Performance improvement of white phosphorescent organic light-emitting diodes by using composite host structure to enhance the carrier injection

• Main Authors: Cheng-Yin Cheng, 鄭程尹
• Other Authors: Fuh-Shyang Juang
• Format: Others
• Language: zh-TW
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/s759an

碩士 === 國立虎尾科技大學 === 光電與材料科技研究所 === 98 === The device characteristics of blue phosphorescent organic light-emitting diodes (PHOLEDs) with hole-buffer structure were investigated by inserting the hole transport-type host (TCTA) between hole transport layer (HTL) and emitting layer (EML). The hole transport-type host has lower hole mobility (μh~1.6×10-4 cm2/Vs) than HTL material, which could be effectively controlled the hole injection current from HTL to EML. Moreover, the highest occupied molecular orbital (HOMO) level of buffer layer material was between HTL and EML, lead to the reduced of hole injection barrier. The distributed recombination zone and balanced charge carrier injection within emissive layer were achieved through the thickness of buffer layer optimization, therefore the device performances were greatly enhanced. In addition, an bipolar transport-type host material (26DCzPPy) and a high triplet energy electron transport material (3TPYMB) with low-lying lowest unoccupied molecular orbital (LUMO) were used to reduce the driving voltage and effective confined the triplet excitons within emissive layer, which resulted in power efficiency effectively improved. At luminance 1000 cd/m2, the driving voltage decreased to 5.1 V , the yield of 23 cd/A and the power efficiency of 13.5 lm/W was achieved. Next, the highly efficiency white PHOLEDs were investigated by doping the orange-red dopant (Os(bpftz)2(PPh2Me)2) into the right-side of blue-EML(exciton generation zone),As the result, the white PHOLED exhibits a yield of 27 cd/A. and power efficiency of 15.5 lm/W at a luminance of 1000 cd/m2.The white device with a maximum yield of 27.8 cd/A, a power efficiency of 17.7 lm/W. and CIE coordinate of (0.33, 0.32) without color-shift can be achieved. Next, the device characteristics of blue PHOLEDs with composite host structure were investigated by co-doping the hole and electron transport-type host materials with an bipolar host (26DCzPPy).From the results, the performances of OLEDs were improved greatly due to the improved charge carrier injection and confined exciton into recombination zone of composite host structure. Moreover, a high efficiency white OLED was also fabricated by doping Os into bule emitting layer. The white OLED shows the efficiencies of 34.5 cd/A and 24 lm/W at a luminance of 1000 cd/m2. Furthermore, the efficiencies can be increased to 42.5 cd/A and 30 lm/W by attaching an outcoupling brightness enhancement film(BEF) onto substrate.