A π-bridge strategy for asymmetric small molecule acceptors in organic photovoltaics†

Asymmetric small molecule acceptors (as-SMAs) offer advantages in photovoltaics via enhanced dipole moments and strong intermolecular interactions, promoting accelerated exciton dissociation and efficient charge transport. We design four A–D–π–A type as-SMAs of IDT-2Cl2F, IDT-S-2Cl2F, IDT-S-2F2Cl, and IDT-S-4F with an indaceno[1,2-b:5,6-b′]dithiophene (IDT) central D-core, asymmetric halogen substituents on their A-end group and an asymmetric alkylthio-thiophene π-bridge between the D-core and A-end group in IDT-S-2Cl2F, IDT-S-2F2Cl, and IDT-S-4F. The O⋯S, O⋯H, and S⋯S bonds ensure a coplanar molecular structure of the as-SMAs. Compared to IDT-2Cl2F, the π-bridge in IDT-S-2Cl2F, IDT-S-2F2Cl, and IDT-S-4F facilitates smoother electron transitions and broader spectral absorption due to their extended conjugation. Organic solar cells (OSCs) with PM6 as a polymer donor and IDT-S-2Cl2F, IDT-S-2F2Cl, or IDT-S-4F as an acceptor exhibit more uniform phase separation, lower surface roughness, faster exciton dissociation, and higher charge collection efficiency, resulting in improved current density, than those devices with IDT-2Cl2F as the acceptor. Additionally, shorter π–π stacking distances and larger dipole moments contribute to a higher open-circuit voltage (V_oc) for the OSCs based on IDT-S-2Cl2F, IDT-S-2F2Cl, and IDT-S-4F. Consequently, the power conversion efficiencies (PCEs) of the OSCs based on IDT-S-2Cl2F, IDT-S-2F2Cl, and IDT-S-4F reach 10.95%, 11.39%, and 12.18%, respectively, significantly surpassing the 5.01% PCE of the device based on IDT-2Cl2F. This study proposes π-bridge engineering strategies to optimize molecular packing and energy alignment, for developing high-efficiency as-SMAs.