Organic solar cells (OSCs) generally degrade seriously due to the rapid intake of oxygen and water if exposed to the air. In this paper, we investigated the extrinsic degradation of OSCs by showing the remarkable acceleration effect of fluorescent lamps. We attributed this effect to the accelerated lateral diffusion of oxygen in the fullerene layer under light. We simulated the dynamics of this process using a finite-element method. The simulation suggests that the diffusion constant of oxygen in cells increases approximately sevenfold under fluorescent lamps. We finally reported a recovering phenomenon of an unencapsulated OSC in dark, which demands more investigations. (C) 2012 Elsevier B.V. All rights reserved.