The design of flexible PTB7:PC71BM bulk-heterojunction and P3HT:SiNW hybrid organic solar cells are optimized for maximum photovoltaic performance. The thickness of each layer is optimized for maximum photon absorption in the active layer of a bulkheterojunction organic solar cell with structure, PET/PEDOT:PSS/TFB/PTB7:PC71BM/Ca and that of a hybrid solar cell with structure, PET/PEDOT:PSS/TFB/P3HT:SiNW/Ca. The optimal design thus obtained produces a power conversion efficiency of 12.87% in the bulk-heterojunction and 4.70% in hybrid solar cell. High photon absorbance is found to occur within a wide range of the solar spectrum for PTB7:PC71BM bulk-heterojunction organic solar cell while a high transmittance and reflectance is found in the P3HT:SiNW hybrid solar cell. This difference may be attributed to the wide band gap of P3HT and mismatch between the electron and hole mobilities in the hybrid solar cell. Nevertheless, the optimized design of the hybrid solar produces a power conversion efficiency greater than 50% measured experimentally.