We report on the photovoltaic properties of organic solar cells based on pentacene and C-60 thin films with a focus on their spectral responses and the effect of thermal annealing. Spectra of external quantum efficiency (EQE) are measured and analyzed with a one-dimensional exciton diffusion model dependent upon the complex optical functions of pentacene films, which are measured by spectroscopic ellipsometry. An improvement in EQE is observed when the thickness of the bathocuproine (BCP) layer is decreased from 12 nm to 6 nm. Detailed analysis of the EQE spectra indicates that large exciton diffusion lengths in the pentacene films are responsible for the overall high EQE values near wavelengths of 668 nm. Analysis also shows that improvement in the EQE of devices with the thinner BCP layer can be attributed to a net gain in optical field distribution and improvernent in carrier collection efficiency. An improvement in open-circuit voltage (V-OC) is also achieved through a thermal annealing process, leading to a net increase in power conversion efficiency. Integration of the EQE spectrum with an AM 1.5 G spectrum yields a predicted power conversion efficiency of 1.8 +/- 0.2%. The increase in V-OC is attributed to a significant reduction in the diode reverse saturation current upon annealing. (C) 2007 Elsevier Ltd. All rights reserved.