Tswen-Hsin Liu

Doped red organic electroluminescent devices based on a cohost emitter system

We have developed an efficient red emitter in organic light-emitting diodes based on the fluorescent dye 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetra-methyljulolidyl-9-enyl)-4H-pyran doped in a cohost emitter system of 5,6,11,12-tetraphenylnathacene/tris(8-hydroxyquinolinato)aluminum which achieved an electroluminescence (EL) efficiency of 4.4 cd/A and 2.1 lm/W at 20 mA/cm2 and 6.8 V, with a near saturated Commission Internationale d’Eclairage coordinates (x=0.65, y=0.35). This cohost emitter system has the advantage of alleviating the current-induced fluorescence quenching often encountered in red organic EL devices and greatly improves the EL efficiency over a wide range of drive current conditions. From accelerated degradation tests, a device half-life of about 33 800 h can be projected in this system at an initial device luminance of 100 cd/m2.

Highly efficient yellow and white organic electroluminescent devices doped with 2,8-di(t-butyl)-5,11-di[4-(t-butyl)phenyl]-6,12-diphenylnaphthacene

We describe the applications of a sterically-hindered yellow dopant, 2,8-di(t-butyl)-5,11-di[4-(t-butyl)phenyl]-6,12-diphenylnaphthacene (TBRb) which, when compared to 5,6,11,12-tetraphenylnaphthacene (Rb) in either tris(8-hydroxyquinolinato)aluminum or 1,4-bis[N-(1-naphthyl)-N′-phenylamino]−4,4′ diamine (NPB) as host emitter, shows a 50%–34% increase in luminance efficiency over that of Rb device without significantly affecting its color. In addition, we have incorporated the TBRb doped yellow NPB emitter into the two-element white organic light-emitting diodes based on p-bis(p-N,N-di-phenyl-aminostyryl)benzene doped 2-methyl-9,10-di(2-naphthyl) anthracene sky-blue emitter which improved the luminance efficiency by 44% over that of Rb to 12.8cd∕A and 4.3lm∕W at 20mA∕cm2 with CIEx,y=[0.31,0.38].

4-(Dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran doped red emitters in organic light-emitting devices

We have studied the device optimization of red 4-(Dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) dye doped Alq3 emitters in organic light-emitting devices (OLED). By varying the hole injection materials, thickness of hole-transport layer and by evaporation with rubrene as co-dopant, efficiencies of 3.24 cd/A and near saturated CIE color coordinates of x=0.643, y=0.354 was achieved without needing additional doping of hole-trap with NPB. From experiments with film thickness variation, it is concluded that the improved electrofluorescent (EL) performance and color saturation are due to the synergistic effects of microcavity, device structural optimization as well as optimal doping of DCJTB with rubrene. We believe this luminance efficiency for red is by far one of the best ever reported for dye-doped electrofluorescent OLEDs.

Doped blue emitters of 9,10-di(2-naphthyl)anthracene in organic electroluminescent devices

Blue EL emission of 9,10-di(2-naphthyl)anthracene (ADN) based emitter in OLED is highly dependent upon its thickness and attenuated by the microcavity effect of the emitter. By carefully tuning the thickness of ADN and optimizing the doping concentration of TBP, one of the highest efficiencies reported for the anthracene based blue emitter at 6.6 cd/A with a CIE of x = 0.13; y = 0.21 is achieved.

Phosphorescence of red Os(fptz)(2)(PPh2Me)(2) doped organic light-emitting devices with n and p hosts

We have applied a stericly-hindered red phosphorescent dopant Os(fptz)2(PPh2Me)2 [fptz=3-trifluoromethyl-5-(2-pyridyl)-1,2,4-triazole, PPh2Me=phosphine ligand] in p-type 4,4′-N,N′-dicarbazole-biphenyl or n-type bis(2-methyl-8-quinolinolato)(p-phenylphenolato) aluminum host and found that the latter produced higher luminance efficiency at lower doping concentration. We present a model to rationalize this phenomenon in which the n-type host impedes hole transport, which leads to narrower recombination zone near the hole transport layer/emission layer interface than the p-type host, hence, more effective recombination.

Lithium manganese oxide as an effective buffer layer between organic and metal layers in organic light-emitting devices

Tris(8-hydroxyquinolinato)aluminum (Alq3)-based organic light-emitting devices using a thermally deposited lithium manganese oxide layer between aluminum (Al) cathode and Alq3 have been fabricated. The highest luminance efficiency obtained with a 1-nm-thick LiMnxOy layer is very similar to that of the device with 1-nm-thick LiF. However, the device with an 18nm LiMnxOy layer obtained a longer operational stability although the luminance efficiency is lower. The improvements are attributed to lithium extractions of the lithium manganese oxide layer and the interfacial properties between Alq3 and Al are discussed.

27.3: Efficient Red Organic Electroluminescent Devices Based on a Novel co-Host Emitter System

We have developed an efficient red OLED device based on the fluorescent dye DCJTB doped in a novel co-host matrix system of 9,10-di(2-naphthyl) anthracene (ADN) and Alq3, which achieved a luminance efficiency of 4.2– 5.5 cd/A at 20 mA/cm2 with a CIEx, y color coordinate of [0.64, 0.35] – [0.62, 0.37]. At high ADN concentration (>60%), this co-hosted emitter system has the advantage of alleviating the current-induced fluorescence quenching problem often encountered in red organic EL devices and greatly improves the EL efficiency over a wide range of drive current conditions.