The maximum energy conversion efficiency of a single junction solar cell is limited to ~31% (Shockley�Queissar limit)1 due to thermalization loss of the excess energy of hot carriers. To exceed the Shockley-Queissar limit, two approaches for utilizing the excess energy are under active investigation. One is to extract the multiple excitons produced by one absorbed high energy photon (multiple exciton generation, MEG). The other one is to capture the energy of hot excitons before they relax to bandedge via hot carrier extraction. Although both concepts have been demonstrated, the mechanism and the efficiency of these approaches remain poorly understood. In this paper, we report recent studies that are aimed at characterizing and understanding the efficiency of multi-exciton dissociation (MED) and hot electron transfer (HET) from PbS quantum dots. In the study, we demonstrated that, in PbS /Methylene blue (PbS-MB+) model QD/acceptor complexes, the yield of the dissociated exciton to absorbed photon (yield of dissociation) reached 116�2 %. This value was the same as the MEG yield (ratio of photogenerated excitons to absorbed photons) of the free QDs measured under the same experimental condition, suggesting that the MEG efficiency is not affected by the presence of electron acceptor and charged carrier produced by MEG in QDs could be completely extracted. In the second study, we examined hot electron transfer from PbS QDs to TiO2 nanocrystalline thin film, a model system for QD solar cells. The electron transfer rate from QDs to the film was estimated as 6.4�0.4 fs, indicating the probability of extracting multiple excitons or hot electrons in the solar cell based on these materials.2 We also showed that the recombination of electron in TiO2 and holes in QDs could be greatly retarded by coating alumina insulating layers on the TiO2 films, providing a possible way for improving the cell performance. We will discuss ongoing studies that are examining the dependence of charge separation, charge recombination and hot electron transfer yield on the thickness of the insulating layer.  1.         Shockley, W.; Queisser, H. J. J. Appl. Phys. 1961,  32, (3), 510.2.         Yang, Y.; Rodr�guez-C�rdoba, W.; Xiang, X.; Lian, T. Nano Lett. 2011,  12, (1), 303-309.

Efficient multiple exciton dissociation and hot electron extraction from PbS QD

244th National Meeting (2012)

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