CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. Ser. No. 08/794,595, filed Feb. 3, 1997 now U.S. Pat. No. 6,091,193. 
    
    
     GOVERNMENT RIGHTS 
     This invention was made with Government support under Contract No. F33615-94-1-1414 awarded by DARPA. The government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to display devices which use organic light emitting devices (OLED&#39;s), and more particularly to display devices which are designed to minimize the problem of waveguiding. 
     BACKGROUND OF THE INVENTION 
     The electronic display is used in such devices as television sets, computer terminals, telecommunications equipment and a host of other applications as well. No other communication medium offers its speed, versatility and interactivity. Among the types of electronic displays currently available, there is no doubt that the technology concerning flat panel displays is of a significant interest and progress is continuously being made in this field. For example, according to S. W. Depp and W. E. Howard, (“Flat Panel Displays”,  Scientific American 90-97 (March 1993)), incorporated herein by reference, flat panel displays were expected to form a market of between 4 and 5 billion dollars in 1995 alone. Desirable factors for any display technology include the ability to provide a high resolution, full color display at good light level and at competitive pricing. 
     Organic light emitting devices (OLED&#39;s), which make use of thin film materials which emit light when excited by electric current, are becoming an increasingly popular form of flat panel display technology. Presently, the most favored organic emissive structure is referred to as the double heterostructure (DH) OLED, shown in FIG.  1 A. In this device, a substrate layer of glass  10  is coated by a thin layer of indium-tin-oxide (ITO)  11 . Next, a thin (100-500 Å) organic hole transporting layer (HTL)  12  is deposited on ITO layer  11 . Deposited on the surface of HTL  12  is a thin (typically, 50 Å-500 Å) emission layer (EL)  13 . The EL  13  provides the recombination site for electrons injected from a 100-500 Å thick electron transporting layer  14  (ETL) with holes from the HTL  12 . Examples of prior art ETL, EL and HTL materials are disclosed in U.S. Pat. No. 5,294,870, the disclosure of which is incorporated herein by reference. 
     Often, the EL  13  is doped with a highly fluorescent dye to tune color and increase the electroluminescent efficiency of the OLED. The device as shown in FIG. 1A is completed by depositing metal contacts  15 ,  16  and top electrode  17 . Contacts  15  and  16  are typically fabricated from indium or Ti/Pt/Au. Electrode  17  is often a dual layer structure consisting of an alloy such as Mg/Ag  17 ′ directly contacting the organic ETL  14 , and a thick, high work function metal layer  17 ″ such as gold (Au) or silver (Ag) on the Mg/Ag. The thick metal  17 ″ is opaque. When proper bias voltage is applied between top electrode  17  and contacts  15  and  16 , light emission occurs from emissive layer  13  through the glass substrate  10 . An LED device of FIG. 1A typically has luminescent external quantum efficiencies of from 0.05% to 2% depending on the color of emission and the device structure. 
     Another known organic emissive structure is referred to as a single heterostructure (SH) OLED, as shown in FIG.  1 B. The difference between this structure and the DH structure is that multifunctional layer  13 ′ serves as both EL and ETL. One limitation of the device of FIG. 1B is that the multifunctional layer  13 ′ must have good electron transport capability. Otherwise, separate EL and ETL layers should be included as shown for the device of FIG.  1 A. 
     Yet another known LED device is shown in FIG. 1C, illustrating a typical cross sectional view of a single layer (polymer) OLED. As shown, the device includes a glass substrate  1  coated by a thin ITO layer  3 . A thin organic layer  5  of spin-coated polymer, for example, is formed over ITO layer  3 , and provides all of the functions of the HTL, ETL, and EL layers of the previously described devices. A metal electrode layer  6  is formed over organic layer  5 . The metal is typically Mg, Ca, or other conventionally used low work function metal. 
     An example of a multicolor electroluminescent image display device employing organic compounds for light emitting pixels is disclosed in U.S. Pat. No. 5,294,870. This patent discloses a plurality of light emitting pixels which contain an organic medium for emitting blue light. Fluorescent media are positioned between the blue OLED and the substrate in certain parts of the pixels. The fluorescent media absorb light emitted by the blue OLED and emit red and green light in different regions of the same pixel. One drawback of this display is that waveguiding of light through the glass substrate from one pixel to adjacent pixels of different color can result in blurring, color bleeding, lack of image resolution and the loss of waveguided light. This problem is schematically shown in FIG. 1D for a device shown in FIG. 1A, and is further described in D. Z. Garbuzov et al., “Photoluminescence Efficiency and Absorption of Aluminum Tri-Quinolate (Alq 3 ) Thin Films,” 249  Chemical Physics Letters  433 (1996), incorporated herein by reference. A further problem in this device is that the ITO used as a transparent, conductive layer is a high-loss material, thus resulting in absorption of waveguided light by ITO layers. One additional problem encountered in this and other prior art devices is that the LED interconnect lines can be seen by the viewer as black lines surrounding individual pixels, thus increasing the granularity of the display and limiting resolution. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a monochromatic or multicolor organic light emitting device with improved efficiency. 
     It is a further object of the present invention to provide a display device that is characterized by reduced or negligible waveguiding. 
     It is an additional object of the present invention to provide a display device wherein the LED interconnect lines are not visible to an observer of the display. 
     It is a further object of this invention to provide a high-definition display device in which the LED&#39;s are arranged in distinct, reflective angle-walled mesas, wherein each mesa appears as a truncated pyramid, so to substantially prevent the waveguiding of light from one pixel to another, thereby increasing display brightness and efficiency. 
     It is another object of the present invention to provide an organic light emitting device which is extremely reliable, relatively inexpensive to produce, compact, efficient and requires low drive voltages. 
     The present invention comprises monochromatic and multicolor display devices comprising a plurality of pixels, each of said plurality of pixels comprising a substrate and at least one angle-walled mesa connected to said substrate. The mesas used in the present invention appear as truncated pyramids, each having a top portion which is narrow relative to its bottom portion such that light is directed via reflection in a direction from its top portion to its bottom portion. 
     In a first embodiment of the present invention, each pixel comprises light-emitting devices arranged in three mesas on a transparent substrate, wherein the first of said mesas serves as a blue light emitter, the second of said three mesas serves as a green light emitter and the third of said three mesas serves as a red light emitter. In this embodiment, the bottom portion of each mesa is immediately adjacent the substrate such that the light emitted by each mesa is directed towards the substrate. 
     In a second embodiment of the invention, each pixel comprises light-emitting devices arranged in three inverted angle-walled mesas wherein the first of said three inverted mesas serves as a blue light emitter, the second of said three inverted mesas serves as a green light emitter and the third of said three inverted mesas serves as a red light emitter. In this embodiment, the mesas are referred to as being “inverted” because the top portion of each mesa is immediately adjacent the substrate such that the light emitted by each mesa is directed away from the substrate. 
     In a third embodiment of the present invention, each pixel comprises light-emitting devices arranged in a single mesa or inverted mesa which serves as an emitter of blue, green or red light, alone or in combination, by virtue of a stacked structure of blue, green and blue OLED&#39;s. 
     To prevent waveguiding and resulting color cross-talk, each mesa of all embodiments of the present invention is separated one from the other and is at least partially surrounded by reflective materials. The present invention also includes methods for making each display device embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a cross sectional view of a typical organic double heterostructure light emitting device (OLED) according to the prior art. 
     FIG. 1B is a cross sectional view of a typical organic single heterostructure light emitting device (LED) according to the prior art. 
     FIG. 1C is a cross sectional view of a known single layer polymer LED structure according to the prior art. 
     FIG. 1D illustrates the problem of waveguiding in conventional structures LED structures. 
     FIGS. 2A,  2 B and  2 C are cross sectional views of integrated three color pixels utilizing blue organic light emitting devices (OLED&#39;s) and red and green down conversion phosphors according to embodiments of this invention. 
     FIG. 2D illustrates a stacked arrangement of blue, green and red OLED&#39;s in a mesa pixel configuration according to an embodiment of the present invention. 
     FIG. 3 shows a top view of one embodiment of the present invention. 
     FIGS. 4A to  4 D illustrate the method of making a first embodiment of the present invention as shown in FIG.  2 A. 
     FIGS. 5A to  5 E illustrate the method of making a second embodiment of the present invention as shown in FIG.  2 B. 
     FIGS. 6A to  6 D illustrate the method of making a third embodiment of the present invention as shown in FIG.  2 D. 
    
    
     DETAILED DESCRIPTION 
     Organic thin film phosphors can convert violet-blue radiation into green and red with efficiencies close to 100% at room temperature. The absorption coefficient for these phosphors at energies higher than their optical bandgap exceeds 2 to 3×10 4  cm −1  such that films with thickness of 1 μm are sufficient for full absorption and down-conversion of short wavelength radiation. These phosphor films in combination with short wavelength (blue or violet OLED&#39;s) are used in the fabrication of the color displays of the present invention. 
     One example of a three color display with green and red down converter luminescent layers is described in U.S. Pat. No. 5,294,870. As an improvement, the present invention solves the problems of refractive index-matching and waveguiding effects in the plane of the structure. The present invention achieves such results by careful index matching of LED layers and by configuring LED&#39;s in a truncated pyramidal mesa structure. Waveguiding effects within each LED of the present invention are therefore insignificant because OLED and/or phosphor layers are laterally bound by angle-walled mesas. This results in improved radiation coupling out of the device with resulting increased total display brightness. 
     The first embodiment of the present invention can be seen in FIG. 2A, which shows a schematic cross section of a highly compact, integrated RGB pixel structure which is implemented by grown or vacuum deposited organic layers. In this embodiment, waveguiding is reduced and radiation output is increased due to the collimating effect of the mesa structure. This collimation is achieved either by total reflection from the angled walls of the mesa structure or by mirror reflection from metal deposited on the angled walls. 
     It should be noted that the device shown in FIG. 2A is not intended to be drawn to scale. This embodiment includes pixels having three mesa stacks on a common substrate  37 , the stacks comprising a blue OLED layer  20  and red and/or green down conversion phosphors, designated as  21  and  22 , respectively, depending on the particular stack. Substrate  37  is generally fabricated from a transparent material, such as glass, quartz, sapphire or plastic. The OLED layer can be of either DH or SH configuration, or a single layer of polymer-based OLED, as is well-known in the art. 
     Device  24  is a blue emitter that includes a blue OLED  20  that extends horizontally to also form a top portion of each of devices  27  and  28 , as shown. Device  27  is a green emitter that includes the blue OLED  20  and a green down conversion phosphor  22  in a bottom portion of the stack, with the phosphor  22  down converting the blue light from OLED  20  to green light. The third device  28  contains a red down conversion phosphor  21  disposed between the blue OLED  20  and the green down conversion phosphor  22  that converts the blue light from OLED  20  to red light. In this case, the red light passes unabsorbed through green phosphor  22 , which is transparent to red light. The green down conversion phosphor  22  is left in device  28  for ease of fabrication. Alternatively, the third device  28  includes the blue OLED  20  and a red down conversion phosphor  21  in a bottom portion of the stack, with the red phosphor down converting the blue light from OLED  20  to red light without passing through a green down conversion phosphor layer. In yet another alternative arrangement of the third device  28 , a layer of green down conversion phosphor is positioned between blue OLED  20  and red down conversion phosphor  21 . In this arrangement, the green down conversion phosphor  22  will convert the blue light emitted from OLED  20  to green light, and then red down conversion phosphor  21  converts this green light to red light. This arrangement, however, is not generally preferred because device efficiency tends to decrease with an increased number of down conversion steps. 
     The mesa walls of any of devices  24 ,  27  and  28  can be configured at any acute angle so as to minimize or prevent waveguiding, although 35°-45° relative to the substrate is preferred. The collimating dielectric layer  19  making up the bottom-most portion of devices  24 ,  27  and  28  is angled so that the light that would normally be waveguided into lateral pixels, and contribute to color bleeding and loss of resolution and brightness, is instead directed out of the substrate  37  by reflection off of the mesa side walls and reflectors  47 . This reflection is shown as beams R 2  which emerge from the dielectric layer  19  through the substrate  37  and act to augment beam R. 
     Reflectors  47  may be made of aluminum, silver, Mg/Al and the like. In addition to serving as reflectors, reflectors  47  can be used as interconnects by extending reflectors  47  to connect with metal layers  26  as shown in FIG.  3 . One distinct advantage to using reflectors  47  as interconnects is that such interconnects are positioned between adjoining mesas such that they are hidden from the viewer. The resulting display therefore has none of the dark lines between adjoining pixels as are frequently found in conventional display devices. 
     For simplicity, the OLED device used in the present invention is shown as a single layer in the drawings. As is well-known in the art, however, the layer actually comprises multiple sub-layers if the OLED is not a single-layer polymer, the arrangement of which sub-layers depends on whether the device is DH or SH configuration. 
     If, for example, a DH OLED is used in the present invention, OLED device  20  will consist of an HTL vacuum-deposited or grown on or otherwise deposited onto the surface of an ITO layer. A top ETL sandwiches an EL between the former and the HTL. Each of the HTL, ETL, ITO and organic EL layers are transparent because of their composition and minimal thickness. Each HTL may be 50-1000 Å thick; each EL may be 50-500 Å thick; each ETL may be 50-1000 Å thick; and the ITO layer may be 1000-4000 Å thick. For optimum performance and low voltage operation, each of the organic layers should preferably be kept towards the lower ends of the above ranges. Each device  24 ,  27  and  28  (excluding ITO/metal layers) is preferably close to 500 Å thick. Examples of suitable organic ETL, EL and HTL materials can be found in U.S. Pat. No. 5,294,870. 
     Formed on top of ETL is a low work function (preferably, &lt;4 eV) metal layer  26 M. Suitable candidates for metal layer  26 M include Mg, Mg/Ag, and Li/Al. Deposited on the top of metal layer  26 M is another conductive layer  26 I suitable for forming an electrical contact. Conductive layer  26 I can be made of ITO, Al, Ag or Au, for example. For convenience, the double layer structure of metallic layers  26 M and  26 I is referred to as metal layer  26 . Terminal  26 T is formed on metal layer  26  for electrical connection thereto, and may be made from In, Pt, Au, Ag, and combinations thereof, or any suitable material as is known in the art. 
     If a SH OLED structure is used for providing devices  24 ,  27  and  28 , rather than DH OLED devices, the ETL and EL layers are provided by a single multifunctional layer, such as layer  13 ′, as previously described for the SH of FIG.  1 B. This layer  13 ′ is Al-quinolate or other known materials which can accomplish the multifunctional purpose of layer  13 ′. However, an advantage of DH OLED stacks over SH OLED stacks is that DH OLED stacks generally permit higher efficiencies. 
     Each device  24 ,  27  and  28  emits light through glass substrate  37  in a substantially omnidirectional pattern. The voltages across the OLED&#39;s in each of devices  24 ,  27  and  28  are controlled to provide a desired resultant emission color and brightness for the particular pixel at any instant of time. Obviously, device  24  emits blue light, device  27  emits green light and device  28  emits red light. Moreover, different combinations of devices  24 ,  27  and  28  can be activated to selectively obtain a desired color of light for the respective pixel partly dependent upon the magnitude of current in each of the devices  24 ,  27  and  28 . 
     Devices  24 ,  27  and  28  can be forward biased by batteries  32 ,  31  and  30 , respectively. In FIG. 2A, current flows from the positive terminal of each battery  32 ,  31  and  30 , into the cathode terminal  26 T of its associated device, through the layers of each respective device, and from anode terminals  35 T formed on conductive layers  35  to negative terminals of each battery  32 ,  31 , and  30 . As a result, light is emitted from the OLED layer in each of the devices  24 ,  27  and  28 . Isolation layer  25  prevents the cathode and anode layers from shorting. When the pixels are aggregated into a display, anode and cathode terminals are brought out at an edge of the display, for example. 
     Each device  24 ,  27  and  28  can optionally comprise a layer  36  of low-loss, high refractive index dielectric material, such as TiO 2 , between contact  35  and layer  21 ,  22  and  19 , respectively. Layer  36  is especially preferred when contact  35  is made from ITO, which is a high-loss material such that light from blue OLED layer  20  can be easily waveguided in and absorbed by contact  35 . The refractive indexes for TiO 2  and ITO are approximately 2.6 and 2.2, respectively. Layer  36  therefore substantially eliminates waveguiding and absorption in the ITO, the light emitted from blue OLED layer  20  now being either transmitted through layer  36  or waveguided within layer  36  and reflected by the mesa side walls and reflectors  47 . 
     In the second embodiment of the present invention, the device of FIG. 2A is constructed in an opposite or inverted manner, for providing light emission out of the top of the stack rather than the bottom. In accordance with this second embodiment, as shown in FIG. 2B (not drawn to scale), the collimating action of the “inverted” angle-walled mesa structure suppresses waveguiding along the structure layers. In this embodiment, the mesas are referred to as being “inverted” because the top portion of each mesa is immediately adjacent the substrate such that the light emitted by each mesa is directed away from the substrate. Without the inverted mesa structure of FIG. 2B, waveguiding along the structure layers could lead to inadvertent optical pumping of down converting layers in one pixel by light emitted from a nearby pixel, a phenomenon known as “cross-talk” or color bleeding. 
     In the embodiment shown in FIG. 2B, a layer of dielectric material such as SiOx, SiNx, polyimide, etc. is deposited on substrate  51  and etched to form regions  50 , between which are left flat-bottom pits. Regions  50  allow for the formation of the inverted mesa configuration of devices  24 ′,  27 ′ and  28 ′ by the deposition of the layers comprising these devices. 
     Each of the inverted mesas of devices  24 ′,  27 ′ and  28 ′ include reflective metal contact layer  56 , isolation layer  53 , blue OLED layer  20 , dielectric layer  55 , and red or green phosphors  21  and  22  for inverted mesas devices  28 ′ and  27 ′, respectively. Inverted mesa device  28 ′ can alternatively have a layer of green phosphor positioned either i) between blue OLED layer  20  and red phosphor  21 , or ii) over red phosphor layer  21 . Metal contact layer  56  may be made of aluminum, silver, Mg/Al and the like. In addition to serving as a reflector, metal contact layer  56  is used as an interconnect. One distinct advantage to using metal contact layer  56  as an interconnect is that it is positioned beneath devices  24 ′,  27 ′ and  28 ′ and is thus hidden from the viewer. The resulting display therefore has none of the dark lines between adjoining pixels as are frequently found in conventional display devices. 
     Each inverted mesa further includes transparent contact area  52  with a thin (about 50-200 Å) low work function metal layer  52 A and a thicker (about 500-4000 Å) ITO coating  52 B. In comparison to the first embodiment, the polarities of batteries  30 ,  31 , and  32  are reversed. As a result, the current flowing through devices  24 ′,  27 ′ and  28 ′ is in the opposite direction relative to the embodiment of FIG. 2A, when forward biased for emitting light. 
     The embodiment shown in FIG. 2B is generally capable of higher resolutions than the embodiment shown in FIG.  2 A. This is because the embodiment shown in FIG. 2A can result in a relatively broad light beam emitting from each of devices  24 ,  27  and  28  due to the relatively large distance between the light emitting regions and the substrate surface. In comparison, the light beams emitting from each of the inverted mesa structures of FIG. 2B do not pass through collimating dielectric layers or substrate material. The result is that relatively narrow light beams emit from inverted mesa devices  24 ′,  27 ′ and  28 ′ of FIG. 2B when compared to those light beams emitted from each of the mesa devices shown in FIG.  2 A. 
     In addition to the configuration shown in FIG. 2B in which inverted mesas are formed by virtue of etched dielectric layer to form regions  50 , inverted mesas can be fabricated by forming devices  24 ′,  27 ′ and  28 ′ on a patterned substrate  60  as shown in FIG.  2 C. Patterned substrate  60  has pits therein, each pit having a substantially flat bottom surface and slanted side walls. Each side wall is slanted so as to create an obtuse angle with the bottom surface, although 135°-145° is preferred. The depth of the pit can be relatively shallow, on the order of 1000-3000 Å, and as wide as desired. For example, patterned substrate  60  can be made of Si having the pits therein formed by a standard directional etching process. In addition to the pit structure shown in FIGS. 2B and 2C, which has straight, slanted side walls, other cross-sections are also possible. For example, pits having a semicircular cross-section are contemplated. Additionally, the mesas or inverted mesas of any embodiment of the present invention can be arranged, from a top view, in virtually any shape such as squares, triangles, circles, and hexagons. 
     In any of the embodiments as shown in FIGS. 2A,  2 B or  2 C, there are no materials with smaller refractive indexes relative to and between the emitting layer of the OLED and the phosphor layers, such that all photons emitted by the OLED are absorbed by the phosphor. This results in an increase in the efficiency of blue radiation transport into the red and green phosphors. 
     When used in multicolor applications, each pixel used in the displays of the present invention can emit red, green, and blue light, respectively, either simultaneously or separately. Alternatively, when used in monochrome applications, each pixel emits a single color. In either case, the emitted light is from substantially the entire width of the mesa(s) used in the pixels. In multicolor applications, the addition or subtraction of colors is integrated by the eye causing different colors and hues to be perceived. This is well-known in the field of color vision and display colorimetry. In any of the configurations shown in FIGS. 2A,  2 B and  2 C, the red, green and blue beams of light are substantially coincident. Depending on the size of the display and its intended viewing distance, any combination of the primary colors can be produced from the stack. However, it will appear as one color originating from a single pixel. 
     A method for making multicolor LED&#39;s on a common substrate  37  will now be described, for the embodiment of the invention shown in FIG.  2 A. This method is schematically illustrated in FIGS. 4A-4D, which are not intended to be drawn to scale. The following steps can be used to obtain the multicolor organic device array: 
     1) Depositing a transparent 5-10 μm dielectric layer  19  onto substrate  37 . Dielectric layer  19  should preferably have a refractive index less than or equal to that of substrate  37 . Layer  19  can be SiO x  or Teflon, for example. 
     2) Depositing a green phosphor layer  22 . 
     3) Depositing a thin, etch-stop, dielectric layer  23 , such as SiO x . 
     4) Depositing a red phosphor layer  21 . The device after this step appears as is shown in FIG.  4 A. 
     5) Photolithographic patterning via reactive ion or wet chemical etching to create a two dimensional mesa-structure as shown in FIG.  4 B. 
     6) Patterning and etching via appropriate chemical or reactive ion etch to remove said red phosphor  21  from one third of the mesas. 
     7) Patterning and etching via appropriate chemical or reactive ion etch to remove said green phosphor  22  from the second third of the mesas. 
     8) Depositing a transparent, conductive material such as ITO to make square contacts  35  on the top of said mesas. 
     9) Depositing metals (not shown) and patterning of stripe contacts to ITO to form stripe-contact metal columns. Such patterning can be done by shadow masking, lift-off or chloride reactive-ion etching of, for example, Al. 
     10) Depositing an isolation dielectric  25 , such as SiN x . The device after this step appears as is shown in FIG.  4 C. 
     11) Etching windows in the isolation dielectric via reactive ion or wet etching to obtain contact for blue EL  20 . 
     12) Depositing a blue OLED layer  20  over everything. Layer  20  can be of either SH or DH structure, as previously described. 
     13) Depositing metallization  26 M and  26 I over everything and patterning of row-metals stripe contacts and metal reflectors  47  on the sides of the mesas, as shown in FIG.  4 D. 
     Although the above-listed method can be used to make the embodiment shown in FIG. 2A, other alternative steps are possible. For example, instead of depositing and etching layer  19  to form the mesa base for each of devices  24 ,  27  and  28 , it is possible to directly etch substrate  37  for this same purpose, thus eliminating the need for layer  19 . As another example, the phosphor and OLED layers can be deposited by aligned shadow masks onto pre-etched layer  19  or a pre-etched substrate  37 . 
     A method for making inverted multicolor LED&#39;s on a common substrate  51  will now be described, for the embodiment of the invention shown in FIG.  2 B. This same method can be used to form the embodiment shown in FIG. 2C, with the exception that a patterned substrate  60  is used instead of flat substrate  51  with dielectric regions  50  thereon. This method is schematically illustrated in FIGS. 5A-5E, which are not intended to be drawn to scale. The following steps can be used to arrive at the embodiment shown in FIG.  2 B: 
     1) Depositing a dielectric coating  50  onto substrate  51 , which may be a metal foil, plastic layer, or other suitable substrate material. Layer  50  should be amenable to selective etching, and can be SiO x , SiN x , polyimide or Teflon, for example. 
     2) Etching dielectric coating to leave regions  50  such that flat-bottomed pits are formed therebetween. 
     3) Depositing metallization  56  over everything and metal patterning to create mesa-reflectors and row-metal stripe contacts. 
     4) Depositing an isolation layer  53 , such as SiO 2 . After this step, the device appears as shown in FIG.  5 A. 
     5) Opening windows in said isolation coating for blue EL  20  contacts. 
     6) Depositing a blue OLED layer  20  over everything. Layer  20  can be of either SH or DH structure, as previously described for the embodiment shown in FIG. 2A but inverted in layering. 
     7) Depositing a transparent ITO-contact  52 . 
     8) Patterning said transparent ITO  52  for fabrication of column strip-contacts. 
     9) Depositing a layer  55  of dielectric material, such as SiO x . After this step, the device appears as shown in FIG. 5B 
     10) Depositing a red phosphor layer  21 , resulting in a configuration as shown in FIG.  5 C. 
     11) Patterning and etching to remove said red phosphor  21  from a first two-thirds of the mesas. 
     12) Depositing a green phosphor layer  22 , resulting in a configuration as shown in FIG.  5 D. 
     13) Patterning and etching to remove said green phosphor  22  from a second two-thirds of the mesas, resulting in a configuration as shown in FIG.  5 E. 
     It should be recognized that although FIGS. 2A,  2 B and  2 C are directed to multicolor displays, the mesa and inverted mesa configurations of these figures can be applied to monochromatic displays in which each pixel includes a single mesa or inverted mesa structure which is only able to emit a single color. 
     In a further embodiment of the present invention, blue, green and red OLED are arranged in a stacked configuration  100  as shown in FIG.  2 D. Such a stacked arrangement is described in co-pending U.S. application Ser. No. 08/354,674, filed Dec. 13, 1994, and PCT International Application Publication No. WO 96/19792, filed Dec. 6, 1995, the disclosures of which are incorporated herein by reference. The present invention makes use of this stacked arrangement in conjunction with a mesa structure to minimize waveguiding and maximize efficiency as previously discussed. In the embodiment shown in FIG. 2D, blue  20 , green  110  and red  111  OLED&#39;s are stacked one upon the other, with each OLED separated one from the other by a transparent conductive layer  26  to enable each device to receive a separate bias potential to emit light through the stack. Each OLED  20 ,  110  and  111  may be of either SH or DH type, as previously described. As shown in FIG. 2D, the stacked arrangement of OLED&#39;s  20 ,  110  and  111  is positioned on conductive layer  112 , dielectric layer  19  and transparent substrate  37 . 
     Each conductive layer  26  comprises a low work function (preferably, &lt;4 eV) metal layer  26 M, such as Mg, Mg/Ag, and Li/Al, and an additional conductive layer  26 I suitable for forming an electrical contact. Of course, all conductive layers between OLED&#39;s  20 ,  110  and  111 , and between substrate  37  and OLED  20 , must be substantially transparent. Conductive layer  26  on OLED  111 , however, being at the top of the stack, need not be transparent and is preferably reflective. Terminal  26 T is formed on metal layer  26  for electrical connection thereto, and may be made from In, Pt, Au, Ag, and combinations thereof, or any suitable material as is known in the art. 
     The stacked configuration  100  may also comprise a layer  36  of low-loss, high refractive index dielectric material, such as TiO 2 , between conductive layer  112  and dielectric layer  19 . Layer  36  is especially preferred when conductive layer  112  is made from ITO, which is a high-loss material such that light from OLED layers  20 ,  110  and  111  can be easily waveguided in and absorbed by conductive layer  112 . Layer  36  substantially eliminates waveguiding and absorption in the ITO, the light emitted from the OLED&#39;s  20 ,  110  and  111  now being substantially transmitted through layer  36 . Moreover, layer  36  can have slanted side walls to reflected any waveguided light towards substrate  37 . 
     The fabrication of a stacked OLED pixel  100  may be accomplished by either a shadow masking process or a dry etching process, as are known in the art. For example, stacked OLED pixel  100  can be made from the following steps, as schematically shown in FIGS.  6 A- 6 D: 
     1) Depositing a transparent 5-10 μm dielectric layer  19  onto transparent substrate  37 . Dielectric layer  19  should have a refractive index less than or equal to that of substrate  37 . Layer  19  can be SiO x  or Teflon, for example. 
     2) Depositing a layer of transparent conductive material  112 , such as ITO. After this step, the device appears as shown in FIG.  6 A. 
     3) Etching to form a mesa structure, as shown in FIG.  6 B. 
     4) Depositing a blue OLED layer  20 . Layer  20  can be of either SH or DH structure, as previously described. 
     5) Depositing conductive layers  26 M and  26 I. 
     6) Depositing a green OLED layer  110 . Layer  110  can be of either SH or DH structure, as previously described. 
     7) Depositing conductive layers  26 M and  26 I. 
     8) Depositing a red OLED layer  111 . Layer  111  can be of either SH or DH structure, as previously described. 
     9) Depositing conductive layers  26 M and  26 I. After this step, the device appears as shown in FIG.  6 C. 
     10) Depositing reflectors  47  on mesa side walls and terminals  26 T on each of layers  26 I. The final device appears as shown in FIG.  6 D. 
     Although FIG. 2D shows the use of a stacked OLED configuration in conjunction with a mesa structure, the present invention also contemplates the use of a stacked OLED within an inverted mesa structure. Such an inverted mesa structure can be formed by depositing the necessary OLED and conductive layers onto, for example, a substrate having an etched dielectric layer thereon or a substrate having etched pits therein, as previously described for the embodiments shown in FIGS. 2B and 2C, respectively. To accomplish an inverted mesa, stacked OLED device, the layering sequence as described above for the embodiment shown in FIG. 2D would be reversed. 
     The deposition techniques for any of the above-listed methods are well-known in the art. For example, the preferred method of depositing the OLED layers is by thermal evaporation (or spin coating if a single-layer polymer LED is used); the preferred method of depositing metal layers is by thermal or electron-beam evaporation; the preferred method of depositing ITO is by electron-beam evaporation or sputtering; the preferred method of depositing the phosphor layers is by sputtering; and the preferred method of depositing dielectrics is by plasma-enhanced chemical vapor deposition or electron-beam evaporation. 
     OLED layers  20 ,  110  or  111  of any embodiment of the present invention can be made from any suitable light-emissive organic compounds such as, for example, trivalent metal quinolate complexes, Schiff base divalent metal complexes, metal acetylacetonate complexes, metal bidentate ligand complexes, bisphosphonates, metal maleontriledithiolate complexes, molecular charge transfer complexes, aromatic and heterocyclic polymers and rare earth mixed chelates. 
     The metal bidentate complexes which may be used for layer  20  have the formula MDL 4   2  wherein M is selected from trivalent metals of Groups 3-13 of the Periodic Table and Lanthanides. The preferred metal ions are Al +3 , Ga −3 , In +3  and Sc +3 . D is a bidentate ligand such as 2-picolylketones, 2-quinaldylkentones and 2-(o-phenoxy)pyridine ketones. The preferred groups for L 4  include acetylacetonate, compounds of the formula OR 3 R wherein R 3  is selected from Si and C, and R is selected from hydrogen, substituted and unsubstituted alkyl, aryl and heterocyclic groups; 3,5-di(t-bu)phenol; 2,6-di(t-bu)phenol; 2,6-di(tbu)cresol; and H 2 Bpz 2 . By way of example, the wavelength resulting from measurement of photoluminescence in the solid state of aluminum (picolymethylketone)bis [2,6-di(tbu)phenoxide] is 420 nm. The cresol derivative of this compound also measured 420 nm. Aluminum (picolylmethylketone)bis (OsiPh 3 ) and scandium (4-methoxy-picolylmethylketone)bis (acetylacetonate) each measured 433 nm, while aluminum [2-(O-phenoxy)pyridine]bis [2,6-di(t-bu) phenoxide] measured 450 nm. 
     Examples of green OLED emissive materials include tin (iv) metal complexes, such as those having the formula SnL 1   2 L 2   2  where L 1  is selected from salicylaldehydes, salicyclic acid or quinolates (e.g. 8-hydroxyquinoline). L 2  can be substituted and unsubstituted alkyl, aryl and heterocyclic groups. When L 1  is a quinolate and L 2  is a phenyl, for example, the tin (iv) metal complex will have an emission wavelength of 504 nm. 
     Examples of red OLED emissive materials include divalent metal maleonitriledithiolate (“mnt”) complexes, such as those described by C. E. Johnson et al. in “Luminescent Iridium(I), Rhodium(I), and Platinum(II) Dithiolate Complexes,” 105  Journal of the American Chemical Society  1795 (1983). For example, the mnt [Pt (Pph 3 ) 2 ] has a characteristic wavelength emission of 652 nm. 
     Additional OLED materials are known in the art (see, e.g., U.S. Pat. No. 5,294,870 to Tang et al., entitled “Organic Electroluminescent Multicolor Image Display Device”; Hosokawa et al., “Highly efficient blue electroluminescence from a distyrylarylene emitting layer with a new dopant,” 67  Applied Physics Letters  3853-55 (December 1995); Adachi et al., “Blue light-emitting organic electroluminescent devices,” 56  Applied Physics Letters  799-801 (February 1990); Burrows et al., “Color-Tunable Organic Light Emitting Devices,” 69  Applied Physics Letters  2959-61 (November 1996)). The entire disclosures of these references are hereby incorporated by reference. Distyrylarylene derivatives such as those described in Hosokawa et al. are a preferred class of compounds. Other preferred OLED&#39;s are described in the copending applications discussed below. 
     The red and green emitting fluorescent media used in the present invention are well-known in the art. U.S. Pat. Nos. 4,769,292 and 5,294,870, the disclosures of which are hereby incorporated by reference, are illustrative. These fluorescent dyes can be dissolved in matrix polymer such as polymethylmethacrylate and many of the suitable dyes were originally developed for plastic lasers. Examples of red fluorescent dyes are 4-dicyanomethylene-4H-pyrans and 4-dicyanomethylene-4H-thiopyrans. Examples of green fluorescent dyes include the polymethine dyes such as cyanines, merocyanines and tri-, tetra, and polynuclear cyanines and merocyanines, oxonols, hemioxanols, styryls, merostyryls and streptocyanines. 
     As is well-known in the art, a complete display comprises a plurality of pixels. The pixels are arranged as a XY matrix to cover the entire surface area of a given display. Instead of having fixed bias means as batteries  30 ,  31  and  32 , each of the lines of terminals for the device structures can be brought out at an edge of the display, for example, and coupled to suitable horizontal and vertical scan processors, all under control of a display generator which may be a TV unit. Accordingly, each matrix of LED&#39;s has at least two axes (x,y), and each device is at the intersection of at least two of the axes. Also, the x-axis may represent a horizontal axis, and the y-axis a vertical axis. The displays of the present invention may include passive or active matrix driving as is well-known in the art. 
     The devices of the present invention can be used to provide a low cost, high resolution, high brightness, monochromatic or multicolor, flat panel display of any size. This widens the scope of this invention to include displays as small as a few millimeters to as large as the size of a building. The images created on the display could be text or illustrations in full color, in any resolution depending on the size of the individual LED&#39;s. Display devices of the present invention are therefore appropriate for an extremely wide variety of applications including billboards and signs, computer monitors, telecommunications devices such as telephones, televisions, large area wall screens, theater screens and stadium screens. Embodiments of the present invention in which the emitted light is directed in a direction away from the substrate are particularly useful for xerographic applications, as such embodiments allow for close positioning to print paper without the use of lenses. 
     The subject invention as disclosed herein may be used in conjunction with co-pending applications: “High Reliability, High Efficiency, Integratable Organic Light Emitting Devices and Methods of Producing Same”, Ser. No. 08/774,119 (filed Dec. 23, 1996); “Novel Materials for Multicolor LED&#39;s”, provisional application Ser. No. 60/072,095 (filed Dec. 23, 1996); “Electron Transporting and Light Emitting Layers Based on Organic Free Radicals”, Ser. No. 08/774,120 (filed Dec. 23, 1996); “Multicolor Display Devices”, Ser. No. 08/772,333 (filed Dec. 23, 1996); “Red-Emitting Organic Light Emitting Devices (LED&#39;s)”, Ser. No. 08/774,087 (filed Dec. 23, 1996); “Driving Circuit For Stacked Organic Light Emitting Devices”, Ser. No. 08/792,050 (filed the same day as the application herein); “High Efficiency Organic Light Emitting Device Structures”, Ser. No. 08/772,332 (filed Dec. 23, 1996); “Vacuum Deposited, Non-Polymeric Flexible Organic Light Emitting Devices”, Ser. No. 08/789,319 (filed Jan. 23, 1997); and “Stacked Organic Light Emitting Devices”, Ser. No. 08/792,046 (filed the same day as the application herein); each co-pending application being incorporated herein by reference in its entirety. The subject invention may also be used in conjunction with the subject matter of each of co-pending U.S. patent application Ser. Nos. 08/354,674, 08/613,207, 08/632,322 and 08/693,359 and provisional patent application Ser. Nos. 60/010,013, 60/024,001 and 60/025,501, each of which is also incorporated herein by reference in its entirety. 
     Those with skill in the art may recognize various modifications to the embodiments of the invention described and illustrated herein. Such modifications are meant to be covered by the spirit and scope of the appended claims. For example, multilayer stacks of alternate high and low refractive index layers can be formed in regions of collimating dielectric layer  19  at the interface with the underlying substrate  37  for enhancing the efficiency of layer  19 . Such MLS stacks are taught by H. A. MacLeod in his book  Thin Film Optical Filters  94-110 (1969), incorporated herein by reference.
A multicolor organic light emitting display device employs angle-walled blue, green and red emitting mesas, with optional metal reflectors on the angled walls, in a plurality of pixels. The angle-walled mesas, which resemble truncated pyramids, direct light out of the mesa by reflection from the mesa side walls or by mirror reflection. The device of the present invention reduces waveguiding, thus simultaneously increasing both display brightness and resolution.