This work presents an easy and controllable method for introducing I-3(-) ions into organic cation dripping solution via chemical reaction of I- +I-2 -> I-3(-) to effectively decrease the concentration of defects in organic -inorganic lead halide perovskite films. The perovskite solar cells based on additional I-3(-) ions exhibited better photovoltaic performance, with a maximum power conversion efficiency of 18.7%, being about 15% higher than that of the control device. The elucidation of defects and its suppression by employing additional I-3(-) ions provide an important direction for defects elimination and future enhancement of perovskite solar cells efficiency.