Display panels using a two-terminal display device that emits light at luminous intensity corresponding to a direct current level, such as an organic electroluminescence element or organic light-emitting diode (OLED); namely, organic electroluminescence (EL) devices, have recently been developed.
Implementation of a high-luminance, high-contrast image display requires an active drive using a switching element, for example, a thin film transistor (TFT). Typically, an OLED is a light emitting device that has a laminated structure of a thin organic material and changes emission intensity according to the level of an electric current running through the OLED. A display unit that displays an image can be configured by using TFTs to pass an electric current through the OLED provided on each pixel or sub-pixel disposed on a matrix and thereby allow it to emit light.
FIG. 1 shows a basic configuration of a display unit that employs an OLED (see Non Patent Literature NPL1). An OLED display unit is composed of a backplane 100 having a TFT (or other switching element) 20, a capacitive element (capacitor) 30, and a wire 40 disposed on a glass substrate 10, and a frontplane 150 having an OLED 120 disposed thereon. The OLED display unit is classified into two types according to the direction in which light is emitted by the OLED 120; one is bottom emission type (see FIG. 1(a)) in which is light emitted by the OLED mainly through the backplane 100, and the other is top emission type (see FIG. 1(b)) in which light is emitted by the OLED mainly through the frontplate 150, rather than through the backplane 100.
In the bottom emission type, an indium tin oxide (ITO) transparent conductive film 60 is formed on the backplane 100 provided on the glass substrate 10 for injecting holes, and then the OLED 120 is formed on the transparent conductive film 60. This type allows the manufacture of display units using processes that are at low temperature and free from oxygen following the formation of the OLED 120, providing advantages of implementing a relatively simplified manufacturing process that gives no damage to the organic material. However, this type has a structure where light passes through the backplane 100 having the TFT 20 and the wire 40 disposed thereon, placing restrictions on an opening area. In particular, this type has the disadvantage of an extremely small aperture ratio in high-definition display units.
The top emission type has to have an ITO transparent conductive film 61 formed on an organic material, due to the necessity to emit light through the frontplate 150′ (opposite side of the backplane 100′). Formation of the ITO transparent conductive film uses oxygen plasma processes including a heating process in some instances, and as a result the organic material on the OLED 120′ tends to sustain damage. In addition, oxygen/water content absorbers must be provided on the side through which light is taken out, resulting in a complicated structure which has disadvantages of making the manufacturing process relatively complicated. However, this type has fewer restrictions caused by the backplane 100′ on an opening area through which light is emitted, since light does not pass through the backplane 100′ having the TFT 20′ and the wire 40′ disposed thereon. In particular, this type has the potential advantage of a relatively large aperture ratio in the high-definition display units. In practice, however, a large aperture ratio has not been achieved to date, since a planarization layer formed between the backplane 100′ and the frontplane 150′ has difficulty in functioning at an end of the wire.
On the other hand, an attempt has been made to make the aperture ratio greater in the bottom emission type, as proposed in some patent literature (see Patent Literature PTL1, PTL2 and PTL 3). These documents propose that the capacitive element 30 and wire 40 disposed on the backplane 100 are made transparent to achieve an increased aperture ratio. The transparent capacitive element 30 has been found to have the effect of ensuring such an increased aperture ratio. The transparent wire 40, however, has not been successfully implemented, due to the difficulty in withstanding micro-fabrication and providing sufficient conductivity.
Recently, demand has been increasing for higher definition display units.
OLED display units designed for use in mobile devices which have recently been gaining attention require high definition that ranges from 300 ppi (pixels per inch) up to 400 ppi. In a high-definition OLED display unit of 400 ppi, for example, one pixel has a size of 60 microns square, making it very difficult to simultaneously achieve large aperture ratio, low cost, high reliability, and stable manufacturing processes. Currently known technologies achieve an aperture ratio of about 20 percent for the top emission type, but this is not satisfactory in ensuring low cost and reliability.
It would be desirable in a high-definition OLED display unit of 300 ppi or more, to provide a high-definition display panel pixel structure and an organic electroluminescence display using such a pixel structure which achieves a light-extraction aperture ratio of 20 percent or more despite the use of a bottom emission type that provides a relatively simplified manufacturing process with less damage to an organic material.