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High-efficiency carrier multiplication through direct photogeneration of multi-excitons via virtual single-exciton states


We have previously demonstrated that absorption of a single photon by a nanocrystal quantum dot can generate multiple excitons with an efficiency of up to 100%. This effect, known as carrier multiplication, should lead to substantial improvements in the performance of a variety of optoelectronic and photocatalytic devices, including solar cells, low-threshold lasers and entangled photon sources. Here we present detailed analysis of the dynamics that govern the ultrafast growth of multi-exciton populations in CdSe and PbSe nanocrystals and propose a model of how such populations arise. Our analysis indicates that the generation of multi-excitons in these systems takes less than 200 fs, which suggests that it is an instantaneous event. We explain this in terms of their direct photogeneration via multiple virtual single-exciton states. This process relies on both the confinement-enhanced Coulomb coupling between single excitons and multi-excitons and the large spectral density of high-energy single- and multi-exciton resonances that occur in semiconductor nanocrystals.

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Figure 1: Evolution of the electron–hole system when photoexcited in the regime of CM.
Figure 2: Buildup dynamics of the lowest 1S electron state in 3.2-nm-radius CdSe nanocrystals with and without CM (the same sample as in Fig. 1b).
Figure 3: Buildup dynamics of the lowest 1S electron state in 3.9-nm-radius PbSe nanocrystals (E g=0.64 eV) with and without CM.
Figure 4: Direct photoexcitation of bi-excitons via virtual single-exciton states.

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This work was supported by the Chemical Sciences, Biosciences and Geosciences Division of the Office of Basic Energy Sciences, Office of Science, US Department of Energy and Los Alamos LDRD funds. R.D.S. is supported by a Frederick Reines Fellowship. We thank J. M. Pietryga and M. A. Petruska for fabrication of nanocrystal samples.

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Correspondence to Victor I. Klimov.

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Schaller, R., Agranovich, V. & Klimov, V. High-efficiency carrier multiplication through direct photogeneration of multi-excitons via virtual single-exciton states. Nature Phys 1, 189–194 (2005).

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