  In this work, we use pump-probe Kelvin Probe Force Microscopy (pp-KPFM) to
investigate light-induced surface potential dynamics in alumina-passivated
crystalline silicon, and in an organic bulk heterojunction thin film based on
the PTB7-PC71BM tandem. In both cases, we demonstrate that it is possible to
identify and separate the contributions of two different kinds of photo-induced
charge distributions that give rise to potential shifts with opposite
polarities, each characterized by different dynamics. The data acquired on the
passivated crystalline silicon are shown to be fully consistent with the
band-bending at the silicon-oxide interface, and with electron trapping
processes in acceptors states and in the passivation layer. The full sequence
of events that follow the electron-hole generation can be observed on the
pp-KPFM curves. Two dimensional dynamical maps of the organic blend
photo-response are obtained by recording the pump-probe KPFM curves in data
cube mode, and by implementing a specific batch processing protocol. Sample
areas displaying an extra positive SPV component characterized by decay
time-constants of a few tens of microseconds are thus revealed, and are
tentatively attributed to specific interfaces formed between a polymer-enriched
skin layer and recessed acceptor aggregates. Decay time constant images of the
negative SPV component confirm that the acceptor clusters act as
electron-trapping centres. Whatever the photovoltaic technology, our results
exemplify how some of the SPV components may remain completely hidden to
conventional SPV imaging by KPFM, with possible consequences in terms of
photo-response misinterpretation. This work furthermore highlight the need of
implementing time-resolved techniques that can provide a quantitative
measurement of the time-resolved potential.
