Classifications

• • 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
  • H10K50/854—Arrangements for extracting light from the devices comprising scattering means
• • 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
  • H10K2102/331—Nanoparticles used in non-emissive layers, e.g. in packaging layer

Definitions

• OLEDs organic light emitting diodes
• the OLED includes a stack of thin organic layers disposed between two charged electrodes (anode and cathode).
• the organic layers may include a hole injection layer, a hole transport layer, an emissive layer, an electron transport layer, and an electron injection layer.
• the injected positive and negative charges recombine in the emissive layer to produce light.
• the light emitted by the emissive layer may transmit away from the emissive layer and out of the OLED to be viewed by a user and/or used by the optoelectronic device for various lighting or display applications.
• not all of the light emitted by the emissive layer may transmit out of the OLED.
• some light e.g., 20% in conventional OLEDs
• Such inefficiencies may translate into higher operating power requirements and/or shorter lifespan of the device as the application of higher voltages may be necessary to obtain a desired luminescence of the OLED.
• Some attempts at increasing the efficiency of the OLED include manipulating the layers of the OLED to increase the light emitted out of the device. For example, some techniques involve optimizing the thickness and possibly the refractive indexes of the OLED layers (commonly referred to as optimizing the OLED cavity), or incorporating scattering particles in the OLED layers to increase the scattering of otherwise trapped light out of the OLED.
• an organic light emitting diode (OLED) device includes a bottom electrode, a plurality of organic layers disposed over the bottom electrode, and a top electrode disposed over the plurality of organic layers.
• One or more of the plurality of organic layers comprises dispersed hollow objects configured to scatter light.
• a method of forming an organic light emitting diode (OLED) device includes dispersing hollow objects over a plane of the OLED device and depositing a layer of the OLED device over the dispersed hollow objects.
• OLED organic light emitting diode
• the OLED stack includes a transparent substrate, a conductive transparent material disposed over the transparent substrate, first non-emissive organic materials disposed over the conductive transparent material, electroluminescent materials disposed over the first non-emissive organic materials, second non-emissive organic materials disposed over the electroluminescent materials, hollow objects dispersed in the first non-emissive organic materials, the second non-emissive organic materials, or both, and a conductive reflective material disposed over the second non-emissive organic materials.
• FIG. 1 depicts a side view of an organic light emitting diode (OLED) stack, in accordance with one embodiment of the present disclosure
• FIG. 2 is a flow chart of a method of forming hollow objects and implementing the hollow objects in the OLED stack of FIG. 1, in accordance with one embodiment of the present disclosure;
• FIG. 3 is a schematic diagram representing the formation of hollow objects, in accordance with one embodiment of the present disclosure
• FIG. 4 is a side view of an organic layer of an OLED stack having a hollow object, in accordance with one embodiment of the present disclosure
• FIG. 5 is a side view of an organic layer of an OLED stack having relatively smaller hollow objects, in accordance with one embodiment of the present disclosure.
• FIG. 6 depicts a side view of an OLED stack having hollow objects, in accordance with one embodiment of the present disclosure.
• Organic materials are becoming increasingly utilized in circuit and lighting area technology due to the low cost and high performance offered by organic electronic devices and optoelectronic devices.
• typical OLED devices may not be operating at a desired level of efficiency, as a significant amount of light generated in the electroluminescent materials of the device may be trapped in the device.
• One or more embodiments of the present disclosure increases the amount of light that is transmitted out of an OLED device through the use of hollow objects configured to scatter otherwise trapped light out of the device.
• the refractive index of gas-filled hollow objects may be sufficiently different from the refractive index of typical layers in the OLED device which may improve the scattering of otherwise trapped light out of the device layers, thereby improving the efficiency of the OLED device.
• the OLED stack 10 may include a top electrode (i.e., cathode) 12 and a bottom electrode (i.e., anode) 14 disposed over a substrate 28, with organic layers 16 disposed between the cathode 12 and the anode 14.
• the organic layers 16 may include a hole injection layer 26 which may be disposed over the anode 14.
• a hole transport layer 24 may be disposed over the hole injection layer 26, and an emissive layer 22 may be disposed over the hole transport layer 24.
• An electron transport layer 20 may be disposed over the emissive layer 22, and an electron injection layer 18 may be disposed over the electron transport layer 20.
• the anode 14 may include a substantially transparent doped thin metal oxide film, such as indium tin oxide (ITO), tin oxide, indium oxide, zinc oxide, indium zinc oxide, zinc indium tin oxide, antimony oxide, and mixtures thereof.
• ITO indium tin oxide
• the thickness of the anode 14 may range from approximately 10 nm to 200 nm, though other thicknesses are also contemplated.
• Examples of materials suitable for the hole injection layer 26 disposed over the anode 14 may include proton-doped (i.e., "p-doped") conducting polymers, such as p-doped polythiophene or polyaniline, and p-doped organic semiconductors, such as tetrafluorotetracyanoquinodimethane (F4-TCQN), doped organic and polymeric semiconductors, and triarylamine-containing compounds and polymers.
• p-doped proton-doped conducting polymers
• p-doped polythiophene or polyaniline such as p-doped polythiophene or polyaniline
• organic semiconductors such as tetrafluorotetracyanoquinodimethane (F4-TCQN), doped organic and polymeric semiconductors, and triarylamine-containing compounds and polymers.
• the hole transport layer 24 disposed over the hole injection layer 26 may include, for example, triaryldiamines, tetraphenyldiamines, aromatic tertiary amines, hydrazone derivatives, carbazole derivatives, triazole derivatives, imidazole derivatives, oxadiazole derivatives including an amino group, polythiophenes, and like materials.
• materials suitable for a hole-blocking layer may include poly N-vinyl carbazole, and like materials.
• the emissive layer 22 may include any electroluminescent organic materials that emit radiation in the visible spectrum upon electrical stimulation.
• such materials may include electroluminescent organic materials which emit light in a determined wavelength range.
• the electroluminescent organic materials in the emissive layer 22 may include small molecules, oligomers, polymers, copolymers, or a mixture thereof.
• suitable electroluminescent organic materials 28 may include Tris(8- hydroxyquinolinato)aluminium (Alq3) and its derivatives; poly N-vinylcarbazole (PVK) and its derivatives; polyfluorene and its derivatives, such as polyalkylfluorene, for example poly-9,9-dihexylfluorene, polydioctylfluorene, or poly-9,9-bis-3,6- dioxaheptyl-fluorene-2,7-diyl; polypara-phenylene and its derivatives, such as poly-2- decyloxy-l,4-phenylene or poly-2,5-diheptyl-l,4-phenylene; polyp-phenylene vinylene and its derivatives, such as dialkoxy-substituted PPV and cyano-substituted PPV; polythiophene and its derivatives, such as poly-3-alkylthioph
• a suitable electroluminescent material is poly-9,9- dioctylfluorenyl-2,7-diyl end capped with N,N-bis4-methylphenyl-4-aniline. Mixtures of these polymers or copolymers based on one or more of these polymers may be used.
• Other suitable materials may include polysilanes, or linear polymers having a silicon-backbone substituted with an alkyl and/or aryl side groups. Polysilanes are quasi one-dimensional materials with delocalized sigma-conjugated electrons along polymer backbone chains.
• polysilanes examples include poly di-n- butylsilane, poly di-n-pentylsilane, poly di-n-hexylsilane, polymethyl phenylsilane, and poly bis p-butyl phenylsilane.
• the electron transport layer 20 disposed over the emissive layer 22 may include small molecules or low-to-intermediate molecular weight organic polymers, for example, organic polymers having weight average molecular weights of less than about 200,000 grams per mole as determined using polystyrene standards. Such polymers may include, for example, poly-3,4-ethylene dioxy thiophene (PDOT), polyaniline, poly-3,4-propylene dioxythiophene (PProDOT), polystyrene sulfonate (PSS), polyvinyl carbazole (PVK), and other like materials.
• the electron injection layer 18 disposed over the electron transport layer 20 may include, for example, sodium fluoride or potassium fluoride, or other like materials.
• the cathode 12 may include conductive, reflective materials such as aluminum, silver, indium, tin, zinc, other suitable metals, and combinations thereof. In some embodiments, the cathode 12 may also be relatively thin (e.g., 30 nm) and may be transparent. The cathode 12 may be deposited over the electron injection layer 18 by, for example, physical vapor deposition, chemical vapor deposition, sputtering or liquid coating. [0022] In some embodiments, the OLED stack 10 may also include different or additional non-emissive materials which may improve the performance or lifespan of the electroluminescent materials in the emissive layer 22.
• the stack 10 may also include layers such as a hole injection enhancement layer, an electron injection enhancement layer or any combinations thereof.
• the layers of the OLED stack 10 may be arranged in different orders or in different combinations, and additional layers may be disposed between the layers illustrated in FIG. 1.
• a voltage may be applied across the OLED stack 10.
• the voltage may charge the anode 14 to a positive charge and the cathode 12 to a negative charge, and electrons may flow through the stack 10 from the negatively charged cathode 12 to the positively charged anode 14. More specifically, electrons may be withdrawn from the organic materials adjacent to the anode 14 and injected to the organic materials adjacent to the cathode 12.
• the process of withdrawing electrons from the anode-side organic materials may also be referred to as hole injection and hole transport, and the process of injecting the electrons to the cathode-side organic materials may also be referred to as electron transport and electron injection.
• Electrostatic forces may combine the electrons and holes in the emissive layer 22 to form an excited bound state (i.e., an exciton) which upon de-excitation, emits radiation having frequencies in the visible region of the electromagnetic spectrum (e.g., visible light).
• the frequency of the emitted radiation and the colors and/or characteristics of visible light may vary in different embodiments depending on the properties of the particular materials used in the OLED stack 10.
• the visible light emitted at the emissive layer 22 may be transmitted (as indicated by the arrow 30) through the organic layers 24 and 26 and through the transparent anode 14 and substrate 28.
• the light which travels from the emissive layer 22 through the organic layers 20 and 18 may be reflected (as indicated by the arrow 32) by a reflective cathode 12 and transmitted out through the substrate 28.
• the visible light may then be used to illuminate a display or lighting application of the optoelectronic device.
• the light emitted by the emissive layer may not all pass through the substrate 28 to illuminate the device.
• emitted light may sometimes become "trapped" in, or may not pass through the organic layers 18, 20, 24, and/or 26.
• about 20% of light emitted by emissive materials may be trapped in the layers of the device. The trapped light decreases the efficiency of the device, as not all of the generated radiation is emitted through the substrate 28 to be utilized by the device or perceived by a user. Therefore, not all of the voltage applied through the stack 10 results in useful radiation.
• hollow objects may be dispersed in one or more layers of an OLED stack 10 to increase the scattering of light.
• the air- or gas-filled hollow objects used in some embodiments have cores characterized by a refractive index of approximately 1 , which is sufficiently different from the refractive index of typical OLED layers (-1.4 to 2.0), thereby affecting the path of light through the layers of the OLED stack 10. Because otherwise trapped light has a relatively longer path length within the OLED stack 10 relative to light which is emitted perpendicularly out of the stack 10, the otherwise trapped light may be scattered by such hollow objects. Such scattering may improve light extraction out of the OLED stack 10.
• the hollow objects may be surface treated to increase particle dispersion and compatibility with the matrix. This surface treatment may be hydrophilic or hydrophobic.
• FIG. 2 is a flow chart of a process 40 for making hollow objects and implementing the particles in an OLED stack 10
• FIG. 3 is a schematic diagram representing intermediate stages of the hollow objects formed in the process 40 of FIG. 2.
• the process 40 may begin with preparing (block 42) a template particle 44.
• a template particle 44 For a discussion of various techniques involved in forming the hollow objects, see U.S. Application No. 2007/0036705, incorporated herein by reference in its entirety.
• a process 40 for making hollow objects involves preparing a template particle 44 by polymerizing styrene under certain conditions. Such conditions may include heat treatment and/or addition of reactants in certain concentrations under an appropriate temperature and other processing conditions. For example, an emulsion, dispersion, or suspension polymerization reaction may be carried out at around 70°C.
• the template particles 44 formed may have a particle size between approximately 40 nm to 700 nm in diameter.
• the process 40 involves forming (block 46) a silica coating over the template particle 44 by treating the template particles 44 with a coupling agent followed by silicon-containing compounds or mixture of compounds under certain pH and temperature conditions, resulting in silica coated template particles 48.
• silica is one example of a suitable material for forming the hollow objects 54, it should be noted that suitable materials may include any material which does not increase non-radiative decay, create electrical shorts within the OLED stack 10, or decrease the operational life of organic elements used in the OLED stack 10.
• other suitable materials for making the hollow objects 52 may include co- and terpolymer systems made from vinylidene chloride and acrylonitrile, or from vinylidene chloride, acrylonitrile, and methylmethacrylate, styrene, acrylate, or combinations thereof.
• a suitable surfactant e.g., alkyl sulfates, alkyl sulfonates, linear alkyl arylsulfonates, or combinations thereof
• the pH and temperature may be used, and the pH and temperature may adjusted to approximately 9 to 11 and approximately 45°C to 90°C, respectively.
• the silica coated template particles 48 may have a substantially uniform silica-containing coating.
• the process 40 then involves eliminating (block 50) the core by isolating and heating the silica coated template particle 48 at sufficiently high temperatures to remove the core (e.g., the template particle 44), leaving a hollow object 52.
• the hollow objects 52 may be substantially filled with gases 60 (e.g., air or a combination of atmospheric gases).
• the silica coated template particles 48 may be isolated by centrifugation or filtration and heated to a temperature at which template depolymerization and volatilization occurs, such as at approximately 325°C to 525°C.
• the thickness of the hollow objects 52 may be sufficiently stable against mechanical stresses which may be applied against the objects 52 during the manufacture, formulation, and use of the OLED stack 10 or OLED device.
• the thickness of the wall of the hollow objects 52 may be approximately lOnm to lOOnm. Further, in some embodiments, the thickness of the wall of the hollow objects may be approximately lOnm to 50nm.
• the process 40 may involve forming (block 58) an OLED stack layer having dispersed hollow objects 52.
• forming (block 58) the OLED stack layer may involve dispersing and depositing (block 54) the hollow objects 52 over one or more layers of the OLED stack 10 and depositing (block 56) an OLED stack layer over the deposited hollow objects.
• the hollow objects 52 may be dispersed over the emissive layer 22, and the electron transport layer 20 may be deposited over the hollow objects 52.
• the hollow objects 52 may be deposited over the anode 14, and the hole injection layer 26 may be deposited over the hollow objects 52.
• a suitable medium may be employed to aid in the dispersion and deposition of the hollow objects 52 in the deposited OLED stack layer.
• forming (block 58) an OLED stack layer with dispersed hollow objects 52 may include dispersing the hollow objects 52 in a formulation having one or more layer components or precursors to layer components and then depositing the formulations having the dispersed hollow particles 52 onto one or more layers of the OLED stack 10.
• the layers with dispersed hollow objects 52 may be processed to be appropriately shaped or sized in the OLED stack 10. For example, spin casting, rollers, or blades may be used to form layers with dispersed hollow objects 52.
• approximately 10% or less of a plane through an OLED stack layer may include hollow objects 52.
• the hollow objects 52 may be denser in one or more layers of the OLED stack 10.
• the hollow objects 52 may cover approximately 10%- 30% of a plane of one or more OLED stack layers.
• the air-filled hollow objects 52 may present a substantially hydrophilic outer surface. Therefore, when implementing such hollow objects 52 into layers of the OLED stack 10 which are substantially hydrophobic (e.g., the hole transport layer 24 or the electron transport layer 20), the hollow objects 52 may be treated for improved suspension in the layers.
• the hollow objects 52 may be hydrophobized using a suitable hexamethyldisilazane (HMDZ) treatment.
• HMDZ hexamethyldisilazane
• the hollow objects are substantially hollow and non- conductive and may have a refractive index of approximately 1, or that which is characteristic of gases.
• the hollow objects may be spherical in shape (i.e., hollow spheres) or, in some embodiments, the hollow objects may be tubular in shape.
• spherical hollow objects may have a diameter of approximately 50 nm to 750 nm, and in some embodiments, tubular hollow objects may have a diameter of approximately 50 nm and may be 750 nm or greater in length.
• the hollow objects 52 may be small enough to not be perceivable by a viewer of the OLED stack 10, as particles which are too large may appear as dark spots under high optical magnification in the stack 10.
• the hollow objects 52 may also be large enough to scatter visible light.
• the determination of the optimal size and shape of the scattering hollow objects 52 in the OLED stack 10 depends on the thicknesses of the organic layers in the OLED stack 10, the choice of electrode materials, the fractional coverage of the hollow objects 52 in the layers in which they are dispersed, and the refractive index contrast of the hollow objects 52 with the layers of the OLED stack 10.
• the relative importance of these different parameters can be estimated using a combination of both Mie scattering theory (H.C. Van De Hulst, Light Scattering by Small Particles, (Dover, New York, 1981)) and radiative transport theory (J.J. Shiang, A.R. Duggal, Journal of Applied Physics, vol. 95, pg 2880, (2004) ) or calculated for certain model cases using finite-difference time-domain (FDTD) methods.
• Mie scattering theory H.C. Van De Hulst, Light Scattering by Small Particles, (Dover, New York, 1981)
• radiative transport theory J.J. Shiang, A.R
• the scattering cross- section increased in a manner approximately proportional to the geometric cross- section. Furthermore, the anisotropy, as defined by the expectation value of the cosine of the deviation angle, remained around 0.8.
• the cross-section and anisotropy results were used in a radiative transport model that calculated the fraction of light emerging from a 200nm thick slab of optical index 1.7 into air, assuming a Lambertian emitter input at the side opposite to the air. The side opposite to the air was assumed to be a mirror with a reflectivity of 0.73 and the particle loading assumed to be 5% volume fraction, evenly dispersed throughout the layer.
• the fraction of light that is scattered by a light beam making a single traverse through the slab is relatively small. Small loading fractions may reduce the degree to which the electrically inert spheres interfere with the electrical properties of the OLED.
• the total amount light extracted increases until about 200 nm diameter particles (i.e. comparable to the thickness of the layer).
• the particles may be approximately 200 nm in diameter, and the calculated light output may increase from 0.33 (10 nm spheres) to 0.36 (50 nm spheres) up to 0.50 (200 nm spheres).
• particle size of the hollow objects 52 ranges from about 0.01 microns in diameter to about 10 microns in diameter, particularly from about 0.01 microns in diameter to about 1 micron in diameter, more particularly from about 0.05 microns in diameter to about 0.2 microns in diameter.
• the hollow objects 52 may be spherical (i.e., hollow spheres) or tubular in shape. Tubular hollow objects 52 may have a diameter of approximately 50 nm and may be 750 nm or greater in length.
• the organic layers of the OLED stack 10 may be approximately 100 nm thick, though in some embodiments, the thickness of the layers in the OLED stack 10 may vary depending on the configuration of the device and/or the desired wavelength of light to be emitted.
• the layer 20 may form a dome over the hollow object 52, such as the spherical hollow object illustrated in FIG. 4. Layers which are deposited over the dome formed over the hollow object 52 may likewise be domed, which may significantly enhance light scattering, especially for a reflective (metallic) top electrode layer.
• the hollow objects 52 may be suspended in the layer 20. While the hollow objects 52 are illustrated in FIGS. 4 and 5 as being dispersed in the electron transport layer 20, in accordance with the present embodiments, the hollow objects 52 may be implemented in any layer or multiple layers of the OLED stack 10.
• FIG. 6 is a side view illustration representing an OLED stack 10 having hollow objects 52 dispersed in multiple different layers of the stack 10.
• the hollow objects 52 may be dispersed in the electron transport layer 20, the hole transport layer 24, the hole injection layer 26, and/or the substrate 28.
• the hollow objects 52 may scatter light emitted by the emissive layer 22, as represented by the arrows 62, thereby improving the percentage of light emitted by the emissive layer 20 which transmits out of the substrate 28.
• the luminescence of an OLED stack 10 implementing hollow objects for light scattering may improve by approximately 20% in comparison to the luminescence of a comparable stack without hollow objects, using the same activating voltage.
• the OLED stack 10 may include various additional layers, and the hollow objects 52 may be dispersed in one or more of the illustrated layers or other additional layers of an OLED device.
• the OLED stack 10 may include a functional layer, such as an encapsulation layer, disposed over the cathode 12.
• the OLED stack 10 may include a planarizing layer or smoothing layer disposed over the substrate 28, beneath the anode 14.
• the hollow objects 52 may be dispersed in the functional layer or smoothing layer, and/or other layers of the OLED stack 10 to increase light scattering out of the OLED stack 10.

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