The authors report on a highly efficient small-molecule organic photovoltaic device employing a planar-mixed heterojunction structure with subphthalocyanine (SubPc) as the donor and C-70 as the acceptor. By changing the mixing ratio of SubPc and C-70, the power conversion efficiency can be significantly improved from 1.8% to 6.0% under air mass 1.5 G solar illumination. From the internal quantum efficiency measurement, the highest carrier extraction is observed when an optimized weight ratio for SubPc to C-70 of 1:7 is used. Besides, atomic force microscopy images demonstrate that a good phase separation of the blend layer is realized under such an optimized mixing ratio. This phenomenon may contribute to the improved fill factor and hence the bulk resistance. As a result, the J(SC) of our device is significantly enhanced to 13.0 mA/cm(2), which is almost twice the conventional value for planar heterojunction SubPc devices. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.041301jes] All rights reserved.