New Aspects of Carrier Multiplication in Semiconductor Nanocrystals

Abstract

One consequence of strong spatial confinement of electronic wave functions in semiconductor nanocrystals (NCs) is a significant enhancement in carrier-carrier Coulomb interactions. This effect leads to a number of novel physical phenomena including ultrafast decay of multiple electron-hole pairs (multiexcitons) by Auger recombination and high-efficiency generation of mutiexcitons by single photons via carrier multiplication (CM). Significant recent interest in multiexciton phenomena in NCs has been stimulated by studies of NC lasing, as well as potential applications of CM in solar-energy conversion. The focus of this Account is on CM. In this process, the kinetic energy of a "hot" electron (or a "hot" hole) does not dissipate as heat but is, instead, transferred via the Coulomb interaction to the valence-band electron, exciting it across the energy gap. Because of restrictions imposed by energy and translational-momentum conservation, as well as rapid energy loss due to phonon emission, CM is inefficient in bulk semiconductors, particularly at energies relevant to solar energy conversion. On the other hand, the CM efficiency can potentially be enhanced in zero-dimensional NCs because of factors such as a wide separation between discrete electronic states, which inhibits phonon emission ("phonon bottleneck"), enhanced Coulomb interactions, and relaxation in translational-momentum conservation. Here, we investigate CM in PbSe NCs by applying time-resolved photoluminescence and transient absorption. Both techniques show clear signatures of CM with efficiencies that are in good agreement with each other. NCs of the same energy gap show moderate batch-to-batch variations (within approximately 30%) in apparent multiexciton yields and larger variations (more than a factor of 3) due to differences in sample conditions (stirred vs static solutions). These results indicate that NC surface properties may affect the CM process. They also point toward potential interference from extraneous effects such as NC photoionization that can distort the results of CM studies. CM yields measured under conditions when extraneous effects are suppressed via intense sample stirring and the use of extremely low pump levels (0.02-0.03 photons absorbed per NC per pulse) reveal that both the electron-hole creation energy and the CM threshold are reduced compared with those in bulk solids. These results indicate a confinement-induced enhancement in the CM process in NC materials. Further optimization of CM performance should be possible by utilizing more complex (for example, shaped-controlled or heterostructured) NCs that allow for facile manipulation of carrier-carrier interactions, as well as single and multiexciton energies and dynamics.

Ultrafast Charge Dynamics in Trap‐Free and Surface‐Trapping Colloidal Quantum Dots

“…It has previously been recognized that the effects of trapping could produce subnanosecond decay in the pump‐induced bleach. However, it was thought that these effects could be prevented by flowing or stirring the sample15, 19, 20 or at least easily detected by the presence of a broad photoinduced absorption feature at wavelengths longer than the band edge that is comparable in magnitude to the bleach 24. In contrast, we show that large surface‐mediated bleach decays can be found for well‐stirred or flowed samples that exhibit a narrow band edge photoinduced absorption feature with a magnitude of only <5% of the bleach peak.…”

Multiple exciton generation in quantum dot-based solar cells

“…is an accepted mechanism, it is not yet clear if the same process is responsible for MEG in quantum-confined materials. The increased efficiency of MEG in these structures has not yet been fully explained and different models have been proposed where the multi-excitonic state is created through an excited virtual multi-excitonic state [42,43] or via an instantly generated superposition of single-and multi-excitonic state [41,44].…”

“…However, surface-trapped carriers can be long-lived species and may therefore nominally charge the QD. 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].…”

Multiple Exciton Generation and Dynamics in InP/CdS Colloidal Quantum Dots

“…These trions decay on a similar time-scale to biexcitons, and their presence can lead to an over-estimation of the MEG QY if they are mis-identified. 39 However, it has also been shown 40,41 that sufficient stirring or flowing of the sample can prevent trion formation by refreshing the pumped sample volume. Transients obtained when the sample was static and when it was stirred are compared in Figure S1 of the Supporting Information.…”