Carrier multiplication (CM) or multiexciton generation (MEG) is a process whereby absorption of a single photon produces multiple electron-hole pairs (excitons). CM could benefit a number of solar-energy conversion technologies, most notably photocatalysis and photovoltaics. This presentation overviews recent progress in understanding the CM process in semiconductor nanocrystals, motivated by an outstanding challenge in this field - the lack of capability to quickly discern between candidate nanomaterials for enhanced CM performance. We present a possible solution to this problem by showing that, using measured biexciton Auger lifetimes and intraband relaxation rates as surrogates for, respectively, CM time constants and non-CM energy-loss rates, we can rationalize relative changes in CM yields as a function of composition. Indeed, by studying PbS, PbSe, and PbTe NCs for a variety of sizes we determine that the significant difference in CM yields for these compounds comes from the dissimilarities in their non-CM relaxation channels, i.e., the processes that compete with CM. We further explore the role of nanostructure shape in the CM process. We observe that via a moderate elongation (aspect ratio of 6�7) of PbSe NCs we can obtain up to an approximately two-fold increase in the multiexciton yield compared to spherical nanoparticles. Beyond this appreciable improvement in CM, increased Auger lifetimes and improved charge transport properties (generally associated with elongated nanostructures) suggest that lead chalcogenide nanorods are a promising system for testing CM concepts in practical photovoltaics. Finally, we discuss how this newly developed understanding can help in the development of tailored nanostructures with CM performance approaching that for the energy-conservation defined limit.

New insights into carrier multiplication in semiconductor nanocrystals: Influence of size, shape, and composition

245th National Meeting (2013)

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