Hidden surface photovoltages revealed by pump probe KPFM

TL;DR

This study employs pump-probe Kelvin Probe Force Microscopy to reveal hidden surface photovoltages and charge dynamics in silicon and organic photovoltaic materials, emphasizing the importance of time-resolved measurements for accurate analysis.

Contribution

It introduces a novel application of pump-probe KPFM to distinguish and analyze different photo-induced charge contributions and their dynamics in photovoltaic materials.

Findings

Identification of two opposite-polarity charge contributions with distinct dynamics.

Observation of electron trapping in acceptor states and passivation layers.

Revelation of hidden SPV components undetectable by conventional KPFM.

Abstract

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.