Tuning the Molecular Packing of Low-Cost Non-Fullerene Acceptors via Asymmetric Terminal Groups

Benefiting from the simple building blocks and facile synthetic processes, IDIC derivatives based on indacenodithiophene (IDT) core and 2-(2,3-dihydro-3-oxo-1H-inden-1-ylidene) propanedinitrile (IC) terminal groups have been widely studied since the early emerging of non-fullerene acceptors (NFAs) in organic solar cells (OSCs). Although the pristine IDIC molecule possesses a low material cost, its photovoltaic performance is unsatisfactory. Thus, this work conducts the modification of IDIC by alternating the terminal groups and the molecular symmetry. Differing from the typical NFAs in which the fluorinated terminal groups can promote photon harvest and charge mobility to optimize the photovoltaic properties, the material ID4F-C8 with the fluorine substitution exhibits inferior performance compared with the IDIC-C8 with the normal IC groups. Such unexpected results are rationalized by the varied molecular packing in the solid state, i.e., IDIC-C8 has a typical three-dimensional (3D) reticular motif and ID4F-C8 possesses a two-dimensional (2D) packing without an obvious π–π stacking. When two different groups with and without fluorine substitution are utilized as the terminals, the asymmetric ID2F-C8 retains both of the merits of the IDIC-C8 and ID4F-C8, exhibiting strong absorption, suitable energy levels, and highly ordered packing. Thus, the OSC device based on ID2F-C8 acquires the highest PCE of 11.67%. This encouraging result suggests the potential of the asymmetric strategy to develop high-performance and low-cost photovoltaic materials.