A detailed parametric study of the influence of silver nanospheres on the optical absorption of a poly [2-méthoxy-5-(2-éthyl-hexyloxy)-1,4-phénylène-vinylène]: 6,6-phenyl C61-butyric acid methyl ester (MEH-PPV:PCBM) bulk heterojunction is performed via a numerical analysis based on the Finite Difference Time Domain (FDTD) rigorous method. The metallic nanoparticles are placed inside this interpenetrated organic layer while a 50 nm-thick organic layer without particles is juxtaposed. We investigate the role of critical parameters such as period, diameter and position of nanospheres in enhancing absorption in MEH-PPV:PCBM when they are placed either in the front area directly exposed to light or in the back area. We calculate intrinsic absorption in the whole active bi-layer after subtracting the intrinsic absorption inside particles. A greater increase in absorption (up to 3) is observed for structures with nanospheres than for structures without nanospheres. Furthermore, absorption of a 150 nm-thick active layer including nanoparticles is stronger than that of a 330 nm-thick active layer without nanoparticles. This absorption enhancement is due to the localized plasmon polariton and scattering. The calculated absorption spectra in this paper could give useful data for further manufacturing of efficient solar cells.