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-l-ylidene)malononitrile (2FIC) as end groups, have been designed and synthesized. Both two acceptors show unique photo physical 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 (V-OC) 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.