Graded donor-acceptor heterojunctions for efficient organic photovoltaic cells are fabricated using boron subphthalocyanine chloride (SubPc) and C-60. The graded heterojunction significantly increases the exciton diffusion efficiency relative to a planar architecture, and improves the charge collection efficiency relative to a uniformly mixed architecture. A power conversion efficiency of (4.2 +/- 0.1)% is realized in a graded heterojunction OPV at 100 mW cm(-2) AM1.5G illumination intensity.