AbstractIn recent years, active-matrix organic light-emitting diode (AMOLED) displays have developed rapidly and attracted much attention. However, the pixel circuits are composed of thin-film transistors (TFTs) whose electrical characteristics may vary or shift because of either fabrication process variation or long-term operation. Additionally, the threshold voltage of OLED increases due to the degradation of organic materials, resulting in the decay of the pixel circuit current. Consequently, these problems reduce the image quality of AMOLED displays.
In order to resolve the issues mentioned above, an external compensating system for the AMOLED pixel circuit is proposed in this work which can compensate for the threshold voltage shifts of driving TFTs and OLEDs. The proposed compensating system has the advantages of high aperture ratio and low energy consumption because it adopts the conventional 2T1C pixel circuit structure. The compensating system can be divided into two parts, including the field-programmable gate array (FPGA) and the external circuit. The control unit which is built for FPGA is utilized to handle control signals and decide the calibration of data voltage during the programming stage of the pixel circuit operation. In addition, the memory unit built for FPGA stores the system parameters and the calibration of data voltage. Conversely the external circuit is composed of a constant current source and comparator. The constant current source generates a constant current (Iref), and the current comparator than senses the difference between the Iref and the OLED current of the pixel circuit. Meanwhile, the control unit increases the gate voltage of the driving TFT until the OLED current is equal to the Iref. The proposed system therefore can detect the threshold voltage shifts of TFTs and OLEDs so as to ameliorate the decay of the pixel circuit current.
The experimental results show that the current error rates of the proposed system are all below 15%, which shows an apparent improvement compared to conventional 2T1C circuits whose error rates are as high as 65%. Therefore, the proposed system can indeed compensate for TFT threshold voltage variations and OLED material degradation to improve the current uniformity of AMOLED displays.
|Date of Award
|Chih-Lung Lin (Supervisor)