© Published under licence by IOP Publishing Ltd. Modified modulation photocurrent (MPC) measurements were carried out to investigate the carrier transport process in organic photovoltaics based on polymer and fullerene derivatives. In conventional MPC measurements, a photocurrent component modulating at the same frequency as the modulated incident light is recorded; however, in this study, a component modulating at twice the modulation frequency was detected. A linear process does not contribute to the detected signals; thus, this technique allows for focus on a process that converts incident photons into photocurrent in a nonlinear manner. An analysis using rate equations suggested that the nonlinearity resulted from the carrier density dependence of the internal electric field, which is the driving force of the carriers under the short-circuit condition. Furthermore, features due to both electron and hole transports were more clearly observed in the modified MPC measurements than in the conventional MPC measurements.