Patent Publication Number: US-9424772-B2

Title: High resolution low power consumption OLED display with extended lifetime

Description:
PRIORITY 
     This application is a non-provisional of U.S. Patent Application Ser. No. 62/003,269, filed May 27, 2014. This application is also a continuation-in-part of U.S. application Ser. No. 14/243,145, filed Apr. 2, 2014, which is a continuation-in-part of U.S. application Ser. No. 13/744,581, filed Jan. 18, 2013, the disclosure of each of which is incorporated by reference in its entirety. 
    
    
     The claimed invention was made by, on behalf of, and/or in connection with one or more of the following parties to a joint university corporation research agreement: Regents of the University of Michigan, Princeton University, The University of Southern California, and the Universal Display Corporation. The agreement was in effect on and before the date the claimed invention was made, and the claimed invention was made as a result of activities undertaken within the scope of the agreement. 
     FIELD OF THE INVENTION 
     The present invention relates to light emitting devices such as OLED devices and, more specifically, to devices that include full-color pixel arrangements that have sub-pixels that emit not more than two colors and/or incorporate not more than two color filters. 
     BACKGROUND 
     Opto-electronic devices that make use of organic materials are becoming increasingly desirable for a number of reasons. Many of the materials used to make such devices are relatively inexpensive, so organic opto-electronic devices have the potential for cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate. Examples of organic opto-electronic devices include organic light emitting devices (OLEDs), organic phototransistors, organic photovoltaic cells, and organic photodetectors. For OLEDs, the organic materials may have performance advantages over conventional materials. For example, the wavelength at which an organic emissive layer emits light may generally be readily tuned with appropriate dopants. 
     OLEDs make use of thin organic films that emit light when voltage is applied across the device. OLEDs are becoming an increasingly interesting technology for use in applications such as flat panel displays, illumination, and backlighting. Several OLED materials and configurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and 5,707,745, which are incorporated herein by reference in their entirety. 
     One application for phosphorescent emissive molecules is a full color display. Industry standards for such a display call for pixels adapted to emit particular colors, referred to as “saturated” colors. In particular, these standards call for saturated red, green, and blue pixels. Color may be measured using CIE coordinates, which are well known to the art. 
     One example of a green emissive molecule is tris(2-phenylpyridine)iridium, denoted Ir(ppy)3, which has the following structure: 
     
       
         
         
             
             
         
       
     
     In this, and later figures herein, we depict the dative bond from nitrogen to metal (here, Ir) as a straight line. 
     As used herein, the term “organic” includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic opto-electronic devices. “Small molecule” refers to any organic material that is not a polymer, and “small molecules” may actually be quite large. Small molecules may include repeat units in some circumstances. For example, using a long chain alkyl group as a substituent does not remove a molecule from the “small molecule” class. Small molecules may also be incorporated into polymers, for example as a pendent group on a polymer backbone or as a part of the backbone. Small molecules may also serve as the core moiety of a dendrimer, which consists of a series of chemical shells built on the core moiety. The core moiety of a dendrimer may be a fluorescent or phosphorescent small molecule emitter. A dendrimer may be a “small molecule,” and it is believed that all dendrimers currently used in the field of OLEDs are small molecules. 
     As used herein, “top” means furthest away from the substrate, while “bottom” means closest to the substrate. Where a first layer is described as “disposed over” a second layer, the first layer is disposed further away from substrate. There may be other layers between the first and second layer, unless it is specified that the first layer is “in contact with” the second layer. For example, a cathode may be described as “disposed over” an anode, even though there are various organic layers in between. As used herein, two layers or regions may be described as being disposed in a “stack” when at least a portion of one layer or region is disposed over at least a portion of the other. 
     As used herein, “solution processible” means capable of being dissolved, dispersed, or transported in and/or deposited from a liquid medium, either in solution or suspension form. 
     A ligand may be referred to as “photoactive” when it is believed that the ligand directly contributes to the photoactive properties of an emissive material. A ligand may be referred to as “ancillary” when it is believed that the ligand does not contribute to the photoactive properties of an emissive material, although an ancillary ligand may alter the properties of a photoactive ligand. 
     As used herein, and as would be generally understood by one skilled in the art, a first “Highest Occupied Molecular Orbital” (HOMO) or “Lowest Unoccupied Molecular Orbital” (LUMO) energy level is “greater than” or “higher than” a second HOMO or LUMO energy level if the first energy level is closer to the vacuum energy level. Since ionization potentials (IP) are measured as a negative energy relative to a vacuum level, a higher HOMO energy level corresponds to an IP having a smaller absolute value (an IP that is less negative). Similarly, a higher LUMO energy level corresponds to an electron affinity (EA) having a smaller absolute value (an EA that is less negative). On a conventional energy level diagram, with the vacuum level at the top, the LUMO energy level of a material is higher than the HOMO energy level of the same material. A “higher” HOMO or LUMO energy level appears closer to the top of such a diagram than a “lower” HOMO or LUMO energy level. 
     As used herein, and as would be generally understood by one skilled in the art, a first work function is “greater than” or “higher than” a second work function if the first work function has a higher absolute value. Because work functions are generally measured as negative numbers relative to vacuum level, this means that a “higher” work function is more negative. On a conventional energy level diagram, with the vacuum level at the top, a “higher” work function is illustrated as further away from the vacuum level in the downward direction. Thus, the definitions of HOMO and LUMO energy levels follow a different convention than work functions. 
     Layers, materials, regions, and devices may be described herein in reference to the color of light they emit. In general, as used herein, an emissive region that is described as producing a specific color of light may include one or more emissive layers disposed over each other in a stack. 
     As used herein, a “red” layer, material, or device refers to one that emits light in the range of about 580-700 nm; a “green” layer, material, or device refers to one that has an emission spectrum with a peak wavelength in the range of about 500-600 nm; a “blue” layer, material, or device refers to one that has an emission spectrum with a peak wavelength in the range of about 400-500 nm. In some arrangements, separate regions, layers, materials, or devices may provide separate “deep blue” and a “light blue” light. As used herein, in arrangements that provide separate “light blue” and “deep blue”, the “deep blue” component refers to one having a peak emission wavelength that is at least about 4 nm less than the peak emission wavelength of the “light blue” component. Typically, a “light blue” component has a peak emission wavelength in the range of about 465-500 nm, and a “deep blue” component has a peak emission wavelength in the range of about 400-470 nm, though these ranges may vary for some configurations. Similarly, a color altering layer refers to a layer that converts or modifies another color of light to light having a wavelength as specified for that color. For example, a “red” color filter refers to a filter that results in light having a wavelength in the range of about 580-700 nm. In general there are two classes of color altering layers: color filters that modify a spectrum by removing unwanted wavelengths of light, and color changing layers that convert photons of higher energy to lower energy. 
     More details on OLEDs, and the definitions described above, can be found in U.S. Pat. No. 7,279,704, which is incorporated herein by reference in its entirety. 
     SUMMARY OF THE INVENTION 
     Pixel arrangements for light emitting devices are provided, which include sub-pixels that emit not more than two colors and/or not more than two color altering layers. Multiple sub-pixels within a full-color pixel arrangement may emit the same color light initially, which is then converted to one or more other colors by various color filter techniques. 
     According to an embodiment, a full-color pixel arrangement for an OLED device includes a plurality of sub-pixels, each of which includes an emissive region. The arrangement may include emissive regions that emit light of not more than two colors, and may include not more than two color altering layers. Each color altering layer may be disposed in a stack with an emissive region associated with a sub-pixel. The sub-pixels, and/or the corresponding emissive regions, may have different physical sizes, and each emissive region may include one or more emissive devices, layers, or materials. 
     In an embodiment, a full-color pixel for an OLED device may include a plurality of sub-pixels, including a first sub-pixel having an emissive region configured to emit blue light and a second sub-pixel having an emissive region configured to emit yellow light. The pixel arrangement may include emissive regions of not more than two colors, and/or not more than two color altering layers. 
     According to an embodiment, a full-color pixel arrangement for an OLED device may include first, second, and third sub-pixels. The first region may be configured to emit a first color, and the second and third regions each configured to emit a second color. A color altering layer may be disposed in a stack with the second and/or the third emissive region. The arrangement also may include a fourth sub-pixel having an emissive region configured to emit the second color. A third color altering layer, which may provide a color different than those disposed in a stack with the second and/or third emissive regions, also may be disposed in a stack with the fourth emissive region. 
     In an embodiment, a full-color OLED pixel arrangement may be fabricated by depositing a first emissive material through a mask over a substrate, and depositing a second emissive material through a mask over the substrate, where the second emissive material is configured to emit a different color than the first emissive material. A first color filter may be disposed in a stack with a portion of the second emissive material. In an embodiment, not more than two masking steps may be used to fabricate the arrangement. 
     According to an embodiment, a display pixel including at least four sub-pixels may be driven. A first projection of an original color signal onto a color space maybe defined by a first and second sub-pixel of the four sub-pixels. A second projection of the first projection onto a second color space defined by a third sub-pixel may be determined. The third sub-pixel may be driven based on the magnitude of the second projection. Here, the third sub-pixel may emit a color equivalent to a combination of colors emitted by the first and second sub-pixels. The second sub-pixel may be driven based on the difference between the first projection and the second projection. The fourth sub-pixel may be driven based on a component of the original color signal. The first sub-pixel may not be activated while driving the third sub-pixel based on the magnitude of the second projection and driving the second sub-pixel based on the difference between the first projection and the second projection. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an organic light emitting device. 
         FIG. 2  shows an inverted organic light emitting device that does not have a separate electron transport layer. 
         FIG. 3  shows a schematic illustration of an example masking arrangement suitable for fabricating a pixel arrangement as disclosed herein. 
         FIG. 4  shows a schematic illustration of a pixel arrangement according to an embodiment disclosed herein. 
         FIG. 5  shows a schematic illustration of a pixel arrangement according to an embodiment disclosed herein. 
         FIG. 6  shows a schematic illustration of a pixel arrangement according to an embodiment disclosed herein. 
         FIG. 7  shows a schematic illustration of an example masking arrangement suitable for fabricating a pixel arrangement as disclosed herein. 
         FIG. 8  shows a schematic illustration of a pixel arrangement according to an embodiment disclosed herein. 
         FIG. 9  shows a schematic illustration of an example masking arrangement suitable for fabricating a pixel arrangement as disclosed herein. 
         FIG. 10  shows a schematic illustration of a pixel arrangement according to an embodiment disclosed herein. 
         FIG. 11  shows the 1931 CIE diagram that highlights a set of points outside the RG line according to an embodiment disclosed herein. 
         FIG. 12  shows the 1931 CIE diagram with coordinates for pure red, green, and blue, and for a multi-component yellow source that lies outside the RG line according to an embodiment disclosed herein. 
         FIG. 13  illustrates an example color point rendered without the use of a red sub-pixel according to an embodiment disclosed herein. 
         FIG. 14  shows a CIE diagram that identifies red, green, blue, and yellow points, an established white point, and various color regions according to an embodiment disclosed herein. 
         FIG. 15  shows a schematic illustration of a pixel arrangement including a blue color change layer disposed over a blue emissive region according to an embodiment disclosed herein. 
         FIG. 16  shows a schematic illustration of a pixel arrangement including a blue color change layer disposed over a blue emissive region according to an embodiment disclosed herein. 
         FIG. 17  shows a schematic illustration of a pixel arrangement including a blue color change layer disposed over a blue emissive region according to an embodiment disclosed herein. 
         FIG. 18  shows a diagram that identifies red, green, blue and yellow points and associated color spaces, according to an embodiment disclosed herein. 
         FIG. 19  shows another diagram that identifies red, green, blue and yellow points and associated color spaces, according to an embodiment disclosed herein. 
         FIG. 20 a    shows a color space defined by two sub-pixels, according to an embodiment disclosed herein. 
         FIG. 20 b    shows a color space defined by a single sub-pixel, according to an embodiment disclosed herein. 
     
    
    
     DETAILED DESCRIPTION 
     Generally, an OLED comprises at least one organic layer disposed between and electrically connected to an anode and a cathode. When a current is applied, the anode injects holes and the cathode injects electrons into the organic layer(s). The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, an “exciton,” which is a localized electron-hole pair having an excited energy state, is formed. Light is emitted when the exciton relaxes via a photoemissive mechanism. In some cases, the exciton may be localized on an excimer or an exciplex. Non-radiative mechanisms, such as thermal relaxation, may also occur, but are generally considered undesirable. 
     The initial OLEDs used emissive molecules that emitted light from their singlet states (“fluorescence”) as disclosed, for example, in U.S. Pat. No. 4,769,292, which is incorporated by reference in its entirety. Fluorescent emission generally occurs in a time frame of less than 10 nanoseconds. 
     More recently, OLEDs having emissive materials that emit light from triplet states (“phosphorescence”) have been demonstrated. Baldo et al., “Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices,” Nature, vol. 395, 151-154, 1998; (“Baldo-I”) and Baldo et al., “Very high-efficiency green organic light-emitting devices based on electrophosphorescence,” Appl. Phys. Lett., vol. 75, No. 3, 4-6 (1999) (“Baldo-II”), which are incorporated by reference in their entireties. Phosphorescence is described in more detail in U.S. Pat. No. 7,279,704 at cols. 5-6, which are incorporated by reference. 
       FIG. 1  shows an organic light emitting device  100 . The figures are not necessarily drawn to scale. Device  100  may include a substrate  110 , an anode  115 , a hole injection layer  120 , a hole transport layer  125 , an electron blocking layer  130 , an emissive layer  135 , a hole blocking layer  140 , an electron transport layer  145 , an electron injection layer  150 , a protective layer  155 , a cathode  160 , and a barrier layer  170 . Cathode  160  is a compound cathode having a first conductive layer  162  and a second conductive layer  164 . Device  100  may be fabricated by depositing the layers described, in order. The properties and functions of these various layers, as well as example materials, are described in more detail in U.S. Pat. No. 7,279,704 at cols. 6-10, which are incorporated by reference. 
     More examples for each of these layers are available. For example, a flexible and transparent substrate-anode combination is disclosed in U.S. Pat. No. 5,844,363, which is incorporated by reference in its entirety. An example of a p-doped hole transport layer is m-MTDATA doped with F 4 -TCNQ at a molar ratio of 50:1, as disclosed in U.S. Patent Application Publication No. 2003/0230980, which is incorporated by reference in its entirety. Examples of emissive and host materials are disclosed in U.S. Pat. No. 6,303,238 to Thompson et al., which is incorporated by reference in its entirety. An example of an n-doped electron transport layer is BPhen doped with Li at a molar ratio of 1:1, as disclosed in U.S. Patent Application Publication No. 2003/0230980, which is incorporated by reference in its entirety. U.S. Pat. Nos. 5,703,436 and 5,707,745, which are incorporated by reference in their entireties, disclose examples of cathodes including compound cathodes having a thin layer of metal such as Mg:Ag with an overlying transparent, electrically-conductive, sputter-deposited ITO layer. The theory and use of blocking layers is described in more detail in U.S. Pat. No. 6,097,147 and U.S. Patent Application Publication No. 2003/0230980, which are incorporated by reference in their entireties. Examples of injection layers are provided in U.S. Patent Application Publication No. 2004/0174116, which is incorporated by reference in its entirety. A description of protective layers may be found in U.S. Patent Application Publication No. 2004/0174116, which is incorporated by reference in its entirety. 
       FIG. 2  shows an inverted OLED  200 . The device includes a substrate  210 , a cathode  215 , an emissive layer  220 , a hole transport layer  225 , and an anode  230 . Device  200  may be fabricated by depositing the layers described, in order. Because the most common OLED configuration has a cathode disposed over the anode, and device  200  has cathode  215  disposed under anode  230 , device  200  may be referred to as an “inverted” OLED. Materials similar to those described with respect to device  100  may be used in the corresponding layers of device  200 .  FIG. 2  provides one example of how some layers may be omitted from the structure of device  100 . 
     The simple layered structure illustrated in  FIGS. 1 and 2  is provided by way of non-limiting example, and it is understood that embodiments of the invention may be used in connection with a wide variety of other structures. The specific materials and structures described are exemplary in nature, and other materials and structures may be used. Functional OLEDs may be achieved by combining the various layers described in different ways, or layers may be omitted entirely, based on design, performance, and cost factors. Other layers not specifically described may also be included. Materials other than those specifically described may be used. Although many of the examples provided herein describe various layers as comprising a single material, it is understood that combinations of materials, such as a mixture of host and dopant, or more generally a mixture, may be used. Also, the layers may have various sublayers. The names given to the various layers herein are not intended to be strictly limiting. For example, in device  200 , hole transport layer  225  transports holes and injects holes into emissive layer  220 , and may be described as a hole transport layer or a hole injection layer. In one embodiment, an OLED may be described as having an “organic layer” disposed between a cathode and an anode. This organic layer may comprise a single layer, or may further comprise multiple layers of different organic materials as described, for example, with respect to  FIGS. 1 and 2 . 
     Structures and materials not specifically described may also be used, such as OLEDs comprised of polymeric materials (PLEDs) such as disclosed in U.S. Pat. No. 5,247,190 to Friend et al., which is incorporated by reference in its entirety. By way of further example, OLEDs having a single organic layer may be used. OLEDs may be stacked, for example as described in U.S. Pat. No. 5,707,745 to Forrest et al, which is incorporated by reference in its entirety. The OLED structure may deviate from the simple layered structure illustrated in  FIGS. 1 and 2 . For example, the substrate may include an angled reflective surface to improve out-coupling, such as a mesa structure as described in U.S. Pat. No. 6,091,195 to Forrest et al., and/or a pit structure as described in U.S. Pat. No. 5,834,893 to Bulovic et al., which are incorporated by reference in their entireties. 
     Unless otherwise specified, any of the layers of the various embodiments may be deposited by any suitable method. For the organic layers, preferred methods include thermal evaporation, ink-jet, such as described in U.S. Pat. Nos. 6,013,982 and 6,087,196, which are incorporated by reference in their entireties, organic vapor phase deposition (OVPD), such as described in U.S. Pat. No. 6,337,102 to Forrest et al., which is incorporated by reference in its entirety, and deposition by organic vapor jet printing (OVJP), such as described in U.S. Pat. No. 7,431,968, which is incorporated by reference in its entirety. Other suitable deposition methods include spin coating and other solution based processes. Solution based processes are preferably carried out in nitrogen or an inert atmosphere. For the other layers, preferred methods include thermal evaporation. Preferred patterning methods include deposition through a mask, cold welding such as described in U.S. Pat. Nos. 6,294,398 and 6,468,819, which are incorporated by reference in their entireties, and patterning associated with some of the deposition methods such as ink jet and OVJD. Other methods may also be used. The materials to be deposited may be modified to make them compatible with a particular deposition method. For example, substituents such as alkyl and aryl groups, branched or unbranched, and preferably containing at least 3 carbons, may be used in small molecules to enhance their ability to undergo solution processing. Substituents having 20 carbons or more may be used, and 3-20 carbons is a preferred range. Materials with asymmetric structures may have better solution processibility than those having symmetric structures, because asymmetric materials may have a lower tendency to recrystallize. Dendrimer substituents may be used to enhance the ability of small molecules to undergo solution processing. 
     Devices fabricated in accordance with embodiments of the present invention may further optionally comprise a barrier layer. One purpose of the barrier layer is to protect the electrodes and organic layers from damaging exposure to harmful species in the environment including moisture, vapor and/or gases, etc. The barrier layer may be deposited over, under or next to a substrate, an electrode, or over any other parts of a device including an edge. The barrier layer may comprise a single layer, or multiple layers. The barrier layer may be formed by various known chemical vapor deposition techniques and may include compositions having a single phase as well as compositions having multiple phases. Any suitable material or combination of materials may be used for the barrier layer. The barrier layer may incorporate an inorganic or an organic compound or both. The preferred barrier layer comprises a mixture of a polymeric material and a non-polymeric material as described in U.S. Pat. No. 7,968,146, PCT Pat. Application Nos. PCT/US2007/023098 and PCT/US2009/042829, which are herein incorporated by reference in their entireties. To be considered a “mixture”, the aforesaid polymeric and non-polymeric materials comprising the barrier layer should be deposited under the same reaction conditions and/or at the same time. The weight ratio of polymeric to non-polymeric material may be in the range of 95:5 to 5:95. The polymeric material and the non-polymeric material may be created from the same precursor material. In one example, the mixture of a polymeric material and a non-polymeric material consists essentially of polymeric silicon and inorganic silicon. 
     Devices fabricated in accordance with embodiments of the invention may be incorporated into a wide variety of consumer products, including flat panel displays, computer monitors, medical monitors, televisions, billboards, lights for interior or exterior illumination and/or signaling, color tunable or color temperature tunable lighting sources, heads up displays, fully transparent displays, flexible displays, laser printers, telephones, cell phones, personal digital assistants (PDAs), laptop computers, digital cameras, camcorders, viewfinders, micro-displays, vehicles, a large area wall, theater or stadium screen, or a sign. Various control mechanisms may be used to control devices fabricated in accordance with the present invention, including passive matrix and active matrix. Many of the devices are primarily intended for use in a temperature range comfortable to humans, such as 18 degrees C. to 30 degrees C., and more preferably at room temperature (20-25 degrees C.), but can operate at temperatures outside this range, such as −40 C to +85 C or higher. 
     The materials and structures described herein may have applications in devices other than OLEDs. For example, other optoelectronic devices such as organic solar cells and organic photodetectors may employ the materials and structures. More generally, organic devices, such as organic transistors, may employ the materials and structures. 
     The terms halo, halogen, alkyl, cycloalkyl, alkenyl, alkynyl, arylkyl, heterocyclic group, aryl, aromatic group, and heteroaryl are known to the art, and are defined in U.S. Pat. No. 7,279,704 at cols. 31-32, which are incorporated herein by reference. 
     Current display architectures and manufacturing capabilities typically do not allow for low power consumption and high resolution OLED displays. For example, side by side (SBS) architecture typically can achieve relatively low power consumption (and therefore good lifetime), but this architecture may require relatively high resolution shadow masking. Such techniques often are limited to around 250 dpi resolution. To achieve higher resolutions, architectures using white devices in conjunction with color filters may be used to avoid patterning the OLED emissive layers. However, such techniques typically suffer from relatively lower efficiency and therefore higher power consumption, which also reduces lifetime. These constraints may be somewhat overcome by employing a RGBW pixel architecture that uses both an unfiltered white sub-pixel and devices that emit at individual colors by employing color filters over other white sub-pixels. This architecture generally is considered to result in poorer image quality, and typically still has a lower power consumption and poorer lifetime than a comparable RGB SBS display. 
     The present disclosure provides arrangements of pixel components that allow for full-color devices, while using emissive devices that emit not more than two colors, and/or a limited number of color altering layers. Embodiments disclosed herein may provide improved performance over conventional RGBW displays, such as lower power consumption and longer lifetime, with fewer high resolution masking steps, and at a lower resolution, in comparison to a conventional RGB SBS display That is, although an arrangement as disclosed herein may include any number of sub-pixels or other emissive devices or regions, within the arrangement there may be a limited number of colors emitted by emissive devices or regions within the arrangement. As a specific example, an arrangement as disclosed herein may include three sub-pixels. Two of the sub-pixels may include emissive regions, such as OLEDs, that emit light of the same color, with one of the sub-pixels being filtered or otherwise modified to produce a different color after light is emitted by the emissive region. The third sub-pixel may include an emissive region that emits light of a different color than the first emissive regions within the two sub-pixels. Thus, although the sub-pixels overall may produce light of three or more colors, the emissive regions within the arrangement need only initially emit light of two colors. Devices disclosed herein also may be achieved using simplified fabrication techniques compared to conventional SBS arrangements, because fewer masking steps may be required. 
     In an embodiment, two masking steps may be used. This may provide for simplified fabrication when compared to the three masking steps required for a conventional RGB SBS display. Each mask opening area may be approximately half the pixel area, as opposed to a third in a conventional SBS display. The increased area of the shadow mask opening relative to a conventional SBS display of the same pixel size may allow for higher pixel density. For example, the same size opening will allow for up to about a 50% increase in display resolution compared to a conventional SBS technique. In some configurations, the exact size of the mask openings may be determined based upon lifetime matching considerations, such as to optimize current flow through each sub-pixel and thus improve overall display lifetime. 
     An increase in fill factor also may also be possible using techniques disclosed herein, particularly for top emitting AMOLED displays, which may allow for higher efficiency relative to a conventional three-mask pixilation approach of the same resolution. This is due to the relatively increased area of the three sub-pixels in a two-mask approach as disclosed, compared to a conventional three-mask approach. With a two-mask approach as disclosed, less current may be required for at least some sub-pixels, to render the same luminance from a display. This may result in higher device efficiency, lower voltage, and/or longer display lifetime. 
       FIG. 3  shows a schematic illustration of an example masking arrangement suitable for fabricating a pixel arrangement as disclosed herein. In a first masked deposition, an emissive layer structure, or a stacked device structure including multiple emissive layers, may be deposited in a first region  310 . The first region contains one or more emissive layers that emit light of a first color. A second masked deposition may be performed in an adjacent or otherwise nearby region  320 , to deposit an emissive layer or stacked structure that emits light of a second color different from the first. Two color altering layers  330 ,  340 , may then be disposed over the second emissive region, such that light passing through each filter may be converted from the second color to third and fourth colors, respectively, each different from the first and second. In some configurations, a portion of the second emissive region may be left uncovered, such that the illustrated arrangement may provide light having four distinct colors. In some configurations, the color altering layers  330 ,  340  may cover the entirety of the second region, such that the illustrated arrangement provides light of three distinct colors. Although shown as disposed with some distance between them in  FIG. 3  for illustration purposes, it will be understood that in general the color filters may be disposed immediately adjacent to one another, such that no yellow light is emitted in the region between the filters. Similarly, each filter may extend to the appropriate edges of the yellow emissive region, such that no yellow light is emitted from the edge immediately adjacent to the color altering layer. Each of the emissive layers or stacked structures may include one or more emissive materials, each of which may be phosphorescent or fluorescent. More generally, each emissive material may include any of the emissive materials, layers, and/or structures disclosed herein. 
     As a specific example, the first mask deposition  310  may provide a blue device, which may be a single EML structure or a stacked device containing more than one EML. As is known in the art, a stacked device may be desirable to provide extended lifetime and/or reduced image sticking; in other arrangements, a single-layer emissive device may be preferred to reduce fabrication cost and complexity. The blue OLED may be phosphorescent or fluorescent. The second mask deposition  320  may provide a yellow device, which may be made, for example, by combining red and green emitters. More generally, the yellow device may be provided using any suitable combination of emissive materials and/or layers. As specific examples, separate red and green emitters may be provided in one mixed layer; in separate layers within a two-EML device; in a stacked device with a red EML in one OLED within the stack and a green EML in the other; in a yellow device using a single EML containing a yellow emitter; or in a stacked device containing two yellow EMLs. Thus, in some configurations, an emissive region may be provided by multiple emissive materials, each of which has an emission spectrum or peak emission wavelength that differs from the ultimate color of the region as a whole. Various combinations also may be used, though advantageously any selected combination may be deposited using the same second mask arrangement. In the completed example configuration, the blue device is controlled by one anode and associated active matrix control circuit. The yellow device is divided into three sub-pixels, yellow, green and red. Each sub-pixel is then controlled by its own anode and associated active matrix control circuit. The yellow sub-pixel uses the unfiltered yellow light from the yellow OLED. The green sub-pixel is obtained by placing a green color filter over the yellow OLED, and, similarly, a red sub-pixel is obtained by placing a red color filter over the yellow OLED. Thus, the resulting pixel arrangement has four sub-pixels, red, green, blue, and yellow (RGBY). Such an arrangement may be advantageous, because the blue performance may not be limited by a color filter as in a conventional RGBW display, but may have the same optimized lifetime as in a conventional RGB SBS display. Further, in a conventional RGBW arrangement, the green color filter is configured to prevent transmission of as much blue and red light as possible. Thus, a band-pass filter typically is used as the green color filter. In an RGBY arrangement as disclosed herein where yellow light is used as a multi-component light source, the green color filter may be configured to prevent transmission only of red light since the multi-component light does not include a blue component. Thus, a cut-off filter may be used instead of a band-pass filter, which may provide relatively greater efficiency and color saturation. 
     Embodiments disclosed herein may use unfiltered yellow light to improve display efficiency at times when highly saturated red or green is not required. In operation, the unfiltered yellow device may be used in a similar manner to white in conventional a RGBW display, and similar algorithms may be employed for signal processing. To render a specific unsaturated color, yellow light can be mixed with the three individual primary red, green or blue colors, which may provide higher efficiency than just using the red, green or blue primary colors alone. A full-color display using this technique may have only about a 12% higher power consumption than a conventional SBS RGB arrangement, in contrast to a conventional RGBW arrangement which typically has about a 50% higher power consumption than the SBS RGB arrangement. This level of power reduction may be achieved even if the overall red and green sub-pixel efficiency is reduced by 25%. For example, color filters may reduce the efficiency for the red and green alone by 50%, but the unfiltered yellow sub-pixel may restore much of this loss. 
     Embodiments disclosed herein similarly may allow for increased display color range. For example, referring to  FIG. 11 , a yellow multi-component source may be configured such that it emits light having CIE coordinates that lie on the “RG line” between the identified pure red and green points, such as the illustrated point  1104 . In some embodiments, the identified red and green points may correspond to the “pure” colors emitted by emissive regions in the corresponding sub-pixels. Alternatively, the yellow multi-component source may be configured to emit light that lies outside the RG line, such as point  1108 , any point along the illustrated curve  1100 , or the like. The use of such a multi-component source may increase the available display color gamut, by allowing for use of the CIE region outside the RG line. The increase in color gamut may be achieved or used when the yellow multi-component source is filtered to provide red and/or green light, or it may be used when the yellow source is used unfiltered, according to the various arrangements disclosed herein. Thus, in some configurations, it may be desirable for the yellow multi-component source to have CIE coordinates that lie outside the RG line on the 1931 CIE diagram. 
       FIG. 4  shows a schematic illustration of a pixel arrangement according to an embodiment disclosed herein. As described with respect to  FIG. 3 , the arrangement includes four sub-pixels  410 ,  420 ,  430 ,  440 . One sub-pixel  410  includes one or more emissive devices or regions that emit light of a first color. The other sub-pixels  420 ,  430 ,  440  are constructed using emissive regions that emit light of a second color. A color altering layer  432 ,  442  may be disposed over each of two of the emissive regions  434 ,  444 . The third sub-pixel  420  is left unfiltered, resulting in a pixel arrangement that has four sub-pixels, each providing light of a different color. 
     In some configurations, additional color altering layers may be used. For example, a blue color altering layer may be disposed over the blue emissive region  410  to modify the spectral output resulting at the blue sub-pixel. An example of such a configuration is shown in  FIG. 15 , in which a blue color altering layer is disposed over the blue emissive region, while the other emissive regions and color altering layers are the same as shown in  FIG. 4 . Although shown generally as a blue color altering layer and a blue emissive region for clarity, the blue color altering layer may be a light blue or deep blue color altering layer. Similarly, the blue emissive region may be a deep blue or light blue emissive region. 
     As described with respect to  FIG. 3 , each sub-pixel may be controlled by an associated control circuit. Example control circuits are shown in  FIG. 4  for purposes of illustration, with various control elements shaded to match the controlled emissive regions. The specific arrangement of control circuitry is provided by way of example only, and any suitable control circuitry may be used as will be readily apparent to one of skill in the art. 
     In general parlance in the art, a “sub-pixel” may refer to the emissive region, which may be a single-layer EML, a stacked device, or the like, in conjunction with any color altering layer that is used to modify the color emitted by the emissive region. For example, the sub-pixel  430  includes an emissive region  434  and a color altering layer  432 . As used herein, the “emissive region” of a sub-pixel refers to any and all emissive layers, regions, and devices that are used initially to generate light for the sub-pixel. A sub-pixel also may include additional layers disposed in a stack with the emissive region that affect the color ultimately produced by the sub-pixel, such as color altering layers disclosed herein, though such color altering layers typically are not considered “emissive layers” as disclosed herein. An unfiltered sub-pixel is one that excludes a color modifying component such as a color altering layer, but may include one or more emissive regions, layers, or devices. 
     In some configurations, fewer sub-pixels may be used to achieve a full-color device or pixel arrangement.  FIG. 5  shows an example arrangement that uses three sub-pixels  510 ,  520 ,  530 . Similarly to the example shown in  FIG. 4 , a first sub-pixel  510  may be created by depositing one or more emissive regions through a mask, in a single emissive layer or a stacked arrangement, and leaving the resulting sub-pixel unfiltered. The other two sub-pixels  520 ,  530 , may be deposited during a single masked deposition. As previously described, each may include one or more emissive materials and/or layers, and may be individual emissive layers or stacked devices. A color altering layer  532  may then be disposed over one or more of the emissive regions, to result in a full-color arrangement having three sub-pixels of different colors. 
     As a specific example, the two masking steps may be blue and green. That is, in a first masked deposition technique, a blue layer or stack may be deposited in a region corresponding to the first sub-pixel  510 . Green layers or stacked devices may be deposited in regions corresponding to the second and third sub-pixels  520 ,  530  in a second masked deposition. The green sub-pixel  520  provides unfiltered green light. The red sub-pixel  530  uses a color altering layer  532 , such as a green-to-red color changing layer having a relatively high conversion efficiency, to convert the green light emitted by the green device  530  to red light. Such a configuration may result in a display that with up to 50% higher resolution than a comparable conventional RGB SBS display, with little or no increase in power consumption or associated decrease in lifetime. Such an approach also may improve the display efficiency by not “losing” as much light due to use of a conventional color filter, instead using a color changing layer to provide the third color. 
     As another example, a blue color altering layer may be disposed over the blue emissive region as previously described. Such a configuration is shown in  FIG. 16 . As previously described, the blue color altering layer may be a light blue or deep blue color altering layer, and the blue emissive region may be a deep blue or light blue emissive region. 
       FIG. 6  shows a schematic illustration of a configuration in which the two masking steps are blue and yellow, i.e., one or more blue emissive layers are deposited during one masked deposition, and one or more yellow emissive layers are deposited during another. As with  FIG. 6 , the illustrated configuration uses only three sub-pixels, red, green, and blue. In this example, the green sub-pixel uses a green color filter to convert light from the yellow OLED to green, the red sub-pixel uses a red color filter to convert light from the yellow OLED to red, and the blue sub-pixel uses unfiltered light from the blue OLED. Similar configurations may use color altering layers other than or in addition to the specific color filters shown. 
     As another example, a blue color altering layer may be disposed over the blue emissive region as previously described. Such a configuration is shown in  FIG. 17 . As previously described, the blue color altering layer may be a light blue or deep blue color altering layer, and the blue emissive region may be a deep blue or light blue emissive region. 
     In some configurations, the efficiency of one or more sub-pixels may be enhanced by using a color changing layer instead of, or in addition to, a conventional color filter as a color altering layer as disclosed herein. For example, referring to the example shown in  FIG. 6 , a red color changing layer with a relatively high conversion efficiency from yellow to red may be placed between the yellow OLED and red color filter. Such a configuration may enhance the red sub-pixel efficiency. More generally, the use of a color changing layer disposed in a stack with an OLED, or an OLED and a color filter, may enhance the efficiency of that sub-pixel. 
     Other configurations disclosed herein may use additional color altering layers, and may include color altering layers disposed over multiple emissive regions or types of emissive region.  FIG. 7  shows an example masking arrangement that uses emissive regions of two colors, light blue  710  and white  720 . The masked areas may be used to deposit emissive regions of each color as previously disclosed. Various color altering layers also may be disposed over the resulting emissive regions to create a full-color pixel arrangement. In the example shown in  FIG. 7 , a deep blue color filter  730 , a red color filter  740 , and a green color filter  750  are disposed over corresponding white emissive regions, to form three sub-pixels of those colors, while the light blue emissive region remains unfiltered to form a light blue sub-pixel. Such a configuration may be used to enhance overall blue lifetime by using the light blue and deep blue sub-pixels as needed, as described in U.S. Patent Publication No. 2010/0090620, the disclosure of which is incorporated by reference in its entirety. As previously described, other color altering layers may be used in addition to or instead of the specific color filters described with respect to  FIG. 7 . 
       FIG. 8  shows an example pixel arrangement corresponding to the deposition arrangement shown in  FIG. 7 . Similarly to the arrangement shown in  FIG. 4 , the arrangement of  FIG. 7  includes an unfiltered sub-pixel  810 , and three sub-pixels  820 ,  830 ,  840  each of which is formed from an emissive region  821 ,  831 ,  841  and a color filter  822 ,  832 ,  842 , respectively. In the example shown in  FIG. 7 , the unfiltered sub-pixel  810  is a light blue sub-pixel, and the color filters  822 ,  832 ,  842  are deep blue, red, and green, respectively. The specific emission colors and color filters shown are illustrative only, and various other colors, color altering layers, and combinations may be used without departing from the scope of embodiments disclosed herein. 
     In an embodiment, each emissive region deposited during each of two masked deposition operations may be combined with a color altering layer to form one or more pixels.  FIG. 9  shows an example arrangement in which each type of emissive region is combined with one or more color altering layers to create multiple sub-pixels. For example, the two masked regions may correspond to a light blue emissive region  910  and a yellow emissive region  920 , each of which may be deposited in one of two masking steps as previously described. A deep blue color altering layer  930  may be combined with a light blue emissive region  910  to form a deep blue sub-pixel. Red and green color altering layers  930 ,  940 , respectively, each may be combined with a portion of a yellow emissive region to form red and green sub-pixels. A light blue emissive region also may be left unfiltered to form a light blue sub-pixel. As described with respect to  FIG. 7 , use of the light and deep blue sub-pixels may improve performance and device lifetime. In addition, the relatively long light blue lifetime can extend overall display operation and provide improved power efficiency, since the light blue sub-pixel is unfiltered. 
       FIG. 10  shows a pixel arrangement corresponding to the mask and color altering layer arrangement described with respect to  FIG. 9 . As shown, four sub-pixels are created from two emissive regions of a first color, one of which is unfiltered, and two emissive regions of a second color. Following the example of  FIG. 9 , a light blue sub-pixel is formed from an unfiltered light blue emissive region, a deep blue sub-pixel is formed from a light blue emissive region and a deep blue color altering layer, a red sub-pixel is formed from a yellow emissive region and a red color altering layer, and a green sub-pixel is formed from a yellow emissive region and a green color altering layer. The specific emission colors and color altering layers shown are illustrative only, and various other colors, color altering layers, and combinations may be used without departing from the scope of embodiments disclosed herein. 
     Embodiments of the invention disclosed herein may use a variety of drive schemes. In many embodiments, four sub-pixels may be available to render each color. Typically, only three sub-pixels may be needed to render a particular color; thus there are multiple options available for the electrical drive configuration used to render the color. For example,  FIG. 12  shows the 1931 CIE diagram with coordinates for pure red, green, and blue, and for a multi-component yellow source that lies outside the RG line according to an embodiment disclosed herein. In a four sub-pixel arrangement as disclosed herein, when rendering a color in the GBY space  1210 , the red sub-pixel is not required; similarly, if the color to be rendered is within the RBY space  1220 , the green sub-pixel is not required.  FIG. 13  illustrates an example color point rendered without the use of a red sub-pixel, i.e., a point which lies within the GBY space  1210  in  FIG. 12 . As shown the initial contribution of pixels for the example point may be R 0 , G 0 , B 0  for the red, green, and blue sub-pixels, respectively, in a RGBY arrangement. In an RGBY arrangement as disclosed herein, the equivalent contributions for yellow, green, and blue sub-pixels may be Y′, G′, B′, respectively. Notably, the red sub-pixel need not be used to render the desired color. 
     Another drive arrangement according to embodiments disclosed herein is to fix a white point using yellow and blue sub-pixels. A desired color may then be rendered through use of the green or red sub-pixel, depending on whether the color lies within the GBY or RBY space.  FIG. 14  shows a CIE diagram that identifies red, green, blue, and yellow points as previously disclosed. A white point  1404  may be established along the BY line using a combination of only the blue and yellow sub-pixels, as shown. A color point  1400  falling in the GBY space, i.e., on the green side of the BY line, thus may be rendered by using the green sub-pixel in addition to the blue and yellow sub-pixels. Similarly, a point in the BYR space, i.e., on the red side of the BY line, may be rendered using the blue, yellow, and red sub-pixels. Thus, embodiments disclosed herein allow for a variety of drive arrangements, and may provide additional flexibility, efficiency, and color range compared to conventional RGBW and similar arrangements. 
     An emissive region, layer, or device disclosed herein may be a single-layer emissive layer, or it may be a stacked device. Each emissive region, layer, or device may also include multiple emissive materials which, when operated in conjunction, provide the appropriate color light for the component. For example, a yellow emissive region may include both red and green emissive materials in an appropriate proportion to provide yellow light. Similarly, any emissive region or device may be a stacked device or otherwise include emissive sub-regions of sub-colors that are used to provide the desired color for the region or device, such as where a stacked configuration with red and green devices is used to provide a yellow emissive region. Each also may include multiple emissive materials that provide light of the same color or in the same region. Further, each emissive material used in any of the configurations disclosed herein may be phosphorescent, fluorescent, or hybrid, unless indicated specifically to the contrary. Although a single emissive region as disclosed herein may include multiple emissive materials, it may be described as emitting only a single color, since typically it will not be configured to allow for only one of the emissive materials to be used. For example, a yellow emissive region may include both red and green emissive materials. Such a region is described herein as a yellow emissive region and is considered a single-color emissive region, since the red and green emissive materials cannot be activated independently of one another. 
     According to embodiments of the disclosed subject matter, a pixel including at least four sub-pixels may be driven based on projections associated with a color signal. The sub-pixels may correspond to or include emissive regions containing two or fewer colors, such as a blue emissive region and a yellow emissive region. Multiple sub-pixels may be formed from a single emissive region as previously described, such as by the use of color altering layers optically coupled to portions of one of the sub-pixels. Such a pixel arrangement may include no more than two color altering layers. 
     In operation, a color signal that defines or otherwise provides an intended color to be generated by the pixel is used to drive the pixel. As an example, a display containing thousands of pixels may be configured to display an image of an automobile at a given time. A particular pixel may be located in an area of the display such that, to display the automobile, the intended color output by the pixel is an orange represented by the hex value #FFA500. Instead of driving all four sub-pixels, three of the four sub-pixels may be driven such that the light that would have been emitted by a primary color sub-pixel in a conventional display (e.g., a green sub-pixel) may be emitted instead by a secondary color pixel (e.g., a yellow sub-pixel). Further, of the three of four sub-pixels that are driven, a primary color sub-pixel (e.g., a red sub-pixel) may be driven at a lower magnitude based on a secondary sub-pixel (e.g., the yellow sub-pixel) emitting a portion of the light that the primary color sub-pixel (e.g., red sub-pixel) would have emitted if driven without the secondary sub-pixel. 
     As disclosed herein, a color space may be defined by one or more sub-pixels. A color space may correspond to the range of colors available based on the color emission range of one or more sub-pixels. An example of a three sub-pixel color space is shown in  FIG. 18 . The three axes (Red, Blue, and Green) represent the color space available based on the three respective red, blue, and green sub-pixels, as shown by the box  1800 . Notably, any color point within the color space box  1800  may be emitted by driving the Red, Blue, and Green sub-pixels at levels corresponding to the projection of the color point onto each respective sub-pixel. An example of a two sub-pixel color space, such as a “slice” of the color space illustrated in  FIG. 18 , is shown in  FIG. 20 a   . Here the x axis  2020  is represented by the emission capability of a red sub-pixel and the y axis  2010  is represented by the emission capability of a green sub-pixel. Notably, any color point within the color space represented by the two axes may be emitted by driving the red and the green sub-pixel at levels corresponding to the projection of the color point onto each respective sub-pixel. As a more specific example, a color point  2030  within the color space represented by the red and green color space may be projected onto the red axis as shown by projection  2032  and may be projected on to the green axis as shown by projection  2034 . The color point  2030  may be emitted by driving the red sub-pixel at a value corresponding to  2033  (i.e., the projection point of the color point  2030  onto the red sub-pixel axis) and the green sub-pixel at a value corresponding to  2031  (i.e., the projection point of the color point  2030  onto the green sub-pixel axis). As an example of a single sub-pixel color space is shown in  FIG. 20 b   . Here, the only axis  2040  corresponds to the emission capability of a red sub-pixel. Notably, any color point within this red pixel&#39;s color space may be represented by a point on the axis  2040 . As a specific example, point  2045  is a color point that can be emitted by driving the red sub-pixel at the point  2045  itself. It will be understood that the same sub-pixel or sub-pixel type used to show the single sub-pixel color space may be a part of the double and/or triple sub-pixel color space. As a specific example, a red color pixel may be have the emission capability represented in  FIG. 20 b    as the red color pixel in  FIGS. 20 a    and  18 . 
     As previously described, a display pixel as disclosed herein may include at least four sub-pixels. A projection of an original color signal onto a color space defined by two of the four sub-pixels may be determined. As an example, one way of producing the color specified in an original color signal is to emit a red, green, and blue light from red, green, and blue sub-pixels, respectively. As disclosed herein, alternatively, the original color signal may be projected onto a color space defined by two of the sub-pixels such as, for example, the red and the green color space. Notably, the projection of the original color signal onto a color space defined by two of the sub-pixels may correspond to the magnitude of emission, corresponding to the two sub-pixels, required to emit the portion of the original color signal corresponding to the two sub-pixels. 
       FIG. 18  shows an illustrative example of projecting an original color signal onto a color space defined by two sub-pixels. As shown in  FIG. 18 , an original color signal may be represented by point  1810 . The point  1810  may indicate that the original color signal is one that can be emitted by driving the red, green, and blue sub-pixels based on the direction and magnitude of the point  1810 . As disclosed herein, to produce the specified color using two sub-pixels, a projection of the original color signal onto the color space defined by the red and green sub-pixels, may be determined. This projection may be represented by the first projection vector  1802  as shown in  FIG. 18 . Projecting the original color signal onto the red and green color space may result in a component of the original color signal that is based only on the red and green color space. In other words, the projection represents the red/green component of the original color signal. 
     As previously described, a color space also may be defined for a single sub-pixel. As shown in  FIG. 20 b   , a color space for a single sub-pixel may be represented by a line as the color space is only single dimensional (as a result of being defined for driving a single sub-pixel). The color space for a single sub-pixel may be for a primary color (e.g., blue, green, red) or may be for a secondary color such as yellow, magenta, cyan, or the like. As an example, a yellow sub-pixel may have a color space that is a factor of red and green color spaces. As an illustrative example, as shown in  FIG. 18 , the y-vector  1803  represents the color space for a yellow sub-pixel. As shown, the color space for the yellow sub-pixel can be represented by points within the red green color space such that the red and/or green sub-pixel may be driven to emit the color points represented by the yellow color space. 
     After determining the first projection of the color signal, a second projection of the first projection onto a color space defined by a single sub-pixel may be determined. The color space defined by the single sub-pixel may be for a sub-pixel associated with a secondary color such that the secondary color is within the color space of two other sub-pixels in the pixel onto which the first projection was determined. In other words, the color space for the single sub-pixel may be a combination of at least two other colors emitted by two other respective sub-pixels in a pixel. As an example, the second projection may be onto a color space defined by a yellow sub-pixel such that the light emitted by the yellow sub-pixel may be light that is within the color space of the red and green sub-pixel. In an illustrative example, as shown in  FIG. 18 , the color space for a yellow sub-pixel may be defined by the Y-vector  1804 . As shown, the Y-vector  1804  may represent the color space defined by a yellow sub-pixel. The first projection represented by the first projection vector  1802  may have an end at point  1811 . The first projection vector  1802  may be projected on the Y-vector  1804  such that the second projection corresponds to point  1805  on the Y-vector  1804 . 
     A second projection of a first projection may represent a single sub-pixel vector that corresponds to components of both components of the two other sub-pixels that the two sub-pixel color space is associated with. As an example, as shown in  FIG. 18 , the projection onto the Y-vector represented by point  1805  represents a color point associated with both red and green components of the first projection. In this example, the point  1805  represents the entire green component of the first projection vector and represents the entire red component less the R-vector  1803  (i.e., RedComponent−R-vector=Red_portion_of_Y-vector). As a more specific example, the point  1811  of the first projection vector corresponds to a red value of 200 and a green value of 150 (as shown in  FIG. 18 ). The projection of the first projection vector onto the Y-vector  1804  results in point  1805 . Point  1805  on the Y-vector represents the entire green component (i.e., 150) and represents a value of 100 for the red component. Here, the R-vector may represent the remaining value of 100 such that the overall red value of 200 is represented by the R-vector plus the red component of the Y-vector at point  1805 . 
     According to embodiments of the disclosed subject matter, a sub-pixel corresponding to the second projection may be driven based on the magnitude of the second projection. Driving the third sub-pixel based on the magnitude of the second projection may result in emission of a light that is representative of one or more of the components of the first projection onto the color space represented by two color pixels. Continuing the previous illustrative example, as shown in  FIG. 18 , the yellow sub-pixel may be driven based on the magnitude associated with the second projection point  1805  such that the yellow sub-pixel emits all of the green component (150) associated with the first projection vector  1802  and a portion of the red component (100) of the total red component (200) of the first projection vector  1802 . Notably, driving the yellow sub-pixel at the magnitude associated with the second projection may effectively eliminate the need to drive the green sub-pixel as the green component value (150) is emitted by the yellow sub-pixel. Additionally, driving the yellow sub-pixel at the magnitude associated with the second projection may effectively reduce the intensity at which the red pixel needs to be driven to produce the desired color. The reduction may correspond to the magnitude of the R-vector such that, when aggregated, the red component of the yellow emitted light plus the R-vector equal or are substantially close to the red component of the first projected vector. Such a configuration may be more energy efficient than a conventional three-color arrangement in which each of the red, green, and blue sub-pixels would need to be driven at levels corresponding to the red, green, and blue components of the desired color. 
     More generally, according to embodiments disclosed herein, at least a one sub-pixel in a pixel may not be activated to emit an original color signal. For example, as described above, a desired color may be achieved without activating a green sub-pixel at all, by using a yellow sub-pixel to obtain the green component of the desired color. As described above, a second sub-pixel may be driven based on the difference between a first projection and a second projection, such as the red sub-pixel in the above example. A third sub-pixel may be driven based on the magnitude of the second projection as previously described, such as the yellow sub-pixel in the above example. Finally, a fourth sub-pixel may be driven based on the respective color component of the original color signal. For example, a blue sub-pixel may be driven at a level corresponding to the blue component of a color signal in the example described with respect to  FIG. 18 . Notably, there may be no need to drive the first sub-pixel at all, since the third sub-pixel can be driven at a magnitude that includes the necessary color component of the first sub-pixel. Additionally, the second sub-pixel may be driven at a reduced level as a result of the third sub-pixel being driven at a magnitude that includes at least a portion of the necessary color component of the second sub-pixel. Notably, the third sub-pixel may emit a color equivalent to a combination of colors emitted by the first and second sub-pixels. 
     According to embodiments disclosed herein, an original color signal may be defined by three primary colors. For example, the original color signal may be defined by red, green, and blue coordinates (R, G, B). Each primary color may have a sub-pixel associated with it. Additionally, a secondary sub-pixel may emit light having color coordinates (r′, g′, 0) such that it contains components of both R and G. The secondary sub-pixel may be driven at ((G×r′)/g′, G, 0) where (G×r′)/g′ corresponds to the red color component of the original signal (R, G, B) and G corresponds to the entire green component of the original signal. The red sub-pixel may be driven at (R−(G×r′)/g′, 0, 0), where R−(G×r′)/g′ corresponds to the remaining fraction of R such that (G×r′)/g′ and R−(G×r′)/g′ (i.e., the component of the secondary sub-pixel and the component of the red sub-pixel) in combination are equivalent to R, the red component of the original color signal. Accordingly, between the secondary sub-pixel and the red sub-pixel, both the R and the G component of the original color signal are covered. The sub-pixel corresponding to a blue emitted color may be driven at (0, 0, B) such that, among the secondary sub-pixel, the red sub-pixel, and the blue sub-pixel, the original color signal (R, G, B) is reproduced. Notably, the green sub-pixel in this example may not be activated to emit the original color signal (R, G, B). Here, the green sub-pixel in this example may not be activated because g′/r′ is greater than or equal to G/R, which corresponds to the secondary sub-pixel being capable of emitting the entire green component but not the entire red component. Described another way, in this case the projection onto the green/red color space corresponds to a point lying on the “red side” of the “yellow line”, i.e., the side on which the R-vector illustrated in  FIG. 18  falls. 
     As another illustrative example, as shown in  FIG. 19 , an original color signal may be represented by vector  1901  and a first projection of the original color signal may be determined onto the color space defined by the red and green sub-pixels. The first projection may correspond to the first projected vector  1902 . A second projection onto the Y-vector  1904  may be determined and may correspond to the point  1905  on the Y-vector. The point  1905  may correspond to the entire red component of the first projected vector  1902  and a fraction of the green component of the first projected vector  1902 . The entire green component may be represented by the point  1905  plus the G-vector  1903 , as shown in  FIG. 19 . 
     According to embodiments disclosed herein, an original color signal may be defined by three primary colors. For example, the original color signal may be defined by red, green, and blue coordinates (R, G, B). Each primary color may have a sub-pixel associated with it. Additionally, a secondary sub-pixel may emit light having color coordinates (r′, g′, 0) such that it contains components of both R and G. The secondary sub-pixel may be driven at (R, (R×g′)/r′, 0) where (R×g′)/r′ corresponds to a fraction of G, the green color component of the original signal (R, G, B) and R corresponds to the entire red component of the original signal. The green sub-pixel may be driven at (0, G−(R×g′)/r′, 0), where G−(R×g′)/r′ corresponds to the fraction of G remaining such that (R×g′)/r′ and G−(R×g′)/r′ (i.e., the component of the secondary sub-pixel and the component of the green sub-pixel) combine to form G, the green component of the original color signal. Accordingly, among the secondary sub-pixel and the green sub-pixel, both the R and the G component of the original color signal are covered. The sub-pixel corresponding to a blue emitted color may be driven at (0, 0, B) such that, between the secondary sub-pixel, the green sub-pixel, and the blue sub-pixel, the original color signal (R, G, B) is reproduced. Notably, the red sub-pixel in this example may not be activated to emit the original color signal (R, G, B). Here, the red sub-pixel in this example may not be activated because g′/r′ is less than G/R which corresponds to the secondary sub-pixel being capable of emitting the entire red component but not the entire green component. This case corresponds to that in which the projection lies on the “green side” of the Y-vector shown in  FIG. 19 , in contrast to the point illustrated in  FIG. 18 . 
     According to an embodiment of the disclosed subject matter, a secondary color sub-pixel (e.g., a yellow sub-pixel) may have a color space that can be defined by three or more primary colors. As an example,  FIG. 20 a    shows a color point  2030  which corresponds to a red component  2033  and a green component  2031 . Color point  2030  is defined by two primary colors. Similarly, a color space for a sub-pixel may be defined by two primary colors. As shown in  FIG. 18 , the color space for the yellow sub-pixel (Y-vector) is defined within the red green color space. The points on the yellow sub-pixel color space are all within the red green color space. Alternatively, if, for example, a color signal is such that it contains a red component, green component, and a blue component, it may be originally emitted by three sub-pixels corresponding to primary colors. Similar to the color signal that is defined by three primary colors sub-pixels, a color space for a sub-pixel may be defined by three or more primary colors. The techniques disclosed herein may be applied by first projecting the color space, for the sub-pixel defined by three or more primary colors, onto a color space defined by two colors (e.g, the Y-vector in  FIG. 18  would be a projection of the yellow sub-pixels color space rather than the actual color space itself). The sub-pixel defined by three or more primary colors may be driven based on the first and second projections onto the sub-pixel projection. Additionally, two sub-pixels may be operated at reduced values based on driving the sub-pixel defined by three or more primary colors. As an illustrative example, a yellow sub-pixel may have a color space defined by red, green, and blue colors. A projection of the yellow sub-pixels color space may correspond to the Y-vector shown in  FIG. 18 . Based on the techniques disclosed herein, the green sub-pixel may not be activated and the red sub-pixel may be driven at a reduced level based on driving the yellow sub-pixel at a magnitude associated with a projection onto the Y-vector. Additionally, the blue sub-pixel may operate at a reduced level based on the difference between the yellow sub-pixel&#39;s color space and the Y-vector (i.e., the projection of the yellow sub-pixel&#39;s color space onto the red-green color space). 
     If an original color signal is within the color space of two sub-pixels (i.e., instead of three sub-pixels), only a first projection may be needed and the original color signal may be emitted using only two sub-pixels. Here, a secondary sub-pixel color space may also be contained within the color space of the same two sub-pixels that the original color signal is in. Accordingly, the original color signal may be projected onto the color space of the secondary sub-pixel. The secondary sub-pixel may be driven based on the magnitude of the projection. The secondary sub-pixel may be driven such that it contains an entire first color component of the original color signal and a portion of a second color component of the original signal. A different sub-pixel may be driven to compensate for the remaining portion of the second color component of the original signal. Accordingly, only the secondary sub-pixel and the other sub-pixel may be driven of the at least four sub-pixels within the pixel. 
     It will be understood that at any given color signal, a given sub-pixel may be activated or not activated based on the color signal. As an example, using the notation introduced previously, when g′/r′ is greater than or equal to G/R, then a green sub-pixel may not be activated whereas when it is less than G/R, a red sub-pixel may not be activated. It will be understood that for two different color signals, a given sub-pixel may be active or not active. It will also be understood that although colors such as red, green, blue, yellow, magenta, and cyan are disclosed herein, the techniques disclosed herein may be applied to any color signals and/or sub-pixels associated with any colors. 
     As used herein, various components may be used as color altering layers as disclosed. Suitable components include color conversion layers, color filters, color changing layers, microcavities, and the like. The dyes used in color conversion layers as disclosed herein are not particularly limited, and any compounds may be used as long as the compound is capable of converting color of light emitted from a light source to a required color, which is basically a wavelength conversion element capable of converting the wavelength of the light from the light source to a wavelength 10 nm or more longer than that of the light of the light source. It may be an organic fluorescent substance, an inorganic fluorescent substance, or a phosphorescent substance, and may be selected according to the objective wavelength. Examples of the material include, but not limit to the following classes: xanthen, acridine, oxazine, polyene, cyanine, oxonol, benzimidazol, indolenine, azamethine, styryl, thiazole, coumarin, anthraquinone, napthalimide, aza[18]annulene, porphin, squaraine, fluorescent protein, 8-hydroxyquinoline derivative, polymethin, nanocrystal, protein, perylene, phthalocyanine and metal-ligand coordination complex. 
     Examples of the fluorescent dye for converting luminescence of from UV and higher energy light to blue light include, but not limit to the styryl-based dyes such as 1,4-bis(2-methylstyryl)benzene, and trans-4,4′-diphenylstilbene, and coumarin based dyes such as 7-hydroxy-4-methylcoumarin, and combinations thereof. 
     Examples of the fluorescent dye for converting luminescence of from blue light to green light include, but not limit to the coumarin dyes such as 2,3,5,6-1H,4H-tetrahydro-8-trifluoromethylquinolizino(9,9a,1-gh) coumarin, 3-(2′-benzothiazolyl)-7-diethylaminocoumarin, 3-(2′-benzimidazolyl)-7-N,N-diethylaminocoumarin, and naphthalimide dyes such as Basic Yellow 51, Solvent yellow 11 and Solvent Yellow 116, and pyrene dyes such as 8-Hydroxy-1,3,6-pyrenetrisulfonic acid trisodium salt (HPTS), and combinations thereof. 
     Examples of the fluorescent dye for converting luminescence of from blue to green light to red include, but not limit to the perylene based dyes such as N,N-bis(2,6-diisopropylphenyl)-1,6,7,12-tetraphenoxyperylene-3,4:9,10-tetracarboxdiimide (Lumogen Red F300), cyanine-based dyes such as 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl-4H-pyran, pyridine-based dyes such as 1-ethyl-2-(4-(p-dimethylaminophenyl)-1,3-butadienyl)-pyridinium perchlorate, and rhodamine-based dyes such as Rhodamine Band Rhodamine 6G, and oxazine-based dyes, and combinations thereof. 
     Examples of the inorganic fluorescent substance include, but not limit to an inorganic fluorescent substance comprising a metal oxide or metal chalcogenide doped with a transition metal ion, including a rare-earth metal ion. 
     Many metal-ligand coordination complexes can be used as dyes, they can be both fluorescent and phosphorescent substance. 
     It may be preferred to use a color conversion layer in the state that the layer is stacked on a color filter. The stacked structure thereof on the color filter makes it possible to make better color purity of light transmitted through the color conversion layer. In some configurations, a “color altering layer” as disclosed herein may include multiple components, such as a color filter disposed in a stack with a color conversion layer, or just a color conversion layer alone, or just a color filter alone. 
     The material used for color filters is not particularly limited. A filter may be made of, for example, a dye, a pigment and a resin, or only a dye or pigment. The color filter made of a dye, a pigment and a resin may be a color filter in the form of a solid wherein the dye and the pigment are dissolved or dispersed in the binder resin. 
     Examples of the dye or pigment used in the color filter include, but not limit to perylene, isoindoline, cyanine, azo, oxazine, phthalocyanine, quinacridone, anthraquinone, and diketopyrrolo-pyrrole, and combinations thereof. 
     As used herein, and as would be understood by one of skill in the art, a “color conversion layer” (e.g. a “down conversion layer”) may comprise a film of fluorescent or phosphorescent material which efficiently absorbs higher energy photons (e.g. blue light and/or yellow light) and reemits photons at lower energy (e.g. at green and/or red light) depending on the materials used. That is, the color conversion layer may absorb light emitted by an organic light emitting device (e.g. a white OLED) and reemit the light (or segments of the wavelengths of the emission spectrum of the light) at a longer wavelength. A color conversion layer may be a layer formed by mixing the fluorescent medium material contained in the above-mentioned color conversion layer with the color filter material. This makes it possible to give the color conversion layer a function of converting light emitted from an emitting device and further a color filter function of improving color purity. Thus, the structure thereof is relatively simple. 
     Embodiments disclosed herein may be incorporated into a wide variety of products and devices, such as flat panel displays, smartphones, transparent displays, flexible displays, televisions, portable devices such as laptops and pad computers or displays, multimedia devices, and general illumination devices. Displays as disclosed herein also may have relatively high resolutions, including 250 dpi, 300 dpi, 350 dpi, or more. 
     It is understood that the various embodiments described herein are by way of example only, and are not intended to limit the scope of the invention. For example, many of the materials and structures described herein may be substituted with other materials and structures without deviating from the spirit of the invention. The present invention as claimed may therefore include variations from the particular examples and preferred embodiments described herein, as will be apparent to one of skill in the art. It is understood that various theories as to why the invention works are not intended to be limiting.