主動矩陣有機發光二極體於薄膜電晶體之補償驅動電路研究

Abstract

有機發光二極體(organic light-emitting diodes，OLEDs)顯示器是使用有機化合物堆疊，利用電子電洞對在發光層結合而能夠發出光線之平面顯示器，有機發光二極體顯示器具有體積小、重量輕、可視範圍廣、高對比、高反應速度、低操作電壓、可撓以及簡易製作等優點。主動矩陣式有機發光二極體顯示器是利用電晶體與OLED結合驅動的面板，OLED需要由電流源來驅動電致發光，流經電晶體之電流量的變異對於主動矩陣式有機發光二極體顯示器的亮度均勻有負面的影響，因此，畫素驅動電路的品質對於主動矩陣式有機發光二極體顯示器之畫質非常重要。傳統的AMOLED顯示器的操作模式為兩個薄膜電晶體(thin film transistor，簡稱TFT)，分別作為開關與驅動元件，另外搭配一個儲存電容，然而無論使用有機薄膜電晶體(Organic TFTs，簡稱OTFT),非晶態薄膜電晶體 (hydrogenated amorphous TFTs，簡稱a-TFT:H )，或者是低溫複晶態薄膜電晶體 (low temperature poly TFTs，簡稱LTPS )，都有其無法克服的缺點；LTPS電子遷移率高，使得元件尺寸可以縮小，提升開口率，降低操作電壓，達到省電目的，但其製程成本較高，而且均勻度不佳。另一方面，a-TFT:H擁有較高的均勻度和較低的製程成本，然而a-TFT:H本身遷移率不高，以及所需驅動開關之跨壓很大，而OTFT是很新穎的元件，不只成本較低且可在低溫下製作，所以其為可撓式基板驅動元件，但相對均勻性以及遷移率在此三者中是最差的，當面板在長時間操作下，各種驅動元件(driving TFT)會因為劣化造成臨界電壓(threshold voltage)的漂移以及遷移率的劣化，使得整個驅動電流越來越小，面板壽命變短，因此用以補償臨界電壓與對於不同電晶體形態設計適當驅動電路正被廣範研究中 在本篇論文，提出了三種驅動補償電路，分別使用LTPS和OTFTs為驅動元件，且設計是利用元件本身特性的優缺點與結合系統於玻璃上之技術(SOG)，以其有效改善面板畫面亮度不均勻，以及可靠度不佳再加上高成本的問題，此將成為大型顯示與可撓式面板的前瞻思想。

The emission mechanism of organic light-emitting diode (OLED) is using stacked organic material while electron-hole pairs combine in the emission layer then it scatters the light. OLED displays have recently attracted much attention. Because OLED has thin module, light weight , high contrast ratio, wide viewing angle, fast response time, low operation voltage, flexible and simple fabrication. Active matrix organic light emitting diode (AMOLED) displays that integrate OLED and transistors are quite popular today. OLED needs stable current source which is given by transistors to drive, thereby, it will have negative effects for AMOLEDs uniformity of illumination if the driving current varies with the transistors degradation. So, the quality of pixel driving circuit is very important. The operation of conventional AMOLED display panels are two thin film transistors (TFTs) , which are individually for switching, driving function, and accompany with a storage capacitor. However, no matter organic TFTs (OTFTs) ,hydrogenated amorphous TFTs (a-TFTs:H) or low temperature poly TFTs (LTPS) we use, they have their own obstacles. The significant advantages over LTPS are high mobility, large aperture ratio and lower operation voltage makes power conservation. But the drawbacks are higher cost on fabrication and lower uniformity. In other hand, a-TFTs:H have good uniformity and low cost, nevertheless, the lower mobility (about 1 cm2/v.s) and large on/off switch operation voltge are the shortcomings. Finally, OTFTs are not only low cost but also could be fabricated in low temperature which really suit for novel flexible applications. But the uniformity and mobility are the worst over the three types before. All of the driving TFT would cause threshold voltage shift and electron properties degradation after long time operation. It leads the driving current becomes lower, so the pixel compensation circuits for such defects have been extensively studied nowadays. In this thesis, we introduce three Voltage-Programmed Pixel Circuits (VPPCs). The transistors are employing LTPS and OTFTs respectively. The design rules are based on the device properties and combined with the concept of System On Glass (SOG). We except for improving panels’ uniformity, reliability and reduce the cost. It will give advanced cerebration for the large size and flexible displays.