Exciton dissociation at the indium tin oxide-N,N′-bis(naphthalen-1-yl)N,N′-bis(phenyl) benzidine interface:: A transient photovoltage study

Transient photovoltage measured from the device of indium tin oxide (ITO)/ N,N'-Bis(naphthalene-1-yl)-N,N'-bis(phenyl) benzidine (NPB) (600 nm)/Al (Al grounded) exhibits an abnormal polarity change from negative to positive upon pulsed laser irradiation. A simple model including interfacial exciton dissociation is proposed to describe the phenomenon observed. The initial negative signal is interpreted as a result of more electrons than holes injected into ITO by dissociation of excitons at the ITO-NPB interface, and the subsequent positive signal can be attributed to carrier separation by the built-in field in NPB. Further experiments confirm that it is the combination of interfacial exciton dissociation and built-in field that determines the polarity of the transient photovoltage. The amount of excitons dissociated at the ITO-NPB interface is much larger than that of free carriers created in other processes, with the ratio in the order of 10(3) for the device studied. (c) 2006 American Institute of Physics.