1. Field of the Invention
The present invention relates to an organic electroluminescent display element, a display device having the organic electroluminescent display element, and a manufacturing method of the organic electroluminescent display element.
2. Description of the Background Art
Flat panel display elements have less power consumption than that of conventionally used cathode ray tubes (CRTs) and enable reduction in thickness of display elements. Flat panel display elements have therefore been increasingly demanded in response to recent diversification of information processing equipments. Examples of flat panel display elements include liquid crystal display elements and electroluminescent (EL) display elements. In particular, organic EL display elements are characterized by their lower driving voltage, all solid-state structure, faster response speed, and self light emission. Organic EL display elements have been actively studied and developed due to these characteristics.
Organic EL display elements are roughly divided into two driving types: a passive matrix type (hereinafter, referred to as “PM type”); and an active matrix type (hereinafter, referred to as “AM type”).
PM-type organic EL elements are driven in a line-sequential manner. In order to implement high-luminance PM-type organic EL elements, large instantaneous power must be applied to each pixel (light-emitting layer). Application of such large instantaneous power significantly degrades the light-emitting layers of PM-type organic EL elements. PM-type organic EL elements thus have shorter service life. Especially in organic EL elements having a large number of scanning electrodes (high-definition organic EL elements), a higher voltage is applied to each pixel. Therefore, high-definition organic EL elements have particularly short service life.
On the other hand, AM-type organic EL elements have switching elements (e.g., thin film transistors (TFTs)) respectively corresponding to pixels and the switching elements can therefore be turned ON/OFF on a pixel by pixel basis. In theory, the number of scanning electrodes is not limited in AM-type organic EL elements, and AM-type organic El elements can therefore provide image display which is close to approximately 100% of a frame period. AM-type organic EL elements can thus implement high luminance, high image quality display even when instantaneous luminance is lower than that of PM-type organic EL elements. Since instantaneous luminance of AM-type organic EL elements can be made lower than that of PM-type organic EL elements, a lower driving voltage and longer service life can be implemented in AM-type organic EL elements. Accordingly, AM-type organic EL elements have been particularly actively studied and developed.
FIG. 7 is a schematic cross-sectional view of a conventional organic EL display device 300.
The organic EL display device 300 includes a substrate 301, an organic layer 303 formed on the substrate 301, and a first electrode 302 and a second electrode 304 with the organic layer 303 interposed therebetween.
The first electrode 302 injects holes into the organic layer 303. The second electrode 304 injects electrons into the organic layer 303. The organic layer 303 emits light when holes injected from the first electrode 302 and electrons injected from the second electrode 304 rebond with each other in the organic layer 303. In the organic EL display device 300, the substrate 301 and the first electrode 302 have a light transmitting property and the second electrode 304 has a light reflecting property. Light emitted in the organic layer 303 is therefore output from the organic EL display device 300 through the first electrode 302 and the substrate 301 (bottom emission type).
When the organic EL display device 300 is of AM type, TFTs and electrodes must be provided on the substrate 101. TFTs and electrodes are generally formed from materials having a less light transmitting property. For example, TFTs may be formed from silicon having a low light transmittance. The AM-type organic EL display device 300 therefore has a small ratio of a light-emitting area to a pixel area (aperture ratio).
AM-type organic EL elements can be divided into two types: a current driven type; and a voltage driven type. Current-driven organic EL elements can suppress variation in display capability among pixels and can effectively suppress degradation in display capability due to degradation of light-emitting materials. However, current-driven organic EL elements have a larger number of TFTs provided for pixels, as compared to voltage-driven organic EL elements. Therefore, current-driven organic EL elements have a smaller aperture ratio.
Top-emission type organic EL elements have been proposed in view of the above problems. In top-emission type organic EL elements, a second electrode has a light transmitting property and a first electrode has a light reflecting property. Therefore, light emitted in an organic light-emitting layer can be retrieved from the second electrode side, the opposite side from the substrate having TFTs and electrodes which have a low light transmittance. Top-emission type organic EL display devices therefore have a larger aperture ratio than that of bottom-emission type organic EL display devices. As a result, high-luminance organic EL display devices can be implemented.
In top-emission type organic EL display devices, light emitted in the organic layer is retrieved from the second electrode side. The second electrode is therefore preferably formed from a transparent conductive material having a high light transmittance. Examples of the transparent conductive material include indium tin oxide (ITO), indium zinc oxide (IZO), and the like. However, transparent conductive materials such as ITO have a higher electric resistance than that of low-resistance metal materials, such as silver (Ag) and aluminum (Al), which have been used as an electrode material. The second electrode formed from a transparent conductive material therefore has a high surface resistance, causing a high driving voltage.
When the second electrode has a high surface resistance, voltage drop occurs in a part of the second electrode. Accordingly, when the second electrode of the organic EL display device is formed from a transparent conductive material having a high electric resistance, a uniform voltage is not applied to the second electrode. As a result, uniform image display cannot be provided.
In view of the above problems, a top-emission type organic EL display device has been proposed in which a second electrode is formed from a main electrode of a transparent conductive material and an auxiliary electrode of a low-resistance metal material (for example, U.S. Pat. No. 6,538,374).
FIG. 8 shows a top-emission type organic EL display device 200 described in the above U.S. Patent.
The organic EL display device 200 includes a substrate 211, an electrically insulating film 260 formed on the substrate 211, TFTs 214 embedded in the electrically insulating film 260, a smoothing film 215 formed over the TFTs 214, an organic EL element 201, and an electric connection portion 223. The organic EL element 201 is formed from an upper electrode 270, a lower electrode 220, and an organic light-emitting medium 240. The electric connection portion 223 electrically connects the TFTs 214 with the organic EL display element 201. In the organic EL display device 200, light emitted in the organic EL element 201 is retrieved from the upper electrode 270 side. In the organic EL display device 200, the upper electrode 270 is formed from a main electrode 272 of a transparent conductive material and an auxiliary electrode 271 of a low resistance metal material and therefore the upper electrode 270 has a low surface resistance. As a result, voltage drop is suppressed in the central portion of the screen and uniform image display can therefore be provided. Moreover, a lower driving voltage of the organic EL display device 200 enables reduction in power consumption.
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