Isomeric small molecule acceptors based on perylene diimide and spirobifluorene for non-fullerene organic solar cells

Abstract Two isomeric small molecules based on perylene diimide with 4,4′-/2,4′-substituted spirobifluorene as the core were designed and synthesized as acceptor materials for organic solar cells. According to density functional theory calculations, the different sites of the substitution, either the 4-position or the 2-position of the spirobifluorene led to significant distinction in both molecular structure and electronic distribution. The dihedral angles between the two perylene diimide units were calculated as well, indicating the 2-position linkage molecule had a more isotropic spatial arrangement. X-ray diffraction results showed the more ordered molecular packing of the 2-position linked molecule. In consequence, when blended with a donor, these two acceptors exhibited different device performance, especially the short circuit currents. The solar cell based on the 2-position linkage molecule achieved the best power conversition efficiency of 4.56% with 2% 1-chloronaphthalene as an additive. The hole and electron mobilities were also improved greatly after adding 2% 1-chloronaphthalene, which benefited the short circuit currents.