Trap-controlled hole transport in small molecule organic semiconductors

The influence of trap concentration on hole transport is investigated by an optical time-of-flight method for the amorphous small molecule organic semiconductor N,N'-bis(1- naphtyl)-N, N'-diphenyl-(1,1'-biphenyl)-4,4'-diamin (alpha-NPD) doped with neutral hole traps by codeposition of 4,4',4 ''-tris-[N-(1-naphtyl)-N-(phenylamino)]-triphenylamine (1-NaphDATA). alpha-NPD doped with 120 ppm 1-NaphDATA exhibits nondispersive hole transport like undoped alpha-NPD, but trap-controlled with reduced mobility. The trap depth derived from the mobility decrease coincides with the ionization potential difference of alpha-NPD and 1-NaphDATA. The transition to dispersive transport for increasing trap concentration to 1160 ppm is explained by an energetic relaxation of optically generated charge carriers within a density of states broadened by traps. (c) 2007 American Institute of Physics.