Carrier multiplication yields in PbS and PbSe nanocrystals measured by transient photoluminescence

TL;DR

This study measures carrier multiplication yields in PbS and PbSe nanocrystals using transient photoluminescence, finding lower efficiencies than previously reported and questioning the significance of nanoscale effects in CM.

Contribution

It provides a quantitative analysis of biexciton and exciton dynamics in nanocrystals, challenging prior claims of high CM efficiencies in such materials.

Findings

Maximum of 0.25 e-h pairs generated per photon at >5Eg

Previous literature reports higher CM efficiencies that are not supported by this study

Data suggests nanoscale-specific phenomena may not significantly enhance CM

Abstract

We report here an assessment of carrier multiplication (CM) yields in PbSe and PbS nanocrystals (NCs) by a quantitative analysis of biexciton and exciton dynamics in transient photoluminescence decays. Interest in CM, the generation of more than one electron and hole in a semiconductor after absorption of one photon, has renewed in recent years because of reports suggesting greatly increased efficiencies in nanocrystalline materials compared to the bulk form, in which CM was otherwise too weak to be of consequence in photovoltaic energy conversion devices. In our PbSe and PbS NC samples, however, we estimate using transient photoluminescence that at most 0.25 additional e-h pairs are generated per photon even at energies hv > 5Eg, instead of the much higher values reported in the literature. We argue by comparing NC CM estimates and reported bulk values on an absolute energy basis, which we justify as appropriate on physical grounds, that the data reported thus far are inconclusive with respect to the importance of nanoscale-specific phenomena in the CM process.