A highly soluble amorphous fullerene derivative substituted with dihexylfluorene (DHFCBM) was synthesized and used as an electron acceptor material for P3HT-based bulk heterojunction solar cells. By fitting the experimental J-V curves with space charge limited current equation, the electron mobility of DHFCBM was determined to be 4 x 10(-4) cm(2)/Vs, possibly leading to balanced charge transport with P3HT. From structural and morphological analysis using X-ray diffraction, UV-vis absorption, and atomic force microscopy, we found that the amorphous nature of DHFCBM stabilized the nanomorphology of P3HT:DHFCBM blend films under high temperature annealing. By optimizing blend ratios and annealing conditions, P3HT:DHFCBM-based solar cells yielded power conversion efficiencies in excess of 3%. In addition, the fabricated cells maintained their initial performances even after high temperature annealing for long times, as predicted from the stable nanomorphology. We believe that the use of thermally stable amorphous fullerene as an electron acceptor can be a promising strategy for commercialization of organic solar cells. (C) 2010 Elsevier B.V. All rights reserved.