A double-heterostructure boron subphthalocyanine chloride (SubPc)/C60 thin-film photovoltaic cell exhibits a more than doubled Voc compared to a conventional copper phthalocyanine (CuPc)/C60 cell under 1 sun AM1.5G simulated illumination. The lower oxidation potential of SubPc results in an increase in the energy difference between the lowest unoccupied molecular orbital of the acceptor-like material and the highest occupied molecular orbital of the donor-like material (referred to as the interface gap, Ig) by 400 meV. We attribute the significant increase in Voc to this increase in Ig.