Anomalous Charge Carrier Decay Spotted by Clustering of a Time-Resolved Microscopic Phase Image Sequence

Abstract

We have developed a new type of time-resolved phase microscope through which the dynamics of photo-excited charge carriers at local positions are visualized by a refractive index change. To categorize the charge carrier responses at local positions, we introduced a classification method for all of the local temporal responses based on their similarity (cluster analysis). We analyzed the local responses for a nanoparticulate-TiO2 film. By categorizing the charge carrier responses for different positions, the categories of the temporal responses were mapped out. Although the average responses of photo-excited charge carriers were caused by recombination processes, we found a spotty fast decay response (50 ± 10 ns), which could not be recognized after averaging in space. The process was assigned as the formation and nonradiative decay of self-trapped excitons. The clustering mapping of the charge carrier responses could reveal the local charge dynamics, which may not be recognized otherwise.