Size and Composition Dependent Multiple Exciton Generation Efficiency in PbS, PbSe, and PbS_xSe_1–x Alloyed Quantum Dots

Abstract

Using ultrafast transient absorption and time-resolved photoluminescence spectroscopies, we studied multiple exciton generation (MEG) in quantum dots (QDs) consisting of either PbSe, PbS, or a PbSxSe1-x alloy for various QD diameters with corresponding bandgaps (Eg) ranging from 0.6 to 1 eV. For each QD sample, we determine the MEG efficiency, ηMEG, defined in terms of the electron-hole pair creation energy (εeh) such that ηMEG = Eg/εeh. In previous reports, we found that ηMEG is about two times greater in PbSe QDs compared to bulk PbSe, however, little could be said about the QD-size dependence of MEG. In this study, we find for both PbS and PbSxSe1-x alloyed QDs that ηMEG decreases lineally with increasing QD diameter within the strong confinement regime. When the QD radius is normalized by a material-dependent characteristic radius, defined as the radius at which the electron-hole Coulomb and confinement energies are equivalent, PbSe, PbS, and PbSxSe1-x exhibit similar MEG behaviors. Our results suggest that MEG increases with quantum confinement, and we discuss the interplay between a size-dependent MEG rate versus hot exciton cooling.

Carrier multiplication in germanium nanocrystals

“…Therefore, the stronger the confinement, the more efficient is the CM process. 19 Moreover, apart from widening the electronic bandgap (in silicon scaling with the square of the diameter 20 ), the same quantum confinement relaxes the momentum conservation requirement in optical transitions through Heisenberg's uncertainty relation, thus considerably enhancing the probability of band-to-band transitions for indirect bandgap materials (such as silicon and germanium), lowering the radiative recombination time constant down to the 10-ms range. This relaxation of momentum conservation also enhances CM and Auger recombination.…”

Multiple exciton generation in quantum dot-based solar cells

“…The consequence of both effects is that, in QDs, a significant proportion of generated charge carriers will get trapped at surface sites that are energetically removed from the semiconductor's charge transport level [72]. This effect is exacerbated for MEG devices due to the requirement for strong confinement [73,74]. It is generally accepted that the timescale for relaxing carriers into trap states does not compete with MEG itself (trap filling between 100 s of ps to microseconds depending on QD size and material) [75,76].…”

Advancing colloidal quantum dot photovoltaic technology

“…To further understand and increase the MEG efficiency, studies have been done on nanocrystals with various shapes, sizes and surface ligands [253,[257][258][259]. Related studies on measuring hot electron cooling rates using transient absorption spectroscopy have also been conducted [260].…”