Classifications

• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B33/00—Electroluminescent light sources
  • H05B33/02—Details
  • H05B33/06—Electrode terminals
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y10/00—Nanotechnology for information processing, storage or transmission, e.g. quantum computing or single electron logic
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B33/00—Electroluminescent light sources
  • H05B33/10—Apparatus or processes specially adapted to the manufacture of electroluminescent light sources
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B33/00—Electroluminescent light sources
  • H05B33/12—Light sources with substantially two-dimensional radiating surfaces
  • H05B33/26—Light sources with substantially two-dimensional radiating surfaces characterised by the composition or arrangement of the conductive material used as an electrode
• • 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
• • 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
  • H10K50/82—Cathodes
  • H10K50/828—Transparent cathodes, e.g. comprising thin metal layers
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
  • H10K71/00—Manufacture or treatment specially adapted for the organic devices covered by this subclass
  • H10K71/60—Forming conductive regions or layers, e.g. electrodes
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
  • H10K85/00—Organic materials used in the body or electrodes of devices covered by this subclass
  • H10K85/20—Carbon compounds, e.g. carbon nanotubes or fullerenes
  • H10K85/221—Carbon nanotubes
• • 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
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
  • H10K71/00—Manufacture or treatment specially adapted for the organic devices covered by this subclass
  • H10K71/20—Changing the shape of the active layer in the devices, e.g. patterning
  • H10K71/231—Changing the shape of the active layer in the devices, e.g. patterning by etching of existing layers
  • H10K71/233—Changing the shape of the active layer in the devices, e.g. patterning by etching of existing layers by photolithographic etching
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/24—Structurally defined web or sheet [e.g., overall dimension, etc.]
  • Y10T428/24479—Structurally defined web or sheet [e.g., overall dimension, etc.] including variation in thickness

Definitions

• the present invention relates to electronic devices that incorporate organic layers. More particularly, the present invention relates to structured cathodes for devices such as organic light emitting diodes (“OLEDs”), solar cells, and the like.
• OLEDs organic light emitting diodes
• Efficient film devices including organic photovoltaic and organic electroluminescent devices have been the subject of much advancement recently.
• organic light emitting small molecules and polymers have attracted increasing interest for manufacture of large area, low cost light emitting devices. They can be used as the light-producing elements in television screens, computer displays, advertising and information board applications, and the like. OLEDs can also be used in lighting devices.
• OLEDs use an electroluminescent conductive polymer or small molecule that emits light when subjected to an electric current. They can be used to make full-spectrum color displays and require a relatively small amount of power for the light produced. No vacuum is required, and the emissive materials can be deposited onto a substrate by a technique derived from commercial inkjet printing or vapor deposition processes. The diodes can be made on either rigid or flexible substrates.
• OLEDs can be printed onto any suitable substrate using inkjet printer or even screen printing technologies, which can result in a significantly lower cost than LCDs or plasma displays.
• printing OLEDs onto flexible substrates opens the door to new applications, such as roll-up displays and displays embedded in curtains, clothing, and the like.
• OLEDs enable a greater range of colors, brightness, and viewing angle than LCDs, because OLED pixels emit light directly. OLED pixel colors appear correct and unshifted, even as the viewing angle approaches 90 degrees from normal. LCDs use a backlight and cannot show true black, while an "off OLED element produces no light and consumes no power. Energy is wasted in LCDs because they require polarizers which filter out about half of the light emitted by the backlight. Additionally, color filters in color LCDs filter out two-thirds of the light.
• an OLED 100 has an emissive layer 110, a transport layer 120, an anode 130 and a cathode 140, all on a substrate 150.
• the layers 110, 120 are made of organic semiconducting small molecules or polymers.
• the cathode 140 injects electrons 145 into the emissive layer 110 and the anode 130 injects holes 135 into the transport layer 120.
• the electrons 145 and the holes 135 move toward each other and they recombine.
• the recombination produces an emission of radiation 160 whose frequency is typically in the visible, may also be in the infrared and ultraviolet regions.
• the electron orbitals in electroluminescent organic small molecules or polymers are analogous to the valence and conduction band edges in an inorganic semiconductor; states below the highest occupied molecular orbital (HOMO) are occupied and those above the lowest unoccupied molecular orbital (LUMO) are empty.
• the HOMO and LUMO are separated by an energy gap, normally in the optical energy range.
• ITO Indium tin oxide
• anode material is commonly used as the anode material. It is transparent to visible light and has a high work function, which promotes injection of holes into the organic layer.
• Metals such as aluminum and calcium are often used for the cathode as they have relatively low work functions, which promote injection of electrons into the organic light emitting layer.
• electroluminescent polymers are long chain hydrocarbon-based conjugated molecules with molecular weights of several hundred thousand atomic units. They can be applied to substrates by spin coating or printing to form amorphous films.
• Typical polymers used in OLED displays include derivatives of poly(p-phenylene vinylene) and poly(fluorene). Substitution of side chains onto the polymer backbone may determine the color of emitted light or the stability and solubility of the polymer for desired performance and ease of processing.
• alkali or alkali earth metals display the lowest work function (2 - 3 eV) of all elemental metals, thus suitable for cathode applications, their reactive nature and the associated fabrication cost have been a major challenge for the realization of low cost, high efficient OLED devices. Therefore, development of cathodes with sub-wavelength size structures such as carbon nanotubes and nanotube-alkali composite networks, and arrays of stable low work function alloy nanoclusters may improve device deficiency by lowering the driving voltage and increasing device stability and light extraction.
• Embodiments of the invention make significant improvements in the charge injection characteristics of the cathode of OLED devices using field emission structures, thus increasing electrical-optical energy conversion efficiency.
• the cathode structures disclosed herein are also substantially insensitive to oxygen and water, making processing and packaging of OLEDs easier and increasing the useful lifetime of the devices.
• Field emission is an effective source for electron ejection into OLED layers.
• electrons near the Fermi level of the cathode material can overcome the energy barrier to escape to the vacuum level.
• the basic physics of electron emission is well known. Low operating voltages can be used to extract electrons when the field emitter is made of a low work function material and/or has a very small tip radius.
• cathodes are developed based on structures having dimensions that are less than the wavelength of visible light ( ⁇ 0.5 ⁇ m).
• the cathode structure elements are smaller than the OLED emission wavelength, so essentially the cathode layer cannot absorb light.
• sub-wavelength structures can reduce heating effects at the metal-polymer interface due to their large surface area-to-volume ratio, therefore mitigating the damaging effects of heat in these devices and achieving greater device reliability.
• Carbon nanotubes are basically carbon molecule sheets organized into a cylindrical shape. "Carbon nanotube” as used herein is meant to include single wall and multi-wall carbon nanotubes unless specified otherwise. Any size or diameter, including metallic and semiconducting n-type and p-type nanotubes, non-cylindrical nanotubes, and mixtures of carbon and other atoms are all contemplated as useful for the embodiments of the invention.
• nanostructure means a structure with at least one dimension in the nanometer range.
• nanostructure is used more loosely to include structures with at least one dimension in the micron range.
• FIG. 2 is a schematic cross section drawing that shows an embodiment of the invention that uses a nanotube-based cathode.
• An OLED 200 has a cathode 240, a light-emitting organic layer 210, a transport layer 220, an anode 230, and a substrate 250.
• the anode 230 may be made of a transparent material, such as indium tin oxide (ITO) and the substrate 250 may be any known substrate such as plastic, glass, and the like.
• Light may be emitted in the direction of arrows 260 or in the opposite direction.
• ITO indium tin oxide
• the cathode 240 has a plurality of nanostructures 242 extending outwardly into the light- emitting organic layer 210.
• the nanostructures 242 may be any type of structure such as nanotubes, nonorods, or nanoclusters.
• the nanostructures 242 can be nanotubes grown out from a cathode substrate 240. Alternatively, the nanostructures 242 can be nanoclusters deposited onto the cathode substrate 240.
• Nanotubes are good field emitters because of their small tip radii, which can range from approximately one nanometer to as much as a micron. The smaller the tip radius the stronger the concentration of the electric field at the tip. A high electric field at the tip causes a high electron ejection rate, which results in very efficient ejection of electrons.
• the small tips and even distribution of the nantotubes 242 provide a balanced charge distribution in the device, reduce exciton quenching near the cathode 240, and allow for the use of lower voltages to achieve electron emission.
• carbon nanotubes are chemically stable, decreasing the environmental sensitivity of the cathode 240.
• the nanotubes used herein may be n-type or p-type for the purpose of balancing the charge transfer in the conductive small molecule or polymer matrix.
• One having ordinary skill in the art will appreciate how to optimize this balance.
• the nanotubes may be associated with the conductive polymer such that the polymer side chains wrap around the nanotube and are bonded to them by Van der Waal forces.
• doped nanotube-alkali composites can also improve electron injection into the light emitting polymers. It has been shown previously that the electronic properties of Li-doped carbon-46 molecules display metallic behavior with a very low work function, approximately 1.5 eV. Thus, carbon nanotubes doped with composites such as Li, Mg, or their alloys have very low work functions, which can provide further improvements in the electron injection efficiency.
• a template-assisted alignment of nanotube arrays is used to form a structured cathode for an OLED device.
• the template approach which is based on confined deposition of materials inside nanometer scale pores of self-organized anodic aluminum oxide or other materials, is becoming increasingly important for fabricating arrays of aligned nanostructures. This method enables easy integration of nanostructured electrodes with functional OLED devices.
• FIG. 3 is a flow diagram that describes steps for forming a structured electrode with an aligned nanotube array, according to an embodiment of the invention.
• an aluminum film is deposited onto a substrate.
• the substrate can be any of a variety of materials.
• the substrate with film is placed in an electrochemical cell in step 302.
• the cell can contain, for example, nitric acid.
• a voltage is applied in step 304.
• the voltage can be between approximately 10 and 500 volts.
• a cathode e.g., platinum
• nanoclusters are deposited into the pores in step 310.
• Precursor gas is used with either chemical vapor deposition or ultra fast laser ablation.
• transport of a carbon-alkali gas into the holes or pores may be through laser ablation of a graphite-alkali composite sample.
• unwanted residue is removed from the template using an acidic solution.
• mechanical polishing or ion milling of the resulting structure provides a thin film of aligned nanostructure arrays on a substrate, ready to be bonded to the emissive layer of an OLED.
• a metal catalyst e.g., Ni, Fe, Co
• step 320 catalyst on the surface of the alumina is removed by ion milling, leaving catalyst in the pores of the template on the substrate.
• step 324 an array of nanotubes is grown from the remaining catalyst in the pores. The cathode is ready to be bonded to the emissive layer of an OLED.
• Figure 4 shows a diagram of an alumina template on a substrate as described above.
• a template 405 with pores 415 is positioned over a substrate 425. Only small regions 435 of the substrate 425 are available for receiving catalyst and subsequent nucleation and growth of nanotubes.
• FIGS 5A, 5B, and 5C show performance data collected from OLED devices with carbon nanotube structured cathodes compared with data from standard devices with non- structured cathodes.
• the devices have the same structure except for the cathodes.
• the substrate 205 is glass
• the anode 230 is ITO
• the transport layer 220 is poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)
• the emissive layer 210 is poly[2-methoxy-5-(2'-ethyl-hexyloxy)-l,4-phenylene vinylene] (MEH-PPV).
• the cathode 240 is simply aluminum for the standard device and is aluminum with carbon nanotubes 242 extending into the emissive layer 210 for the device with the structured electrode.
• Figure 5 A is a plot of light intensity as a function of wavelength for the standard device 510 and for the device with the carbon nanotube cathode 520. There is no significant change in the emission spectrum with the use of the structured cathode with the nanotube layer.
• Figure 5B is a plot of current density as a function of voltage for the standard device 530 and the device with the carbon nanotube cathode 540. There is a significant increase in the current density for the structured cathode. This suggests enhanced electron injection from the structured nanotube cathode.
• Figure 5C is a plot of brightness as a function of voltage for the standard device 550 and the device with the carbon nanotube cathode 560. There is a significant increase in the brightness for the structured cathode.
• FIG. 6 is a schematic cross section drawing that shows an embodiment of the invention that uses a cathode with lithographically-defined microstructures or nanostructures.
• Cathodes with lithographically-defined microstructures can improve OLED device performance due to enhanced electron injection as discussed above for nanostructures.
• nanostructures is used in the following discussion, it should be understood that this is meant to include microstructures also.
• An OLED 600 has a cathode 640, a light-emitting organic layer 610, a transport layer 620, an anode 630, and a substrate 650.
• the anode 630 may be made of a transparent material, such as indium tin oxide (ITO) and the substrate 650 may be any known substrate such as plastic, glass, and the like. Light may be emitted in the direction of arrows 660 or in the opposite direction.
• the cathode 640 has a plurality of lithographically-defined nanostructures (or microstructures) 642 extending outwardly into the light-emitting organic layer 610.
• the cathode 640 has a plurality of lithographically-defined nanostructures 642 extending outwardly from the cathode 640 into the emissive layer 610.
• the nanostructures 642 are low work function metal or metallic alloy materials.
• Alkali metals have very low work functions and therefore can be expected to enhance electron emission if they are used as OLED cathodes. However, it has been found that alkali metal layers provide maximum enhancement of electron emission when they are less than about one monolayer thick. Unfortunately, alkali metals are physically and chemically unstable, and it is extremely difficult to fabricate and maintain such thin alkali metal layers.
• Lithographically-defined nanostructures 642 can be formed from metals such as aluminum (Al), or metallic alloys such as copper (Cu)-lithium (Li). Alloy cathodes overcome the drawbacks exhibited in conventional alkali film cathodes.
• Low work function (e.g., Li) layers on the surface of a host metal (e.g., Cu) can increase electron emission current density and lower threshold voltage. It has been shown that in Cu-Li alloys, a monolayer of Li can segregate to the surface of the alloy to form a thermodynamically stable, low-work function, alkali metal layer.
• a Li monolayer on the surface of the alloy leads to charge transfer between the metal conduction band and the Li adsorbate atom, giving rise to an effective work function even lower than that of the elemental alkali metal alone.
• FIG. 7 is a flow diagram that shown the steps for forming a structured electrode with lithographically-defined nanostructures.
• Photoresist is applied to a metal substrate in step 700.
• the metal substrate can be aluminum, or an alloy suitable as OLED cathode.
• the substrate can be any kind of solid material suitable for lithographic processing.
• step 702 using a standard photolithography process, a mask is used to pattern the photoresist and the photoresist is developed and cleaned.
• the remaining pattern on the metal substrate is etched to form trenches.
• the remaining photoresist and any etch residue is removed.
• the pattern is not limited to long lines that form trenches when etched. Any pattern that can result in nanostructures protruding from the substrate after etching can be used.
• the etched metal substrate is the lithographically-defined nanostructure cathode and can bonded to an emissive polymer layer in an OLED.
• step 720 the etched metal substrate is used as a stamp.
• step 722 the etched metal substrate is pressed against an emissive polymer layer in an OLED to form a pattern of ridges and trenches.
• step 724 a metal, such as aluminum, is deposited onto the stamped organic layer to fill the trenches and form a lithographically-defined nanostructure cathode.
• the photoresist can be applied to the emissive polymer layer of an OLED, and subsequent steps 702, 704, 706, can performed on the polymer layer, leaving the polymer layer with a pattern of ridges and trenches. Then a metal, such as aluminum, can be deposited onto the polymer layer to fill the trenches and form a lithographically-defined nanostructure cathode, as in step 724.
• a metal such as aluminum
• Figure 8 is an atomic force microscope image that shows a metal cathode or stamp surface that has been etched to form long microstructured ridges. The bright areas are ridges, and the dark areas are the trenches.
• Figures 9A, 9B, and 9C show performance data collected from OLED devices with lithographically-defined structured cathodes compared with data from standard devices with non-structured cathodes.
• the devices have the same structure except for the cathodes.
• the substrate 605 is glass
• the anode 630 is ITO
• the transport layer 660 is poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)
• the emissive layer 610 is poly[6-methoxy-5-(6'-ethyl-hexyloxy)-l,4-phenylene vinylene] (MEH-PPV).
• the cathode 640 is simply aluminum for the standard device and is aluminum with lithographically- defined aluminum nanostructures 644 extending into the emissive layer 610 for the device with the structured electrode.
• Figure 9A is a plot of light intensity as a function of wavelength for the standard device (solid line) 910 and for the device with the carbon nanotube cathode (dashed line) 960. There is no significant change in the emission spectrum with the use of the structured cathode with the lithographically-defined nanostructures .
• Figure 9B is a plot of current density as a function of voltage for the standard device 930 and the device with the lithographically-defined nanostructure cathode 940. There is a significant increase in the current density for the structured cathode. This suggests enhanced electron injection from the structured nanotube cathode.
• Figure 9C is a plot of brightness as a function of voltage for the standard device 950 and the device with the lithographically-defined nanostructure cathode 960. There is a significant increase in the brightness for the structured cathode.
• Figure 1 is a schematic drawing that shows the basic components of an OLED.
• Figure 2 is a schematic drawing that shows the basic components of an OLED with a nanotube structured cathode according to an embodiment of the invention.
• Figure 3 is a flow diagram that outlines the steps for forming a nanotube structured cathode, according to an embodiment of the invention.
• Figure 4A is a diagram showing an oxidized aluminum template with pores for growing a nanotube array.
• Figure 4B is a top-view SEM image showing a template for growing a nanotube array.
• Figures 5A, 5B, 5C show performance data from a standard OLED with a non-structured cathode and from a nanotube structured cathode.
• Figure 6 is a schematic drawing that shows the basic components of an OLED with a lithographically-defined nanostructure cathode according to an embodiment of the invention.
• Figure 7 is a flow diagram that outlines step for forming lithographically-defined nanostructure cathodes, according to embodiments of the invention.
• Figure 8 is an atomic force microscopy image of an etched aluminum substrate for a lithographically-defined nanostructure cathode.
• Figures 9A, 9B, 9C show performance data from a standard OLED with a non-structured cathode and from a lithographically-defined nanostructure cathode.
• OLED technology is used in commercial applications such as small screens for mobile phones and portable digital audio players (MP3 players), car radios, digital cameras and high- resolution displays for head-mounted displays.
• MP3 players portable digital audio players
• Such portable applications favor OLEDs because of their high light output, their readability in sunlight, and their low power drain.
• Prototypes have been made of flexible and rollable displays which use unique OLEDs characteristics.
• OLEDs can also be used as solid-state light sources and there is much interest in using them for general illumination. OLED efficiencies and long life already exceed those of incandescent light bulbs,

Landscapes

• Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
• Physics & Mathematics (AREA)
• Nanotechnology (AREA)
• Optics & Photonics (AREA)
• Materials Engineering (AREA)
• Manufacturing & Machinery (AREA)
• Mathematical Physics (AREA)
• Theoretical Computer Science (AREA)
• Crystallography & Structural Chemistry (AREA)
• Electroluminescent Light Sources (AREA)

Priority Applications (3)

Applications Claiming Priority (2)

Publications (2)

Family

ID=38656390

Family Applications (1)

Country Status (5)

Cited By (6)

Families Citing this family (8)

Family Cites Families (12)

• 2007
  • 2007-04-26 US US12/297,615 patent/US8324651B2/en not_active Expired - Fee Related
  • 2007-04-26 JP JP2009507958A patent/JP2009535767A/ja active Pending
  • 2007-04-26 KR KR1020087027656A patent/KR20090009240A/ko not_active Ceased
  • 2007-04-26 EP EP07761379A patent/EP2013926A4/en not_active Withdrawn
  • 2007-04-26 WO PCT/US2007/067548 patent/WO2007127870A2/en not_active Ceased

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events