Device and Electrode Architectures for High-Efficiency Organic Light-Emitting Diodes

• Main Authors: Ting-Yi Cho, 卓庭毅
• Other Authors: Chung-Chih Wu
• Format: Others
• Language: en_US
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/25708311627645531537

博士 === 國立臺灣大學 === 電子工程學研究所 === 95 === Organic light-emitting devices (OLEDs) have attracted much attention for its applications in high-efficiency, large-scale, full-color flat-panel displays and solid-state lighting. In this thesis, we focused on developing device structures and electrode structures to obtain improvement of emission efficiency of OLEDs. We investigated theoretically and experimentally the optical characteristics of noncavity and microcavity tandem OLEDs. By use of rigorous electromagnetic modeling of OLEDs, radiation characteristics of tandem OLEDs as a function of device structures are analyzed and correspondingly the guidelines for optimizing performances of tandem devices are suggested. Making use of the analysis results, we show that with well designed microcavity conditions and device structures, a five-fold enhancement in luminance can be achieved with cavity tandem devices having only two emitting units. A very high efficiency of 200 cd/A has been demonstrated with a phosphorescent cavity two-unit device. We demonstrated ultra-thin tandem WOLEDs by locating yellow and blue emitters at corresponding first antinodes to the metal electrode, instead of 1st and 2nd antinodes in conventional tandem OLEDs. This attempt leads to the substantial reduction of operating voltage improvement in power efficiency, compared to conventional tandem WOLEDs. Besides, it shows stable EL spectra under various bias conditions. The thermally evaporated molybdenum oxide (MoOx) was investigated as the effective hole-injection material for organic light-emitting diodes (OLEDs). The use of MoOx significantly lowers the operating voltage of OLEDs. According to ultraviolet photoemission spectroscopy (UPS), there exists gap states in MoOx layer, such that the Fermi level is pinned and hole-injection barrier through MoOx layer is reduced. As such, OLEDs using MoOx as the hole-injection layer are rather insensitive to the anode materials utilized. Finally, we investigated the derivative of oligofluorene, ter(9,9-diarylfluorene), as an efficient electron-transport layer in OLEDs. In interface engineering of the cathode, we found that the introduction of CsF effectively reduces the electron injection barrier. Combined with high mobility of oligofluorene (i.e. electron-transport layer), the operating voltage of OLEDs can be reduced and power efficiency is improved accordingly.