Perovskite solar cells (PSCs) have reached an impressive efficiency over 23%. One of its promising characteristics is the low-cost solution printability, especially for flexible solar cells. However, printing large area uniform electron transport layers on rough and soft plastic substrates without hysteresis is still a great challenge. Herein, we demonstrate slot-die printed high quality tin oxide films for high efficiency flexible PSCs. The inherent hysteresis induced by the tin oxide layer is suppressed using a universal potassium interfacial passivation strategy regardless of fabricating methods. Results show that the potassium cations, not the anions, facilitate the growth of perovskite grains, passivate the interface, and contribute to the enhanced efficiency and stability. The small size flexible PSCs achieve a high efficiency of 17.18% and large size (5 × 6 cm2) flexible modules obtain an efficiency over 15%. This passivation strategy has shown great promise for pursuing high performance large area flexible PSCs.