The graded bulk heterojunction (GBHJ) organic solar cell (OSC), with an active layer of donor-blend-acceptor structure, has recently paid much attention in developing the performance of OSCs. In this work, a numerical simulation has been performed for a GBHJ solar cell using SCAPS simulator to provide some design guidelines for this type of OSC. Firstly, the validation of the simulation is done by comparing the simulation with experimental results. Then, a comparative study is performed between the presented GBHJ with conventional bulk heterojunction (BHJ) and bi-layer solar cells. Simulation results indicate that GBHJ has the highest power conversion efficiency (PCE) of 10.15%, while BHJ has the highest collection efficiency (zeta(coll)) of 96.71%. The GBHJ performance is optimized by inspecting each layer individually and the interfacial trap states. It has been found that the main layer, having the most crucial impact on the performance, is the blend layer and its optimum thickness is 70 nm. Further, an improvement of the performance of GBHJ is achieved by using other candidates of nonfullerene acceptors (NFAs) with higher reported blend mobilities. A short circuit current density (J(sc)) of 19.88 mA/cm(2), open circuit voltage (V-oc) of 0.798 V, fill factor (FF) of 78.68%, PCE of 12.49% and zeta(coll) of 95.12% is obtained.