Fully-unfused electron acceptors based on a steric boron dipyrromethene (BODIPY) core for organic solar cells

In this study, two electron acceptors with fully-unfused conformations, namely MBTIC-4F and MBTIC-4Cl, are facilely developed with a steric boron dipyrromethene (BODIPY) derivative as core, single thiophene as bridges, and fluorinated and chlorinated dicyanoindanone as endcaps. Benefiting from the steric BODIPY core, both the fully-unfused electron acceptors (FUEAs) show twisted geometries to avoid excessive intermolecular stacking and achieve satisfactory sunlight capture contributed by the inherent high extinction coefficient of BODIPY. Blending with the polymer donor PBDB-T, MBTIC-4Cl affords a better power conversion efficiency of 3.47% than MBTIC-4F of 2.03% in the corresponding organic solar cells. The disparities in crystallinity and miscibility are unraveled to be responsible for differential photovoltaic performance. More specifically, chlorinated MBTIC-4Cl demonstrates a stronger crystalline structure, lower miscibility with PBDB-T, and large-scale pure acceptor domains in the relevant blends, resulting in favorable morphology to guarantee sufficient exciton dissociation, high electron mobility, and balanced charge transport. This study reveals great potential for achieving cost-effective FUEAs by employing a BODIPY core via an appropriate molecular design.