Theoretical maximum quantum efficiency in red phosphorescent organic light-emitting diodes at a low doping concentration using a spirobenzofluorene type triplet host material

Red phosphorescent organic light-emitting diodes (PHOLEDs) with a 100% internal quantum efficiency at a low doping concentration were fabricated using a spirobenzofluorene type phosphine oxide as the host material. A spirobenzofluorene type phosphine oxide compound, 2,7-bis(diphenylphosphoryl)spiro[fluorene-7,11′-benzofluorene] (SPPO21), was synthesized as the electron transport type host material for the red phosphorescent organic light-emitting diodes. The device performances of the SPPO21 based red PHOLEDs were optimized at a low doping concentration of 1–2% because of energy transfer, little concentration quenching and charge trapping. A high external quantum efficiency of 20% was achieved for the red PHOLEDs with the SPPO21 host.