All-inorganic perovskite cesium lead iodine (CsPbI₃) without any volatile organic components has attracted much attention due to its superior stability, high absorption efficiency and suitable band gap. However, the power-conversion efficiencies of CsPbI₃ based perovskite solar cells (PSCs) are substantially low compared with those of the organic-inorganic hybrid lead halide PSCs. The surface passivation of the CsPbI₃ film by long-chain halide salts has been found to be an effective method of improving the performance. In this paper, we report the concentration effect of an inexpensive 2-bromoterephthalic acid (BBr) as passivation material on the performance of CsPbI₃ perovskite solar cells. The experimental results show that the conversion efficiency of perovskite solar cells first increases and then decreases as the concentration of BBr increases from 0 to 2 mg/mL. The best conversion efficiency of CsPbI₃ perovskite solar cells reaches 13.5% at 0.2 mg/mL BBr. The results from X-ray diffraction and scanning electron microscopy suggest that there is no change in the phase or microstructure of the CsPbI₃ perovskite film after surface passivation by BBr. By further analyzing the photoluminescence data of the CsPbI₃ film with and without capping hole transport layer, it can be found that the passivation of BBr with the concentration of 0.2 mg/mL can enhance the fluorescence excitation intensity of the CsPbI₃ film and accelerate the exciton separation at the interface between CsPbI₃ film and hole transport layer. Based on the electrochemical impedance spectroscopy data, we find that the electron transport ability at the interface between TiO₂ and CsPbI₃ can be significantly improved after surface passivation, which is induced by the acceleration of the exciton separation at the interface between CsPbI₃ film and hole transport layer. The decrease of the PSCs performance when the concentration of the BBr precursor increases from 0.5 mg/mL to 2 mg/mL can be attributed to the local agglomeration of the BBr material, resulting in the block of charge transportation. This research is expected to provide basic support for the low-cost development of the passivation materials for perovskite solar cells.