Considering that Si-based materials are easily mixed with Fe impurities and Cu impurities during the fabrication process, we establish Si models with two different impurities according to the first-principles and photoelectric response theory. The effects of impurity atoms at different interstitial sites on the energy band structure and response characteristics of Si are further compared. Results show that the mixing of the two impurities can lead to changes in the energy band structure of the silicon material, resulting in an out-of-band response and a decrease in the saturation threshold of the photosensitive unit. Specifically, when the Fe impurity occupies the tetrahedral interstitial site, the energy band structure of silicon is significantly affected, and its band gap is reduced to 0.013 eV, resulting in an out-of-band absorption peak at about 1 560 nm. The Cu impurity has an obvious effect on silicon material at the hexagonal inter-stitial site, so that the band gap disappears, and an out-of-band absorption peak appears at about 1 700 nm. In these two cases, the saturation threshold of the silicon- based photosensitive unit also decreases most significantly. When irradiated by a 1 550 nm laser, the saturation thresholds are 0.001 65 W·cm-2 and 0.002 54 W·cm-2, respectively. The analysis results provide reference for the application and development of optoelectronic devices.