We propose models of the incoherent optical absorption and exciton transport to simulate the processes of absorption and exciton diffusion in organic photovoltaic devices according to the optical principle and diffusion theory. The functional relation of optical absorption, and the power conversion efficiency versus the thickness of the film has been obtained. We demonstrate that the developed model can be useful for the modelling of the optical absorption and exciton separation in the optimization of devices of organic photovoltaic cells with high power conversion efficiency.