Abstract:
An OLED device including an array of light emitting pixels, each pixel including subpixels having organic layers including at least one emissive layer that produces light and spaced electrodes, and wherein there are at least three gamut subpixels that produce colors which define a color gamut and at least one subpixel that produces light within the color gamut produced by the gamut subpixels; and wherein at least one of the gamut subpixels includes a reflector and a semitransparent reflector which function to form a microcavity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly-assigned U.S. Publication No. 2004/0140757, published Jul. 22, 2004, entitled “Microcavity OLED Devices” by Tyan et al, U.S. Publication No. 2004/0149984, published Aug. 5, 2004, entitled “Color OLED Display with Improved Emission” by Tyan et al, U.S. Pat. No. 6,861,800, issued Mar. 1, 2005, entitled “Tuned Microcavity OLED Display” by Tyan et al, U.S. patent application Ser. No. 10/412,534, filed Apr. 11, 2003, entitled “A Color OLED Display With Improved Power Efficiency” by Arnold et al and U.S. Pat. No. 6,919,681 issued Jul. 19, 2005, entitled “A Color OLED Display With Improved Power Efficiency” by Cok et al, the disclosures of which are incorporated herein. 
     FIELD OF INVENTION 
     This invention relates to microcavity organic electroluminescent (EL) devices. 
     BACKGROUND OF THE INVENTION 
     Full color organic electroluminescent (EL), also known as organic light-emitting devices or (OLED), have been recently been demonstrated as a new type of flat panel display. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic EL medium sandwiched between these electrodes to support charge recombination that yields emission of light. An example of an organic EL device is described in commonly-assigned U.S. Pat. No. 4,356,429. In order to construct a pixilated display device such as is useful, for example, as a television, computer monitor, cell phone display or digital camera display, individual organic EL elements may be arranged as an array of pixels in a matrix pattern. To produce a multi-color display, the pixels are further arranged into subpixels, with each subpixel emitting a different color. This matrix of pixels can be electrically driven using either a simple passive matrix or an active matrix driving scheme. In a passive matrix, the organic EL layers are sandwiched between two sets of orthogonal electrodes arranged in rows and columns. An examples of a passive matrix driven organic EL devices is disclosed in commonly-assigned U.S. Pat. No. 5,276,380. In an active matrix configuration, each pixel is driven by multiple circuit elements such as transistors, capacitors, and signal lines. Examples of such active matrix organic EL devices are provided in U.S. Pat. No. 5,550,066 (commonly-assigned); U.S. Pat. Nos. 6,281,634 and 6,456,013. 
     Full color OLED devices are also known in the art. Typical full color OLED devices are constructed of pixels having three subpixels that are red, green, and blue in color. Such an arrangement is known as an RGB design. An example of an RGB design is disclosed in U.S. Pat. No. 6,281,634. Full color organic electroluminescent (EL) devices, have also recently been described that are constructed of pixels having four subpixels that are red, green, blue, and white in color. Such an arrangement is known as an RGBW design. An example of an RGBW device is disclosed in commonly-assigned U.S. Patent Publication 2002/0186214A1. In an RGBW device, a high efficiency white emitting pixel is used to display a portion of the digital image content. This results in improved power consumption relative to an RGB constructed of similar OLED materials. However, the red, green, and blue subpixels do not improved in efficiency in this design. Therefore, no power savings are achieved for displaying any portion of the digital image content that is pure red, pure blue, or pure green in color such as, for example, icons and toolbars commonly used in many personal digital assistant (PDA), cell phone, or digital camera applications. Furthermore, the addition of the fourth subpixel results in all red, green, and blue subpixels having to be made small to fit the same total number of subpixels per unit area and achieve the same device pixel resolution as a comparable RGB device. This results in the current density per unit area of the associated red, green, and blue subpixels having to be increased in order to display the same brightness for this pure red, pure blue, or pure green content. It is known that OLED devices degrade, or become less efficient, more quickly as current density is increased. For an RGBW display, this may result in content which is pure red, pure green, or pure blue, and appearing frequently such as, for example, icons and toolbars causing image burn-in quicker than in an equivalent RGB display and therefore overall device lifetime is reduced. 
     Therefore, an RGBW device with improved efficiency and lifetime for the red, green, and blue subpixels is desired. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to make use of an OLED display device that has pixels having gamut subpixels and within gamut subpixels that can substantially improve the efficiency of the gamut subpixels. 
     This object is achieved by an OLED device comprising: 
     (a) an array of light emitting pixels, each pixel including subpixels having organic layers including at least one emissive layer that produces light and spaced electrodes, and wherein there are at least three gamut subpixels that produce colors which define a color gamut and at least one subpixel that produces light within the color gamut produced by the gamut subpixels; and 
     (b) wherein at least one of the gamut subpixels includes a reflector and a semitransparent reflector which function to form a microcavity. 
     By constructing a gamut subpixels with a microcavity structure the present invention provides improved efficiency and lifetime. It is a further advantage that such a device can be constructed using a common OLED organic layers for all subpixels which does not require precision patterning to from the subpixels. It is a further advantage that such a device can be constructed without the need for color filter elements, which thereby lowers cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1   a–d  depict RGBW pixel pattern layouts that can be used in accordance with the present invention; 
         FIG. 2  depicts a cross sectional view of a pixel according to an embodiment of the present invention; 
         FIG. 3  depicts a cross sectional view of a pixel according to an another embodiment of the present invention; and 
         FIG. 4  depicts a cross sectional view of a pixel according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     RGBW displays are one example of a type of display that utilizes within-gamut emission to improve power consumption. Such display devices are capable of displaying color images through the use of pixels having at least four different color subpixels. At least three of the subpixels are gamut subpixels that emit different colors, which define the color gamut of the display. For example, gamut subpixels might emit light that is either Red, Green, or Blue in color. By illuminating two or more of the gamut subpixels to varying intensities, other colors can be generated. These new colors are within-gamut colors. Such a display device also has at least one additional subpixel that is a within-gamut subpixel, which emits a within-gamut color light such as white. The term white is used in the present invention to indicate any light emission which is perceived as approximately white to a viewer. These within-gamut subpixels generally are more efficient than the gamut subpixels.  FIG. 1  illustrates example pixel configurations for an RGBW device. 
       FIG. 1   a  shows a stripe pattern configuration of an RGBW device pixel  20 . Pixel  20  includes gamut subpixels  21   a ,  21   b , and  21   c  as well as within gamut pixel  21   d . These subpixels can have, for example, the colors Red (R), Green (G), Blue (B), and White (W) respectively. 
       FIG. 1   b  shows a quad pattern configuration of an RGBW device pixel  20 . Pixel  20  includes gamut subpixels  21   a ,  21   b , and  21   c  as well as within gamut pixel  21   d . These subpixels can have, for example, the colors Red (R), Green (G), Blue (B), and White (W) respectively. 
       FIG. 1   c shows another pattern configuration of an RGBW device pixel  20 . Pixel  20  includes gamut subpixels  21   a ,  21   b , and  21   c  as well as within gamut pixel  21   d . These subpixels can have, for example, the colors Red (R), Green (G), Blue (B), and White (W) respectively. 
       FIG. 1   d  shows another pattern configuration of an RGBW device pixel  20 . Pixel  20  includes gamut subpixels  21   a ,  21   b , and  21   c  as well as within gamut pixel  21   d . These subpixels can have, for example, the colors Red (R), Green (G), Blue (B), and White (W) respectively. 
     Other patterns of RGBW can be applied to the present invention. Furthermore, patterns with more than 4 subpixels can also be applied. While in the above mentioned examples, the subpixels are shown to be arranged in a certain order, the subpixels can be arranged in other embodiments having different orders. Furthermore, while the subpixels are shown as being all the same size and shape, those skilled in the art will recognize that other embodiments can have subpixels with differing sizes and shapes. 
     This type of display is more efficient than conventional OLED displays because the with-gamut subpixels tend to have higher efficiency than at least one of the gamut subpixels. Typically, the within gamut subpixel is more efficient than all of the gamut subpixels. Each subpixel can be fabricated using a separate OLED material designed to emit a different color light. However, a preferred configuration uses a common broadband or white emitting OLED material common to all subpixels. The use of a broadband or white emitting OLED material eliminates the need to pattern the OLED materials for each pixel. In this case, color filters could be used for some of the subpixels to convert a broadband or white emission to individual colors. For example, red, green, and blue color filters could be used in the gamut subpixels of RGBW devices to form red, green, and blue colors and the within-gamut subpixel is left unfiltered to emit white. Because the within-gamut subpixel does not have a filter, it is more efficient than the gamut subpixels. 
       FIG. 2  shows the cross section of one pixel of a device according to the present invention having, for example, a stripe pattern with three gamut subpixels  21   a ,  21   b , and  21   c  and one within-gamut subpixel  21   d . These subpixels emit light  30   a , 30   b , and  30   c,  which have gamut colors and light  30   d , which has a within-gamut color. The device shown in  FIG. 2  is shown to be active matrix with active matrix circuitry  110 , however, alternate embodiments which are passive matrix which do not have active matrix circuitry can be applied to the present invention  FIG. 2  also shows a configuration that is bottom emitting, that is where light ( 30   a ,  30   b ,  30   c , and  30   d ) is extracted in the direction of the substrate. Gamut subpixels  21   a ,  21   b , and  21   c  are formed using a microcavity structure. Microcavity structures are formed using a reflector and semi-transparent reflector. The organic EL medium is formed between the reflector and the semi-transparent reflector. The layers between the reflector and the semi-transparent reflector create an optical cavity, which is then adjusted in thickness and refractive index to resonate at a desired wavelength. Examples of microcavity structures are shown in (U.S. Pat. Nos. 6,406,801 B1; 5,780,174 A1, and JP 11288786). Preferred materials for the reflector that are highly reflective include Ag, Al, Au or alloys composed of one or more of these materials. The semi-transparent reflector is partially reflective and partially transmissive. Preferred materials for the semi-transparent reflector include Ag, Au or alloys composed of one or both of these materials. These materials have a thickness selected to cause them to be semi-transparent, that is partially transmitting and partially reflective. This thickness may be in the range of, for example, between 5 nm and 50 nm, and is more preferably between 15 nm and 30 nm. If conductive material are used to form the reflector or the semi-transparent reflector, then the reflector, the semi-transparent reflector, or both, can also serve the function of an electrode, either the anode or the cathode, for the organic EL medium. An alternate semi-transparent reflector structure composed of a quarter wave stack (QWS) of transparent materials with alternating high and low refractive indexes is also known and can be applied to the present invention by someone skilled in the art In a bottom emission configuration, such as shown, where the light is viewed through the substrate, the semi-transparent reflector is located between the organic EL layers and the substrate and the reflector is located above the substrate, the semi-transparent reflector, and the organic EL layers. Alternately, in a top emission configuration, that is where the light is viewed in the direction opposite the substrate, the reflector is located between the organic EL layers and the substrate and the semi-transparent reflector is located above the substrate, the reflector, and the organic EL layers. 
     The active matrix circuitry  110  is formed above a substrate  100 . Active matrix circuitry  110  includes a first thin film transistor (TFT) comprised of a semiconductor active layer  111 , a gate dielectric  112 , a gate conductor  113 , a first insulating layer  114 , and a second insulating layer  117 . Active matrix circuitry  110  further includes one signal line  116  that carries the luminance signal and one power line  115  to supply power to the transistor. Methods of fabricating the TFT Circuitry are well known in the art. While only a single transistor, signal line, and power line are shown for each subpixel, typically, each subpixel also has a second transistor (not shown) as well as a capacitor (not shown) and additional select lines (not shown). Many types of circuits having different numbers and configurations of circuit components are known in the art and it is understood that a wide variety of these circuits will work with this invention. Examples of active matrix configurations include U.S. Pat. Nos. 5,550,066; 6,281,634, and 6,501,466. While, the TFTs shown are fabricated with a thin film semiconductor active layer  111 , it is understood that with semiconducting substrates, the substrate may actually serve this function. A top gate structure, that is where the gate conductor  113  and the gate dielectric  112  are above the semiconductor active layer  111 , is shown, however, it is also known in the art that TFTs with an inverse structure known as bottom gate can be used to drive organic EL devices. 
     Above the active matrix circuitry, semi-transparent reflectors  120   a ,  120   b , and  120   c  are formed in gamut subpixel  21   a ,  21   b , and  21   c  respectively. These semi-transparent reflectors  120   a ,  120   b , and  120   c  can be formed of a reflective metal such as Ag, Au, Al, and alloys thereof, which are made to be thin so as to also be semi-transparent. It is possible, although not required that the semi-transparent reflector also serves as one of the electrodes for the organic EL media. 
     The within-gamut subpixel  21   d  does not have a semi-transparent reflector but instead has only a transparent electrode  130 . The transparent electrode  130  is typically constructed of a metal oxide such as, but not limited to, indium-tin oxide (ITO), zinc-tin oxide (ZTO), tin-oxide(SnOx), indium oxide (InOx), molybdnum oxide (MoOx), tellurium oxide (TeOx), antimony oxide (SbOx), and zinc oxide (ZnOx). Transparent electrode  130  is also electrically connected down to the active matrix components either directly as shown or with an intermediate conductor. The use of a transparent electrode without a semi-transparent reflector results in the within-gamut subpixel not having a microcavity structure. 
     For gamut subpixel  21   a , transparent cavity-spacer layer  140   a  is formed above semi-transparent reflector  120   a . The transparent cavity-spacer layer  140   a  can be constructed of a metal oxide such as, but not limited to, indium-tin oxide (ITO), zinc-tin oxide (ZTO), tin-oxide(SnOx), indium oxide (InOx), molybdnum oxide (MoOx), tellurium oxide (TeOx), antimony oxide (SbOx), and zinc oxide (ZnOx). In this case the transparent cavity-spacer layer  140   a  also serves as an electrode for the organic EL media  210  so that transparent cavity-spacer layer  140   a  must be electrically connected down to the active matrix components. This can be achieved either directly or if the semi-transparent reflector  120   a  is of a conducting material such as Ag, Al, Au or alloys thereof, through the semi-transparent reflector  120   a  as shown or with other intermediate conductors. If transparent cavity-spacer layer  140   a  is not conductive, then the semi-transparent reflector  120   a  can serve as the electrode for the organic EL media  210 , and would therefore be connected down to the active matrix circuitry  110 . Alternately, the transparent electrode  130 , transparent cavity spacer layer  140   a , and semi-transparent reflector  120   a  could be formed of three (or more) different layers in which case the transparent electrode would be in electrical contact with the active matrix circuitry and the cavity-spacer layer could be between the transparent electrode and the semi-transparent reflector. The thickness and index of refraction of transparent cavity-spacer layer  140   a  is optimized in conjunction with the thickness and index of refraction of the organic EL media  210  in order to tune the cavity to resonate at the wavelength desired for the color of light for subpixel  21   a , for example red. Tuning thickness and refractive index to achieve the desired microcavity emission is well known in the art. 
     Gamut subpixel  21   b  is similarly constructed using cavity-spacer layer  140   b  above semi-transparent reflector  120   b . In this case, the thickness and index of refraction of cavity-spacer layer  140   b  is optimized in conjunction with the thickness and index of refraction of the organic EL media  210  in order to tune the cavity to resonate at a different wavelength desired for the color of light for subpixel  21   b , for example green. 
     Gamut subpixel  21   c  is shown here as not having a cavity-spacer layer. In this case, only the thickness and index of refraction of the organic EL media  210  is optimized in order to tune the cavity to resonate at a different wavelength desired for the color of light for subpixel  21   c , for example blue. If the organic EL media  210  is common to all subpixels, as shown, then organic EL media  210  is optimized only for this gamut subpixel, and the other gamut subpixels are separately tuned using their respective cavity-spacer layers. This configuration minimizes the number of different cavity-spacer layers which need to be deposited and patterned. However, in an alternate embodiment, all gamut subpixels could include a cavity-spacer layer with each separately optimized to tune to the desired colors. Use of these above mentioned cavity spacer layer is preferred in order to allow the organic EL media  210  to be deposited without patterning for each subpixel. However, in an alternate embodiment, one or more of the organic layers of the organic EL media can be pattered and separately adjusted in thickness or refractive index to tune the microcavity of each of the gamut subpixels. In this configuration, the cavity space layer can be used or eliminated. However, this requires precision patterning of at least one of the organic layers. 
     As described above, in order to minimize the number of deposition steps and patterning steps, one of the gamut subpixels may have no cavity-spacer layer. Another preferred method to reduce the number of deposition and patterning steps is to use the same material, thickness, and refractive index to form one of the cavity-spacer layers, for example, cavity-spacer layer  140   b  and the transparent electrodes of the non-gamut subpixels, for example transparent electrode  130 . 
     An interpixel dielectric layer  160 , as such as is described in U.S. Pat. No. 6,246,179, is preferably used to cover the edges of the transparent electrodes to prevent shorts or strong electric fields in this area. The interpixel dielectric layer  160  can also cover the cavity spacer as shown if the cavity spacer is conductive or forms part of the electrode. While use of the interpixel dielectric layer  160  is preferred, it is not required for successful implementation of the present invention. 
     Each of the subpixels further includes organic layers forming an organic EL media  210 . There are numerous configurations of the organic EL media  210  layers wherein the present invention can be successfully practiced. Examples of organic EL media layers that emit broadband or white light are described, for example, in commonly-assigned EP 1 187 235, U.S. Patent Publication No. 20020025419 (commonly-assigned), EP 1 182 244, U.S. Pat. No. 5,683,823 (commonly-assigned); U.S. Pat. Nos. 5,503,910; 5,405,709 (commonly-assigned), and U.S. Pat. No. 5,283,182. As shown in commonly-assigned EP 1187235A2, a white emitting organic EL media can be achieved by the inclusion of the following layers; a hole injecting layer  211 , a hole transporting layer  212  that is disposed over the hole injecting layer  211  and is doped with a rubrene compound for emitting light in the yellow region of the spectrum, a light emitting layer  213  doped with a blue light-emitting compound disposed over the hole transporting layer  212 , and an electron transporting layer  214  disposed over the light emitting layer  213 . Alternate embodiments where one or more different organic EL media materials are used for different subpixels can also be applied to the present invention. 
     Above the organic EL media  210 , the reflector  220  is formed. The reflector  220  can be formed of materials such as Al, Ag, Au, or alloys thereof. The reflector  220  can also serve as the second electrode for the organic EL media  210 . 
     While the transparent cavity spacers are shown here as being between the semi-transparent reflectors and the organic EL media, in alternate embodiments, the cavity step spacers can be formed instead between the organic EL media and the reflector. 
     In another alternate embodiment where one or more of the layers of the organic EL media  210  is not common to all subpixels but is instead individually patterned for each subpixel, then the cavity-spacer layers can be eliminated and the microcavity for the gamut subpixels can be tuned by tuning the thickness, refractive index, or both of one or more of the layers forming the organic EL media  210  separately for each gamut subpixel. In this case, if the organic EL media  210  is designed to emit broadband light, it may be preferable to use the same thickness for one or more of the organic EL media  210  layers for the within-gamut subpixels and one or more of the gamut subpixels to minimize the number of deposition steps. 
       FIG. 3  shows a cross section of a further embodiment of the present invention, which is top emitting, that is where light ( 30   a ,  30   b ,  30   c , and  30   d ) is extracted in the direction away from the substrate. In order to realize this top emitting configuration, reflectors  150   a ,  150   b ,  150   c , and  150   d  are located between the organic EL media  210  and the substrate  100 . These reflectors  150   a ,  150   b ,  150   c , and  150   d  can be formed of materials such as Ag, Au, Al, or alloys thereof. These reflectors  150   a ,  150   b ,  150   c , and  150   d  can also serve as an electrode for the organic EL media  210  as shown in which case it must be connected down to the active matrix circuitry. In this embodiment, the semi-transparent reflector  230  must be formed above organic EL media  210  forth gamut subpixels  21   a ,  21   b , and  21   c . However, the semi-transparent reflector  230  must patterned so as to not be present in the within-gamut subpixel  21   d . A transparent electrode  240  must be used over the within-gamut subpixel  21   d . The transparent electrode can also be present over the other subpixels as shown to reduce the need for patterning, although this is not required. The transparent electrode  240  could be constructed of a metal oxide such as, but not limited to, indium-tin oxide (ITO), zinc-tin oxide (ZTO), tin-oxide(SnOx), indium oxide (InOx), molybdnum oxide (MoOx), tellurium oxide (TeOx), antimony oxide (SbOx), and zinc oxide (ZnOx). 
     While in  FIG. 3 , the reflectors ( 150   a ,  150   b ,  150   c , and  150   d ) form one electrode for the organic EL media  210 , in an alternate embodiment, a separate electrode could be formed over the reflector and below the organic EL media  210 , which then becomes part of the microcavity cavity for the gamut subpixels  21   a ,  21   b , and  21   c . This electrode could be constructed of a metal oxide such as, but not limited to, indium-tin oxide (ITO), zinc-tin oxide (ZTO), tin-oxide(SnOx), indium oxide (InOx), molybdnum oxide (MoOx), tellurium oxide (TeOx), antimony oxide (SbOx), and zinc oxide (ZnOx). 
     While the transparent electrode  240  is shown in  FIG. 3  as being above the semi-transparent reflector  230 , in another alternate embodiment, it could be located between the semi-transparent reflector  230  and the organic EL media  210 . In this case, the transparent electrode  240  would form one of the electrodes for the organic EL media  210  for all the subpixels and become part of the microcavity cavity. 
     The embodiments described above illustrate examples where the efficiency and lifetime of the all gamut subpixels have been improved by using microcavity while still maintaining the ability to generate broadband or white emission from the within-gamut subpixels. However, other embodiments are possible whereby only some of the gamut subpixels are improved by using a microcavity structure. That is, some of the gamut subpixels are not constructed as to form a microcavity. An example of such a configuration is shown if  FIG. 4  which has two microcavity gamut subpixels  22   b  and  22   c , one non-microcavity gamut subpixel  22   a  and a within-gamut subpixel  22   d . In this case, non-microcavity gamut subpixel  22   a , is formed using a reflector  220  and a transparent electrode  130   a . The transparent electrode can be of the same material and thickness as used for the transparent electrode  130   d  of the within-gamut subpixel  22   d . If the organic EL media  210  used is white or broadband emitting, then a color filter  301  may be used to achieve the desired gamut color for these subpixels. The use of color filters to convert broadband emission to narrow band emissions of a specific color is known in the art. As long as at least one of the gamut subpixels is constructed as a microcavity, then some improvement in lifetime and efficiency will be realized according to the present invention. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     Parts List 
     
         
           20  Pixel 
           21   a  Gamut Subpixel 
           21   b  Gamut Subpixel 
           21   c  Gamut Subpixel 
           21   d  Within-Gamut Subpixel 
           22   a  Non-Microcavity Gamut Subpixel 
           22   b  Microcavity Gamut Subpixel 
           22   c  Microcavity Gamut Subpixel 
           22   d  Within-Gamut Subpixel 
           30   a  Light 
           30   b  Light 
           30   c  Light 
           30   d  Light 
           100  Substrate 
           110  Active Matrix Circuitry 
           111  Semiconductor Active Layer 
           112  Gate Dielectric 
           113  Gate Conductor 
           114  First Insulating Layer 
           115  Power Line 
           116  Signal Line 
           117  Second Insulating Layer 
           120   a  Semi-Transparent Reflector 
           120   b  Semi-Transparent Reflector 
           120   c  Semi-Transparent Reflector 
           130  Transparent Electrode 
           130   a  Transparent Electrode 
           130   d  Transparent Electrode 
           140   a  Transparent Cavity-Spacer Layer
 
Parts List Cont&#39;d
 
           140   b  Transparent Cavity-Spacer Layer 
           150   a  Reflector 
           150   b  Reflector 
           150   c  Reflector 
           150   d  Reflector 
           160  Interpixel Dielectric 
           210  Hole Injecting Layer 
           212  Hole Transporting Layer 
           213  Light Emitting Layer 
           214  Electron Transporting Layer 
           220  Reflector 
           230  Semi-Transparent Reflector 
           240  Transparent Electrode 
           310  Color Filter