Lifetime mapping of photo-excited charge carriers by the transient grating imaging technique for nano-particulate semiconductor films.

Makoto Ebihara,Kenji Katayama,Woon Yong Sohn

doi:10.1063/1.5111418

Makoto Ebihara, Kenji Katayama + Show 1 more

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https://doi.org/10.1063/1.5111418

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Abstract

The transient grating (TG) imaging technique has been developed, where the refractive index change due to the photoexcited charge carriers excited with a stripe patterned light can be visualized. The spatiotemporal imaging of photoexcited charge carriers was demonstrated for a nanoparticulate TiO2 film. In the analytical procedures to map out the time constant distribution, the averaged response of photoexcited carriers in each image was obtained from the Fourier transform of the TG images since the image has a spatial modulation with a stripe pattern of light. The oscillation response due to the acoustic grating, the decay of the surface trapped electrons (until 1 μs), and thermal diffusion (until 100 µs) were observed. In order to obtain the lifetime imaging of the photoexcited electrons, the target time region (0-1 µs) for the response was selected and fitted with an exponential function, and the time constants were mapped out. We found that the time constants showed a wide range of distribution (68-920 ns), dependent on the sample positions.

Highlights

In many photodevices for solar cells,1–3 water splitting,4–7 and photocatalytic reactions,8,9 oxide semiconductor particles such as titanium dioxide (TiO2), zinc oxide, hematite, copper oxide, lead sulfur, and cadmium sulfur have been utilized for generation of photoexcited carriers and for charge transport and charge separation at interfaces
Since most of the semiconductor particles are provided in the form of particles from manufacturers or synthesized in the form of powder in the lab, they are coated on substrates and calcined, which potentially have a tremendous number of defects with different origins, ranging wide spatial orders from atomic to macroscopic scales
Crystalline TiO2 shows a single decay due to the recombination of photoexcited carriers in the transient absorption (TA) response because the properties of photoexcited carriers are homogeneous in nature,10 the lifetime of photoexcited carriers of nanoparticulate films extends for multiple temporal orders,11–13 indicating that the sample possesses different types of trap states relevant for different lifetimes

Summary

INTRODUCTION

In many photodevices for solar cells, water splitting, and photocatalytic reactions, oxide semiconductor particles such as titanium dioxide (TiO2), zinc oxide, hematite, copper oxide, lead sulfur, and cadmium sulfur have been utilized for generation of photoexcited carriers and for charge transport and charge separation at interfaces. We have developed a new imaging technique called the TG imaging technique, where the image sequence of the photoexcited charge carriers generated by a spatially patterned pulse light was imaged directly by a camera, which provides local responses of charge carriers, and the time constant mapping for one of charge carrier dynamics for a TiO2 nanoparticulate film was demonstrated. The pump pulse excites the sample, causing the generation of the photoexcited carriers and the following heat due to the carrier decay These processes induce the changes in the refractive index, and its pattern was the same as the pump-light pattern, called as TG. ACN was used as a liquid phase as an inert liquid, meaning that there is no charge transfer from TiO2 to the liquid side, otherwise photoexcited electrons are quickly scavenged by oxygen in air or water

RESULT

CONCLUSION