Patent Description:
Organic light-emitting diodes (OLEDs), which are self-emitting devices, have a wide viewing angle, excellent contrast, quick response, high brightness, excellent driving voltage characteristics, and color reproduction. A typical OLED includes an anode, a hole transport layer (HTL), an emission layer (EML), an electron transport layer (ETL), and a cathode, which are sequentially stacked on a substrate. In this regard, the HTL, the EML, and the ETL are thin films formed from organic compounds.

<CIT> refers to an organic light emitting device that includes a first electrode, a second electrode, and two or more light emitting units provided between the first electrode and the second electrode, wherein a charge generation layer is provided between, among the light emitting units, two light emitting units that are adjacent to each other, an electron transport layer is provided between the charge generation layer and the light emitting unit placed closer to the first electrode of the two adjacent light emitting units, and the electron transport layer includes a first electron transport layer doped with an n-type dopant, and a second electron transport layer doped with a metal salt, metal oxide or organic metal salt.

<CIT> refers to an organic light emitting device comprising a first electrode, a second electrode, and one or more organic material layers disposed between the first electrode and the second electrode, and having an excellent life-span property by changing a dipole moment of a compound comprised in the organic material layers.

It is one object to provide organic light-emitting diode having an increased external quantum efficiency (EQE) of OLEDs, especially for blue emitting OLEDs but also, for example for red, green or white, and/or an increased lifetime, in particular for top and/or bottom emission organic light-emitting diodes (OLED).

According to the invention, there is provided an organic light-emitting diode (OLED) comprising an emission layer, and an electron transport layer stack of at least two electron transport layers, wherein a first electron transport layer and a second electron transport layer comprises at least one matrix compound and in addition,.

The phosphine oxide matrix compound may be same or different selected for each electron transport layer.

In the context that the electron transport layer or layers comprising a lithium halide or a lithium organic complex is free of an elemental metal, the term "free of" means that the electron transport layer or layers containing lithium halide/organic complex may comprise impurities of <NUM> wt. -% or less, preferably <NUM> wt. -% or less, and more preferably <NUM> wt. -% or less of a metal dopant and most preferred no metal dopant.

In the context that the electron transport layer or layers that comprises an elemental metal is free of a metal salt and/or a metal organic complex, the term "free of" means that the electron transport layer or layers containing an elemental metal may comprise of <NUM> wt. -% or less of a lithium halide and/or a lithium organic complex, preferably <NUM> wt. -% or less, and more preferably <NUM> wt. -% or less, and even more preferably <NUM> wt. -% or less of a lithium halide and/or a lithium organic complex and most preferred no lithium halide and/or a lithium organic complex.

According to various aspects, there is provided an organic light-emitting diode comprising at least one emission layer, and an electron transport layer stack of at least two electron transport layers, wherein a first electron transport layer and a second electron transport layer comprises at least one matrix compound, wherein the matrix compound is a phosphine oxide based compound, preferably selected from the group comprising (<NUM>-(dibenzo[c,h]acridin-<NUM>-yl)phenyl)diphenylphosphine oxide, <NUM>-phenyl-<NUM>-benzo[b] dinaphtho [<NUM>,<NUM>-d:<NUM>',<NUM>'-f] phosphepine-<NUM>-oxide, bis(<NUM>-(anthracen-<NUM>-yl)phenyl)- (phenyl)phosphine oxide, phenyldi(pyren-<NUM>-yl)phosphine oxide, (<NUM>-(<NUM>,<NUM>-di(naphthalen-<NUM>-yl)anthracen-<NUM>-yl)phenyl) diphenylphosphine oxide, phenyldi(pyren-<NUM>-yl)phosphine oxide, diphenyl(<NUM>-(pyren-<NUM>-yl)pyridin-<NUM>-yl)phosphine oxide, diphenyl(<NUM>'-(pyren-<NUM>-yl)-[<NUM>,<NUM>'-biphenyl]-<NUM>-yl)phosphine oxide, diphenyl(<NUM>'-(pyren-<NUM>-yl)-[<NUM>,<NUM>'-biphenyl]-<NUM>-yl)phosphine oxide, (<NUM>'-(dibenzo[c,h] acridin-<NUM>-yl)-[<NUM>,<NUM>'-biphenyl]-<NUM>-yl)diphenylphosphine oxide and/or phenyl bis(<NUM>-(pyren-<NUM>-yl)phenyl)phosphine oxide; and in addition,.

wherein the second electron transport layer comprising a lithium halide or a lithium organic complex is free of an elemental metal, and the first electron transport layer that comprises an elemental metal is free of a metal salt and/or a metal organic complex.

According to various aspects, there is provided an organic light-emitting diode comprising at least one emission layer, and an electron transport layer stack of at least three electron transport layers, wherein.

wherein for an electron transport layer stack comprising at least two electron transport layers comprising a lithium halide or a lithium organic complex the lithium halide or lithium organic complex of each electron transport layer are selected same or different, and preferably different and more preferred selected the same; and/or wherein for an electron transport layer stack comprising at least two electron transport layers comprising an elemental metal selected from the group of lithium, magnesium and/or ytterbium; the elemental metal selected from the group of lithium, magnesium and/or ytterbium of each electron transport layer are selected same or different, and preferably selected the same.

According to various aspects, wherein for an electron transport layer stack comprising at least three electron transport layers, at least the second or third electron transport layers comprising a lithium organic complex, the lithium organic complex of each electron transport layer are selected same or different, and preferably the same; and the remaining first electron transport layers, comprising an elemental metal selected from the group of lithium, magnesium and/or ytterbium, preferably magnesium and/or ytterbium, wherein each electron transport layer comprising an elemental metal, the elemental metal are selected same or different, and preferably the same.

Since the first electron transport layer is arranged directly on the emission layer and the second electron transport layer is formed directly on the first electron transport layer, for an electron transport layer stack of two electron transport layers the first electron transport layer is arranged closest to the at least one emission layer, and the second electron transport layer is arranged closest to a cathode.

According to various aspects, wherein for an electron transport layer stack of three electron transport layers the first electron transport layer is arranged closest to an emission layer, the second electron layer is sandwiched between the first and the third electron transport layer and the third electron transport layer is arranged closest to a cathode.

The organic light-emitting diode can be a bottom emission OLED or a top emission OLED.

The external quantum efficiency, also named EQE, is measured in percent (%).

The lifetime, also named LT, between starting brightness and <NUM> % of the original brightness is measured in hours (h).

The voltage, also named V, is measured in Volt (V) at <NUM> milliAmpere per square centimeter (mA/cm<NUM>) in bottom emission devices and at <NUM> mA/cm<NUM> for top emission devices.

The colour space is described by coordinates CIE-x and CIE-y (International Commission on Illumination <NUM>). For blue emission the CIE-y is of particular importance. A smaller CIE-y denotes a deeper blue color.

The efficiency of top emission OLEDs is recorded in candela per Ampere, also named cd/A. Candela is a SI base unit of luminous intensity and describes the power emitted by a light source in a particular direction, weighted by the luminosity function (a standardized model of the sensitivity of the human eye to different wavelengths). The human eye is particularly insensitive to deep blue and deep red colors. Therefore, the efficiency measured in cd/A is corrected for the emission color, in the case of blue emission, the CIE-y. For example, a deeper blue OLED would have a lower cd/A efficiency even if the quantum efficiency (photons in compared to photons out) is the same. By dividing the efficiency measured in cd/A by the CIE-y, the efficiency of OLEDs with slightly different shades of blue can be compared. The efficiency, also named Eff. , is measured in Candela per Ampere (cd/A) and divided by the CIE-y.

The highest occupied molecular orbital, also named HOMO, and lowest unoccupied molecular orbital, also named LUMO, are measured in electron volt (eV).

The term "not the same as" with respect to a lithium halide or a lithium organic complex means that the electron transport layers comprising lithium halide or a lithium organic complex, differs in its containing lithium halide or lithium organic complex.

The term "not the same as" with respect to elemental metal selected from the group of lithium, magnesium and/or ytterbium means that the electron transport layers containing elemental lithium, magnesium and/or ytterbium, differs in its containing elemental metal.

The term "OLED" and "organic light-emitting diode" is simultaneously used and having the same meaning.

As used herein, "weight percent", "wt. -%", "percent by weight", "% by weight", and variations thereof refer to an elemental metal, a composition, component, substance or agent as the weight of that elemental metal, component, substance or agent of the respective electron transport layer divided by the total weight of the respective electron transport layer thereof and multiplied by <NUM>. It is understood that the total weight percent amount of all elemental metal, components, substances or agents of the respective electron transport layer are selected such that it does not exceed <NUM> wt.

As used herein, "volume percent", "vol. -%", "percent by volume", "% by volume", and variations thereof refer to an elemental metal, a composition, component, substance or agent as the volume of that elemental metal, component, substance or agent of the respective electron transport layer divided by the total volume of the respective electron transport layer thereof and multiplied by <NUM>. It is understood that the total volume percent amount of all elemental metal, components, substances or agents of the respective electron transport layer are selected such that it does not exceed <NUM> vol.

As used herein, the term "about" refers to variation in the numerical quantity that can occur. Whether or not modified by the term "about", the claims include equivalents to the quantities.

It should be noted that, as used in this specification and the appended claims, the singular forms "a", "an", and "the" include plural referents unless the content clearly dictates otherwise.

The term "free of', "does not contain", "does not comprise" does not exclude impurities. Impurities have no technical effect with respect to the object achieved by the present invention.

The cathode is not an electron transport layer or electron layer stack. In particular the second electron transport layer is not a cathode. According to the present invention the cathode and the electron transport layer differs in their chemical composition.

Further, the electron layer stack is arranged between the anode and cathode layer.

Further, for reasons of clarification, the electron transport layer differs in their chemical composition from the cathode described in <CIT>.

The term "alkyl" refers to straight-chain or branched alkyl groups. The term "<NUM> to <NUM> carbon atoms" as used herein refers to straight-chain or branched alkyl groups having <NUM> to <NUM> carbon atoms. The alkyl groups can be selected from the group comprising methyl, ethyl and the isomers of propyl, butyl or pentyl, such as isopropyl, isobutyl, tert. -butyl, sec. -butyl and/or isopentyl. The term "aryl" refers to aromatic groups for example phenyl or naphthyl.

Herein, when a first element is referred to as being formed or disposed "on" a second element, the first element can be disposed directly on the second element or one or more other elements may be disposed there between. When a first element is referred to as being formed or disposed "directly on" a second element, no other elements are disposed there between.

According to various embodiments of the organic light-emitting diode the amount of the lithium halide or lithium organic complex in an electron transport layer stack, that is in at least the second electron transport layer, is in the range of ≥ <NUM> mol-% to ≤ <NUM> mol-%, preferably ≥ <NUM> mol-% to ≤ <NUM> mol-% and also preferred ≥ <NUM> mol-% to ≤ <NUM> mol-%, of the second electron transport layer; or the amount of the lithium halide or lithium organic complex in an electron transport layer stack of at least three electron transport layers, at least two of the electron transport layers, namely the second electron transport layer and the at least third electron transport layer, is in the range of ≥ <NUM> mol-% to ≤ <NUM> mol-%, preferably ≥ <NUM> mol-% to ≤ <NUM> mol-% and also preferred ≥ <NUM> mol-% to ≤ <NUM> mol-%, of the corresponding electron transport layers.

According to various embodiments of the OLED of the present invention the lithium halide or lithium halide can be selected from the group comprising LiF, LiCl, LiBr and LiJ. However, most preferred is LiF.

According to various embodiments of the organic light-emitting diode comprising an electron transport layer stack, preferably of at least two electron transport layer or three electron transport layers, wherein.

wherein the weight percent of the elemental metal selected from the group of lithium, magnesium and/or ytterbium is based on the total weight of the corresponding electron transport layer.

According to various embodiments of the organic light-emitting diode (OLED) of the present invention the organic ligand of the lithium organic complex can be a quinolate. Preferably the lithium organic complex is a lithium organic complex of formula I, II or III:
<CHM>
wherein.

Quinolates that can be suitable used are disclosed in <CIT>.

According to various embodiments of the organic light-emitting diode (OLED) of the present invention the organic ligand of the lithium organic complex can be a borate based organic ligand, Preferably the lithium organic complex is a lithium tetra(<NUM>-pyrazol-<NUM>-yl)borate. Borate based organic ligands that can be suitable used are disclosed in <CIT>.

According to various embodiments of the organic light-emitting diode (OLED) of the present invention the organic ligand of the lithium organic complex can be a phenolate ligand, Preferably the lithium organic complex is a lithium <NUM>-(diphenylphosphoryl)phenolate. Phenolate ligands that can be suitable used are disclosed in <CIT>.

Further, phenolate ligands can be selected from the group of pyridinolate, preferably <NUM>-(diphenylphosphoryl)pyridin-<NUM>-olate. Pyridine phenolate ligands that can be suitable used are disclosed in <CIT>.

In addition, phenolate ligands can be selected from the group of imidazol phenolates, preferably <NUM>-(<NUM>-phenyl-<NUM>-benzo[d]imidazol-<NUM>-yl)phenolate. Imidazol phenolate ligands that can be suitable used are disclosed in <CIT>.

Also, phenolate ligands can be selected from the group of oxazol phenolates, preferably <NUM>-(benzo[d]oxazol-<NUM>-yl)phenolate. Oxazol phenolate ligands that can be suitable used are disclosed in <CIT>.

Lithium Schiff base organic complexes can be use. Lithium Schiff base organic complexes that can be suitable used having the structure <NUM>, <NUM>, <NUM> or <NUM>:
<CHM>.

According to various embodiments of the organic light-emitting diode (OLED) of the present invention the organic ligand of the lithium organic complex is a quinolate, a borate, a phenolate, a pyridinolate or a Schiff base ligand;.

According to various embodiments of the organic light-emitting diode (OLED) of the present invention the first electron transport layer, the second electron transport layer and/or in case of three electron transport layers the third electron transport layer as well, may comprises at least one matrix compound each.

According to various embodiments of the organic light-emitting diode (OLED) of the present invention comprising an electron transport layer stack of at least two electron transport layers or at least three electron transport layers, wherein each electron transport layer comprises at least one matrix compound, whereby the matrix compound of the electron transport layers are selected same or different.

According to various embodiments of the organic light-emitting diode (OLED) the at least two electron transport layers (<NUM>/<NUM>) or the at least three electron transport layers (<NUM>/<NUM>/<NUM>) comprise at least one matrix compound, whereby the matrix compound of the electron transport layers are selected same or different, and wherein the electron matrix compound is selected from:.

According to various embodiments of the organic light-emitting diode (OLED) of the present invention the thicknesses of each electron transport layer, preferably the at least first electron transport layer (<NUM>) and the at least second electron transport layer (<NUM>) and/or the at least third electron transport layer (<NUM>), are same or each independently, in the range of ≥ <NUM> to ≤ <NUM>, preferably of ≥ <NUM> to ≤ <NUM>, further preferred of ≥ <NUM> to ≤ <NUM>, also preferred of ≥ <NUM> to ≤ <NUM>, in addition preferred ≥ <NUM> to ≤ <NUM> and more preferred of ≥ <NUM> to ≤ <NUM>.

According to various embodiments of the organic light-emitting diode (OLED) the thicknesses of the electron transport layer stack can be in the range of ≥ <NUM> to ≤ <NUM>, preferably of ≥ <NUM> to ≤ <NUM>, further preferred of ≥ <NUM> to ≤ <NUM>, and more preferred of ≥ <NUM> to≤ <NUM>.

According to various embodiments of the organic light-emitting diode (OLED) of the present invention the electron transport layer stack has <NUM> to <NUM> electron transport layers and more preferred <NUM> to <NUM> electron transport layers.

According to various embodiments of the organic light-emitting diode (OLED) of the present invention the second electron transport layer is formed directly on the first electron transport layer and an optional third electron transport layer can be formed directly on the second electron transport layer, so that the second electron transport layer is sandwiched between the first and third electron transport layers.

According to various embodiments of the organic light-emitting diode (OLED) of the present invention:.

According to various embodiments of the OLED of the present invention:.

According to another aspect, there is provided an organic light-emitting diode comprising: a substrate; a first anode electrode formed on the substrate; a second cathode electrode formed on the first anode electrode; and an electron transport layer stack arranged between the first anode electrode and the second cathode electrode, comprising or consisting of at least two electron transport layers or at least three electron transport layers.

According to various embodiments, the organic light-emitting diode (OLED) may further include at least one layer selected from the group consisting of a hole injection layer, a hole transport layer, an emission layer, and a hole blocking layer, arranged between the first anode electrode and the electron transport layer.

According to another aspect, there is provided an organic light-emitting diode comprising in addition: at least one layer selected from the group consisting of a hole injection layer, a hole transport layer, an emission layer, and a hole blocking layer, arranged between the first anode electrode and the electron transport layer stack.

According to various aspects, there is provided an organic light-emitting diode further comprising an electron injection layer arranged between the electron transport layer and the second cathode electrode.

According to various embodiments of the OLED of the present invention, the OLED may not comprise an electron injection layer.

According to another aspect, there is provided a method of manufacturing an organic light-emitting diode (OLED), the method using:.

According to various aspects, there is provided a method using:.

at least one electron transport layer comprises a lithium halide or a lithium organic complex and is free of an elemental metal; and at least one electron transport layer comprises an elemental metal and is free of a lithium halide or a lithium organic complex.

According to various aspects, the method may further include forming on the first anode electrode at least one emission layer, and at least one layer selected from the group consisting of forming a hole injection layer, forming a hole transport layer, or forming a hole blocking layer, between the first anode electrode and the electron transport layer stack.

According to various aspects, the method may further include the steps for forming an organic light-emitting diode (OLED), wherein.

According to various aspects, the method may further include forming an electron injection layer on the electron transport layer stack.

Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.

These and/or other aspects and advantages of the present invention will become apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings, of which <FIG> are embodiments of the invention and <FIG> for illustration only and not part of the invention.

Reference will now be made in detail to the exemplary aspects, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The exemplary embodiments are described below, in order to explain the aspects, by referring to the figures.

Herein, when a first element is referred to as being formed or disposed "on" a second element, the first element can be disposed directly on the second element, or one or more other elements may be disposed there between. When a first element is referred to as being formed or disposed "directly on" a second element, no other elements are disposed there between.

<FIG> is a schematic sectional view of an organic light-emitting diode <NUM>. The OLED <NUM> includes an emission layer <NUM> and an electron transport layer stack (ETL) <NUM> comprising a first electron transport layer <NUM> directly on the light emission layer <NUM> and a second electron transport layer <NUM>, whereby the second electron transport layer <NUM> is disposed directly on the first electron transport layer <NUM>.

<FIG> is a schematic sectional view of an organic light-emitting diode <NUM>. The OLED <NUM> includes an emission layer <NUM> and an electron transport layer stack (ETL) <NUM> comprising a first electron transport layer <NUM>, a second electron transport layer <NUM>, and a third electron transport layer <NUM>, whereby the first electron transport layer <NUM> is directly on the light emission layer <NUM>, the second electron transport layer <NUM> is disposed directly on the first electron transport layer <NUM> and the third electron transport layer <NUM> is disposed directly on the second electron transport layer <NUM>.

<FIG> is a schematic sectional view of an organic light-emitting diode <NUM>, which is not part of the invention. The OLED <NUM> includes a substrate <NUM>, a first electrode <NUM>, a hole injection layer (HIL) <NUM>, a hole transport layer (HTL) <NUM>, an emission layer (EML) <NUM>, an electron transport layer (ETL) <NUM>, an electron injection layer (EIL) <NUM>, and a second electrode <NUM>. The electron transport layer (ETL) <NUM> includes a first electron transport layer <NUM> including a matrix material and a lithium halide or a lithium organic complex and a second electron transport layer <NUM> including an elemental metal selected from the group comprising of lithium, magnesium and/or ytterbium. The second electron transport layer <NUM> is directly formed on the first electron transport layer <NUM>. The first layer <NUM> may be formed directly on the EML <NUM>.

<FIG> is a schematic sectional view of an organic light-emitting diode <NUM>, which is not part of the invention. The OLED <NUM> includes a substrate <NUM>, a first electrode <NUM>, a hole injection layer (HIL) <NUM>, a hole transport layer (HTL) <NUM>, an emission layer (EML) <NUM>, an electron transport layer (ETL) <NUM>, an electron injection layer (EIL) <NUM>, and a second electrode <NUM>. The electron transport layer (ETL) <NUM> includes a first electron transport layer <NUM> and a third electron transport layer <NUM> including a matrix material and a lithium halide or a lithium organic complex, wherein the lithium halide or the lithium organic complex of the first electron transport layer <NUM> is not the same as the lithium halide or the lithium organic complex of the third electron transport layer <NUM>; and the second electron transport layer <NUM> including an elemental metal of lithium, magnesium and/or ytterbium. The second electron transport layer <NUM> is directly formed on the first electron transport layer <NUM> and the third electron layer <NUM> is directly formed on the second electron layer <NUM>. The first layer <NUM> may be formed directly on the emission layer (EML) <NUM>.

The substrate <NUM> may be any substrate that is commonly used in manufacturing of organic light-emitting diodes. If light is emitted through the substrate, the substrate <NUM> may be a transparent material, for example a glass substrate or a transparent plastic substrate, having excellent mechanical strength, thermal stability, transparency, surface smoothness, ease of handling, and waterproofness. If light is emitted through the top surface, the substrate <NUM> may be a transparent or non-transparent material, for example a glass substrate, a plastic substrate, a metal substrate or a silicon substrate.

The first anode electrode <NUM> may be formed by depositing or sputtering a compound that is used to form the first anode electrode <NUM>. The compound used to form the first anode electrode <NUM> may be a high work-function compound, so as to facilitate hole injection. If a p-doped HIL is used, the anode material may also be selected from a low work function material (i.e. Aluminum). The first anode electrode <NUM> may be a transparent or reflective electrode. Transparent conductive compounds, such as indium tin oxide (ITO), indium zinc oxide (IZO), tin-dioxide (SnO<NUM>), and zinc oxide (ZnO), may be used to form the first anode electrode <NUM>. The first anode electrode <NUM> may also be formed using magnesium (Mg), aluminum (Al), aluminum-lithium (Al-Li), calcium (Ca), magnesium-indium (Mg-In), magnesium-silver (Mg-Ag), silver (Ag), gold (Au), or the like.

The HIL <NUM> may be formed on the first anode electrode <NUM> by vacuum deposition, spin coating, printing, casting, slot-die coating, Langmuir-Blodgett (LB) deposition, or the like. When the HIL <NUM> is formed using vacuum deposition, the deposition conditions may vary according to the compound that is used to form the HIL <NUM>, and the desired structure and thermal properties of the HIL <NUM>. In general, however, conditions for vacuum deposition may include a deposition temperature of <NUM>° C to <NUM>° C, a pressure of <NUM>-<NUM> to <NUM>-<NUM> Torr (<NUM> Torr equals <NUM> Pa), and a deposition rate of <NUM> to <NUM>/sec.

When the HIL <NUM> is formed using spin coating, printing, coating conditions may vary according to a compound that is used to form the HIL <NUM>, and the desired structure and thermal properties of the HIL <NUM>. For example, the coating conditions may include a coating speed of <NUM> rpm to <NUM> rpm, and a thermal treatment temperature of <NUM>° C to <NUM>° C. Thermal treatment removes a solvent after the coating is performed.

The HIL <NUM> may be formed of any compound that is commonly used to form an HIL. Examples of compounds that may be used to form the HIL <NUM> include a phthalocyanine compound, such as copper phthalocyanine (CuPc), <NUM>,<NUM>',<NUM>"-tris (<NUM>-methylphenylphenylamino) triphenylamine (m-MTDATA), TDATA, 2T-NATA, polyaniline/dodecylbenzenesulfonic acid (Pani/DBSA), poly(<NUM>,<NUM>-ethylenedioxythiophene)/poly(<NUM>-styrenesulfonate) (PEDOT/PSS), polyaniline/camphor sulfonic acid (Pani/CSA), and polyaniline)/poly(<NUM>-styrenesulfonate (PANI/PSS).

The HIL <NUM> may be a pure layer of p-dopant or may be selected from a hole-transporting matrix compound doped with a p-dopant. Typical examples of known redox doped hole transport materials are: copper phthalocyanine (CuPc), which HOMO level is approximately -<NUM> eV, doped with tetrafluoro-tetracyanoquinonedimethane (F4TCNQ), which LUMO level is -<NUM> eV; zinc phthalocyanine (ZnPc) (HOMO = -<NUM> eV) doped with F4TCNQ; α-NPD (N,N'-Bis(naphthalen-<NUM>-yl)-N,N'-bis(phenyl)-benzidine) doped with F4TCNQ. α-NPD doped with <NUM>,<NUM>'-(perfluoronaphthalene-<NUM>,<NUM>-diylidene) dimalononitrile (PD1). α-NPD doped with <NUM>,<NUM>',<NUM>"-(cyclopropane-<NUM>,<NUM>,<NUM>-triylidene)tris(<NUM>-(p-cyanotetrafluorophenyl)acetonitrile) (PD2). Dopant concentrations can be selected from <NUM> to <NUM> wt. -%, more preferably from <NUM> wt. -% to <NUM> wt.

The thickness of the HIL <NUM> may be in the range of <NUM> to <NUM>, and for example, <NUM> to <NUM>. When the thickness of the HIL <NUM> is within this range, the HIL <NUM> may have excellent hole injecting characteristics, without a substantial increase in driving voltage.

The hole transport layer (HTL) <NUM> may be formed on the HIL <NUM> by vacuum deposition, spin coating, slot-die coating, printing, casting, Langmuir-Blodgett (LB) deposition, or the like. When the HTL <NUM> is formed by vacuum deposition or spin coating, the conditions for deposition and coating may be similar to those for the formation of the HIL <NUM>. However, the conditions for the vacuum or solution deposition may vary, according to the compound that is used to form the HTL <NUM>.

The HTL <NUM> may be formed of any compound that is commonly used to form a HTL. Compound that can be suitably used is disclosed for example in<NPL> and incorporated by reference. Examples of the compound that may be used to form the HTL <NUM> are: a carbazole derivative, such as N-phenylcarbazole or polyvinylcarbazole; an amine derivative having an aromatic condensation ring, such as N,N'-bis(<NUM>-methylphenyl)-N,N'-diphenyl-[<NUM>,<NUM>-biphenyl]-<NUM>,<NUM>'-diamine (TPD), or N,N'-di(naphthalene-<NUM>-yl)-N,N'-diphenyl benzydine (alpha -NPD); and a triphenylamine-based compound, such as <NUM>,<NUM>',<NUM>"-tris(N-carbazolyl)triphenylamine (TCTA). Among these compounds, TCTA can transport holes and inhibit excitons from being diffused into the EML.

The thickness of the HTL <NUM> may be in the range of <NUM> to <NUM>, preferably, <NUM> to <NUM>, further <NUM> to <NUM>, further <NUM> to <NUM>, further <NUM> to <NUM>, further <NUM> to <NUM>, further <NUM> to <NUM>, further <NUM> to <NUM>. A preferred thickness of the HTL <NUM> may be <NUM> to <NUM>.

When the thickness of the HTL <NUM> is within this range, the HTL <NUM> may have excellent hole transporting characteristics, without a substantial increase in driving voltage.

The EML <NUM> may be formed on the HTL <NUM> by vacuum deposition, spin coating, slot-die coating, printing, casting, LB, or the like. When the EML <NUM> is formed using vacuum deposition or spin coating, the conditions for deposition and coating may be similar to those for the formation of the HIL <NUM>. However, the conditions for deposition and coating may vary, according to the compound that is used to form the EML <NUM>.

The emission layer (EML) <NUM> may be formed of a combination of a host and a dopant. Example of the host are Alq3, <NUM>,<NUM>'-N,N'- dicarbazole-biphenyl (CBP), poly(n-vinylcarbazole) (PVK), <NUM>,<NUM>-di(naphthalene-<NUM>-yl)anthracene (ADN), TCTA, <NUM>,<NUM>,<NUM>-tris(N-phenylbenzimidazole-<NUM>-yl)benzene (TPBI), <NUM>-tert-butyl-<NUM>,<NUM>-di-<NUM>-naphthylanthracenee (TBADN), distyrylarylene (DSA), Bis(<NUM>-(<NUM>-hydroxyphenyl)benzothiazolate)zinc (Zn(BTZ) <NUM>), E3 below, ADN and referred to as Formula <NUM>, Compound <NUM> below, and Compound <NUM> below. <CHM>
<CHM>
<CHM>
<CHM>.

The dopant may be a phosphorescent or fluorescent emitter. Phosphorescent emitters are preferred due to their higher efficiency.

Examples of a red dopant are PtOEP, Ir(piq) <NUM>, and Btp 2lr(acac), but are not limited thereto. These compounds are phosphorescent emitters, however, fluorescent red dopants could also be used.

Examples of a phosphorescent green dopant are Ir(ppy) <NUM> (ppy = phenylpyridine), Ir(ppy) <NUM>(acac), Ir(mpyp) <NUM> are shown below. Compound <NUM> is an example of a fluorescent green emitter and the structure is shown below. <CHM>
<CHM>.

Examples of a phosphorescent blue dopant are F<NUM>Irpic, (F<NUM>ppy) <NUM>Ir(tmd) and Ir(dfppz) <NUM>, ter-fluorene, the structures are shown below. <NUM>'-bis(<NUM>-diphenyl amiostyryl)biphenyl (DPAVBi), <NUM>,<NUM>,<NUM>,<NUM>-tetra-tert-butyl perylene (TBPe), and Compound <NUM> below are examples of fluorescent blue dopants. <CHM>
<CHM>.

The amount of the dopant may be in the range of <NUM> to <NUM> parts by weight, based on <NUM> parts by weight of the host. The EML <NUM> may have a thickness of <NUM> to <NUM>, for example, <NUM> to <NUM>. When the thickness of the EML <NUM> is within this range, the EML <NUM> may have excellent light emission, without a substantial increase in driving voltage.

When the EML <NUM> comprises a phosphorescent dopant, a hole blocking layer (HBL) (not shown) may be formed on the EML <NUM>, by using vacuum deposition, spin coating, slot-die coating, printing, casting, LB deposition, or the like, in order to prevent the diffusion of triplet excitons or holes into the ETL <NUM>. When the HBL is formed using vacuum deposition or spin coating, the conditions for deposition and coating may be similar to those for the formation of the HIL <NUM>. However, the conditions for deposition and coating may vary, according to the compound that is used to form the HBL. Any compound that is commonly used to form a HBL may be used. Examples of compounds for forming the HBL include an oxadiazole derivative, a triazole derivative, and a phenanthroline derivative.

The HBL may have a thickness of <NUM> to <NUM>, for example, <NUM> to <NUM>. When the thickness of the HBL is within this range, the HBL may have excellent hole-blocking properties, without a substantial increase in driving voltage.

The ETL <NUM> may be formed on the EML <NUM> or on the HBL if the HBL is formed. The ETL <NUM> includes a first layer <NUM> including a lithium halide or a lithium organic complex; and the second electron transport layer <NUM> comprises an elemental metal selected from the group of lithium, magnesium and/or ytterbium; or vice versa.

The ETL <NUM> has a stacked structure, preferably of two ETL-layers (<NUM>/<NUM>), so that injection and transport of electrons may be balanced and holes may be efficiently blocked. In a conventional OLED, since the amounts of electrons and holes vary with time, after driving is initiated, the number of excitons generated in an emission area may be reduced. As a result, a carrier balance may not be maintained, so as to reduce the lifetime of the OLED.

However, in the ETL <NUM>, the first layer <NUM> and the second layer <NUM> may have similar or identical energy levels. Surprisingly, it was found that the lifetime (LT) of the OLED <NUM> is improved irrespective of HOMO level of the matrix compounds.

In general the matrix compound for the first electron layer (<NUM>) and second electron layer (<NUM>) can be identical or different.

Matrix compound for the first electron layer (<NUM>) and second electron layer (<NUM>) that can be suitable used are selected from the group comprising anthracene compounds, preferably <NUM>-(<NUM>-(<NUM>,<NUM>-di(naphthalen-<NUM>-yl)anthracen-<NUM>-yl)phenyl)-<NUM>-phenyl-<NUM>-benzo[d] imidazole.

Anthracene compounds that can be used as matrix materials are disclosed in <CIT>.

Other matrix compounds that can be used are diphenylphosphine oxide, preferably (<NUM>-(dibenzo[c,h]acridin-<NUM>-yl)phenyl)diphenylphosphine oxide, phenylbis(<NUM>-(pyren-<NUM>-yl)phenyl) phosphine oxide, <NUM>-phenyl-<NUM>-benzo[b]dinaphtho[<NUM>,<NUM>-d:<NUM>',<NUM>'-f]phosphepine-<NUM>-oxide, bis(<NUM>-(anthracen-<NUM>-yl)phenyl)(phenyl)phosphine oxide, phenyldi(pyren-<NUM>-yl)phosphine oxide.

Diphenylphosphine oxide compounds that can be used as matrix materials are disclosed in <CIT>, <CIT>, <CIT>, <CIT> and <CIT>. Other suitable matrix compounds that can be used are phenanthroline compounds, preferably selected from the group comprising of <NUM>,<NUM>,<NUM>,<NUM>-tetraphenyl-<NUM>,<NUM>-phenanthroline and <NUM>,<NUM>-di(biphenyl-<NUM>-yl)-<NUM>,<NUM>-diphenyl-<NUM>,<NUM>-phenanthroline. Phenanthroline compounds that can be used as matrix materials are disclosed in <CIT>.

The matrix compound of the first electron layer (<NUM>) and/or second electron transport layer (<NUM>) may be a compound that efficiently transports electrons, such as an anthracene-based compound, diphenylphosphine oxide based compound, or a phenanthroline based compound, preferably a matrix compound mentioned in Table <NUM>. For example, the matrix compound of the first electron layer (<NUM>) and/or second electron transport layer (<NUM>) may be selected from the group consisting of ADN and referred to as Formula <NUM>, a compound represented by Formula <NUM>, and a compound represented by Formula <NUM> below:
<CHM>
<CHM>
<CHM>
<CHM>.

In Formulae <NUM> and <NUM>, R<NUM> to R<NUM> are each independently a hydrogen atom, a halogen atom, a hydroxy group, a cyano group, a substituted or unsubstituted C<NUM>-C<NUM> alkyl group, a substituted or unsubstituted C<NUM>-C<NUM> alkoxy group, a substituted or unsubstituted C<NUM>-C<NUM> acyl group, a substituted or unsubstituted C<NUM>-C<NUM> alkenyl group, a substituted or unsubstituted C<NUM>-C<NUM> alkynyl group, a substituted or unsubstituted C<NUM>-C<NUM> aryl group, or a substituted or unsubstituted C<NUM>-C<NUM> heteroaryl group. At least two adjacent R<NUM> to R<NUM> groups are optionally bonded to each other, to form a saturated or unsaturated ring. L<NUM> is a bond, a substituted or unsubstituted C<NUM>-C<NUM> alkylene group, a substituted or unsubstituted C<NUM>-C<NUM> arylene group, or a substituted or unsubstituted C<NUM>-C<NUM> hetero arylene group. Q<NUM> through Q<NUM> are each independently a hydrogen atom, a substituted or unsubstituted C<NUM>-C<NUM> aryl group, or a substituted or unsubstituted C<NUM>-C<NUM> hetero aryl group, and "a" is an integer from <NUM> to <NUM>.

For example, R<NUM> to R<NUM> may be each independently selected from the group consisting of a hydrogen atom, a halogen atom, a hydroxy group, a cyano group, a methyl group, an ethyl group, a propyl group, a butyl group, a methoxy group, an ethoxy group, a propoxy group, a butoxy group, a phenyl group, a naphthyl group, an anthryl group, a pyridinyl group, and a pyrazinyl group.

In particular, in Formula <NUM> and/or <NUM>, R<NUM> to R<NUM> may each be a hydrogen atom, R<NUM> may be selected from the group consisting of a halogen atom, a hydroxy group, a cyano group, a methyl group, an ethyl group, a propyl group, a butyl group, a methoxy group, an ethoxy group, a propoxy group, a butoxy group, a phenyl group, a naphthyl group, an anthryl group, a pyridinyl group, and a pyrazinyl group. In addition, in Formula <NUM>, R<NUM> to R<NUM> may each be a hydrogen atom.

For example, in Formula <NUM> and/or <NUM>, Q<NUM> to Q<NUM> are each independently a hydrogen atom, a phenyl group, a naphthyl group, an anthryl group, a pyridinyl group, and a pyrazinyl group. In particular, in Formulae <NUM> and/or <NUM>, Q<NUM>, Q<NUM>-Q<NUM>, Q<NUM> and Q<NUM> are hydrogen atoms, and Q<NUM> and Q<NUM> may be each independently selected from the group consisting of a phenyl group, a naphthyl group, an anthryl group, a pyridinyl group, and a pyrazinyl group.

For example, L<NUM>, in Formula <NUM> and/or <NUM>, may be selected from the group consisting of a phenylene group, a naphthylene group, an anthrylene group, a pyridinylene group, and a pyrazinylene group. In particular, Li may be a phenylene group or a pyridinylene group. For example, "a" may be <NUM>, <NUM>, or, <NUM>.

The matrix compound may be further selected from Compound <NUM> or <NUM> below:
<CHM>.

Reduction and oxidation potentials are determined via cyclic voltammetry, using the Ferrocene/Ferrocenium (Fc/Fc+) redox couple as internal reference. A simple rule is very often used for the conversion of redox potentials into electron affinities and ionization potential: IP (in eV) = <NUM> eV + e*Eox (wherein Eox is given in Volt vs. ferrocene/ferrocenium (Fc/Fc+) and EA (in eV) = <NUM> eV + e*Ered (Fred is given in Volt vs. Fc/Fc+) respectively (see <NPL>)), e* is the elemental charge. It is common practice, even if not exactly correct, to use the terms "energy of the HOMO" E(HOMO) and "energy of the LUMO" E(LUMO), respectively, as synonyms for the ionization energy and electron affinity (Koopmans Theorem).

Table <NUM> below shows the energy levels of matrix compounds which may be suitably used according to the invention.

According to the invention the first electron transport layer <NUM> directly on the emission layer <NUM> comprises an elemental metal selected from the group of lithium, magnesium and/or ytterbium; and the second electron transport layer <NUM> directly on the first electron transport layer <NUM> comprises a lithium halide or a lithium organic complex.

According to another aspect the first electron transport layer <NUM> comprises an elemental metal selected from the group of lithium, magnesium and/or ytterbium; and the second electron transport layer <NUM> comprises a lithium halide or a lithium organic complex; and the third electron transport layer <NUM> comprises an elemental metal selected from the group of lithium, magnesium and/or ytterbium that is the same or differs from the elemental metal selected from the group of lithium, magnesium and/or ytterbium of the first electron transport layer <NUM>.

Suitable organic ligands to form a lithium organic complex that can be used for the first electron transport layer or the second electron transport layer are disclosed, for example in <CIT>, <CIT> and <CIT> and <NPL>.

The organic ligand of the lithium organic complex of at least one electron transport one of which is the second electron transport layer may be selected from the group comprising a quinolate, a borate, a phenolate, a pyridinolate or a Schiff base ligand, or Table <NUM>;.

The lithium halide of at least one electron transport layer may be selected from the group comprising a LiF, LiCl, LiBr or LiJ, and preferably LiF.

The ETL layer stack thickness can be adjusted such that the light out coupling is maximized. Further ETL layer stack thickness can be adjusted for the desired color tuning, for example to achieve a deeper shade of blue, i.e. smaller CIEy.

The thicknesses of the first electron transport layer <NUM>, second electron transport layer <NUM> and/or third electron transport layer <NUM> may be the same or each independently in the range of ≥ <NUM> to ≤ <NUM>, preferably of ≥ <NUM> to ≤ <NUM>, further preferred of ≥ <NUM> to ≤ <NUM>, also preferred of ≥ <NUM> to ≤ <NUM>, in addition preferred ≥ <NUM> to ≤ <NUM> and more preferred of ≥ <NUM> to ≤ <NUM>.

When the thicknesses of the first electron transport layer <NUM>, second electron transport layer <NUM> and/or third electron transport layer <NUM> within this range, preferably of ≥ <NUM> to ≤ <NUM>, the electron transport layer stack <NUM> may effectively inject and transport electrons, without a substantial increase in driving voltage.

For blue emitting OLEDs, the thickness of the ETL layer stack is <NUM> to <NUM>, preferably <NUM> to <NUM>. For red and green emitting OLEDs, the thickness of ETLs is <NUM> to <NUM>, preferably <NUM>-<NUM> and more preferably <NUM>-<NUM>. The thickness is selected so as to maximize efficiency of light emission.

The amount of the lithium organic complex in at least one electron transport layer (<NUM>) may be in the range of ≥ <NUM> mol-% to ≤ <NUM> mol-%, preferably ≥ <NUM> mol-% to ≤ <NUM> mol-% and also preferred ≥ <NUM> mol-% to ≤ <NUM> mol-%, of the first electron transport layer <NUM>.

The amount of an elemental metal selected from the group of lithium, magnesium, and/or ytterbium in the second electron transport layer <NUM> may be in the range of ≥ <NUM> wt. -% to ≤ <NUM> wt. -%, preferably ≥ <NUM> wt. -% to ≤ <NUM> wt. -% and also preferred ≥ <NUM> wt. -% to ≤ <NUM> wt. -%, by weight of the second electron transport layer <NUM>.

The amount of the lithium halide or lithium organic complex in the first electron transport layer <NUM> may be in the range of ≥ <NUM> mol-% to ≤ <NUM> mol-%, preferably ≥ <NUM> mol-% to ≤ <NUM> mol-% and also preferred ≥ <NUM> mol-% to ≤ <NUM> mol-%, of the first electron transport layer <NUM>.

The amount of an elemental metal selected from the group of lithium, magnesium, and/or ytterbium in the second electron transport layer <NUM> may be in the range of ≥ <NUM> mol-% to ≤ <NUM> mol-%, preferably ≥ <NUM> mol-% to ≤ <NUM> mol-% and also preferred ≥ <NUM> mol-% to ≤ <NUM> mol-%.

The above organic light-emitting diode is not claimed, however, an OLED with the first electron transport layer <NUM> directly on the light emission layer and comprising the elemental metal and the second electron transport layer <NUM> directly on the first electron transport layer and comprising the lithium halide or lithium organic complex as defined in the claims falls within the definition of the claims.

The ETL <NUM> may be formed on the EML <NUM> by vacuum deposition, spin coating, slot-die coating, printing, casting, or the like. When the ETL <NUM> is formed by vacuum deposition or spin coating, the deposition and coating conditions may be similar to those for formation of the HIL <NUM>. However, the deposition and coating conditions may vary, according to a compound that is used to form the ETL <NUM>.

The deposition sources are positioned relative to one another, such that a mixed deposition region of the second electron transport layer <NUM> is formed directly on the first electron transport layer <NUM>.

The stacking process is more simply and quickly performed, as compared to prior methods. In particular, since a plurality of ETL layers may be almost simultaneously deposited in a single chamber, the chamber may not be required to be exhausted after the formation of each layer.

The EIL <NUM>, which facilitates injection of electrons from the cathode, may be formed on the ETL <NUM>, preferably directly on the second electron transport layer <NUM>. Examples of materials for forming the EIL <NUM> include LiF, NaCl, CsF, Li2O, BaO, Ca, Ba, Yb, Mg which are known in the art. Deposition and coating conditions for forming the EIL <NUM> are similar to those for formation of the HIL <NUM>, although the deposition and coating conditions may vary, according to a material that is used to form the EIL <NUM>.

The thickness of the EIL <NUM> may be in the range of <NUM> to <NUM>, for example, in the range of <NUM> to <NUM>. When the thickness of the EIL <NUM> is within this range, the EIL <NUM> may have satisfactory electron-injecting properties, without a substantial increase in driving voltage.

The second cathode electrode <NUM> is formed on the EIL <NUM> if present. The second cathode electrode <NUM> may be a cathode, which is an electron-injecting electrode. The second electrode <NUM> may be formed of a metal, an alloy, an electrically conductive compound, or a mixture thereof. The second electrode <NUM> may have a low work function. For example, the second electrode <NUM> may be formed of lithium (Li), magnesium (Mg), aluminum (Al), aluminum (Al)-lithium (Li), calcium (Ca), barium (Ba), ytterbium (Yb), magnesium (Mg)-indium (In), magnesium (Mg)-silver (Ag), or the like. In addition, the second electrode <NUM> may be formed of a transparent conductive material, such as ITO or IZO.

The thickness of the cathode electrode <NUM> may be in the range of <NUM> to <NUM>, for example, in the range of <NUM> to <NUM>. When the cathode electrode <NUM> is in the range of <NUM> to <NUM>, the electrode will transparent even if a metal or metal alloy is used.

Since the layers of the ETL <NUM> have similar or identical energy levels, the injection and transport of the electrons may be controlled, and the holes may be efficiently blocked. Thus, the OLED <NUM> may have long lifetime.

<FIG> is a schematic sectional view of an OLED <NUM>, and are not part of the invention. <FIG> and <FIG> differ from <FIG> in that the OLED <NUM> has no electron injection layer (ElL) <NUM>. Referring to <FIG> the OLED <NUM> includes a substrate <NUM>, a first electrode <NUM>, a HIL <NUM>, a HTL <NUM>, an EML <NUM>, an ETL <NUM>, and a second electrode <NUM>. The ETL stack <NUM> of <FIG> includes a first ETL layer <NUM> and a second ETL layer <NUM> and the ETL stack <NUM> of <FIG> includes a first ETL layer <NUM>, a second ETL layer <NUM> and a third ETL layer <NUM>. The electron transport layer stack <NUM> of <FIG> comprises of at least two electron transport layers <NUM> and <NUM>, wherein a first electron transport layer <NUM> and a second electron transport layer <NUM> comprises at least one matrix compound and in addition,.

The layers of the ETL <NUM> and <NUM> or of the ETL <NUM>, <NUM> and <NUM> have similar or identical energy level. Further, the OLED <NUM> have an improved lifetime (LT). The substrate <NUM>, the first electrode <NUM>, the hole injection layer <NUM>, the hole transport layer <NUM>, the emission layer <NUM>, and the electron transport layer <NUM> and <NUM> of the OLED <NUM> of <FIG> or the electron transport layer <NUM>, <NUM> and <NUM> of the OLED <NUM> of <FIG> are similar to corresponding elements described with reference to <FIG> and <FIG>, respectively. Even though the structure of the OLED <NUM> and the method of manufacturing the OLED <NUM> are described with reference to <FIG> and <FIG>, other methods known in the art can be used. For example, the ETL stack <NUM> may include three or more ETL layers but two ETL layers of ETL <NUM> and <NUM> may be preferred.

In the description above the method of manufacture an OLED of the present invention is started with a substrate <NUM> onto which a first anode electrode <NUM> is formed, on the first anode electrode <NUM> an emission layer <NUM> is formed. An electron transport layer stack <NUM> is formed on the emission layer <NUM>, wherein the first electron transport layer <NUM> is formed directly on the emission layer <NUM> and the second electron transport layer <NUM> is formed directly on the first electron transport layer <NUM>, on the electron transport layer stack <NUM>, in this case on the second electron transport layer <NUM>, a cathode electrode <NUM> is formed, optional a hole injection layer <NUM>, and a hole transport layer <NUM>, are formed in that order between the first anode electrode <NUM> and the electron transport layer stack <NUM>, an optional hole blocking layer is arranged between the emission layer and the ETL layer stack, and optionally an electron injection layer <NUM> is arranged between the electron transport layer <NUM> and the second cathode electrode <NUM>.

However, the OLED of the present invention can be manufactured also the other way around, starting with the second cathode electrode <NUM> onto which optionally an electron injection layer <NUM> is formed. On the second cathode electrode <NUM> or on the electron injection layer <NUM>, if present, the second electron transport layer <NUM> is formed and directly on the second electron transport layer <NUM> the first electron transport layer <NUM> is formed and so on.

In case of a three layer electron transport layer stack <NUM>, the second electron layer <NUM> is formed on the first electron layer <NUM> and the third electron layer <NUM> is formed on the second electron layer <NUM>. Then a cathode electrode <NUM> is formed, optional a hole injection layer <NUM>, and a hole transport layer <NUM>, are formed in that order between the first anode electrode <NUM> and the electron transport layer stack <NUM>, an optional hole blocking layer is arranged between the emission layer and the ETL layer stack, and optionally an electron injection layer <NUM> is arranged between the electron transport layer <NUM> and the second cathode electrode <NUM>.

According to another embodiment the electron transport layer stack <NUM> may form part of a charge generation layer (CGL).

The term "charge generation layer" also named "CGL" stands for a layer structure of a second electron transport layer adjacent arranged to a hole injection layer, or a layer structure of a second electron transport layer adjacent arranged to an interlayer that is adjacent arranged to a hole injection layer.

The optional interlayer comprises CN-HAT (<NPL>) or CuPc. The interlayer can be used to increase lifetime of the OLED-device. Suitable interlayer compounds are disclosed in <CIT>.

According to one aspect an OLED can comprise a layer structure of a substrate that is adjacent arranged to an anode electrode, the anode electrode is adjacent arranged to a first hole injection layer, the first hole injection layer is adjacent arranged to a first hole transport layer, the first hole transport layer is adjacent arranged to a first emission layer, the first emission layer is adjacent arranged to a first electron transport layer, the first electron transport layer is adjacent arranged to a second electron transport layer, the second electron transport layer is adjacent arranged to an interlayer, the interlayer is adjacent arranged to a second hole injection layer, the second hole injection layer is adjacent arranged to a second hole transport layer, the second hole transport layer is adjacent arranged to a second emission layer, between the second emission layer and the cathode electrode an optional electron transport layers and/or an optional injection layer are arranged.

According to another aspect an OLED can comprise a layer structure of a substrate that is adjacent arranged to an anode electrode, the anode electrode is adjacent arranged to a first hole injection layer, the first hole injection layer is adjacent arranged to a first hole transport layer, the first hole transport layer is adjacent arranged to a first emission layer, the first emission layer is adjacent arranged to a first electron transport layer, the first electron transport layer is adjacent arranged to a second electron transport layer, the second electron transport layer is adjacent arranged to a second hole injection layer, the second hole injection layer is adjacent arranged to a second hole transport layer, the second hole transport layer is adjacent arranged to a second emission layer, between the second emission layer and the cathode electrode an optional electron transport layers and/or an optional injection layer are arranged.

According to another aspect the OLED may comprise the following layers arranged in the order:
substrate / anode electrode / first hole injection layer / first hole transport layer / first emission layer / first ETL / second ETL / optional interlayer / second hole injection layer / second hole transport layer / second emission layer / optional electron transport layers / optional injection layers / cathode electrode.

According to another aspect the OLED may comprise the following layers arranged in the order of a substrate, an anode electrode, a first hole injection layer, a first hole transport layer, a first emission layer, a first electron transport layer, a second electron transport layer, an optional interlayer, a second hole injection layer, a second hole transport layer, optional a triplet control layer a second emission layer, an optional electron transport layer, optional a third electron transport layer, an optional injection layer, and a cathode electrode.

The function of the triplet control layer is to reduce quenching of triplets if a phosphorescent green or blue emission layer is used. Thereby, higher efficiency of light emission from a phosphorescent emission layer can be achieved. The triplet control layer is selected from triarylamine compounds with a triplet level above the triplet level of the phosphorescent emitter in the adjacent emission layer. Suitable triplet control layer, in particular the triarylamine compounds, are described in <CIT>.

<FIG> shows a tandem OLED <NUM> not according to the claims that comprises a layer structure in the following order:
Substrate <NUM> / anode electrode <NUM> / first hole injection layer <NUM> / first hole transport layer <NUM> / first emission layer fluorescent blue emitting layer <NUM> / first electron transport layer <NUM> / second electron transport layer <NUM> / second hole injection layer <NUM> / second hole transport layer <NUM> / triplet control layer <NUM> / second emission layer phosphorescent green emitting layer <NUM>/ third electron transport layer <NUM> / electron injection layer <NUM> / and a cathode electrode <NUM>.

According to another aspect the OLED may comprise the following layers arranged in the order of a substrate, an anode electrode, a first hole injection layer, a first hole transport layer, optional a first triplet control layer, a first emission layer, a first electron transport layer, a second electron transport layer, an optional interlayer, a second hole injection layer, a second hole transport layer, optional a second triplet control layer, a second emission layer, an optional electron transport layer, optional a third electron transport layer, an optional injection layer, and a cathode electrode.

An OLED comprising two or more emission layers and one or more charge generation layers may be described as tandem OLED or stacked OLED.

While not shown in <FIG>, a sealing layer may further be formed on the second electrodes <NUM>, in order to seal the OLEDs <NUM>, <NUM>, <NUM>. In addition, various other modifications may be applied thereto.

In conclusion, high efficiency and LT are obtained for tandem OLED devices which comprise a first and second ETL.

The double ETL of a first electron transport layer and of a second electron transport layer as well as the triple ETL of a first electron transport layer, a second electron transport layer and a third electron transport layer could also be employed for other emission colors, for example green, red, and white-light emitting devices.

Claim 1:
An organic light-emitting diode (OLED) (<NUM>) comprising an emission layer (<NUM>) and an electron transport layer stack (<NUM>) of at least two electron transport layers (<NUM>/<NUM>),
wherein a first electron transport layer (<NUM>) and a second electron transport layer (<NUM>) comprises at least one matrix compound and in addition,
- the first electron transport layer (<NUM>) comprises an elemental metal selected from the group of lithium, magnesium and/or ytterbium; and the second electron transport layer (<NUM>) comprises a lithium halide or a lithium organic complex;
wherein the electron transport layer or layers comprising a lithium halide or a lithium organic complex is free of an elemental metal, and the electron transport layer or layers that comprises an elemental metal is free of a metal salt and/or a metal organic complex,
whereby the matrix compound of the electron transport layers are selected same or different, and
wherein the electron matrix compound is selected from:
- an anthracene based compound or a heteroaryl substituted anthracene based compound;
- a phosphine oxide based compound;
- a substituted phenanthroline compound;
whereby the second electron transport layer (<NUM>) is disposed directly on the first electron transport layer (<NUM>) and the first electron transport layer (<NUM>) is arranged directly on the emission layer (<NUM>).