Organic light-emitting diode (OLED) display devices typically include a substrate having one or more OLED light-emitting elements including a first electrode formed thereon, one or more OLED light-emitting layers located over the first electrode, and a second electrode located over the OLED light-emitting layers, and an encapsulating cover located over the second electrode and affixed to the substrate. Such devices may be top-emitting, where the light-emitting elements are intended to be viewed through the cover, and/or bottom-emitting, where the light-emitting elements are intended to be viewed through the substrate. Accordingly, in the case of a bottom-emitting OLED device, the substrate must be largely transparent, and in the case of a top-emitting OLED device, the cover must be largely transparent.
A variety of materials may be used to construct suitable substrates and encapsulating covers for OLED devices. The desirable material properties and/or characteristics of an OLED substrate include low cost, very flat, low coefficient of thermal expansion (CTE), high strength and stability under a variety of environmental stresses, and electrically non-conductive (or able to be coated with electrically non-conductive material on the substrate surface contacting the OLED). It is important that the material forming the substrate on which electrical circuitry is deposited is electrically non-conductive so that any electrical circuitry formed thereon is not shorted. The material used most often for such substrates is glass, typically borosilicate glass, because it is transparent, very stable, can be made at low-cost, and has a very smooth surface suitable for the deposition and processing of semiconductor and organic materials. Other substrate materials have been described in the art, for example ceramics, plastics, and metals such as stainless steel (see U.S. Pat. No. 6,641,933 B1 to Yamazaki et al entitled “Light-emitting EL display device”). However, metals are conductive and so their use typically requires additional electrically non-conductive insulating layers. Metal supports typically also have a relatively high CTE, which may cause stresses in any devices deposited on the substrate. Metal and glass materials are also used in OLED encapsulating covers, for example in products demonstrated and sold by the Eastman Kodak Company.
Organic light-emitting diodes can generate efficient, high-brightness displays. However, heat generated during the operation of the display in high-brightness modes can limit the lifetime of the display, since the light-emitting materials within an OLED display degrade more rapidly when used at higher temperatures. While it is important to maintain the overall brightness of an OLED display, it is even more important to avoid localized degradation within a display. The human visual system is acutely sensitive to differences in brightness in a display. Hence, differences in uniformity are readily noticed by a user. Such localized differences in uniformity in an OLED display may occur as a consequence of displaying static patterns on the display, for example, graphic user interfaces often display bright icons in a static location. Such local patterns will not only cause local aging in an OLED display, but will also create local hot spots in the display, further degrading the light-emitting elements in the local pattern. Glass and plastic supports, the use of which is advantageous in view of their relative electrical non-conductivity, may not be sufficiently thermally conductive to provide a uniform temperature across the substrate when the display is in operation. Hence, improved thermal management techniques may significantly improve the life expectancy of an organic display device.
One method of removing heat from an organic light emitting display device is described in U.S. Pat. No. 6,265,820, entitled, “Heat removal system for use in organic light emitting diode displays having high brightness.” The '820 patent describes a heat removal system for use in organic light emitting diode displays. The heat removal assembly includes a heat dissipating assembly for dissipating heat from the organic light emitting device, a heat transfer assembly for transferring heat from the top organic light emitting device to the heat dissipating assembly and a cooling assembly for cooling the organic light emitting display device. While the system of the '820 patent provides a means for heat removal in an OLED application, its efficiency is limited by the presence of a glass substrate having poor thermal conductivity characteristics through which heat generated by the OLED devices must transfer for removal. Moreover, the structure described in the '820 patent is complex, requiring multiple layers and specific, heat transfer materials in contact with delicate OLED layers.
U.S. Pat. No. 6,480,389 to Shie et al entitled “Heat dissipation structure for solid-state light emitting device package” describes a heat dissipation structure for cooling inorganic LEDs and characterized by having a heat dissipating fluidic coolant filled in a hermetically sealed housing where at least one LED chip mounted on a metallic substrate within a metallic wall erected from the metallic substrate. Such an arrangement is complex, requires fluids, and is not suitable for area emitters such as OLEDs.
Heat sinks are also well known in the integrated circuit industry and are applied to cooling large integrated circuits. Such sinks typically are thick and are unsuitable for displays in which limiting the thickness of the display is an important goal.
It is therefore an object of the invention to provide a more uniform distribution of heat within an OLED display and to optimize the removal of heat from an OLED display device to improve the lifetime of the display.
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