Patent Application
20060197441
The present invention relates to electroluminescence (EL) devices, and more particularly to array substrates for active matrix organic light emitting diodes.
The new generation of flat panel devices, electroluminescent displays, for example organic light emitting diode (OLED) displays, have self-luminescence, wide-viewing angle, thin profile, light weight, low driving voltage, and a simple manufacturing process. In OLED displays with a laminated structure, organic compounds such as dyes, polymers, or other luminescent materials serve as an organic luminescent layer and are disposed between a cathode and an anode. OLED displays can be classified as passive matrix and active matrix types, according their driving mode.
Active matrix OLEDs (AM-OLED) are driven by an electric current where each of the matrix-array pixel areas has at least one thin film transistor (TFT) that serves as a switch. The TFTs modulate the driving current based on the variation of capacitor storage potential so as to control the brightness and gray level of various pixel areas.
In an AM-OLED, an electric current is applied to a specific organic lamination to cause luminescence. The AM-OLED has panel luminescence with thin and lightweight characteristics, spontaneous luminescence with high luminance efficiency and low driving voltage, increased viewing angle, high contrast, rapid response, full color, and flexibility.
Indium tin oxide (ITO) has been widely used as an anode electrode material for AM-OLED applications due to its transparency, good conductivity, and high work function. However, the luminescent characteristics of an AM-OLED can depend on the surface roughness of the anode electrode. The surface roughness of the ITO film should be smooth enough to avoid large or unwanted leakage currents and/or point discharge which can cause pixel defects.
The average roughness of ITO films formed by sputtering deposition is less than 1 nm. Typically the ITO film is formed on an under-layer in an AM-OLED process, thereby the surface roughness of the ITO film depends strongly on different under-layers.
Generally, a transparent and insulating organic material is used as the under-layer in conventional AM-OLED process. However, the surface roughness of the ITO film on the organic materials is 3˜4 times larger than that on a smooth glass plate. For example, the average roughness (Ra) of an ITO film on organic materials is about 3˜4 nm. Such a rough surface may result in a large leakage current and can cause point discharge. As a result, the luminance efficiency and lifetime of the device are adversely affected.
Moreover, after formation of an insulating organic material, processes such as chemical vapor deposition (CVD) cannot be utilized due to, for example, a tool contamination. This limits the methods available for forming subsequent layers over the ITO film and increases costs for an AM-OLED.
Array substrates for electroluminescent (EL) devices and methods of forming the same are provided. An exemplary embodiment of an array substrate for an electroluminescent device comprises a substrate with at least one thin film transistor formed thereon. A planarization layer covers the substrate and the thin film transistor and a first dielectric passivation layer with a contact hole therein covers parts of the planarization layer to expose a source/drain electrode of the thin film transistor. A transparent electrode covers a portion of the first dielectric passviation layer and fills the contact hole. A second dielectric passivation layer covers a first portion of the transparent electrode and exposes a second portion of the transparent electrode. A plurality of spacers covers portions of the second dielectric passivation layer to define an organic electroluminescent area with an exposed transparent electrode. An organic electroluminescent layer covers the transparent electrode and the second dielectric passivation layer in the organic electroluminescent area and a metal electrode covers the organic electroluminescent layer.
An exemplary embodiment of a method of forming an array substrate for an electroluminescent display comprises providing a substrate with at least one thin film transistor thereon. A planarization layer is formed over the substrate and the thin film transistor. A first dielectric passivation layer is formed over the planarization layer with a contact hole therein, exposing a source/drain electrode of the thin film transistor. A transparent electrode is formed over the first dielectric passivation layer, filling the contact hole. A second dielectric passivation layer is formed over portions of the transparent electrode to expose a portion the transparent electrode. A plurality of spacers is formed on the second dielectric passivation layer defining an organic electroluminescent area with an exposed transparent electrode therein. An organic electroluminescent layer is formed over the exposed transparent electrode in the organic electroluminescent area and a metal electrode is formed over the organic electroluminescent layer.
A detailed description is given in the following with reference to the accompanying drawings.
The invention can be more fully understood by reading the subsequent detailed description and examples with reference made to the accompanying drawings, wherein:
a to 1f are schematic cross section view showing an array substrate of an electroluminescent (EL) device according to various embodiments of the present invention;
Electroluminescent (EL) devices and methods of forming the same will now be described in greater detail. The present invention can prevent tool contamination during fabrication of electroluminescence devices and increase process flexibility of an EL device process. In some embodiments, the above can be accomplished by providing a dielectric layer under a transparent electrode, using an insulating organic layer thereunder, and/or reducing the surface roughness of the transparent electrode.
In this specification, expressions such as “overlying the substrate”, “above the layer”, or “on the film” denote a relative positional relationship with respect to the surface of the base layer, regardless of the existence of intermediate layers. Accordingly, these expressions may indicate not only the direct contact of layers, but also, a non-contact state of one or more laminated layers.
a to 1f are cross sectional representations of methods for forming an array substrate for electroluminescent (EL) devices, according to embodiments disclosed herein.
In
Suitable materials for the planarization layer 34 can include insulating oxides, nitrides, carbides or combinations thereof. Exemplary materials can include silicon nitride, silicon oxide, aluminum oxide, magnesium oxide, aluminum nitride, or magnesium fluoride. The source/drain electrode 21 can comprise a conductive layer such as a metal layer.
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According to various embodiments, the material of the second passivation layer 38 can comprise an insulating dielectric material, such as silicon oxide, silicon nitride, or combinations thereof. According to various embodiments, the material of second passivation layer 38 also provides good surface flatness. For example, the average surface flatness or Ra can be less than about 2.5 nm, and in certain cases, can be less than about 2.0 nm.
Next, spacers 40 are formed on the second passivation layer 38 to more clearly define at least one predetermined organic electroluminescent area 45. Then, tri-color organic electroluminescent materials, such as red, green and blue color, can be formed isolated by spacers 40, the problem of color blending can be reduced. According to various embodiments, spacers 40 can be dam-shaped structures and can be formed separately. Exemplary materials for the spacers 40 can include, for example, photosensitive material such as silane, acrylic, polyimide, siloxane, or epoxy. Thus, spacers 40 can be formed by photolithography and subsequent development without additional etching. Accordingly damage to the exposed transparent electrode 50 can be reduced.
In
Next, an electrode 54, such as a metal electrode, serving as the cathode electrode of the OLED is formed on the surface of the organic electroluminescent layer 52 in the predetermined organic electroluminescent area, for example, by sputtering or evaporation. To meet OLED cathode requirements, a material capable of injecting electrons into organic electroluminescent material is preferably used. Exemplary materials include, for example, low work function materials such as Ca, Ag, Mg, Al, Li, or alloys thereof.
In a conventional AM-OLED process, portions of a transparent electrode are formed directly on an insulating organic material such as PC403, available from JSR. Problems such as poor roughness of the transparent electrode caused by moisture uptake of the organic material, and organic contamination can occur in subsequent process tools such as a chemical vapor deposition (CVD) tool. Thus utilizing organic insulating material can be problematic.
However, according to various embodiments disclosed herein, the transparent electrode 50 is formed directly on a first passivation layer 36 of dielectric material with reduced moisture uptake. Surface roughness of the transparent electrode 50 is thus significantly reduced and the luminance efficiency and reliability of the electroluminescent devices are improved. For example, the average surface flatness can be less than about 2.5 nm, and in some cases less than about 2.0 nm. The organic electroluminescent layer 52 is formed in organic electroluminescent area 45 which defined by second passivation layer 38 and spacers 40, not only reduce moisture uptake but also avoid color blending problem.
Furthermore, the illustrated pixel structures in
While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.
