Organic Light-Emitting Diodes Based on 2-(Stilben-4-yl)benzoxazole Derivatives: An Implication on the Emission Mechanism

A series of 2-(stilben-4-yl)benzoxazole derivatives (BOXSB-X) was prepared and used as a dopant in the fabrication of organic light-emitting diodes. With a device structure of ITO/NPB/CBP/TPBI:3%BOXSB-X/TPBI/Mg:Ag, where NPB, CBP, and TPBI stand for 4,4‘-bis[ N -(1-naphthyl)- N -phenyl-amino]-biphenyl, 4,4‘-dicarbazolyl-1,1‘-biphenyl, and 2,2‘,2‘ ‘-(1,3,5-phenylene)tris-[1-phenyl-1H-benzimidazole], respectively, light emission from the dopant was observed under electric bias, presumably due to energy transfer between the host TPBI exciton and the dopant. Bright blue emission with a luminance ranging between 8 000 and 13 000 cd/m 2 was obtained depending on the substituent on the stilbene moiety. However, with a device structure of ITO/NPB/TPBI:3%BOXSB-X/TPBI/Mg:Ag, where no CBP layer was present, the recombination/emission region shifted to the NPB layer except in the device doped with dimethylamino-substituted benzoxazole derivative, in which case the carrier trap mechanism is suggested to be responsible for the emission from the dopant.