Size Dependence of the Multiple Exciton Generation Rate in CdSe Quantum Dots

TL;DR

This study investigates how the size of CdSe quantum dots influences the rate of multiple exciton generation, revealing size-dependent effects on carrier multiplication efficiency and Coulomb interactions.

Contribution

It provides a detailed atomistic analysis of size effects on carrier multiplication rates, considering Coulomb interactions and energy dependence, which contrasts with previous studies.

Findings

Holes have higher multiplication rates than electrons at the same excess energy.

Net carrier multiplication is dominated by photogenerated electrons due to higher excess energy.

Smaller dots show decreased multiplication rate at fixed photon energy but enhanced rate when scaled by optical gap.

Abstract

The multiplication rates of hot carriers in CdSe quantum dots are quantified using an atomistic pseudopotential approach and first order perturbation theory. Both excited holes and electrons are considered, and electron-hole Coulomb interactions are accounted for. We find that holes have much higher multiplication rates than electrons with the same excess energy due to the larger density of final states (positive trions). When electron-hole pairs are generated by photon absorption, however, the net carrier multiplication rate is dominated by photogenerated electrons, because they have on average much higher excess energy. We also find, contrary to earlier studies, that the effective Coulomb coupling governing carrier multiplication is energy dependent. We show that smaller dots result in a decrease in the carrier multiplication rate for a given absolute photon energy. However, if the photon energy is scaled by the volume dependent optical gap, then smaller dots exhibit an enhancement in carrier multiplication for a given relative energy.