We discuss the photovoltaic effect at a p-n heterojunction, in which the illuminated side is a doped Mott insulator, using the simplest description of a Mott insulator within the Hubbard model. We find that the internal quantum efficiency of such a device, if we choose an appropriate narrow-gap Mott insulator, can be significantly enhanced due to impact ionization caused by the photoexcited ``hot'' electron/hole pairs. Namely, the photoexcited electron and/or hole can convert its excess energy beyond the Mott-Hubbard gap to additional electrical energy by creating multiple electron/hole pairs in a time scale which can be shorter than the time characterizing other relaxation processes.