Emergence of New Materials for Exploiting Highly Efficient Carrier Multiplication in Photovoltaics

In conventional solar cell semiconductor materials (predominantlySi)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 itcan 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 abovetwice the band gap enhancesthe photocurrentofa solar cell. In this review, we discuss recent progress in CM research in terms of fundamental understanding, emergenceof 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 MoTe2fulfill these requirements to a large extent. These findings pointtowards promising prospects for further development of new materials for highly efficient photovoltaics.