The interfaces between a halide perovskite absorber film and its charge transporting layers have been identified as one of the key bottlenecks for achieving high performance and stability. Herein we report a benzothieno[3,2-b]benzothiophene (BTBT) derivative and a carbazole derivative to passivate the interface between the sputtered NiO_x and the halide perovskite in p-i-n perovskite solar cells (PSCs). The treatment of the NiO_x with the BTBT derivative significantly enhances the open-circuit voltage (V_oc), boosting the champion power conversion efficiency (PCE) from 16.6% to 18.6%. We carry out an in-depth analysis of the interface employing a lift-off procedure revealing that the BTBT molecules reside at the bottom interface and successfully suppress a (redox) reaction at this interface. As a result of the improved interface quality, the stability of the PSCs is superior under maximum power point (MPP) conditions and in ambient conditions, retaining 89% of the initial PCE after 250 hours at 25% relative humidity and 95% after 1 hour under continuous MPP tracking (MPPT).