The Ideal Doping Concentration in Phosphorescent Organic Light Emitting Devices

概要

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The method for prediction of an ideal doping concentration in phosphorescent organic light emitting devices (PHOLEDs) is proposed by consideration of filling imaginary spherical molecules in 4\times 4\times 4 face-centered cubic lattice. Calculated ideal doping concentration is about 0.93 mol % in similar spherical size of host and guest molecules. Two different host materials are selected to demonstrate this concept. The ideal doping concentration of 4,4'-N,N\aku '-dicarbazolebiphenyl (CBP) host with fac-tris(2-phenyl-pyridinato)iridium(III) [Ir(ppy)3] guest system is predicted to be 1.19 wt % (0.93 mol %), which is observed at relatively thin ({\sim}10 nm) emitting layer (EML) condition presumably due to a deep trapping nature of the hole carriers at dopant molecules. The external quantum efficiency (EQE) is improved with increasing the doping concentration and thickness due to a preliminary trap filling process at the dopant molecules. Hence, we obtain the maximum EQE of 17.3% at a slightly over-doped and thicker condition (3%, 20 nm EML). Conversely, maximum EQE value of 20.7% is obtained at 1.5% doping concentration which is relatively less value from that of the calculated ideal doping concentration condition ({\sim}1.93 wt %) from bis[2-(2-hydroxyphenyl)-pyridine] beryllium (Bepp2) host with Ir(ppy)3 guest system.
2011-06-25