The Study of Hybrid Double Host Phosphorescent Organic Light Emitting Diodes

Autor: Lin-Ann Hong, 洪鄰安
Rok vydání: 2015
Druh dokumentu: 學位論文 ; thesis
Popis: 103
This study is carried out to discuss how to reduce the driving voltage of blue phosphorescent organic light-emitting diodes (PHOLEDs) by using a thin double-emission layer. Experimental results show that when TCTA (10 nm) is used as the buffer layer, the luminance efficiency of the blue device increases up to 35.1 cd/A at a luminance of 1000 cd/m2, the power efficiency reaches 25.6 lm/W, and the operating voltage of 4.4 V. Furthermore, in this study, we also utilize the interface between double light-emitting layers of devices by codoping them with a red phosphorescent dopant [Os(bpftz)2(PPh2Me)2]. An Os complex with a high-lying highest occupied molecular orbital (HOMO) energy level (trapping holes) is codoped at the interface between emitting layers and an exciton-formation zone is expanded to obtain a white PHOLED with high efficiency. From the results, the optimal structure of the white device exhibits a operating voltage of 4.9 V, a current efficiency of 34.4 cd/A, a power efficiency of 22.0 lm/W, and Commission Internationale de L’Eclairage (CIE) coordinates of (0.328, 0.377) at a luminance of 1000 cd/m2. The CIE coordinates shited form 4 to 10 V is (∆x=+0.005, ∆y=+0.004). Furthermore, the power efficiency can be improved to 30.4 lm/W by attaching the outcoupling enhancement film. PHOLEDs with high efficiency and low driving voltage were achieved by incorporating an electron transport material (3TPYMB) into a hole transport-type host (TCTA) as a mixed-host structure. For electrons, the emitting layer is nearly barrier-free until they reach the region of exciton formation, which keeps the driving voltage low. As the results shown, when the thickness of ETL was adjusted to 50 nm, it exhibited the driving 4.2 V, yield of 36.0 cd/A, and power efficiency of 27.5 lm/W at a luminance of 1000 cd/m2. Through the results of above, Os is doped in the middle of the mixed-host. White PHOLED exhibits current efficiency of 36.1 cd/A, and power efficiency of 26.4 lm/W. The CIE coordinates shited form 4 to 10 V is (∆x=+0.019, ∆y=+0.005). Furthermore, the power efficiency can be improved to 35.0 lm/W, and luminance efficiency to 46.8 cd/A by attaching a brightness enhancement film (BEF). In this research complex emitting layers were fabricated using TCTA doping hole-transport material in the front half of bipolar 26DCzPPy as well as PPT doping electron-transport material in the back half of 26DCzPPy. Blue dopant FIrpic was also mixed inside the complex emitting layer to produce a highly efficient blue phosphorescent organic light emitting diode. The hole and electron injection and carrier recombination rate were effectively increased. The fabricated complex emitting layers exhibited current efficiency of 41.7cd/A, and power efficiency of 30 lm/W. A white OLED component was then manufactured by doping red dopant [Os(bpftz)2(PPh2Me)2] (Os) in proper locations. When the Os dopant was doped in between the complex emitting layers, excitons were effectively confined within, increasing the recombination rate and therefore reducing the color shift. The resulting CIE coordinates shifted from 4 to 10 V is (∆x=-0.004, ∆y=+0.001). The component had a current efficiency of 35.7 cd/A, a power efficiency of 24 lm/W. Attaching an outcoupling enhancement film was applied to increase the luminance efficiency to 30 lm/W. Hybrid white organic light emitting diodes (HWOLEDs) with fluorescence and phosphorescence hybrid structures are studied in this work. HWOLEDs were fabricated with blue/red emitting layers: fluorescent host material doped with sky blue material, and bipolar phosphorescent host emitting material doped with red dopant material. Spacer layers were also inserted to expand the recombination zone, increase efficiency and reduce energy quenching along with roll-off effects. The resulting high efficiency warm white OLED device has the lower highest occupied molecule orbital level red guest material, current efficiency of 17.5 cd/A, and power efficiency of 12.5 lm/W, and CIE coordinates of (0.362,0.410) at a luminance of 1000 cd/m2.
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