1. Field of the Invention
The present invention relates to a light emitting display, and more particularly to an organic light emitting diode (OLED) light emitting display utilizing electroluminescent (EL) light emission of an organic material.
2. Discussion of the Related Art
Generally, OLED displays emit light by electrically exciting an organic compound. Such an OLED display includes N×M organic light emitting diodes arranged in the form of a matrix, and displays an image by driving the organic light emitting cells, using voltage or current.
Such organic light emitting cells are also called “organic light emitting diodes (OLEDs)” because they have diode characteristics. As shown in FIG. 1, each organic light emitting diode has a structure including an anode electrode layer (e.g., ITO), an organic layer, and a cathode electrode layer (e.g., metal). The organic layer has a multi-layer structure including an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL), to achieve an improved balance between electrons and holes, and thus, to achieve an enhancement in light emitting efficiency. The organic layer also includes an electron injecting layer (EIL) and a hole injecting layer (HIL). Such organic light emitting diodes are arranged in the form of an N×M matrix to form an OLED display panel.
For driving methods for such an OLED display panel, there are a passive matrix type of driving method and an active matrix type of driving method using thin film transistors (TFTs). In accordance with the passive matrix type of driving method, anodes and cathodes are arranged to be orthogonal to each other so that a desired line to be driven is selected. In accordance with the active matrix type of driving method, thin film transistors are coupled to respective indium tin oxide (ITO) pixel electrodes in an OLED display panel so that the OLED display panel is driven by a voltage maintained by a capacitor coupled to the gate of each thin film transistor.
FIG. 1 shows a circuit diagram for representing one of N×M pixels as a conventional pixel circuit, equivalently representing a pixel arranged in a first row and a first column.
As shown in FIG. 1, a pixel 10 includes three sub-pixels 10r, 10g, and 10b. The sub-pixels 10r, 10g, and 10b respectively include OLED elements OLED_r, OLED_g, and OLED_b for respectively emitting red, green, and blue lights. In a configuration in which sub-pixels are arranged in a stripe pattern, the sub-pixels 10r, 10g, and 10b are respectively coupled to data lines D1r, D1g, and D1b, and are commonly coupled to a scan line S1.
The sub-pixel 10r for emitting the red light includes two transistors M1r and M2r, and a storage capacitor C1r for driving the OLED element OLED_r. The sub-pixel 10g for emitting the green light includes two transistors M1g and M2g, and a storage capacitor C1g. The sub-pixel 10b for emitting the blue light includes two transistors, M1b and M2b, and a storage capacitor C1b. Operations of the sub-pixels 10r, 10g, and 10b are substantially the same as each other, and therefore only the operation of the sub-pixel 10r will be described.
The driving transistor M1r is coupled between a first power source VDD and an anode of the OLED element OLED_r, and transmits a current to the OLED element OLED_r to emit the OLED element OLED_r. The cathode of the OLED element OLED_r is coupled to a second power source VSS which provides a voltage lower than that of the first power source. Current of the driving transistor M1r is controlled by a data voltage applied through a switching transistor M2r. The capacitor C1r is coupled between a source and a gate of the transistor M1r, and it maintains an applied voltage for a predetermined period of time. A gate of the transistor M2r is coupled to the scan line S1 for transmitting a switching signal, and a source of the transistor M2r is coupled to the data line D1r for transmitting a data voltage corresponding to the sub-pixel 10r for emitting a red light.
A data voltage VDATA from the data line D1r is applied to the gate of the transistor M1r when the switching transistor M2r is turned on in response to a selection signal applied to the gate of the transistor M2r. A current of IOLED flows to the transistor M1r correspondingly to a voltage of VGS charged between the gate and the source by the capacitor C1r, and the OLED element OLED_r is emitted corresponding to the current of IOLED. At this time, the current of IOLED flowing through the OLED element OLED_r is given as Equation 1.
                                                                        I                OLED                            =                                                β                  2                                ⁢                                                      (                                                                  V                        GS                                            -                                              V                        TH                                                              )                                    2                                                                                                        =                                                β                  2                                ⁢                                                      (                                                                  V                        DD                                            -                                              V                        DATA                                            -                                                                                                V                          TH                                                                                                              )                                    2                                                                                        [                  Equation          ⁢                                          ⁢          1                ]            
In the pixel circuit shown in FIG. 1, when a current corresponding to the data voltage is supplied to the OLED element OLED_r, the OLED element OLED_r is emitted with a brightness corresponding to the supplied current. At this time, the applied data voltage has various values within a predetermined range in order to express predetermined gray scales.
As shown, the OLED light emitting display includes the pixel 10 including the three sub-pixels 10r, 10g, and 10b. The respective sub-pixels include a driving transistor, a switching transistor, and a capacitor for driving an OLED element. A data line for transmitting a data signal and a power line for applying the first power source VDD are formed for each sub-pixel. Accordingly, the OLED light emitting display must include a large number of lines and other elements. The lines are difficult to arrange in a limited display area, and aperture efficiency corresponding to an emitting pixel area is reduced. Therefore a pixel circuit for reducing the number of lines and elements for driving a pixel should be developed.