Star-shaped small molecule acceptors with a subphthalocyanine core for solution-processed non-fullerene solar cells

Two star-shaped electron acceptors (STIC and STFIC) based on subphthalocyanine (SubPc) as core and 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (IC) or 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (2FIC) as end groups, have been designed and synthesized. Both two acceptors show unique photophysical properties different from those of traditional SubPc based non-fullerene acceptors (NFAs): first, two strong absorption bands in the region of 400–700 nm with high extinction coefficients are observed; second, their spectra in films exhibit full width at half maximum (FWHM) over 260 nm, more than two-fold larger than those of traditional SubPc based NFAs; third, their optical bandgaps are around 1.6 eV, among the narrowest bandgaps for SubPc-based NFAs. Compared with STFIC with 2FIC end groups, the higher-lying lowest unoccupied molecular orbital (LUMO) level of STIC with IC end groups leads to the higher values of open circuit voltage (VOC) and power conversion efficiency (PCE) in organic solar cells (OSCs). Using PBDB-T as donor, the PCEs of 4.69% and 3.80% are achieved by OSCs based on STIC and STFIC, respectively. The PCE of 4.69% is also among the highest PCE values for SubPc-based solution-processed BHJ OSCs.