Side-chain engineering to develop phenyl-substituted benzodithiophenedione-unit-based polymer donors for efficient non-fullerene polymer solar cells

Abstract

To improve the photovoltaic performance of non-fullerene polymer solar cells (NF-PSCs), it is essential to develop wide bandgap (WBG) polymer donors that can match with narrow bandgap non-fullerene acceptors. Side-chain engineering is a feasible strategy for designing efficient WBG polymer donors. This paper introduces two novel copolymers with linear alkyl chains (n-octyl) or branched alkyl chains (2-ethylhexyl) on the thiophene π-bridge. These copolymers incorporated a benzodithiophene (BDT) as the electron donor unit and a phenyl-substituted benzodithiophenedione (BDD-Ph) as the electron acceptor unit, with the resulting copolymers designated as PBF-C8 and PBF-EH, respectively. Compared with PBF-EH with branched side-chains, PBF-C8 with linear side-chains exhibited red-shifted absorption and stronger intermolecular aggregation. When paired with the narrow-bandgap non-fullerene acceptor L8-BO as the electron acceptor, the PBF-C8-based device achieved a power conversion efficiency of 16.44%, superior to that of the PBF-EH-based device (3.21%). This enhanced performance is attributable to the superior face-on orientation packing, exciton dissociation, and charge transport of the PBF-C8-based blended film. These findings indicate that using the BDD-Ph unit as the electron-deficient unit and optimising π-bridge side-chains can help construct highly efficient polymer donors.