[en] Highlights: • Amorphous niobium-modified tungsten oxide W(Nb)O_x was prepared as ESL in flexible perovskite solar cells. • It was observed that modification using Nb⁵⁺ improved the electron transport in WO_x-based ESL. • The photovoltaic performance of the simple planar flexible PSCs achieved a high PCEs of 15.65%. • A capacitance C_E across the ESL was proposed to indicate the effect of ESL thickness on hysteresis behavior. Flexible perovskite solar cell (PSC) using plastic substrate is one of the most focal points in the studies of thin-film solar cells. Low-temperature preparation of suitable electron selective layer (ESL) is the key issue in the fabrication of flexible PSCs. In this work, amorphous niobium-modified tungsten oxide W(Nb)O_x was prepared as ESL for efficient flexible PSC. Modification using Nb⁵⁺ improved the electron transport of WO_x-based ESL by enhancing donor density, reducing interfacial depletion width, and minimizing trap states in the ESL. Consequently, photovoltaic performance of the simple planar flexible PSCs was improved, and high PCEs of 15.65% and 13.14% were obtained when ESLs were fabricated at 120 °C and room temperature, respectively. In addition, the effect of ESL thickness on the hysteresis behavior of PSCs was carefully analyzed. A capacitance C_E across the ESL in the ITO/ESL/perovskite structure was proposed. This capacitance could well indicate the effect of ESL thickness on hysteresis behavior. The proposed modification strategy and mechanism is expected to facilitate the development of novel and advanced functional materials.