Two new [C60]methanofullerenes containing hydroxyl groups, including a fullerene dimeric derivative and its analogue with one fullerene unit, are synthesized to investigate the influence of covalent linkage of fullerene and the hydroxyl groups on the photovoltaic properties of fullerene derivatives. It is found the hydroxyl groups cause poor compatibility between the methanofullerenes and the donor material P3HT, and both two new compounds show worse photovoltaic performance compared with PCBM. However, dimeric fullerene derivative demonstrates much better photovoltaic properties than mono-fullerene derivative, suggesting that the covalent aggregation of fullerene could significantly influence the performance of fullerene-based solar cells. (C) 2012 Elsevier B. V. All rights reserved.