  In conventional solar cell semiconductor materials (predominantly Si) photons
with energy higher than the band gap initially generate hot electrons and
holes, which subsequently cool down to the band edge by phonon emission. Due to
the latter process, the energy of the charge carriers in excess of the band gap
is lost as heat and does not contribute to the conversion of solar to
electrical power. If the excess energy is more than the band gap it can in
principle be utilized through a process known as carrier multiplication (CM) in
which a single absorbed photon generates two (or more) pairs of electrons and
holes. Thus, through CM the photon energy above twice the band gap enhances the
photocurrent of a solar cell. In this review, we discuss recent progress in CM
research in terms of fundamental understanding, emergence of new materials for
efficient CM, and CM based solar cell applications. Based on our current
understanding, the CM threshold can get close to the minimal value of twice the
band gap in materials where a photon induces an asymmetric electronic
transition from a deeper valence band or to a higher conduction band. In
addition, the material must have a low exciton binding energy and high charge
carrier mobility, so that photoexcitation leads directly to the formation of
free charges that can readily be extracted at external electrodes of a
photovoltaic device. Percolative networks of coupled PbSe quantum dots, Sn/Pb
based halide perovskites, and transition metal dichalcogenides such as MoTe2
fulfill these requirements to a large extent. These findings point towards
promising prospects for further development of new materials for highly
efficient photovoltaics.
