  Employing the interband exciton scattering model, we have performed a
numerical study of the direct photogeneration and population relaxation
processes contributing to carrier multiplication (CM) in nanocrystalline and
bulk PbSe. We argue that in both cases the impact ionization is the main
mechanisms of CM. This explains weak contribution of the direct photogeneration
to the total quantum efficiency (QE). Investigated size scaling of QE in NCs
and comparison to the bulk limit provide microscopic insight in the
experimentally observed trends.
