Molecular Geometry‐Property Relationship of Benzodipyrrole‐Based A‐DA'D‐A Type Acceptors for High‐Performance Organic Solar Cells

Molecular geometry plays a crucial role in determining photovoltaic properties of organic semiconductor materials for organic solar cells (OSCs). In this work, we used dichlorine‐substituted benzene as the A’ unit in A‐DA'D‐A type small molecule acceptors (SMAs) and synthesized four isomers of the benzodipyrrole‐based SMAs (C‐Cl46‐Cl, Ɂ‐Cl46‐Cl, M‐Cl46‐Cl and S‐Cl46‐Cl) with C, Ɂ, M, and S molecular geometries, and the effect of the molecular geometry on their photovoltaic performance was studied. We revealed that the molecular geometry influences the physicochemical and photovoltaic properties in three aspects: (1) intrinsic physicochemical properties, including energy levels, absorption and reorganization energy; (2) molecular stacking pattern, which govern the exciton diffusion and charge transport process; and (3) donor‐acceptor interaction and miscibility. We found that the C‐shaped molecular geometry possesses suitable energy level and absorption range, dense and ordered molecular stacking, and improved donor‐acceptor interactions and miscibility. These advantages enable a record‐high power conversion efficiency (PCE) of 19.94% (certified as 19.54%) for the binary OSCs based on D18:C‐Cl46‐Cl active layer. The other shaped SMAs showed weaknesses in different aspects, such as limited absorption of Ɂ‐shaped SMA, large reorganization energies and loose molecular stacking of M‐shaped SMA, low solubility and strong aggregation of S‐shaped SMA.