Perovskite single crystals (PSCs) are considered the next breakthrough in optoelectronics research due to their free-grain boundary and much lower density of trap states compared to those of their polycrystalline counterparts. However, the inevitable formation of triiodide-based intrinsic defects during high-temperature crystal growth is one of the major challenges impeding the further development of optoelectronic devices based on PSCs. Here, we not only identified the existence of these triiodide ions as hole-trapping centers and their tremendous negative impact on the performance of PSCs, but more importantly, we used a reduction treatment to prevent their formation during crystal growth. The removal of such defect centers resulted in much higher charge carrier mobility and longer carrier lifetime than the untreated counterparts, leading to enhanced photodetection properties. The I3–-free MAPbI3 single crystal (MSC) devices consistently generated a more than 100 times higher photocurrent than that generated by I3–-rich devices under the same light intensity.