In this work we measure the exciton diffusion length (LD) of the electron donor material boron subphthalocyanine chloride (SubPc) as a function of concentration in a wide energy gap host material, effectively modulating the intermolecular separation. It is shown that the LD of neat SubPc (LD = 10.7 nm) can be increased by ∼50% at a concentration of 25 wt.% (LD = 15.4 nm). The enhancement in LD is attributed to the optimization of the parameters that control Förster energy transfer. Furthermore, we show that enhanced LD can be translated to dilute donor OPVs that demonstrate an enhanced power efficiency of ηP = 4.4%, a ∼30% increase relative to OPV devices based on neat SubPc and rivaling the efficiency of corresponding bulk heterojunction devices based on SubPc and C60. Kinetic Monte Carlo modeling of exciton dynamics in these devices suggests that optimal incorporation of dilute donor layers with enhanced LD depends intimately on the interface. Specifically, an imbalance in energy transfer across the dilute donor interface imparts inhomogeneity in the energy transfer landscape, dramatically affecting exciton motion. Overall, this work highlights the opportunity for designing future organic semiconductors that have longer LD as well as OPV architectures that are directly optimized for enhanced exciton diffusion.