Donor–spacer–acceptor monodisperse conjugated co-oligomers for efficient single-molecule photovoltaic cells based on non-fullerene acceptors

Abstract

The current challenges for efficient bulk heterojunction (BHJ) organic photovoltaics (OPVs) based on organic/polymeric (non-fullerene) acceptors involve difficult control of neat phase separation in the nanoscale, severe geminate charge recombination, etc. Herein, a new molecular design concept, that is to construct donor–spacer–acceptor (D–S–A) co-oligomers with self-assembly properties, is proposed in order to realize ideal film morphology and manipulate the exciton dissociation and geminate charge recombination processes simultaneously. Three D–S–A co-oligomers, i.e.F5T8P-C2, F5T8P-C4 and F5T8P-C6 with oligo(fluorene-alt-bithiophene), perylene diimide (PDI) and alkyl as D-, A- and S-segments, respectively, were synthesized. All three D–S–A co-oligomers can form D–A alternating lamellar nanostructures with periods of ∼15 nm, an ideal nanostructure for BHJ OPVs. Compared to D–A co-oligomer F5T8-epP in which the D- and A-segments are directly connected without the alkyl spacer, the D–S–A co-oligomers not only show higher electron mobilities due to closer packing of PDI moieties, but also exhibit longer lifetimes of the charge-transfer states that can potentially restrain the geminate charge recombination and improve the charge generation efficiency. Accordingly, the single-molecule photovoltaic cells based on the D–S–A co-oligomers exhibit an improved fill factor of 0.47 and a high open-circuit voltage of 1.04 V. In particular, an external quantum efficiency of ∼65%, which is the highest for BHJ OPVs based on non-fullerene acceptor materials, has been demonstrated. By further extending the absorption onset of D–S–A co-oligomers to ∼600 nm, a single-molecule photovoltaic device with a power conversion efficiency of 2.70% has been fabricated. These results prove that high-efficiency BHJ OPVs based on non-fullerene acceptors are achievable if both the film morphology of the D–A blend and D–A interfaces are suitably manipulated.