The generation of e-h pairs in silicon by incident light is the underlying principle behind most silicon-based photodetection, imaging, photometry, spectroscopy and photovoltaic technologies. In this work, we show that graphene provides voltage-tunable ways in which the photogenerated carriers in Si can be captured. Combining the photoexcitation in Si with the carrier capturing abilities of graphene results in photodetectors which not only respond with high quantum efficiency values, but, more interestingly, devices whose photocurrent responsivity can be completely tuned using an external voltage. Such tunability is quite useful for detection in variable light conditions, which makes it attractive for imaging and videographic applications, especially because they respond within milliseconds of the incidence. Layer thickening and doping can further enhance the absolute responsivity values, and devices with a few hundred mA/W and quantum efficiency up to ~65% could be fabricated. Most importantly though, another mode of photodetection, using photovoltage, is found to be incredibly sensitive to ultra-low intensity light, with photovoltage responsivity as high as 107 V/W and contrast sensitivity exceeding 106 V/W. This current-free method of detection is able to detect extremely low-absorbing materials, and coupled with scanning-photovoltage measurements, can give rise promising new ways of photodetection and imaging.