Superposition Principle in Auger Recombination of Charged and Neutral Multicarrier States in Semiconductor Quantum Dots

Abstract

Application of colloidal semiconductor quantum dots (QDs) in optical and optoelectronic devices is often complicated by unintentional generation of extra charges, which opens fast nonradiative Auger recombination pathways whereby the recombination energy of an exciton is quickly transferred to the extra carrier(s) and ultimately dissipated as heat. Previous studies of Auger recombination have primarily focused on neutral and, more recently, negatively charged multicarrier states. Auger dynamics of positively charged species remains more poorly explored due to difficulties in creating, stabilizing, and detecting excess holes in the QDs. Here we apply photochemical doping to prepare both negatively and positively charged CdSe/CdS QDs with two distinct core/shell interfacial profiles ("sharp" versus "smooth"). Using neutral and charged QD samples we evaluate Auger lifetimes of biexcitons, negative and positive trions (an exciton with an extra electron or a hole, respectively), and multiply negatively charged excitons. Using these measurements, we demonstrate that Auger decay of both neutral and charged multicarrier states can be presented as a superposition of independent elementary three-particle Auger events. As one of the manifestations of the superposition principle, we observe that the biexciton Auger decay rate can be presented as a sum of the Auger rates for independent negative and positive trion pathways. By comparing the measurements on the QDs with the "sharp" versus "smooth" interfaces, we also find that while affecting the absolute values of Auger lifetimes, manipulation of the shape of the confinement potential does not lead to violation of the superposition principle, which still allows us to accurately predict the biexciton Auger lifetimes based on the measured negative and positive trion dynamics. These findings indicate considerable robustness of the superposition principle as applied to Auger decay of charged and neutral multicarrier states, suggesting its generality to quantum-confined nanocrystals of arbitrary compositions and complexities.

Carrier-doping as a tool to probe the electronic structure and multi-carrier recombination dynamics in heterostructured colloidal nanocrystals

“…These time constants are assigned to the X – lifetimes ( τ X – ) of DIRs, which should contain contributions from both radiative and nonradiative Auger recombinations. According to previous reports, 48 , 70 – 72 the radiative lifetime of X – ( τ X – ,r ) is ∼1/1.7 that of single excitons ( τ X ) for CdSe@CdS systems, with τ X already determined from time-resolved PL measurements (Fig. S3b † ).…”

“…The QYs are in the range of 14% to 24% and are relatively higher in DIRs with more strongly delocalized electron wavefunctions (2.6 nm@thin and 3.4 nm@thick) or with large volume (4.2 nm@thick). These QYs might explain the “gray” states observed in single-particle PL experiments of CdSe@CdS NRs 73 as the photocharged species, usually being X – for CdSe@CdS systems, 70 are reasonably bright rather than completely dark.…”

Electrochemical Charging Effect on the Optical Properties of InP/ZnSe/ZnS Quantum Dots

“…[31][32][33][34][35][36][37] The radiative trion emission lifetime constant (k r,X± ) is easily obtained by the core emission lifetime constant, as theoretically determined from k r,X± = 2•k r,X (See the Supporting information for details). [32,[35][36] The relative PL QY is 98% for InP/5ZnSe/ZnS and 95% for InP/10ZnSe/ ZnS. The core emission lifetime of neutral QDs is 33 ns for InP/5ZnSe/ZnS and 35 ns for InP/10ZnSe/ZnS.…”

“…Several previous studies have reported that the Auger recombination rate of positive trion is faster than that of negative trion. [31,36,38] Meanwhile, Hou et al have recently described in detail the crossover of Auger recombination rates of negative and positive trions in CdSe/CdS QDs. [34] In QDs with a thin shell, the wavefunction of conduction band maximum (CBM) extends to the region where dielectric screening is much weaker.…”

Observation of positive and negative trions in organic-inorganic hybrid perovskite nanocrystals

“…CuSCN has been used in perovskite solar cells to extract photoexcited holes from the perovskite layer [47,48]. Thus, a negatively charged NC is formed upon addition of CuSCN due to the extraction of photogenerated holes from the NCs, i.e., CuSCN enables photochemical doping of perovskite NCs [49,50]. In Figs.…”