Multiexciton absorption and multiple exciton generation in CdSe quantum dots

Efficient multiple-exciton generation (MEG) in semiconductor quantum dots has been recently reported. The MEG efficiency has so far been evaluated assuming that the change (bleaching) of the absorption spectrum due to MEG is linearly proportional to the number of excitons N-X. Here, we critically examine this assumption using atomistic pseudopotential calculations for colloidal CdSe nanocrystals. We find that the bleaching of the first absorption peak depends nonlinearly on N-X, due to carrier-carrier interactions. This nonlinearity mandates an upper bound of 1.5 to the value of the normalized bleaching that can be attributed to MEG, significantly smaller than the limit of 2.0 predicted by the linear scaling assumption. Thus, measured values of the normalized bleaching in excess of 1.5 cannot be due entirely to MEG, but must originate in part from other mechanisms.