We have modeled experimental short-circuit photocurrent action spectra of ${\mathrm{poly(3-(4}}^{\mathrm{\prime }}{\mathrm{-(1}}^{\mathrm{″}}{\mathrm{,4}}^{\mathrm{″}}{\mathrm{,7}}^{\mathrm{″}}\mathrm{-trioxaoctyl)phenyl)thiophene)}$ (PEOPT)/fullerene $({\mathrm{C}}_{\mathrm{60}})$ thin film heterojunction photovoltaic devices. Modeling was based on the assumption that the photocurrent generation process is the result of the creation and diffusion of photogenerated species (excitons), which are dissociated by charge transfer at the ${\mathrm{PEOPT/C}}_{\mathrm{60}}$ interface. The internal optical electric field distribution inside the devices was calculated with the use of complex indices of refraction and layer thickness of the materials as determined by spectroscopic ellipsometry. Contributions to the photocurrent from optical absorption in polymer and fullerene layers were both necessary to model the experimental photocurrent action spectra. We obtained values for the exciton diffusion range of 4.7 and 7.7 nm for PEOPT and ${\mathrm{C}}_{\mathrm{60}},$ respectively. The calculated internal optical electric field distribution and resulting photocurrent action spectra were used in order to study the influence of the geometrical structure with respect to the efficiency of the thin film devices. In this way the photocurrent was optimized.