The organic-inorganic metal halide perovskite materials have excellent optical and electrical properties such as high absorption coefficient, high carrier mobility, long carrier lifetime, tunable bandgap, facile fabrication process, etc. Owing to the above excellent properties, the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has increased significantly from 3.8% to 22.1% in the last few years. The PSCs have attracted intensive interest in recent years and show great commercial potential. Previous approaches to increasing the PCE of PSCs have focused on the optimization of the morphology of perovskite film. However, there are relatively few studies on the electron transport layer (ETL) in the typical p-i-n sandwiched structure. In this work, the PCE of PSCs with device structure of ITO/PEDTO: PSS/CH3NH3PbI3/PCBM/Al is improved from 10.8% to 12.5% by using polystyrene (PS) and 1,8-diiodooctane (DIO) as binary additives during the deposition of phenyl-C61-butyric acid methyl ester (PCBM) layer. With the addition of PS, a highly smooth and uniform PCBM ETL is formed due to the increase of viscosity. The morphologies of the PCBM films prepared with and without PS are analyzed using an atomic force microscope in the tapping mode. The root-mean-square surface roughness decreases from 1.270 to 0.975 nm with the addition of PS increasing, which is more effective in preventing electron and hole from recombining at the interface between the perovskite layer and the top electrode. Addition of DIO improves the morphology of PCBM, which plays an important role in charge dissociation, charge transportation, and charge collection. From the time-resolved photoluminescence decay curves of ITO/CH3NH3PbI3/PCBM (with different additives), it is clear to conclude that the exciton dissociation between the perovskite layer and PCBM layer is faster and faster. Electrons and holes can be quickly separated, indicating that charge transport performances of electron transport layer with the addition DIO turn better. The addition of two additives is a simple and low-cost approach to improving the morphology of the electron transport layer, which provides a path-to the further improvement of the performance of p-i-n PSCs.