Trap engineering in organic hole transport materials

Chemical impurities with known highest occupied molecular orbital (HOMO) an d lowest unoccupied molecular orbital energies were incorporated in organic hole transport materials. The effect of these dopants on quantity and dept h of trap levels, transport properties, and luminescence of organic light e mitting devices was examined. This was achieved by investigating current-vo ltage characteristics, luminance-voltage characteristics, and utilizing the method of thermally stimulated current for trap level detection. It was fo und that 4,4',4"-tris-[N-(1-naphthyl)-N- (phenylamino)]triphenylamine (1-Na phDATA) doped into N,N'-di(1-naphthyl)-N,N'-diphenylbenzidine (alpha -NPD) generates a trap level whose activation energy corresponds to the HOMO ener gy difference between dopant and matrix molecules. Therefore, the detected electronic states can be assigned to hole traps. The influence of those tra ps on the charge transport will be reported. For doping alpha -NPD into 1- NaphDATA no new trap levels could be detected. (C) 2001 American Institute of Physics.