Past Event: Oden Institute Seminar

Distribution of carrier multiplication rates in nanocrystals

Eran Rabani, Professor and Director, Raymond and Beverly Sackler Institute of Chemical Physics, Tel Aviv University

Abstract

Multiexciton generation (MEG) is a process where several excitons are generated upon the absorption of a single photon in semiconductors. This process enjoys great technological ramifications for solar cells and other light harvesting technologies. For example, it is expected that the more charge carriers created shortly after the photon is absorbed, the larger fraction of the photon energy can successfully be converted into electricity, thus increasing the device efficiency. Strict selection rules and competing processes in the bulk allows MEG at energies of five times the band gap. It was suggested that nanocrystals, where quantum confinement effects are important, may exhibit MEG at lower values of (typically 2 to 3 times the band gap). Indeed, MEG in NCs has been reported recently for several systems, showing that the threshold was size and band-gap independent. However, more recent studies have questioned the efficiency of MEG in nanocrystals, in particular for CdSe and InAs. The goal of the present talk is to address this controversy. We present a theoretical study of the problem, where the rates of MEG following photon absorption is calculated for semiconductor nanocrystals using Fermi's golden rule with all relevant Coulomb matrix elements, taking into account proper selection rules within a screened semiempirical pseudopotential approach. In CdSe and InAs nanocrystals we find a broad distribution of biexciton generation rates depending strongly on the exciton energy and size of the nanocrystal. We find that the process becomes inefficient for nanocrystals exceeding 3 nm in diameter in the photon energy range of 2-3 times the band gap.