Ladder-Type Dithienonaphthalene-Based Small-Molecule Acceptors for Efficient Nonfullerene Organic Solar Cells

Two novel small molecule acceptors (DTNIC6 and DTNIC8) based on a ladder-type dithienonaphthalene (DTN) building block with linear (hexyl) or branched (2-ethylhexyl) alkyl substituents are designed and synthesized. Both acceptors exhibit strong and broad absorption in the range from 500 to 720 nm as well as appropriate highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels. Replacing the linear hexyl chains with the branched 2-ethylhexyl chains has a large impact on the film morphology of photoactive layers. In the blend film based on DTNIC8 bearing the branched alkyl chains, morphology with well-defined phase separation was observed. This optimal phase morphology yields efficient exciton dissociation, reduced bimolecular recombination, and enhanced and balanced charge carrier mobilities. Benefited from these factors, organic solar cells (OSCs) based on PBDB-T:DTNIC8 deliver a highest power conversion efficiency (PCE) of 9.03% with a high fill factor (FF) of 72.84%. This unprecedented high FF of 72.84% is one of the highest FF values reported for nonfullerene OSCs. Our work not only affords a promising electron acceptor for nonfullerene solar cells but also provides a side-chain engineering strategy toward high performance OSCs.