Organic photovoltaic cell performance is limited in part by a short exciton diffusion length (LD). While state-of-the-art devices address this challenge using a morphology-optimized bulk heterojunction (BHJ), longer LD would relax domainsize constraints and enable higher efficiency in simple bilayer architectures. One approach to increase LD is to exploit long-lived triplet excitons in fluorescent materials. Though these states do not absorb light, they can be populated using a host-guest triplet-sensitized architecture. Photogenerated host singlets undergo energy transfer to a guest, which rapidly forms triplets that are transferred back to the long-lived host triplet state. Previous efforts have been focused on Pt- and Irbased guests. Here, a host-guest pairing of metal-free phthalocyanine (H2Pc) and copper phthalocyanine (CuPc) is explored, advantageous as the guest also has strong and complementary optical absorption. In optimized devices (20 vol.% CuPc), the short-circuit current is enhanced by 20%. To probe the origin of the enhancement, the exciton LD is measured using a device-based methodology that relies on fitting ratios of donor-to-acceptor internal quantum efficiency as a function of layer thickness. Compared with the neat H2Pc, the LD of the 20 vol.% CuPc doped layer increases from (8.5 ± 0.4) nm to (13.4 ± 1.6 nm), confirming the increased device current comes from enhanced exciton harvesting.