Abstract
Transient photovoltage measured from the device of indium tin oxide (ITO)/N,N′-Bis(naphthalene-1-yl)-N,N′-bis(phenyl) benzidine (NPB) (600nm)∕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 103 for the device studied.
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