1. Field of the Invention
This invention relates to a top-emission type organic EL (electroluminescent) panel having an organic EL device formed by sequentially laying a reflecting electrode, organic compound layers and a transparent electrode on a substrate where a thin film transistor is formed and a protective layer covering the organic EL device. More particularly, the present invention relates to an organic EL panel designed to prevent the thin film transistor from being degraded by light.
2. Description of the Related Art
Massive efforts are being paid to develop organic EL devices. The organic EL device driving system is roughly classified into a passive matrix system and an active matrix system. However, in order to realize a large screen with high definition, an active matrix system with use of a thin film transistor is being developed.
While amorphous silicon and poly silicon that are being employed to form thin film transistors have light susceptibility and hence there is a problem that some of the characteristics of thin film transistors can be changed and/or degraded when they are hit by light coming from organic EL devices and external light.
Japanese Patent Application Laid-Open No. 2002-108250 describes a technique of forming a light-shielding layer that directly covers the thin film transistors on a substrate.
There is another problem about organic EL devices that light striking the organic EL devices is reflected by the reflecting electrodes and some of the undulations of the devices to fall the contrast ratio of the organic EL panel.
Japanese Patent Application Laid-Open No. 2006-236963 discloses a technique of arranging a black adhering member on the device separation layer so as to expose the EL devices of an organic EL panel.
Since Japanese Patent Application Laid-Open No. 2002-108250 describes a technique of forming a light-shielding layer that directly covers the thin film transistors, the technique can prevent some of the characteristics of thin film transistors from being changed or degraded by incident light.
The light-shielding layer may be formed by using a black inorganic insulating layer or a layer that is by turn formed by dispersing a black pigment or a black die in resin. However, the following problem can arise when such a light-shielding material is employed.
Since the light-shielding layer is held in contact with thin film transistors, the light-shielding layer is sandwiched between the thin film transistors and an electro-conductive member when an electro-conductive member is arranged on the light-shielding layer. Then, if the light-shielding layer is formed by using a metal material or a material illustrating a large relative permittivity, the light-shielding layer is electrically charged to produce a capacitance, which by turn changes or degrades some of the characteristics of the thin film transistors.
In short, while the technique of Japanese Patent Application Laid-Open No. 2002-108250 can prevent some of the characteristics of thin film transistors from being changed or degraded by incident light, those characteristics of thin film transistors are changed or degraded due to the fact that a light-shielding layer is held in contact with thin film transistors in an organic EL panel.
Thus, there is not known any practical light-shielding technique for effectively covering thin film transistors.
The technique of Japanese Patent Application Laid-Open No. 2006-236963 can improve the contrast ratio of an organic EL panel by covering the region thereof other than the light emitting regions by means of a black material because the black material operates like a black matrix.
However, since the device separation layer is tapered at the lateral surfaces thereof, which are then smoothly connected to the respective light emitting regions, external light that strikes the lateral surfaces of the device separation layer, which is part of light trying to enter the EL panel, is reflected and simply goes out. Therefore, when the lateral surfaces of the device separation layer are not covered by a black material but exposed, the contrast ratio of the organic EL panel can fall so much. In short, the technique of Japanese Patent Application Laid-Open No. 2006-236963 is not satisfactory for improving the contrast ratio of an organic EL panel because light striking the lateral surfaces of the device separation layer is reflected to become a cause of making the contrast ratio fall.