新型紅色客發光體的設計與有機電激發光元件的應用

Abstract

具有高螢光效率的紅色客發光體材料4-(dicyanomethylene)-2-t- butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (簡稱DCJTB)，2% DCJTB在40% tris-(8-hydroxyquinolinolato) aluminum (Alq3)和60% rubrene共主發光體元件中，可得到4.4 cd/A的高發光效率，和高穩定性的紅光(26,000 h @100 cd/m2)，色度座標為CIEx,y(0.65, 0.35)。但在目前全彩化OLED的技術上，紅光畫素的效率仍是不足的，因此對於紅色客發光體材料的研發與改良有著迫切的需要。於是我們取代DCJTB苯環上兩個氫的位置，加入官能基做衍生變化，合成出三個系列的衍生物，並探討其Alq3單主發光體元件，與Alq3 / rubrene共主發光體元件的特性： (1) 加入推電子基： 加入的烷基或甲氧基形成較大的立體阻礙，以降低濃度驟熄，提高發光效率，並由推電子基的作用以得到相較於DCJTB紅位移的化合物，使顏色更接近NTSC (National Television Standards Committee)標準的紅色。2.5 wt.% DCJTB-M(甲基取代基)和2.5 wt.% DCJTB-E (乙基取代基)的元件，顏色可達到CIEx,y(0.67, 0.33)，飽和的紅色可應用在NTSC的規格。 (2) 加入拉電子基： DCJTB在Alq3為host的元件中容易造成charge trapping的效應，對被動元件驅動IC的設計一直是一個存在的困擾。DCJTB-C(CN取代基)的HOMO (highest occupied molecular orbital)值比DCJTB下降了0.3eV，減少了hole trapping的不良影響。 (3) 加入芳香族取代基： 此系列衍生物是藉由加入芳香族取代基，來提高載子遷移率，並藉由芳香族取代基的立體阻礙，以降低濃度驟熄，提高發光效率。2 wt.% DCJTB-P(苯基取代基)共主發光體的元件發光效率為6.37 cd/A，比 DCJTB提升了24%，在20 mA/cm2時的驅動電壓為6.5 V，比DCJTB低了0.9 V，顏色為CIEx,y(0.64, 0.36)飽和紅光，這是目前為止文獻發表之中螢光紅光發光效率最高的效能，也更能滿足製作全彩面板時紅光畫素的需求。

An archetypical and highly efficient red dopant used for many of today’s organic light emitting diodes (OLED) displays on the market is 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)- 4H-pyran, better known as DCJTB. In co-host emitting system, 2% DCJTB doped in 40% tris-(8-hydroxyquinolinolato) aluminum (Alq3) and 60% rubrene, yielded an electroluminescent (EL) efficiency of 4.4 cd/A with a red Commission Internationale d’Eclairage coordinates (CIEx,y) of (0.65, 0.35) and long lifetime (12600 h @ 100 cd/m2). In the current status of full-color fluorescent OLED technology, the red sub-pixel apparently is still not efficient enough. Continuation of research in design and synthesis of improved red dopants to further boost the red EL efficiency of DCJTB is therefore necessitated. In this thesis, we design and synthesize three series of DCJTB derivatives. Their EL properties in Alq3 single host and Alq3 / rubrene co-host devices will be also presented.

(1) electron-releasing group substituted derivatives: By substitution of alkyl or methoxy groups, concentration quenching could be reduced, and red shift could be caused to get close to NTSC (National Television Standards Committee) red color. The device of 2.5 wt.% DCJTB-M (methyl group substituted) doped in Alq3 can achieve saturated color of CIEx,y(0.67, 0.33), which can be applied in NTSC specifications.

(2) Electron-withdrawing group substituted derivative: DCJTB doped Alq3 device could easily cause charge trapping effect, which is a hurdle in IC design of passive matrix OLED. The HOMO level of DCJTB-C (cyano group substituted) is more low-lying than that of DCJTB by 0.3 eV, thus hole trapping effect could be reduced.

(3) Aryl group substituted derivatives: By substitution of aryl groups, carrier mobility and luminous efficiency could be enhanced, concentration quenching could be also reduced by steric hindrance. The device of 2 wt.% DCJTB-P (phenyl group substituted) doped in Alq3 / rubrene co-host system attained an EL efficiency of 6.37 cd/A with an CIEx,y(0.64, 0.36). Indeed, we believe the EL performance results disclosed herewith are one of the highest ever for a red fluorescent dopant, and therefore can provide a definitive solution and meet the demands of red pixels in full-color OLED.