Photoconversion in planar-heterojunction organic photovoltaic cells (OPVs) is limited by a short exciton diffusion length (L-D) that restricts migration to the dissociating electron donor/acceptor interface. Consequently, bulk heterojunctions are often used to realize high efficiency as these structures reduce the distance an exciton must travel to be dissociated. Here, we present an alternative approach that seeks to directly engineer L-D by optimizing the intermolecular separation and consequently, the photophysical parameters responsible for excitonic energy transfer. By diluting the electron donor boron subphthalocyanine chloride into a wide-energy-gap host material, we optimize the degree of interaction between donor molecules and observe a similar to 50% increase in L-D. Using this approach, we construct planar-heterojunction OPVs with a power conversion efficiency of (4.4 +/- 0.3)%, >30% larger than the case of optimized devices containing an undiluted donor layer. The underlying correlation between L-D and the degree of molecular interaction has wide implications for the design of both OPV active materials and device architectures.