Many-body processes in the photophysics of colloidal semiconductor nanocrystals

Abstract

In this work we have experimentally studied several aspects of two Coulomb processes that change the number of electrons and holes in colloidal semiconductor nanocrystals (NCs). Carrier Multiplication (CM) is the production of additional electron-hole pairs by collision of a highly excited carrier with valence electrons. Efficient CM would improve the performance of solar energy conversion devices, but it is weak in the bulk. Recent reports by several groups suggested highly efficient CM in semi-conductor NCs. We describe here our assessment of CM using transient photoluminescence in CdSe and lead chalcogenide NCs. Biexciton radiative and nonradiative rates were determined. In our study, no detectable CM was found in CdSe NCs photoexcited at a photon energy of up to 5.9 eV, and the CM yields observed for PbSe NCs at 3.1 eV were found consistent with bulk values. Reasons for the strong disagreement with prior measurements are discussed, and the low yields are theoretically accounted for. The second part of the thesis describes two studies of the "Auger" nonradiative recombination process whereby an electron-hole pair recombines while transferring its energy to a third particle. This mechanism is responsible for the short multiexciton lifetimes in NCs. In one study, we demonstrate a direct method for determining biexciton quantum yields in single nanocrystals by photon cross-correlation (antibunching) measurements. We find significant inhomogeneity in these values, indicating a previously obscured variation in Auger recombination rates.

(cont.) Another set of experiments tests the conventional charging model of NC fluorescence intermittency ("blinking") which attributes off-state quenching to Auger decay, by studying single NCs with relatively long multiexciton Auger lifetimes. We find that off-state exciton quantum yields are significantly lower than the quantum yield of a biexciton and we demonstrate that multiexciton emission also shows strong intermittency. Both of these findings contradict the standard charging model. Alternatives are discussed.