Biosensing applications are driving an increase in demand for low-cost photodetectors with sensitivity between 1000 and 1400 nm. Recently, the sensitivity of Ag₂Se colloidal quantum dot based devices in this wavelength range has been observed. In this work we make the first demonstration of an Ag₂Se photodiode device with sensitivity in this entire range. By employing secondary phosphine to elevate the precursor reactivity, we achieved accurate size control of the Ag₂Se nanocrystals with a distinct excitonic absorption peak. These nanocrystals were deposited from solution into a mesoporous TiO₂ scaffold, similar to that used in dye-sensitized solar cells, to increase the light absorption and charge separation and reduce the exciton diffusion length. By incorporation of a suitable hole-transporting layer between the active layer and Ag anode, the resulting devices showed a responsivity of 4.17 mA/W at a wavelength of 1200 nm. These results demonstrate that Ag₂Se colloidal quantum dots offer a low-toxicity route for low-cost fabrication of near-infrared photodetectors.