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.