The Dynamics of Multiple Exciton Generation in Semiconductor Quantum Dots

Abstract

In this paper, the dynamics of multiple exciton generation (MEG) in colloidal PbS quantum dots (QDs) characterized using a transient grating (TG) technique are reported. Only one peak, soon after the absorption of light, can be observed in the TG kinetics when the photon energy of the pump light, hν, is smaller than 2.7E g (E g: the band gap between the LUMO and HOMO in the QDs). When hν is greater than 2.7E g, however, after the initial peak, the TG signal first decreases and then (after as little as 200 fs) increases, and a new peak appears at about 3 ps. The second peak intensity becomes larger with increasing hν. The first peak is considered to be related to hot carriers and the second one is believed to result from MEG. We found that MEG begins after just 200 fs and that the carrier density due to MEG increases and finally saturates after about 2 ps. Then, following the second peak, a fast decay, due to Auger recombination of the generated multiexcitons with a lifetime of about 30 ps, together with a slow decay, due to single exciton relaxation with a lifetime longer than nanoseconds, is observed in the TG kinetics. We succeeded in directly observing the occurrence and dynamics of MEG in semiconductor QDs using the TG technique.