Effect of axial halogen substitution on the performance of subphthalocyanine based organic photovoltaic cells

To probe the influence of molecular dipole on the open circuit voltage ( V OC ) of molecular heterojunction organic solar cells, we study axially fluorinated boron subphthalocyanine/fullerene (SubPc-F/C 60 ) junctions. These exhibit an open-circuit voltage  V OC  = 1.00 V, a value closer to the HOMO–LUMO offset at the donor–acceptor interface = 1.69 eV than the  V OC  = 1.06 V measured for junctions between the archetypal chlorinated SubPc and C 60 , with corresponding HOMO–LUMO offset = 1.84 eV. Aside from the axial halogen substitution, the two compounds exhibit similar molecular structure and optical absorption. The energy levels and structure of the heteromolecular polaron pair are calculated, and the ideal organic diode model for SubPc-Cl is modified accordingly, successfully reproducing the experimental SubPc-F device characteristics. The reproducible difference in  V OC  is attributed to the different electric dipole strength between SubPc-F and SubPc-Cl and its influence on polaron pair dynamics at the heterojunction.