All-inorganic cesium lead halide perovskites (CsPbX3, X = Cl, I, Br) have attracted great attention for developing high-performance photovoltaics due to their superb thermal stability. However, the short electron-hole diffusion lengths (LDs) in CsPbI2Br perovskite are the primary limiting factor for further improvement of the power conversion efficiencies (PCEs) of the CsPbI2Br solar cells. Herein, a simple yet effective method is reported to prolong the LDs in CsPbI2Br via codoping of Zn2+ and acetate (Ac−) ions (i.e., using Zn(Ac)2 as dopant). It is demonstrated that Zn2+ partially replaces Pb2+ to form a more stable black phase, and Ac− coordinates with Pb2+ to passivate the defects at the perovskite grain boundaries which reduces electron-hole recombination within the perovskite. As a result, the LDs of Zn(Ac)2 doped CsPbI2Br are greater than 1 μm, which is the first report on all-inorganic perovskites. The champion PCE of the CsPbI2Br solar cells is drastically increased from 12.31% to 14.93% upon doping with Zn(Ac)2, stemming mainly from the increased open-circuit voltage and fill factor. Furthermore, the non-encapsulated devices with Zn(Ac)2 doped CsPbI2Br show less than 10% efficiency loss after storage in dry air for 30 days.