Direct Visualization and Determination of the Multiple Exciton Generation Rate.

Multiple exciton generation (MEG) takes place in competition to other hot carrier cooling processes. While the determination of carrier cooling rates is well established, direct information on MEG dynamics has been lacking. Here, we present a methodology to obtain the MEG rate directly in the initial ultrafast transient absorption dynamics. This method is most effective to systems with slow carrier cooling rates. Perovskite quantum dots exhibit this property and are used to illustrate this approach. They show a delayed carrier concentration buildup following an excitation pulse above the MEG threshold energy, which is accompanied by a faster carrier relaxation, providing a direct evidence of the MEG process. Numerical modeling within a simple framework of two competing cooling mechanisms allows us to extract the MEG rate and carrier energy cooling rates for this material. The presented methodology could provide new insights in carrier generation physics and valuable information for MEG investigations.