1. Field of the Invention
The present invention relates to a display device, a driving method for the display device, and an electronic apparatus, and, more particularly to a flat (flat panel) display device in which pixels including electro-optic elements are two-dimensionally arranged in a matrix shape, a driving method for the display device, and an electronic apparatus including the display device.
2. Description of the Related Art
In recent years, in the field of a display device that performs image display, flat display devices in which pixels (hereinafter also referred to as “pixel circuits” in some case) including light emitting elements are two-dimensionally arranged in a matrix shape are rapidly spread. As one of the flat display devices, there is a display device in which electro-optic elements of a so-called current drying type, light emission luminance of which changes according to a current value flowing to a device, are used as light emitting elements of pixels. As the electro-optic element of the current driving type, there is known an organic EL (Electro Luminescence) element that makes use of a phenomenon in which an organic thin film emits light when an electric field is applied thereto.
An organic EL display device in which organic EL elements are used as light emitting elements of pixels has characteristics explained below. The organic EL elements consume low power because the organic EL elements can be driven with applied voltage equal to or lower than 10 V. Since the organic EL elements are self-light emitting elements, visibility of an image is high compared with a liquid crystal display device that displays an image by controlling the intensity of light from a light source with a liquid crystal in each of pixels. Further, since a light source such as a backlight is unnecessary, it is easy to reduce weight and thickness of the organic EL display device. Moreover, since response speed of the organic EL elements is extremely high at about several microseconds, a residual image during moving image display does not occur.
In the organic EL display device, as in the liquid crystal display device, a simple (passive) matrix system and an active matrix system can be adopted as a driving system therefor. However, although a display device of the simple matrix system is simple in structure, a light emission period of electro-optic elements decreases according to an increase in scanning lines (i.e., the number of pixels). Therefore, it is difficult to realize a large and high-definition display device.
Therefore, in recent years, the development of a display device of the active matrix system that controls an electric current flowing to electro-optic elements with active elements, for example, insulated-gate field-effect transistors provided in pixels, in which the electro-optic elements are provided, is actively performed. As the insulated-gate field-effect transistors, in general, TFTs (Thin Film Transistors) are used. In the display device of the active matrix system, the electro-optic elements maintain light emission over a period of one frame. Therefore, it is easy to realize a large and high-definition display device.
In general, it is known that an I (current)-V (voltage) characteristic of the organic EL elements deteriorates as time elapses (so-called aged deterioration). In pixel circuits in which, in particular, N-channel TFTs are used as transistors for current-driving the organic EL elements (hereinafter referred to as “driving transistors”), when the I-V characteristic of the organic EL elements deteriorates with time, gate-to-source voltage Vgs of the driving transistors changes. As a result, light emission luminance of the organic EL elements changes. This occurs because the organic EL elements are connected to source electrode sides of the driving transistors.
This is more specifically explained below. Source voltage of the driving transistors depends on operating points of the driving transistors and the organic EL elements. When the I-V characteristic of the organic EL elements deteriorates, since the operating points of the driving transistors and the organic EL elements fluctuate, even if the same voltage is applied to gate electrodes of the driving transistors, the source voltage of the driving transistors changes. Therefore, since the gate-to-source voltage Vgs of the driving transistors changes, a current value flowing to the driving transistors changes. As a result, since a current value flowing to the organic EL elements also changes, light emission luminance of the organic EL elements changes.
In particular, in pixel circuits in which polysilicon TFTs are used, in addition to the aged deterioration of the I-V characteristic of the organic EL elements, transistor characteristics of the driving transistors change as time elapses and the transistor characteristics are different in each of pixels because of irregularity in a manufacturing process. In other words, there is irregularity in the transistor characteristics of the driving transistor in each of the pixels. Examples of the transistor characteristics include threshold voltage Vth of the driving transistors and mobility μ of semiconductor thin films included in channels of the driving transistors (hereinafter simply referred to as “mobility μ of the driving transistors”).
When the transistor characteristics of the driving transistors are different in each of the pixels, irregularity occurs in a current value flowing to the driving transistor in each of the pixels. Therefore, even if the same voltage is applied the gate electrodes of the driving transistors among the pixels, irregularity occurs in light emission luminance of the organic EL elements among the pixels. As a result, uniformity of a screen is spoiled.
Therefore, in order to maintain the light emission luminance of the organic EL elements constant without being affected by the aged deterioration in the I-V characteristic of the organic EL elements, the aged deterioration in the transistor characteristics of the driving transistors, and the like, there is proposed a technique for imparting various correction (compensation) functions to the pixel circuit (see, for example, JP-A-2007-310311).
Examples of the correction functions include a compensation function for the fluctuation in the I-V characteristic of the organic EL elements, a correction function for the fluctuation in the threshold voltage Vth of the driving transistors, and a correction function for the fluctuation in the mobility μ of the driving transistors. In the following explanation, correction for the fluctuation in the threshold voltage Vth of the driving transistors is referred to as “threshold correction” and correction for the fluctuation in the mobility μ of the driving transistors is referred to as “mobility correction”.
By imparting the various correction functions to each of the pixel circuits in this way, it is possible to maintain the light emission luminance of the organic EL elements constant without being affected by the aged deterioration in the I-V characteristic of the organic EL elements and the aged deterioration in the transistor characteristics of the driving transistors. As a result, it is possible to improve a display quality of the organic EL display device.
In the related art disclosed in JP-A-2007-310311, control of light emission and non-light emission of the organic EL elements is performed by appropriately switching the potential of a power supply line, to which drain electrodes of the driving transistors are connected, between first potential Vcc and second potential Vss. The first potential Vcc is power supply potential for supplying an electric current to the driving transistors and the second potential Vss is power supply potential for applying reverse bias to the organic EL elements.