(a) Field of the Invention
The present invention relates to an organic EL (electroluminescence) display unit and, more particularly, to an active-matrix organic EL display unit including a TFT (thin-film transistor) area and a luminescence area receiving therein an organic EL element.
(b) Description of the Related Art
An organic EL device emits light with a spontaneous electroluminescence function wherein positive holes injected from an anode and electrons injected from a cathode are recombined in an organic El film by applying an electric field to the organic EL film. The recombination energy generated by the recombination allows a fluorescent substance or phosphor in the organic EL film to generate electroluminescence. Examples of an organic EL display unit having such an organic EL device include an active-matrix organic EL display unit, wherein a plurality of pixel elements are arranged in a matrix on a glass substrate. Each pixel element includes a TFT area receiving therein a TFT and a luminescence area receiving therein an organic EL element driven by the associated TFT which functions as an active element.
FIG. 13 shows in a sectional view an example of a single pixel of the active-matrix organic EL display unit. The pixel element includes a TFT 20 and an associated organic EL element 40 both formed on a transparent insulator substrate (not shown). The TFT 20 includes source/drain regions formed in a p-type amorphous silicon layer 80, formed on the transparent insulator substrate with an intervention of a SiO2 underlying layer 60, a gate insulation film 10, and a gate electrode 12 connected to an aluminum electrode 14. The TFT area is covered by an edge cover film 30 made of resin. The luminescence area receiving therein the organic EL element 40 include an organic planarizing film 21 formed on the underlying SiO2 film 60 with an intervention of the gate insulation film 10 and inter-layer dielectric films 16 and 18, and a transparent electrode 22, an organic EL film 24 and an aluminum cathode 26, which are consecutively formed on the organic planarizing film 21.
The organic planarizing film 21 is used to alleviate the irregularity on the top surface of the interlayer dielectric film 18. The transparent electrode 22 formed on the organic planarizing film 21 is connected to the TFT 20 via a contact plug 28. The edge cover film 30, which alleviates the irregularity on the surface of the transparent electrode 22 has a window 32 whereat a junction is formed between the organic EL film 24 and the transparent electrode 22. The aluminum cathode 26 covers the entire surface of the organic EL display unit as a topmost layer. The edge cover film 30 is subjected to a tapering processing at the edge thereof adjacent to the window 32 formed in the edge cover film 30 for exposing the luminescence area. The taper processing protects the organic EL film 24 against a crack at the edge 34 of the window 32, thereby preventing a short-circuit failure between the transparent electrode 22 and the aluminum cathode 16.
In the conventional active-matrix organic EL display unit shown in FIG. 13, the organic planarizing film 21 is formed for planarization after the TFTs 20 are formed on the glass substrate. In addition, the tapering processing is conducted for planarization of the irregularity on the top of the TFT 20 caused by the thickness of the transparent electrode 22 after the transparent electrode 22 and the contact plug of the aluminum electrode 14 are formed. This process is conducted for prevention of the aluminum cathode 26 from being damaged or broken at the step difference and for prevention of a short-circuit failure between the transparent electrode 22 and the aluminum cathode 26 caused by the thinner structure of the organic EL film 24.
The conventional active-matrix organic EL display unit shown in FIG. 13 has a drawback wherein degassing from the organic planarization film 21 degrades the organic EL film 24. In fabrication of the organic EL display unit, a wet processing conducted after forming the contact plug of the transparent electrode 22 causes moisture absorption in the organic planarizing film 21, which later discharges the moisture therefrom and degrades the organic EL film 24.
In order to solve the above problem in the conventional organic EL display unit, it may be considered to merely omit the organic planarizing film 21. However, the omission of the organic planarizing film 21, as shown in FIG. 14, necessitates a larger thickness of the edge cover film 30 for planarizing the surface of the TFT area. The larger thickness generates a larger step difference between the TFT area and the luminescence area, and a larger taper angle θ at the edge of the edge cover film 30 near the window 32, which causes a short-circuit failure between the transparent electrode 22 and the aluminum cathode 26. Thus, omission of the organic planarizing film should not be employed.
In view of the above problem in the conventional organic EL display unit, it is an object of the present invention to provide an active-matrix organic EL display unit which is capable of solving the degassing problem to prevent degradation of the organic EL film and preventing a short-circuit failure between the transparent electrode and the aluminum cathode in the vicinity of the edge of the edge cover film.
The present invention provides an organic electroluminescence (EL) display unit including: a transparent insulator substrate; and a plurality of pixels formed thereon and each including a TFT area and a luminescence area, the TFT area receiving therein a TFT and including an edge cover film covering the TFT, the luminescence area having an organic EL element and a window formed in the edge cover film for exposing therefrom the organic EL element, an edge of the edge cover film adjacent to the window having a taper angle equal to or smaller than 30 degrees.
In accordance with the organic EL display unit of the present invention, the smaller taper angle equal to or smaller than 30 degrees prevents a short-circuit failure between electrodes of the organic EL element without degrading the organic EL film. The smaller taper angle may be obtained by a step alleviating film formed in the luminescence area and/or a tapering processing for the edge of the edge cover film as by using a post-baking treatment.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.