Direct Assessment of Auger Recombination Rates of Charged Excitons via Opto-Electrical Measurements

Abstract

Auger recombination (AR), whereby the electron−hole recombination energy is transferred to a third charge carrier, prevails in nanocrystal quantum dots (QDs) and governs the performance of QD-based devices including light-emitting diodes and lasers. Thus, precise AR evaluation of QDs is essential for understanding and improving the characteristics of such applications. So far, conventional charging approaches, such as the stir-versus-static method, photochemistry, or electrochemistry, have been able to assess the AR decay rate of either positively (two holes and one electron, X + ) or negatively (one hole and two electrons, X − ) charged excitons, and the decay dynamics of the other type of charged exciton is presumably estimated by the superposition principle of the biexciton Auger process. Herein, we demonstrate an opto-electrical method that enables us to precisely assess AR rates of X + and X − in core/shell heterostructured QDs. Specifically, we devise electron-only devices and hole-only devices to inject extra charge carriers into QDs without unwanted side reactions or degradation of QDs and probe AR characteristics of these charged QDs via timeresolved photoluminescence measurements. We find that AR rates of charged excitons, both X + and X − , gained from the present method agree well with those attained from conventional approaches and the superposition principle, corroborating the validity of the present approach. This present method permits us to comprehend multicarrier dynamics in QDs, prompting the use of QDs in light-emitting diodes and laser devices based on QDs.

Differences in Electron and Hole Injection and Auger Recombination between Red, Green, and Blue CdSe-Based Quantum Dot Light Emitting Devices

“…As the electric field across the QD layers increases further, the mobility of holes starts to increase, reducing the residence time of the holes on the QDs and thereby reducing the slopes. The stronger PL quenching effect of excess holes compared to that of excess electrons can be attributed to the high density of valence band states and the smaller hole localization radius, which accelerate nonradiative Auger recombination and make quenching by positive trions more effective. ,, …”

Enhancement of Optical Gain in Colloidal CdSe/CdS/ZnS Quantum Dots through Nanosecond Optical Pumping

“…Specifically, the reference sample used for determining the τ A,XX /τ A,X – ratio had a significantly thicker CdS shell layer (3.7 nm) compared to that in our QDs (1.8 nm). Given the similar CdSe core radius (1.7 nm for the reference QDs and 1.8 nm for our QDs), the difference in the CdS shell layer thickness led to more delocalized electrons in the reference sample, which significantly increased τ A,X – while only minimally affecting τ A,X + . ,, Therefore, the scaling factor of 0.17 for the τ A,XX /τ A,X – ratio may be underestimated for our QDs. Nevertheless, we conducted a quantitative analysis of the exciton population per QD during optical pumping, assuming a more restrictive condition with a shorter τ A,XX of 190 ps than the actual case, as described in the following text.…”

“…The biexciton Auger decay process involves positive and negative trion decay pathways, and the biexciton Auger decay rate is expressed as 1/τ A,XX = 2/τ A,X + + 2/τ A,X – , where τ A,XX , τ A,X + , and τ A,X – represent the Auger decay lifetimes of biexcitons, positive trions, and negative trions, respectively. , Moreover, the trion Auger rate is significantly influenced by the density of states and charge carrier distribution. The higher density of state in the valence band and spatially strong confinement of a hole, resulting from the larger effective mass of a hole compared to that of an electron in CdSe QDs, typically lead to the Auger recombination rates of positive trions being higher than those for negative trions. , Recently, Bae et al analyzed the multicarrier Auger rates of QD samples based on PL decay dynamics. The authors obtained a τ A,XX /τ A,X – ratio of 0.17 for quasi-type II CdSe/CdS (C/S) QDs with a core radius of 1.7 nm and shell thickness of 3.7 nm .…”

“…The higher density of state in the valence band and spatially strong confinement of a hole, resulting from the larger effective mass of a hole compared to that of an electron in CdSe QDs, typically lead to the Auger recombination rates of positive trions being higher than those for negative trions. , Recently, Bae et al analyzed the multicarrier Auger rates of QD samples based on PL decay dynamics. The authors obtained a τ A,XX /τ A,X – ratio of 0.17 for quasi-type II CdSe/CdS (C/S) QDs with a core radius of 1.7 nm and shell thickness of 3.7 nm . The Auger decay time of the negatively charged QDs was observed to be 1.1 ns in our study (Figure c).…”

Spectroelectrochemistry of CdSe/CdS Core–Shell Quantum Dots

“…Furthermore, single-particle spectroelectrochemistry of CdSe/CdS nanocrystals has revealed a reduction in PL intensity under positive potentials with no change in the underlying PL dynamics . Electrochemical hole injection into CdTe and CdSe QDs films results in a bleaching of the excitonic transitions. , Bae et al estimated both the positive and negative trion lifetimes of different core–shell QDs incorporated into devices . However, all of these studies have been conducted on thin films, where there is strong interparticle coupling and little control over charge migration.…”