In perovskite based planar heterojunction solar cells, the interface between TiO2 compact layer and perovskite film is critical for high photovoltaic performance. The deep trap states on TiO2 surface induce several challenging issues, such as charge recombination loss and poor stability etc. To solve the problems, we synthesized a tri-block fullerene deriva-tive (PCBB-2CN-2C8) via rational molecular design for interface engineering in the perovskite solar cells. Modifying TiO2 surface with the compound significantly improves charge extraction from perovskite layer. Together with its up-lifted surface work function, open circuit voltage and fill factor are dramatically increased from 0.99 V to 1.06 V, and from 72.2% to 79.1%, respectively, resulting in 20.7% improvement in power conversion efficiency for the best perform-ing devices. Scrutinizing the electrical properties of this modified interfacial layer strongly suggests that PCBB-2CN-2C8 passivates TiO2 surface and thus reduces charge recombination loss caused by the deep trap states of TiO2. The pas-sivation effect is further proved by stability testing of the perovskite solar cells with shelf lifetime in ambient condition improved by a factor of more than 4, from ~40 hours to ~200 hours, by using the PCBB-2CN-2C8 as TiO2 modification layer. This work offers not only a promising material for cathode interface engineering, but also provides a viable ap-proach to address the challenges of deep trap states on TiO2 surface in planar perovskite solar cells.