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

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” <i>versus</i> “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 <i>elementary</i> three-particle Auger events. As one of the manifestations of the <i>superposition principle</i>, 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” <i>versus</i> “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

Room Temperature Single-Photon Emission from Individual Perovskite Quantum Dots

Lead-halide-based perovskites have been the subject of numerous recent studies largely motivated by their exceptional performance in solar cells. Electronic and optical properties of these materials have been commonly controlled by varying the composition (<i>e.g.</i>, the halide component) and/or crystal structure. Use of nanostructured forms of perovskites can provide additional means for tailoring their functionalities <i>via</i> effects of quantum confinement and wave function engineering. Furthermore, it may enable applications that explicitly rely on the quantum nature of electronic excitations. Here, we demonstrate that CsPbX₃ quantum dots (X = I, Br) can serve as room-temperature sources of quantum light, as indicated by strong photon antibunching detected in single-dot photoluminescence measurements. We explain this observation by the presence of fast nonradiative Auger recombination, which renders multiexciton states virtually nonemissive and limits the fraction of photon coincidence events to ∼6% on average. We analyze limitations of these quantum dots associated with irreversible photodegradation and fluctuations (“blinking”) of the photoluminescence intensity. On the basis of emission intensity-lifetime correlations, we assign the “blinking” behavior to random charging/discharging of the quantum dot driven by photoassisted ionization. This study suggests that perovskite quantum dots hold significant promise for applications such as quantum emitters; however, to realize this goal, one must resolve the problems of photochemical stability and photocharging. These problems are largely similar to those of more traditional quantum dots and, hopefully, can be successfully resolved using advanced methodologies developed over the years in the field of colloidal nanostructures

Engineered CuInSe_<i>x</i>S_2–<i>x</i> Quantum Dots for Sensitized Solar Cells

Colloidal CuInSe_<i>x</i>S_2–<i>x</i> quantum dots (QDs) are an attractive less-toxic alternative to PbX and CdX (X = S, Se, and Te) QDs for solution-processed semiconductor devices. This relatively new class of QD materials is particularly suited to serving as an absorber in photovoltaics, owing to its high absorption coefficient and near-optimal and finely tunable band gap. Here, we engineer CuInSe_<i>x</i>S_2–<i>x</i> QD sensitizers for enhanced performance of QD-sensitized TiO₂ solar cells (QDSSCs). Our QD synthesis employs 1-dodecanethiol (DDT) as a low-cost solvent, which also serves as a ligand, and a sulfur precursor; addition of triakylphosphine selenide leads to incorporation of controlled amounts of selenium, reducing the band gap compared to that of pure CuInS₂ QDs. This enables significantly higher photocurrent in the near-infrared (IR) region of the solar spectrum without sacrificing photovoltage. In order to passivate QD surface recombination centers, we perform a surface–cation exchange with Cd prior to sensitization, which enhances chemical stability and leads to a further increase in photocurrent. We use the synthesized QDs to demonstrate proof-of-concept QDSSCs with up to 3.5% power conversion efficiency

Auger Recombination of Biexcitons and Negative and Positive Trions in Individual Quantum Dots

Charged exciton states commonly occur both in spectroscopic studies of quantum dots (QDs) and during operation of QD-based devices. The extra charge added to the neutral exciton modifies its radiative decay rate and also opens an additional nonradiative pathway associated with an Auger process whereby the recombination energy of an exciton is transferred to the excess charge. Here we conduct single-dot spectroscopic studies of Auger recombination in thick-shell (“giant”) CdSe/CdS QDs with and without an interfacial alloy layer using time-tagged, time-correlated single-photon counting. In photoluminescence (PL) intensity trajectories of some of the dots, we resolve three distinct states of different emissivities (“bright”, “gray”, and “dark”) attributed, respectively, to the neutral exciton and negative and positive trions. Simultaneously acquired PL lifetime trajectories indicate that the positive trion is much shorter lived than the negative trion, which can be explained by a high density of valence band states and a small hole localization radius (defined by the QD core size), factors that favor an Auger process involving intraband excitation of a hole. A comparison of trion and biexciton lifetimes suggests that the biexciton Auger decay can be treated in terms of a superposition of two independent channels associated with positive- and negative-trion pathways. The resulting interdependence between Auger time constants might simplify the studies of multicarrier recombination by allowing one, for example, to infer Auger lifetimes of trions of one sign based on the measurements of biexciton decay and dynamics of the trions of the opposite sign or, alternatively, estimate the biexciton lifetime based on studies of trion dynamics

Effect of Interfacial Alloying versus “Volume Scaling” on Auger Recombination in Compositionally Graded Semiconductor Quantum Dots

Auger recombination is a nonradiative three-particle process wherein the electron–hole recombination energy dissipates as a kinetic energy of a third carrier. Auger decay is enhanced in quantum-dot (QD) forms of semiconductor materials compared to their bulk counterparts. Because this process is detrimental to many prospective applications of the QDs, the development of effective approaches for suppressing Auger recombination has been an important goal in the QD field. One such approach involves “smoothing” of the confinement potential, which suppresses the intraband transition involved in the dissipation of the electron–hole recombination energy. The present study evaluates the effect of increasing “smoothness” of the confinement potential on Auger decay employing a series of CdSe/CdS-based QDs wherein the core and the shell are separated by an intermediate layer of a CdSe_<i>x</i>S_1–<i>x</i> alloy comprised of 1–5 sublayers with a radially tuned composition. As inferred from single-dot measurements, use of the five-step grading scheme allows for strong suppression of Auger decay for both biexcitons and charged excitons. Further, due to nearly identical emissivities of neutral and charged excitons, these QDs exhibit an interesting phenomenon of lifetime blinking for which random fluctuations of a photoluminescence lifetime occur for a nearly constant emission intensity

Photocharging Artifacts in Measurements of Electron Transfer in Quantum-Dot-Sensitized Mesoporous Titania Films

Transient absorption and time-resolved photoluminescence measurements of high-performance mesoporous TiO₂ photoanodes sensitized with CuInSe_<i>x</i>S_2–<i>x</i> quantum dots reveal the importance of hole scavenging in the characterization of photoinduced electron transfer. The apparent characteristic time of this process strongly depends on the local environment of the quantum dot/TiO₂ junction due to accumulation of long-lived positive charges in the quantum dots. The presence of long-lived photoexcited holes introduces artifacts due to fast positive-trion Auger decay (60 ps time constant), which can dominate electron dynamics and thus mask true electron transfer. We show that the presence of a redox electrolyte is critical to the accurate characterization of charge transfer, since it enables fast extraction of holes and helps maintain charge neutrality of the quantum dots. Although electron transfer is observed to be relatively slow (19 ns time constant), a high electron extraction efficiency (>95%) can be achieved because in well-passivated CuInSe_<i>x</i>S_2–<i>x</i> quantum dots neutral excitons have significantly longer lifetimes of hundreds of nanoseconds

Role of Solvent–Oxygen Ion Pairs in Photooxidation of CdSe Nanocrystal Quantum Dots

Understanding the mechanisms for photodegradation of nanocrystal quantum dots is an important step toward their application in real-world technologies. A usual assumption is that photochemical modifications in nanocrystals, such as their photooxidation, are triggered by absorption of a photon in the dot itself. Here, we demonstrate that, contrary to this commonly accepted picture, nanocrystal oxidation can be initiated by photoexcitation of solvent–oxygen ion pairs that relax to produce singlet oxygen, which then reacts with the nanocrystals. We make this conclusion on the basis of photolysis studies of solutions of CdSe nanocrystals. Our measurements indicate a sharp spectral onset for photooxidation, which depends on solvent identity and is 4.8 eV for hexane and 3.4 eV for toluene. Importantly, the photooxidation onset correlates with the position of a new optical absorption feature, which develops in a neat solvent upon its exposure to oxygen. This provides direct evidence that nanocrystal photooxidation is mediated by excitation of solvent–oxygen pairs and suggests that the stability of the nanocrystals is defined by not only the properties of their surfaces (as has been commonly believed) but also the properties of their environment, that is, of the surrounding solvent or matrix

Quality Factor of Luminescent Solar Concentrators and Practical Concentration Limits Attainable with Semiconductor Quantum Dots

Luminescent solar concentrators (LSCs) can be utilized as both large-area collectors of solar radiation supplementing traditional photovoltaic cells as well as semitransparent “solar windows” that provide a desired degree of shading and simultaneously serve as power-generation units. An important characteristic of an LSC is a concentration factor (<i>C</i>) that can be thought of as a coefficient of effective enlargement (or contraction) of the area of a solar cell when it is coupled to the LSC. Here we use analytical and numerical Monte Carlo modeling in addition to experimental studies of quantum-dot-based LSCs to analyze the factors that influence optical concentration in practical devices. Our theoretical model indicates that the maximum value of <i>C</i> achievable with a given fluorophore is directly linked to the LSC quality factor (<i>Q</i>_LSC) defined as the ratio of absorption coefficients at the wavelengths of incident and reemitted light. In fact, we demonstrate that the ultimate concentration limit (<i>C</i>₀) realized in large-area devices scales linearly with the LSC quality factor and in the case of perfect emitters and devices without back reflectors is approximately equal to <i>Q</i>_LSC. To test the predictions of this model, we conduct experimental studies of LSCs based on visible-light emitting II–VI core/shell quantum dots with two distinct LSC quality factors. We also investigate devices based on near-infrared emitting CuInSe_<i>x</i>S_2–<i>x</i> quantum dots for which the large emission bandwidth allows us to assess the impact of varied <i>Q</i>_LSC on the concentration factor by simply varying the detection wavelength. In all cases, we find an excellent agreement between the model and the experimental observations, suggesting that the developed formalism can be utilized for express evaluation of prospective LSC performance based on the optical spectra of LSC fluorophores, which should facilitate future efforts on the development of high-performance devices based on quantum dots as well as other types of emitters

Effect of Auger Recombination on Lasing in Heterostructured Quantum Dots with Engineered Core/Shell Interfaces

Nanocrystal quantum dots (QDs) are attractive materials for applications as laser media because of their bright, size-tunable emission and the flexibility afforded by colloidal synthesis. Nonradiative Auger recombination, however, hampers optical amplification in QDs by rapidly depleting the population of gain-active multiexciton states. In order to elucidate the role of Auger recombination in QD lasing and isolate its influence from other factors that might affect optical gain, we study two types of CdSe/CdS core/shell QDs with the same core radii and the same total sizes but different properties of the core/shell interface (“sharp” vs “smooth”). These samples exhibit distinctly different biexciton Auger lifetimes but are otherwise virtually identical. The suppression of Auger recombination in the sample with a smooth (alloyed) interface results in a notable improvement in the optical gain performance manifested in the reduction of the threshold for amplified spontaneous emission and the ability to produce dual-color lasing involving both the band-edge (1S) and the higher-energy (1P) electronic states. We develop a model, which explicitly accounts for the multiexciton nature of optical gain in QDs, and use it to analyze the competition between stimulated emission from multiexcitons and their decay via Auger recombination. These studies re-emphasize the importance of Auger recombination control for the realization of real-life QD-based lasing technologies and offer practical strategies for suppression of Auger recombination via “interface engineering” in core/shell structures

Auger Up-Conversion of Low-Intensity Infrared Light in Engineered Quantum Dots

One source of efficiency losses in photovoltaic cells is their transparency toward solar photons with energies below the band gap of the absorbing layer. This loss can be reduced using a process of up-conversion whereby two or more sub-band-gap photons generate a single above-gap exciton. Traditional approaches to up-conversion, such as nonlinear two-photon absorption (2PA) or triplet fusion, suffer from low efficiency at solar light intensities, a narrow absorption bandwidth, nonoptimal absorption energies, and difficulties for implementing in practical devices. Here we show that these deficiencies can be alleviated using the effect of Auger up-conversion in thick-shell PbSe/CdSe quantum dots. This process relies on Auger recombination whereby two low-energy, core-based excitons are converted into a single higher-energy, shell-based exciton. Compared to their monocomponent counterparts, the tailored PbSe/CdSe heterostructures feature enhanced absorption cross-sections, a higher efficiency of the “productive” Auger pathway involving re-excitation of a hole, and longer lifetimes of both core- and shell-localized excitons. These features lead to effective up-conversion cross-sections that are more than 6 orders of magnitude higher than for standard nonlinear 2PA, which allows for efficient up-conversion of continuous wave infrared light at intensities as low as a few watts per square centimeter