Interface engineering has shown great potential to improve the photovoltaic performance and long-term stability of perovskite solar cells. Herein, RbX (X=Br, I) materials were developed as the interfacial modifiers for CsPbI2Br solar cells that realized power conversion efficiencies (PCE) over 15 % with a high open-circuit voltage (VOC) of 1.27 V. The RbX interlayer having a dendrite-shaped structure could optimize the surface wetting behavior of TiO2 films and thus enabled formation of high-quality perovskite films. More importantly, RbX could better align the devices’ energy levels and passivate surface defects because of its large bandgap. The PCE of a CsPbI2Br device with RbX interlayer remained 87 % of its initial efficiency for 800 h in ambient atmosphere.