It only takes a few picoseconds (10
−12 s) for the hot charge carriers to relax back to
the edges of the electronic bands. The idea of hot carrier cells is to collect the charge
carriers as long as they are still hot. Hence, an energy larger than the bandgap energy can
be utilized per excited charge carrier and the average bandgap utilization would exceed the
bandgap.
The fundamental challenge is to collect the hot carriers before they relax back to the
edges of the electronic bands. Such a concept would require selective contacts, which only
select electrons above a particular energy level in the conduction band and holes below a
certain energy level in the valence band. At the moment the main challenge is to increase
the lifetime of the hot charge carriers, such that they have time to move from the absorber
material to the selective contacts [134].