Patent Publication Number: US-2006003487-A1

Title: Low power consumption OLED material for display applications

Description:
BACKGROUND OF THE INVENTION  
      Liquid crystal displays (LCDs) are commonly used in devices such as flat panel displays for laptop computers, personal digital assistants, cellular phones, and the like. Displays made with LCDs frequently use a cold cathode fluorescent lamp (CCFL) or similar devices as a backlight for the LCD display to show forth an optical image to the viewer. CCFLs and similar devices are fragile, relatively inefficient materials that require an inverter and consume large quantities of power, up to 35 percent of the power within a notebook computer system. The use of CCFLs, which are made of glass or other rigid materials, renders the display module fragile, difficult to manufacture and maintain, and expensive to repair when broken. The specifications of these materials also render the display itself bulky and add to the weight of the system which incorporates the display. Because the displays are typically used in portable devices, users desire devices which are more ruggedized with lighter weight.  
      In an effort to reduce the weight of the display and increase its durability, some manufacturers use organic light emitting diode (OLED) materials as a backlight source in mobile devices. OLEDs are thin film materials which emit light when excited by electric current. Since OLEDs emit light of different colors, they could be used to make displays. Displays made from OLED materials, therefore, do not need additional backlights, thus eliminating the need for the fragile glass CCFL and hence the bulky form factor of the display module. OLEDs are usually lightweight and can operate efficiently at relatively low voltages, thus consuming less power from the system. The versatility of the light emitting OLED materials has led some manufacturers to believe it would be desirable to substitute them for LCDs in mobile display devices in the near future.  
      Although OLEDs can generate light with high efficiency, more than half of the light can be trapped within the device and render the light as useless for the device. Because the light emission from the OLED has no preference in the emitting direction, light is therefore emitted equally in all directions so that some of the light is emitted forward to the viewer, some is emitted to the back of the device and is either reflected forward to the viewer or being absorbed by the ambient, and some of the light is emitted laterally and trapped and absorbed by the various layers comprising the device. In general, up to 80% of the light generated from the OLED materials may be lost within the system and may never reach the viewer.  
      There is a need therefore for an improved organic light emitting diode display structure that avoids the problems noted above and improves the efficiency of power used by the display, especially in portable devices. The present invention is directed to a new way of improving power efficiency of organic emitting diode displays through modification of the device fabrication with respect to the OLED material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows an OLED structure.  
       FIG. 2  shows an OLED structure having a grooved substrate.  
       FIG. 3  shows an OLED structure integrated with a display device. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Some embodiments of the present invention are directed to OLED structures useful in display devices and processes for making such OLED structures. The OLED structures may comprise polar compounds which possess certain dielectric anisotropy and can be aligned with respect to either one or more substrates of the display cell. When the polar compounds are exposed to an applied voltage or electric field, the polar compounds will respond and the molecule aligns in certain orientation with respect to the direction of the electric field or voltage. Such orientation can be calibrated in a manner that may result in the light emitted from the OLED material radiating in a certain dominant direction.  
      Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.  
      An exemplary embodiment of the present invention includes an OLED material comprising polar functional groups and entities as their molecular components which, when subjected to an electric field, orient in a dominant direction as dictated by the electric field, thus orienting the emitted light in a single particular direction.  
      Turning now to the figures, in which like numerals refer to like elements through the several figures,  FIG. 1  (not to scale) illustrates the structure of an OLED material  10  in accordance with some embodiments of the present invention. In an OLED structure  10 , an anode coated conductive layer  20  may be integrated on a substrate  30 . A hole-transport layer  40  may be stacked on the coating of the anode. A layer of polar light-emitting material  50  may be disposed on the hole-transport layer  40 . An electron transport layer  60  may be disposed on the light-emitting layer  50 . Finally, a substrate  90  may support a cathode  70  comprising a conductive film. A cathode  70  may additionally be disposed on the electron transport layer  60 . The anode  20  and cathode  70  may be connected to a power source  80 . When the power source is activated, holes are injected from anode  20  into hole transport layer  40 , the holes combine in a light emitting layer  50  with electrons that travel from cathode  70  and generate visible light.  
      The substrates  30  and  90  may be made from any material capable of supporting the conductive coating of the anode  20  and cathode  70  and may be flexible or rigid. Examples include, but are not limited to, plastic, glass, quartz, plastic films, metals, ceramics, polymers or the like. Non-limiting examples of flexible plastic film and plastic include a film or sheet of polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyethersulfon (PES), polyetherimide, polyetheretherketone, polyphenylene sulfide, polyarylate, polyimide, polycarbonate (PC), cellulose triacetate (TAC), and cellulose acetate-propionate. Additionally, the substrate material  30  is transparent or otherwise light transmissive so that the light generated from the OLED material may pass through the device and be visible.  
      The anode coated conductive layer  20  may be formed by optionally coating the substrate with a transparent and conductive coating material. For example, and not for limitation, transparent and conductive coating materials may include indium-tin oxide (ITO), indium-zinc oxide (IZO), and other tin oxides such as, but not limited to, aluminum- or indium-doped zinc oxide, magnesium-indium oxide, nickel-tungsten oxide, metal nitrides, such as but not limited to gallium nitride, and metal selenides, such as but not limited to zinc selenide, and metal sulfides, such as but not limited to zinc sulfide.  
      Atop the anode coated conductive layer  20  is a hole transporting material  40 . The hole-transporting material may include amines, such as but not limited to aromatic tertiary amines. In one form the aromatic tertiary amine may be an arylamine, such as but not limited to a monoarylamine, diarylamine, triarylamine, or a polymeric arylamine. In addition, polymeric hole-transporting materials may include poly(N-vinylcarbazole) (PVK), polythiophenes, polypyrrole, polyaniline, and copolymers such as poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) also called PEDOT/PSS.  
      A polar light-emitting layer  50  is formed on hole transport layer  40  and may comprise a polar fluorescent and/or phosphorescent material where electroluminescence is produced as a result of electron-hole pair recombination in this region. The polar light-emitting layer  50  can be comprised of a single material or a host material doped with a guest compound or compounds, where light emission comes primarily from the dopant and can be of any color. In an exemplary embodiment, the light-emitting layer emits white light. The host material in the polar light-emitting layer  50  can be an electron-transporting material, as defined below, a hole-transporting material, as defined above, or another material or combination of materials that support hole-electron recombination. The dopant may be chosen from highly fluorescent dyes, but phosphorescent compounds, e.g., transition metal complexes, are also useful. Iridium complexes of phenylpyridine and its derivatives are particularly useful luminescent dopants. The polar light-emitting layer  50  may include dyes or coumarins and may also be polymeric material in nature. Polymeric materials such as polyfluorenes and polyvinylarylenes (e.g., poly(p-phenylenevinylene) (PPV)) can also be used as the host material. Small molecule dopants can be molecularly dispersed into the polymeric host, or the dopant could be added by copolymerizing a minor constituent into the host polymer. Any polar luminescent dopant known to be useful by one of ordinary skill in the art may be used herein.  
      An electron transport layer  60  is formed atop the polar light-emitting layer  50 . The electron transporting material may be any material known to one of ordinary skill in the art to be useful for this purpose. Such compounds help to inject and transport electrons, exhibit high levels of performance, and are readily fabricated in the form of thin films. For example, and not for limitation, metal chelated oxinoid compounds, including chelates of oxine itself (also commonly referred to as 8-quinolinol or 8-hydroxyquinoline), may be used.  
      Finally, a cathode  70  is deposited on electron transport layer  60  and supported by a substrate  90 . The cathode may be transparent or otherwise light transmissive, opaque, or reflective and can comprise nearly any conductive material. Suitable cathode materials have good film-forming properties to ensure good contact with the underlying organic layer, promote electron injection at low voltage, and have good stability. Useful cathode materials often contain a low work function metal (&lt;4.0 eV) or metal alloy.  
      As noted above, substrate  90  may be made from any material capable of supporting the conductive coating of the cathode  70  and may be flexible or rigid. Examples include, but are not limited to, plastic, glass, quartz, plastic films, metals, ceramics, polymers or the like. Non-limiting examples of flexible plastic film and plastic include a film or sheet of polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyethersulfon (PES), polyetherimide, polyetheretherketone, polyphenylene sulfide, polyarylate, polyimide, polycarbonate (PC), cellulose triacetate (TAC), and cellulose acetate-propionate. Additionally, the substrate material  90  may be transparent or otherwise light transmissive, opaque, reflective or variations thereof.  
      When a potential, i.e. voltage, is applied to the device from a power source  80 , electrons are emitted from light-emitting layer  50  where they are injected into electron transport layer  60  and recombined with the holes present therein giving rise to light emission. The cathode  70  reflects the light generated back toward the organic layers. By using multicolored OLED panels known to one of ordinary skill in the art, a white light or images with partial or full color utilizing field sequential color techniques may be formed.  
      Exemplary OLED materials of the present invention comprise polar light-emitting layer materials. By exposing the polar light-emitting layer materials to an electric field or applied voltage, the polar light-emitting layer polarizes, i.e., lines up, in the direction of the electric field. Such polarization orients the polar materials in a certain orientation and directs the light emitted from the light-emitting layer in a uniform dominant direction, thus optimizing the light emitted and reducing problems associated with light scatter and channeling. The polarity of the material may come from the organic light emitting material itself, the dopant host material, or the dopant. Chemical compounds useful as a light-emitting material, dopant host material, or dopant include those noted above as well as those known to one of ordinary skill in the art. Non-limiting examples of organic light-emitting materials include amines, including the aromatic tertiary amines, including arylamines, such as but not limited to a monoarylamine, diarylamine, triarylamine, or a polymeric arylamine polyimides, and polythiophenes including, but not limited to, poly(N-vinylcarbazole) (PVK), polypyrrole, polyaniline, and copolymers such as poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) also called PEDOT/PSS and other amines referenced above.  
      Another exemplary embodiment of the present invention may be shown in  FIG. 2  (not to scale). In an OLED structure  10 , an anode coated conductive layer  20  may be integrated on a substrate  30  having an irregular, non-smooth surface  35 , also known as an alignment layer. The alignment layer  35  may provide an irregular, non-smooth surface for the subsequent layers. A hole-transport layer  40  may be applied on the coating of the anode  20  and upon the alignment layer  35 . A layer of polar light-emitting material  50  may be disposed on the hole-transport layer  40 . An electron transport layer  60  may be disposed on the light-emitting layer  50 . Finally, a cathode  70  comprising a conductive film may be supported on a substrate  90  and disposed on the electron transport layer  60 . The irregular, non-smooth surface of the alignment layer  35  may carry through the deposition process and exist at within all layers of the OLED structure. For example, the light-emitting layer  50  may fill part of the irregular surface of the alignment layer  35 . In one embodiment, the polar light-emitting compounds may fill the alignment layer with portions of the molecules extending below the surface of the alignment layer and portions of the molecule extending above the surface of the alignment layer. The anode  20  and cathode  70  may be connected to a power source  80 , which generates an applied voltage. When the power source is activated, holes may be injected from anode  20  into hole transport layer  40 , the holes may combine in the light emitting layer  50  with electrons that travel from the cathode  70  and generate visible light. Because the light-emitting layer molecules are polar, the applied voltage causes the dipoles of the molecules to orient in a uniform arrangement, e.g., all positive ends of the molecule will be anchoring onto the surface of the alignment layer and all negative ends of the molecule will be pointing away from the surface of the alignment layer or vice-versa during the curing process.  
      Once the chemicals are applied to the alignment layer  35  or the substrate  30 , the chemicals may undergo a curing process. During curing, a voltage is simultaneously applied to the OLED material, aligning the polar light-emitting compounds in all of the layers of the OLED material. The voltage facilitates the alignment of the dipoles of the light-emitting layer within the material during the curing cycle.  
      The applied voltage used to orient the light-emitting dipoles is typically less than about 7 volts. In one embodiment, the voltage ranges from 1 to about 7 volts. In another embodiment, the voltage ranges from about 3 volts to about 5 volts.  
      The irregular, non-smooth surface of the alignment layer  35  may be formed on the substrate  30  by any means known in the art. A non-limiting example of forming the irregular, non-smooth surface of the alignment layer  35  includes the rubbing process or friction transfer. Friction transfer includes preparing the alignment layer by pressing a solid structure, for example and not for limitation, pellets, bars, ingots, rods, sticks, or the like, of the alignment material against the substrate and drawing the solid alignment material across the structure in a selected direction under a pressure sufficient to transfer a thin layer of the alignment material onto the substrate. The selected direction of the friction transfer provides an orientation direction fro the alignment of subsequent layers. The substrate may optionally be heated to optimize the initial action of the alignment layer.  
      The thickness of the alignment layer may be sufficient to impart alignment on subsequent layers. The thickness may be thin enough such that the layer is not completely insulating. Exemplary thicknesses of the alignment layer of the present invention range from 0.1 to 20 microns. One embodiment of the invention provides for an alignment layer with thickness of between 1 to 10 microns, and still another embodiment provides for an alignment layer with thickness of between 5-7 microns.  
      The thickness of the polar light-emitting materials may range from 100 angstroms to 2000 angstroms. In one embodiment of the present invention, the thickness of the polar light-emitting layer ranges from 300 to 2000 angstroms. In another embodiment, the thickness of the polar light-emitting layer ranges from 800 to 2000 angstroms.  
      The polar light-emitting compound  50  may be applied to the irregular, non-smooth surface of the alignment layer  35  of which the topology shows through layer  20  and  40  or to the surface of the substrate  30  at room temperature or under elevated temperatures to enhance the uniformity of the light-emitting compound layer.  
      Other embodiments of the present invention include processes for preparing OLED materials useful in display devices. One exemplary process is illustrated in  FIG. 2  and may include coating a substrate  30  with a conductive layer  20  and/or a hole transport layer  40  to form a coated substrate, rubbing the coated substrate to form grooves or other irregular surfaces of an alignment layer  35 , applying a polar light-emitting compound  50  to the irregular surface of the coated substrate and filling the grooves or irregularities formed by rubbing the substrate with the light-emitting compound  50 , then curing the coated substrate while simultaneously exposing it to an electric field.  
      Another exemplary process of the present invention may include coating a substrate  30  with a conductive layer  20  and/or a hole transport layer  40  to form a coated substrate, applying a polar light-emitting compound  50  to the surface of the coated substrate, then curing the coated substrate while simultaneously exposing it to an electric field.  
      Another exemplary embodiment of the present invention may include the OLED materials integrated into a display device.  FIG. 3  (not to scale) illustrates this exemplary embodiment. When a voltage is applied to the OLED structure  10  from a power source  80 , light  300  emitted from the OLED structure  10  is transmitted in the direction of the applied voltage and toward the display  100 . Because more of the light emitted from the OLED structure  10  is transmitted to the viewer, the display  100  may operate with less power than displays currently known in the art.  
      The display device may include light distributing devices, such as lenses, polarizers, or optical viewing elements. Integrated with the OLED materials of the present invention, the display  100  may be any element which transmits the light from the OLED to the viewer. The display  100  may also comprise other components such as, but not limited to, a processor, memory, power supplies, or other peripheral devices, either alone or in combination.  
      Those skilled in the art will appreciate that other light distributing devices may be used, such as, for example, and not for limitation, light guides, prisms, lenses, Fresnel lenses, diffusers, interferometers, or any other optical element that can distribute white light uniformly and efficiently onto the display device. It is further disclosed that additional optical elements, such as but not limited to polarizers, refractive elements, diffractive elements, bandpass filters, and the like, may be easily positioned exterior to or otherwise located near the OLED structure  10 . By using a plurality of OLED panels as the light source, the size of the OLED structure  10  may be further reduced and the electrical power required may also be minimized. By using multicolored OLED panels, a white light or images with partial or full color utilizing field sequential color techniques may be formed. The light may optionally be passed through a light distributing device, which disperses the light to uniformly illuminate the display device  100 .  
      Those skilled in the art will further appreciate that the OLED structure  10  of the present invention may optionally be present in a display device  100  in combination with other OLED structures. The OLED structure  10  may be arranged randomly or in a pattern and may be stacked or arranged in series or adjacent to one another. The arrangement of the OLED structure  10  may depend on any of several factors including, but not limited to, size of the display, lighting requirements for the display, color, and the like. Additionally, those skilled in the art will appreciate that OLED materials may be, for example, and not for limitation, strips, films, blocks, and the like.  
      The light emitted from the OLED structure  10  of the present invention may be manipulated by the structure of the OLED structure  10  itself and may emit white light or colors. A color-emitting OLED may be combined with white light-emitting OLED, both of which may then be incorporated into a display device  100 .  
      In the embodiments of the present invention, the intensity of the light transmitted to the display device  100  and the intensity of the color may be varied by adjusting the current and driving voltages applied to the OLED structure  10 . Proportional current changes may be applied to each layer of the stack or to each OLED structure  10  in the series to optionally vary the color perceived by the viewer.  
      The current necessary to display light from the OLED structure  10  to a display device  100  may be less than about 15 volts. In one embodiment of the present invention, the current necessary to display light from the OLED structure  10  ranges from about 1 volt to about 12 volts. The intensity of the light displayed from the OLED structure  10  may be varied by varying the voltage applied to the OLED structure  10 .  
      The OLED structure  10  of the present invention may be incorporated into any system benefiting from an image display device. The OLED structure  10  of the present invention may be incorporated into a display device in addition to or in lieu of LCD displays or other display devices known in the art. Systems incorporating display devices include, but are not limited to, those used with laptop computers, personal digital assistants, cellular phones, and the like.  
      In addition to the display device  100 , the system may also include, but is not limited to, a processing unit, a system memory, and a system bus that couples various system components including the system memory to the processing unit. The system bus may be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures. By way of example, and not limitation, such architectures include Industry Standard Architecture (ISA) bus, Micro Channel Architecture (MCA) bus, Enhanced ISA (EISA) bus, Video Electronics Standards Association (VESA) local bus, and Peripheral Component Interconnect (PCI) bus also known as Mezzanine bus.  
      The system memory may include computer storage media in the form of volatile and/or nonvolatile memory such as read only memory (ROM) and random access memory (RAM). A basic input/output system (BIOS), containing the basic routines that help to transfer information between elements within the system, such as during start-up, is typically stored in ROM. RAM typically contains data program modules, and/or computer-executable instructions that are immediately accessible to and/or presently being operated on by processing unit.  
      While the present invention has been particularly shown and described with respect to exemplary embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and detail may be made without departing from the scope and spirit of the present invention. It is therefore intended that the present invention not be limited to the exact forms described and illustrated, but fall within the scope of the appended claims.