We have determined the Auger recombination kinetics of electrons and holes in colloidal CdSe-only and CdSe/CdS/ZnS core/shell nanoplatelets by time-resolved photoluminescence measurements.(1) At high excitation density Auger recombination can be described by second-order kinetics. From this we infer that the majority of electrons and holes are bound in the form of neutral excitons, while the fraction of free charges is much smaller, in agreement with our terahertz conductivity measurements. The biexciton Auger recombination is not diffusion-controlled and is more than one order of magnitude smaller than for quantum dots and nanorods of equal volume. The latter is of advantage for application in lasers, light-emitting diodes and photovoltaics.

The generation of two or more electron-hole pairs for the absorption of a single energetic photon is of interest for development of highly efficient (up to 44%) solar cells. The efficiency of this carrier multiplication (CM) process depends on several factors, including the competition with cooling, the Coulomb interaction between the hot charge carrier and the final trion density of states. All these factors depend on nanocrystal dimensionality. Previously carrier multiplication has been investigated in lead chalcogenide quantum dots (0D), nanorods (1D) and bulk (3D).

We investigated the efficiency of carrier multiplication in two-dimensional PbS nanosheets of 4 to 7 nm thickness using ultrafast optical pump-probe spectroscopy.(2) The efficiency of carrier multiplication in nanosheets is much higher than for quantum dots, nanorods and bulk material. in thin PbS nanosheets virtually the entire excess photon energy above the CM threshold is used for CM, in contrast to quantum dots, nanorods and bulk lead chalcogenide materials.

References:

(1) Kunneman, L. T.; Tessier, M. D.; Heuclin, H.; Dubertret, B.; Aulin, Y. V.; Grozema, F. C.; Schins, J. M.; Siebbeles, L. D. A. J. Phys. Chem. Lett. 2013, 4, 3574.
(2) Aerts, M.; Bielewicz, T.; Klinke, C.; Grozema, F. C.; Houtepen, A. J.; Schins, J. M.; Siebbeles, L. D. A. Nature Commun. 2014, 5, 3789.

Laurens Siebbeles

Delft University of Technology

Appearances