The photocurrent spectral responses of poly[2-methoxy-5-(2'-ethyl-hexoxy-p-pheiiylene vinylene]:fullerene (C60) composites are measured as a function of C60 concentration. At low concentration, the relationship between the external quantum efficiency (EQE) and absorption spectra is exhibited as the strengthened antibatic effect, and the EQE of the composite devices declines with increasing concentration of C60. At higher concentration, however, the maximum EQE gradually coincides with the absorption peak (symbatic response) and the EQE of composite devices begins to increase with increasing C60 concentration. It is proposed that at low concentration, dopant C60 increases the self-absorption rate of composite films, and charge trapping by C60 molecules causes the loss of efficiency. At high C60 concentration, the large-scale aggregations in composite films build pathways for charge carrier transport to respective electrodes, inhibiting the self-absorption effect and charge recombination on C60. (c) 2007 Elsevier B.V. All rights reserved.