Subphthalocyanine-cored star-shaped electron acceptors with perylene diimide wings for non-fullerene solar cells

Two star-shaped electron acceptors (E-SubPc-PDI and S-SubPc-PDI) based on subphthalocyanine (SubPc) as the core and three perylene diimides (PDIs) as wing groups linked with and without acetylene bridges, respectively, have been synthesized. Compared with conventional SubPc-based acceptors with optical absorption full width at half maximum (FWHM) values of less than 100 nm and optical bandgaps over 2 eV, both E-SubPc-PDI and S-SubPc-PDI possess broader absorption spectra with FWHM values of up to 170 nm and smaller optical bandgaps below 1.9 eV. Due to the different linkages between SubPc and PDIs, they exhibit different molecular geometries, optical properties, energy levels and stacking behaviors. With a more twisted molecular structure, the single bond linked S-SubPc-PDI exhibits reduced intermolecular aggregation and a higher lowest unoccupied molecular orbital (LUMO) level compared to the acetylene bridge-linked E-SubPc-PDI. When blending with a donor polymer PBDB-T, the active layer of PBDB-T:S-SubPc-PDI shows more favorable phase separation and more balanced carrier transport. As a result, solution-processed organic solar cells (OSCs) based on S-SubPc-PDI exhibit a power conversion efficiency (PCE) up to 4.53%, which is much higher than that of E-SubPc-PDI-based devices (1.78%).