Flowing versus Static Conditions for Measuring Multiple Exciton Generation in PbSe Quantum Dots

Abstract

Recent reports question the validity of pulsed fs-laser experiments for measuring the photon-to-exciton quantum yields (QYs) that result from multiple exciton generation (MEG). The repetitive nature of these experiments opens up an alternative relaxation pathway that may produce artificially high results. We present transient-absorption (TA) data for 4.6 and 6.6 nm diameter PbSe quantum dots (QDs) at a variety of pump photon energies. The data are collected under laminar flow conditions with volumetric flow rates ranging from 0 to 150 mL/min (resulting in Reynolds numbers up to 460). The results are modeled with a spatially resolved population balance of generation, recombination, convective replacement, and accumulation of long-lived excited QDs. By comparing the simulations and experiments, the steady-state population of the long-lived QD-excited states and their kinetics are determined for different experimental conditions. We also improve upon reported photon-to-exciton QYs for PbSe QDs. We find differences in the observed TA dynamics between flowing and static conditions that depend upon photon fluence, pump photon energy, and quality of the QD surfaces. For excitation energies below 2 E g, independent of QD size or photon fluence, we observe no flow rate dependence in the TA dynamics. At excitation energies of hν > 3 E g, we observe differences between static and flowing conditions that are most pronounced for high photon fluences. At 3.7 E g and for 4.6 nm PbSe QDs we find a QY of 1.2 ± 0.1 and at 4.5 E g the QY is 1.55 ± 0.05. With 6.6 nm QDs excited at 4.7 E g we observe no difference between static and flowing conditions and find a QY of 1.61 ± 0.05. We also find that by treating the surface of QDs, we can decrease the charging probability (P g ≈ 5 × 10−5) by a factor of 3−4. The observed variations suggest that different QD samples vary regarding their susceptibility to the creation of long-lived states.

Multiple Exciton Generation in Nanostructures for Advanced Photovoltaic Cells

“…Here, s is found to be around 2.2 for lead-based QDs. e study by Midgett et al [28] and Mihaylov et al [38] in agreement with previous relevant literature also reports on properties for a variety of PbS x Se 1−x alloy QDs, with varying composition and radius, allowing one to draw conclusions about the size dependence of η for the di erent compositions of QDs ( Figure 6). It is suggested that higher MEG might be achievable in nonspherical crystal structures, although more data need to be collected to explore such con gurations.…”

“…is gives a deceptive MEG-like signature in the spectroscopic data. Subsequently, it was found that this effect could be nullified simply by stirring the sample solution [28]. Another hurdle in the assessment of QYs for specific structures is clearly the large number of variables involved and relevant to these effects, such as the size, composition, shape, and method of observation.…”

Absolute femtosecond measurements of Auger recombination dynamics in lead sulfide quantum dots

“…This is an advantage of exciting hot carriers that interact only weakly; these assumptions of the Auger analysis do not hold for band edge excitation of excitons. A likely explanation for the discrepancies in Figure 2 is charge ejection from the dot into a surface or ligand trap [6,7]. Charge separated dots have an emission lifetime of ~300 ps; the signal at 1 ns can be explained by assuming charge separation in ~4/5 of the dots at <Neh> = 10.…”

Multiple exciton generation in quantum dot-based solar cells

“…Whilst MEG has been detected in a number of materials, including silicon [57][58][59], most studies focused on QDs consisting of lead chalcogenides (PbS, PbSe and PbTe) [26,41,[60][61][62]. The large Bohr radius of lead chalcogenides, relative to, for example, silicon or GaAs [59,63,64], allows strong quantum confinement effects and efficient tuning of the confinement to produce a band gap in the near-IR region (0.7-1.0 eV) [65].…”

“…Such effects have led to difficulties in measuring MEG rates: a trapped charge may stay on a QD, which upon re-excitation by a subsequent pulse, may form a trion species that can be mistakenly interpreted as an MEG signal thereby complicating spectroscopic studies [43]. In solution-based measurements stirring the sample or the use of a flow cell prevents the same set of QDs being excited by consecutive pulses [61]. In films, however, it remains challenging to isolate effects that influence the MEG process itself, its measurement or the extraction of the generated charges.…”