Patent Application
20090162644
This invention relates to an organic light-emitting diode (OLED) electroluminescent (EL) device having a light-emitting layer and a first layer between the light-emitting layer and the cathode containing (a) more than 50 vol % of a salt or complex of an alkali or alkaline earth metal and (b) a carbocyclic fused ring aromatic compound, that is less than 15 nm thick and a second layer, located between the first layer and the cathode, containing a phenanthroline derivative, a metal oxinoid complex and an alkali metal.
While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, 30, 322, (1969); and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often greater than 100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the term “organic EL element” encompasses the layers between the anode and cathode. Reducing the thickness lowered the resistance of the organic layers and has enabled devices that operate at much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, and therefore is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons and is referred to as the electron-transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by C. Tang et al. (J. Applied Physics, Vol. 65, 3610 (1989)). The light-emitting layer commonly consists of a host material doped with a guest material, otherwise known as a dopant. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element comprising a hole injecting layer (HIL), a hole-transporting layer (HTL), a light-emitting layer (LEL) and an electron-transporting/injecting layer (ETL). These structures have resulted in improved device efficiency.
Since these early inventions, further improvements in device materials have resulted in improved performance in attributes such as color, stability, luminance efficiency and manufacturability, e.g., as disclosed in U.S. Pat. No. 5,061,569, U.S. Pat. No. 5,409,783, U.S. Pat. No. 5,554,450, U.S. Pat. No. 5,593,788, U.S. Pat. No. 5,683,823, U.S. Pat. No. 5,908,581, U.S. Pat. No. 5,928,802, U.S. Pat. No. 6,020,078, and U.S. Pat. No. 6,208,077, amongst others.
Notwithstanding these developments, there are continuing needs for organic EL device components, such as light-emitting materials, sometimes referred to as dopants, that will provide high luminance efficiencies combined with high color purity and long lifetimes. In particular, there is a need to be able to adjust the emission wavelength of the light-emitting material for various applications. For example, in addition to the need for blue, green, and red light-emitting materials there is a need for blue-green, yellow and orange light-emitting materials in order to formulate white-light emitting electroluminescent devices. For example, a device can emit white light by emitting a combination of colors, such as blue-green light and red light or a combination of blue light and yellow light.
The preferred spectrum and precise color of a white EL device will depend on the application for which it is intended. For example, if a particular application requires light that is to be perceived as white without subsequent processing that alters the color perceived by a viewer, it is desirable that the light emitted by the EL device have 1931 Commission International d'Eclairage (CIE) chromaticity coordinates, (CIEx, CIEy), of about (0.33, 0.33). For other applications, particularly applications in which the light emitted by the EL device is subjected to further processing that alters its perceived color, it can be satisfactory or even desirable for the light that is emitted by the EL device to be off-white, for example bluish white, greenish white, yellowish white, or reddish white.
White EL devices can be used with color filters in fill-color display devices. They can also be used with color filters in other multicolor or functional-color display devices. White EL devices for use in such display devices are easy to manufacture, and they produce reliable white light in each pixel of the displays. Although the OLEDs are referred to as white, they can appear white or off-white, for this application, the CIE coordinates of the light emitted by the OLED are less important than the requirement that the spectral components passed by each of the color filters be present with sufficient intensity in that light. Thus there is a need for new materials that provide high luminance intensity for use in white OLED devices.
Lee et al U.S. Pat. No. 7,126,271 discloses OLED devices with a first layer next to an electrode containing an organic metal compound including Alq or Liq and a second layer next to the first containing a mixture of charge carrier transport materials and an organic metal compound.
Kido et al., in U.S. Pat. No. 6,396,209 discloses OLED devices with an electron-injection layer adjacent to the cathode with a mixture of an electron-transporting organic compound and an organic metal complex compound.
Lee et al., in US 2007/0020484 discloses OLED devices with an electron-transport layer with mixtures of organic and metal compounds.
Organometallic complexes, such as lithium quinolate (also known as lithium 8-hydroxyquinolate, lithium 8-quinolate, 8-quinolinolatolithium, or Liq) have been used in EL devices, for example see WO 0032717 and US 2005/0106412.
Common assigned US 2006/0286405 discloses electron transporting layers containing (i) more than 10 vol % of a carbocyclic flused ring aromatic compound and (ii) at least one salt or complex of an alkali or alkaline earth metal. Commonly assigned U.S. Ser. Nos. 11/076,821; 11/077,218; and 11/116,096 describe mixing a first compound with a second compound that is a low voltage electron transport material, to form a layer on the cathode side of the emitting layer in an OLED device, which gives an OLED device that has a drive voltage even lower than that of the device with the low voltage electron transport material. In some cases a metallic material based on a metal having a work function less than 4.2 eV is included in the layer. Common assigned US2005233166, US20070092756 and US20070207347 also describe the use of a salt or complex of an alkali or alkaline earth metal, not including complexes where the ligand is a quinolate, in an electron-transporting layer.
Seo et al., in US2002/0086180 discloses OLED devices with a mixture of Bphen and Alq as an electron injection material in a layer adjacent to a cathode.
Kido et al., in U.S. Pat. No. 6,013,384 discloses OLED devices with an organic electron injection layer doped with an alkali metal.
WO2006/070913, U.S. Pat. No. 7,161,291, U.S. Pat. No. 6,639,357, US2007/0222379, EP1227528, US2007/0216292 and WO2001/067825 all disclose electron-injection layers which may contain a combination of Alq and Bphen doped with lithium metal.
However, these devices do not have all desired EL characteristics in terms of high luminance in combination with low drive voltages. Thus, notwithstanding these developments, there remains a need to reduce drive voltage of OLED devices while maintaining good luminance. Moreover, these devices do not have all desired EL characteristics in terms of maintaining high T90 or T95 lifetimes.
The invention provides an OLED device comprising a cathode, a light emitting layer and an anode, in that order, and
(i) a first layer, located between the cathode and the light emitting layer, containing (a) more than 50 vol % of an organic salt or complex of an alkali or alkaline earth metal and (b) a carbocyclic fused ring aromatic compound, that is less than 15 nm thick; and
(ii) a second layer, located between the first layer and the cathode, containing a codeposited phenanthroline derivative, metal oxinoid complex, and alkali or alkaline earth metal.
Devices of the invention provide an improved balance between T95 stability, drive voltage and luminance.
The FIGURE shows a cross-sectional schematic view of one embodiment of the device of the present invention.
OLED displays require low power consumption and high lifetime for many applications such as cell phones, digital cameras, TVs, and monitors for PCs and notebooks. The operational lifetime or stability of the OLED display varies with the type of application. One metric of operational lifetime or stability is the half-life (T50) which is defined as the time taken to drop to half of the initial luminance level of the display. Typical specifications for OLED devices call for T50>10,000-20,000 hrs at normal operating conditions. However, there are other metrics that are used to describe device performance over shorter lifetimes, i.e. T90 or T95 values, and are defined as the time taken to drop its luminance level to the 90% or 95% levels with respect to the initial luminance. T90 and T95 lifetimes are particularly important for OLED displays when a fixed test pattern or image are displayed constantly and continuously on the screen. OLEDs show non-linear dimming with aging and continuously operated pixels will show a “burn-in” effect. With time, pixels that are continuously lit displaying a logo or fixed images will have significantly lower luminance than the immediately adjacent pixels that have been lit for less time. Thus, the pixels that are continuously on will show a different contrast than the surrounding pixels and pixels in another part of the screen. This burn-in effect is a more serious issue for OLEDs than other types of display technologies such as LCD. Unlike OLED displays, LCD displays require an uniform backlight. To reduce or eliminate this burn-in effect, it is required that OLED devices should have high T90 or T95 lifetimes.
It is generally accepted that the device performance parameters such as short-term T90 or T95 lifetimes, longer term lifetimes such as T50 or T60, operational drive voltages and luminance efficiencies are interdependent. Oftentimes, improvements in one or more of these parameters are accompanied by a decline in the performance of one or more of the other parameters. Depending on the requirements of the ultimate device or display, more often than not, a balance has to be reached between the different device performance parameters. In some applications, the elimination of ‘burn-in’ is critical for good viewing performance of the display and can be prevented by extending the T90 or T95 lifetimes. If the device parameters are interdependent, it is desirable that the changes made to the device to extend the short-lifetime stability have minimal effects on the other parameters. For example, improvements in operational stability can be obtained at the expense of increased drive voltage and lower efficiency. However, for some applications, it may be desirable to accept less than the maximum stability improvement to minimize loss or even improve the voltage and efficiency.
The OLED devices in all aspects of this invention include a cathode, a light emitting layer and an anode in that order. As used herein, two layers are “adjacent” if one layer is juxtaposed with and shares a common boundary with the other layer.
In the invention, the OLED device has located between the cathode and the light-emitting layer, a first layer which is an electron-transporting layer. The first layer is preferably located adjacent to the light-emitting layer.
The first layer contains at least one salt or complex of an alkali or alkaline earth metal amounting to more than 50% by volume of all materials present in that layer. A particularly desirable complex of the invention is Liq or one of its derivatives. Liq is a complex of Li+ with 8-hydroxyquinolinate, to give the lithium quinolate complex, also known as lithium 8-quinolate, but often referred to as Liq. Liq can exist as the single species, or in other forms such as Li6q6 and Linqn, where n is an integer and q is the parent 8-hydroxyquinolate ligand or other 8-hydroxyquinolate derivatives.
In one embodiment, the metal complex is represented by formula (1):
(M)m(Q)n (1)
In formula (1), M represents an alkali or alkaline earth metal ion. In one suitable embodiment M is a Group IA metal ion such as Li30 , Na−, K+, Cs+, and Rb+. In one desirable embodiment M represents Li+.
In formula (1), each Q is an independently selected ligand. Desirably, each Q has a net charge of −1. In one suitable embodiment Q is a bidentate ligand. For example Q can represent an 8-quinolate group.
In formula (1), n represents an integer, commonly 1-6. Thus the organometallic complex can form dimers, trimers, tetramers, pentamers, hexamers and the like. However, the organometallic complex can also form a one dimensional chain structure in which case n is greater than 6. In any case, m and n are chosen so that the net charge on the complexes of formula (1) is zero.
In another desirable embodiment, the metal complex is represented by formula (1′):
In formula (1′), Z and the dashed are represent two or three atoms and the bonds necessary to complete a 5- or 6-membered ring with M. Each A represents H or a substituent and each B represents an independently selected substituent on the Z atoms, provided that two or more substituents may combine to form a fused ring or a fused ring system. In formula (1′), j is 0-3 and k is 1 or 2. Also, M represents an alkali metal or alkaline earth metal ion with m and n independently selected integers selected to provide a neutral charge on the complex.
In another desirable embodiment of the invention, the metal complex is represented by Formula (1″):
In formula (1″), M represents an alkali or alkaline earth metal, as described previously. In one desirable embodiment, M represents Li+. Each ra and rb represents an independently selected substituent, provided two substituents may combine to form a fused ring group. Examples, of such substituents include a methyl group, a phenyl group, a fluoro substituent and a fused benzene ring group formed by combining two substituents. In formula (1″), t is 1-3, s is 1-3 and n is an integer from 1 to 6.
Formula (1′″) represents an embodiment of the invention where the ligand of the complex is acetylacetonate or a derivative thereof.
In formula (1′″) Y1, Y2 and Y3 independently represent substituents provided that any of Y1, Y2 and Y3 may combine to form a ring or fused ring system. M is an alkaline or alkaline earth metal ion with m and n representing integers selected to provide a neutral charge on the complex. In one desirable embodiment of formula (1′″), M represents Li+. When the substituents are hydrogen and M represents Li+, formula (1′″) then represents lithium acetylacetonate. In addition to hydrogen, examples of other substituents include carbocyclic groups, heterocyclic groups, alkyl groups such as a methyl group, aryl groups such as a phenyl group, or a naphthyl group. A fused ring group may be formed by combining two substituents.
For the purpose of the different aspects of this invention, the terms complex, organic complex and cyclometallated complex describe the complexation of an alkali or alkaline earth metal ion with an organic molecule via coordinate or dative bonding. The molecule, acting as a ligand, can be mono-, di-, tri- or multi-dentate in nature, indicating the number of potential coordinating atoms in the ligand. It should be understood that the number of ligands surrounding a metal ion should be sufficient to render the complex electrically neutral. In addition, it should be understood that a complex can exist in different crystalline forms in which there can be more than one metal ion present from form to form, with sufficient ligands present to impart electrical neutrality.
The definition of a coordinate or dative bond can be found in Grant & Hackh's Chemical Dictionary, page 91. In essence, a coordinate or dative bond is formed when electron rich atoms such as O or N, donate a pair of electrons to electron deficient atoms such as Al or B. One such example is found in tris(8-quinolinolato)aluminum(III), also referred to as Alq, wherein the nitrogen on the quinoline moiety donates its lone pair of electrons to the aluminum atom thus forming a heterocyclic or cyclometallated ring, called a complex and hence providing Alq with a total of 3 fused rings. The same applies to Liq.
As used herein and throughout this application, the term carbocyclic and heterocyclic rings or groups are generally as defined by the Grant & Hackh's Chemical Dictionary, Fifth Edition, McCraw-Hill Book Company. A carbocyclic ring is any aromatic or non-aromatic ring system containing only carbon atoms and a heterocyclic ring is any aromatic or non-aromatic ring system containing both carbon and non-carbon atoms such as nitrogen (N), oxygen (O), sulfur (S), phosphorous (P), silicon (Si), gallium (Ga), boron (B), beryllium (Be), indium (In), aluminum (Al), and other elements found in the periodic table useful in forming ring systems. Also, for the purpose of the aspects of this invention, also included in the definition of a heterocyclic ring are those rings that include coordinate or dative bonds.
In the first layer, there can be more than one salt or complex, or a mixture of a salt and a complex in the layer. The salt can be any organic or inorganic salt or oxide of an alkali or alkaline earth metal that can be reduced to the free metal, either as a free entity or a transient species in the device. Examples of suitable complexes or salts include, but are not limited to, the alkali and alkaline earth halides, including sodium fluoride (NaF), cesium fluoride (CsF), calcium fluoride (CaF2), lithium benzoate, potassium benzoate and lithium formate. Examples MC-1-MC-30 are further examples of useful salts or complexes for the invention.
The first layer also contains a carbocyclic fused ring aromatic compound. This compound should be present at less than 50% by volume of all materials present in that layer. In one desirable embodiment, the carbocyclic compound is a tetracene, such as for example, rubrene.
Suitably, the carbocyclic fused ring aromatic compound may be represented by formula (2):
In formula (2), R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11, and R12 are independently selected as hydrogen or substituent groups, provided that any of the indicated substituents may join to form farther fused rings. In one desirable embodiment, R1, R4, R7, and R10 represent hydrogen and R5, R6, R11, and R12 represent independently selected aromatic ring groups.
In a further embodiment, the carbocyclic fused ring aromatic compound may be represented by formula (2′):
In formula (2′), Ar1-Ar4 represent independently selected aromatic groups, for example, phenyl groups, tolyl groups, naphthyl groups, 4-biphenyl groups, or 4-t-butylphenyl groups. In one suitable embodiment, Ar1 and Ar4 represent the same group, and independently of Ar1 and Ar4, Ar2 and Ar3 are the same.
R1-R4 independently represent hydrogen or a substituent, such as a methyl group, a t-butyl group, or a fluoro group. In one embodiment R1 and R4 are not hydrogen and represent the same group.
In another embodiment, the carbocyclic compound is an anthracene. Particularly useful anthracene compounds are those of formula (3):
In formula (3), W1-W10 independently represent hydrogen or an independently selected substituent, provided that two adjacent substituents can combine to form rings. In one embodiment of the invention W1-W10 are independently selected from hydrogen, alkyl, aromatic carbocyclic and aromatic heterocyclic groups. In another embodiment of the invention, at least one of W9 and W10 represent independently selected aromatic carbocyclic and aromatic heterocyclic groups. In yet another embodiment of the invention W9 and W10 are independently selected from phenyl, naphthyl and biphenyl groups. For example, W9 and W10 may represent such groups as 1-naphthyl, 2-naphthyl, 4-biphenyl, 2-biphenyl and 3-biphenyl. In a desirable embodiment, at least one of W9 and W10 represents a carbocyclic group selected from an anthracenyl group (derived from anthracene). Particularly useful anthracene derived groups are 9-anthracenyl groups. In a further aspect of the invention, W1-W8 represent hydrogen or alkyl groups. Particularly useful embodiments of the invention are when W9 and W10 are aromatic carbocyclic groups and W7 and W3 are independently selected from hydrogen, alkyl and phenyl groups.
Suitable carbocyclic fused ring aromatic compounds of the naphthacene type can be prepared by methods known in the art. These include forming a naphthacene type material by reacting a propargyl alcohol with a reagent capable of forming a leaving group followed by heating in the presence of a solvent, and in the absence of an oxidizing agent and in the absence of an organic base, to form a naphthacene. See commonly assigned U.S. Ser. Nos. 10/899,821 and 10/899,825 filed Jul. 27, 2004.
In order to provide high T90 and T95 stabilities, the first layer of the invention should contain a high volume % of the salt or complex of an alkali or alkaline earth metal. While the layer should be more than 50% by volume of the salt or complex, even higher amounts are preferred. More desirably, the % volume of the salt can be 70% by volume or more, or most preferably, 90% by volume or more. Suitably, the carbocyclic fused ring aromatic compound is present at less than 50% by volume, more preferably, less than 30% by volume or most preferably, less than 10% by volume. Other materials may also be present in the first layer. All volume % s are relative to the total amount of all materials present in that layer.
In addition, the thickness of the first layer is important to provide high T90 and T95 stabilities. Ideally, the thickness of the first layer should be no more than 15 nm thick, preferably 10 nm or less.
The first layer should be a non-luminescent layer; that is, it should provide less than 25% of the total device emission. Ideally, it should have substantially no light emission.
Examples of useful carbocyclic aromatic fused ring compounds for the invention are as follows:
In this invention, there is a second layer which is an electron-injection layer located between the first layer, which is an electron-transporting layer, and the cathode. It is preferred that the second layer be in direct contact with the first layer or the cathode, or most preferably, located between and in direct contact with both.
The second layer contains a codeposited phenathroline derivative, metal oxinoid compound, and alkali or alkaline earth metal. The % volume of the phenanthroline can be between 0.5-99.5%, but is preferably between 40-60%. The % volume of the metal oxinoid compound can be 0.5-99.5%, but is preferably between 40-60%. The % volume of the alkali or alkaline earth metal can be between 0.1 to 10%, but is preferably between 0.5 to 5% and most preferably, in the range of 1 to 3%.
Suitable substituted phenanthrolines for this application are represented by formula (R):
In formula (R), R1-R8 are independently hydrogen, alkyl groups, aryl or substituted aryl groups, and at least one of R1-R8 is an aryl group or substituted aryl group.
Specific examples of the phenanthrolines useful in the EIL are 2,9-dimethyl-4,7-diphenyl-phenanthroline (BCP) (see formula R-1) and 4,7-diphenyl-1,10-phenanthroline (Bphen) (see formula R-2).
Suitable metal oxinoid compounds, also known as metal-chelated oxinoid compounds, for the second layer are metal complexes of 8-hydroxyquinoline and similar derivatives according to formula E:
wherein
M represents a metal selected from the groups 2b, 3a, 3b, 4a, 4b, 5b, 6b, 7b and 8 of the periodic table not including rare earth metals;
n is an integer of from 1 to 4; and
Z independently in each occurrence represents the atoms completing a nucleus having at least two fused aromatic rings.
Z completes a heterocyclic nucleus containing at least two fused aromatic rings, at least one of which is an azole or azine ring. Additional rings, including both aliphatic and aromatic rings, can be fused with the two required rings, if required. To avoid adding molecular bulk without improving on function the number of ring atoms is usually maintained at 18 or less.
The groups 1b, 2b, 3a, 3b, 4a, 4b, 5b, 6b, 7b and 8 of the periodic table are as listed in the CRC Handbook of Chemistry and Physics, 54th Edition, CRC Press, Cleveland, Ohio. The following are suitable metals: group 2b are Zn, Cd or Hg; group 3a are Al, Ga, In or Tl; group 3b are Sc, Y, La or Ac; group 4a are Ge, Sn or Pb; group 4b are Ti, Zr or Hf; group 5b are V, Nb or Ta; group 6b are Cr, Mo or W; group 7b are Mn, Tc or Re; and group 8 is Fe, Co, Ni, Ru, Rh, Pd, Os, Ir or Pt. Of these, the metal is most suitably a trivalent metal, such aluminum or gallium, or a divalent metal such as zinc or zirconium. Preferably, the metal oxinoid complex is aluminum, gallium or zirconium.
From the foregoing it is apparent that the metal in the metal oxinoid complex of formula (E) cannot be an alkali metal ion, such as lithium, sodium, or potassium or an alkaline earth metal ion, such as magnesium or calcium. The metal oxinoid present in the electron injection layer is different from and cannot be a compound according to formula (1′).
Illustrative of useful chelated oxinoid compounds are the following:
CO-1: Aluminum trisoxine[alias, tris(8-quinolinolato)aluminum(III) or Alq]
CO-3: Bis[benzo{f}-8-quinolinolato]zinc (II)
CO-4: Bis(2-methyl-8-quinolinolato)aluminum(III)-□-oxo-bis(2-methyl-8-quinolinolato)aluminum(III)
CO-5: Indium trisoxine [alias, tris(8-quinolinolato)indium]
CO-6: Aluminum tris(5-methyloxine)[alias, tris(5-methyl-8-quinolinolato)aluminum(III)]
CO-8: Gallium oxine[alias, tris(8-quinolinolato)gallium(III)]
CO-9: Zirconium oxine[alias, tetra(8-quinolinolato)zirconium(IV)]
The preferred metals for the second layer to be combined with the phenanthroline and metal oxinoid complex is lithium, sodium or cesium. The most preferred metal is lithium.
In addition, the thickness of the second layer is important to provide high efficiency. Ideally, the thickness of the second layer should be at least 10 nm but less than 50 nm thick or, preferably 20 nm to 40 nm. The second layer should be a non-luminescent layer; that is, it should provide less than 25% of the total device emission. Ideally, it should have substantially no light emission.
In all described aspects of the invention, it should be understood that the inventive combination of layers applies to OLED devices that emit light by both fluorescence and phosphorescence. In other words, the OLED devices can be triple or singlet in nature. The advantages of the invention can be realized with both fluorescent and phosphorescent devices.
Unless otherwise specifically stated, use of the term “substituted” or “substituent” means any group or atom other than hydrogen. Additionally, when the term “group” is used, it means that when a substituent group contains a substitutable hydrogen, it is also intended to encompass not only the substituent's unsubstituted form, but also its form further substituted with any substituent group or groups as herein mentioned, so long as the substituent does not destroy properties necessary for device utility. Suitably, a substituent group may be halogen or may be bonded to the remainder of the molecule by an atom of carbon, silicon, oxygen, nitrogen, phosphorous, sulfur, selenium, or boron. The substituent may be, for example, halogen, such as chloro, bromo or fluoro; nitro; hydroxyl; cyano; carboxyl; or groups which may be further substituted, such as alkyl, including straight or branched chain or cyclic alkyl, such as methyl, trifluoromethyl, ethyl, t-butyl, 3-(2,4-di-t-pentylphenoxy)propyl, and tetradecyl; alkenyl, such as ethylene, 2-butene; alkoxy, such as methoxy, ethoxy, propoxy, butoxy, 2-methoxyethoxy, sec-butoxy, hexyloxy, 2-ethylhexyloxy, tetradecyloxy, 2-(2,4-di-t-pentylphenoxy)ethoxy, and 2-dodecyloxyethoxy; aryl such as phenyl, 4-t-butylphenyl, 2,4,6-trimethylphenyl, naphthyl; aryloxy, such as phenoxy, 2-methylphenoxy, alpha- or beta-naphthyloxy, and 4-tolyloxy; carbonamido, such as acetamido, benzamido, butyramido, tetradecanamido, alpha-(2,4-di-t-pentyl-phenoxy)acetamido, alpha-(2,4-di-t-pentylphenoxy)butyramido, alpha-(3-pentadecylphenoxy)-hexanamido, alpha-(4-hydroxy-3-t-butylphenoxy)-tetradecanamido, 2-oxo-pyrrolidin-1-yl, 2-oxo-5-tetradecylpyrrolin-1-yl, N-methyltetradecanamido, N-succinimido, N-phthalimido, 2,5-dioxo-1-oxazolidinyl, 3-dodecyl-2,5-dioxo-1-imidazolyl, and N-acetyl-N-dodecylamino, ethoxycarbonylamino, phenoxycarbonylamino, benzyloxycarbonylamino, hexadecyloxycarbonylamino, 2,4-di-t-butylphenoxycarbonylamino, phenylcarbonylamino, 2,5-(di-t-pentylphenyl)carbonylamino, p-dodecyl-phenylcarbonylamino, p-tolylcarbonylamino, N-methylureido, N,N-dimethylureido, N-methyl-N-dodecylureido, N-hexadecylureido, N,N-dioctadecylureido, N,N-dioctyl-N′-ethylureido, N-phenylureido, N,N-diphenylureido, N-phenyl-N-p-tolylureido, N-(m-hexadecylphenyl)ureido, N,N-(2,5-di-t-pentylphenyl)-N′-ethylureido, and t-butylcarbonamido; sulfonamido, such as methylsulfonamido, benzenesulfonamido, p-tolylsulfonamido, p-dodecylbenzenesulfonamido, N-methyltetradecylsulfonamido, N,N-dipropyl-sulfamoylamino, and hexadecylsulfonamido; sulfamoyl, such as N-methylsulfamoyl, N-ethylsulfamoyl, N,N-dipropylsulfamoyl, N-hexadecylsulfamoyl, N,N-dimethylsulfamoyl, N-[3-(dodecyloxy)propyl]sulfamoyl, N-[4-(2,4-di-t-pentylphenoxy)butyl]sulfamoyl, N-methyl-N-tetradecylsulfamoyl, and N-dodecylsulfamoyl; carbamoyl, such as N-methylcarbamoyl, N,N-dibutylcarbamoyl, N-octadecylcarbamoyl, N-[4-(2,4-di-t-pentylphenoxy)butyl]carbamoyl, N-methyl-N-tetradecylcarbamoyl, and N,N-dioctylcarbamoyl; acyl, such as acetyl, (2,4-di-t-amylphenoxy)acetyl, phenoxycarbonyl, p-dodecyloxyphenoxycarbonyl methoxycarbonyl, butoxycarbonyl, tetradecyloxycarbonyl, ethoxycarbonyl, benzyloxycarbonyl, 3-pentadecyloxycarbonyl, and dodecyloxycarbonyl; sulfonyl, such as methoxysulfonyl, octyloxysulfonyl, tetradecyloxysulfonyl, 2-ethylhexyloxysulfonyl, phenoxysulfonyl, 2,4-di-t-pentylphenoxysulfonyl, methylsulfonyl, octylsulfonyl, 2-ethylhexylsulfonyl, dodecylsulfonyl, hexadecylsulfonyl, phenylsulfonyl, 4-nonylphenylsulfonyl, and p-tolylsulfonyl; sulfonyloxy, such as dodecylsulfonyloxy, and hexadecylsulfonyloxy; sulfinyl, such as methylsulfinyl, octylsulfinyl, 2-ethylhexylsulfinyl, dodecylsulfinyl, hexadecylsulfinyl, phenylsulfinyl, 4-nonylphenylsulfinyl, and p-tolylsulfinyl; thio, such as ethylthio, octylthio, benzylthio, tetradecylthio, 2-(2,4-di-t-pentylphenoxy)ethylthio, phenylthio, 2-butoxy-5-t-octylphenylthio, and p-tolylthio; acyloxy, such as acetyloxy, benzoyloxy, octadecanoyloxy, p-dodecylamidobenzoyloxy, N-phenylcarbamoyloxy, N-ethylcarbamoyloxy, and cyclohexylcarbonyloxy; amine, such as phenylanilino, 2-chloroanilino, diethylamine, dodecylamine; imino, such as 1 (N-phenylimido)ethyl, N-succinimido or 3-benzylhydantoinyl; phosphate, such as dimethylphosphate and ethylbutylphosphate; phosphite, such as diethyl and dihexylphosphite; a heterocyclic group, a heterocyclic oxy group or a heterocyclic thio group, each of which may be substituted and which contain a 3 to 7 membered heterocyclic ring composed of carbon atoms and at least one hetero atom selected from the group consisting of oxygen, nitrogen, sulfur, phosphorous, or boron. Such as 2-furyl, 2-thienyl, 2-benzimidazolyloxy or 2-benzothiazolyl; quaternary ammonium, such as triethylammonium; quaternary phosphonium, such as triphenylphosphonium; and silyloxy, such as trimethylsilyloxy.
If desired, the substituents may themselves be further substituted one or more times with the described substituent groups. The particular substituents used may be selected by those skilled in the art to attain desirable properties for a specific application and can include, for example, electron-withdrawing groups, electron-donating groups, and steric groups. When a molecule may have two or more substituents, the substituents may be joined together to form a ring such as a fused ring unless otherwise provided. Generally, the above groups and substituents thereof may include those having up to 48 carbon atoms, typically 1 to 36 carbon atoms and usually less than 24 carbon atoms, but greater numbers are possible depending on the particular substituents selected.
The present invention can be employed in many EL device configurations using small molecule materials, oligomeric materials, polymeric materials, or combinations thereof These include very simple structures comprising a single anode and cathode to more complex devices, such as passive matrix displays comprised of orthogonal arrays of anodes and cathodes to form pixels, and active-matrix displays where each pixel is controlled independently, for example, with thin film transistors (TFTs).
There are numerous configurations of the organic layers wherein the present invention can be successfully practiced. The essential requirements of an OLED are an anode, a cathode, and an organic light-emitting layer located between the anode and cathode. Additional layers may be employed as more fully described hereafter.
A typical structure according to the present invention and especially useful for a small molecule device, is shown in the FIGURE and is comprised of a substrate 101, an anode 103, a hole-injecting layer 105, a hole-transporting layer 107, a light-emitting layer 109, an electron-transporting layer 111, an electron injecting layer 112, and a cathode 113. These layers are described in detail below. Note that the substrate 101 may alternatively be located adjacent to the cathode 113, or the substrate 101 may actually constitute the anode 103 or cathode 113. The organic layers between the anode 103 and cathode 113 are conveniently referred to as the organic EL element. Also, the total combined thickness of the organic layers is desirably less than 500 nm. If the device includes phosphorescent material, a hole-blocking layer, located between the light-emitting layer and the electron-transporting layer, may be present.
The anode 103 and cathode 113 of the OLED are connected to a voltage/current source 150 through electrical conductors 160. The OLED is operated by applying a potential between the anode 103 and cathode 113 such that the anode 103 is at a more positive potential than the cathode 113. Holes are injected into the organic EL element from the anode 103 and electrons are injected into the organic EL element at the cathode 113. Enhanced device stability can sometimes be achieved when the OLED is operated in an AC mode where, for some time period in the AC cycle, the potential bias is reversed and no current flows. An example of an AC driven OLED is described in U.S. Pat. No. 5,552,678.
The OLED device of this invention is typically provided over a supporting substrate 101 where either the cathode 113 or anode 103 can be in contact with the substrate. The electrode in contact with the substrate 101 is conveniently referred to as the bottom electrode. Conventionally, the bottom electrode is the anode 103, but this invention is not limited to that configuration. The substrate 101 can either be light transmissive or opaque, depending on the intended direction of light emission. The light transmissive property is desirable for viewing the EL emission through the substrate 101. Transparent glass or plastic is commonly employed in such cases. The substrate 101 can be a complex structure comprising multiple layers of materials. This is typically the case for active matrix substrates wherein TFTs are provided below the OLED layers. It is still necessary that the substrate 101, at least in the emissive pixelated areas, be comprised of largely transparent materials such as glass or polymers. For applications where the EL emission is viewed through the top electrode, the transmissive characteristic of the bottom support is immaterial, and therefore the substrate can be light transmissive, light absorbing or light reflective. Substrates for use in this case include, but are not limited to, glass, plastic, semiconductor materials such as silicon, ceramics, and circuit board materials. Again, the substrate 101 can be a complex structure comprising multiple layers of materials such as found in active matrix TFT designs. It is necessary to provide in these device configurations a light-transparent top electrode.
When the desired electroluminescent light emission (EL) is viewed through the anode, the anode 103 should be transparent or substantially transparent to the emission of interest. Common transparent anode materials used in this invention are indium-tin oxide (ITO), indium-zinc oxide (IZO) and tin oxide, but other metal oxides can work including, but not limited to, aluminum- or indium-doped zinc oxide, magnesium-indium oxide, and nickel-tungsten oxide. In addition to these oxides, metal nitrides, such as gallium nitride, and metal selenides, such as zinc selenide, and metal sulfides, such as zinc sulfide, can be used as the anode 103. For applications where EL emission is viewed only through the cathode 113, the transmissive characteristics of the anode 103 are immaterial and any conductive material can be used, transparent, opaque or reflective. Example conductors for this application include, but are not limited to, gold, iridium, molybdenum, palladium, and platinum. Typical anode materials, transmissive or otherwise, have a work function of 4.1 eV or greater. Desired anode materials are commonly deposited by any suitable means such as evaporation, sputtering, chemical vapor deposition, or electrochemical means. Anodes can be patterned using well-known photolithographic processes. Optionally, anodes may be polished prior to application of other layers to reduce surface roughness so as to minimize short circuits or enhance reflectivity.
When light emission is viewed solely through the anode 103, the cathode 113 used in this invention can be comprised of nearly any conductive material. Desirable materials have good film-forming properties to ensure good contact with the underlying organic layer, promote electron injection at low voltage, and have good stability. Useful cathode materials often contain a low work function metal (<4.0 eV) or metal alloy. One useful cathode material is comprised of a Mg:Ag alloy wherein the percentage of silver is in the range of 1 to 20%, as described in U.S. Pat. No. 4,885,221. Another suitable class of cathode materials includes bilayers comprising the cathode and a thin electron-injection layer (EIL) in contact with an organic layer (e.g., an electron transporting layer (ETL)), the cathode being capped with a thicker layer of a conductive metal. Here, the EIL preferably includes a low work function metal or metal salt, and if so, the thicker capping layer does not need to have a low work function. One such cathode is comprised of a thin layer of LiF followed by a thicker layer of Al as described in U.S. Pat. No. 5,677,572. An ETL material doped with an alkali metal, for example, Li-doped Alq, is another example of a useful EIL. Other useful cathode material sets include, but are not limited to, those disclosed in U.S. Pat. Nos. 5,059,861, 5,059,862, and 6,140,763.
When light emission is viewed through the cathode, the cathode 113 must be transparent or nearly transparent. For such applications, metals must be thin or one must use transparent conductive oxides, or a combination of these materials. Optically transparent cathodes have been described in more detail in U.S. Pat. No. 4,885,211, U.S. Pat. No. 5,247,190, JP 3,234,963, U.S. Pat. No. 5,703,436, U.S. Pat. No. 5,608,287, U.S. Pat. No. 5,837,391, U.S. Pat. No. 5,677,572, U.S. Pat. No. 5,776,622, U.S. Pat. No. 5,776,623, U.S. Pat. No. 5,714,838, U.S. Pat. No. 5,969,474, U.S. Pat. No. 5,739,545, U.S. Pat. No. 5,981,306, U.S. Pat. No. 6,137,223, U.S. Pat. No. 6,140,763, U.S. Pat. No. 6,172,459, EP 1 076 368, U.S. Pat. No. 6,278,236, and U.S. Pat. No. 6,284,3936. Cathode materials are typically deposited by any suitable method such as evaporation, sputtering, or chemical vapor deposition. When needed, patterning can be achieved through many well known methods including, but not limited to, through-mask deposition, integral shadow masking as described in U.S. Pat. No. 5,276,380 and EP 0 732 868, laser ablation, and selective chemical vapor deposition.
Depending on the aspect of the invention, the device may include a HIL as known in the art. A hole-injecting layer 105 may be provided between anode 103 and hole-transporting layer 107. The hole-injecting layer can serve to improve the film formation property of subsequent organic layers and to facilitate injection of holes into the hole-transporting layer 107. Suitable materials for use in the hole-injecting layer 105 include, but are not limited to, porphyrinic compounds as described in U.S. Pat. No. 4,720,432, plasma-deposited fluorocarbon polymers as described in U.S. Pat. No. 6,208,075, and some aromatic amines, for example, MTDATA (4,4′,4″-tris[(3-methylphenyl)phenylamino]triphenylamine). Alternative hole-injecting materials reportedly useful in organic EL devices are described in EP 0 891 121 A1 and EP 1 029 909 A1. A hole-injection layer is conveniently used in the present invention, and is desirably a plasma-deposited fluorocarbon polymer. The thickness of a hole-injection layer containing a plasma-deposited fluorocarbon polymer can be in the range of 0.2 nm to 15 nm and suitably in the range of 0.3 to 1.5 nm.
In one particular embodiment of the invention, the OLED device also contains HIL containing a compound of Formula (8).
In Formula (8), R independently represents hydrogen or an independently selected substituent, at least one R represents an electron-withdrawing substituent having a Hammett's sigma para value of at least 0.3.
For an explanation of Hammett sigma values and a listing of the values for various substituents see C. Hansch, A. Leo, D. Hoekman; Exploring QSAR: Hydrophobic, Electronic, and Steric Constants. American Chemical Society: Washington, D.C. 1995. Also, C. Hansch, A. Leo; Exploring QSAR: Fundamentals and Applications in Chemistry and Biology. American Chemical Society: Washington, D.C. 1995.
Specific compounds for use in the HIL are as follows:
The thickness of the HIL containing organic materials like Dpq can be 1-100 nm, preferably 5-20 nm.
While not always necessary, it is often useful to include a hole-transporting layer in an OLED device. The hole-transporting layer 107 of the organic EL device contains at least one hole-transporting compound such as an aromatic tertiary amine. An aromatic tertiary amine is understood to be a compound containing at least one trivalent nitrogen atom that is bonded only to carbon atoms, at least one of which is a member of an aromatic ring. In one form the aromatic tertiary amine can be an arylamine, such as a monoarylamine, diarylamine, triarylamine, or a polymeric arylamine. Exemplary monomeric triarylamines are illustrated by Klupfel et al. U.S. Pat. No. 3,180,730. Other suitable triarylamines substituted with one or more vinyl radicals and/or comprising at least one active hydrogen containing group are disclosed by Brantley et al U.S. Pat. No. 3,567,450 and U.S. Pat. No. 3,658,520.
A more preferred class of aromatic tertiary amines is those which include at least two aromatic tertiary amine moieties as described in U.S. Pat. No. 4,720,432 and U.S. Pat. No. 5,061,569. Such compounds include those represented by structural formula (A).
wherein Q1 and Q2 are independently selected aromatic tertiary amine moieties and G is a linking group such as an arylene, cycloalkylene, or alkylene group of a carbon to carbon bond. In one embodiment, at least one of Q1 or Q2 contains a polycyclic fused ring structure, e.g., a naphthalene. When G is an aryl group, it is conveniently a phenylene, biphenylene, or naphthalene moiety.
A useful class of triarylamines satisfying structural formula (A) and containing two triarylamine moieties is represented by structural formula (B):
where
R1 and R2 each independently represents a hydrogen atom, an aryl group, or an 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 (C):
wherein R5 and R6 are independently selected aryl groups. In one embodiment, at least one of R5 or R6 contains a polycyclic fused ring structure, e.g., a naphthalene.
Another class of aromatic tertiary amines is the tetraaryldiamines. Desirable tetraaryldiamines include two diarylamino groups, such as indicated by formula (C), linked through an arylene group. Useful tetraaryldiamines include those represented by formula (D).
wherein
each Are is an independently selected arylene group, such as a phenylene or anthracene moiety,
n is an integer of from 1 to 4, and
Ar, R7, R8, and R9 are independently selected aryl groups.
In a typical embodiment, at least one of Ar, R7, R8, and R9 is a polycyclic fused ring structure, e.g., a naphthalene.
The various alkyl, alkylene, aryl, and arylene moieties of the foregoing structural formulae (A), (B), (C), (D), can each in turn be substituted. Typical substituents include alkyl groups, alkoxy groups, aryl groups, aryloxy groups, and halide such as fluoride, chloride, and bromide. The various alkyl and alkylene moieties typically contain from about 1 to 6 carbon atoms. The cycloalkyl moieties can contain from 3 to about 10 carbon atoms, but typically contain five, six, or seven ring carbon atoms—e.g., cyclopentyl, cyclohexyl, and cycloheptyl ring structures. The aryl and arylene moieties are usually phenyl and phenylene moieties.
The hole-transporting layer can be formed of a single tertiary amine compound or a mixture of such compounds. Specifically, one may employ a triarylamine, such as a triarylamine satisfying the formula (B), in combination with a tetraaryldiamine, such as indicated by formula (D). Illustrative of useful aromatic tertiary amines are the following:
Another class of useful hole-transporting materials includes polycyclic aromatic compounds as described in EP 1 009 041. Tertiary aromatic amines with more than two amine groups may be used including oligomeric materials. In addition, polymeric hole-transporting materials can be used such as poly(N-vinylcarbazole) (PVK), polythiophenes, polypyrrole, polyaniline, and copolymers such as poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) also called PEDOT/PSS. It is also possible for the hole-transporting layer to comprise two or more sublayers of differing compositions, the composition of each sublayer being as described above. The thickness of the hole-transporting layer can be between 10 and about 500 nm and suitably between 50 and 300 nm.
As more fully described in U.S. Pat. Nos. 4,769,292 and 5,935,721, the light-emitting layer (LEL) of the organic EL element includes a luminescent material where electroluminescence is produced as a result of electron-hole pair recombination. The light-emitting layer can be comprised of a single material, but more commonly consists of a host material doped with a guest emitting material or materials where light emission comes primarily from the emitting materials and can be of any color. The host materials in the light-emitting layer can be an electron-transporting material, as defined below, a hole-transporting material, as defined above, or another material or combination of materials that support hole-electron recombination. Fluorescent emitting materials are typically incorporated at 0.01 to 10% by weight of the host material.
The host and emitting materials can be small non-polymeric molecules or polymeric materials such as polyfluorenes and polyvinylarylenes (e.g., poly(p-phenylenevinylene), PPV). In the case of polymers, small-molecule emitting materials can be molecularly dispersed into a polymeric host, or the emitting materials can be added by copolymerizing a minor constituent into a host polymer. Host materials may be mixed together in order to improve film formation, electrical properties, light emission efficiency, operating lifetime, or manufacturability. The host may comprise a material that has good hole-transporting properties and a material that has good electron-transporting properties.
An important relationship for choosing a fluorescent material as a guest emitting material is a comparison of the excited singlet-state energies of the host and the fluorescent material. It is highly desirable that the excited singlet-state energy of the fluorescent material be lower than that of the host material. The excited singlet-state energy is defined as the difference in energy between the emitting singlet state and the ground state. For non-emissive hosts, the lowest excited state of the same electronic spin as the ground state is considered the emitting state.
Host and emitting materials known to be of use include, but are not limited to, those disclosed in U.S. Pat. No. 4,768,292, U.S. Pat. No. 5,141,671, U.S. Pat. No. 5,150,006, U.S. Pat. No. 5,151,629, U.S. Pat. No. 5,405,709, U.S. Pat. No. 5,484,922, U.S. Pat. No. 5,593,788, U.S. Pat. No. 5,645,948, U.S. Pat. No. 5,683,823, U.S. Pat. No. 5,755,999, U.S. Pat. No. 5,928,802, U.S. Pat. No. 5,935,720, U.S. Pat. No. 5,935,721, and U.S. Pat. No. 6,020,078.
Metal complexes of 8-hydroxyquinoline and similar derivatives, also known as metal-chelated oxinoid compounds (Formula E), constitute one class of useful host compounds capable of supporting electroluminescence, and are particularly suitable for light emission of wavelengths longer than 500 nm, e.g., green, yellow, orange, and red.
Derivatives of 9,10-di-(2-naphthyl)anthracene (Formula F1) constitute one class of useful host materials capable of supporting electroluminescence, and are particularly suitable for light emission of wavelengths longer than 400 nm, e.g., blue, green, yellow, orange or red.
wherein: R1, R2, R3, R4, R5, and R6 represent one or more substituents on each ring where each substituent is individually selected from the following groups:
Group 1: hydrogen, or alkyl of from 1 to 24 carbon atoms;
Group 2: aryl or substituted aryl of from 5 to 20 carbon atoms;
Group 3: carbon atoms from 4 to 24 necessary to complete a fused aromatic ring of anthracenyl; pyrenyl, or perylenyl;
Group 4: heteroaryl or substituted heteroaryl of from 5 to 24 carbon atoms as necessary to complete a fused heteroaromatic ring of furyl, thienyl, pyridyl, quinolinyl or other heterocyclic systems;
Group 5: alkoxylamino, alkylamino, or arylamino of from 1 to 24 carbon atoms; and
Group 6: fluorine, chlorine, bromine or cyano.
Illustrative examples include 9,10-di-(2-naphthyl)anthracene and 2-t-butyl-9,10-di-(2-naphthyl)anthracene. Other anthracene derivatives can be useful as a host in the LEL, including derivatives of 9,10-bis[4-(2,2-diphenylethenyl)phenyl]anthracene.
The monoanthracene derivative of Formula (F2) is also a useful host material capable of supporting electroluminescence, and are particularly suitable for light emission of wavelengths longer than 400 nm, e.g., blue, green, yellow, orange or red.
wherein:
R1-R8 are H; and
R9 is a naphthyl group containing no fused rings with aliphatic carbon ring members; provided that R9 and R10 are not the same, and are free of amines and sulfur compounds. Suitably, R9 is a substituted naphthyl group with one or more further fused rings such that it forms a fused aromatic ring system, including a phenanthryl, pyrenyl, fluoranthene, perylene, or substituted with one or more substituents including fluorine, cyano group, hydroxy, alkyl, alkoxy, aryloxy, aryl, a heterocyclic oxy group, carboxy, trimethylsilyl group, or an unsubstituted naphthyl group of two fused rings. Conveniently, R9 is 2-naphthyl, or 1-naphthyl substituted or unsubstituted in the para position; and
R10 is a biphenyl group having no fused rings with aliphatic carbon ring members. Suitably R10 is a substituted biphenyl group, such that is forms a fused aromatic ring system including but not limited to a naphthyl, phenanthryl, perylene, or substituted with one or more substituents including fluorine, cyano group, hydroxy, alkyl, alkoxy, aryloxy, aryl, a heterocyclic oxy group, carboxy, trimethylsilyl group, or an unsubstituted biphenyl group. Conveniently, R10 is 4-biphenyl, 3-biphenyl unsubstituted or substituted with another phenyl ring without fused rings to form a terphenyl ring system, or 2-biphenyl. Particularly useful is 9-(2-naphthyl)-10-(4-biphenyl)anthracene.
Another useful class of anthracene derivatives is represented by general formula (F3)
A1-L-A2 (F3)
wherein A1 and A2 each represent a substituted or unsubstituted monophenyl-anthryl group or a substituted or unsubstituted diphenylanthryl group and can be the same with or different from each other and L represents a single bond or a divalent linking group.
Another useful class of anthracene derivatives is represented by general formula (F4)
A3-An-A4 (F4)
wherein An represents a substituted or unsubstituted divalent anthracene residue group, A3 and A4 each represent a substituted or unsubstituted monovalent condensed aromatic ring group or a substituted or unsubstituted non-condensed ring aryl group having 6 or more carbon atoms and can be the same with or different from each other.
Asymmetric anthracene derivatives as disclosed in U.S. Pat. No. 6,465,115 and WO 2004/018587 are useful hosts and these compounds are represented by general formulas (F5) and (F6) shown below, alone or as a component in a mixture
wherein;
Ar is an (un)substituted condensed aromatic group of 10-50 nuclear carbon atoms;
Ar′ is an (un)substituted aromatic group of 6-50 nuclear carbon atoms;
X is an (un)substituted aromatic group of 6-50 nuclear carbon atoms, (un)substituted aromatic heterocyclic group of 5-50 nuclear carbon atoms, (un)substituted alkyl group of 1-50 carbon atoms, (un)substituted alkoxy group of 1-50 carbon atoms, (un)substituted aralkyl group of 6-50 carbon atoms, (un)substituted aryloxy group of 5-50 nuclear carbon atoms, (un)substituted arylthio group of 5-50 nuclear carbon atoms, (un)substituted alkoxycarbonyl group of 1-50 carbon atoms, carboxy group, halogen atom, cyano group, nitro group, or hydroxy group;
a, b, and c are whole numbers of 0-4; and n is a whole number of 1-3;
and when n is 2 or more, the formula inside the parenthesis shown below can be the same or different.
Furthermore, the present invention provides anthracene derivatives represented by general formula (F6) shown below
wherein:
Ar is an (un)substituted condensed aromatic group of 10-50 nuclear carbon atoms;
Ar′ is an (un)substituted aromatic group of 6-50 nuclear carbon atoms;
X is an (un)substituted aromatic group of 6-50 nuclear carbon atoms, (un)substituted aromatic heterocyclic group of 5-50 nuclear carbon atoms, (un)substituted alkyl group of 1-50 carbon atoms, (un)substituted alkoxy group of 1-50 carbon atoms, (un)substituted aralkyl group of 6-50 carbon atoms, (un)substituted aryloxy group of 5-50 nuclear carbon atoms, (un)substituted arylthio group of 5-50 nuclear carbon atoms, (un)substituted alkoxycarbonyl group of 1-50 carbon atoms, carboxy group, halogen atom, cyano group, nitro group, or hydroxy group;
a, b, and c are whole numbers of 0-4; and n is a whole number of 1-3; and
when n is 2 or more, the formula inside the parenthesis shown below can be the same or different
Specific examples of useful anthracene materials for use in a light-emitting layer include
Benzazole derivatives (Formula G) constitute another class of useful host materials capable of supporting electroluminescence, and are particularly suitable for light emission of wavelengths longer than 400 nm, e.g., blue, green, yellow, orange or red.
wherein:
n is an integer of 3 to 8;
Z is O, NR or S; and
R and R′ are individually hydrogen; alkyl of from 1 to 24 carbon atoms, for example, propyl, t-butyl, heptyl, and the like; aryl or hetero-atom substituted aryl of from 5 to 20 carbon atoms for example phenyl and naphthyl, furyl, thienyl, pyridyl, quinolinyl and other heterocyclic systems; or halo such as chloro, fluoro; or atoms necessary to complete a fused aromatic ring; and
L is a linkage unit consisting of alkyl, aryl, substituted alkyl, or substituted aryl, which connects the multiple benzazoles together. L may be either conjugated with the multiple benzazoles or not in conjugation with them. An example of a useful benzazole is 2,2′,2″-(1,3,5-phenylene)tris[1-phenyl-1H-benzimidazole].
Styrylarylene derivatives as described in U.S. Pat. No. 5,121,029 and JP 08333569 are also useful hosts for blue emission. For example, 9,10-bis[4-(2,2-diphenylethenyl)phenyl]anthracene and 4,4′-bis(2,2-diphenylethenyl)-1,1′-biphenyl (DPVBi) are useful hosts for blue emission.
Useful fluorescent emitting materials include, but are not limited to, derivatives of anthracene, tetracene, xanthene, perylene, rubrene, coumarin, rhodamine, and quinacridone, dicyanomethylenepyran compounds, thiopyran compounds, polymethine compounds, pyrylium and thiapyrylium compounds, fluorene derivatives, periflanthene derivatives, indenoperylene derivatives, bis(azinyl)imine boron compounds, bis(azinyl)methene compounds, and carbostyryl compounds. Illustrative examples of useful materials include, but are not limited to, the following:
Light-emitting phosphorescent materials may be used in the EL device. For convenience, the phosphorescent complex guest material may be referred to herein as a phosphorescent material. The phosphorescent material typically includes one or more ligands, for example monoanionic ligands that can be coordinated to a metal through an sp2 carbon and a heteroatom. Conveniently, the ligand can be phenylpyridine (ppy) or derivatives or analogs thereof. Examples of some useful phosphorescent organometallic materials include tris(2-phenylpyridinato-N,C2′)iridium(III), bis(2-phenylpyridinato-N,C2)iridium(III)(acetylacetonate), and bis(2-phenylpyridinato-N,C2′)platinum(II). Usefully, many phosphorescent organometallic materials emit in the green region of the spectrum, that is, with a maximum emission in the range of 510 to 570 nm.
Phosphorescent materials may be used singly or in combinations other phosphorescent materials, either in the same or different layers. Phosphorescent materials and suitable hosts are described in WO 00/57676, WO 00/70655, WO 01/41512 A1, WO 02/15645 A1, US 2003/0017361 A1, WO 01/93642 A1, WO 01/39234 A2, U.S. Pat. No. 6,458,475 B1, WO 02/071813 A1, U.S. Pat. No. 6,573,651 B2, US 2002/0197511 A1, WO 02/074015 A2, U.S. Pat. No. 6,451,455 B1, US 2003/0072964 A1, US 2003/0068528 A1, U.S. Pat. No. 6,413,656 B1, U.S. Pat. No. 6,515,298 B2, U.S. Pat. No. 6,451,415 B1, U.S. Pat. No. 6,097,147, US 2003/0124381 A1, US 2003/0059646 A1, US 2003/0054198 A1, EP 1 239 526 A2, EP 1 238 981 A2, EP 1 244 155 A2, US 2002/0100906 A1, US 2003/0068526 A1, US 2003/0068535 A1, JP 2003073387A, JP 2003073388A, US 2003/0141809 A1, US 2003/0040627 A1, JP 2003059667A, JP 2003073665A, and US 2002/0121638 A1.
The emission wavelengths of cyclometallated Ir(III) complexes of the type IrL3 and IrL2L′, such as the green-emitting fac-tris(2-phenylpyridinato-N,C2′)iridium(III) and bis(2-phenylpyridinato-N,C2′)iridium(III)(acetylacetonate) may be shifted by substitution of electron donating or withdrawing groups at appropriate positions on the cyclometallating ligand L, or by choice of different heterocycles for the cyclometallating ligand L. The emission wavelengths may also be shifted by choice of the ancillary ligand L′. Examples of red emitters are the bis(2-(2′-benzothienyl)pyridinato-N,C3′)iridium(III)(acetylacetonate) and tris(2-phenylisoquinolinato-N,C)iridium(III). A blue-emitting example is bis(2-(4,6-difluorophenyl)-pyridinato-N,C2′)iridium(III)(picolinate).
Red electrophosphorescence has been reported, using bis(2-(2′-benzo[4,5-a]thienyl)pyridinato-N, C3)iridium(acetylacetonate)[Btp2Ir(acac)] as the phosphorescent material (C. Adachi, S. Lamansky, M. A. Baldo, R. C. Kwong, M. E. Thompson, and S. R. Forrest, App. Phys. Lett., 78, 1622-1624 (2001)).
Other important phosphorescent materials include cyclometallated Pt(II) complexes such as cis-bis(2-phenylpyridinato-N,C2′)platinum(II), cis-bis(2-(2′-thienyl)pyridinato-N,C3′) platinum(II), cis-bis(2-(2′-thienyl)quinolinato-N,C5′) platinum(II), or (2-(4,6-difluorophenyl)pyridinato-N,C2,) platinum (II) (acetylacetonate). Pt (II) porphyrin complexes such as 2,3,7,8,12,13,17,18-octaethyl-21H, 23H-porphine platinum(II) are also useful phosphorescent materials.
Still other examples of useful phosphorescent materials include coordination complexes of the trivalent lanthanides such as Tb3+ and Eu3+ (J. Kido et al., Appl. Phys. Lett., 65, 2124 (1994)).
Suitable host materials for phosphorescent materials should be selected so that transfer of a triplet exciton can occur efficiently from the host material to the phosphorescent material but cannot occur efficiently from the phosphorescent material to the host material. Therefore, it is highly desirable that the triplet energy of the phosphorescent material be lower than the triplet energy of the host. Generally speaking, a large triplet energy implies a large optical bandgap. However, the band gap of the host should not be chosen so large as to cause an unacceptable barrier to injection of charge carriers into the light-emitting layer and an unacceptable increase in the drive voltage of the OLED. Suitable host materials are described in WO 00/70655 A2; 01/39234 A2; 01/93642 A1; 02/074015 A2; 02/15645 A1, and US 20020117662. Suitable hosts include certain aryl amines, triazoles, indoles and carbazole compounds. Examples of desirable hosts are 4,4′-N,N′-dicarbazole-biphenyl, otherwise known as 4,4′-bis(carbazol-9-yl)biphenyl or CBP; 4,4′-N,N′-dicarbazole-2,2′-dimethyl-biphenyl, otherwise known as 2,2′-dimethyl-4,4′-bis(carbazol-9-yl)biphenyl or CDBP; 1,3-bis(N,N′-dicarbazole)benzene, otherwise known as 1,3-bis(carbazol-9-yl)benzene, and poly(N-vinylcarbazole), including their derivatives.
In another embodiment of the invention, the light-emitting layer comprises at least one light emitting compound selected from bis(azinyl)azene boron complex compounds, amine containing monostyryl, amine containing distyryl, amine containing tristyryl and amine containing tetrastyryl compounds.
Preferred bis(azinyl)azene boron complex compounds are according to the structure K:
wherein:
Preferred classes of styryl dopants in this invention includes blue-emitting derivatives of such styrylarenes and distyrylarenes as distyrylbenzene, styrylbiphenyl, and distyrylbiphenyl, including compounds described in U.S. Pat. No. 5,121,029. Among such derivatives that provide blue luminescence, particularly useful are those substituted with diarylamino groups. Examples include bis[2-[4-[N,N-diarylamino]phenyl]vinyl]-benzenes of the general structure L1 shown below:
[N,N-diarylamino][2-[4-[N,N-diarylamino]phenyl]vinyl]biphenyls of the general structure L2 shown below:
and bis[2-[4-[N,N-diarylamino]phenyl]vinyl]biphenyls of the general structure L3 shown below:
In Formulas L1 to L3, X1-X4 can be the same or different, and individually represent one or more substituents such as alkyl, aryl, fused aryl, halo, or cyano. In a preferred embodiment, X1-X4 are individually alkyl groups, each containing from one to about ten carbon atoms
Desirable host materials are capable of forming a continuous film.
It should noted that many of the same materials described as hosts in a light-emitting layer are also suitable for use as the carbocyclic fused ring aromatic compound in the first electron-transporting layer. The same material may be used in both as the host in the light-emitting layer as well as in the first electron-transporting layer of the invention.
In addition to suitable hosts, an OLED device employing a phosphorescent material often requires at least one hole-blocking layer placed between the electron-transporting layer 111 and the light-emitting layer 109 to help confine the excitons and recombination events to the light-emitting layer comprising the host and phosphorescent material. In this case, there should be an energy barrier for hole migration from the host into the hole-blocking layer, while electrons should pass readily from the hole-blocking layer into the light-emitting layer comprising a host and a phosphorescent material. The first requirement entails that the ionization potential of the hole-blocking layer be larger than that of the light-emitting layer 109, desirably by 0.2 eV or more. The second requirement entails that the electron affinity of the hole-blocking layer not greatly exceed that of the light-emitting layer 109, and desirably be either less than that of light-emitting layer or not exceed that of the light-emitting layer by more than about 0.2 eV.
When used with an electron-transporting layer whose characteristic luminescence is green, such as an Alq-containing electron-transporting layer as described below, the requirements concerning the energies of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the material of the hole-blocking layer frequently result in a characteristic luminescence of the hole-blocking layer at shorter wavelengths than that of the electron-transporting layer, such as blue, violet, or ultraviolet luminescence. Thus, it is desirable that the characteristic luminescence of the material of a hole-blocking layer be blue, violet, or ultraviolet. It is further desirable, but not absolutely required, that the triplet energy of the hole-blocking material be greater than that of the phosphorescent material. Suitable hole-blocking materials are described in WO 00/70655A2 and WO 01/93642 A1. Two examples of useful hole-blocking materials are bathocuproine (BCP) and bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III) (BAlq). The characteristic luminescence of BCP is in the ultraviolet, and that of BAlq is blue. Metal complexes other than BAlq are also known to block holes and excitons as described in US 20030068528. In addition, US 20030175553 A1 describes the use of fac-tris(1-phenylpyrazolato-N,C2′)iridium(III) (Irppz) for this purpose.
When a hole-blocking layer is used, its thickness can be between 2 and 100 nm and suitably between 5 and 10 nm.
The invention contains a first layer which is an electron transporting layer as generally described above. In other embodiments it may be desirable to have additional electron-transporting materials or layers as described below.
Desirable thin film-forming materials for use in forming electron-transporting layer of organic EL devices are metal-chelated oxinoid compounds, including chelates of oxine itself (also commonly referred to as 8-quinolinol or 8-hydroxyquinoline). Such compounds help to inject and transport electrons, exhibit high levels of performance, and are readily fabricated in the form of thin films. Exemplary of contemplated oxinoid compounds are those satisfying structural formula (E), previously described.
Other electron-transporting materials suitable for use in the electron-transporting layer include various butadiene derivatives as disclosed in U.S. Pat. No. 4,356,429 and various heterocyclic optical brighteners as described in U.S. Pat. No. 4,539,507. Benzazoles satisfying structural formula (G) are also useful electron transporting materials. Triazines are also known to be useful as electron transporting materials.
If both a hole-blocking layer and an electron-transporting layer 111 are used, electrons should pass readily from the electron-transporting layer 111 into the hole-blocking layer. Therefore, the electron affinity of the electron-transporting layer 111 should not greatly exceed that of the hole-blocking layer. Desirably, the electron affinity of the electron-transporting layer should be less than that of the hole-blocking layer or not exceed it by more than about 0.2 eV.
If an additional electron-transporting layer is used, its thickness maybe between 2 and 100 nm and suitably between 5 and 20 nm.
The invention contains a second layer which is an electron injecting layer as generally described above. In other embodiments it may be desirable to have additional electron-injecting materials or layers as described below.
Electron-injecting layers include those taught in U.S. Pat. Nos. 5,608,287; 5,776,622; 5,776,623; 6,137,223; and 6,140,763; the disclosures of which are incorporated herein by reference. An electron-injecting layer generally consists of an electron-injecting material having a work function less than 4.2 eV or the salt of a metal having a work function less than 4.2 eV. A thin-film containing low work-function alkaline metals or alkaline earth metals, such as Li, Na, K, Rb,Cs, Ca, Mg, Sr and Ba can be employed. In addition, an organic material doped with these low work-function metals can also be used effectively as the electron-injecting layer. Examples are Li— or Cs-doped Alq or Bphen. When included in the layer, the elemental metal is often present in the amount of from 0.1% to 15%, commonly in the amount of 0.1% to 10%, and often in the amount of 1 to 5% by volume of the total material in the layer.
The electron-injecting layer may also include alkali and alkaline earth metal inorganic salts, including their oxides. Also included are alkali and alkaline earth metal organic salts and complexes. In fact, any metal salt or compound which can be reduced in the device to liberate its free metal, either as a free entity or a transient species, are useful in the electron-injecting layer. Examples include, lithium fluoride (LiF), sodium fluoride (NaF), cesium fluoride (CsF), lithium oxide (Li2O), lithium acetylacetonate (Liacac), lithium benzoate, potassium benzoate, lithium acetate, lithium formate or any of the salts or complexes of an alkali or alkaline earth metal previously described as being useful in the first electron-transporting layer of the invention.
In practice, the electron-injecting layer is typically in the range of 0.05-2.0 nm when using a thin interfacial layer of inorganic materials. An interfacial electron-injecting layer in this thickness range will provide effective electron injection into the layer or further layer of the invention. Alternatively, electron-injection layers containing organic materials may be somewhat thicker, preferable 10 nm but less than 50 nm thick or, preferably 20 nm to 40 nm
In some instances, layers 109 through 111 can optionally be collapsed into a single layer that serves the function of supporting both light emission and electron transportation. The hole-blocking layer, when present, and layer 111 may also be collapsed into a single layer that functions to block holes or excitons, and supports electron transport. It also known in the art that emitting materials may be included in the hole-transporting layer 107. In that case, the hole-transporting material may serve as a host. Multiple materials may be added to one or more layers in order to create a white-emitting OLED, for example, by combining blue- and yellow-emitting materials, cyan- and red-emitting materials, or red-, green-, and blue-emitting materials. White-emitting devices are described, for example, in EP 1 187 235, US 20020025419, EP 1 182 244, U.S. Pat. No. 5,683,823, U.S. Pat. No. 5,503,910, U.S. Pat. No. 5,405,709, and U.S. Pat. No. 5,283,182 and can be equipped with a suitable filter arrangement to produce a color emission.
This invention may be used in so-called stacked device architecture, for example, as taught in U.S. Pat. No. 5,703,436 and U.S. Pat. No. 6,337,492.
The organic materials mentioned above are suitably deposited through sublimation, but can be deposited from a solvent with an optional binder to improve film formation. If the material is a polymer, solvent deposition is usually preferred. The material to be deposited by sublimation can be vaporized from a sublimator “boat” often comprised of a tantalum material, e.g., as described in U.S. Pat. No. 6,237,529, or can be first coated onto a donor sheet and then sublimed in closer proximity to the substrate. Layers with a mixture of materials can utilize separate sublimator boats or the materials can be pre-mixed and coated from a single boat or donor sheet. Patterned deposition can be achieved using shadow masks, integral shadow masks (U.S. Pat. No. 5,294,870), spatially-defined thermal dye transfer from a donor sheet (U.S. Pat. No. 5,851,709 and U.S. Pat. No. 6,066,357) and inkjet method (U.S. Pat. No. 6,066,357).
Organic materials useful in making OLEDs, for example organic hole-transporting materials, organic light-emitting materials doped with an organic electroluminescent components have relatively complex molecular structures with relatively weak molecular bonding forces, so that care must be taken to avoid decomposition of the organic material(s) during physical vapor deposition. The aforementioned organic materials are synthesized to a relatively high degree of purity, and are provided in the form of powders, flakes, or granules. Such powders or flakes have been used heretofore for placement into a physical vapor deposition source wherein heat is applied for forming a vapor by sublimation or vaporization of the organic material, the vapor condensing on a substrate to provide an organic layer thereon.
Several problems have been observed in using organic powders, flakes, or granules in physical vapor deposition: These powders, flakes, or granules are difficult to handle. These organic materials generally have a relatively low physical density and undesirably low thermal conductivity, particularly when placed in a physical vapor deposition source which is disposed in a chamber evacuated to a reduced pressure as low as 10−6 Torr. Consequently, powder particles, flakes, or granules are heated only by radiative heating from a heated source, and by conductive heating of particles or flakes directly in contact with heated surfaces of the source. Powder particles, flakes, or granules which are not in contact with heated surfaces of the source are not effectively heated by conductive heating due to a relatively low particle-to-particle contact area; This can lead to nonuniform heating of such organic materials in physical vapor deposition sources. Therefore, result in potentially nonuniform vapor-deposited organic layers formed on a substrate.
These organic powders can be consolidated into a solid pellet. These solid pellets consolidating into a solid pellet from a mixture of a sublimable organic material powder are easier to handle. Consolidation of organic powder into a solid pellet can be accomplished with relatively simple tools. A solid pellet formed from mixture comprising one or more non-luminescent organic non-electroluminescent component materials or luminescent electroluminescent component materials or mixture of non-electroluminescent component and electroluminescent component materials can be placed into a physical vapor deposition source for making organic layer. Such consolidated pellets can be used in a physical vapor deposition apparatus.
In one aspect, the present invention provides a method of making an organic layer from compacted pellets of organic materials on a substrate, which will form part of an OLED.
One preferred method for depositing the materials of the present invention is described in US 2004/0255857 and U.S. Ser. No. 10/945,941 where different source evaporators are used to evaporate each of the materials of the present invention. A second preferred method involves the use of flash evaporation where materials are metered along a material feed path in which the material feed path is temperature controlled. Such a preferred method is described in the following co-assigned patent applications: U.S. Ser. No. 10/784,585; U.S. Ser. No. 10/805,980; U.S. Ser. No. 10/945,940; U.S. Ser. No. 10/945,941; U.S. Ser. No. 11/050,924; and U.S. Ser. No. 11/050,934. Using this second method, each material may be evaporated using different source evaporators or the solid materials may be mixed prior to evaporation using the same source evaporator.
Most OLED devices are sensitive to moisture or oxygen, or both, so they are commonly sealed in an inert atmosphere such as nitrogen or argon, along with a desiccant such as alumina, bauxite, calcium sulfate, clays, silica gel, zeolites, alkaline metal oxides, alkaline earth metal oxides, sulfates, or metal halides and perchlorates. Methods for encapsulation and desiccation include, but are not limited to, those described in U.S. Pat. No. 6,226,890. In addition, barrier layers such as SiOx, Teflon, and alternating inorganic/polymeric layers are known in the art for encapsulation. Any of these methods of sealing or encapsulation and desiccation can be used with the EL devices constructed according to the present invention.
OLED devices of this invention can employ various well-known optical effects in order to enhance their emissive properties if desired. This includes optimizing layer thicknesses to yield maximum light transmission, providing dielectric mirror structures, replacing reflective electrodes with light-absorbing electrodes, providing anti-glare or anti-reflection coatings over the display, providing a polarizing medium over the display, or providing colored, neutral density, or color-conversion filters over the display. Filters, polarizers, and anti-glare or anti-reflection coatings may be specifically provided over the EL device or as part of the EL device.
Embodiments of the invention may provide advantageous features such as higher luminous yield, lower drive voltage, and higher power efficiency, longer operating lifetimes or ease of manufacture. Embodiments of devices useful in the invention can provide a wide range of hues including those useful in the emission of white light (directly or through filters to provide multicolor displays). Embodiments of the invention can also provide an area lighting device.
The architecture of the OLED devices of all aspects of the invention can be constructed, by the careful selection of hosts and dopants (also known as light emitting materials), so that the devices can be made to emit blue, green, red or white light. Additionally, in all of the aforementioned aspects, the device may include two light-emitting layers, for example such as in an EL device that produces white light.
The invention and its advantages are further illustrated by the specific examples that follow. Materials were prepared according to methods known and previously described in the art. The term “percentage” or “percent” and the symbol “%” indicate the volume percent (or a thickness ratio as measured on a thin film thickness monitor) of a particular compound of the total material in the layer.
A series of white EL devices (1.1 through 1.14) were constructed in the following manner:
The above sequence completes the deposition of the EL device. The device is then hermetically packaged in a dry glove box for protection against ambient environment.
Recorded in Table 1 are the times required for the luminance efficiencies of the devices to drop to 50% (T50) or 95% (T95) of their initial value while operating at a current density of 80 mA/cm2. It should be noted that these T50 and T95 measurements are accelerated tests and are estimates of performance under normal operating conditions. In this regard, it is believed that under these accelerated conditions, a minimum of about 20-30 hours in T95 would provide satisfactory performance for some applications where the usable lifetime of the device is short (for example, a cell-phone). However, for other applications (for example, a television) where prevention of ‘burn-in’ is critical over a long lifetime, a desirable T95 would be a minimum of about 100 hours, or greater than about 200 hours, or best, greater than about 400 hours which would be predicted to prevent a ‘burn-in’ effect in excess of 10,000 hours under typical operating conditions. Note that Alq is also designated as CO-1 and Bphen as R-2.
In Table 1, comparative example 1.1 shows that a device with only a single combined ETL/EIL composed only of Alq, Bphen and Lithium does not show good T95 stability. Comparisons of comparative example 1.3 to 1.2 as well as comparative example 1.6 to 1.4 and 1.5 demonstrate some improvement in T95 stability whenever the metal complex (in these examples, MC-1) is present at levels greater than 50% in the first layer ETL with either no or a typical non-inventive second layer EIL formulation. In a similar manner, comparative examples 1.7-1.9 and 1.12 also highlight that some improvements in T95 can seen with other second layer EIL formulations. However, it can seen that with the second layer EIL of the invention (comprising a phenathroline, metal oxinoid and alkali or alkaline metal), examples 1.10 and 1.13, which contain only a 1:1 mixture of metal complex and carbocycle in the first layer do not show much improvement in T95. Example 1.11, with the same inventive second layer EIL, but with a first layer ETL that contains greater than 50% of the metal complex but is thick, shows a small improvement in T95. Only example 1.14, which contains a thin first layer ETL with a high level of metal complex and a second layer EIL consisting of a mixture of phenanthroline, metal oxinoid and alkali or alkaline metal, shows a surprising and unpredicted increase in T95.
The entire contents of the patents and other publications referred to in this specification are incorporated herein by reference. The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
Reference is made to commonly assigned U.S. patent applications U.S. Ser. No. 11/259,472 filed Oct. 26, 2005 which is a continuation-in-part of U.S. Ser. No. 11/156,302 filed Jun. 17, 2005.
