Organic solar cell (OSC) is one of the best promising candidate for harvesting energy mainly due to their simple and economic fabrication process, the reduced manufacturing cost, and easy integration to other products. Bulk heterojunction solar cell in which the active layer is a blend of donor –acceptor materials are one of the best organic photovoltaic device with highest efficiency and a significant improvement in the device performance occur over last years. Bulk heterojunction architecture gives a high interfacial surface area for efficient charge dissociation. In this study, bulk heterojunction solar cell is simulated using General Purpose Photovoltaic Device Model Software. A donor-acceptor blend of Zinc pthalocyanine (ZnPc) fullerene C60 is used as the active layer. The power conversion efficiency for various thickness of the active layer is studied. Optimization of active layer thickness for maximum power conversion efficiency are done. The dependence of various electrical parameters such as short circuit current density(Jsc), Open circuit voltage(Voc), fill factor (FF), average carrier mobility on power conversion efficiency are also studied.