Exciton diffusion in organic semiconductors is often probed using measurements of photoluminescence (PL), a method only applicable to luminescent materials. A more general approach to extract the exciton diffusion length (LD) is from the photocurrent current spectrum of a bilayer organic photovoltaic cell (OPV). This method, however, is limited by often unknown recombination losses that can occur during charge collection. Here, we demonstrate a device-based measurement to extract the intrinsic LD without the need to assume a value for the charge collection efficiency (ηCC). Since ηCC is identical for carriers originating from both the donor and acceptor, a ratio of the donor and acceptor internal quantum efficiencies (ηIQE) cancels this unknown quantity. In this work, a fluorescent material boron subphthalocyanine chloride (SubPc) is used as a test of this method. We measure both donor to acceptor ηIQE ratio and PL emission of SubPc-C60 bilayer OPVs as a function of SubPc thickness. The LD extracted from the thickness dependence of the ηIQE ratio agrees well with that extracted from device PL. The extracted LD is also in agreement with thickness dependent PL quenching measurement taken on glass substrates, further confirming the viability of this technique. We expect this simple, device-based approach for the extraction of LD will have wide applicability to any excitonic material capable of integration into a simple, bilayer photovoltaic device.