To produce better solar cells, deeper knowledge of the physics of photovoltaic absorbers (particularly polymer-fullerene systems) and refined understanding of the experimental characterization of these systems are needed. The authors present a simple model to interpret capacitance-voltage measurements of organic solar cells under illumination. Their discussion of the model's assumptions, limitations, and pitfalls, and demonstration that the charge-carrier mobility can be obtained from data taken under reverse bias, should be of great practical interest to engineers and physicists alike.