| Summary: | 碩士 === 國立臺灣科技大學 === 光電工程研究所 === 97 === Organic Light Emitting Diode (OLED) with fast response, high brightness and high contrast plays an important role gradually in the further market of flat penal display (FPD).
In order to enhance the image quality of AMOLEDs further, the different driving methods have been compared and evaluated. Analog driving circuits can be divided into current programming circuits and voltage programming circuits. Current programming circuits can compensate both of the threshold voltage and mobility variation. Moreover, due to amorphous silicon TFTs reliability issue under the gate bias stress, current programming method is more suitable for wide range threshold voltage variation devices because of its self-compensation function. However, they have the limitation of the programming time issue. The data driver IC also needs more complicated design. Voltage programming circuits show great potential for high resolution and low cost applications in the future because of the simple structure.
This thesis demonstrates two pixel circuits using LTPS-TFT TFTs and a-Si TFTs for AMOLED.
In chapter two, we introduced the conventional 2T1C pixel circuit. The experimental results of conventional pixel design (2T1C) demonstrate the influence caused by the threshold voltage variation of LTPS-TFTs and the power line IR voltage drop effect. As a result, the different current flow through OLED would result in the non-uniform brightness across the panel.
At first, conventional 2T1C circuit is simulated and discussed. Simulation results show that conventional 2T1C pixel circuit has high non-uniformity due to the various characteristics of TFTs. The average error rate of the OLED current is up to 37.8 %.
In order to overcome the non-uniformity problem of conventional 2T1C circuit, B. S. Lin proposed a 3T1C voltage programming AMOLED pixel design. But for conventional row-by-row driving, this pixel circuit cannot conform with AM technique. As the above reason, we modified the 3T1C pixel circuit into two 5T1C pixel circuits. The simulation result shows the average error rate of the OLED current is about 1.67 % in the first 5T1C modified pixel circuit. Apparently, the OLED current is independent of the variation of threshold voltage. In addition, the degradation rate of the output current while Vdd drop of 1 V is improved to about 15 %, while it is about 80% or even more in the conventional p-type 2T1C pixel circuit. In order to improve the degradation rate of the first modified 5T1C pixel circuit, the second modified 5T1C pixel circuit is proposed. It shows that the average error rate of the OLED current is only 1.07 % in the second modified 5T1C pixel circuit. Apparently, the OLED current is independent of the variation of threshold voltage. In addition, the degradation rate of the output current while Vdd drop of 1 V is improved to about 2.5 %.
In chapter three, we introduced the current copy type, current mirror type and proposed 5T1C current copy type pixel circuits. We used a variable power line Vdd design in our pixel circuit to control the ΔVD_DTFT and decrease the IOLED deviation caused by capacitance coupling effect and improve the output current linearity. The average IOLED error rate is about 2.88% in a very wide range of threshold voltage variation (�幀TH = 5 V, -1~+4 V), while it is about 30–40% in the conventional a-Si 2T1C voltage programming pixel in a very small range of threshold voltage variation(�幀TH = ± 0.3 V). The result shows that because of the n-type DTFT design, the proposed pixel circuit is nearly independent of the I-R voltage (＜3%). The IOLED is independent of OLED degradation because when the anode voltage shift by 0.33V, the IOLED is nearly the same.
Therefore, these proposed pixel circuits can all successfully compensate for the threshold voltage variation of LTPS and a-Si DTFTs and the IR drop on the power lines. So these proposed pixel circuits can improve the current non-uniformity for AMOLED compared with conventional pixel circuit. Therefore, they could be the promising candidates for the AMOLED panel application in the future.
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