Patent Application
20060232195
The present invention relates to organic light-emitting diode (OLED) devices and, more particularly, to an OLED device having improved light output and power distribution.
Organic light-emitting diode (OLED) devices, also referred to as organic electroluminescent (EL) devices, have numerous well-known advantages over other flat-panel display devices currently in the market place. Among the potential advantages are brightness of light emission, relatively wide viewing angle, reduced device thickness, and reduced electrical power consumption compared to, for example, liquid crystal displays (LCDs) using backlighting.
Applications of OLED devices include active-matrix image displays, passive-matrix image displays, and area-lighting devices such as, for example, selective desktop lighting. Irrespective of the particular OLED device configuration tailored to these broad fields of applications, all OLEDs function on the same general principles. An organic electroluminescent (EL) medium structure is sandwiched between two electrodes. At least one of the electrodes is at least partially light transmissive. These electrodes are commonly referred to as an anode and a cathode in analogy to the terminals of a conventional diode. When an electrical potential is applied between the electrodes so that the anode is connected to the positive terminal of a voltage source and the cathode is connected to the negative terminal, the OLED is said to be forward biased. Positive charge carriers (holes) are injected from the anode into the EL medium structure, and negative charge carriers (electrons) are injected from the cathode. Such charge carrier injection causes current flow from the electrodes through the EL medium structure. Recombination of holes and electrons within a zone of the EL medium structure results in emission of light from this zone that is, appropriately, called the light-emitting zone or interface. The organic EL medium structure can be formed of a stack of sublayers that can include small molecule layers and polymer layers. Such organic layers and sublayers are well known and understood by those skilled in the OLED art.
Referring to
Full-color OLED devices may employ a variety of organic materials to emit different colors of light. In this arrangement, the OLED device is patterned with different sets of organic materials, each set of organic materials associated with a particular color of light emitted. Each pixel in an active-matrix full-color OLED device typically employs each set of organic materials, for example to form a red, green, and blue sub-pixel. The patterning is typically done by evaporating layers of organic materials through a mask. In an alternative arrangement, a single set of organic materials emitting broadband light may be deposited in continuous layers on either the substrate (for a top-emitter) or cover (for a bottom-emitter) with arrays of differently colored filters employed to create a full-color OLED device.
The emitted light is directed towards an observer, or towards an object to be illuminated, through the light transmissive electrode. If the light transmissive electrode is between the substrate and the light emissive elements of the OLED device, the device is called a bottom-emitting OLED device. Conversely, if the light transmissive electrode is not between the substrate and the light emissive elements, the device is referred to as a top-emitting OLED device. The present invention may be directed to either a top-emitting or bottom-emitting OLED device. However, in one embodiment, because of the limitations on a transparent electrode that are overcome in the present invention, a top-emitting OLED device is preferred.
In top-emitting OLED devices, light is emitted through an upper electrode or top electrode, typically but not necessarily the cathode, which has to be sufficiently light transmissive, while the lower electrode(s) or bottom electrode(s), typically but not necessarily the anode, can be made of relatively thick and electrically conductive metal compositions which can be optically opaque. Because light is emitted through an electrode, it is important that the electrode through which light is emitted be sufficiently light transmissive to avoid absorbing the emitted light. Typical prior-art materials proposed for such electrodes include indium tin oxide (ITO) and very thin layers of metal, for example silver or metal alloys including silver. However, the current carrying capacity of such electrodes is limited, thereby limiting the amount of light that can be emitted from the organic layers.
Applicants have demonstrated that in order to supply adequate uniform current to an OLED while permitting adequate amounts of light to escape from the device without employing additional electrode bussing, the sheet resistance of a transparent electrode suitable for use in a prior-art top-emitter OLED device configuration such as
The use of sufficiently thin metal layers providing at least partial transparency may alternatively be employed to provide adequate conductivity. For instance, microcavity structures employing a highly reflective bottom electrode and a partially reflective, semi-transparent continuous top electrode have been proposed as a means for increasing the light output from the OLED device. By carefully tuning the thickness of the layers between these two electrodes, an appropriate color of light with increased brightness can be emitted from the OLED, even with the metal upper electrode. Applicants have demonstrated good results with a 20 nm thick layer of silver as an upper electrode and an aluminum or silver bottom anode. 20 nm of silver may have a sheet resistance of 0.80 ohms per square. Such microcavity designs are known in the art, see for example US 2004/0140757 and 2004/0155576. US 2005/0073228 describes a white-light emitting OLED apparatus comprising a microcavity OLED device and a light-integrating element, wherein the microcavity OLED device has a white light emitting organic EL element and the microcavity OLED device is configured to have angular-dependent narrow-band emission, and the light-integrating element integrates the angular-dependent narrow-band emission from different angles from the microcavity OLED device to form white light emission. However, such microcavity designs require very precise control of layer thicknesses in order to obtain the desired color, and may also create a strong angular dependence on the color of light emitted, especially if broadband emitters are employed, and do not emit all of the light created in the OLED.
In an alternative approach to overcoming the problem of inadequate transparent electrode materials, a transparent electrode bussing scheme may be considered. Referring to
A typical top-emitter OLED device as proposed in the art uses a glass substrate, a reflective conducting anode comprising a metal, for example aluminum, a stack of organic layers, and a transparent cathode layer, employing, for example indium-tin-oxide (ITO). Light generated from the device is emitted through the top transparent electrode. However, as noted above, applicants have determined that the ITO is insufficiently conductive for most practical applications. Such proposed devices will not be bright enough and will be subject to variability in light output over the surface of the device due to resistance variations at different locations on the device. Moreover, in these typical devices, the index of the ITO layer, the organic layers, and the glass is about 2.0, 1.7, and 1.5 respectively. It has been estimated that nearly 60% of the generated light is trapped by internal reflection in the ITO/organic EL element, 20% is trapped in the glass substrate, and only about 20% of the generated light is actually emitted from the device and performs useful functions.
A variety of techniques have been proposed to improve the out-coupling of light from thin-film light emitting devices. For example, diffraction gratings have been proposed to control the attributes of light emission from thin polymer films by inducing Bragg scattering of light that is guided laterally through the emissive layers; see “Modification of polymer light emission by lateral microstructure” by Safonov et al., Synthetic Metals 116, 145-148 (2001) and “Bragg scattering from periodically microstructured light emitting diodes” by Lupton et al., Applied Physics Letters. 77, 3340-3342 (2000). Brightness enhancement films having diffractive properties and surface and volume diffusers are described in WO0237568 A1 entitled “Brightness and Contrast Enhancement of Direct View Emissive Displays” by Chou et al., published May 10, 2002. However, such diffractive techniques cause a significant frequency dependence on the angle of emission so that the color of the light emitted from the device changes with the viewer's perspective.
Scattering techniques are also known to improve light output from an OLED device. Chou (International Publication Number WO 02/37580 A1) and Liu et al. (U.S. Patent Application Publication No. 2001/0026124 A1) taught the use of a volume or surface scattering layer to improve light extraction. The scattering layer is applied next to the organic layers or on the outside surface of the glass substrate or cover and has an optical index that matches these layers. Light emitted from the OLED device at higher than critical angle that would have otherwise been trapped can penetrate into the scattering layer and be scattered out of the device.
Light-scattering layers used externally to an OLED device are described in U.S. Patent Application Publication No. 2005/0018431 entitled “Organic electroluminescent devices having improved light extraction” by Shiang and U.S. Pat. No. 5,955,837 entitled “System with an active layer of a medium having light-scattering properties for flat-panel display devices” by Horikx, et al. These disclosures describe and define properties of scattering layers located on a substrate in detail. Likewise, U.S. Pat. No. 6,777,871 entitled “Organic ElectroLuminescent Devices with Enhanced Light Extraction” by Duggal et al. describes the use of an output coupler comprising a composite layer having specific refractive indices and scattering properties.
The use of light scattering techniques may increase the light-output efficiency of an OLED device, but have not been proposed to address the difficulty of providing a sufficiently transparent top electrode with adequate conductivity.
There is a need therefore for an improved organic light-emitting diode device structure that increases the light output, that does not exhibit a color dependence on angle of emission, and that provides improved conductivity of the transparent electrode without a burdensome manufacturing process.
In accordance with one embodiment, the invention is directed towards an organic light-emitting diode (OLED) device, comprising: a reflective element, a partially reflective semi-transparent electrode through which light from the OLED is emitted, and at least one layer of organic light-emitting material disposed between the reflective element and the partially reflective semi-transparent electrode; wherein either the reflective element comprises a reflective electrode or a transparent electrode is positioned between the reflective element and the organic light emitting material, and further comprising a light scattering element optically integrated into the OLED device for scattering light emitted by the light-emitting layer and reflected by the reflective element and the partially reflective semi-transparent electrode.
The present invention has the advantage that it increases the light output from, and reduces the manufacturing cost of, an OLED device.
It will be understood that the figures are not to scale since the individual layers are too thin and the thickness differences of various layers too great to permit depiction to scale.
Referring to
The OLED device may further comprise a substrate 10 whereon the OLED is formed and an encapsulating cover 20 to seal and protect the OLED device. The reflective element electrode 12 or transparent electrode 13 may be pixelated to form distinct light emitting areas. The partially reflective semi-transparent electrode 16 may also be patterned or it may be a continuous, unpatterned layer (as shown). The partially reflective semi-transparent electrode 16 preferably has a transparency of at least 20%, more preferably at least 50%, and most preferably at least 70% at 550 nm. In a particular embodiment, the partially reflective semi-transparent electrode may have a transparency of from 20% to 90% at 550 nm. The organic material layers 14 may comprise organic materials known in the art, for example, hole-injection, hole-transport, light-emitting, electron-injection, and/or electron-transport layers. Such organic material layers are well known in the OLED art.
As employed herein, a light scattering element is an optical layer or surface that tends to randomly redirect any light that impinges on the layer or surface from any direction. The light scattering element 22 is optically integrated into the OLED device for scattering light emitted by the light-emitting layer and reflected by the reflective element 12 or 15 and the partially reflective semi-transparent electrode 16. The presence of an optically integrated scattering layer in accordance with the present invention defeats standing waves that might otherwise form between the reflective element 12 or 15 and the partially reflective semi-transparent electrode 16, and thereby substantially prevents an optical microcavity from forming. This, in turn, reduces or eliminates any dependency on angle for the light that is emitted from the OLED.
If the light scattering element 22 is located between the reflective element 12 or 15 and the partially reflective semi-transparent electrode 16, optical integration means that light reflected between the reflective element 12 or 15 and the partially reflective semi-transparent electrode 16 is modified so as to be redirected. If the light scattering element 22 is a part of either the reflective element 12 or 15 or the partially reflective semi-transparent electrode 16, optical integration means that the reflective optical behavior of the reflective element 12 or 15 or partially reflective semi-transparent electrode 16 is modified. For example, a light scattering element integrated into the reflective element causes the reflective element to scatter the reflected light and may be accomplished by constructing a reflector that has a rough surface rather than a smooth planar surface. If the light scattering element is integrated into a transparent, partially reflective, or reflective electrode, it causes the electrode to scatter the light that passes through or is reflected from the electrode and may be accomplished by constructing the transparent electrode with a transparent conductive scattering layer, a partially reflective, or a reflective scattering layer. The scattering element may also be optically integrated with a reflective or partially reflective layer if it is formed on the side of the reflective or partially reflective layer opposite the side adjacent to the light emitting layer and the reflective or partially reflective layer is sufficiently thin that the light encounters the scattering elements when it impinges on the reflective or partially reflective layer, for example if the reflective or partially reflective layer has a thickness less than the wavelength of the light reflected.
A transparent low-index layer 18 (possibly an air gap) having a refractive index lower than the refractive index of the cover 20 and organic layers 14 may be located between the scattering element 22 and the encapsulating cover 20. The use of a transparent low-index layer 18 having a refractive index lower than the refractive index of the encapsulating cover 20 and organic layers 14 to enhance the sharpness of an OLED device having a scattering element 22 is described in co-pending, commonly assigned U.S. Ser. No. 11/065,082 filed Feb. 24, 2005, the disclosure of which is hereby incorporated by reference, and may be employed in concert with the present invention.
In various embodiments the scattering element 22 may be adjacent to the partially reflective semi-transparent electrode 16 opposite the organic layers 14 or between the reflective element 12 and the partially reflective semi-transparent electrode 16 as illustrated in
Partially reflective semi-transparent electrode 16 is preferably made of metal, metal oxides, or metal alloys, for example aluminum, silver, ytterbium, magnesium silver, or indium tin oxide, or combinations of layers of such materials, and may incorporate other dopants and/or layers such as lithium and molybdenum to enhance the conductivity or electron-injection capabilities of the partially reflective semi-transparent electrode 16. To provide the advantage of increased conductivity and lower sheet resistance (e.g., preferably of less than 3.20, more preferably less than 2.0 and most preferably less than 1.0 ohms per square) while obtaining sufficient transparency, semi-transparent electrode 16 preferably comprises a metal layer of at least about 5 nanometers to provide adequate current-carrying capability, and may more preferably have a thickness of 10 nm or 20 nm or more to provide additional conductivity. The metal layer thickness is also preferably less than 50 nm, however, to enable sufficient transparency. Such metal layers will typically reflect at least a few percent, and more typically at least 10 percent, of emitted light. In the absence of employing a scattering element 22 in accordance with the invention, such reflected light will create an undesired dependence on viewing angle for the color of light emitted from the device. In a preferred embodiment, partially reflective semi-transparent electrode 16 comprises silver.
According to an embodiment of the present invention, the partially reflective semi-transparent electrode 16 is unpatterned and does not include additional patterned material, for example a grid pattern of thick conductors either formed directly on the partially reflective semi-transparent electrode 16 or connected to it. Likewise, in this embodiment, the partially reflective semi-transparent electrode 16 would not employ vias to electrode busses to provide additional current distribution in an OLED device. Such patterned elements require additional manufacturing process steps that raise the cost of such OLED devices and may not need to be employed in the present invention. Alternatively, patterned elements such as a grid conductors and vias with busses may be employed in concert with the present invention to further improve the conductivity of the partially reflective semi-transparent electrode 16. The partially reflective semi-transparent electrode 16 is preferably deposited directly over the organic layers 14 in a single continuous deposition step, for example by sputtering and does not require masking within the light-emitting area of the OLED device.
In operation, a voltage differential is supplied to the electrodes in the OLED device. Current flows through the organic layers 14 and light is emitted in every direction from the organic layers 14. Because of the scattering element 22, emitted light 1 in
In
To improve transparency, a partially reflective semi-transparent metal layer electrode 16 may be used in combination with an absorption-reduction layer (ARL) 33 (
Scattering element 22 may comprise a volume scattering layer or a surface scattering layer. In certain embodiments, e.g., scattering element 22 may comprise materials having at least two different refractive indices. The scattering element 22 may comprise, e.g., a matrix of lower refractive index and scattering elements having a higher refractive index. Alternatively, the matrix may have a higher refractive index and the scattering elements may have a lower refractive index. For example, the matrix may comprise silicon dioxide or cross-linked resin having indices of approximately 1.5, or silicon nitride with a much higher index of refraction. If scattering element 22 has a thickness greater than approximately one-tenth the wavelength of the emitted light, then it is desirable for the index of refraction of at least one material in the scattering element 22 to be approximately equal to or greater than the refractive indices of the organic layers 14. This is to insure that all of the light trapped in the organic layers 14 and partially reflective semi-transparent electrode 16 can experience the direction altering effects of scattering element 22. If scattering element 22 has a thickness less than approximately one-tenth the wavelength of the emitted light, then the materials in the scattering layer need not have such a preference for their refractive indices.
In an alternative embodiment shown in
The scattering element 22 is typically adjacent to and in contact with, or close to, an electrode to defeat total internal reflection in the organic layers 14 and partially reflective semi-transparent electrode 16. However, if the scattering element 22 is between the reflective element 12 and partially reflective semi-transparent electrode 16, it may not be necessary for the scattering element to be in contact with reflective element 12 or partially reflective semi-transparent electrode 16 so long as it does not unduly disturb the generation of light in the organic layers 14. According to an embodiment of the present invention, light emitted from the organic layers 14 can waveguide along the organic layers 14 and partially reflective semi-transparent electrode 16 combined. The scattering element 22 or surface 25 disrupts the total internal reflection of light in the combined layers 14 and 16 and redirects some portion of the light out of the combined layers 14 and 16.
Whenever light crosses an interface between two layers of differing index, a portion of the light is reflected and another portion is refracted (except for the case of total internal reflection). Unwanted reflections can be reduced by the application of standard thin anti-reflection layers. Use of anti-reflection layers may be particularly useful on both sides of the encapsulating cover 20 for top emitters or on the substrate 10 for a bottom emitter. Referring to
Referring back to
The scattering element 22 can employ a variety of materials. For example, randomly located spheres of titanium dioxide may be employed in a matrix of polymeric material. Alternatively, a more structured arrangement employing ITO, silicon oxides, or silicon nitrides may be used. In a further embodiment, the refractive materials may be incorporated into the electrode itself so that the electrode is a scattering layer. Shapes of refractive elements may be cylindrical, rectangular, or spherical, but it is understood that the shape is not limited thereto. The difference in refractive indices between materials in the scattering element 22 may be, for example, from 0.3 to 3, and a large difference is generally desired. The thickness of the scattering layer, or size of features in, or on the surface of, a scattering layer may be, for example, 0.03 to 50 μm. It is generally preferred to avoid diffractive effects by the scattering layer. Such effects may be avoided, for example, by locating features randomly or by ensuring that the sizes or distribution of the refractive elements are not the same as the wavelength of the color of light emitted by the device from the light-emitting area.
The scattering element 22 should be selected to get the light out of the OLED device as quickly as possible so as to reduce the opportunities for re-absorption by the various layers of the OLED device. If the scattering element 22 is to be located between the organic layers 14 and a transparent low-index element 18, or between the organic layers 14 and a reflector 15, then the total diffuse transmittance of the same layer coated on a glass support should be high (preferably greater than 80%). In other embodiments, where the scattering layer 22 is itself desired to be reflective, then the total diffuse reflectance of the same layer coated on a glass support should be high (preferably greater than 80%). In all cases, the absorption of the scattering layer should be as low as possible (preferably less than 5%, and ideally 0%).
Materials of the light scattering element 22 can include organic materials (for example polymers or electrically conductive polymers) or inorganic materials. The organic materials may include, e.g., one or more of polythiophene, PEDOT, PET, or PEN. The inorganic materials may include, e.g., one or more of SiOx (x>1), SiNx (x>1), Si3N4, TiO2, MgO, ZnO, Al2O3, SnO2, In2O3, MgF2, and CaF2. The scattering element 22 may comprise, for example, silicon oxides and silicon nitrides having a refractive index of 1.6 to 1.8 and containing particles of titanium dioxide having a refractive index of 2.3 to 3. Polymeric materials having refractive indices in the range of 1.4 to 1.6 may be employed having a dispersion of particles or other refractive elements of material with a higher refractive index, for example titanium dioxide.
Conventional lithographic means can be used to create the scattering layer using, for example, photo-resist, mask exposures, and etching as known in the art. Alternatively, coating may be employed in which a liquid, for example polymer having a dispersion of titanium dioxide, may form a scattering element 22.
One problem that may be encountered with such scattering layers is that the electrodes may tend to fail open at sharp edges associated with the scattering elements in the element 22. Although the scattering layer may be planarized, typically such operations do not form a perfectly smooth, defect-free surface. To reduce the possibility of shorts between the electrodes, a short-reduction layer may be employed between the electrodes. Such a layer is a thin layer of high-resistance material (for example having a through-thickness resistance (defined as a product of the bulk resistivity and the film thickness) between 10−7 ohm-cm2 to 103 ohm-cm2). Because the short-reduction layer is very thin, device current can pass between the electrodes through the device layers but leakage current through the shorts is much reduced. Such layers are described in co-pending, commonly assigned U.S. Ser. No. 10/822,517, filed Apr. 12, 2004, the disclosure of which is incorporated herein by reference.
Most OLED devices are sensitive to moisture or oxygen, or both, so they are commonly sealed in an inert atmosphere such as nitrogen or argon, along with a desiccant such as alumina, bauxite, calcium sulfate, clays, silica gel, zeolites, alkaline metal oxides, alkaline earth metal oxides, sulfates, or metal halides and perchlorates. Methods for encapsulation and desiccation include, but are not limited to, those described in U.S. Pat. No. 6,226,890 issued May 8, 2001 to Boroson et al. In addition, barrier layers such as SiOx (x>1), Teflon, and alternating inorganic/polymeric layers are known in the art for encapsulation.
In particular, as illustrated in
In one embodiment of the present invention, the organic layers are patterned with a variety of organic materials and produce a variety of colored light defining the colored sub-pixels of a full-color OLED device. In an alternative embodiment, the light emitted from the light-emitter layer is broadband light, for example white, and color filters may be located over the light-emitting layers 14 to provide different colors of light. Referring to
OLED devices of this invention can employ various well-known optical effects in order to enhance their properties if desired. This includes optimizing layer thicknesses to yield maximum light transmission, providing anti-glare or anti-reflection coatings over the display, providing a polarizing medium over the display, or providing colored, neutral density, or color conversion filters over the display. Filters, polarizers, and anti-glare or anti-reflection coatings may be specifically provided over the cover or as part of the cover.
The present invention may also be practiced with either active- or passive-matrix OLED devices. It may also be employed in display devices or in area illumination devices. In a preferred embodiment, the present invention is employed in a flat-panel OLED device composed of small molecule or polymeric OLEDs as disclosed in but not limited to U.S. Pat. No. 4,769,292, issued Sep. 6, 1988 to Tang et al., and U.S. Pat. No. 5,061,569, issued Oct. 29, 1991 to VanSlyke et al. Many combinations and variations of organic light-emitting displays can be used to fabricate such a device, including both active- and passive-matrix OLED displays having either a top- or bottom-emitter architecture.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
