Improving the luminous efficiency of blue phosphorescent organic light emitting diodes by a multiple emission layer structure

• Main Authors: Ming-Hao Chou, 周明豪
• Other Authors: none
• Format: Others
• Language: zh-TW
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/96807636582680183075

碩士 === 義守大學 === 電子工程學系碩士班 === 99 === In this study, the blue phosphorescent organic light-emitting diodes (PHOLEDs) have been fabricated, in which the multiple emission layer (EML) consists of various host materials of N, N’-dicarbazolyl-3,5-benzene (mCP) and 1,3,5-tris(N-phenylbenzimidazole-2-yl) benzene (TPBi) doped with iridium(III)bis [(4,6-di-fluorophenyl)-pyridinato-N,C2’] picolinate (FIrpic). A stepwise multiple EML structure is discussed for optimizing the device’s characteristics. Blue PHOLEDs with a typical double EML structure are fabricated. The structure is ITO/m-MTDATA(30 nm)/α-NPB(10 nm)/mCP:FIrpic(5 wt%, 15 nm)/TPBi:FIrpic(5 wt%, 15 nm)/TPBi(30 nm)/LiF/Al .When the doping concentration of FIrpic reaches 5 wt %, the device exhibits the maximum luminance of 10480 cd/m2 at 11.2 V, and the maximum luminous efficiency of 18.9 cd/A at 0.5 mA/cm2. Unlike typical double EML structure, experimental results demonstrate that luminous efficiency can be improved by adjusting the thickness of two hosts in the EML. By examining the changes of recombination zone in a partially doped EML, we find that most of the hole/electron recombination occurs at the interface of two different transporting hosts. We can improve the confined ability by adjusting the thickness of two hosts with different barrier and transported characteristics that can efficiently expand the recombination region in the EML to improve performance of blue PHOLEDs. Blue PHOLEDs with a stepwise multiple EML structure are successfully fabricated and the luminous efficiency of them is efficiently improved. The optimum structure is ITO/m-MTDATA(30 nm)/α-NPB(10 nm)/mCP:FIrpic(5 wt%, 10 nm)/TPBi:FIrpic(5 wt%, 5 nm)/mCP:FIrpic(5 wt%, 5 nm)/TPBi:FIrpic(5 wt%, 10 nm)/TPBi(30 nm)/LiF/Al. It shows the maximum luminance of 11790 cd/m2 at 11.35 V, and the maximum luminous efficiency of 22.64 cd/A at 0.3 mA/cm2.