Multiple-trapping carrier transport due to modulated photogeneration

A modified time-of-flight technique for investigating the localized states in disordered solids is proposed. The method consists of generating the charge carriers near the sample surface by sinusoidally modulated light. The conduction current at the opposite surface is monitored by interdigital electrodes. Simple formulae are derived, from the multiple-trapping model, for the phase shift between the output current and the exciting light as well as for the current amplitude. The formulae enable us to determine directly the energetic profile of the traps from experimental data.