Although the efficiency of perovskite solar cells is over 25%, there is still a big difference between the open-circuit voltage and its theoretical value, which is ascribed to defects in the perovskite films. Here, 4-iodo-1H-imidazole is introduced into a perovskite precursor to prevent defect formation in final films. The open-circuit voltage is significantly elevated to 1.209 V from 1.085 V. 4-Iodo-1H-imidazole, as a kind of small organic molecule, can not only reduce pin-holes in perovskite films by participating in the growth of perovskite crystals, but also suppress the generation of defects, resulting in decreased nonradiative recombination. Finally, the device efficiency becomes 21.68%, which is increased by 20% in comparison with that of the control device (17.87%).