We propose a theoretical study of the role of a two-dimensional photonic crystal in an organic solar cell. In particular, we use some specific resonant modes of the photonic crystal, that are not guided modes, to favor light trapping in the absorbing medium. The increase of optical path in the active layer causes an increase of absorption at some particular frequencies. A frequential - temporal analysis has been made to highlight the correspondence between absorption enhancement and photonic crystal resonant modes, in a thin absorbing slab. In addition we present possible designs that would broaden the absorption spectrum of the bulk material. Finally, we study a complete solar cell made of a 50nm-thick active layer, and an ITO electrode. In such device, the well known trade-off between carrier extraction and light absorption can appear as a constraint. We show that we can obtain a satisfying absorption level, provided the electrode is structured as a photonic crystal.