We have studied the energy level of the trap states in P3HT: PCBM-71 bulk heterojunction solar cells by using the PICTS (photo-induced current transient spectroscopy) method. The trap energy level is extracted to be 73.1 meV, which is larger than that of bulk P3HT (similar to 54 meV). This difference, we believe, is that fullerene anion of PCBM plays as a deep potential well for holes. (C) 2010 Elsevier B. V. All rights reserved.