We report on highly efficient organic solar cells based on polycrystalline thin films of pentacene, a material that has been widely investigated for p-type transport layers in organic field-effect transistors (OFET). The spectral measurement of external quantum efficiencies (EQE) shows that these high efficiencies are due to the efficient light-harvesting occurring throughout the visible spectrum. In particular, the peak EQE of 69% has been measured at a wavelength of 668 nm where most of the excitons are generated inside the pentacene layer. This suggests that the polycrystalline nature of pentacene films leading to high field-effect mobilities in OFET also results in relatively large exciton diffusion lengths which are desired in multilayer organic solar cells. In an effort to understand these devices, we model the external quantum efficiencies as a function of wavelength based on the exciton diffusion model using the complex indices (n, k) of participating materials. This study provides information on the correlation of optical properties of photoactive materials to the spectral responses and allows one to estimate exciton diffusion lengths. Based on this information, we discuss the optimization of layer structures that can lead to maximization of the photocurrent under standard illumination condition.