Recently, the problem of device stability has become an important issue for the commercialization of organic solar cells. The stability problems arise from the reduction of device performance due to various causes such as heat, light, moisture, and oxygen under the condition that the device is driven, and the all-polymer solar cells (all-PSCs) show excellent characteristics in terms of stability. Especially, the mechanical stability of the active layer is very important for the application to the flexible device, and unlike the fullerene-based organic solar cells which is occurred crack even at a low tensile strain, the all-PSCs have excellent mechanical properties. However, all-PSCs show relatively low power conversion efficiency (PCE) of 6 to 8% compared with the fullerene-based organic solar cells showing more than 10% PCE. In this thesis, we introduced a new small molecule additive into all-PSCs, and investigated the electrical properties, morphological properties, and optical properties of the all-PSCs. Finally, based on the results, we successfully enhanced performance of all-PSCs by control of the crystalline packing structure of polymer blends.