CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national stage application of PCT/EP2016/064448, filed Jun. 22, 2016, which claims priority to European Application No. 15173443.1, filed Jun. 23, 2015. The contents of these applications are incorporated herein by reference.

The present invention concerns organic light emitting device with improved electrical properties, particularly the device comprising, in combination with a silver cathode, an improved electron transporting and/or electron injecting layer and/or an OLED stack comprising the improved transporting and/or electron injecting layer and, optionally, an improved charge generating layer.

I. BACKGROUND OF THE INVENTION

Among the electronic devices comprising at least a part based on material provided by organic chemistry, organic light emitting diodes (OLEDs) have a prominent position. Since the demonstration of efficient OLEDs by Tang et al. in 1987 (C. W. Tang et al., Appl. Phys. Lett. 51 (12), 913 (1987)), OLEDs developed from promising candidates to high-end commercial displays. An OLED comprises a sequence of thin layers substantially made of organic materials. The layers typically have a thickness in the range of 1 nm to 5 μm. The layers are usually formed either by means of vacuum deposition or from a solution, for example by means of spin coating or jet printing.

OLEDs emit light after the injection of charge carriers in the form of electrons from the cathode and in form of holes from the anode into organic layers arranged in between. The charge carrier injection is effected on the basis of an applied external voltage, the subsequent formation of excitons in a light emitting zone and the radiative recombination of those excitons. At least one of the electrodes is transparent or semitransparent, in the majority of cases in the form of a transparent oxide, such as indium tin oxide (ITO), or a thin metal layer.

Among the matrix compounds used in OLED light emitting layers (LELs) or electron transporting layers (ETLs), important position have the compounds that comprise at least one polar group selected from phosphine oxide and diazole. The reason why such polar group often significantly improves the electron injecting and/or electron transporting properties of the semiconducting material is not yet fully understood. It is believed that the high dipole moment of the polar group plays somehow the positive role. Especially recommended for this use are triaryl phosphine oxides comprising at least one condensed aromatic or heteroaromatic group attached directly to the phosphine oxide group, see e.g. JP 4 876 333 B2. Among diazole groups, especially phenylbenzimidazole groups have been widely utilized for the design of new electron transport matrix compounds, for example TPBI described in U.S. Pat. No. 5,645,948, and some compounds comprising benzimidazolyl structural moieties linked with other structural moieties comprising delocalized pi-electrons in two or more aromatic or heteroaromatic rings nowadays are considered to be industrial standard, e.g. compound LG-201 (for example U.S. Pat. No. 6,878,469)



Electrical doping of charge transporting semiconducting materials for improving their electrical properties, especially conductivity, is known since 1990s, e.g. from U.S. Pat. No. 5,093,698 A. An especially simple method for n-doping in ETLs prepared by the thermal vacuum deposition, which is currently the standard method most frequently used, e.g. in industrial manufacture of displays, is evaporation of a matrix compound from one evaporation source and of a highly electropositive metal from another evaporation source and their co-deposition on a solid substrate. As useful n-dopants in triaryl phosphine oxide matrix compounds, alkali metals and alkaline earth metals were recommended in JP 4 725 056 B2, with caesium as the dopant successfully used in the given examples. Indeed, caesium as the most electropositive metal offers the broadest freedom in the choice of a matrix material, and it is likely the reason why solely caesium was the n-doping metal of choice in the cited document.

For an industrial use, caesium as a dopant has several serious drawbacks. First, it is very reactive, moisture and highly air sensitive material that renders any handling difficult and incurs significant additional costs for mitigating the high safety and fire hazard unavoidably linked with its use. Second, its quite low normal boiling point (678° C.) indicates that it may be highly volatile under high vacuum conditions. Indeed, at pressures below 10−2Pa used in industrial equipment for vacuum thermal evaporation (VTE), caesium metal evaporates significantly already at slightly elevated temperature. Taking into account that the evaporation temperatures for typical matrix compounds used in organic semiconducting materials at pressures below 10−2Pa are typically between 150-400° C., avoiding an uncontrolled caesium evaporation, resulting in its undesired deposition contaminating the colder parts of the whole equipment (e.g. the parts that are shielded against heat radiation from the organic matrix evaporation source), is a really challenging task.

Several methods for overcoming these drawbacks and enabling industrial applicability of caesium for n-doping in organic electronic devices have been published. For safe handling, caesium may be supplied in hermetic shells that open just inside the evacuated evaporation source, preferably during heating to the operational temperature. Such technical solution was provided e.g. in WO 2007/065685, however, it does not solve the problem of caesium high volatility.

U.S. Pat. No. 7,507,694 B2 and EP 1 648 042 B1 offer another solution in form of caesium alloys that melt at low temperature and show significantly decreased caesium vapour pressure in comparison with the pure metal. Bismuth alloys of WO2007/109815 that release caesium vapours at pressures of the order 10−4Pa and temperatures up to about 450° C. represent another alternative. Yet, all these alloys are still highly air and moisture sensitive. Moreover, this solution has further drawback in the fact that the vapour pressure over the alloy changes with the decreasing caesium concentration during the evaporation. That creates new problem of an appropriate deposition rate control, e.g. by programming the temperature of the evaporation source. So far, quality assurance (QA) concerns regarding robustness of such process on an industrial scale hamper a wider application of this technical solution in mass production processes.

A viable alternative to Cs doping represent highly electropositive transition metal complexes like W2(hpp)4that have ionisation potentials comparably low as caesium and volatilities comparable with volatilities of usual organic matrices. Indeed, these complexes disclosed as electrical dopants first in WO2005/086251 are very efficient for most electron transporting matrices except some hydrocarbon matrices. Despite their high air and moisture sensitivity, these metal complexes provide satisfactory n-doping solution for an industrial use, if supplied in the shells according to WO 2007/065685. Their main disadvantage is their high price caused by relative chemical complexity of comprised ligands and necessity of a multistep synthesis of the final complex, as well as additional costs incurred by necessity of using the protective shells and/or by the QA and logistic issues linked with shell recycling and refilling.

Another alternative represent strong n-dopants created in situ in the doped matrix from relatively stable precursors by an additional energy supplied e.g. in form of ultraviolet (UV) or visible light of an appropriate wavelength. Appropriate compounds for this solution were provided e.g. in WO2007/107306 A1. Nevertheless, state-of-the-art industrial evaporation sources require materials with very high thermal stability, allowing their heating to the operational temperature of the evaporation source without any decomposition during the whole operating cycle (e.g., for a week at 300° C.) of the source loaded with the material to be evaporated. Providing organic n-dopants or n-dopant precursors with such long-term thermal stability is a real technical challenge so far. Moreover, the complicated arrangement of the production equipment that must ensure a defined and reproducible additional energy supply for achieving reproducibly the desired doping level (through the in situ activation of the dopant precursor deposited in the matrix) represents an additional technical challenge and a potential source of additional CA issues in mass production.

Yook et al (Advanced Functional Materials 2010, 20, 1797-1802) successfully used caesium azide in laboratory as an air-stable Cs precursor. This compound is known to decompose under heating above 300° C. to caesium metal and elemental nitrogen. This process is, however, hardly applicable in contemporary industrial VTE sources, due to difficult control of such heterogeneous decomposition reaction in a larger scale. Moreover, release of nitrogen gas as a by-product in this reaction brings a high risk that, especially at higher deposition rates desired in the mass production, the expanding gas will expel solid caesium azide particles from the evaporation source, causing thus high defect counts in the deposited layers of doped semiconducting materials.

Another alternative approach for electrical n-doping in electron transporting matrices is doping with metal salts or metal complexes. The most frequently used example of such dopant is lithium 8-hydroxy-quinolinolate (LiQ). It is especially advantageous in matrices comprising a phosphine oxide group, see e.g. WO 2012/173370 A2. The main disadvantage of metals salt dopants is that they improve basically only electron injection to the adjacent layers and do not increase the conductivity of doped layers. Their utilization for decreasing the operational voltage in electronic devices is thus limited on quite thin electron injecting or electron transporting layers and does hardly allow e.g. an optical cavity tuning by using ETLs thicker than approximately 25 nm, what is well possible with redox-doped ETLs having high conductivity. Furthermore, metal salts typically fail as electrical dopants in cases wherein creation of new charge carriers in the doped layer is crucial, e.g. in charge generating layers (CGL, called also p-n junctions) that are necessary for the function of tandem OLEDs.

For the above reasons, and especially for electrical doping in ETLs thicker than approximately 30 nm, the current technical practice prefers lithium as an industrial redox n-dopant (see e.g. U.S. Pat. No. 6,013,384 B2). This metal is relatively cheap and differs from other alkali metals by its somewhat lower reactivity and, especially, by its significantly lower volatility (normal boiling point about 1340° C.), allowing its evaporation in the VTE equipment at temperatures between 350-550° C.

Nevertheless, quite in accordance with its high n-doping power allowing Li to dope majority of usual types of electron transporting matrices, this metal possesses also a high degree of reactivity. It reacts under ambient temperature even with dry nitrogen and for its use in a highly reproducible manufacturing process complying with contemporary industrial QA standards, it must be stored and handled exclusively under high purity noble gases. Moreover, if Li is co-evaporated with matrix compounds that have evaporation temperatures in the range 150-300° C., its significantly higher evaporation temperature in comparison with the matrix evaporation temperature already causes cross-contamination problems in the VIE equipment.

Many documents suggest as alternative n-dopants almost any known metallic element including weakly reductive and highly volatile Zn, Cd, Hg, weakly reductive Al, Ga, In, Tl, Bi, Sn, Pb, Fe, Co, Ni, or even noble metals like Ru, Rh, Ir and/or refractory metals with highest known boiling points like Mo, W, Nb, Zr (see e.g. JP 2009/076508 or WO 2009/106068). Unfortunately, not only in these two documents cited here as examples but throughout the scientific and patent literature overall, there is in fact lack of any evidence that some of these suggestions have ever been experimentally tested.

To be more specific, even WO 2009/106068 that does not merely mention all imaginable dopants but really strives to claim all the named metalloid elements as n-dopants in organic electronic devices due their alleged applicability through a high-temperature decomposition of a gaseous precursor compound in a heated nozzle, does not bring any single numeric value documenting the physical parameters of allegedly prepared doped materials and/or technical performance of allegedly prepared devices.

On the other hand, US2005/0042548 published before the date of priority of WO 2009/106068 teaches in paragraph 0069 (see namely the last two lines of the left column and first three lines of the right column on page 7) that iron pentacarbonyl can be used for n-doping in organic ETMs if the compound is activated by UV radiation which splits off a carbon monoxide ligand. The coordinatively unsaturated iron compound then reacts with the matrix, what results in the observed doping effects. In the light of this previous art showing that the metal carbonyls that were used in the alleged working example of WO 2009/106068 are known n-dopants in organic matrices if activated by supply of additional energy, it seems quite likely that if the applicants of WO2009/106068 really obtained with their jet of iron pentacarbonyl flowing through a ceramic nozzle electrically heated to a white glow (see the last paragraph of the German text on page 12 of the cited PCT application) any doping effect in the target bathocuproin layer, this effect was caused rather by the same coordinatively unsaturated iron carbonyl complex as produced by UV irradiation in US2005/0042548, than with elemental iron as they suggest. This suspicion is further supported by the fourth paragraph on page 13 of the cited PCT application which teaches that the same result can be obtained with a cold nozzle, if the stream of iron pentacarbonyl is irradiated with an infrared laser having the wavelength fitting with the absorption frequency of the CO groups in the iron pentacarbonyl complex. Here, it is even more likely that the laser activation resulted not in naked metal atoms or clusters of metal atoms but in a reactive, coordinately unsaturated iron complex still bearing some carbonyl ligands, analogously to the reactive complex formed by activation with the UV light.

Despite metals with strongly negative standard redox potentials like alkali earth metals or lanthanides are recited as alternative n-dopants besides alkali metals basically in each document dealing with redox n-doping, the record of the proven n-doping with any metal different from alkali metals is very scarce.

Magnesium is in comparison with alkaline metals much less reactive. It reacts even with liquid water at the ordinary temperature very slowly and in air it keeps its metallic luster and does not gain weight for months. It may be thus considered as practically air-stable. Moreover, it has low normal boiling point (about 1100° C.), very promising for its VTE processing in an optimum temperature range for co-evaporation with organic matrices.

On the other hand, the authors of the present application confirmed in a screening done with dozens of state-of-the-art ETMs that Mg does not possess a sufficient doping strength for common ETMs which are free of strongly polar groups like phosphine oxide group. The only favourable result has been achieved in OLEDs comprising thin electron injection layers consisting of a specific kind of triaryl phosphine oxide matrix (comprising a special tris-pyridyl unit designed for chelating metals), doped with magnesium, as shown in EP 2 452 946 A1. Despite the structural specificity and very favourable (in terms of its LUMO level which is quite deep under the vacuum level in the absolute energy scale) dopability of the exemplary matrix tested with magnesium in EP 2 452 946 A1, the positive results achieved with this n-doped semiconducting material encouraged further research focused on n-doping with substantially air stable metals.

It is an object of the invention to overcome the drawbacks of the prior art and to provide organic light emitting diodes with better characteristics, especially with low voltage and, more specifically, OLEDs with low voltage and high efficiency, preferably utilizing substantially air stable metals as n-dopants.

II. SUMMARY OF THE INVENTION

The object is achieved by an electronic device comprising at least one light emitting layer between an anode and substantially silver cathode, the device further comprising between the cathode and the anode at least one mixed layer (which may also be referred to as substantially organic layer) comprising(i) at least one substantially covalent electron transport matrix compound comprising at least one polar group selected from phosphine oxide group or diazole group, and(ii) in substantially elemental form, an electropositive element selected from substantially non-radioactive alkali metals, alkaline earth metals, rare earth metals, and transition metals of the fourth period of the Periodic table having proton numbers 22, 23, 24, 25, 26, 27, 28, 29.It is to be understood that “substantially silver” means silver content at least 99 weight %. Preferably, the silver content in the cathode is at least 99.5 weight %, more preferably at least 99.8 weight %, even more preferably at least 99.9 weight %, most preferably at least 99.95 weight %.If the polar group is the phosphine oxide group, the reduction potential of the electron transport matrix compound, if measured by cyclic voltammetry under the same conditions, has preferably the value which is more negative than the value obtained for tris(2-benzo[d]thiazol-2-yl)phenoxyaluminum, preferably more negative than for 9,9′,10,10′-tetraphenyl-2,2′-bianthracene or 2,9-di([1,1′-biphenyl]-4-yl)-4,7-diphenyl-1,10-phenanthroline, more preferably more negative than for 2,4,7,9-tetraphenyl-1,10-phenanthroline, even more preferably more negative than for 9,10-di(naphthalen-2-yl)-2-phenylanthracene, even more preferably more negative than for 2,9-bis(2-methoxyphenyl)-4,7-diphenyl-1,10-phenanthroline, even more preferably more negative than for 9,9′-spirobi[fluorene]-2,7-diylbis(diphenylphosphine oxide), even more preferably more negative than for 4,7-diphenyl-1,10-phenanthroline, even more preferably more negative than for 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene, most preferably more negative than for pyrene and still preferably more negative than the value obtained for [1,1′-binaphthalen]-2,2′-diylbis(diphenylphosphine oxide).If the polar group is the imidazole group, the reduction potential of the electron transport matrix compound, if measured by cyclic voltammetry under the same conditions, has preferably the value which is more negative than the value obtained for 4,7-diphenyl-1,10-phenanthroline, preferably more negative than for (9-phenyl-9H-carbazole-2,7-diyl)bis(diphenylphosphine oxide), more preferably more negative than for (9,9-dihexyl-9H-fluorene-2,7-diyl)bis(diphenylphosphine oxide), highly preferably more negative than for 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene, even more preferably more negative than for 3-phenyl-3H-benzo[b]dinaphto[2,1-d:1′,2′-f]phosphepine-3-oxide, most preferably more negative than for pyrene and still preferably more negative than for [1,1′-binaphthalen]-2,2′-diylbis(diphenylphosphine oxide).

It is to be understood that “substantially covalent” means compounds comprising elements bound together mostly by covalent bonds. Examples of substantially covalent molecular structures may be organic compounds, organometallic compounds, metal complexes comprising polyatomic ligands, metal salts of organic acids. In this sense, the term “substantially organic layer” is to be understood as a layer comprising a substantially covalent electron transport matrix compound.

It is further understood that the term “substantially covalent compound” encompasses materials which can be processed by usual techniques and equipment for manufacturing organic electronic devices, like vacuum thermal evaporation or solution processing. It is clear that purely inorganic crystalline or glassy semiconducting materials like silicon or germanium which cannot be, due to their extreme high vaporization temperatures and insolubility in solvents, prepared in equipment for organic electronic devices, are not encompassed by the term “substantially covalent compound.

Preferably, the phosphine oxide polar group is part of a substantially covalent structure comprising at least three carbon atoms directly attached to the phosphorus atom of the phosphine oxide group and having overall count of covalently bound atoms, preferably selected from C, H, B, Si, N, P, O, S, F, Cl, Br and I, in the range 16-250, more preferably in the range 32-220, even more preferably in the range 48-190, most preferably in the range 64-160.

Also preferably, the electron transport matrix compound comprises a conjugated system of at least ten delocalized electrons.

Examples of conjugated systems of delocalized electrons are systems of alternating pi- and sigma bonds. Optionally, one or more two-atom structural units having the pi-bond between its atoms can be replaced by an atom bearing at least one lone electron pair, typically by a divalent atom selected from O, S, Se, Te or by a trivalent atom selected from N, P, As, Sb, Bi. Preferably, the conjugated system of delocalized electrons comprises at least one aromatic ring adhering to the Hückel rule.

More preferably, the substantially covalent electron transport matrix compound comprises at least two aromatic or heteroaromatic rings which are either linked by a covalent bond or condensed. Also preferably, the phosphine oxide polar group is selected from phosphine oxide substituted with three monovalent hydrocarbyl groups or one divalent hydrocarbylene group forming with the phosphorus atom a ring and one monovalent hydrocarbyl group, and the overall count of carbon atoms in the hydrocarbyl groups and the hydrocarbylene group is 8-80, preferably 14-72, more preferably 20-66, even more preferably 26-60, most preferably 32-54. In another preferred embodiment, there are two substantially covalent compounds comprised in the semiconducting material, the first compound comprising the polar group selected from the phosphine oxide group and the diazole group, wherein the first compound is free of a conjugated system of delocalized electrons, or does comprise a conjugated system of less than 10 delocalized electrons; and the second compound that does comprise a conjugated system of at least 10 delocalized electrons. More preferably, the second compound is free of a polar group selected from the phosphine oxide group and/or the diazole group.

Examples of compounds comprising a condensed aromatic skeleton comprising at least 10 delocalized electrons are e.g. naphthalene, anthracene, phenanthrene, pyrene, quinoline, indole or carbazole. The conjugated system of delocalized electrons may also consist of at least two directly attached aromatic rings, the simplest examples of such systems being biphenyl, bithienyl, phenylthiophene, phenylpyridine and like. On the other hand, it is known that a pentavalent phosphorus atom, for example the phosphorus atom in the phosphine oxide group, does not participate in conjugation in systems of delocalized electrons attached to the pentavalent P atom, and in this sense resembles sp3hybridized carbon, for example the carbon atom in a methylene group. Most preferably, the conjugated system of at least 10 delocalized electrons comprised in the second compound is comprised in a C14-C50-arene or a C8-C50heteroarene structural moiety, wherein the overall count of carbon atoms comprises also possible substituents. It is in the spirit of the present invention that the count, topology and spatial arrangement of the substituents on the structural moiety comprising the conjugated system of delocalized electrons is not decisive for the function of the invention. Preferred heteroatoms in the heteroarene structural moiety are B, O, N and S. In the second compound, both the core atoms bearing the comprised system of delocalized electrons as well as the polyvalent atoms like C, Si, B, which preferably form the peripheral substituents attached to the core atoms, may be substituted with terminal atoms of elements which are typically monovalent in organic compounds and which are more preferably selected from H, F, Cl, Br and I. The conjugated system of at least 10 delocalized electrons is preferably comprised in a C14-C50-arene or a C8-C50heteroarene structural moiety. Also preferably, the diazole polar group is an imidazole group.

In one of preferred embodiments, the electronic device further comprises a metal salt additive consisting of at least one metal cation and at least one anion. Preferably, the metal cation is Li+or Mg2+. Also preferably, the metal salt additive is selected from metal complexes comprising a 5-, 6- or 7-membered ring that contains a nitrogen atom and an oxygen atom attached to the metal cation and from complexes having the structure according to formula (II)



wherein A1is a C6-C30arylene or C2-C30heteroarylene comprising at least one atom selected from O, S and N in an aromatic ring and each of A2and A3is independently selected from a C6-C30aryl and C2-C30heteroaryl comprising at least one atom selected from O, S and N in an aromatic ring. Equally preferably, the anion is selected from the group consisting of phenolate substituted with a phosphine oxide group, 8-hydroxyquinolinolate and pyrazolylborate. The metal salt additive preferably works as a second electrical n-dopant, more preferably, it works synergistically with the metallic element present in the elemental form and works as the first electrical n-dopant.

It is preferred that the electropositive element is selected from Li, Na, K, Be, Mg, Ca, Sr, Ba, Sc, Y, La, Sm, Eu, Tb, Yb, Lu, Ti, V and Mn, more preferably, from Li, Mg, Ca, Sr, Ba, Sm, Eu, Tb, Yb, most preferably from Li, Mg and Yb. Preferably, the molar ratio of the electropositive element to the first compound is lower than 0.5, preferably lower than 0.4, more preferably lower than 0.33, even more preferably lower than 0.25, even more preferably lower than 0.20, even more preferably lower than 0.17, most preferably lower than 0.15, still preferably lower than 0.13, still but less preferably lower than 0.10.

It is further preferred that the molar ratio of the electropositive element to the first compound is higher than 0.01, preferably higher than 0.02, more preferably higher than 0.03, even more preferably higher than 0.05, most preferably higher than 0.08.

Preferably, the electropositive element is comprised in an electron transporting, electron injecting, or charge generating layer. More preferably, the electron transporting or electron injecting layer is adjacent to a layer consisting of compounds that have their reduction potentials, if measured by cyclic voltammetry under the same conditions, more negative than the electron transport matrix compounds of the adjacent electron transporting or electron injecting layer. In one of preferred embodiments, the layer adjacent to the layer made of inventive semiconducting material is the emitting layer.

It is further preferred that the light emitting layer emits blue or white light. In one of preferred embodiments, the light emitting layer comprises at least one polymer. More preferably, the polymer is a blue light emitting polymer.

Also preferably, the electron transporting or electron injecting layer is thicker than 5 nm, preferably thicker than 10 nm, more preferably thicker than 15 nm, even more preferably thicker than 20 nm, most preferably thicker than 25 nm, still preferably thicker than 50 nm, and still preferably thicker than 100 nm.

In one of preferred embodiments, the electron transporting or electron injecting layer is adjacent to the cathode.

Still another embodiment of the invention is a tandem OLED stack comprising a metal-doped pn-junction comprising(i) in substantially elemental form, an electropositive element selected from substantially non-radioactive alkali metals, alkaline earth metals, rare earth metals, and transition metals of the fourth period of the Periodic table having proton numbers 22, 23, 24, 25, 26, 27, 28, 29, and

(ii) at least one substantially covalent electron transport matrix compound comprising at least one polar group selected from the phosphine oxide group and from the diazole group. The device of the invention is preferably prepared by process comprising, at least one step wherein(i) an electropositive element selected from substantially non-radioactive alkali metals, alkaline earth metals, rare earth metals, and transition metals of the fourth period of the Periodic table having proton numbers 22, 23, 24, 25, 26, 27, 28, 29, and(ii) at least one substantially covalent electron transport matrix compound comprising at least one polar group selected from the phosphine oxide group and from the diazole group

are coevaporated and codeposited under reduced pressure to form the substantially organic layer, and the electropositive element is deposited in its elemental or substantially elemental form. Preferably, the electropositive element is evaporated from its elemental or substantially elemental form, more preferably from a substantially air stable elemental or substantially elemental form. Also preferably, the pressure is lower than 10−2Pa, more preferably lower than 10−3Pa, most preferably lower than 10−4Pa.

Preferably, the electropositive element has normal boiling point lower than 3000° C., more preferably lower than 2200° C., even more preferably lower than 1800° C., most preferably lower than 1500° C. Under normal boiling point, it is to be understood the boiling point at normal atmospheric pressure (101.325 kPa).

It is to be understood that the term “substantially air stable” refers to metals and their substantially elemental form (e.g. alloys with other metals) which reacts under ambient conditions with atmospheric gases and moisture slowly enough to avoid quality assurance issues if it is handled under ambient conditions in an industrial process. More specifically, a form of a metal shall be assigned as substantially air stable for the purposes of this application, if a sample of this form having the weight at least 1 g and a surface exposed to air at least 1 cm2may be kept under standard temperature 25° C., pressure 101 325 Pa and relative humidity 80% for at least an hour, preferably for at least 4 hours, more preferably for at least 24 hours, and most preferably for at least 240 hours, without showing a statistically significant weight gain, provided that the accuracy of weighing is at least 0.1 mg.

Most preferably, the electropositive element is evaporated from a linear evaporation source. In one of preferred embodiments, the inventive electronic device is an inverted device.

In another preferred embodiment, the device is a top-emitting device with the cathode having thickness less than 30 nm, preferably less than 25 nm, more preferably less than 20 nm, most preferably less than 15 nm.

In one of preferred embodiments, the silver cathode is corrugated and/or combined with a light-scattering layer.

