Realization of fast hole extraction is an effective route for the performance enhancement of inverted perovskite solar cells (PSCs). The precise bandgap tuning in perovskite layer has been expected to accelerate hole extraction. Therefore, in this report, we demonstrate a modified one-step fabrication method for the bandgap tuning of perovskite layer by designing iodide concentration gradient. Prior to the perovskite precursor solution spin coated, different concentration of methanaminium iodide (MM) solution was pre-coated on PEDOT:PSS layer to form iodide concentration gradient in perovskite layer. Fast hole extraction and short lifetime due to iodide concentration gradient was confirmed by photoluminescence measurement. By optimizing the MAI concentration with 4 mg/ml, the modified PSCs exhibited a power conversion efficiency of 16.67% due to an increase in short-circuit current density (J(sc)) from 19.66 mA/cm(2) to 23.52 mA/cm(2) and an increase in open circuit voltage from 0.97 V to 1.01 V. These values are significantly higher than PSCs using conventional one-step fabrication method without pre-coated MM layer, which exhibit efficiency of only 14.8%. We think that this new approach is an effective way to enhance the performance of PSCs.