Organic solar cells present a promising alternative for the generation of solar energy at lower material and production costs compared to widely used silicon-based solar cells. The major drawback of organic solar cells currently is a lower rate of energy conversion. Thus many research projects aim to improve the achievable efficiency. In this work a phase field model is used to mathematically describe the morphology evolution of the active layer composed of polymer as electron-donor and fullerene as electron-acceptor. The derivation of a chemical potential term and a surface energy term for the polymer-fullerene solution using the Flory-Huggins theory forms the basis to employ the Cahn-Hilliard equation. After including several specifics of the application in this non-linear partial differential equation of fourth order, an implementation of the model using the FEM solver software FEniCS provides some simulation results that qualitatively match results from the literature.