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[Display omitted] • The interfacial redox reaction between NiO x and perovskite is obstructed. • SaC-100 is developed as a modifier layer in NiO x -based perovskite solar cells. • SaC-100 can efficiently reduce the open-circuit voltage loss of device. • SaC-100 can inhibit perovskite decomposition ascribed from interfacial reaction. In NiO x -based perovskite solar cells (PVSCs), the interfacial redox reaction between Ni3+ (on the surface of NiO x) and A-site cation salt (MAI in perovskite precursor solution) is invariably ignored. This adverse reaction will generate PbI 2 -rich hole extraction barriers at the NiO x -perovskite interface, which limits hole transmission and increases charge recombination, thus resulting in open-circuit voltage (V oc) loss. Furthermore, it will accelerate perovskite degradation by deprotonating the precursor amine and oxidizing iodide to interstitial iodine, which induces the severe instability of devices. Herein, a physical separation strategy by introducing a modifier layer to obstruct the detrimental reaction is explored. The results demonstrate that the trimethylolpropane tris(2-methyl-1-aziridinepropionate) (SaC-100) depositing onto NiO x can suppress the reaction between Ni3+ and MAI to endow the improvement of conductivity and reduction of interfacial defects, thus reducing V oc loss and enhancing device stability. Moreover, the interfacial energy level alignment and the morphology of perovskite are also optimized. As a result, the PVSCs device based on NiO x /SaC-100 presents the best power conversion efficiency (PCE) of 20.21% with a superior V oc value of 1.12 V. Furthermore, the device shows better light and thermal stability because of the hindering effect and defect passivation effect of SaC-100. [ABSTRACT FROM AUTHOR]