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 (50–1000nm) 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, 1.63×10−5cm2∕Vs, at 50nm is smaller than the value 7.64×10−4cm2∕Vs at 1000nm (electric field at 0.1MV∕cm). The authors suggest that the lower mobility is caused by the interfacial trap states.