Conventionally, as for a display element provided in a display device, there exist an electro-optical element whose luminance is controlled by an applied voltage and an electro-optical element whose luminance is controlled by a flowing current. A liquid-crystal display element is a representative example of the electro-optical element whose luminance is controlled by an applied voltage. On the other hand, an OLED is a representative example of the electro-optical element whose luminance is controlled by a flowing current. The OLED is also called an organic electro-luminescence (EL) element. An organic EL display device using the OLED as a self light emission-type electro-optical element can achieve reduction in thickness, low power consumption, and high luminance as compared with the liquid crystal display device that generally requires a backlight, a color filter, and the like. Therefore, development of the organic EL display device has been progressed actively in recent years.
As driving systems of the organic EL display device, there are two kinds of driving systems, i.e., a passive matrix system (also called a simple matrix system), and an active matrix system. The organic EL display device employing the passive matrix system (hereinafter, referred to as a “passive matrix-type organic EL display device”) has a simple structure, but realization of a large size and high precision is difficult. On the other hand, the organic EL display device employing the active matrix system (hereinafter, referred to as an “active matrix-type organic EL display device”) can realize a large size and high precision easily as compared with the passive matrix-type organic EL display device.
The organic EL display device includes a plurality of pixel circuits arranged in a matrix form. Further, the pixel circuit of the active matrix-type organic EL display device typically includes an input transistor that selects a pixel, and a drive transistor that controls supply of a current to the OLED. In the following, the current that flows from the drive transistor to the OLED may be also referred to as a “drive current”.
It is known that degradation of the OLED progresses as a light emission time of the OLED becomes long, and as a result, emission luminance becomes lower than that at the beginning. That is, when degradation of the OLED has progressed, luminance does not reach desired emission luminance even when a drive current having the same amount as that at the beginning is made to flow. In the following, the degradation that progresses with increase in the light emission time of the OLED is referred to as “time degradation”. FIG. 16 is a diagram for describing an influence that time degradation of the OLED exterts on a display. As shown in FIG. 16, when attempting to make the OLED emit light in an area where white display is continued for a long time, light can be emitted only in lower emission luminance than desired emission luminance. This is because in the area where white display has been continued for a long time, time degradation of the OLED has progressed. On the other hand, in an area where black display has been continued for a long time, the time degradation of the OLED has not progressed. Therefore, the OLED can be made to emit light in the desired emission luminance. As described above, in a certain pixel, when the time degradation of the OLED has progressed more than that in surrounding pixels, there occurs a phenomenon called “burn-in” in which a difference in luminance between the pixels is recognized visually.
In Patent Document 1, there is disclosed an organic EL display device that detects reduction in capacitance of an OLED from a pixel circuit, and applies a drive current larger than an original drive current to the OLED in which time degradation has progressed, based on a correlation between a degree of time degradation of the OLED and the reduction in the capacitance of the OLED, so that the organic EL display device compensates for reduction in luminance. In the organic EL display device disclosed in Patent Document 1, there is provided, in a data driver, a read block for reading a voltage and the like from a pixel circuit to detect the reduction in the capacitance of the OLED. The read block transmits the read voltage to a controller.
In Patent Document 2, there is disclosed an organic EL display device that estimates an OLED in which time degradation has progressed most, by continuously or periodically sampling a video signal supplied to a controller, and compensates for reduction in luminance by applying a drive current larger than an original drive current to the estimated OLED.
In Patent Document 3, there is disclosed an organic EL display device that accumulates, in a capacitor in a pixel circuit, a voltage between terminals of an OLED which increases with progress of time degradation of the OLED, and compensates for reduction in luminance by using the accumulated voltage between the terminals of the OLED.