Modulated photoconductivity study of nanocrystalline CdSe films

Carrier transport in thermally-evaporated cadmium selenide films has been investigated using the modulated photoconductivity technique. At lower temperatures and high photogeneration rates the low-frequency ac photocurrent exhibits a phase lead over the optical excitation, which varies systematically with temperature and mean excitation. This behavior is linked to ‘sensitizing’ centers in the recombination path although it may be observed in the absence of accompanying features in the steady-state photoconductivity. The high-frequency (trap-limited) response suggests a gradual increase in defect state density from mid-gap towards the conduction band edge, a trend mirrored by the low-frequency behavior when phase lead effects are either absent or allowance for them is made. Computer modeling of a system containing band tails and two species of defects predicts a phase lead, and the correct qualitative dependence on temperature and generation rate.