Photocurrent of an organic solar cell under chopped monochromatic light was studied at different stages, i.e. just after fabrication (without degradation) and after serious degradation. A large photocurrent under chopped light was observed for a seriously degraded device, unexpected from intuitionistic reckoning. This work demonstrates that both light bias and chopper frequency affect the photocurrent measurement of an organic solar cell, especially for a degraded device. The influence of light bias and chopper frequency is explained as the result of traps produced during the degradation of an organic solar cell. Thus measuring photocurrent under one sun light bias at low chopper frequency is suggested to more accurately determine the external quantum efficiency. (C) 2009 Elsevier B.V. All rights reserved.