Strong Soliton-Like Characteristics in Non-Fullerene Acceptors – A New Photochemical Insight for Future Molecular Design

Abstract

Among the spectral features of photovoltaic materials, the large molar extinction coefficients and low band-tailing absorption ensure efficient photon utilization and reduced energy loss. Non-fullerene acceptors (NFAs), such as ITIC and Y6, significantly propel photovoltaic performance due to their strong absorption and steep band edges. The frontier orbitals of ITIC and Y6 mainly distribute on central conjugated backbones while little on electron donor and terminal acceptor segments, indicating a more localized electronic signature rather than a charge transfer state. The single crystal structures of ITIC and Y6 indicate that the π-conjugated backbones appear homogeneous carbon-carbon bond lengths, and the calculated atomic charge distribution demonstrates that the carbon atoms of π-conjugated backbones carry alternating positive and negative partial charges, revealing soliton-like electronic structural characteristics in these molecules. Inspired by cyanine dyes, these soliton-like features are considered to be the origin of ITIC and Y6 strong and tailless absorption spectra. Further analysis shows that the introduction of an acceptor group (benzothiadiazole) induces more extensive soliton waves in the conjugated backbones (Y6) compared to ITIC, resulting in stronger absorption and steeper band edge. The analysis suggests that soliton-type NFA materials are expected to provide new insight for designing high-performance organic photovoltaic materials.