High-efficiency fullerene-free single-heterojunction (SHJ) organic photovoltaic (OPV) cells consisting of tetracene (Tc) as a typical donor material and boron subphthalocyanine chloride (SubPc) as an acceptor material are reported. Cells containing SubPc as a direct replacement for C60 exhibit an similar to 60% improvement in open circuit voltage (Voc) achieving a maximum Voc of 1.24 V, which is amongst the highest values acheived to date for SHJ devices. This resulted in an overall improvement of similar to 60% in power conversion efficiency from 1.8%, for Tc/C60 cells, to 2.9% for Tc/SubPc. The OPV device results are complemented by soft Xray photoelectron spectroscopy (PES) measurements of the interfacial energetics of both systems. The results demonstrate that SubPc shows considerable promise as an electron acceptor material for future cell designs.