用以改善劣化之新式有機發光二極體具負偏壓 驅動電路

Abstract

有機發光二極體擁有許多優點，例如高對比、高反應速度、自發光、可視角高、低功耗等優點，被視為次世代具有潛力的平面顯示器之一。然而其尚未能投入大量生產的主要原因在於面板均勻度無法提升以及元件劣化的壽命問題，面板均勻度問題主要為像素(Pixel)裡的驅動薄膜電晶體(TFT)經過長時間的驅動，其特性會產生不同程度的飄移；元件劣化的壽命問題在於持續固定電流密度驅動下，元件內部發光層電子與電洞不斷累積，使得再結合發光的效應變差，造成元件劣化，進而使得亮度下降。本論文將以一電流鏡架構以補償TFT特性飄移的基礎下，設計以脈寬調變(PWM)原理來達到降低電流密度以改善元件劣化效應，同時在OLED元件無電流通過時，加以負偏壓於OLED元件上，減少發光層內電子電洞的濃度，提高元件發光效率之具有補償面板均勻度及改善劣化之主動式有機發光二極體像素電路。另外針對傳統電流驅動之畫素電路之電流源製作不易之缺點，改以製作較簡單的電壓驅動型，以降低電路製作時的複雜度。本論文使用LTPS TFT Model Card作為模擬基準，驗證所設計之電路特性，最後透過友達之LTPS SPC製程製作成QVGA面板並進行劣化量測以驗證改善元件劣化程度。

Organic light-emitting diode(OLED) owns advantages such as high contrast, fast response time, self-luminous, wide viewing angle and low power consumption. It is regarded as a viable display option in the future. However the reason that OLED is not popular is due to emission non-uniformity and degradation of an OLED component. The problem of non-uniformity is caused by the shifts of threshold voltage of thin film transistor (TFT) under long time stress. The OLED degradation is caused by the accumulation of electrons and holes in the emission layer under constant current stress, which enables recombination effects to be weakened. A pixel driving circuit is designed in this study based on a current mirror structure to compensate shifts of the threshold voltage of TFT, and also a reverse bias to alleviate OLED degradation. This circuit introduces a driving method similar to pulse width modulation (PWM) to reduce the current density through the OLED component such that it improves the OLED lifetime. This circuit further applies a reverse bias on OLED component when the driving TFT turns off for improving OLED lifetime. The pixel circuit is designed as a voltage driving scheme to ease the difficulty of fabrication. It is simulated by HSPICE using LTPS TFT models to verify the predicted performance. The pixel circuit is fabricated and integrated to a practical QVGA panel using LTPS SPC process and finally verifies the capability of alleviating OLED degradation.