Fused nonacyclic electron acceptors with additional alkyl side chains for efficient polymer solar cells

Three new non-fullerene acceptors with the core of a fused nonacyclic unit, and the end-capping groups of 3-dicycanovinylindan-1-one are designed and synthesized for polymer solar cells. Those acceptor molecules bear four additional aliphatic side chains at the nonacyclic core, which endow them excellent solubility and crystallization-morphology tunability. Meanwhile, the effect of side chains' size on the optical absorptions, electrochemical properties, and photovoltaic performance are carefully investigated. The results show that the bulk of side chains just slightly affects optical absorption and electronic energy levels, however, it significantly influences the molecular stacking and morphology separation, as well as the eventual device performance. When they are matched with the donor polymer poly [(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b'] dithiophene))-alt-(5,5 -(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione))] (PBDB-T), the device based on the acceptor with octyl group can deliver the best PCE of 8.57%, which overwhelms those based on other two acceptor materials. This can be attributed to the preferable nanophase separation for their blend films, and high and balanced charge mobility. The results highlight the important role of backbone and side-chain tailoring in designing non-fullerene acceptors towards high-performance polymer solar cells.