1. Field of the Invention
The present invention relates to an OLED panel in which an organic light emitting element formed on a substrate is enclosed between the substrate and a cover member. Also, the present invention relates to an OLED module in which an IC or the like is mounted on the OLED panel. Note that, in this specification, the OLED panel and the OLED module are generically called light emitting devices. The present invention further relates to a method of driving the light emitting device and an electronic appliance using the light emitting device.
2. Description of the Related Art
A light-emitting element emits light by itself, and thus, has high visibility. The light-emitting element does not need a backlight necessary for a liquid crystal display device (LCD), which is suitable for a reduction of a light-emitting device in thickness. Also, the light-emitting element has no limitation on a viewing angle. Therefore, the light-emitting device using the light-emitting element has recently been attracting attention as a display device that substitutes for a CRT or the LCD.
Incidentally, the light-emitting element means an element of which a luminance is controlled by electric current or voltage in this specification. The light emitting element includes an OLED (organic light emitting diode), an MIM type electron source element (electron emitting elements) used to a FED (field emission display) and the like.
The OLED includes a layer containing an organic compound in which luminescence generated by application of an electric field (electroluminescence) is obtained (organic light emitting material) (hereinafter, referred to as organic light emitting layer), an anode layer and a cathode layer. A light emission in returning to a base state from a singlet excitation state (fluorescence) and a light emission in returning to a base state from a triplet excitation state (phosphorescence) exist as the luminescence in the organic compound. The light-emitting device of the present invention may use one or both of the above described light emissions.
Note that, in this specification, all the layers provided between an anode and a cathode of the OLED are defined as the organic light emitting layers. The organic light emitting layers specifically include a light emitting layer, a hole injecting layer, an electron injecting layer, a hole transporting layer, an electron transporting layer and the like. These layers may have an inorganic compound therein. The OLED basically has a structure in which an anode, a light emitting layer, a cathode are laminated in order. Besides this structure, the OLED may take a structure in which an anode, a hole injecting layer, a light emitting layer, a cathode are laminated in order or a structure in which an anode, a hole injecting layer, a light emitting layer, an electron transporting layer, a cathode are laminated in order.
FIG. 23 exemplifies the constitution of an individual pixel of a conventional light emitting device. The conventional pixel shown in FIG. 23 includes TFTs (thin-film transistors) 50 and 51, a storage capacitor 52, and a light emitting element 53.
A gate of the TFT 50 is connected to a scanning line 55. Either of a source and a drain of the TFT 50 is connected to a signal line 54, and the other is connected to the gate of the TFT 51. The source of the TFT 51 is connected to a power supply 56, and the drain is connected to an anode of a light emitting element 53. A cathode of the light emitting element 53 is connected to a power supply 57. The storage capacitor 52 is provided in order to preserve a predetermined voltage between the gate and the source of the TFT 51.
When the TFT 50 is turned ON by a predetermined voltage of the scanning line 55, a video signal fed to the signal line 54 is delivered to the gate of the TFT 51. Upon the input of video signal, based on the voltage of the input video signal, the gate voltage (i.e., the potential difference between the gate and the source) of the TFT 51 is determined. Then, the drain current of the TFT 51 driven by the gate voltage thereof is fed to the light emitting element 53, thereby enabling the light emitting element 53 to emit light with the input current.
The TFT composed of polysilicon exerts a field-effect mobility higher than that of the TFTs composed of amorphous silicon, and it has a large amount of an ON current. Because of the above reasons, the TFT composed of polysilicon is better suited for forming the transistor components of a light emitting element panel.
However, even when forming the TFT by applying polysilicon, its electrical characteristics are by no means comparable to the electrical characteristics of a MOS transistor formed on a monocrystalline silicon substrate. For example, field-effect mobility of the TFT composed of polysilicon is rated to be equal to or lower than one tenth the field-effect mobility of monocrystalline silicon. Further, because of a certain defect generated in crystal grain boundaries, the characteristics of the TFT composed of polysilicon is easily subject to variation, which is a problem.
Referring to FIG. 23, when electrical characteristics such as a threshold value and the ON current of the TFT 51 are variable per pixel, even though a voltage of the video signal is the same, a magnitude of the drain current in the TFT 51 varies between individual pixels, thus resulting in the uneven luminance of the light emitting element 53.
When industrially and commercially providing such a light emitting device utilizing an OLED (organic light-emitting display), there was such a critical problem in terms of the short service duration of the OLED caused by degradation of organic light-emitting layers. Generally, an organic light-emitting material is vulnerable to water, oxygen, light, and heat, which expedite possible degradation of the organic light-emitting layers. More particularly, the degrading rate is dependent on the constitution of a device for driving a light emitting device, electrical characteristics of the organic light-emitting material, a material of electrodes, a condition in the manufacturing processes, and the method of driving the light-emitting device.
Even though the voltage applied to the organic light-emitting layers is constant, once degradation occurs in the organic light-emitting layers, the luminance of the OLED is lowered to result in an obscure image on a display panel.
Further, a temperature of the organic light-emitting layers is variable by the outside temperature and heat generated by an OLED panel itself. However, generally, actual value of current flowing through the OLED is variable by the temperature. More particularly, when the temperature of organic light-emitting layers rises while the voltage is constant, a greater amount of current flows into the OLED. Further, inasmuch as the current flowing into the OLED and the luminance of the OLED are in the proportional relationship, the greater the amount of current flowing into the OLED, the brighter the luminance of the OLED. In this way, the luminance of the OLED is variable by the temperature of organic light-emitting layers, and thus, it is quite difficult to display desired gradation. In consequence, relative to the rise of the temperature, a greater amount of current is consumed by the light-emitting device.