In perovskite solar cells, I- tends to oxidize to I2 due to its low redox potential. The generated I2 has been proved to be detrimental to device performance. Herein, for the first time, an amidine DBU is introduced as an additive into the precursor of the perovskite layer. The reductive DBU can suppress the formation of iodine impurity, resulting in a highly pure perovskite polycrystalline film. The efficiency and stability of the perovskite solar cells are thus improved.