Inorganic perovskite solar cells (PSCs) suffer from serious carrier recombination and open-circuit voltage loss because of surface defects and unfavorable energy level alignment. Herein, a polylactic acid (PLA) modification approach to improve the performance of mixed-halide inorganic perovskites is reported. First, the surface defects are effectively passivated through strong interaction between CO in PLA and undercoordinated Pb2+. Second, secondary grain growth is induced by PLA modification, resulting in larger grain sizes. Third, PLA modification makes the surface region of perovskite change from n- to p-type, favoring charge transport from perovskite to the hole transport layer (HTL). The PLA modified films enable PSCs with less nonradiative recombination and lower energy loss. Consequently, record PCEs of 19.12% and 18.05% are achieved for CsPbI2.25Br0.75 and CsPbI2Br PSCs, respectively. The PSC with an active area of 1 cm(2) shows a PCE of 16.41%. A PCE of 14.70% is achieved for HTL-free PSC with carbon electrode. In addition, the PSC with PLA modification shows significantly improved air stability due to the hydrophobic PLA coating. This work suggests that PLA surface modification is an effective approach to achieving efficient, stable, scalable, and low-cost inorganic PSCs.