Low temperature solution-processed organic–inorganic hybrid perovskite films inevitably encounter defects especially on surface and at grain boundaries. Anti-solvent washing has been widely practiced in fabricating uniform and dense perovskite films, but unfortunately it could cause even more defects on surface. Herein, polymer PTB7 was introduced into anti-solvent as an additive to significantly enhance perovskite film surface crystallinity with fewer defects, as evidenced by depth sensitive grazing incidence X-ray diffraction. Such improvements could be attributed to the Lewis base coordination bonds formed between PTB7 and Pb atoms in the surface region which slow down crystallization dynamics for enhanced perovskite surface crystallinity and effectively passivate defects at the perovskite grain boundary and film surface. With PTB7 in anti-solvent, the fabricated perovskite solar cells (PSCs) achieved a champion power conversion efficiency (PCE) of 21.41% from that of 19.79% for the control devices without PTB7. In addition, the hydrophobic alkyl chains in PTB7 were shown to improve film surface hydrophobicity with enhanced environmental stability of both the fabricated films and PSCs. Thus, addition of PTB7 in anti-solvent is proven to effectively improve the performance and stability of PSCs which could be easily applied in the fabrication of PSCs and other perovskite-based optoelectronics.