Center for Functional Nanomaterials Seminar

Carrier multiplication (CM) is a phenomenon in which one absorbed photon gives rise to multiple electron-hole pairs. Harnessing this phenomenon with high efficiencies could yield dramatic improvements in the efficiency of photovoltaic cells. Unfortunately, the efficiency of CM in bulk semiconductors is generally low at photon energies at which the solar spectral density is high. In the last few years there have been numerous reports of enhanced CM efficiencies in semiconductor nanocrystal quantum dots (NQDs).1,2 However, a recent controversy has emerged due to discrepancies between the reported NQD CM efficiencies from different research groups studying NQD samples that are nominally the same (i.e., same chemical composition and size).3 This has raised questions about the sources of the discrepancies and suggestions of differing sensitivities of different techniques to CM or sample-to-sample variations of unknown cause. We have applied both ultrafast transient absorption and photoluminescence upconversion spectroscopy to study CM in PbSe NQDs across a range of sizes and under varying conditions (i.e., static versus stirred solutions and high- versus low-repetition-rate excitation). The two techniques yield equivalent values of the CM quantum efficiency. However, our investigations of different measurement conditions suggest a resolution of the controversy in the important role of competing processes such as NQD ionization.4 If time permits I shall also discuss temporal constraints on making use of multiexcitons in NQDs due to the rapid scaling with exciton number of Auger recombination.

1 R. D. Schaller and V. I. Klimov, Phys. Rev. Lett. 92, 186601 (2004).
2 R. J. Ellingson, M. C. Beard, J. C. Johnson et al., Nano Lett. 5, 865 (2005).
3 G. Nair, S. M. Geyer, L.-Y. Chang et al., Phys. Rev. B 78, 125325 (2008).
4 J. A. McGuire, J. Joo, J. M. Pietryga et al., Accs. Chem. Res., accepted (2008).