In this paper we present a theoretical model for simulating the behavior of a polymer solar cell with a poly(3-hexylthiophene):1-(3-methoxycarbonyl) propyl-1-phenyl-[6, 6]-methanofullerene (P3HT:PCBM) active layer. Two different types of boundary conditions were considered, Dirichlet’s and mixed. For Dirichlet’s boundary conditions we have achieved an excellent agreement with the experiment. The influence of boundary conditions on the appearance of the s-shaped current-voltage characteristic (sometimes observed in experiments) has been investigated. When mixed boundary conditions are applied, calculated current-voltage characteristics are inevitably s-shaped. By altering the boundary carrier concentration, an s-shaped deformation in current-voltage characteristics is numerically simulated by using Dirichlet’s boundary conditions.