The ideal doping concentration in phosphorescent organic light emitting devices

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 × 4 × 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'-dicarbazolebiphenyl (CBP) host with fac-tris(2-phenyl-pyridinato)iridium(III) [Ir(ppy)3] guest system is predicted to be 1.19wt% (0.93mol%), which is observed at relatively thin (∼10nm) 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 (∼1.93wt%) from bis[2-(2-hydroxyphenyl)- pyridine] beryllium (Bepp₂) host with Ir(ppy)₃ guest system.