Unveiling the Molecular Symmetry Dependence of Exciton Dissociation Processes in Small-Molecular Heterojunctions

Abstract

Understanding the dependence of molecular symmetry on exciton (EX) dissociation could facilitate optimization of overall power conversion efficiencies (PCEs) in solution-processed small-molecule (SM) solar cells. In this work, a series of high-performance oligomer molecules DRCNnT (n = 4–8) containing different numbers of thiophene units is systematically investigated to clarify the dependence of molecular symmetry on EX dissociation dynamics. Femtosecond transient absorption spectroscopy is employed to track the evolution of photogenerated electron–hole pairs across the DRCNnT:PC71BM interfaces, and faster intermolecular charge transfer is observed in the odd-numbered (axisymmetric) molecules relative to the even-numbered (centrosymmetric) molecules. It is found that those molecules with axisymmetric structures exhibit larger local dipole (Δμge) and better interpenetrating morphology than the centrosymmetric even-numbered molecules. Furthermore, a charge separation efficiency up to ∼80% is observed, whic...