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 organic bulk heterojunction solar cell made of a 50 nm-thick active layer, and a patterned ITO electrode. In such device, the well known trade-off between carrier extraction and light absorption can appear as a constraint. Our challenge is to obtain a high absorption in an ultra thin active layer solar cell, by using a light trapping effect. (C) 2013 Elsevier B.V. All rights reserved.