主動式有機發光二極體顯示器之畫素電路必須採用薄膜電晶體作為驅動與開關元件，然而不同製程的薄膜電晶體元件臨界電壓會因為製程上的誤差或者因長時間的使用而產生漂移。此外，有機發光二極體材料隨著長時間使用而老化的現象導致臨界電壓上升及發光效率下降，因而對於畫面顯示品質造成影響。
本論文即針對上述問題提出四種新式畫素補償電路。第一個電路是利用傳統2T1C電路架構並結合外部補償電路進行改善，具有較高的開口率並可以有效維持OLED電流穩定度以改善面板亮度均勻性，但依然會受到發光材料老化導致發光效率下降的影響，從模擬結果可知其驅動電流幾乎維持一致，但其發光亮度卻會隨著時間而逐漸下降，相較於傳統畫素電路，面板發光亮度能有效改善15%。因此在第二個電路中，為了有效維持面板亮度均勻性同時簡化電路操作，畫素內元件數增為3T1C，利用畫素內部元件的設計，採用動態平衡的方式使驅動電流不受到臨界電壓變異的影響，同時利用外部補償電路偵測電流衰減幅度動態調整偵測電壓，以達到面板均勻性的目標，由實驗結果顯示在60 ℃下，經過50000秒的測試，電流衰減度只有原來的5%，而實際接上有機發光二極體測試，可發現其發光亮度幾乎一致。第三個電路採用電壓回授的概念進行設計，畫素內元件數為3T1C，利用外部電路可以達到消除TFT臨界電壓變異的影響，同時利用OLED材料老化衰減臨界電壓上升的現象改善亮度下降的問題，由模擬結果可知當有機發光二極體臨界電壓漂移量為0.9V時，本電路亮度可以維持在原先的96.4%，而傳統P型與N型2T1C分別為79.4%與47.4%，因此本電路可以有效改善面板亮度衰減的問題。由於前三個電路皆是採用外部補償的方式進行面板亮度的改善，其操作方式較為複雜，因此在第四個電路中結合反向偏壓的概念進行設計，畫素元件數維持3T1C，可以同時有效降低電源線內阻效應與元件臨界電壓的影響，由實驗結果可知，當發光時間為傳統驅動方式的20%時，其亮度可以維持在原先的94.3%，而傳統發光方式則為76.4%，此外，當反向偏壓由0V上升到6V時，其亮度亦由77.15%提升至94.3%，因此反向偏壓的方式的確可以有效改善有機發光二極體老化的現象，由模擬結果可得知電路中相關節點電流變化值都幾乎維持一致，所以本電路可以補償薄膜電晶體臨界電壓變異和降低電源線內阻效應的影響，並且增加有機發光二極體的顯像壽命。

For an active matrix organic light emitting diode (AMOLED) display, the pixel circuit utilizes a thin film transistor (TFT) as the driving and switching component. However, variation in the VTH of the driving TFT due to process variation or long-term operation, in addition to luminance decay caused by OLED aging, directly influence the image quality of the AMOLED display.
This dissertation proposes four novel pixel circuits and verifies their effectiveness by simulations and experiments. The first system integrates a simplified 2T1C pixel structure and external compensation circuit to ameliorate VTH variations and OLED degradation. The pixel current in the proposed system remains constant, whereas current degradation in the conventional pixel circuit is over 20%. Additionally, the luminance degrades by 15% in the proposed pixel circuit versus 30% in the conventional 2T1C circuit. The second circuit can stabilize the OLED current against TFT degradation and provide an additional driving current to compensate for the brightness of the AMOLED. Measurement results indicate that the current degradation of the proposed circuit is less than 5%, over more than 50000s at 60 ℃, whereas that of a conventional 2T1C pixel circuit is over 34%. Additionally, the luminance of the fabricated OLED device with the proposed 3T1C driving scheme can remain at the initial value. The third proposed pixel circuit with three TFTs utilizes the external compensation circuit to detect VTH variations, and employs the increase in VTH_OLED as voltage feedback to compensate for luminance degradation. Simulation results indicate that the normalized luminance of this work is 96.4%, while ΔVTH_OLED is 0.9 V, while those of the conventional 2T1C circuit with P-type driving TFT and N-type driving TFT are 79.4% and 47.4%, respectively. Thus, the proposed driving scheme can ameliorate the luminance degradation via the detection of VTH_OLED through the external compensated circuit. Because of the complexity in the operation of the external detection method, the fourth proposed 3T1C circuit adopts a novel ac driving method to ameliorate the brightness uniformity. Experimental results indicate that adjusting the duty ratio of the ac driving scheme can ameliorate the brightness uniformity; meanwhile, the normalized luminance of a 20% duty ratio after a 3600s stress is 94.3% and that of dc stress is 76.4%. The normalized luminance of measured OLED is 94.3% under a higher voltage (-6V), while it is only 77.15% under a lower voltage (0V). Therefore, the applications of a reversed-bias voltage under an ac driving scheme ameliorate luminance uniformity variation. The proposed pixel circuit has high immunity to variations of electrical characteristics of TFTs and can compensate for OLED degradation.

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