Hole mobility of N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl) benzidine investigated by using space-charge-limited currents

The hole mobility of N,N-'-bis(naphthalen-1-yl)-N,N-'-bis(phenyl)-benzidine (NPB) at various thicknesses (50-1000 nm) has been estimated by using space-charge-limited current measurements. A thin layer of buckminsterfullerene has been used for a quasi-Ohmic contact between NPB and indium tin oxide. The mobility at bulk property dominant thickness is in excellent agreement with the results from time-of-flight method. For the typical thickness of organic light-emitting devices, the hole mobility of NPB, 1.63x10(-5) cm(2)/V s, at 50 nm is smaller than the value 7.64x10(-4) cm(2)/V s at 1000 nm (electric field at 0.1 MV/cm). The authors suggest that the lower mobility is caused by the interfacial trap states. (C) 2007 American Institute of Physics.