Severe nonradiative recombination originating from interfacial defects together with the pervasive energy level mismatch at the interface remarkably limits the performance of CsPbI3 perovskite solar cells (PSCs). These issues need to be addressed urgently for high-performance cells and their applications. Herein, we demonstrate an interfacial gradient heterostructure based on low-temperature post-treatment of quaternary bromide salts for efficient CsPbI3 PSCs with an impressive efficiency of 21.31% and an extraordinary fill factor of 0.854. Further investigation reveals that Br- ions diffuse into the perovskite films to heal undercoordinated Pb2+ and inhibit Pb clusters formation, thus suppressing nonradiative recombination in CsPbI3 . Meanwhile, a more compatible interfacial energy level alignment resulting from Br- gradient distribution and organic cations surface termination has been also achieved, hence promoting charge separation and collection. Consequently, we also demonstrate the printed small-size cell with an efficiency of 20.28% and 12-cm2 printed CsPbI3 minimodules with a record efficiency of 16.60%. Moreover, the unencapsulated CsPbI3 films and devices exhibit superior stability. This article is protected by copyright. All rights reserved.