Abstract
Non-fullerene acceptors (NFAs) are attracting much attention as a material for efficient organic photovoltaics (OPVs). However, the photoinduced dynamics of NFAs has yet to be fully revealed. It is still questionable whether free carriers can be generated in the film consisting of the stacking of small NFA molecules. Transient absorption (TA) spectroscopy is a powerful method to investigate photoexcited states with ultrafast time resolution. TA spectroscopy in the visible and NIR region enables us to observe strongly bound photoexcitation states, but cannot detect free carriers. On the other hand, broadband MIR TA measurement makes it possible to observe free carriers. However, this is not commonly employed for the study of OPV. Herein, we investigated the photoexcited dynamics in the film of ITIC, one of the most well-known NFAs, by using MIR to visible TA measurement. In the MIR TA spectra, a peak around 3100 cm-1 and a broader component extending to lower energy region were observed. The latter broadband component got increased by blending ITIC with a donor, suggesting that this can be attributed to free carriers generated by charge separation. The observation of the vibrational frequency shift of CN group also supported the charge separation in the ITIC film. This study highlights the utility of MIR-TA spectroscopy in the study of OPVs.
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