably weak. The organic EL device constructed starting with C-34 exhibited a peak wavelength of emission of 491 nm. Efficiency was 0.028 W/A with an initial light output of 0.56 mW/cm², but the device declined to 1/2 its initial light output in only 18 hours, indicating inadequate stability. Comparative Example 35 Chelate with Phenolato Ligand With Fused Noncarbocylic Ring The purpose of this comparison is to demonstrate the importance of the ring fused with the hydroxybenzene ring in the phenolato ligand being a carbocyclic ring. An organic EL device was constructed similarly as in Examples 1-23, except that the phenolato ligand containing aluminum chelate was replaced by the following aluminum chelate: C-35 Bis(2-methyl-8-quinolinolato)(5-quinolinolato)aluminum(III) The organic EL device exhibited a relatively long maximum emission wavelength of 500 and was judged unacceptable in performance based on a decline to 1/2 initial light output in less than one hour. Examples 36-39 Doping to Attain Shorter Wavelengths of Emission A series of organic EL devices were constructed identically as in Example 7, except that PC-7 was doped with varied amounts of perylene (FD-1), ranging from 0.5 to 3 mole percent, based on PC-7. The results are summarized below in Tables IV and V. From Table IV it is apparent that all concentrations of FD-1, ranging from 0.5 to 3 mole per percent, based on PC-7, were effective to shift the emission hues of the organic EL devices of Examples 36 to 39 to shorter wavelengths. The points E-7 (x-0.20,y=0.30)and E-36 (x=0.16, y=0.21) in Figure 1 demonstrate the hue shift that can be provided by FD-1. The data indicate that a concentration range of from 0.2 to 3 mole percent is a preferred range, with from 0.5 to 2 mole percent being an optimum range. Turning to Table V, it is apparent that the overall efficiency of the organic EL devices first increased and then declined somewhat as the level of dopant increased, but this was more than offset by very dramatic increases in stability being realized. Examples 40-45 Doping Varied Phenolato Ligand Aluminum Chelates A series of organic EL devices were constructed similarly as in Examples 1-23, except that FD-1 was either included in the electron transporting layer in a concentration of 1 mole percent, based on the phenolato ligand aluminum chelate host, or omitted. The results are summarized below in Tables VI and VII. From Table VI it is apparent that a hypsochromic shift in the hue of emission was achieved with each of the varied phenolato ligand aluminum chelate hosts. Turning to Table VII, it apparent that the dopant in every instance produced a marked increase in the stability of the organic EL devices. Compound Preparations Each of the bis(8-quinolinolato)phenolatoalumium(III) mixed ligand chelates compounds are novel compounds and the specific subject matter of the Applicant's simultaneously filed European Patent Application corresponding to U.S. Patent Application Serial No. 738,751 filed 1 August 1991. The following is a description of the preparation and characterization of compounds PC-1 to P-23 employed in the Examples above. PC-1 A sample of 2-methyl-8-quinolinol (Eastman Kodak Company) was recrystallized from ethanol/water. Then 0.8 g (0.005 mole) of the recrystallized ligand was heated and stirred in 40 mL of absolute ethanol with 1.0 g (0.005 mole) of 99.995% aluminum isopropoxide (Aldrich Chemical Company). After about 30 minutes the solution was filtered through a celite mat to remove a small amount of insoluble material. Then an ethanol solution containing 0.8 g (0.005 mole) of recrystallized 2-methyl-8-quinolinol and 1.0 g (0.01 mole) of phenol (Eastman Kodak Company) was added to the original solution. The resulting solution was heated and stirred at reflux for 4 hours and allowed to cool to room temperature. The solid was collected and washed with ethanol, then ether and allowed to air dry. The solid weighed 1.0 g, which represented a 46% yield. PC-2 to PC-18 The procedure described above for the preparation of PC-1 was used to prepare the title compounds, except that phenol was replaced with the appropriate substituted phenol. The substituted phenols used to prepare PC-2, PC-4, PC-7, PC-12, PC-13, P-14 and PC-15 were obtained from Aldrich with the remainder of the substituted phenols being obtained from the Eastman Kodak Company. The results are summarized in Table VIII. PC-19 This aluminum chelate was prepared similarly as PC-1, except that 2,4-dimethyl-8-quinolinol was substituted for 2-methyl-8-quinolinol. The ortho -phenylphenol used was from the Eastman Kodak Company. The title compound yield was 73%. PC-20 This aluminum chelate was prepared similarly as PC-1, except that 2,4-dimethyl-8-quinolinol was substituted for 2-methyl-8-quinolinol. The para -phenylphenol used was from the Eastman Kodak Company. The title compound yield was 94%. PC-21 A 1.74 g (0.010 mole) sample of 2,4-dimethyl-8-quinolinol was stirred in 65 mL of anhydrous ether with 1.0 g (0.005 mole) of 99.995% aluminum isoproxide (Aldrich Chemical Company) and 1.7 g (0.010 mole) of meta -phenylphenol (Eastman Kodak Company). The impure solid was collected after 3 hours (0.94 g). PC-22 A 1.74 g (0.010 mole) sample of 2,4-dimethyl-8-quinolinol was stirred in 75 mL of anhydrous ether with 1.0 g (0.005 mole) of 99.995% aluminum isoproxide (Aldrich Chemical Company) and 1.2 g (0.010 mole) of 3,5-dimethylphenol (Aldrich Chemical Company). The impure solid was collected after 6 hours (2.3 g). PC-23 A 0.87 g (0.005 mole) sample of 2,4-dimethyl-8-quinolinol was stirred in 40 mL of anhydrous ether with 0.5 g (0.0025 mole) of 99.995% aluminum isoproxide (Aldrich Chemical Company) and 1.0 g (0.005 mole) of 3,5-di- t -butylphenol (Aldrich Chemical Company). The impure solid was collected after 5 hours (0.83 g). Compound Characterizations The compounds prepared were analyzed and compared to theoretical compositions as shown in Table IX. This provided confirmation that the intended compounds had been synthesized. The next task was to determine that the compounds were capable of undergoing vacuum evaporation followed by deposition while retaining their intended structure. For compounds that are capable of undergoing vacuum evaporation without decomposition this procedure has the desirable effect of purifying the materials. In this technique a powder sample was placed in a porcelain boat which was then inserted into a 2.54 cm diameter Pyrex™ tube. Argon was flowed through the tube at a pressure of about 2 torr while the center of the tube was heated in a tube furance. Each of the samples was treated in this way. The solids condensed from the vapor phase were analyzed, and the results are reported in Table IX. The compounds were further evaluated to determine that each was fluorescent. The fluorescence spectrum was recorded for each of the powders as initially prepared. The ultraviolet excited emission spectrum of each powder sample was obtained by packing the powder into a 2.48 cm diameter by 0.24 cm deep aluminum planchet and placing the loaded planchet into a sample chamber of a spectrofluorometer. Each sample was exposed to ultraviolet light with a 4 nm bandwidth centered at 355 nm from a xenon arc lamp that had passed through a monochromator and bandpass filter. The emitted light was collected, passed through an order-sorting filter and detected by a spectrometer which was calibrated to within ±1 nm with a resolution of approximately 4 nm (full width at half maximum). The wavelength of maximum intensity emission is provided in Table IX.

Claims: An internal junction organic electroluminescent device comprised of, 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 electron injecting and transporting zone is comprised of an electron injecting layer in contact with the cathode and, interposed between the electron injecting layer and the organic hole injecting and transporting zone, a blue emitting luminescent layer comprised of an aluminum chelate of the formula: (R s -Q)₂-Al-O-L where Q in each occurrence represents a substituted 8-quinolinolato ligand, R s represents an 8-quinolinolato ring substituent chosen to block sterically the attachment of more than two substituted 8-quinolinolato ligands to the aluminum atom, O-L is phenolato ligand, and L is a hydrocarbon of from 6 to 24 carbon atoms comprised of a phenyl moiety. An internal junction organic electroluminescent device according to claim 1 further characterized in that R s is a 2-position substituent of the 8-quinolinolato ring nucleus. An internal junction organic electroluminescent device according to claim 2 further characterized in that R s is an electron donating substituent. An internal junction organic electroluminescent device according to claim 3 further characterized in that R s is chosen from the group consisting of amino, oxy and hydrocarbon groups containing up to 10 carbon atoms. An internal junction organic electroluminescent device according to claim 1 further characterized in that the 8-quinolinolato ring nucleus includes a 4-position ring substituent that is electron donating. An internal junction organic electroluminescent device according to claim 1 further characterized in that the 8-quinolinolato ring nucleus includes in at least one of its 5, 6 and 7 ring positions a substituent that is electron accepting. An internal junction organic electroluminescent device according to claim 1 further characterized in that the aluminum chelate satisfies the formula: where R² represents an electron donating substituent, R³ and R⁴ each independently represent hydrogen or an electron donating substituent, R⁵, R⁶ and R⁷ each independently represent hydrogen or an electron accepting substituent, O-L is phenolato ligand, and L is a hydrocarbon of from 7 to 18 carbon atoms comprised of a phenyl moiety. An internal junction organic electroluminescent device according to any one of the preceding claims further characterized in that the hydrocarbon includes at least one aliphatic substituent of the phenyl moiety. An internal junction organic electroluminescent device according to any one of claims 1 to 7 further characterized in that a benzo ring is fused with the phenyl moiety. An internal junction organic electroluminescent device according to any one of claims 1 to 7 further characterized in that the hydrocarbon includes at least one aromatic substituent of the phenyl moiety. An internal junction organic electroluminescent device according to claim 1 further characterized in that the aluminum chelate satisfies the formula: where Q in each occurrence represents a substituted 8-quinolinolato ring nucleus, R² in each occurrence represents a 2-position electron donating substituent of the 8-quinolinolato ring nucleus, and L¹, L², L³, L⁴ and L⁵ collectively contain 12 or fewer carbon atoms and each independently represent hydrogen or hydrocarbon groups of from 1 to 12 carbon atoms, with the proviso that L¹ and L² together or L² and L³ together can form a fused benzo ring. An internal junction organic electroluminescent device according to any of the preceding claims further characterized in that the luminescent layer additionally includes a fluorescent dye. An internal junction organic electroluminescent device according to any of the preceding claims further characterized in that the electron injecting and transporting zone and the hole injecting and transporting zone together exhibit a thickness of less than 1 µm. An internal junction organic electroluminescent device according to any of the preceding claims further characterized in that a metal oxinoid compound forms the electron injecting layer.

Inventors: BRYAN PHILIP STEVEN C O EASTMA, LOVECCHIO FRANK VITO C O EASTM, VANSLYKE STEVEN ARLAND C O EAS, BRYAN, PHILIP STEVEN, C/O EASTMAN KODAK COMPANY, LOVECCHIO, FRANK VITO, C/O EASTMAN KODAK COMPANY, VANSLYKE, STEVEN ARLAND, C/O EASTMAN KODAK COMPANY
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1992
Main classification: H05B33
IPC codes: H01L51, H05B33, C09K11, C07F5