Self-powered, all-inorganic photodetectors (PDs) have attracted ever-increasing research attention recently, because of their upgraded stability, promising performance, and free of external power supply. However, most of the inorganic perovskite films used for such PDs are prepared by solution methods, for which the crystal sizes vary significantly among different areas. This issue is more serious among different samples and eventually stands a substantial hindrance to PDs' applications. In this paper, we introduced an amount of polyethyleneimine (PEI) species into the CsPbIBr2 precursor, which enabled a thin film with the homogeneous grain growth. Moreover, with the PEI modification, the conduction band energy level of resulting CsPbIBr2 film was also reduced, which leads to a more favored interfacial energy alignment between it and the TiO2 electron transporting layer (ETL). Consequently, the champion PD yielded the dark current of 3.12 x 10(-10) A, the peak responsivity (R) of 0.20 A/W, the maximum specific detectivity (D*) of 6.01 x 10(12) Jones, and the response time of 2.07 mu s, which are far beyond those of the one prepared without PEI additive.