Abstract:
An organic light emitting device for a multicolor image display. The elements of the device are arranged laterally along the substrate surface in a pillar and channel arrangement, rather than the traditional method of vertical stacking. The invention&#39;s pillar and channel architecture provides a rugged structure which can be efficiently encapsulated. The lateral arrangement of the various elements along the substrate reduces sheet resistance and increases external efficiency by allowing the typical transparent conductor layer to be replaced with a conventional metal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application Ser. No. 60/060,687 entitled “LATERALLY STRUCTURED, HIGH RESOLUTION MULTICOLOR ORGANIC ELECTROLUMINESCENCE DISPLAY DEVICE,” filed on Sep. 22, 1997. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed to an organic light emitting device (“OLED”). In particular, the device of the present invention is an innovative advancement over previous OLED designs due to its arrangement transversely along the substrate surface, rather than vertically stacked upon it. Additionally, the design of the present invention yields improved operating characteristics and is easier to fabricate. 
     BACKGROUND OF THE INVENTION 
     A typical OLED generates multicolor, intense light emission from vertical stacks of organic thin films sandwiched between charge injecting conductors. The OLED&#39;s organic materials are typically arranged in single or multilayer stacks. The conductors supply, upon the application of voltages with sufficient amplitude and polarity, both negative and positive charge carriers which recombine in the organic stack to release pure energy in the form of light. This phenomenon is called electroluminescence. In a vertically stacked OLED, at least one of the conductors must be optically transparent in order to couple light in the viewer&#39;s direction. 
     Typically, conductor materials for vertically stacked OLEDs are indium-tin-oxide (ITO) for the hole-injecting transparent conductor, and co-evaporated magnesium and silver (Mg:Ag) for the electron-injecting conductor. ITO is a problematic material for a number of reasons. First, ITO is normally deposited in a high-temperature sputtering process which can damage the organic material. Second, the deposition process for an ITO conductor is variable and can yield an unacceptably rough microstructure. Third, it is difficult to control the etching of the ITO layer in sub-micron dimensions using traditional wet etching techniques. Finally, the surface condition of ITO is unfavorable for injection and, as a result, oxidation is required to achieve high performance. Furthermore, in the traditional vertically stacked OLED architecture (e.g., ITO on one side with Mg:Ag on the opposite side of the organic material) the moisture-sensitive magnesium conductor has similar problems with patterning and environmental exposure. 
     In vertical OLEDs, the organic material is sequentially deposited in layers of molecular film. This sequential process is more time consuming and less efficient than the deposition of one layer of organic material comprised of a polymeric blend of the required material constituents. The fabrication process for the vertically stacked OLED also typically includes the need for mask changes during the sequential deposition of materials. The mask changes add further complexity and cost to the manufacturing process. 
     Vertically stacked prior art OLEDs, which employ a continuous (unpatterned) stack of organic materials, face the additional problem of unavoidable increases in lateral charge leakage. This charge leakage phenomenon typically occurs when one display device contains numerous OLEDs or pixel elements mounted on a substrate. The lateral charge conduction or leakage occurs along the substrate direction as the individual OLEDs decrease in size to submicron dimensions. These lateral leakage currents destroy the integrity of the overall display by creating crosstalk between the individual OLEDs or pixel elements. 
     Light piping is another problem associated with vertically stacked OLEDs. Light piping, a phenomenon also referred to as total internal reflection, results when light passes from a high dielectric constant material to a low dielectric constant material over a set of incidence angles determined by the laws of geometric optics and the frequency response of the materials. Light is confined to the high dielectric constant material. Light piping forms the basis for all fiber optic communications technologies. Light piping in the glass substrate creates optical losses. Both light piping and current leakage reduce the overall external efficiency of the display device. Accordingly, there is a need for microcavity structure within the OLED to provide output enhancement. 
     The typical applications of OLEDs include, for example, flat panel displays, optical interconnects, optical fiber communications and LED printing. Given the nature of these fields, the need exists for an OLED which is reliable and capable of withstanding forces associated with its use. The vertically stacked OLED does not provide any inherent structural stability. The horizontal layers are merely stacked upon one another without any means for resisting lateral or forces. Accordingly, there is a need for an OLED structure which provides increased support and stability for the organic layers. 
     The present invention addresses the above problems, in whole or in part, via a display device, which utilizes transversely stacked OLEDs. Each OLED of the present invention typically serves as one color component of a color pixel. The present invention also addresses the problems associated with patterning a multicolor display device due to the degradation or destruction of organic light-emitting materials through exposure to moisture, oxygen, light, temperature and chemicals. This exposure unavoidably results from traditional semiconductor fabrication methods. The present invention solves the aforementioned problems with traditional OLEDs, and provides other benefits as well. 
     OBJECT OF THE INVENTION 
     It is therefore an object of the present invention to reduce sheet resistance and increase device efficiency by replacing the transparent conductor material with a conventional metal. 
     It is another object of the present invention to increase device efficiency through reduction of optical piping losses. 
     It is a further object of the present invention to provide chromaticity enhancement through the formation of a microcavity structure. 
     It is still another object of the present invention to provide an OLED with a rugged architecture with enhanced structural stability. 
     It is a further object of the present invention to provide encapsulation protection of organic light emitting materials through the use of a buried channel architecture. 
     It is still another object of the present invention to accommodate either polymeric or molecular film approaches to organic electroluminescence. 
     It is yet another object of the present invention to facilitate sequential color patterning of adjacent pixels through the use of buried channel architecture. 
     Additional objects and advantages of the invention are set forth, in part, in the description which follows, and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention. 
     SUMMARY OF THE INVENTION 
     In response to the foregoing challenges and to achieve the objects set forth above and other objects that will become apparent in the following description, an innovative organic light emitting device is provided. The organic light emitting device may comprise: a substrate, wherein the substrate is comprised of a substantially planar base, a microcavity stack overlying the base, a layer of conducting film overlying the microcavity stack, and a thin insulator layer overlying the layer of conducting film; plural electrode stacks overlying the insulator layer wherein the electrode stacks comprise a bottom layer of conductive material and a top layer of non-conductive material; a light emitting stack disposed between the electrode stacks, wherein the light emitting stack comprises a layer of light emitting organic material overlying the insulator layer and contacting the bottom layer of conductive material; a layer of filler material overlying the layer of organic material; a mirror overlying the filler layer; and an encapsulation layer overlying the mirror. The layer of light emitting organic material may be comprised of a multilayer stack of organic material, and the base may be either transparent or opaque. 
     Another embodiment of the organic light emitting device of the present invention may comprise: a substantially planar substrate; a plurality of conductors laterally spaced on the planar substrate, the conductors having sidewalls; and a layer of light emitting organic material overlying the planar substrate and contacting a sidewall of each of two of the plurality of conductors. The planar substrate may comprise a layer of insulator overlying a layer of glass. The planar substrate may further comprise a layer of dielectric material located between the insulated material and the layer of glass. The planar substrate may further comprise a layer of conducting film located between the insulator and the layer of glass. The device may further comprise a layer of nonconducting material overlying each of the plurality of conductors. The layer of light emitting organic material may be comprised of a multi-layer stack of organic material and may be covered by a layer of filler material. Each of the plurality of conductors may be composed of a metallic material. The plurality of conductors may overlie the substrate in an interdigitation pattern. 
     A further embodiment of the organic light emitting device of the present invention may comprise: a substantially planar substrate; a plurality of conductors laterally spaced on the planar substrate; a layer of light emitting organic material overlying the planar substrate between two of the plurality of conductors; a layer of nonconducting material overlying each of the plurality of conductors; and a layer of filler material overlying the layer of light emitting organic material. The device may further comprise a mirror overlying the filler layer. The layer of light emitting organic material may be comprised of a multilayer stack of organic material. Each of the plurality of conductors may be comprised of a metallic material. The planar substrate may be comprised of a layer of insulator overlying a layer of glass. The planar substrate may further comprises a layer of dielectric material located between the insulator and the layer of glass. The planar substrate may further comprise a layer of conducting film located between the insulator and the layer of glass. The plurality of conductors may overlie the planar substrate in an interdigitation pattern, which may be a spiral pattern. 
     The present invention further includes an innovative method of fabricating an organic light emitting device comprising the steps of: providing a substantially planar substrate; providing a plurality of laterally spaced conductors on the planar substrate; forming a layer of light emitting organic material between two of the plurality of conductors. The step of forming a layer of light emitting organic material may be comprised of depositing multiple layers of organic material sequentially. The step of providing a plurality of conductors may be comprised of arranging the conductors on the planar substrate in an interdigitation pattern. The method of the present invention may further comprise the step of depositing a layer of filler material overlying the layer of light emitting organic material. The method may also include the step of depositing a mirror layer on top of the layer of filler material. The method may further comprise the step of covering each of the plurality of conductors with a layer of nonconducting material. The method may include the step of encapsulating the device following the deposition of the organic material, and applying photoresist to the device following the encapsulation step. Following the encapsulating step the method may include the step of removing accumulated materials from the nonconducting pillars. 
     The present invention further may include an organic light emitting device wherein each light emitting diode comprises two interdigitated patterns of conductive material in contact with an intermediate pattern of light emitting organic material. The interdigitated patterns may comprise spirals. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed. The accompanying drawings, which are incorporated herein by reference and which constitute a part of this specification, illustrate certain embodiments of the invention, and together with the detailed description serve to explain the principles of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in conjunction with the following drawings in which like reference numerals designate like elements and wherein: 
     FIG. 1 is a cross sectional view of a laterally structured light emitting device according to the present invention; 
     FIG. 2 is a cross sectional view of an alternative embodiment device according to the present invention; 
     FIG. 3 is a cross sectional view of another embodiment of a device according to the present invention; 
     FIG. 4 is a cross sectional view of another embodiment of a device according to the present invention; 
     FIG. 5 is a partial cross sectional view of one color component of a color pixel, along the line A—A in FIG. 6; 
     FIG. 6 is a top view of an interdigitated pixel element; 
     FIG. 7 is a top view of an alternative design of an interdigitated pixel element; 
     FIGS. 8 through 11 illustrate a method of fabricating a color OLED display; and 
     FIG. 12 illustrates a single-transistor active matrix layout. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, there is shown an organic light emitting device with transversely spaced conductors. FIG. 1 is a cross sectional view of a light emitting device in which conductors  200  and  210  overlay the substrate  100 . Light emitting organic material  300  is sandwiched between the conductors  200  and  210 . When a voltage is applied to the conductors  200  and  210 , holes and electrons may move from the conductors into the organic material  300 . When the holes and electrons combine in the organic material  300  light may be produced. The substrate  100  may be formed from a material which allows the emitted light to pass through to a viewer, such as glass. Organics  300  are transparent to the light they emit, allowing for the possibility of a two-way viewable display. If substrate  100  is opaque (e.g., a silicon wafer) the device is solely up-emitting. If substrate  100  is transparent and a mirror element  700  (see description of FIG. 5) is included, the device is solely down emitting. 
     The present invention&#39;s innovative arrangement of the conductors  200  and  210  transversely along the coated substrate  100 , rather than in a vertical stack as in a conventional OLED, allows a wide variety of metals to be considered as conductor material. Transparent conducting material (e.g., Indium Tin Oxide (ITO)) is not required. The conductors  200  and  210  are preferably formed from metals such as Cu, Mo, Ni, Al, Cr, or Au. These materials are preferred because they are usually easier to process. The metal conductors also manifest reduced energy band offsets relative to the neighboring organic material  300 . As a result of the present invention&#39;s use of standard metallic conductors  200 ,  210  the fabrication problems associated with ITO, Mg and Ca conductors are avoided. Hi-resolution lithography (e.g., x-ray, electron beam) may be used to pattern the conductors  200  and  210 . 
     It is also within the scope of the present invention that the conductors are comprised of metallic alloys. Alloys may provide better injection characteristics and/or increased stability. The present invention also includes the use of doped silicon injectors as conductors. These conductors may be integrated onto silicon wafers using standard semiconductor technology as described in “SILICON COMPATIBLE ORGANIC LIGHT EMITTING DIODE,” Kim et. al.,  Journal of Lightwave Technology,  Vol. 12, No. Dec. 12, 1994, which is hereby incorporated by reference. 
     Light emitting organic material  300  may be located between the conductors  200  and  210 . The organic material  300  can be either a single organic layer or a stack of organic layers, as shown in FIG.  2 . When deposited in a stack each organic layer may accomplish a different function such as hole transport, electron transport or light emission. When a single organic layer is used, that layer may be a blend of organic materials capable of performing the required transport and emission functions. The organic layer  300  may be constructed using either a polymer approach or a molecular film approach. The preferred method uses the molecular film approach to deposit hole transport, emission and/or electron transport layers of organic material sequentially in a repeating pattern until the vertical height of the organic layers  300  reaches the top of the metal conductors  200  and  210 . The height of the conductors  200  and  210  should be greater than or equal to the organic stack height. Typically, the conductors would be between 1200 to 2500 Å in height. 
     The device disclosed in FIG. 2 further includes an additional layer within the substrate  100 . The substrate  100  comprises a substantially planar base of transparent glass or silicon  101  and a layer of thin insulator material  102 . The base  101  is substantially planar and generally smooth to allow for application of uniform layers above. The insulator layer  102  electrically isolates the conductors  200  and  210  from each other, and from the base  101 . The insulator layer  102  may be transparent to allow emitted light to pass through. The insulator layer  102  preferably is comprised of SiO 2  with a thickness of approximately 3000 Å. 
     The substrate may further include a layer of conducting film  103 , as shown in FIG.  3 . The conducting film  103  may be either transparent or opaque. ITO, or other structures from the indium, zinc oxide families (e.g. In 2 O 3 :x, ZnO:x with x=Ga, Al or Sn) may be used to form a transparent film, and Al, Cu, Ni, etc. may be used to form an opaque film. The conducting film  103  has a thickness generally less than 1500 Å. One purpose of the transparent conducting film  103  is to couple the emitted light to the viewer through a transparent substrate. The conducting film  103  also may charge the thin insulator layer  102  when a suitable low voltage (e.g., less than 25V) is applied. The insulator layer  102 , when charged, is capable of manipulating the presence of charge carriers at the interfaces between the organic material layers  300 . The charged insulator layer  102  may further enhance the likelihood that laterally moving charge carriers will recombine to produce electroluminescence at the organic interfaces between the layers of organic material. When a single blended organic layer is used instead of a multilayer stack, the insulating layer  102  may not be required to promote charge carrier recombination. 
     The device in FIG. 3 further comprises nonconducting pillars  400  positioned on top of the conductors  200  and  210 . The pillars  400  are at least equal in height to the conductors  200  and  210  on which they reside. The pillars  400  are composed of inorganic materials such as oxides (e.g. SiO, SiO 2 ) and may be deposited using standard semiconductor fabrication methods (e.g., thermal, sputter, CVD, etc.). The pillars  400  partition the light emitting device, creating vertical channels  500  in the interpillar space. The organic material  300  is buried at the bottom of the vertical channels  500 . 
     The pillar design of the present invention provides certain advantages. The nonconducting pillars  400  cover the top surface of the metal conductors  200  and  210  partitioning the matrix and protecting the conductors. When used in combination with the filler layer  600 , the pillars  400  also may prevent the permeation of moisture into the vertical channels  500  where light emission takes place. The pillars  400  also provide stability and rigidity to the organic layers  300 , making the device less susceptible to damage. The walls of the pillars  400  also serve to induce light piping in the interpillar spaces or vertical channels  500 . 
     Light piping may be encouraged by depositing a filler layer  600  with a suitably larger dielectric material constant on top of the organic material  300 , as shown in FIG.  4 . Preferably, the filler layer  600  is formed from high index of refraction a materials such as barium titanate (BaTiO 3 ) or silicon nitride (Si 3 N 4 ). 
     The device in FIG. 4 has a substrate layer  100  comprised of a substantially planar base  101 , an insulator layer  102 , and multiple layers of dielectric material which comprise a microcavity stack  104 . The microcavity stack  104  resides directly over the transparent glass layer  101  and may serve as a reflection enhancing coating. The microcavity stack  104  typically consists of multilayer stacks of SiO 2 /TiO 2 , where each layer has a thickness typically less than 1000 Å. The multiple layers of dielectric have alternating indices of refraction, creating a quarter wave mirror effect. The light emitting device of the present invention may include either the conducting film  103  or the microcavity stack  104 , or both. When used in combination with the conducting film  103 , the microcavity stack  104  is positioned above the glass layer  101 , but below the transparent conducting film  103 . 
     A typical color pixel  910  has four color components  250  (e.g. Red, Green, Green, Blue or RGGB). The display colors are generated through the illumination of a combination of the color components. The single transistor active matrix layout  900 , shown in FIG. 12, is indicative of the color pixel  910  with its four color components  250 . A top view of the color component  250 , is shown in FIG.  6 . The preferred embodiment of present invention with its associated elements is shown in FIG. 5, which represents a partial cross-sectional view of one color component  250 , of a color pixel  910 . 
     In addition to those materials previously described, the device shown in FIG. 5 may further comprise a thin film  205  deposited on top of each of the conductors  200  and  210 . The thin film  205  is preferably composed of oxides or fluorides such as MgO, SiO 2 , BaTiO 3 , or LiF and MgF 2 . The thin film  205  which overlies conductor  210  may be comprised of a different material than the film overlying conductor  200 . The film  205  generally has a thickness no greater than 100 Å. However, thicker films are considered to be within the scope of the invention. The thin film  205  provides further reductions in band offset complementing the underlying metal conductors. 
     The thin film  205  imposes an additional voltage drop across the interface between the conductors  200  and  210  and the organic material  300 . This helps to decrease the band offset of these elements, or, stated differently, it promotes alignment of the electronic energy levels in these elements, and therefore assists in charge injection of the interface. The thin element  205  can serve to improve the adhesion of the pillar  400  to the conductors  200  and  210  in addition to assisting injection of charge from said conductors into the organic  300 . The use of the thin film is further described in the Kim article cited above. In addition to reducing band offset, the thin film  205  protects the conductors  200  and  210  from moisture and reduces exciton quenching at the conductors&#39; metal atoms. 
     The external efficiency of the device in FIG. 5 is further improved by depositing a thin metal layer  700  in the vertical channel  500  on top of the filler layer  600 . The primary purpose of the metal layer  700  is to create a mirror surface for use with a transparent substrate that reflects the emitted light toward the substrate  100  and the viewer. When the mirror surface of the thin metal layer  700  is combined with the pillars  400  and the filler layer  600 , a microcavity is formed. The microcavity serves to increase light yield in the view direction, through the transparent insulator  102  and the glass substrate  101 . The microcavity traps light between two reflective surfaces, and places restrictions on the frequencies of light which can exit the structure. As a result, the microcavity may also be used to spectrally shape the output light for chromaticity enhancement. The metal layer  700  further provides a barrier to moisture penetration in the interpillar region. 
     Despite all of its advantages, the metal layer is an optional component of the invention. In fact, the omission of the metal mirror layer  700  may result in a two-way viewable display. An encapsulation layer  800  fills the interpillar space  500  and buries the entire structure shown in FIG.  5 . For top-emitting or two-way viewable display, the encapsulation layer  800  may be a transparent oxide such as SiO 2  or SiO. In this embodiment, the organic layer  300  is well protected at the bottom of the vertical channel  500  beneath a filler layer  600 , a mirror  700  and the encapsulation layer  800 . 
     The top view of a pixel component  250  is shown in FIG.  6 . The conductors or leads  200  and  210  of the color component  250  are spaced apart in alternating fashion in a layout known as interdigitation. An alternative interdigitation design  255  is shown in FIG.  7 . Each pixel color component is approximately 3 μm by 3 μm. Therefore, a full color pixel (RGGB)  501  will occupy at least a 12 μm by 12 μm area, plus the additional space for the active matrix elements shown in FIG.  12 . The present invention includes many possible patterns for interdigitation. A comb design is shown in FIG. 6, while FIG. 7 discloses a spiral design. FIGS. 6 and 7 are merely two examples of microstructured patterns which cannot be resolved by the viewer. Any pattern which the viewer cannot resolve is an acceptable alternative, and within the scope of the present invention. FIG. 12 illustrates a single-transistor active matrix  900  layout comprising at least one full color pixel  910  containing RGGB components  250 . 
     The spacing between the conductors  200  and  210  should be small (e.g., less than 0.2 μm) in order to permit charge carrier conduction upon the application of the required voltages. Current technology in the area of microlithography fabrication is capable of supporting construction of a device with the required clearances. The conductors  200  and  210  are either electron or hole injectors, which due to the interdigitation scheme alternate positionally. 
     The invention includes an innovative method of fabricating an organic light emitting device. The device is shown at different stages of the process in FIGS. 8-11. First, as shown in FIG. 8, the substrate  100  is formed. The insulator layer  102  is deposited on the coated substrate comprising a glass layer  101 , a multilayer dielectric  103 , and a layer of conducting film  104 . Second, the conductors  200 ,  210 , are placed on the substrate  100 . The non-conducting pillars  400  are stacked on the conductors  200 ,  210 . FIG. 9 discloses the component structure following the inclusion of the conductors  200 ,  210  and the non-conducting pillars  400 . Next, the organic material  300 , filler layer  600 , mirror  700  are deposited in the vertical channels  500 . The device is encapsulated and the device shown in FIG. 10 results. During fabrication of the various layers of interpillar materials, material accumulates on top of the pillars  400  (see FIG.  10 ). The material accumulation on top of the pillar  400  occurs as a result of the deposition of materials in the channel  500 . A standard solvent wash which does not affect the integrity of the encapsulation layer  800  may be used to remove the materials which accumulate atop the vertical pillars  400 . Compositions comprising acetone, isopropyl alcohol (IPA), N-methyl pyrolidone (NMP) may be used. Following the lift-off wash and the removal of accumulated materials on top of the pillars  400 , the device appears as shown in FIG.  11 . 
     After encapsulation and washing, photoresist is spun over the pixel color component  250 . Photoresist completes the fabrication and protects the component  250  from any damage or contamination. Since each pixel component is protected following fabrication, materials may be deposited in adjacent areas of the matrix  900  during the fabrication of subsequent components without adversely affecting a pixel component  250  already completed. Conventional (i-line) photolithography is used to define elements  250  for processing of color materials. The process of the present invention provides efficient sequential color display fabrication. 
     Following the application of photoresist, the procedure may be repeated to make additional color pixel components. The buried channel architecture of the present invention facilitates sequential color patterning of adjacent pixels. Subsequent fabrication of adjacent pixels will not disturb the pixel components already in place, since the vital organic materials  300  are buried and protected in the vertical channel  500 . 
     It will be apparent to those skilled in the art that various modifications and variations may be made in the preparation and configuration of the present invention without departing from the scope and spirit of the present invention. Thus, it is intended that the present invention covers the modifications and variations of the invention, provided they come within the scope of the appended claims and their equivalents.