WO1997047051A1

WO1997047051A1 - Gallium nitride anodes for organic electroluminescent devices and displays

Info

Publication number: WO1997047051A1
Application number: PCT/IB1997/000559
Authority: WO
Inventors: Samuel Clagett Strite; Ching Wan Tang
Original assignee: International Business Machines Corporation; Eastman Kodak Company
Priority date: 1996-06-05
Filing date: 1997-05-16
Publication date: 1997-12-11
Also published as: DE69637686D1; EP0902983B1; JP3254221B2; JP2000503798A; WO1997047050A1; KR20000016407A; KR100324869B1; JPH11511895A; EP0902983A1; US6433355B1

Abstract

An organic light emitting device is provided which comprises a substrate (60), a cathode (64), an anode (61), and an organic region (62, 63) in which electroluminescence takes place if a voltage is applied between said anode (61) and cathode (64). The anode (61) comprises a III-Nitride such as Gallium Nitride (GaN) or an alloy of Gallium Nitride (GaN) with InN and/or AlGaN.

Description

DESCRIPTION

Gallium Nitride Anodes for Organic Electroluminescent Devices and Displays

TECHNICAL FIELD

The present invention pertains to organic electroluminescent devices, arrays, displays and methods for making the same

BACKGROUND OF THE INVENTION

Organic electroluminescence (EL) has been studied extensively because of its possible applications in discrete light emitting devices, arrays and displays. Organic materials investigated so far can potentially replace conventional inorganic materials in many applications and enable wholly new applications. The ease of fabrication and extremely high degrees of freedom in organic EL device synthesis promises even more efficient and durable materials in the near future which can capitalize on further improvements in device architecture.

Organic EL at low efficiency was observed many years ago in metal/organic/metal structures as, for example, reported in Pope et al , Journal Chem. Phys., Vol. 38, 1963, pp 2024, and in "Recombination Radiation in Anthracene Crystals", Helfπch et al , Physical Review Letters, Vol. 14, No. 7, 1965, pp. 229-231. Recent developments have been spurred largely by two reports of high efficiency organic EL These are C W Tang et al., "Organic electroluminescent diodes", Applied Physics Letters, Vol 51 , No. 12, 1987, pp. 913-915, and by a group from Cambridge University in Burroughs et al., Nature, Vol. 347, 1990, pp. 539 Tang et al. made two-layer organic light emitting devices using vacuum deposited molecular dye compounds, while Burroughs used spin coated poly(p-phenylenevιnylene) (PPV), a polymer

The advances described by Tang and in subsequent work by the Cambridge group, for example in "Efficient LEDs based on polymers with high electron affinities", N. Greenham et al , Nature, Vol. 365, 1993, pp. 628-630, were achieved mainly through improvements in the design of EL devices derived from the selection of appropriate organic multilayers and contact metals.

Organic EL light emitting devices (OLEDs) function much like inorganic LEDs, except that light is commonly extracted through a transparent electrode deposited on a transparent glass substrate. The simplest possible structure, schematically illustrated in Figure 1 A, consists of an organic emission layer 10 sandwiched between two electrodes 1 1 and 12 which inject electrons (e^~) and holes (h⁺), respectively. Such a structure has been described in the above mentioned paper of Burroughs et al., for example. The electrons and holes meet in the organic layer 10 and recombme producing light. It has been shown in many laboratories, see for example: "Conjugated polymer electroluminescence", D. D. C. Bradley, Synthetic Metals, Vol. 54, 1993, pp. 401 -405, 'The effect of a metal electrode on the electroluminescence of Poly(p-phenylvιnylene)", J. Peng et al , Japanese Journal of Applied Physics, Vol. 35, No. 3A, 1996, pp. L317-L319, and "Carrier tunneling and device characteristics in polymer LEDs", I. D. Parker, Journal of Applied Physics, Vol. 75, No. 3, 1994, pp. 1656-1666, that improved performance can be achieved when the electrode material work functions are chosen to match the respective molecular orbitals of the organic material forming the organic layer 10 Such an improved structure is shown in Figure 1 B. By choosing the optimized electrode materials 13 and 14, the energy barriers to injection of carriers are reduced, as illustrated Still, such simple structures perform poorly because little stops electrons from traversing the organic layer 1 0 and reaching the anode 14, or the hole from reaching the cathode 13.

Figure 2A illustrates a device with a large electron barrier 16 such that few electrons are injected, leaving the holes no option but to recombme in the cathode 15.

A second problem, illustrated in Figure 2B, is that the mobilities of electrons and holes in most known organic materials, especially conductive ones, differ strongly. Figure 2B illustrates an example where holes injected from the anode 18 quickly traverse the organic layer 19, while the injected electrons move much slower, resulting in recombination near the cathode 17. If the electron mobility in the organic layer 19 were larger than the holes', recombination would occur near the anode 18. Recombination near a metal contact is non-radiative, which limits the luminous efficiency of such flawed devices.

Tang, as shown in Figure 3, separated electron and hole transport functions between separate organic layers, an electron transport layer 20 (ETL) and a hole transport layer (HTL) 21 , mainly to overcome the problems described above. In "Electroluminescence of doped organic thin films", CW. Tang et al., Journal of Applied Physics, Vol. 65, No. 9, 1989, pp. 3610-3616, it is described that higher carrier mobility was achieved in the two-layer design, which led to reduced device series resistance enabling equal light output at lower operating voltage. The contact metals 22, 23 could be chosen individually to match to the ETL 20 and HTL 21 molecular orbitals, respectively, while recombination occurred at the interface 24 between the organic layers 20 and 21 , far from either electrode 22, 23. As electrodes, Tang used a MgAg alloy cathode and transparent Indium-Tin-Oxide (ITO) as the anode. Egusa et al. in "Carrier injection characteristics of organic electroluminescent devices", Japanese Journal of Applied Physics, Vol. 33, No. 5A, 1994, pp. 2741 -2745 have shown experimentally that the proper selection of the organic multilayer can lead to a blocking of both electrons and holes at an organic interface remote from either electrode This effect is illustrated by the structure of Figure 3 which blocks electrons from entering the HTL 21 and vice versa by a clever choice of HTL and ETL materials This feature eliminates non-radiative recombination at the metal contacts as described in Figure 1 A and also promotes a high density of electrons and holes in the same volume leading to enhanced radiative recombination.

With multilayer device architectures now well understood and commonly used, the major performance limitation of OLEDs is the lack of ideal contact electrodes. The main figure of merit for electrode materials is the position of the bands relative to those of the organic materials (see Bradley, Peng and Parker above for detailed discussion). In some applications it is also desirable for the electrode material to be either transparent or highly reflective The electrode should also be chemically inert and capable of forming a dense uniform film to effectively encapsulate the OLED. It is also desirable that the electrode not strongly quench organic EL

Much attention has been paid to the choice of the cathode electrode material, largely because those materials which are good electron injectors are also chemically reactive, low work function metals which are unstable in atmosphere, and limit the overall device reliability and lifetime.

Much less attention has been paid to the optimization of the anode contact, since ITO or Au anodes generally outperform the cathode contact. However, if the anode electrode could be improved, it would have a similarly positive effect on device performance and reliability as improved cathodes.

Indium-tin-oxide has been the anode of choice for years Its major advantage is that it is a transparent conductor which also has a large work function (roughly 4.7 eV), and is therefore well suited for the formation of a transparent anode on glass. However, ITO is known to have a barrier to hole injection into preferred organic HTL material. Parker showed that, by replacing ITO with Au in an otherwise identical organic LED structure, the device efficiency is doubled He attributed this to the elimination of the ITO/organic hole injection barrier which is achieved using Au, which has a higher work function ITO is also responsible for device degradation as a result of In diffusion emanating from the ITO into the OLED which can eventually cause short circuiting In diffusion from ITO into PPV was clearly identified in "Characterization of polymeric light emitting diodes by SIMS depth profiling analysis", G. Sauer, M. Kilo, A. Wokaun, S. Karg, M Meier, M Schwoerer, H Suzuki, J. Simmerer, H. Meyer, D Haarer, Fresenius J Anal Chem., pp. 642-646, Vol. 353 (1995). ITO is a polycrystallme material in the form commonly used for OLEDs The abundance of grain boundaries provides ample pathways for contaminant diffusion through the ITO. Finally, ITO also is a reservoir of oxygen which is known to have a detrimental effect on common organic materials. Despite all of these known problems related to ITO anodes, they are still favored in the art because no other transparent electrode material of similar or better quality is yet known in the art. At least one transparent electrode is necessary for a practical OLED, since the light must be efficiently extracted.

While Au has a large (5.2 eV) work function, long-lived OLED devices cannot be made using Au cathodes because of the very high diffusivity of Au in organic materials. Like In from ITO, only worse, Au from the contact diffuses through the OLED and eventually short circuits the device. In addition, Au is not a practical anode material for most architectures because it is not transparent For the lack of a transparent cathode material, the anode must be the transparent contact for present day OLEDs

Other semiconductors besides ITO have been tried as OLED anodes I D Parker and H. H. Kim, "Fabrication of polymer light-emitting diodes using doped silicon substrates", Applied Physics Letters, Vol. 64, No. 14, 1994, pp 1774-1776, showed that, depending on the semiconductor doping, the Si/ Sι0₂ is capable of either hole or electron injection into organic thin films. Si electrodes, due to the small bandgap and moderate work function of Si, have both a barrier to electron and hole injection into organic LED materials, and therefore represent no marked improvement over conventional metals. Parker and Kim got avoided this by adding a Si0₂ interlayer between the Si contact and OLED While the voltage drop across the Sι0₂ insulator permitted the Si bands to line up with their organic molecular orbital counterpart, electrons were not directly injected, rather forced to tunnel through the Sι0₂ insulator Such OLEDs had turn-on voltages of > 10 V, too high for efficient device operation

The lack of inert, stable, energetically matched, and transparent electrode materials for low voltage, efficient and stable OLED operation remains a major obstacle to OLED development.

Organic LEDs have great potential to outperform conventional inorganic LEDs in many applications. One important advantage of OLEDs and devices based thereon is the price since they can be deposited on large, inexpensive glass substrates, or a wide range of other inexpensive transparent, semitransparent or even opaque crystalline or non-crystalline substrates at low temperature, rather than on expensive crystalline substrates of limited area at comparatively higher growth temperatures (as is the case for inorganic LEDs). The substrates may even be flexible enabling pliant OLEDs and new types of displays To date, the performance of OLEDs and devices based thereon is inferior to inorganic ones for several reasons-

1 High operating voltage. Organic devices require more voltage to inject and transport the charge to the active region (emission layer) which in turn lowers the power efficiency of such devices High voltage results from the need for high electric fields to inject carriers over energy barriers at the electrode/organic interfaces, and from the low mobility of the carriers in the organic transport layers (ETL and HTL) which leads to an ohmic voltage drop and power dissipation

Low brightness Today's OLEDs can produce nearly as many photons per electron as common inorganic LEDs, i.e. their quantum efficiency is good OLEDs lag inorganic LEDs in brightness mainly because comparatively little charge can be injected, or conducted through the resistive transport layers (HTL or ETL). This latter is a well known effect is referred to as Space Charge Limited Current. Simply put, due to the low mobility of carriers in organic materials, a traffic jam develops which restricts the flux of electrons and holes reaching the emission layer. Better emitter materials cannot offer greatly improved brightness until efficiently injecting electrodes and high conductance transport layers are also available.

Reliability: Organic LEDs degrade in air and during operation. Several problems are known to contribute.

A) Efficient low field electron injection requires low work function cathode metals like Mg, Ca, F, Li etc. which are all highly reactive in oxygen and water. Ambient gases, and gases coming out of the organic materials during ohmic heating degrade the contacts.

B) Conventional AgMg and ITO contacts still have a significant barrier to carrier injection in preferred ETL and HTL materials, respectively. Therefore, a high electric field is needed to produce significant injection current. Stress from the high field and ohmic heating at the resistive electrode/organic interface contribute to device degradation.

C) The high resistance of carrier transport layers heats the device under operation.

D) Thermal stability of most OLED materials is poor making them sensitive to heating Upon heating, many amorphous organic materials crystallize into grains The crystallites have less volume and pack less uniformly than the amorphous solid The resulting gaps and odd shapes of the crystallites make conduction from one crystallite to the next difficult, increasing resistance and heating in a positive feedback loop, while opening further channels for gaseous contaminants to penetrate, or for neighboring materials to diffuse

4 Poor chemical stability: Organic materials commonly used in OLEDs are vulnerable to degradation caused by reaction with and diffusion of contact electrode materials and the ambient atmosphere.

OLEDs are mainly limited by their contacts and transport layers, and feedback from the transport layer heating. It is thus highly desirable to replace the low work function metal based cathodes and ITO anodes with stable, (and at least one transparent) contacts characterized by barπerless charge injection into OLEDs.

However, present day solutions inhibit performance and degrade device reliability. The price of distancing the active layer from the contacts, desirable to isolate the active region from diffusing impurities and quenching by the electrodes, are ohmic voltage drops across the HTL/ETL, leading to heating and power consumption

As can be seen from the above examples, and the description of the state of the art, the contact materials need to be improved to realize OLEDs, and displays based thereon, with superior characteristics.

It is an object of the present invention to provide new and improved organic EL devices, arrays and displays based thereon It is a further purpose of the present invention to provide new and improved organic EL devices, arrays and displays based thereon with improved efficiency, lower operating voltage, and increased reliability.

It is another object of the present invention to provide new and improved anodes for organic EL devices, arrays and displays based thereon

It is a further object to provide a method for making the present new and improved organic EL devices, arrays and displays

SUMMARY OF THE INVENTION

The above objects have been accomplished by providing an OLED having an anode composed primarily of the Ill-nitride, e g GaN wide-bandgap semiconductor in contact with the HTL layer

The inventive approach capitalizes primarily on the favorable valence band energy of lll-N compounds, as well as their good conductivity, transparency in the visible spectrum, chemical inertness, hardness, and ability to be deposited in the amorphous state at extremely low temperatures on glass, organic thin films, or other amorphous or crystalline substrates This is particularly true for GaN Our experiments have shown that GaN is conductive, even when deposited at room temperature in the amorphous state, highly transparent in the visible spectrum, and has a favorable band alignment for hole injection into preferred OLED materials. Furthermore, GaN is an excellent encapsulant for OLEDs due to the extremely low diffusivity of impurities in GaN and the nearly amorphous state of material deposited at low temperature. In addition, the considerable strength of the Ga-N chemical bond makes GaN chemically inert, even capable of resisting the attack of highly corrosive acids and bases We have experimentally observed that GaN - having all of the above favorable properties - can be deposited onto glass, or even directly onto an OLED multilayer structure, and produces a device with improved performance and stability. In the following, GaN is particularly addressed. However, all statements also apply to other lll-N compounds.

In the main embodiment of the present invention, a single or multilayer OLED structure having a GaN anode directly in contact with the corresponding organic layer, and a conventional opposite contact electrode is envisioned.

The introduction of a GaN anode leads to the following advantages- Low voltage carrier injection is realized through the highly favorable band energies of GaN with respect to preferred OLED materials

GaN is highly transparent to visible light

GaN is chemically inert and thermally stable and therefore has no undesirable solid state interactions with the organic layers with which it is in contact or close proximity

GaN is an outstanding encapsulant and mechanical protectant material for OLEDs, due to it's amorphous-like structure when deposited at low temperature, hardness and low impurity diffusion constants

GaN can be deposited at conditions required for OLED formation (e.g low temperature, amorphous substrates, minimum damage to the growth surface) in a conductive state

GaN, as a semiconductor, quenches optical recombination in nearby organic layers less strongly than metals, enabling reduced transport layer thicknesses.

DESCRIPTION OF THE DRAWINGS

The invention is described in detail below with reference to the following schematic drawings:

FIG. 1A shows a known OLED having an emission layer and two electrodes.

FIG. 1 B shows another known OLED having an emission layer and two metal electrodes, with work functions chosen such that the energy barrier for carrier injection is reduced

FIG. 2A shows another known OLED having an emission layer and two metal electrodes, the work function of the anode being chosen such that the energy barrier for hole injection is low, whereas the work function of the cathode poorly matches the emission layer yielding little electron injection and little radiative recombination in said emission layer.

FIG. 2B shows another known OLED having an emission layer with low electron mobility compared to hole mobility such that the recombination occurs close to the cathode where it is quenched.

FIG. 3 shows another known OLED having an electron transport layer and hole transport layer.

FIG. 4 shows an optical absorption spectrum of a GaN/glass thin film.

The onset of absorption occurs around 360nm in the near ultraviolet, which is responsible for the high transparency of GaN grown at low substrate temperatures on all types of substrates. FIG. 5 shows the energy alignments of GaN and Alq3 molecular levels measured by ultraviolet photoemission spectroscopy for a GaN/Alq3 heterojunction. Experimental error is included in the Alq3 bands as drawn. To within experimental error, the HOMO of Alq3 lies at equal or higher energy compared to the GaN valence band minimum, meaning that hole injection from GaN to Alq3 is barπerless. Typical hole transporting materials have higher HOMO energies than Alq3, so also enjoy barπerless hole injection from GaN.

FIG. 6 shows a band structure of the GaN anode device which exhibited comparable performance compared to a conventional ITO anode device. The band structure, deduced from FIG. 5, and values available in the open literature, indicate that a barrier exists to hole injection from ITO into the GaN anode. This barrier is responsible for the performance of the GaN anode based device being only comparable as opposed to superior to the conventional ITO anode device structure, which also has a barrier to hole injection, but in this case between the ITO anode and the CuPc.

FIG. 7 shows a cross-section of the main embodiment of the present invention, in which a GaN layer is inserted between an ITO layer and the HTL in an OLED.

FIG. 8 shows a cross-section of a second embodiment of the present invention, in which the GaN anode is deposited on top of the OLED device in an anode-up configuration. FIG. 9 shows a cross-section of a display or array, according to the present invention, in which a GaN-based anode-up device structure is deposited onto an opaque Si IC substrate, with light emitted through the anode into the plane above the substrate.

FIG. 10 shows a cross-section of a display or array, according to the present invention, in which a GaN-based anode-down device structure is deposited onto a transparent glass substrate which also contains Si-based display related circuitry, with light emitted through the anode into the plane below the substrate

GENERAL DESCRIPTION

GaN is an ideal material for hole injection into OLEDs, based both on what is known of the material in the literature, and what we have discovered in our laboratory. The physical properties of GaN are catalogued in: S. Stπte and H. Morkoc, "GaN, AIN and InN: a review", Journal of Vacuum Science and Technology B, Vol. 10, 1992, pp. 1237-1266 and "Properties of Group III Nitrides", edited by James H. Edgar, (The Institution of Electrical Engineers, London 1994). The properties of GaN as a transparent conductor have been described by H. Sato et al., "Transparent and conductive GaN thin films prepared by an electron cyclotron resonance plasma metalorganic chemical vapor deposition method", Journal of Vacuum Science and Technology A, Vol. 1 1 , No. 4, 1993, pp. 1422-1425. Measurements on films grown by the authors are presented in Figures 4 and 5 to verify this fact. As reviewed above, an ideal contact electrode material should be characterized by:

1 Depositability onto organic layers, amorphous, crystalline or polycrystalline substrates at low temperatures with no damage to the underlying material. 2. Favorable energy band levels for injection of charge into preferred OLED materials.

3. Sufficient electrical conductivity so that total vertical device series resistance is unaffected by the electrode.

4. High transparency in the visible spectrum (for low quenching and flexibility in light extraction).

5. Chemical inertness

6. Low diffusivity of impurities

7. Mechanical hardness

GaN can be deposited at low temperatures via magnetron sputtering, laser ablation, plasma enhanced molecular beam deposition (PEMBD) or other related techniques in which the energy required to create a reactive nitrogen radical is supplied from some external stimulus, and not thermal energy at the substrate. PEMBD, in which thermally evaporated Ga atoms react with low kinetic energy nitrogen radicals at the substrate surface, is our preferred method due to the small amount of chemical and/or kinetic damage endured by the substrate. Using this method, undoped GaN has a resistivity of 10 Ohm-cm or less when grown at room temperature.

We have studied the band structure of GaN deposited onto glass at low temperature. Figure 4 is a transmission spectrum of one such thin film These data indicate that low temperature amorphous GaN, much like crystalline GaN, has a wide bandgap energy of 3.3 - 3.4 eV, making it highly transparent to visible light. In order to determine the energy position of the GaN valence band (VB) relative to common OLED materials, a clean GaN surface was prepared onto which a thin layer of Alq3 was vacuum deposited. In this arrangement, Ultraviolet Photoemission Spectroscopy can resolve the relative positions in energy of the GaN valence band maximum and the Alq3 highest occupied molecular orbital (HOMO). These data, depicted in Figure 5 and combined with our knowledge of the GaN and Alq3 bandgap, indicate that the GaN valence band is positioned at equal or higher energy with respect to the Alq3 HOMO, meaning that hole injection from GaN into Alq3 can proceed with no or even a negative intermediate barrier. This also indicates that the GaN VB is positioned favorably in energy to barrierless hole injection into preferred OLED HTL materials which are characterized by HOMO energies higher than the Alq3 HOMO energy level.

Further confirmation of the favorable energy positioning of the GaN valence band comes from measurements of actual OLED device structures, e.g. a GaN anode structure having the following layer sequence from the glass substrate up: Glass/ITO/GaN/CuPc/NPB/Alq3/MgAg. The performance of these devices is equivalent to conventional Glass/ITO/CuPc/NPB/Alq3/MgAg OLED devices, both in terms of operating voltage and external efficiency. Figure 6 shows the band structure of the

Glass/ITO/GaN/CuPc/NPB/Alq3/MgAg device using values taken from the above described measurements and number available in the open literature. Figure 6 clearly indicates that there is a barrier to hole injection from the ITO to the GaN in this device, which is roughly equivalent to the ITO/CuPc barrier in the conventional device structure. Because GaN is a conductive semiconductor, it is probable that band bending reduces the effective barrier height between ITO/GaN which accounts for the comparable performance of the GaN anode which we observed experimentally. Furthermore, devices having better ohmic contact to the GaN than that provided by ITO (e.g. if Au or Pt were used instead of ITO) would outperform conventional device structures significantly. CuPc on the other hand cannot be highly doped, and therefore the barrier it presents to hole injection cannot be altered. In this way, the GaN anode greatly increases the flexibility of device design High work function metals can be introduced (e.g Au/GaN/CuPc anode) for very low voltage hole injection with high stability resulting from the GaN diffusion barrier The GaN can also be made conductive by doping or varied deposition conditions which will permit low voltage injection using conventional ITO in an ITO/GaN/CuPc anode configuration Even without the ohmic contact to GaN, the device structure of Figure 6 is superior to the conventional device because of the greater potential stability of GaN compared to ITO

Our experimental results have shown that GaN fulfills the first four points of the above list describing an ideal contact electrode That GaN fulfills the next three points ( 5-7) is apparent from the technical literature on GaN which is readily available, e.g. in the above referenced review paper by Stπte and Morkoc or the book entitled "Properties of Group III Nitrides."

Similar or even improved performance is realized if the GaN is alloyed

with either AIN or InN. Alloying with AIN increases the bandgap of GaN This has the favorable effect of further reducing the absorption of the anode in the short visible wavelengths. AIGaN also has a lower valence band energy position than GaN due to the larger bandgap The lower valence band can be used to reduce or eliminate a barrier to hole injection into an organic material which presents a barrier to a simple GaN anode. Alloying with InN increases the conductivity and the free electron concentration of GaN. An InGaN anode has a higher valence band energy, and therefore presents a smaller barrier to hole injection from ITO The increased electron concentration of InGaN will also promote an ohmic contact to ITO since holes can tunnel through the thinner barrier at the ITO/lnGaN heterojunction Furthermore, an InGaN anode will have less series resistance than a GaN anode. It is conceivable the the optimal anode material for a given device design could be a triple alloy of InN, AIN and GaN (i.e. InAIGaN) for all of the reasons stated above. For simplicity, we refer to AIGaN, InGaN or InAIGaN anodes as GaN-based anodes.

To overcome the problems of conventional ITO anodes for discrete light emitters, light emitting arrays and display applications, improved structures which capitalize on the favorable physical properties of GaN, as illustrated in Figures 7-8, are provided, enabling new array and display applications as illustrated in Figures 9-10.

Two embodiments of improved OLEDs incorporating GaN anodes are now detailed in connection with Figures 7-8

The simplest embodiment of a GaN anode OLED, already improved with respect to the state of the art is depicted in Figure 7 From the substrate up, listed in the order of deposition, is a glass/GaN/HTL/ETL/Metal OLED structure. In addition to the lower barrier to hole injection afforded by the GaN anode 61 formed on the glass substrate 60, the HTL 62 thickness may be reduced as a result of reduced optical quenching and diffusion emanating from the anode. We note that the GaN anode could be further improved by alloying with InN, AIN or both materials We also note here that the structure depicted in Figure 7 might also benefit from the addition of an additional layer 61.1 (e.g. ITO) between the GaN 61.2 and glass 60 to lower the lateral sheet resistance of the anode 61. Finally, any substrate, even an opaque one, can replace the glass substrate 60 depicted. In this case, the preferred embodiment require a transparent top contact 64. The organic region 65 of the first embodiment comprises an ETL 63 and HTL 62 It is to be noted that the present Figure and all other Figures are not drawn to scale. Table 1 : Exemplary details of the first embodiment

Layer No Material Width present example

substrate 60 glass 0.1 mm-5mm 1 mm

outer anode 61.1 ITO 10-300 nm 50nm

anode 61 2 GaN 10-300nm 50nm

HTL 62 TAD 5-500nm 50nm

ETL and EL 63 Alq3 20-1000nm 80nm

cathode 64 AgMg 10-2000nm 50nm

A second embodiment of a GaN-based anode device is depicted in Figure 8. From the substrate 70 up, listed in the order of deposition, is a glass/Metal/ETL/EL/HTL/AIGaN OLED structure. The major difference between Figure 8 and Figure 7 is that the GaN-based anode is deposited last on top of the organic layer stack 72-74, which in this case includes a separate emission layer 73 (EL) as is sometimes practiced in the art We note that the AIGaN anode layer could be replaced by a GaN, InGaN or InAIGaN anode within the spirit of the invention We further note that structures in which the AIGaN layer 75 1 being part of the anode 75 is directly deposited onto the OLED stack 72-74 have similarly improved performance in comparison to structures such as that depicted in Figure 7 The anode 75 might comprise additional layer or layers, e g. an Al or ITO layer 75.2 might be grown on top of the AIGaN layer 75 1 in order to reduce the lateral sheet resistance of the contact Any substrate can be chosen, even an opaque one. In the latter case, the combined anode 75 is preferably designed to be fully transparent for ease of light extraction The organic region 76 of the second embodiment comprises an ETL 74, a layer 73 suited for electroluminescence (EL) and HTL 72

Table 2: Exemplary details of the second embodiment

Layer No Material Width present example

substrate 70 Si 0 1 mm-5mm 1 mm

cathode 71 AI LI 10-300 nm 50 nm

ETL 72 Alq3 M OOnm 20nm

EL 73 coumaπne- doped Alq3 20-1000 nm 70 nm

HTL 74 TAD 5-500nm 50nm

anode contact 75.1 AIGaN 10-2000nm 50nm

lateral transport layer 75.2 ITO 10-2000nm 50nm

In the following, some display embodiments, based on and enabled by the present invention, are disclosed.

It would be advantageous if one could integrate OLEDs onto Si substrates because prior to OLED deposition, the substrate could be fabricated to contain active Si devices, such as for example an active matrix, drivers, memory and so forth. Such a structure can be a very inexpensive small area organic display with high resolution and performance realized in the Si. An OLED, OLED arrays or an OLED display may either by grown directly on such a Si substrate carrying Si devices, or it may be fabricated - 21 -

separately and flipped onto the Si substrate later One problem is the Si metallization which is typically Al, a poor OLED anode or cathode metal Another problem is that a transparent upper contact is needed because Si is opaque The present invention offers a solution to this problem The disclosed GaN-based anode permits a stable, low voltage hole contact to be formed on top of the standard Si process metallizations For example, an InGaN anode layer could be deposited directly onto the Al metallization, and a cathode up OLED structure could be deposited directly on top of the InGaN anode GaN-based anodes are also useful if an anode-up design is chosen In such an approach, the Si metallization must be modified by some means to function as an efficient cathode A cathode down OLED structure could then be deposited directly on top of the modifi

An organic array or display structure formed on a Si substrate is illustrated in Figure 9 and described in the following This display comprises a Si substrate 1 10 which has integrated circuits comprising active and/or passive devices such as memory cells, drivers, capacitors, transistor etc (these devices are not shown) On top of the Si integrated circuit, an OLED cathode (e.g MgAg, Ca, AILi, Al) material 1 1 1 is patterned or blanket deposited in a thin layer which is sufficient to modify the Si metallization for efficient electron injection, but too thin to introduce lateral short circuiting (5 - 100 A), to connect the Si devices to the OLEDs 1 12 An OLED, in the anode-up geometry is deposited on the patterned cathodes 1 1 1 and Si substrate 1 10 Finally, a GaN anode 1 13 is provided. It is to be noted that no details of the OLED(s) are shown for sake of simplicity, but the OLED may be blue, white or any other color Blue light may be desirable because it can be converted to red and green light efficiently by patterned organic dyes to achieve full color White light may be desirable because it can be passed through a color filter array to achieve full color

For example, an Al-metal zed Si chip 1 10 on which thin Al-Li cathodes 1 1 1 are blanket deposited shortly before OLED growth may serve as substrate for an OLED array or display 1 12. One such OLED comprises (from the bottom to the top): a cathode layer, e.g. Al-Li 1 1 1 , an ETL, an organic doped or undoped active region, a HTL, and a GaN-based anode 1 13. This anode 1 13 may for example be composed of the following stack of 'layers': InGaN/ITO.

The organic region of the present devices may - in addition to charge transport layers if needed at all - either comprise:

• a stack of more than one organic emission layers (EL), or

• an organic compound doped with one or more impurities, organic or inorganic, chosen to dominate and enhance the electroluminescence, or

• a stack of more than one organic emission layer, some of which may be doped to dominate or enhance the electroluminescence of that particular organic emission layers, or

• a stack of more than one organic layer, in which the role of one or more of said organic layers is to electrically confine one or more carrier types to improve the emission of an adjacent organic layer.

Another possible display embodiment, illustrated in Figure 10, is described below. This display comprises a transparent substrate 130 on top of which amorphous-Si or poly-Si structures are formed using the same technology developed for active matrix liquid crystal displays. Usually the Si is structured to provide thin-film-transistors 131 (TFTs) and other devices, to produce an active matrix. The Si devices 131 formed may then be covered or planarized by special layers 134. Color filters or color converting dyes 132 can be provided, in addition, if the OLEDs 135 emit white or blue light, respectively. The Si devices 131 include structured GaN-based anodes 133, for example, onto which the OLEDs 135 can be deposited. An advantage of this approach is that entrenched active matrix liquid crystal display (AMLCD) technology can be leveraged in combination with OLEDs to realize inexpensive, high performance AM displays over large areas Furthermore, clever design permits light to be emitted through the glass substrate 130 so no transparent top contact (cathode 136) is needed. The cathode 136 may be covered by a cap layer 137 for the purpose of encapsulation or lateral conduction We note that the substrate in Figure 10 could in principle still be a Si IC, as in Figure 9, but in this case a transparent top cathode electrode would have to be provided.

In the following some examples of the different organic materials which can be used are given Alq, also known as Alq3 may be replaced by other 8-hydroxyquιnolιne metal complexes such as Znq2, Beq2, Mgq2, ZnMq2, BeMq2, and AlPrq 3, for example. These materials can be used as ETL or emission layer. Other materials which can be used as ETL are: cyano-substituted polymers, didecyl sexithiophene (DPS6T), bis-trnsopropylsilyl sexithiophene (2D6T), Azomethin-zinc complexes, pyrazine (e.g. BNVP), strylanthracent derivatives (e.g BSA-1 , BSA-2), 1 ,2,4-trιazole derivative (TAZ).

The following materials are particularly well suited as emission layers: Anthracene, phyπdine derivatives (e.g. ATP), Azomethin-zinc complexes, pyrazine (e.g. BNVP), strylanthracent derivatives (e.g. BSA-1 , BSA-2), Coronene (also suited as dopant), Coumaπn (also suited as dopant), DCM compounds (DCM1 , DCM2; both also suited as dopants), distyryl arylene derivatives (DSA), alkyl-substituted distyrylbenzene derivatives (DSB), benzimidazole derivatives (e.g. NBI), naphthostyrylamine derivatives (e.g NSD), oxadiazole derivatives (e.g. OXD, OXD-1 , OXD-7), N,N,N',N'-tetrakιs(m-methylphenul)-1 ,3-dιamιnobenzene (PDA), Perylene, phenyl-substituted cyclopentadiene derivatives, 12-phthalopeπnone derivatives (PP), squarilium dye (Sq), 1 ,1 ,4,4-tetraphenyl-1 ,3-butadιene (TPBD), poly(2-methoxyl,5-(2'ethyl-hexoxy)- 1 ,4-phenylene-vιnylene

(MEH-PPV), sexithiophene (6T), poly(2,4-bιs(cholestanoxyl)-1 ,4-phenylene-vιnylene (BCHA-PPV),

Polythiophenes, poly(p-phenylenevιnylene) (PPV)

The below materials are suited as HTL Cu(ll) phtalocyanme (CuPc), NPB (important!), distyryl arylene derivatives (DSA), naphthalene, naphthostyrylamine derivatives (e.g NSD), Quinacπdone (QA; also suited as dopant) , poly(3-methylthιophene) family (P3MT), Perylene, polythiophene (PT), 3,4,9, 10-perylenetetracarboxylιc dianhydπde (PTCDA) (also suited as isolator), tetra phenyldiaminodiphenyl (TPD- 1 , TPD-2, or TAD), poly(2-methoxyl,5-(2'ethyl-hexoxy)-1 ,4-phenylene-vιnylene (MEH-PPV), poly(9-vιnylcarbazole) (PVK).

There are many other organic materials known as being good light emitters, and many more will be discovered. These materials can be used as well for making light emitting structures according to the present invention Examples of such materials are given in the publications cited in the introductory portion of the present description. The contents of these publications is herewith incorporated by means of reference

Monomeric devices are routinely made by vacuum evaporation This is compatible with PEMBD of GaN. Evaporation can be performed in a Bell jar type chamber with independently controlled resistive and electron-beam heating of sources. It can also be performed in a Molecular Beam

Deposition System incorporating multiple effusion cells and electron-beam evaporators In each case, GaN deposition can occur in the same chamber, a vacuum connected chamber, or even a separate chamber if some atmospheric contamination is tolerable

Oligomeπc and Polymeric organics can also be deposited by evaporation of their monomeric components with later polymerization via heating or plasma excitation at the substrate It is therefore possible to alloy these by co-evaporation also, and they are fully compatible with monomeric compounds

In general, polymer containing devices are made by dissolving the polymer in solvent and spreading it over the substrate, either by spin coating or a blade In this case, the inorganic must also be suspended or dissolved in solvent After coating the substrate, the solvent is dissolved by heating This method is not promising for the development of multilayer structures such as described herein since one would have to have a solvent heatinq ^a cycle for each layer, as well as a new solvent which does not redissolve any of the previously deposited layers More interesting to use is the possibility of making a polymer/inorganic transport layer on top of which monomeric layers are evaporated, possibly also incorporating alloys If the polymer is handled in an inert atmosphere prior to introduction to vacuum, sufficient cleanliness for device fabrication is maintained. In any case, the chemical inertness of GaN makes it highly tolerant of polymer OLED processing

Claims

1 Organic light emitting device having a) a substrate (60, 70; 1 10, 130), b) an anode (61 , 75; 1 13; 133), c) a cathode (64; 71 ; 1 1 1 ; 136), and d) an organic region (65; 76; 1 12; 135) in which electroluminescence takes place if a voltage is applied between said anode and cathode, said device being characterized in that said anode comprises a I ll - nitride, preferably Gallium Nitride (GaN).

2 The light emitting device of claim 1 , wherein said anode comprises an alloy of Gallium Nitride (GaN) with InN and/or AIGaN or the InAIN alloy.

3. The light emitting device of claim 1 or 2, wherein the sequence of layers is: substrate/cathode/organic region/anode.

4. The light emitting device of claim 3, wherein light generated by said electroluminescence is either emitted from said organic region through said anode, or from said organic region through said cathode and substrate.

5. The light emitting device of claim 1 or 2, wherein the sequence of layers is: substrate/anode/organic region/cathode.

6. The light emitting device of claim 5, wherein light generated by said electroluminescence is either emitted from said organic region through said cathode, or from said organic region through said anode and substrate.

7. The light emitting device of claim 1 or 2, wherein said organic region comprises a single organic layer or a stack of organic layers.

The light emitting device of claim 1 or 2, wherein said organic region (65, 76) comprises a hole transport layer (62, 74) being arranged such that said anode (61 , 75) is in direct contact with said hole transport layer

The light emitting device of claim 1 or 2, wherein said substrate (60, 130) is transparent or semitransparent.

10 The light emitting device of claim 1 or 2, wherein said substrate consists of Silicon or glass

1 1 The light emitting device of claim 1 or 2, wherein said substrate is flexible

12. The light emitting device of claim 1 or 2, wherein said substrate is a Silicon substrate (130) comprising integrated circuits (131 )

13. The light emitting device of claim 1 or 2, wherein said anode is either crystalline or amorphous

14 The light emitting device of claim 1 or 2, wherein said anode comprises an Indium-Tin-Oxide (ITO) layer separating said Gallium Nitride from said substrate.

15. The light emitting device of claim 1 or 2, wherein said anode is capped with an Indium-Tin-Oxide (ITO) layer separating said Gallium Nitride from said substrate for the purpose of augmenting the lateral conductivity of the light emitting device

16 The light emitting device of claim 1 or 2, wherein said organic region comprises either • a stack of more than one organic emission layers (EL), or • an organic compound doped with one or more impurities, organic or inorganic, chosen to dominate and enhance the electroluminescence, or

• a stack of more than one organic layer, in which the role of one or more of said organic layers is to electrically confine one or more carrier types to improve the emission of an adjacent organic layer

Organic light emitting array or display comprising more than one light emitting device pursuant to any of the preceding claims

The organic light emitting array or display of claim 17, wherein said substrate is a a Silicon substrate comprising devices and/or circuits and/or electrical connections

The organic light emitting array or display of claim 18, wherein said devices and/or circuits and/or electrical connections are designed for driving and controlling at least one of said light emitting devices

The organic light emitting array or display of claim 18, comprising color filters (132) providing for the emission of light at different wavelengths

The organic light emitting array or display of claim 1 8, wherein said light emitting devices are deposited cathode first onto said Silicon substrate, and wherein said anode is transparent or semitransparent such that light emitted by said light emitting devices is emitted into the half space above the Silicon substrate plane

22. The organic light emitting array or display of claim 18, wherein said light emitting devices are deposited anode first onto said Silicon substrate, and wherein

• said anode injects holes efficiently into the organic region of said light emitting devices, and

• the cathode of said light emitting devices is transparent or semitransparent, arranged such that light emitted by said light emitting devices is emitted into the half space above the Silicon substrate plane.

23. The organic light emitting array or display of claim 18, wherein said light emitting devices are formed such that light is emitted from said organic region through said anode and substrate into the half space below said substrate plane.

24. The organic light emitting array or display of claim 18, wherein the substrate and the entire array or display are flexible.

25. Method for making organic light emitting devices having a substrate, a cathode metallization an anode, and an organic region in which electroluminescence takes place if a voltage is applied between said anode and cathode, comprising the steps of: a) forming said anode such that it comprises Gallium Nitride (GaN), or an alloy of Gallium Nitride (GaN) with InN and/or AIGaN.

26. The method of claim 25, whereby said anode is formed using

• an electron cyclotron resonance plasma metalorganic chemical vapor deposition method, or

• magnetron sputtering, or

• laser ablation, or

• plasma enhanced molecular beam deposition.

27. The method of claim 23, whereby said anode is formed at low temperature below 350 degree centigrade, and preferably at a temperature in the range of the room temperature