Light emitting display device

#The present invention relates to a light emitting display device.

#Organic electroluminescent devices such as organic light emitting diodes (OLEDs) are presently of great interest due to their potential application in high efficiency, flat panel displays. OLEDs are described in U.S. Pat. Nos. 6,013,384 and 6,579,029 each of which are hereby incorporated by reference as if set forth in their entireties. OLEDs were originally demonstrated as the light emitting component in passively addressed display products. Passive matrix displays demonstrate the feasibility of OLEDs in many applications, but encounter a fundamental barrier as the display, size and pixel density of the display device increases. Because the luminescent output of an OLED is proportional to the charge injected through the device, the current densities required to operate passively addressed displays rapidly rise as the time available to drive each pixel decreases with increasing display resolution. These higher current densities cause large voltage drops in the indium tin oxide (ITO) or similar interconnect lines of the passive matrix array, push the OLED operation to higher voltages and create display driver issues that are not easily resolved.

#In response to such issues, active matrix drive schemes have been developed and active matrix OLEDs (AMOLEDs) are therefore of particular interest. Active matrix technology is a method of sending charges to pixels of a light emitting display. A common example of an active matrix display is a TFT, thin film transistor, commonly formed using polysilicon technology. Whereas a passive matrix display uses a simple conductive grid to deliver current to the pixels of the matrix, an active matrix display uses a grid of transistors with the ability to hold a charge for a limited period of time. Because of the switching action of transistors, only the desired pixel receives a charge, improving image quality over a passive matrix. Furthermore, because of the thin film transistor's ability to hold a charge, the pixel remains active until the next refresh. A goal in AMOLED display technology is to generate a constant current source at each pixel using such thin film transistors. Each pixel is programmed to provide a constant current during the entire frame time, eliminating the high currents encountered in the passive matrix approach. However, unlike LCD-thin film transistor technology, for example, the lighting control of OLEDs is self generated and all of the current supplied to the TFTs to enable the TFTs to drive the OLEDs, flows along a thin layer of conductive material formed on the surface of the substrate and which forms a power transmission line and various interconnect lines within the array. For small arrays such as a 2 square inch array, the current required in the power transmission line that delivers current to the array, may be at manageably low levels such as 300 milliamps. For large displays such as a 30-inch display, the total current needed may increase to 5-10 amps which affects the dependability of the thin power transmission lines conventionally formed on the substrate/array surface. Due to the resistance of the very thin material typically used as conductive interconnects such as ITO (indium tin oxide) aluminum or copper, such a current may result in a potential difference of as much as 27V across the power transmission lines and interconnect lines. Such a potential voltage may cause the interconnect material to burn out, resulting in opens that render the display device inoperable.

#The present invention is best understood from the following detailed description when read in conjunction of the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not necessarily to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Like numerals denote like features throughout the specification and drawing.

#FIGS. 1A and 1B show an exemplary display device of the invention. FIG. 1A is a perspective view and FIG. 1B is a front view;

#FIG. 2 is a rear view of an exemplary display device according to the present invention;

#The present invention provides an active light emitting display. In particular, the present invention provides a display device with an array of pixels. In one exemplary embodiment, each pixel may include an OLED and a thin film transistor to provide electroluminescence. A patterned thin film forms an interconnect material that connects the pixels to a common power transmission line and a conductive material of substantial cross-sectional area is electrically connected to the common power transmission line and capable of accommodating large currents. The patterned thin film also forms part of the common power transmission line. In one embodiment, the pixel array of the display device is covered by a cover that includes a trench that is at least partially filled with conductive material. The conductive material is electrically and physically coupled to the common power transmission line that provides power from an external power source to each of the pixels. The conductive material therefore assists the common power transmission line in transmitting power. An exemplary arrangement is shown in FIG. 1A, which shows substrate 2, upon which the pixel array (not shown in FIG. 1A) is disposed, and cover 4. Cover 4 includes trench 24 that is least is partially filled with conductive material 18. When contact surface 30 of cover 4 contacts surface 22 of substrate 2, trench 24 and surface 22 form a channel in which conductive material 18 can transmit power. FIG. 1B is an exemplary front view of the display device shown in FIG. 1A and illustrates cover 4 and conductive material 18 disposed behind substrate 2 and shown by dotted lines. A cross-sectional area of the channel may be about 0.005 to 5 mm2. A cross-sectional area of the conductive material may be about 0.005 to 5 mm2. A thickness of the conductive material may be about 0.01 to 1 mm, but the preceding values are exemplary only and other areas and thicknesses may be used in other exemplary embodiments.

#FIG. 2 is a top, rear view of an exemplary display device that has a display face on the front, and FIG. 3 is a cross-sectional view showing relevant portions of an exemplary display device. Substrate 2 may be a light transmissive material such as glass, quartz, or various light transmissive plastics. Pixel array 6 is formed on surface 22 of substrate 2. Each pixel comprises an OLED formed on the substrate and associated TFT, according to the exemplary embodiment in which the display device is an active organic light emitting display. Each pixel is connected to common power transmission line 10 by conductive interconnect materials such as interconnect leads 12. The conductive interconnect materials may be formed of indium tin oxide (ITO), aluminum, copper or other suitable conductive materials. The conductive interconnect material formed on substrate 2 and interconnecting the pixels of pixel array 6, are formed from a thin film having a thickness of 500 nanometers (0.5 microns) or less in one embodiment. Cover 4 extends around edge 8 of substrate 2 and common power transmission line 10 is greater in length than the width of pixel array 6 shown in FIG. 2. The display device may be an active light emitting display such as active organic light emitting display or a plasma display.

#Common power transmission line 10 may include conductive lead 20 formed within a channel formed between cover 4 and substrate 2 as shown in FIG. 3. Cover 4 includes trench 24 that extends inwardly from contact surface 28 of cover 4. When contact surface 28 of cover 4 contacts surface 22 of substrate 2, trench 24 forms channel 26. In one exemplary embodiment, trench 24 may be defined as the area between two substantially parallel ribs 30 and referred to as a trough. In the illustrated embodiment, channel 26 is substantially filled with conductive material 18, which is electrically connected to common power transmission line 10 and can transmit power. Conductive material 18 may be various conductive materials and in various exemplary embodiments, conductive material 18 may be a liquid or other fluid, a gel, a powder, or other deformable material that can advantageously be placed within, and substantially fill, trench 24. In one exemplary embodiment, conductive material 18 may be a conductive epoxy. Conductive material 18 may be formed by using a metal brazing process, melting a conductive glass frit, performing an eutectic solder or an eutectic brazing process. The conductive material may also be a curable conductive adhesive, which may be cured using ultraviolet radiation or thermal radiation. Various means may be used to cause cover 4 to adhere to substrate 2. In one exemplary embodiment, conductive lead 20 of common power transmission line 10 forms a continuous boundary with conductive material 18. Conductive lead 20 of common power transmission line 10 is also coupled to a plurality of leads (not shown) that extend to pixel array 6 and connect each pixel of pixel array 6 to common power transmission line 10. Conductive lead 20 of common power transmission line 10 may include a thickness of 0.3 to 0.5 microns in an exemplary embodiment.