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• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
  • H10K50/00—Organic light-emitting devices
  • H10K50/80—Constructional details
  • H10K50/805—Electrodes
  • H10K50/81—Anodes
  • H10K50/818—Reflective anodes, e.g. ITO combined with thick metallic layers
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
  • H10K2102/00—Constructional details relating to the organic devices covered by this subclass
  • H10K2102/301—Details of OLEDs
  • H10K2102/302—Details of OLEDs of OLED structures
  • H10K2102/3023—Direction of light emission
  • H10K2102/3026—Top emission
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
  • H10K50/00—Organic light-emitting devices
  • H10K50/10—OLEDs or polymer light-emitting diodes [PLED]
  • H10K50/17—Carrier injection layers
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
  • H10K50/00—Organic light-emitting devices
  • H10K50/80—Constructional details
  • H10K50/85—Arrangements for extracting light from the devices
  • H10K50/856—Arrangements for extracting light from the devices comprising reflective means
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
  • H10K50/00—Organic light-emitting devices
  • H10K50/80—Constructional details
  • H10K50/86—Arrangements for improving contrast, e.g. preventing reflection of ambient light

Definitions

• the present invention pertains to organic electroluminescent displays and methods for making the same.
• Organic electroluminescence (EL) has been studied extensively because of its possible applications in discrete light emitting diodes (LED), arrays and displays. Organic materials can potentially replace semiconductors in many LED applications and enable wholly new applications. The ease of organic LED (OLED) fabrication and the continuing development of improved organic materials promise novel and inexpensive OLED display possibilities.
• Tang used vacuum deposition of molecular compounds to form OLEDs with two organic layers.
• poly(p-phenylenevinylene) to form a single-organic-layer OLED.
• the simplest possible OLED structure depicted in Figure 1A, consists of an organic emission layer 10 sandwiched between cathode 11 and anode 12 electrodes which inject electrons (e " ) and holes (h + ), respectively, which meet in the emission layer 10 and recombine producing light. It has been shown (D. D. C. Bradley, Synthetic Metals, Vol. 54, 1993, pp. 401-405, J. Peng et al., Japanese Journal of Applied Physics, Vol. 35, No. 3A, 1996, pp. L317-L319, and I. D. Parker, Journal of Applied Physics, Vol. 75, No. 3, 1994, pp.
• Balanced charge injection is also important.
• an excellent anode is of limited use if the cathode has a large energy barrier to electron injection.
• Figure 2 illustrates a device with a large electron barrier 16 such that few electrons are injected, leaving the holes no option but to recombine non-radiatively in or near the cathode 15.
• the anode and cathode materials should be evenly matched to their respective MOs to provide balanced charge injection and optimized OLED efficiency.
• Electrodes With multilayer device architectures now well understood and widely used, a remaining performance limitation of OLEDs is the electrodes.
• the main figure of merit for electrode materials is the position of the electrode Fermi energy relative to the relevant organic MO.
• ITO is by no means an ideal anode, however. ITO is responsible for device degradation as a result of In diffusion into the OLED eventually causing short circuits as identified by G. Sauer et al., Fresenius J. Anal. Chem., pp. 642-646, Vol. 353 (1995). ITO is polycrystalline and its abundance of grain boundaries provides ample pathways for contaminant diffusion into the OLED. Finally, ITO is a reservoir of oxygen which is known to have a detrimental effect on many organic materials (see J.C. Scott, J.H. Kaufman, P.J. Brock, R. DiPietro, J. Salem, and J.A. Goita, J. Appl. Phys., Vol. 79, p. 2745, 1996). Despite all of these problems, ITO anodes are favored because no better transparent electrode material is known and ITO provides adequate stability for many applications.
• a TC OLED thin, semi-transparent low work function metal layer, e.g. Ca or MgAg, followed by ITO or another transparent, conducting material or materials, e.g. as reported in Bulovic et al., Nature, Vol. 380, No. 10, 1996 p. 29, or in the co-pending PCT patent application PCT/IB96/00557, published on 11 December 1997 (publication number WO97/47050).
• a highly reflective anode which can direct more light out through the TC is desired. Consequently, the low reflectivity of ITO is a disadvantage in TC OLEDs.
• a TC OLED could benefit from a non-reflective, highly absorbing anode. Again the optical characteristics of ITO are a disadvantage.
• High work function metals could form highly reflective anodes for TC OLEDs.
• Some of these metals, e.g. Au have a larger work function than ITO (5.2 eV vs. 4.7 eV), but lifetime may be compromised because of high diffusivity in organic materials. Like In from ITO, only worse, Au diffuses easily through many organic materials and can eventually short circuit the device. Efforts have been made to fabricate OLEDs on Si substrates (Parker and Kim, Applied Physics Letters, Vol. 64, No. 14, 1994, pp. 1774-1776). Si, due to its small bandgap and moderate work function, has a large barrier for both electron and hole injection into organic MOs, and therefore performs poorly as an electrode.
• electrode materials must be improved to realize OLEDs, and displays based thereon, with superior reliability and efficiency, and to enable novel architectures, such as devices emitting through a TC.
• an improved anode compatible with Si IC technology is required for optimized TC OLED architectures.
• an OLED having a cathode, an anode, and an organic region sandwiched in between, said anode being composed of
• anode being arranged such that said anode modification layer is in contact with said organic region and light is extracted through said cathode.
• metal layer in connection with the present invention.
• metal layer any kind of metal is suited as metal layer in connection with the present invention. Examples are Al, Cu, Mo, Ti, Pt, Ir, Ni, Au, Ag, and any alloy thereof, or any metal stack such as Pt on Al and the like.
• the inventive approach is specifically designed for the fabrication of OLEDs on top of Si, preferably Si crystalline wafers incorporating pre-processed integrated display circuitry (herein referred to as Si IC).
• Si IC pre-processed integrated display circuitry
• the present invention is designed to modify the existing Si device metallization into a stable OLED anode having good hole injection properties.
• the metal layer in the present invention is generally the final metallization layer of the Si IC process, which consistent with present Si technology is normally Al, Cu or an alloy thereof. Neither Al, Cu or Al:Cu alloys perform well as OLED anodes, but they do provide excellent visible spectrum reflectivity which increases the amount of light extracted through a TC.
• the Si IC metallization surface can vary widely in terms of oxide thickness, roughness and surface contaminants depending on numerous factors, including the fabrication process, the time between IC fabrication and OLED deposition, and the environment in which the Si IC was stored and shipped.
• the Si metallization anode properties must be improved and effects arising from variations of the initial state of the metal surface must be eliminated.
• the inventive approach is also suited for use with pixel and drive circuitry comprising polysilicon or amorphous silicon devices.
• the anode modification layer in the present invention is mainly selected for its high work function which provides efficient hole injection into OLEDs.
• the anode modification layer must form a stable interface with the adjacent organic layer being part of the so-called organic region (e.g. the organic HTL) to insure consistent OLED performance over an extended time period.
• the anode modification layer can be conductive or insulating, but it should be sufficiently thin that it contributes negligibly both to the OLED series resistance and optical absorption losses. Oxides are well suited as anode modification layers.
• the thickness of the anode modification layer is preferably between 0.5 nm and 10 nm.
• the barrier layer or layers in the present invention isolates the anode modification layer from the metal layer by forming a physical and chemical barrier, while permitting charge to pass freely through its interfaces with the metal layer and anode modification layer.
• the barrier layer(s) provides a consistent and reproduceable surface for the deposition or formation of the anode modification layer regardless of the metal layer composition or initial state of its surface.
• the barrier layer(s) can be conductive or insulating, but it (they) should be sufficiently thin that it (they) contributes negligibly to the OLED series resistance. Alternatively, the barrier layer(s) can be highly reflective which avoids absorption losses.
• the thickness of the barrier layer is preferably between 5 nm-100 nm. Well suited are barrier layers comprising TiN or TiNC, for example.
• a single or multilayer OLED structure having a TC fabricated on a Si substrate incorporates a multilayer anode structure comprising a metal layer, an anode modification layer, and an intermediate barrier layer(s), such that the anode is stable and efficient at hole injection.
• the barrier layer(s) provides between the metal layer and anode modification layer, and the stability of the anode modification layer interface with the adjacent organic HTL.
• FIG. 1 A shows the band structure of a known OLED having an emission layer and two electrodes (Prior Art).
• FIG. IB shows the band structure of 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 (Prior Art).
• FIG. 2 shows the band structure of 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 (Prior Art).
• FIG. 3 shows the band structure of another known OLED having an electron transport layer and hole transport layer (Prior Art).
• FIG. 4 is a schematic cross section of a first embodiment of the present invention.
• FIG. 5 is a schematic cross section of a second embodiment of the present invention.
• FIG. 6 is a schematic cross section of a third embodiment of the present invention. 11
• a first embodiment is depicted in Figure 4.
• a TC OLED is shown which is formed on a substrate 45. Since in the present configuration the electroluminescent light 52 is emitted through the top electrode (cathode 51), almost any kind of substrate 45 can be used. Examples are Si, glass, quartz, stainless steel, and various plastics.
• the inventive anode comprising a metal layer 46, a barrier layer 47, and an anode modification layer 48 is situated on said substrate 45.
• Any kind of metal is suited as metal layer 46. Examples are Al, Cu, Mo, Ti, Pt, Ir, Ni, Au, Ag, and any alloy thereof, or any metal stack such as Pt on Al and the like. Depending on the embodiment, particularly well suited are metals which provide visible spectrum reflectivity.
• the barrier layer 47 isolates the anode modification layer 48 from the metal layer 46 by forming a physical and chemical barrier, while permitting charge to pass freely through its interfaces with the metal layer 46 and anode modification layer 48.
• the inventive anode might comprise one or more barrier layers.
• the barrier layer(s) 47 provides a consistent and reproduceable surface for the formation or deposition of the anode modification layer 48 regardless of the metal layer composition or initial state of its surface.
• the barrier layer(s) 47 can be conductive or insulating, but it (they) should be sufficiently thin that it (they) contributes negligibly to the OLED series resistance. Alternatively, the barrier layer (s) 47 can 12
• the thickness of the barrier layer is preferably between 5 nm-100 nm.
• the anode modification layer 48 is mainly selected for its high work function which provides efficient hole injection into the organic region of the OLED.
• the anode modification layer 48 must form a stable interface with the adjacent organic emission layer (EML) 49 to insure . consistent OLED performance over an extended time period.
• EML organic emission layer
• the anode modification layer 48 can be conductive or insulating, but it must be sufficiently thin that it contributes negligibly to both the OLED series resistance and the optical absorption losses.
• the thickness of the anode modification layer is preferably between 0.5 nm and 10 nm.
• a TC 51 is situated on the EML 49.
• the electroluminescence takes place within the EML 49.
• part of the light is emitted directly through the EML 49 and the TC 51 into the half space above the OLED.
• Another part of the light travels towards the inventive anode structure.
• the anode structure reflects the light such that it is also emitted into the half space above the OLED.
• Table 1 Exemplary details of the first embodiment Layer No. Material Thickness present example
• Substrate 45 Quartz 0.05 - 5 mm 800 ⁇ m
• Metal Layer 46 Ti/Al 0.01-0.7 ⁇ m / 0.05-3 ⁇ m 2 nm
• Anode modification Layer 48 ITO 0.003 - 2 ⁇ m 7 nm
• Emission Layer 49 PPV 50 - 500 nm 200 nm
• Transparent Cathode 51 Li:Al alloy 50 - 1000 nm 120 nm
• a second embodiment of the inventive anode for TC OLEDs fabricated on Si substrates is depicted in Figure 5.
• the Si IC 25 in Figure 5 comprises an Al top metal contact pad 26 which also serves as the metal layer in the inventive anode.
• an InN barrier layer 27 is deposited directly onto the substrate such that it overlaps at least the contact pad 26.
• the sample is then oxidized in an oxygen plasma, or equivalently in an air, steam, ozone or other oxidizing environment to prepare an InNO * surface anode modification layer 28 capable of low voltage hole injection into the organic HTL 29.
• Electrons injected into the ETL 30 from the TC 31 recombine with the holes in the organic region producing EL 32 which is extracted through the TC 31.
• the organic region in the present embodiment comprises an HTL 29 and an ETL 30. It is to be noted that the organic region in any case at least comprises one organic layer (see first embodiment, for example).
• the Si IC 25 might comprise integrated circuitry which is not illustrated in Figure 5, for sake of simplicity. Instead of InN other group III nitrides might be used, for example.
• InN is an excellent barrier layer material because it is a degenerate semiconductor which has both excellent transparency and is conductive, but not so conductive that lateral conduction through the InN barrier layer 27 between adjacent Al metal pads 26 on the Si IC 25 causes electrical crosstalk.
• InN having these properties can be deposited at or near room temperature as described in Beierlein et al., Materials Research Society Internet Journal of Nitride Semiconductor Research, Vol. 2, Paper 29.
• InN is also a convenient choice because its surface work function can be increased by oxidation thus forming an InNO x anode modification layer 28 directly on the InN barrier layer. Equivalently, the InNO x anode modification layer 28 could be directly deposited onto the InN.
• oxide-based anode modification layers Several methods of deposition of oxide-based anode modification layers are listed below:
• PECVD sputter deposition or reactive (e.g. in an oxygen environment) sputter deposition; thermal evaporation; electron-beam evaporation; • oxygen plasma (plasma-assisted oxidation); thermal annealing in an oxidizing environment; 14
• anode modification layer 28 could also be substituted, e.g. ITO, ZnO,
• a completely different metal layer 26 could also be substituted.
• the degeneracy of the InN semiconductor insures that charge can pass easily from the metal layer 26 into the InN barrier layer 27, regardless of the composition or initial state of the metal layer 26 surface. For the same reason, charge can also traverse the InN barrier layer 27 and into the anode modification layer 28 without significant series resistance.
• the strength of the highly polar In-N bond insures that InN 27 acts as an excellent chemical and physical barrier to corrosion or diffusion between the metal layer 26 and the anode modification layer 28.
• the device depicted in Figure 5 benefits from the high reflectivity of the Al metal layer 26, and the low visible spectrum absorption of the InN barrier layer 27 and the InNO x anode modification layer 28, which permits much of the EL 32 emitted towards the substrate to be reflected back through the TC 31.
• Devices having the anode structure depicted in Figure 5 exhibit higher quantum and power efficiencies than comparable structures having conventional ITO anodes as a result of more balanced charge injection between the Al InN/InNO ⁇ anode and the TC.
• Metal Layer 26 Aluminum 0.05 - 2 ⁇ m 500 nm
• a third embodiment of the inventive anode for TC OLEDs fabricated on Si substrates is depicted in Figure 6. From the substrate 33 up, listed in the order of deposition, is a Si
• the Si IC 33 in Figure 6 comprises an Al:Cu alloy top metal contact 34 which serves as the metal layer in the inventive anode.
• a two layer Ni 35/NiO x 36 barrier layer is deposited in sequence directly onto those parts of the substrate 33 which are covered by the metal contact layer 34, or alternatively, the Ni 35 barrier layer is deposited, and its surface is subsequently oxidized to form the NiO x 36 barrier layer.
• the inventive anode is completed by the deposition of the V 2 O 5 anode modification layer 37 capable of injecting holes into the HTL 38. Electrons injected into the ETL 40 from the TC 41 recombine in the organic emission layer 39 producing EL 42 which is extracted through the TC 41.
• the circuitry in the Si substrate is not shown for simplicity.
• barrier layers 35, 36 can be inserted between the metal layer 34 and the anode modification layer 37 of the inventive anode provided that they do not reduce device efficiency through excessive series resistance at their interfaces or in their bulk.
• the Ni 35/NiO x 36 barrier layer structure of the present embodiment relies on the high reflectivity of the Ni 35 metal and the transparency and thinness of the insulating NiO x 36 layer to insure good device efficiency. 16
• the third embodiment without the Ni and NiO x layers is unstable due to a chemical reaction between the Al:Cu alloy 34 and the anode modification layer 37.
• the oxidation of the Ni 35 surface, or alternatively the deposition of an additional barrier layer further chemically isolates the Ni metal from the anode modification layer 37.
• the Ni 35 barrier layer is highly conductive, it must be very thin, or patterned (as shown in Figure 6) to avoid lateral conduction between adjacent IC metallizations 34 (not shown in Figure 6).
• Devices having the inventive metal/Ni/NiO x /V 2 O 5 anode structure depicted in Figure 6 exhibit steeper current/voltage characteristics than conventional ITO anodes and similar power efficiencies.
• Transparent Cathode 41 Ca/InGaN/ITO 1-20 nm / 10-100 nm 10 nm / 80 nm / / 10-lOOOnm 150nm
• the Si substrate might comprise integrated circuitry which drives the pixels of the OLEDs formed thereon.
• the inventive anode might be connected to the metal contact of an active matrix element formed in the Si IC substrate. If the circuitry is patterned appropriately, individual pixels or groups of pixels can be turned on and off. 17
• Arrays and displays can be realized with high quantum and power efficiencies, lower threshold voltages, and/or steep current/voltage characteristics.
• the inventive anode is compatible with many known approaches.
• the substrate can be fabricated to contain the active Si devices, such as for example an active matrix, drivers, memory and so forth.
• active Si devices such as for example an active matrix, drivers, memory and so forth.
• Such an OLED on Si structure can be a very inexpensive small area organic display with high resolution and performance.
• An OLED or OLED array may either be grown directly on such a Si substrate carrying Si devices, or it may be fabricated separately and assembled in a flip-chip fashion onto the Si circuitry later.
• the Si metallization is typically Al or an Al alloy which are poor OLED anode or cathode materials.
• the inventive anode permits a stable, low voltage hole contact to be formed on top of the standard Si process metallizations.
• Electron transport/Emitting materials
• electron transporting materials are electron-deficient nitrogen-containing systems, for example oxadiazoles like PBD (and many derivatives), and triazoles, for example TAZ (l,2,4-triazole). These functional groups can also be incorporated in polymers, starburst and spiro compounds. Further classes are materials containing pyridine, pyrimidine, pyrazine and pyridazine functionalities.
• DPS6T didecyl sexithiophene
• 2D6T bis-triisopropylsilyl sexithiophene
• azomethin-zinc complexes pyrazine (e.g. BNVP), styrylanthracene derivatives (e.g. BSA-1, BSA-2), non-planar distyrylaiylene derivatives, for example DPVBi (see C. Hosokawa and T.
• cyano-substituted polymers such as cyano-PPV (PPV means poly(p-phenylenevinylene)) and cyano-PPV derivatives.
• Anthracene pyridine derivatives (e.g. ATP), Azomethin-zinc complexes, pyrazine (e.g. BNVP), styrylanthracene derivatives (e.g. BSA-1, BSA-2), Coronene, Coumarin, DCM compounds (DCM1, DCM2), distyryl arylene derivatives (DSA), alkyl-substituted distyrylbenzene derivatives (DSB), benzimidazole derivatives (e.g. NBI), naphthostyrylamine derivatives (e.g. NSD), oxadiazole derivatives (e.g.
• MEH-PPV poly(2-methoxy)-5-(2'-ethylhexoxy)- 1 ,4-phenylene-vinylene), poly(2,4-bis(cholestanoxyl)-l,4-phenylene-vinylene) (BCHA-PPV), and segmented PPVs (see for example: E. Staring in International Symposium on Inorganic and Organic Electroluminescence, 1994, Hamamatsu, 48, and T. Oshino et al. in Sumitomo Chemicals, 1995 monthly report).
• the following materials are suited as hole injection layers and hole transport layers.
• Materials containing aromatic amino groups like tetraphenyldiaminodiphenyl (TPD- 1 , TPD-2, or TAD) and NPB (see C. Tang, SID Meeting San Diego, 1996, and C. Adachi et al. Applied Physics Letters, Vol. 66, p. 2679, 1995), TPA, NIPC, TPM, DEH (for the abbreviations see for example: P. Borsenberger and D.S. Weiss, Organic Photoreceptors for Imaging Systems, Marcel Dekker, 1993).
• aromatic amino groups can also be incorporated in polymers, starburst (for example: TCTA, m-MTDATA, see Y. Kuwabara et al., Advanced Materials, 6, p. 677, 1994, Y. Shirota et al., Applied Physics Letters, Vol. 65, p. 807, 1994) and spiro compounds.
• NSD quinacridone
• P3MT poly(3-methylthiophene)
• PT polythiophene
• PTCDA 3,4,9, 10-perylenetetracarboxylic dianhydride
• PPV poly(9-vinylcarbazole)
• HPT discotic liquid crystal materials
• blend (i.e. guest-host) systems containing active groups in a polymeric binder are also possible.
• the concepts employed in the design of organic materials for OLED applications are to a large extent derived from the extensive existing experience in organic photoreceptors. A brief overview of some organic materials used in the fabrication of organic photoreceptors is found in the above mentioned publication of P. Brosenberger and D.S. Weiss, and in Teltech, Technology Dossier Service, Organic Electroluminescence (1995), as well as in the primary literature.
• OLEDs have been demonstrated using polymeric, oligomeric and small organic molecules.
• the devices formed from each type of molecule are similar in function, although the deposition of the layers varies widely.
• the present invention is equally valid in all forms described above for organic light emitting devices based on polymers, oligomers, or small molecules.
• 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 sputter sources. Oligomeric 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 copolymerize or create mixtures by co-evaporation. 21
• polymer containing devices are made by dissolving the polymer in a solvent and spreading it over the substrate either by spin coating or the doctor blade technique. After coating the substrate, the solvent is removed by evaporation or otherwise. This method allows the fabrication of well defined multilayer organic stacks, provided that the respective solvents for each subsequent layer do not dissolve previously deposited layers.
• hybrid devices containing both polymeric and evaporated small organic molecules are possible.
• the polymer film is generally deposited first, since evaporated small molecule layers often cannot withstand much solvent processing.

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