We have investigated how bulk photoconductivity influences the photovoltaic parameters of zincphthalocyanine-fullerene (ZnPc/C-60) p-n heterojunction solar cells. The results indicate that the photocurrent action spectrum of a cell depends strongly on the photoconductivity and spectral characteristics of each component material. We therefore propose a model that simulates the action spectra of the short-circuit photocurrent in the molecular organic solar cells, and our model is based on the assumption that the photocurrent action spectrum depends on the bulk layer photoconductivity. c 2005 The Electrochemical Society. [DOI: 10.1149/1.2126576] All rights reserved.