Defects at the surface of perovskite films are considered as the main origins of efficiency loss and instability, which has severely limited the commercialization of perovskite solar cells (PSCs). Herein, a novel functional ionic compound, guanidinium tetrafluoroborate (GuaBF 4 ), is introduced into the CH 3 NH 3 PbI 3 (MAPbI 3 )/[6,6]-phenyl-C 61 -butyric acid methyl ester (PC 61 BM) interface for the surface defect passivation of perovskite films. Thanks to the synergistic effect of Gua + and BF 4 − , the GuaBF 4 surface treatment can endow the MAPbI 3 film with larger grains, smoother surface, higher crystallinity, efficiently-passivated surface defects and optimized interfacial energy level alignment. Meanwhile, systematical optical and electrical studies have demonstrated that the GuaBF 4 surface treatment can suppress trap-assisted carrier recombination loss and reduce the charge accumulation at the MAPbI 3 /PC 61 BM interface, thus promoting charge carrier transport and extraction. Based on the GuaBF 4 surface treatment, the power conversion efficiency (PCE) of devices increases from 18.09% to 20.54% for small devices (active area 0.10 cm 2 ) and from 12.35% to 15.52% for large devices (active area 1.0 cm 2 ). Moreover, PSCs undergone GuaBF 4 surface treatment display the reduced hysteresis behavior, increased moisture stability and high reproducibility. Our work provides a new surface treatment approach by functional ionic compounds for passivating surface-defects, thus achieving high-performance PSCs. A novel surface treatment enabled by guanidinium tetrafluoroborate (GuaBF 4 ) is developed to improve the efficiency and stability of inverted planar PSCs. • A novel and effective surface treatment strategy enabled by GuaBF 4 is developed. • The synergistic effect of Gua + and BF 4 − can improve the quality of CH 3 NH 3 PbI 3 films. • The surface treatment can promote the interfacial charge transport and extraction. • Device efficiency and stability are effectively improved using this strategy.