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Monday, November 06, 2006
3:00 PM - 4:00 PM
CNLS Conference Room


Exciton-Exciton Interactions in Semiconductor Nanocrystals from the Perspective of Solar-Energy Conversion

Victor Klimov

The efficient conversion of photon energy into electrical charges is a central goal of much research in physics, chemistry, and biology. A usual assumption is that absorption of a single photon by a material produces a single electron-hole pair (exciton), while the photon energy in excess of the energy gap is dissipated as heat. In 2004, we reported for the first time that nanocrystals (NCs) of PbSe could respond to absorption of a single photon by producing two or more excitons with the unity probability (Phys. Rev. Lett. 92, 186601, 2004). This presentation reviews our recent follow-up work that addresses such issues as the generality and the mechanism of this carrier multiplication phenomenon, the limits on photon-to-exciton conversion efficiencies, and implications of carrier multiplication in photovoltaics and photocatalysis.

One remarkable feature of carrier multiplication in NCs is that it can produce multiple charges with quantum efficiencies that correspond to the ultimate limit dictated by energy conservation. For example, for photon energy of 7.8 energy gaps, a maximal possible number of photogenerated excitons based on energy conservation is 7, which is exactly the number measured in our experiments (Nano Lett. 6, 424, 2006). In this case, 90% of the photon energy is used to produce a useful effect (multiple charges) and only 10% is lost as heat. In the normal scenario (one exciton per photon), 90% of the photon energy would be dissipated as heat. Another unexpected feature of carrier multiplication is that it results in unusual distributions of carrier populations that cannot be described by Poissonian statistics. Specifically, by selecting certain photon energies, we obtain photoexcited NC ensembles with nearly pure single multiplicities (i.e., all excited NCs contain the same number of excitons) that can be tuned in the controlled way from 1 to 7 (Phys. Rev. Lett. 96, 097402, 2006).

While the exact mechanism for carrier multiplication in NCs is still under debate, one factor, which certainly contributes to high efficiencies for this process, is a unique property of NCs to produce significant exciton-exciton interactions. We believe that because of these strong interactions a high-energy exciton initially excited by a photon only exists in its “virtual,” short-lived state, which rapidly decays to produce a more stable state that comprises two or more excitons (Nature Phys. 1, 189, 2005).

Based on measured carrier-multiplication efficiencies, we project that a power-conversion limit of a single-junction solar cell can be increased up to ~42% (Appl. Phys. Lett. 89, 123118, 2006), which is roughly a 40% improvement compared to the situation without multiple-exciton generation. Carrier multiplication can also significantly improve the performance of photocatalytic structures particularly in reactions that involve multiple reduction/oxidation steps such as water splitting. Other potential applications include nonlinear optics, lasing, and quantum information.

Host: Andrei Piryatinski