Dimerized small molecule acceptors featuring isomeric benzodithiophene linkers for organic solar cells

Abstract

Dimerized small molecule acceptors (SAMs) with different π-linking units have received significant attention due to their application in organic solar cells (OSCs). Herein, we designed and synthesized three dimerized SMAs, namely DM1, DM2, and DM3, by integrating centrosymmetric benzodithiophene (BDT) and its axisymmetric isomer (i-BDT) as linking units with a Y-series acceptor unit featuring fluorine and chlorine-substituted terminal groups. For two dimerized SMAs containing fluorine-substituted terminal groups, the i-BDT-linked DM2 possesses reduced molecular symmetry, increased dipole moments, and enhanced molecular backbone coplanarity compared to the BDT-linked DM1, which leads to stronger intermolecular interactions and optimized phase-separated morphology. The device based on DM2 achieves a power conversion efficiency (PCE) of 11.86%, outperforming the DM1-based device (PCE = 10.64%). Moreover, the chlorinated counterpart DM3 exhibits further red-shifted absorption spectrum with its bandgap down to 1.38 eV, and tighter π–π stacking as well. Consequently, the DM3-based OSC achieves an improved short-circuit current density, resulting in the highest PCE of 12.60% among the three dimerized SMAs. These findings not only demonstrate an effective method for designing novel oligomeric acceptor architectures through the linking unit isomerization strategy but also highlight the potential of the i-BDT linker for the construction of efficient dimerized SMAs for high-performance OSCs.