Modeling of organic photovoltaic (OPV) cells can be achieved by adaptation of drift-diffusion models. Replacement of traditional crystalline solid state materials by organic materials leads to much lower carrier mobility and to a new carrier, the exciton, which is a bound electron-hole pair. The Buxton-Clarke model includes electrons, holes, and excitons, together with generation, dissociation, and recombination mechanisms connecting these carriers, partially induced by device illumination. Device materials consist of a polymer:fullerene blend, poly(3-hexylthiophene): 6,6-phenyl C61-butyric acid methyl ester (P3HT:PCBM). In this article, the model is used to simulate an active layer of 20 nm; results include I-V curves and carrier current densities.