After numerically solving the Poisson and continuity equations that characterize the electrical behavior of semiconductor devices, we investigated the loss mechanism in bulk heterojunction organic solar cells (OSCs) by simulating intensity-dependent current-voltage curves. Comparison of simulation results with real experimental measurements reveals that monomolecular recombination is the primary loss mechanism in OSCs. This conclusion is consistent with direct observations that bimolecular recombination is significantly suppressed in bulk heterojunction OSCs. The determination of recombination mechanism is important to model and predict the operation of OSCs precisely and further optimize their performance. (C) 2011 Elsevier B.V. All rights reserved.