We could directly observe “hole transfer in a microscopic scale” between a perovskite and hole transfer layer (HTL), using our newly developed microscopic time-resolved technique combined with the informatics calculation. Needless to say, perovskite solar cells (PSCs) are one of the key photovoltaic cells for future energy generation, and tremendous efforts have been made in the last 20 years. However, the mechanism of PSCs is still not much clear because of the lack of appropriate measurement techniques.
The key to understanding the mechanism of PSCs is to observe the photo-excited charge carrier behavior, and the transient absorption, time-resolved photoluminescence, and time-resolved microwave conductivity have been used to observe the charge carrier dynamics. However, many processes of charge carriers are invisible, in the case of recombination, trapping, diffusion, etc., and this is the main reason why the knowledge could not reach the necessary level. The other issue is the structure dependence of the charge carrier behavior because they depend on the domain structure, which makes its understanding more difficult.
We have developed our new microscopic time-resolved technique, called the pattern-illumination time-resolved phase microscopy (PI-PM) method, which has provided various invisible charge carrier dynamics, and this time, we could successfully observe the charge transfer between perovskite and a hole transport layer. (Figure) We could succeed in the direct observation of the holes in HTL for the first time, and furthermore, with a micro-scale resolution. We could recognize the localized holes or unlocalized holes depending on the HTL materials. Especially in the case of P3HT, we could observe the unlocalized hole state as long as a millisecond time range. This explains a lot about the high efficiency of the MAPbI₃/P3HT photovoltaic system.

The categorized map of the charge carrier responses of P3HT / MAPbI<sub>3.</sub> (a) microscopic image, (b) categorized map, (c) overlap between (a) and (b). The responses for three categories are shown in (d).

The categorized map of the charge carrier responses of P3HT / MAPbI_3. (a) microscopic image, (b) categorized map, (c) overlap between (a) and (b). The responses for three categories are shown in (d).

Kenji Katayama

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