Hybrid organic-inorganic metal halide perovskite solar cells (PSCs) have been regarded as a low-cost, high-efficiency photovoltaic technology; however, the device performance seriously depends on interfacial properties between each contact layer. The interface plays an important role in PSCs as it induces defects, trap states, charge recombination sites etc., which are detrimental to the device performance and stability. Herein, a post-treatment of self-assembling large cations of tert-butylammonium (tBA) on the surface of the perovskite layer is demonstrated as an effective strategy to passivate interface and grain boundary defects and thus improve device performance and stability. In particular, a normal hysteresis is turned into an inverted hysteresis by increasing the amount of tert-butylammonium iodide (tBAI), which can be ascribed to modified interface states. An impressive power conversion efficiency (PCE) over 20% with a significantly suppressed hysteresis is achieved via the careful control of post-treatment conditions, which shows great promise for commercialization in the future.