We investigate the dark low-frequency noise characteristics of P3HT:PCBM bulk heterojunction organic solar cells in both forward and reverse bias conditions. The current noise power spectral density (S1) is "1/f"-like and is compared among cells annealed at different temperatures (60 degrees C to 140 degrees C). The asymmetric relationship of S-I versus DC dark current (Inc) can be explained by the competition between the recombination current noise and tunneling current noise. Among the different annealing temperatures, we find that higher annealing temperature yields smaller ratio of the Hooge parameter to the carrier recombination lifetime, which is reflected in the forward bias S-I versus I-DC relationship. We demonstrate that the low-frequency noise can serve as a non-destructive diagnostic indicator of the performance of organic solar cells. (C) 2014 Elsevier B.V. All rights reserved.