Recently, planar-structure perovskite solar cells processed by lowtemperature solution method have attracted tremendous attention. However, the interfacial carrier recombination limits the further improvement in efficiency and causes serious hysteresis. Therefore, the effective interface passivation techniques are necessary to further improve the efficiency of perovskite solar cells (PSCs). In this work, we demonstrate that an ultrathin mixture layer of PMMA:PCBM with the optimization ratio of 1:2 can effectively passivate defects at the electron transport layer/perovskite interface, thus significantly inhibiting the interfacial carrier recombination, contributing to the improved open-circuit voltage (V-oc) and negligible hysteresis effect. The PMMA: PCBM mixture passivation layer elevates the Voc of triple-cation perovskite (Cs-0.05(FA(0.85)MA(0.15))(0.95)Pb(I0.85Br0.15)(3)) solar cells by 40 mV, with champion device achieving a Voc of 1.17 V. The maximum efficiency of the passivated PSCs is 18.63% with negligible hysteresis. Moreover, the interfacial passivation method improves the long-term stability of PSCs, which retains 82% initial performance after over 768 h of storage at room temperature. This passivation approach successfully provides an avenue for further improving the efficiency and stability of PSCs.