17% efficiency for linear-shaped ADA-type nonfullerene acceptors enabled by 3D reticulated molecular packing

The fundamental principles governing photovoltaic properties of nonfullerene acceptors (NFAs) are essential for developing high-performance polymer solar cells. In this work, using three heteroheptacene-based acceptors (M-series) as model compounds, we systematically study the implication of heteroatoms in the heteroheptacene core on the molecular packing, morphology, optoelectronic, photophysical, and photovoltaic properties of the NFAs. It is found that replacing the oxygen atoms at the inner positions of the heteroheptacene core with sulfur atoms leads to the molecular packing mode change from a linear two-dimensional (2D) brickwork structure to a three-dimensional (3D) reticulated motif, which facilitates the formation of a desirable active layer with nanostructured phase separation morphology and improved charge transport. Meanwhile, the down-shifted highest occupied molecular orbital energy level of M36 induced by the sulfur substitution, matches better with that of polymer donor PM6 thereby leading to more efficient exciton dissociation and charge transfer. As a result, the best-performing photovoltaic device based on M36 affords an outstanding efficiency of 17%, which is among the highest values reported for all the ADA-type NFAs. Our work reveals the important role of the heteroatoms in the heteroheptacene core in constructing the 3D network of ADA-type NFAs, and the structure-property relationships herein shall provide an important guidance for designing high-performance NFAs.