Patent Application
20080117362
The present invention relates to edge-emitting organic light emitting diode (OLED) devices with optical microstructures and methods to make them.
Solid-state lighting uses light-emitting diodes (LEDs) or OLEDs as the light source instead of the traditional incandescent and fluorescent light bulbs. LEDs or OLEDs provide for much higher energy efficiency than incandescent and fluorescent lighting, which can result in significant energy savings through a reduction in consumption of electricity. They can also provide for many more lighting effects. For example, they can be formed in a flexible substrate and thus formed around three-dimensional objects. Also, they can be controlled to selectively provide various colors of lights.
One challenge of solid-state lighting is the light extraction of the LEDs or OLEDs. Significant losses of light occur as a result of lightguiding in the substrate due to total internal reflection (TIR) and waveguiding losses in the high index layers. Losses due to TIR are typically resolved using conventional methods of antireflection, such as index gradients, surface or bulk diffusers, multilayered optical stacks, and microlenses. However, the solution to the loss of approximately 50% of light in waveguided modes is more difficult. The high optical index organic and transparent conductive oxide (TCO) layers of an OLED act similar to an optical fiber with a glass cladding. Evanescent modes emitted into the device plane are trapped within this layer and travel only a very short distance before self-absorption and conversion of the optical energy into heat.
Certain approaches to address lightguiding in solid-state lighting have been developed. These solutions to extracting this light include the use of a low index hydrophobic aerogel layer between the indium tin oxide (ITO) and glass, the use of distributed feedback (DFB) gratings, and the optimization of microcavity effects.
Another approach includes use of two-dimensional (2D) photonic crystal gratings at the high index TCO/glass interface (related to the DFB gratings). This method was initially developed for extracting light from inorganic LEDs and has been modified for OLEDs. This method involves use of photolithography to pattern a regular array of nanoholes in the glass substrate and filling them with high index silicon nitride. Finally, ITO matched to the silicon nitride is deposited to complete the substrate.
Yet another approach involves use of fluorescent dye elements to trap edge-emitted light. This method consists of an array of standard OLEDs configured in stripes that are interspersed with stripes of fluorescent material. The fluorescent material absorbs any edge-emitted light and re-emits it at a lower energy. The result is a more efficient, albeit two-color, OLED device.
Finally, another approach involves use of a vertical cylindrical OLED device with metallic electrodes and a dielectric layer.
Accordingly, a need exists for other solutions to increasing light extraction in solid-state lighting devices and other light-emitting devices.
An edge-emitting OLED device, consistent with the present invention, includes a substrate and an organic electroluminescent layer overlaying the substrate surface. The organic electroluminescent layer forms an edge-emitting OLED. A plurality of optical microstructures are formed on the substrate and separated from the organic electroluminescent layer. Each of the optical microstructures defines turning optics for reflecting and redirecting light from the edge-emitting OLED.
A method of fabricating an edge-emitting OLED device, consistent with the present invention, includes the following steps: providing a substrate; applying a curable organic layer to the substrate surface for use in forming an edge-emitting OLED; forming a plurality of optical microstructures on the substrate and separated from the organic electroluminescent layer; curing the curable organic layer; and applying a metallic layer over the organic electroluminescent layer to form an electrode and over the optical microstructures to form turning optics for reflecting and redirecting light from the edge-emitting OLED.
The accompanying drawings are incorporated in and constitute a part of this specification and, together with the description, explain the advantages and principles of the invention. In the drawings,
a is a cross-sectional diagram illustrating a first exemplary pattern for fabricating edge-emitting OLEDs with optical microstructures;
b is a cross-sectional diagram illustrating a second exemplary pattern for fabricating edge-emitting OLEDs with optical microstructures;
a-8d are top views illustrating the appearance of various optical microstructures for use with edge-emitting OLEDs;
a-9d are perspective views illustrating the appearance of the optical microstructures shown in
e is a top view diagram of a serpentine layout of optical microstructures;
a-10j are block diagrams illustrating an exemplary sequence of fabrication steps for producing an edge-emitting OLED with optical microstructures;
Embodiments can provide for increased light extraction in OLED devices for higher light output in solid-state lighting or other devices such as displays. A microreplicated substrate includes edge-emitting OLEDs and microstructured turning optics to extract the light and reduce the amount of losses due to lightguiding. This type of microreplicated substrate can be provided as a rigid or flexible sheet for solid-state lighting applications, for example.
An OLED is typically a thin film structure formed on a substrate such as glass, a transparent flexible film, or a flexible metal foil. The substrate for an OLED may comprise, for example, one of the following: an organic polymeric material; polyethylene terephthalate (“PET”); polyacrylates; polycarbonates; silicone; epoxy resins; silicone-functionalized epoxy resins; polyesters; polyimides; polyethersulfones; polyetherimide; polyethylene naphthalate (“PEN”); or other opaque or translucent materials. The substrate may also comprise impermeable materials such as, for example, stainless steel and metal foils. Examples of display device substrates, including flexible metal foils, are described in the following papers, all of which are incorporated herein by reference: H. S. Shin et al., “4.1 inch Top-Emission AMOLED on Flexible Metal Foil,” SID 05 DIGEST, pp. 1642-1645 (2005); X. L. Zhu et al., “Very Bright and Efficient Top-Emitting OLED with Ultra-Thin Yb as Effective Electron Injector,” SID 06 DIGEST, pp. 1292-1295 (2006); J. H. Cheon et al., “A 2.2-in. Top-Emission AMOLED on Flexible Metal Foil with SOG Planarization,” SID 06 DIGEST, pp. 1354-1357 (2006); H. K. Chung et al., “AMOLED Technology for Mobile Displays,” SID 06 DIGEST, pp. 1447-1450 (2006); D. U. Jin et al., “5.6-inch Flexible Full Color Top Emission AMOLED Display on Stainless Steel Foil,” SID 06 DIGEST, pp. 1855-1857 (2006); and A. Chwang et al., “Full Color 100 dpi AMOLED Displays on Flexible Stainless Steel Substrates,” SID 06 DIGEST, pp. 1858-1861 (2006).
A light-emitting layer of an organic electroluminescent (EL) material, such as a light-emitting polymer, and optional adjacent semiconductor layers are located between a cathode and an anode. An example of a light-emitting polymer layer is described in U.S. Pat. No. 6,605,483, which is incorporated herein by reference. The EL material can be sandwiched or interdigitated, for example, between the cathode and anode. The semiconductor layers may be hole injection (positive charge), electron injection (negative charge), hole blocking, electron blocking, hole transporting, or electron transporting, and they also comprise organic materials. The material for the light-emitting layer may be selected from many organic EL materials. The light emitting organic layer may itself include multiple sublayers, each comprising a different organic EL material. Organic EL materials can emit electromagnetic (“EM”) radiation having narrow ranges of wavelengths in the visible spectrum.
To achieve white light in one particular example, devices incorporate closely arranged OLEDs emitting blue, green, and red light, or complementary colors of light (e.g., blue and yellow). These colors are mixed to produce white light. In one configuration described in U.S. Pat. Nos. 5,294,869 and 5,294,870, these OLEDs are formed as adjacent pixels that are individually addressable. Separated areas of one or more organic EL materials and color conversion layers coupled to them are deposited on a patterned electrode that provides the capability to electrically control the individual pixels. The deposition of these layers is effected by a shadow mask comprising a plurality of walls that are first formed on the substrate by photolithography. The portions of the surface in the shadow of the walls do not receive vapor directed at an angle at such surface. Thus, the locations of the deposited areas are controlled by the heights of the walls and the angle of deposition.
OLEDs can include topographical features and can be formed as a matrix of rows and column, or as a series of long rows. The dimension of each topographical feature and the distance between two adjacent features typically correspond to the desired dimension of a pixel of the matrix display. For high-density, high-resolution information displays, the pixel dimension can be in the range from about 5 microns to about 100 microns. On the other hand, for general lighting purposes, the pixel area can be several square centimeters or greater. The height of the raised features is typically in the range from about 1 micron to about 100 microns, and more typically less than 10 microns. The OLEDs with turning optics can also be used as a backlight unit for a liquid crystal display (LCD) device, optionally spatially modulated.
Individual OLEDs can be built on top of the ridges (raised features) or in valleys between them. The OLEDs thus built can be addressed individually to display information or images represented by the collection of activated OLEDs for display devices. Typically, an OLED comprises at least an organic EL layer capable of emitting light when activated by a current with the organic EL layer being sandwiched or interdigitated between two conductor layers serving as an anode and a cathode.
To make an OLED, successive layers of an anode, an organic EL, and a cathode material are deposited on the substrate. ITO is typically used as the anode material, which should typically be a high work function in the range of about 4.5 eV to about 5.5 eV. ITO is substantially transparent to light transmission and allows at least 80% light transmission. Therefore, light emitted from the organic electroluminescent layer can easily escape through the ITO anode layer without being substantially attenuated. Other materials suitable for use as the anode layer include, for example, tin oxide, indium oxide, zinc oxide, indium zinc oxide, cadmium tin oxide, and mixtures thereof. In addition, materials used for the anode may be doped with aluminum or fluorine to improve charge injection property. Electrode layers may be deposited on the underlying element by physical vapor deposition, chemical vapor deposition, ion beam-assisted deposition, or sputtering. A thin, substantially transparent layer of a metal is also suitable.
More than one organic EL layer may be formed successively one on top of another, each layer comprising a different organic EL material that emits in a different wavelength range. Such a construction can facilitate a tuning of the color of the light emitted from the overall light-emitting display device. Furthermore, one or more additional layers may be included between electrodes to increase the efficiency of the overall device. These additional layers may also be deposited by physical vapor deposition or chemical vapor deposition. For example, these additional layers can serve to improve the injection (electron or hole injection enhancement layers) or transport (electron or hole transport layers) of charges into the organic EL layer. The thickness of each of these layers is typically kept to below about 500 nanometers (nm), more preferably below about 100 nm. Materials for these additional layers are typically low-to-intermediate molecular weight (less than about 2000) organic molecules. In some cases, a hole injection enhancement layer is formed between the anode layer and organic EL layer to provide a higher injected current at a given forward bias and/or a higher maximum current before the failure of the device. Thus, the hole injection enhancement layer facilitates the injection of holes from the anode. Suitable materials for the hole injection enhancement layer include, for example, materials disclosed in U.S. Pat. No. 5,998,803, incorporated herein by reference.
As an alternative, each OLED can further include a hole transport layer which is disposed between the hole injection enhancement layer and organic EL layer. The hole transport layer has the functions of transporting holes and blocking the transportation of electrons so that holes and electrons are optimally combined in organic EL layer. Materials suitable for the hole transport layer include, for example, materials disclosed in U.S. Pat. No. 6,023,371, incorporated herein by reference.
As another alternative, each OLED can include an additional layer disposed between the cathode layer and the organic EL layer. This additional layer has the combined function of injecting and transporting electrons to the organic EL layer. Materials suitable for the electron injecting and transporting layer include, for example, materials disclosed in U.S. Pat. No. 6,023,371.
In another aspect, a mixture of an organic EL material and a dye (host and emitter) is deposited between the electrodes of individual OLEDs. The dye absorbs a portion of EM radiation emitted by the organic EL material and emits EM radiation in a different wavelength range. Different dyes are used for different OLEDs to generate different colors. For example, dyes can be selected such that three adjacent OLEDs emit blue, green, and red colors, which in combination result in white light.
The following paper, incorporated herein by reference, describes an OLED that emits light laterally: A. Mikami et al., “High Efficiency Phosphorescent Organic Light-Emitting Devices Coupled with Lateral Color Conversion Layer,” SID 06 DIGEST, pp. 1376-1379 (2006).
The following papers, all of which are incorporated herein by reference, describe methods to extend the cavity dimensions of OLEDs: L.-S. Liao et al., “High-Efficiency Tandem Blue OLEDs,” SID 06 DIGEST, pp. 1197-1200 (2006); T. Maindron et al., “High Performance and High Stability PIN OLED,” SID 06 DIGEST, pp. 1189-1192 (2006); and T. K. Hatwar et al., “Low-Voltage White Tandem Structures for Fabricating RGBW AMOLED Displays,” SID 06 DIGEST, pp. 1964-1967 (2006).
The following papers, all of which are incorporated herein by reference, describe use of edge-emitting thick film electroluminescent (TFEL) devices: Y.-H. Lee et al., “A New Light Emitting Structure Using Edge Emission Reflected by Micro-Tip Reflectors,” Korean Institute of Science and Technology, pp. 41-42; Z. Kun et al., “TFEL Edge Emitter Array for Optical Image Bar Applications,” SID 86 DIGEST, pp. 270-272 (1986); and Y. H. Lee et al., “White-Light Emitting Thin-Film Electroluminescent Device Using Micromachined Structure,” IEEE Transaction on Electronic Devices, vol. 44, no. 1, pp. 40-44 (1997).
An aspect of this structure, as illustrated by unit cell 100, is self-alignment and compatibility with vacuum OLED material deposition. During the deposition process, the optional gap 101 can create the light-emitting edge of OLED 104 as well as the electrical isolation of the optical microstructures 102. In some cases, the OLED material is deposited uniformly over the microstructured area, even on the prism surfaces of optical microstructures 102, and a metallic layer is then deposited over the OLED material. The metallic layer provides a dual use as both an electrode for OLED 104 and reflective surface for the prisms, optical microstructures 102. One or both of the electrodes on optical microstructures 102 may functions as a reflective surface, and the OLED layers on the turning optics facet are inactive.
The construction of the unit cells illustrated in
The size of the gap (101, 201, 301, 401, 501, and 601) between the OLED and optical microstructures can be adjusted based on the divergence of the emitted beam from the OLED edge. If the divergence is large, then the gap need be relatively small. The gap may also be adjusted based on the deposition angle and the shadowing observed during deposition. The size of the gap can be, for example, between about 0.01 micron and 100 microns. In certain cases, no gap is needed, meaning the gap can have a distance of zero or substantially zero.
The size of the depth (103, 203, 303, 403, 503, and 603) between the base of the optical microstructures and the bottom of the gap, when used, can be adjusted. The size of the depth can be, for example, between about 0.01 micron and 100 microns. In certain cases, no depth is needed, meaning the depth can have a distance of zero or substantially zero, particularly if no gap is used.
The angle of the turning optics in each of the unit cells (100, 200, 300, 400, 500, and 600), such as angle 105, can be adjusted for a desired reflection and redirection of light from the OLEDs in each unit cell. Also, the angles can be constant or variable over the device area, meaning that the turning optics can all have substantially the same angle or they can have varying angles. In this manner, the entire device can be configured for directing light in different directions by fine-tuning the various angles for the turning optics. In addition, the angles can be selectively varied across the device, along with use of concave or convex turning optics as shown in
The turning optics (102, 202, 302, 402, 502, 602, and 606) for the unit cells can be formed from a variety of materials. For example, they can be formed from a metallic layer providing a reflective surface for reflecting and redirect the light from the OLEDs. Alternatively, they can formed from any material for provides for TIR. For example, if light 108 from OLED 104 impinges upon optical microstructure 102 at an angle less than the critical angle for TIR, then optical microstructure 102 can reflect and redirect light 108 as shown in
The size of the light generation region of the OLEDs can be optimized so that the emitting area is a large fraction of the total surface, effectively creating a large aperture ratio device. The brightness of the device or regions within a device can be determined geometrically by considering the size of the gap, the light generation region, and the optical microstructures, and by altering the ratio of light emitted from the face and edges of the light emitting region by changing the optical density of the metallic cathode. Another method to determine the device or region brightness involves changing the edge-to-land ratio in the two-dimensional design of the device, for exampling increasing the edge-to-land ratio by making the edge a structured shape, for example a zig-zag pattern. The use of a structured edge also increases the amount of light emitted through the edge in the plane of the device. With such a structure, the distance from the point of emission to the nearest edge is reduced and, therefore, the number of reflections before the light escapes from the edge is decreased. Use of these methods allows for a range of brightness within a single device operating at a single current density and voltage. This range of brightness can be useful in solid-state lighting, for example.
Another variant of the unit cells involves the combination of the edge-emitting structure with a tandem or stacked OLED device construction. This structure can be used as a way of creating a white light emitting device from red, green, and blue light-emitting layers. It is also a way to extend the vertical cavity dimensions for a monochromatic emitter so that the cavity between reflective anode and cathode is tuned to the wavelength of emission. The monochromatic emitter can be useful in solid-state lighting, for example, to obtain white light for indoor lighting applications. Alternatively, the unit cells can be constructing of OLEDs emitting one substantially uniform color for solid-state lighting when a particular color is desired. In addition, if the unit cells are made with a repeating microreplicated structure of red, green, and blue light-emitting OLEDs, the individual unit cells can be controlled to produce a variety of colors for lighting effects in solid-state lighting. For example, through control of which unit cells are activated, a variety of colors can be selectively produced, providing a user with the flexibility to select particular colors of light in solid-state lighting applications.
Several patterns are possible for edge-emitting microstructured OLED devices based upon the same structural motif 700, as shown in
b is a cross-sectional diagram illustrating an alternative pattern, structural motif 705, for fabricating edge-emitting OLEDs with optical microstructures. The one-dimensional alternative pattern 705 comprises a series of linear OLED surfs 707 with turning optics 709 opposite one edge of each OLED surf. Each surf and turning optic together form a unit cell such as those illustrated in
The structural motifs 700 and 705 can be used to generate a number of two-dimensional patterns including a linear array, a circular array, and several arrays in which the prismatic elements are isolated grid-like structures. In the circular array case, a linear grid is superimposed over the array as a means of conducting current to the periphery of the device.
The structural motifs 700 and 705 can be formed as a microreplicated sheet as the substrate, for example. In particular, the unit cells shown in
a-8d are top views illustrating examples of the appearance of various optical microstructures for use in unit cells according to various OLED embodiments. In particular,
a, 7b, 8a-8d, and 9a-9e are not shown to scale and the dimensions can be adjusted based upon particular implementations.
An exemplary device fabrication method to make edge-emitting OLEDs with optical microstructures can involve use of conventional shadow mask deposition of each layer, the anode and cathode contacts, EL material, and turning optics.
Table 1 describes steps for another exemplary device fabrication method to make edge-emitting OLEDs with optical microstructures, and
For this exemplary fabrication method, the substrate can be any barrier substrate such as stainless steel, foil laminated polyimide, glass, ceramic, or others as described above. The organic coating may be a photopolymerizable polyimide, a photopolymerizable benzocyclobutane, or any other material that is compatible with both the microreplication process and OLEDs. The material should be thermally stable and free of volatile components.
The microstructured devices may be encapsulated. Encapsulation techniques known in the art for top and bottom emitting OLEDs may be used, for example multilayer coatings or vapor deposited inorganic coatings. The limited exposure of the edge should decrease device degradation regardless of encapsulation technique.
Table 2 describes steps for another exemplary device fabrication method to make edge-emitting OLEDs with optical microstructures using a laser induced thermal imaging (LITI) process. The LITI process involves use of a donor film having a donor substrate, a light-to-heat conversion (LTHC) layer over the substrate, and a transfer layer over the LTHC layer. For thermal transfer using radiation (e.g., light) in a LITI process, a variety of radiation-emitting sources can be used with a LITI donor film. For analog techniques (e.g., exposure through a mask), high-powered light sources (e.g., xenon flash lamps and lasers) are useful. For digital imaging techniques, infrared, visible, and ultraviolet lasers are particularly useful. Suitable lasers include, for example, high power (e.g. ≧100 mW) single mode laser diodes, fiber-coupled laser diodes, and diode-pumped solid state lasers (e.g., Nd:YAG and Nd:YLF). Laser exposure dwell times can be in the range from, for example, about 0.1 microsecond to 100 microseconds and laser fluences can be in the range from, for example, about 0.01 J/cm2 to about 1 J/cm2. During imaging, the thermal transfer layer is typically brought into intimate contact with a permanent receptor (substrate for the unit cells) adapted to receive at least a portion of the transfer layer. In at least some instances, pressure or vacuum may be used to hold the thermal transfer layer in intimate contact with the receptor. A radiation source may then be used to heat the LTHC layer or other layers containing radiation absorbers in an image-wise fashion (e.g., digitally or by analog exposure through a mask) to perform image-wise transfer of the transfer layer from the thermal transfer layer to the receptor according to a pattern.
Various layers of an exemplary LITI donor film, and methods to image it, are more fully described in U.S. Pat. Nos. 6,866,979; 6,586,153; 6,468,715; 6,284,425; and 5,725,989, all of which are incorporated herein by reference.
Spatial Modulation and Backlights
