Abstract
In conventional solar cell semiconductor materials, the excess energy of electrons and holes beyond the bandgap is wasted as heat, because they cool down to the band edge due to phonon emission. If the excess energy is more than the bandgap, it can in principle be utilized through a process known as carrier multiplication (CM) in which a single photon generates two (or more) electron-hole pairs. In this way, CM can enhance the photocurrent of a photovoltaic device. We provide an overview of experimental and theoretical methods used to study CM. Next, we consider the effects of composition and nanostructure of materials, on the threshold photon energy and efficiency of CM. Results for percolative networks of coupled PbSe quantum dots, Sn/Pb based halide perovskites, and two-dimensional transition metal dichalcogenides such as MoTe2 are discussed. Based on our current understanding, the CM threshold can get close to the minimal value of twice the bandgap in materials where a photon induces an asymmetric electronic transition from a deeper valence band or to a higher conduction band. We then address the effects of the exciton binding energy and charge carrier mobility on the photogeneration of free charges and their extraction at external electrodes of a photovoltaic device. Finally, we discuss future directions toward the development of new materials to realize a low threshold photon energy and high efficiency of CM.
Highlights
(b) band and create an electron-hole pair
If the excess energy is more than the bandgap, it can in principle be utilized through a process known as carrier multiplication (CM) in which a single photon generates two electron-hole pairs
We address the effects of the exciton binding energy and charge carrier mobility on the photogeneration of free charges and their extraction at external electrodes of a photovoltaic device
Summary
During the process of CM, a hot charge carrier with energy exceeding the bandgap (either an electron in a conduction band or a hole in a valence band) relaxes by excitation of a valence band electron. Soon after the work of Nozik, Schaller et al reported experimental observation of efficient CM in PbSe QDs.[3] controversy arose with opposing results of the efficiency of CM in QDs from different laboratories.[32,56–60] Careful experimental procedures designed to avoid artifacts have shown the CM efficiency to be lower than the initial results in QDs, but still of promise for solar cell applications.[9,16,61] Later on the research of CM was extended to 1-D nanorods, 2-D nanosheets, complex heterostructures, and assemblies showing both a decrease in the CM threshold and an increase in the QY, see Sec. VI of this paper.
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