Field of the Invention
This disclosure relates to an active matrix type organic light emitting display.
Discussion of the Related Art
An active matrix type organic light emitting display covers an organic light emitting diode (hereinafter, referred to as “OLED”) which emits light by itself, and has advantages of a fast response speed, high light emitting efficiency, high brightness, and a wide viewing angle.
The OLED, which is a self light emitting device, includes an anode electrode, a cathode electrode, and organic compound layers (HIL, HTL, EML, ETL, and EIL) formed therebetween. The organic compound layers include a hole injection layer (HIL), a hole transport layer (HTL), an emission layer (EML), an electron transport layer (ETL), and an electron injection layer (EIL). When a driving voltage is applied to the anode electrode and the cathode electrode, holes passing through the hole transport layer (HTL) and electrons passing through the electron transport layer (ETL) move into the emission layer (EML) to form an exciton, and as a result, the emission layer (EML) emits a visible light.
In the organic light emitting display, pixels each including an OLED are arranged in a matrix type, and the brightness of the pixels is controlled according to the grayscale of video data. Each of the pixels includes a driving thin film transistor (TFT) for controlling a driving current flowing through the OLED. The organic light emitting display has a problem in that electric characteristics of TFT, such as threshold voltage and mobility of the driving TFT, are not uniform for the pixels, and thus the current levels with respect to the same data voltage, that is, the light emission amounts of OLEDs are different for the pixels, causing a difference in brightness.
For solving the problem, a hybrid compensation manner is proposed that non-uniform brightness due to a difference in threshold voltage of the diving TFT is compensated in an external compensation manner, and non-uniform brightness due to a difference in mobility of the driving TFT is compensated in an internal compensation manner. Specifically, according to the hybrid compensation manner, the difference in the threshold voltage of the driving TFT is compensated by sensing threshold voltages of driving TFTs for the respective pixels and correcting input data according to the sensed values.
In addition, according to the hybrid compensation manner, the difference in mobility of the driving TFT is compensated by raising the source voltage level of the driving TFT in a source follower manner while the gate voltage level of the driving TFT is fixed to a data voltage during the sensing period. The driving current determining the light emission amount (brightness) of the pixel is proportional to the mobility of the driving TFT and the gate-source voltage of the driving TFT programmed in the sensing period. With respect to a pixel having large mobility, the source voltage of the driving TFT promptly is raised toward the gate voltage, which is higher than the source voltage, during the sensing period, so that the gate-source voltage of the driving TFT is programmed to be small. On the contrary to this, with respect to a pixel having small mobility, the source voltage of the driving TFT is slowly raised toward the gate voltage, which is higher than the source voltage, during the sensing period, so that the gate-source voltage of the driving TFT is programmed to be large. As a result, the difference in brightness due to the difference in mobility between pixels is compensated.
With respect to this hybrid compensation manner, the sensing period during which the difference in mobility of the driving TFT is internally compensated may be determined by a gate signal applied to each pixel. However, the gate signal varies depending on the display position due to RC delay, causing a difference in the sensing period depending on the display position. Meanwhile, the difference in the sensing period may be shown depending on the display grayscale. In the case where the sensing period varies depending on the display position or the display grayscale, the performance of compensating the mobility of the driving TFT also varies accordingly, and thus the uniformity in brightness of the display panel may deteriorate.