  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.
