The carrier collection efficiency (eta(c)) and energy conversion efficiency (eta(e)) of polymer photovoltaic cells were improved by blending of the semiconducting polymer with C-60 or its functionalized derivatives. Composite films of poly(2-methoxy-5-(2’-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) and fullerenes exhibit eta(c) of about 29 percent of electrons per photon and eta(e) of about 2.9 percent, efficiencies that are better by more than two orders of magnitude than those that have been achieved with devices made with pure MEH-PPV. The efficient charge separation results from photoinduced electron transfer from the MEH-PPV (as donor) to C-60 (as acceptor); the high collection efficiency results from a bicontinuous network of internal donor-acceptor heterojunctions.