This article presents a simple and effective method of functionalizing hydrogen-terminated silicon (Si) nanocrystals (NCs) to form a high-quality colloidal Si NC ink with short ligands that allow charge transport in nanocrystal solid films. Si NCs fabricated by laser-pyrolysis and acid etching are passivated with allyl disulfide via ultraviolet (UV)-initiated hydrosilylation to form a stable colloidal Si NC ink. Then a Si NC-based photodiode is directly fabricated in air from this ink. Only a solution-processed poly(3,4-ethylenedioxy-thiophene):poly(styrene sulfonate) (PEDOT: PSS) electron blocking layer and top- and bottom-contacts are needed along with the Si NC layer to construct the device. A Schottky-junction at the interface between the Si NC absorber layer and aluminum (Al) back electrode drives charge separation in the device under illumination. The unpackaged Si NC-based photodiode exhibites a peak photoresponse of 0.02 A W-1 to UV light in air, within an order of magnitude of the response of commercially available gallium phosphide (GaP), gallium nitride (GaN), and silicon carbide (SiC) based photodetectors. This provides a new pathway to large-area, low-cost solution-processed UV photodetectors on flexible substrates and demonstrates the potential of this new silicon nanocrystal ink for broader applications in solution-processed optoelectronics.