This manuscript explores the formation of well-defined molecular p-n junctions in solution-processed self-assembled hetero-junction solar cells using dodecyloxy-substituted contorted hexabenzocoronene (12-c-HBC) as a donor material and phenyl-C70-butyric acid methyl ester (PC70BM) as an acceptor. We find that the contorted 12-c-HBC molecules effectively assemble in solution to form a nested structure with the ball-shaped PC70BM. The result is a self-assembled molecular-scale p-n junction. When this well-defined p-n junction is embedded in active films we make efficient self-assembled solar cells with minimal amounts of donor material relative to the acceptor. The power conversion efficiency is drastically enhanced by the mode of donor and acceptor as-sembly within the film.