Photoexcitation and Carrier Collection Dynamic Behavior

In this section, we briefly consider the response of nanociystalline semiconductor-electrolyte interfaces to either pulsed or periodic photoexcitation. Several points are noteworthy in this respect (1) The photocurrent rise-time in response to an illumination step is nonlinear. Further, the response is faster when the light intensity is higher. (2) The decay profiles exhibit features on rather slow timescales extending up to several seconds. (3) The photoctir-rent decay transients exhibit a peaking behavior. The time at which this peak 

These observations have been interpreted within the framework of two distinct models, one involving trapping or detrapping of the photogenerated electrons [231, 232) and the other based on electron diffusion (or field-assisted diffusion) not attenuated by electron localization [233, 234). The millisecond transit times also mean that the transit times are very long compared to equilibration of majority carriers in a bulk semiconductor or electron-hole pair separation within the depletion layer of a flat electrode. The slow transport is rationalized by a weak driving force and by invoking percolation effects [223). 

---