Patent Description:
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 <CIT>, <CIT>, and <CIT>.

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(<NUM>-phenylpyridine) iridium, denoted Ir(ppy)<NUM>, which has the structure of Formula I:
<CHM>.

In this, and later figures herein, we depict the dative bond from nitrogen to metal (here, Ir) as a straight line.

Document <CIT> relates to an organic electroluminescent display apparatus including a plurality of pixels constituting a display portion.

Document <CIT> relates to an OLED device which includes an array of light-emitting pixels having organic layers.

Document <CIT> relates to a display device.

Document <CIT> relates to an organic electroluminescent display.

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.

Similarly, a higher LUMO energy level corresponds to an electron affinity (EA) having a smaller absolute value.

(an EA that is less negative).

A device that may be used as a multi-color pixel is provided. The device has a first organic light emitting device, a second organic light emitting device, a third organic light emitting device, and a fourth organic light emitting device. The device may be a pixel of a display having four sub-pixels.

The first organic light emitting device emits red light, the second organic light emitting device emits green light, the third organic light emitting device emits light blue light, and the fourth organic light emitting device emits deep blue light. The peak emissive wavelength of the third and fourth devices differ by at least <NUM>. As used herein, "red" means having a peak wavelength in the visible spectrum of <NUM> - <NUM>, "green" means having a peak wavelength in the visible spectrum of <NUM> - <NUM>, "light blue" means having a peak wavelength in the visible spectrum of <NUM> - <NUM>, and "deep blue" means having a peak wavelength in the visible spectrum of <NUM> - <NUM>.

The first, second and third organic light emitting devices each have an emissive layer that includes a phosphorescent organic material that emits light when an appropriate voltage is applied across the device. The fourth organic light emitting device has an emissive layer that includes an organic emissive material, which may be phosphorescent or fluorescent, that emits light when an appropriate voltage is applied across the device.

The first, second, third and fourth organic light emitting devices may have the same surface area, or may have different surface areas. The first, second, third and fourth organic light emitting devices may be arranged in a quad pattern, in a row, or in some other pattern.

The device may be operated to emit light having a desired CIE coordinate by using at most three of the four devices for any particular CIE coordinate. Use of the deep blue device may be significantly reduced compared to a display having only red, green and deep blue devices.

Non- radiative mechanisms, such as thermal relaxation, may also occur, but are generally considered undesirable.

Device <NUM> may include a substrate <NUM>, an anode <NUM>, a hole injection layer <NUM>, a hole transport layer <NUM>, an electron blocking layer <NUM>, an emissive layer <NUM>, a hole blocking layer <NUM>, an electron transport layer <NUM>, an electron injection layer <NUM>, a protective layer <NUM>, and a cathode <NUM>. <NUM>- <NUM>.

More examples for each of these layers are available. For example, a flexible and transparent substrate-anode combination is disclosed in <CIT>. An example of a p-doped hole transport layer is m- MTDATA doped with F. <NUM>-TCNQ at a molar ratio of <NUM>:<NUM>, as disclosed in <CIT>. Examples of emissive and host materials are disclosed in <CIT>. An example of an n- doped electron transport layer is BPhen doped with Li at a molar ratio of <NUM>:<NUM>, as disclosed in <CIT>. <CIT> and <CIT>, 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 <CIT> and <CIT>. Examples of injection layers are provided in <CIT>. A description of protective layers may be found in <CIT>.

The simple layered structure illustrated in <FIG> and <FIG> 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 <NUM>, hole transport layer <NUM> transports holes and injects holes into emissive layer <NUM>, 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 <FIG> and <FIG>.

Structures and materials not specifically described may also be used, such as OLEDs comprised of polymeric materials (PLEDs) such as disclosed in <CIT>. By way of further example, OLEDs having a single organic layer may be used. OLEDs may be stacked, for example as described in <CIT> The OLED structure may deviate from the simple layered structure illustrated in <FIG> and <FIG>. For example, the substrate may include an angled reflective surface to improve out-coupling, such as a mesa structure as described in <CIT>, and/or a pit structure as described in <CIT>.

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 <CIT> and <CIT>, organic vapor phase deposition (OVPD), such as described in <CIT>, and deposition by organic vapor jet printing (OVJP), such as described in <CIT>. 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 <CIT> and <CIT>, 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 <NUM> carbons, may be used in small molecules to enhance their ability to undergo solution processing. Substituents having <NUM> carbons or more may be used, and <NUM>-<NUM> 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 invention may be incorporated into a wide variety of consumer products, including flat panel displays, computer monitors, televisions, billboards, lights for interior or exterior illumination and/or signalling, 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, theatre 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 intended for use in a temperature range comfortable to humans, such as <NUM> degrees C. to <NUM> degrees C, and more preferably at room temperature (<NUM>-<NUM> degrees C).

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 <CIT> at cols. <NUM>-<NUM>.

One application for phosphorescent emissive molecules is a full color display, preferably an active matrix OLED (AMOLED) display. One factor that currently limits AMOLED display lifetime and power consumption is the lack of a commercial blue phosphorescent OLED with saturated CIE coordinates.

<FIG> shows the <NUM> CIE chromaticity diagram, developed in <NUM> by the International Commission on Illumination, usually known as the CIE for its French name Commission Internationale de l'Eclairage. Any color can be described by its x and y coordinates on this diagram. A "saturated" color, in the strictest sense, is a color having a point spectrum, which falls on the CIE diagram along the U-shaped curve running from blue through green to red. The numbers along this curve refer to the wavelength of the point spectrum. Lasers emit light having a point spectrum.

<FIG> shows another rendition of the <NUM> chromaticity diagram, which also shows several color "gamuts. " A color gamut is a set of colors that may be rendered by a particular display or other means of rendering color. In general, any given light emitting device has an emission spectrum with a particular CIE coordinate. Emission from two devices can be combined in various intensities to render color having a CIE coordinate anywhere on the line between the CIE coordinates of the two devices. Emission from three devices can be combined in various intensities to render color having a CIE coordinate anywhere in the triangle defined by the respective coordinates of the three devices on the CIE diagram. The three points of each of the triangles in <FIG> represent industry standard CIE coordinates for displays. For example, the three points of the triangle labelled "NTSC / PAL / SECAM / HDTV gamut" represent the colors of red, green and blue (RGB) called for in the sub-pixels of a display that complies with the standards listed. A pixel having sub-pixels that emit the RGB colors called for can render any color inside the triangle by adjusting the intensity of emission from each subpixel.

A full color display using phosphorescent OLEDS is desirable for many reasons, including the theoretical high efficiency of such devices, cost advantages, and device flexibility. While the industry has achieved red and green phosphorescent devices that have high efficiency and long lifetimes suitable for use in displays, there may still be issues with blue phosphorescent devices, and particularly blue phosphorescent devices that have the CIE coordinates called for by industry standards such as HDTV and NTSC. The CIE coordinates called for by NTSC standards are: red (<NUM>, <NUM>); green (<NUM>, <NUM>); blue (<NUM>, <NUM>). There are devices having suitable lifetime and efficiency properties that are close to the blue called for by industry standards, but far enough from the standard blue that the display fabricated with such devices instead of the standard blue would have noticeable shortcomings in rendering blues. The blue called for industry standards is a "deep" blue as defined below, and the colors emitted by efficient and long-lived blue phosphorescent devices are generally "light" blues as defined below.

A display is provided which allows for the use of highly efficient and long-lived phosphorescent devices, including red, green and light blue phosphorescent devices, while still allowing for the rendition of colors that include a deep blue component. This is achieved by using a quad pixel, i.e., a pixel with four devices. Three of the devices are highly efficient and long-lived phosphorescent devices, emitting red, green and light blue light, respectively. The fourth device emits deep blue light, and may be less efficient or less long lived that the other devices. However, because many colors can be rendered without using the fourth device, its use can be limited such that the overall lifetime and efficiency of the display does not suffer much from its inclusion.

A device is provided. The device has a first organic light emitting device, a second organic light emitting device, a third organic light emitting device, and a fourth organic light emitting device. The device may be a pixel of a display having four sub-pixels. A preferred use of the device is in an active matrix organic light emitting display, which is a type of device where the shortcomings of deep blue OLEDs are currently a limiting factor.

The first organic light emitting device emits red light, the second organic light emitting device emits green light, the third organic light emitting device emits light blue light, and the fourth organic light emitting device emits deep blue light. The peak emissive wavelength of the third and fourth devices differ by at least <NUM>. As used herein, "red" means having a peak wavelength in the visible spectrum of <NUM> - <NUM>, "green" means having a peak wavelength in the visible spectrum of <NUM> - <NUM>, "light blue" means having a peak wavelength in the visible spectrum of <NUM> - <NUM>, and "deep blue" means having a peak wavelength in the visible spectrum of <NUM> - <NUM>. Preferred ranges include a peak wavelength in the visible spectrum of <NUM> - <NUM> for red and <NUM> - <NUM> for green.

To add more specificity to the wavelength-based definitions, "light blue" may be further defined, in addition to having a peak wavelength in the visible spectrum of <NUM> - <NUM> that is at least <NUM> greater than that of a deep blue OLED in the same device, as preferably having a CIE x-coordinate less than <NUM> and a CIE y-coordinate less than <NUM>, and "deep blue" may be further defined, in addition to having a peak wavelength in the visible spectrum of <NUM> - <NUM>, as preferably having a CIE y-coordinate less than <NUM> and preferably less than <NUM>, and the difference between the two may be further defined such that the CIE coordinates of light emitted by the third organic light emitting device and the CIE coordinates of light emitted by the fourth organic light emitting device are sufficiently different that the difference in the CIE x-coordinates plus the difference in the CIE y- coordinates is at least <NUM>. As defined herein, the peak wavelength is the primary characteristic that defines light and deep blue, and the CIE coordinates are preferred.

More generally, "light blue" means having a peak wavelength in the visible spectrum of <NUM> - <NUM>, and "deep blue" means having a peak wavelength in the visible spectrum of <NUM> - <NUM>, and at least <NUM> less than the peak wavelength of the light blue.

In another embodiment, "light blue" means having a CIE y coordinate less than <NUM>, and "deep blue" means having a CIE y coordinate at least <NUM> less than that of "light blue.

In another embodiment, the definitions for light and deep blue provided herein may be combined to reach a narrower definition. For example, any of the CIE definitions may be combined with any of the wavelength definitions. The reason for the various definitions is that wavelengths and CIE coordinates have different strengths and weaknesses when it comes to measuring color. For example, lower wavelengths normally correspond to deeper blue. But a very narrow spectrum having a peak at <NUM> may be considered "deep blue" when compared to another spectrum having a peak at <NUM>, but a significant tail in the spectrum at higher wavelengths. This scenario is best described using CIE coordinates. It is expected that, in view of available materials for OLEDs, that the wavelength-based definitions are well-suited for most situations. In any event, embodiments of the invention include two different blue pixels, however the difference in blue is measured.

"Red" and "green" phosphorescent devices having lifetimes and efficiencies suitable for use in a commercial display are well known and readily achievable, including devices that emit light sufficiently close to the various industry standard reds and greens for use in a display. Examples of such devices are provided in <NPL>); see also <NPL>).

"Light blue" phosphorescent devices having desirable lifetime and efficiency properties are also readily achievable. Such devices may be used in displays without adversely affecting the lifetime or efficiency of the display. However, a display relying on a light blue device may not be able to properly render colors requiring a deep blue component. An example of a suitable light blue device has the structure:.

Such a device has been measured to have a lifetime of <NUM>. 3khrs from initial luminance 1000nits at constant dc current to <NUM>% of initial luminance, <NUM> CIE coordinates of CIE (<NUM>, <NUM>), and a peak emission wavelength of <NUM> in the visible spectrum.

"Deep blue" devices are also readily achievable, but not necessarily having the lifetime and efficiency properties desired for a display suitable for consumer use. One way to achieve a deep blue device is by using a fluorescent emissive material that emits deep blue, but does not have the high efficiency of a phosphorescent device. An example of a deep blue fluorescent device is provided in <NPL>). Funahashi discloses a deep blue fluorescent device having CIE coordinates of (<NUM>, <NUM>) and a peak wavelength of <NUM>. Another way is to use a phosphorescent device having a phosphorescent emissive material that emits light blue, and to adjust the spectrum of light emitted by the device through the use of filters or microcavities. Filters or microcavities can be used to achieve a deep blue device, as described in <NPL>), but there may be an associated decrease in device efficiency. Indeed, the same emitter may be used to fabricate a light blue and a deep blue device, due to microcavity differences. Another way is to use available deep blue phosphorescent emissive materials, such as described in <CIT> and for compounds shown at pages <NUM>-<NUM>. However, such devices may have lifetime issues. An example of a suitable deep blue device using a phosphorescent emitter has the structure:
ITO (<NUM>)/Compound C(<NUM>)/NPD (<NUM>)/Compound A: Emitter B (<NUM>:<NUM>%)/Compound A (<NUM>) / Alq3 (<NUM>)/LiF(lnm)/Al (l00nm).

Such a device has been measured to have a lifetime of <NUM> hrs from initial luminance l000nits at constant dc current to <NUM>% of initial luminance, <NUM> CIE coordinates of CIE: (<NUM>, <NUM>), and a peak emissive wavelength of <NUM>.

The difference in the performance of available deep blue and light blue devices may be significant. For example, a deep blue device may have good efficiency, but a lifetime that is less than <NUM>% or less than <NUM>% of that of a light blue device. Such a difference in lifetime may describe many deep blue phosphorescent devices. A standard way to measure lifetime is LT<NUM> at an initial luminance of 1000nits, i.e., the time required for the light output of a device to fall by <NUM>% when run at a constant current that results in an initial luminance of 1000nits. Or, a deep blue device may have a good lifetime, but an efficiency that is less than <NUM>% or less than <NUM>% of that of a light blue device. Such a difference in efficiency may describe many fluorescent deep blue devices, or deep blue devices obtained by color-shifting the emission from a light blue phosphorescent emitter, for example by using microcavities.

A device or pixel having four organic light emitting devices, one red, one green, one light blue and one deep blue, may be used to render any color inside the shape defined by the CIE coordinates of the light emitted by the devices on a CIE chromaticity diagram. <FIG> illustrates this point. <FIG> should be considered with reference to the CIE diagrams of <FIG> and <FIG>, but the actual CIE diagram is not shown in <FIG> to make the illustration clearer. In <FIG>, point <NUM> represents the CIE coordinates of a red device, point <NUM> represents the CIE coordinates of a green device, point <NUM> represents the CIE coordinates of a light blue device, and point <NUM> represents the CIE coordinates of a deep blue device. The pixel may be used to render any color inside the triangle defined by points <NUM>, <NUM> and <NUM>. If the CIE coordinates of points <NUM>, <NUM> and <NUM> correspond to, or at least encircle, the CIE coordinates of devices called for by a standard gamut - such as the corners of the triangles in <FIG> - the device may be used to render any color in that gamut.

However, because there is also a light blue device that emits light having CIE coordinates represented by point <NUM>, many of the colors inside the triangle defined by points <NUM>, <NUM> and <NUM> can be rendered without using the deep blue device. Specifically, any color inside the triangle defined by points <NUM>, <NUM> and <NUM> may be rendered without using the deep blue device. The deep blue device would only be needed for colors inside the triangle defined by points <NUM>, <NUM> and <NUM>, or inside the triangle defined by points <NUM>, <NUM> and <NUM>. Depending upon the color content of the images in question, only minimal use of the deep blue device may be needed. In addition, for colors where the deep blue device is needed, i.e., for colors inside the triangle defined by points <NUM>, <NUM> and <NUM>, or inside the triangle defined by points <NUM>, <NUM> and <NUM>, the contribution needed from the deep blue device may be much less when the light blue device is being used, because, for many of these colors, much of the blue contribution can come from the light blue device. For example, for a color having a CIE coordinate much closer to point <NUM> than any of the other points, most or all of the contribution would come from the light blue device. In a comparable pixel without a light blue device, most or all of the contribution would come from the deep blue device, which is closest to point <NUM>.

Although <FIG> shows a "light blue" device having CIE coordinates <NUM> that are inside the triangle defined by the CIE coordinates <NUM>, <NUM> and <NUM> of the red, green and deep blue devices, respectively, the light blue device may have CIE coordinates that fall outside of said triangle. For example, CIE coordinates for a light blue device may fall to the left of the line between coordinates <NUM> and <NUM>, and still meet the definition of "light blue" included herein.

A preferred way to operate a device having a red, green, light blue and deep blue device, or first, second, third and fourth devices, respectively, as described herein is to render a color by using only the devices that define the smallest triangle in CIE space that include the color. Thus, points <NUM>, <NUM> and <NUM> define a first triangle <NUM> in the CIE space; points <NUM>, <NUM> and <NUM> define a second triangle <NUM> in the CIE space; and points <NUM>, <NUM> and <NUM> define a third triangle <NUM> in the CIE space. If a desired color has CIE coordinates falling within the first triangle, only the first, second and third devices, but not the fourth device, are used to render the color. If a desired color has CIE coordinates falling within the second triangle, only the first, third and fourth devices, but not the second device, are used to render the color.

If a desired color has CIE coordinates falling within the third triangle, only the second, third and fourth devices, but not the first device, are used to render the color.

Such a device could be operated in other ways as well. For example, light having the CIE coordinates of the light blue device may be achieved by using a combination of the deep blue device, the red device and the green device, and such light could be used in whole or in part to replace the contribution of the light blue device. However, such use would not achieve the purpose of minimizing use of the deep blue device.

Algorithms have been developed in conjunction with RGBW (red, green, blue, white) devices that may be used to map a RGB color to an RGBW color. Similar algorithms may be used to map an RGB color to RG Bl B2. Such algorithms, and RGBW devices generally, are disclosed in <NPL>); <NPL>) ("Spindler"); <NPL>) ("Peng"); <NPL>). RGBW displays are significantly different from those disclosed herein because they still need a good deep blue device. Moreover, there is teaching that the "fourth" or white device of an RGBW display should have particular "white" CIE coordinates, see Spindler at <NUM> and Peng at <NUM>.

A device having four different organic light emitting devices, each emitting a different color, may have a number of different configurations. <FIG> illustrates some of these configurations. In <FIG>, R is a red-emitting device, G is a green-emitting device, Bl is a light blue emitting device, and B2 is a deep blue emitting device.

Configuration <NUM> shows a quad configuration, where the four organic light emitting devices making up the overall device or multicolor pixel are arranged in a two by two array. Each of the individual organic light emitting devices in configuration <NUM> has the same surface area. In a quad pattern, each pixel could use two gate lines and two data lines.

Configuration <NUM> shows a quad configuration where some of the devices have surface areas different from the others. It may be desirable to use different surface areas for a variety of reasons. For example, a device having a larger area may be run at a lower current than a similar device with a smaller area to emit the same amount of light. The lower current may increase device lifetime. Thus, using a relatively larger device is one way to compensate for devices having a lower expected lifetime.

Configuration <NUM> shows equally sized devices arranged in a row, and configuration <NUM> shows devices arranged in a row where some of the devices have different areas. Patterns other than those specifically illustrated may be used.

Other configurations may be used. For example, a stacked OLED with four separately controllable emissive layers, or two stacked OLEDs each with two separately controllable emissive layers, may be used to achieve four subpixels that can each emit a different color of light.

Various types of OLEDs may be used to implement various configurations, including transparent OLEDs and flexible OLEDs.

Displays with devices having four sub-pixels, in any of the various configurations illustrated and in other configurations, may be fabricated and patterned using any of a number of conventional techniques. Examples include shadow mask, laser induced thermal imaging (LITI), ink-jet printing, organic vapor jet printing (OVJP), or other OLED patterning technology. An extra masking or patterning step may be needed for the emissive layer of the fourth device, which may increase fabrication time. The material cost may also be somewhat higher than for a conventional display. These additional costs would be offset by improved display performance.

A single pixel,in an example not forming part of the claimed invention, may incorporate more than the four sub-pixels disclosed herein, possibly with more than four discrete colors. However, due to manufacturing concerns, four sub-pixels per pixel is preferred.

Claim 1:
A device having a four sub-pixels pixel, the pixel comprising: a first organic light emitting device that emits light having a peak wavelength in the visible spectrum of <NUM> - <NUM>, further comprising a first emissive layer having a first emissive material, wherein the first emissive material emits light from its triplet state;
a second organic light emitting device that emits light having a peak wavelength in the visible spectrum of <NUM> - <NUM>, further comprising a second emissive layer having a second emissive material, wherein the second emissive material emits light from its triplet state;
a third organic light emitting device that emits light having a peak wavelength in the visible spectrum of <NUM> - <NUM>, further comprising a third emissive layer having a third emissive material, wherein the third emissive material emits light from its triplet state; and
a fourth organic light emitting device that emits light having a peak wavelength in the visible spectrum of <NUM> to <NUM>, further comprising a fourth emissive layer having a fourth emissive material;
wherein the third and fourth organic light-emitting devices have different emissive materials;
wherein at least one of the following (a) and (b) holds true:
(a) the peak wavelength in the visible spectrum of light emitted by the fourth organic light emitting device is at least <NUM> less than the peak wavelength in the visible spectrum of light emitted by the third organic light emitting device; and
(b) the third organic light emitting device emits light having a CIE y coordinate less than <NUM> and the fourth organic light emitting device emits light having a CIE y coordinate at least <NUM> less than that of light emitted by the third organic light emitting device.