We will discuss on the ultimate efficiency of OLEDs with and without extra light extraction layers. We will demonstrate using optical simulation based on the classical dipole model and OLEDs with almost perfect electron-hole balance that a maximum EQE of 46% can be achieved in normal ITO based bottom emitting OLEDs without any extra outcoupling layers using a phosphorescent dye with the horizontally oriented dipoles and the PL quantum yield of 1. Driving voltage and efficiency roll-off will also be discussed. We also present an OLED with an EQE value of 63% by integrating a micro-lens array on one side and a half-sphere lens on the other side of a surface plasmon loss minimized transparent OLED. The optical simulation indicated that the EQEs over 60% are possible by using the microstructures on both surfaces without any internal light extraction layer and over 90% if the guided modes in OLEDs are extracted.

Jang-Joo Kim received the B.S. and M.S. degree from the Department of Chemical Engineering, Seoul National University, Seoul, Korea, in 1977 and 1980, respectively, and the Ph.D. degree from the Department of Materials Science and Engineering, Stanford University, USA in 1987. He is currently a professor in the Department of Materials Science and Engineering and the director of the “Center for Organic Light Emitting Diodes” at Seoul National University since 2004. He is a member of Korea Academy of Science and Technology and serving as an editor of the journal “Organic Electronics; Materials, Physics, Chemistry and Applications” and as a Vice president of the Korea Information Display Society. His research areas include electronic processes in organic semiconductors, electroluminescence from organic materials, organic photovoltaics, and organic electronics.