Amorphous niobium-modified tungsten oxide W(Nb)Ox was prepared as ESL in flexible perovskite solar cells. It was observed that modification using Nb5+ improved the electron transport in WOx-based ESL. The photovoltaic performance of the simple planar flexible PSCs achieved a high PCEs of 15.65%. A capacitance CE across the ESL was proposed to indicate the effect of ESL thickness on hysteresis behavior.