Organic solar cells made from bi-layer thin-film heterojunctions having poly((2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene)vinylene) (MEH-PPV) as an electron donor and fulleropyrrolidine derivatives as an electron acceptor were investigated. We synthesized soluble fulleropyrrolidine derivatives substituted different chain lengths for the organic solar cell. Due to the high solubility and sufficiently long chain length of fulleropyrrolidine derivatives, though those are monomers, a thin film (about 80-90 nm) could be fabricated individually by the spin-coating method. The fill factor of the bi-layer device was achieved to be 0.46, which is higher than that of the single-layer device by a polymer/fulleropyrrolidine derivative blend film of 0.37, due to the decrease of the recombination. (C) 2007 Elsevier B.V. All rights reserved.