高效率p-i-n串列式白光有機發光二極體之研究

Abstract

一個高效率、操作壽命長的元件，可由串接的方式（Tandem Structure）來實現，而摻雜p型和n型的物質(p-i-n Structure)，亦可使元件的驅動電壓大幅降低，進而達到高效率。基於上述，本論文探討兼具此兩結構特性之元件，即高效率p-i-n串列式白光有機發光二極體。 在串列式（Tandem）元件結構中，連接層的特性是非常關鍵且重要的，影響的將是元件的操作特性、效率、色彩穩定度、視角、壽命….等。在本論文中，我們找出了一個穩定、有效且非常適合作為連接層的材料NPB:WO3。 接著我們分別將本實驗室目前所發表過，不論在電壓、效率及色穩定度上具有最佳效能的1-unit雙波段p-i-n白光元件及1-unit三波段p-i-n白光元件加以串聯，並且分別將它們最佳化。 （1）在雙波段白光部分：當1-unit 元件操作於20 mA/cm2下，其驅動電壓、電流效率、能量效率以及色座標分別為 4.5 V，10.5 cd/A，7.4 lm/W以及 (0.31, 0.42)，當操作於200~2000 nits 的亮度下，色偏 (x, y)值小於0.02；經串接並將其最佳化後，該元件在相同的電流密度下操作，驅動電壓約為1-unit元件的兩倍，此Tandem元件之圖譜幾乎與1-unit元件圖譜重合，而在電流效率上更有顯著的提升，達1-unit元件的2.3倍之高，功率效率為7.8 lm/W，NTSC Ratio為62%，不論在亮度400~4000 nits下及視角0~60˚下，其色偏值皆小於(0.02, 0.02)。 （2）在三波段白光部分：我們發現在串列式OLED中連接層的厚度對於白光在色溫上的改變影響非常大。因此我們可針對不同的應用方向，如顯示或照明，藉由調變白光元件中連接層相對的厚度，以控制色溫獲得所需的光色；除此之外我們也模擬了串列式白光OLED搭配高階彩色濾光片的結果，所模擬出的NTSC Ratio值高達88%，並且比較白光經傳統的LCD彩色濾光片及高階彩色濾光片後色彩上的差異，作為日後相關元件結構設計的參考。

In addition to the introduction of phosphorescent dopants, which is well-known for enhancing efficiency, both the architectures of p-i-n and tandem Organic Light Emitting Diodes (OLEDs) provide interesting strategies to increase the brightness of displays as well. In this thesis, we integrate both the structural features of p-i-n and the tandem device into one White Organic Light Emitting Diodes (WOLEDs) and demonstrate for the first time the apparent benefit in exploiting these two technologies. In the architecture of tandem OLEDs, an effective interconnecting layer is critical as if ineffective material were employed, it may increase the driving voltage and lower the efficiency. In this part of the research we found that BPhen:Cs2CO3/NPB:WO3 is an excellent interconnecting layer. After connecting two p-i-n WOLEDs of the two-components system with the highly transparent BPhen:Cs2CO3/NPB:WO3, the driving voltage (at 20 mA/cm2), current efficiency and CIEx ,y of the tandem OLEDs can be improved to 9.6 V, 23.9 cd/A and (0.30, 0.43) as compared to the single-unit OLED with 4.5 V, 10.5 cd/A and (0.31, 0.42), respectively. The CIEx,y coordinates of the tandem 2-unit device reveals little change (Δx,y <0.02) from brightness of 400 cd/m2 to 4000 cd/m2 and from forward viewing angle of ±60 degree (Δx,y <0.02). This overall performance is considerably better than what have been reported in the literature. In the second part of my research with three-components system, it is found that the optical length of interconnecting layer plays an important role in changing the color temperature of tandem WOLEDs and the desired color can be obtained by adjusting the thickness of interconnecting layer. Besides, we also got a high NTSC ratios (88 %) from the simulation of our tandem WOLEDs coupling with an advanced color filter (CF). The comparison of our tandem WOLEDs transmitted through conventional LCD CF with the advanced CF has been made to aid future device structure design