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 the physical and chemical properties of organic semiconductor materials. However, there is little research on the molecular geometry-property relationship of A-DA'D-A type small molecule acceptors (SMAs), which are the most representative organic semiconductor materials used in organic solar cells (OSCs). In this work, we used dichlorine-substituted benzene as the A' unit to overcome the geometry constraints imposed by the conventional A' unit that has a large size. Four isomers of benzodipyrrole-based SMAs (C-Cl46-Cl, Ɂ-Cl46-Cl, M-Cl46-Cl, and S-Cl46-Cl) with C-, Ɂ-, M-, and S-shaped molecular geometries were synthesized, and the effect of 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 governs the exciton diffusion and charge transport processes; and 3) donor-acceptor interaction and miscibility. We found that the C-shaped molecular geometry possesses a suitable energy level and absorption range, dense and ordered molecular stacking, and improved donor-acceptor interaction 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 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. These properties resulted in inferior photovoltaic device performance. These findings on the molecular geometry-property relationships will be instructive for future molecular design of high-performance A-DA'D-A type SMAs.