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Perovskite nanoplatelets possess extremely narrow absorption and emission line widths, which are crucial characteristics for many optical applications. However, their underlying intrinsic and extrinsic line-broadening mechanisms are poorly understood. Here, we apply multidimensional coherent spectroscopy to determine the homogeneous line broadening of colloidal perovskite nanoplatelet ensembles. We demonstrate a dependence of not only their intrinsic line widths but also of various broadening mechanisms on platelet geometry. We find that decreasing nanoplatelet thickness by a single monolayer results in a 2-fold reduction of the inhomogeneous line width and a 3-fold reduction of the intrinsic homogeneous line width to the sub-millielectronvolts regime. In addition, our measurements suggest homogeneously broadened exciton resonances in two-layer (but not necessarily three-layer) nanoplatelets at room-temperature.

We investigate the operational instability of quantum dot (QD)-based light-emitting diodes (QLEDs). Spectroscopic analysis on the QD emissive layer within devices in chorus with the optoelectronic and electrical characteristics of devices discloses that the device efficiency of QLEDs under operation is indeed deteriorated by two main mechanisms. The first is the luminance efficiency drop of the QD emissive layer in the running devices owing to the accumulation of excess electrons in the QDs, which escalates the possibility of nonradiative Auger recombination processes in the QDs. The other is the electron leakage toward hole transport layers (HTLs) that accompanies irreversible physical damage to the HTL by creating nonradiative recombination centers. These processes are distinguishable in terms of the time scale and the reversibility, but both stem from a single origin, the discrepancy between electron versus hole injection rates into QDs. Based on experimental and calculation results, we propose mechanistic models for the operation of QLEDs in individual quantum dot levels and their degradation during operation and offer rational guidelines that promise the realization of high-performance QLEDs with proven operational stability.

Cesium lead halide perovskite quantum dots (PQDs) have emerged as a promising new platform for lighting applications. However, to date, light emitting diodes (LED) based on these materials exhibit limited efficiencies. One hypothesized limiting factor is fast nonradiative multiexciton Auger recombination. Using ultrafast spectroscopic techniques, we investigate multicarrier interaction and recombination mechanisms in cesium lead halide PQDs. By mapping the dependence of the biexciton Auger lifetime and the biexciton binding energy on nanomaterial size and composition, we find unusually strong Coulomb interactions among multiexcitons in PQDs. This results in weakly emissive biexcitons and trions, and accounts for low light emission efficiencies. We observe that, for strong confinement, the biexciton lifetime depends linearly on the PQD volume. This dependence becomes sublinear in the weak confinement regime as the PQD size increases beyond the Bohr radius. We demonstrate that Auger recombination is faster in PQDs compared to CdSe nanoparticles having the same volume, suggesting a stronger Coulombic interaction in the PQDs. We confirm this by demonstrating an increased biexciton binding energy, which reaches a maximum of about 100 meV, fully three times larger than in CdSe quantum dots. The biexciton shift can lead to low-threshold optical gain in these materials. These findings also suggest that materials engineering to reduce Coulombic interaction in cesium lead halide PQDs could improve prospects for high efficiency optoelectronic devices. Core-shell structures, in particular type-II nanostructures, which are known to reduce the bandedge Coulomb interaction in CdSe/CdS, could beneficially be applied to PQDs with the goal of increasing their potential in lighting applications.

The influence of a CdSexS1-x interfacial alloyed layer on the photophysical properties of core/shell CdSe/CdS nanocrystal quantum dots (QDs) is investigated by comparing reference QDs with a sharp core/shell interface to alloyed structures with an intermediate CdSexS1-x layer at the core/shell interface. To fully realize the structural contrast, we have developed two novel synthetic approaches: a method for fast CdS-shell growth, which results in an abrupt core/shell boundary (no intentional or unintentional alloying), and a method for depositing a CdSexS1-x alloy layer of controlled composition onto the CdSe core prior to the growth of the CdS shell. Both types of QDs possess similar size-dependent single-exciton properties (photoluminescence energy, quantum yield, and decay lifetime). However the alloyed QDs show a significantly longer biexciton lifetime and up to a 3-fold increase in the biexciton emission efficiency compared to the reference samples. These results provide direct evidence that the structure of the QD interface has a significant effect on the rate of nonradiative Auger recombination, which dominates biexciton decay. We also observe that the energy gradient at the core-shell interface introduced by the alloyed layer accelerates hole trapping from the shell to the core states, which results in suppression of shell emission. This comparative study offers practical guidelines for controlling multicarrier Auger recombination without a significant effect on either spectral or dynamical properties of single excitons. The proposed strategy should be applicable to QDs of a variety of compositions (including, e.g., infrared-emitting QDs) and can benefit numerous applications from light emitting diodes and lasers to photodetectors and photovoltaics.

Carrier multiplication (CM) is the process in which absorption of a single photon produces multiple electron-hole pairs. Here, we evaluate the effect of particle shape on CM efficiency by conducting a comparative study of spherical nanocrystal quantum dots (NQDs) and elongated nanorods (NRs) of PbSe using a time-resolved technique that is based on photon counting in the infrared using a superconducting nanowire single-photon photodetector (SNSPD). Due to its high sensitivity and low noise levels, this technique allows for accurate determination of CM yields, even with the small excitation intensities required for quantitative measurements, and the fairly low emission quantum yields of elongated NR samples. Our measurements indicate an up to ∼60% increase in multiexciton yields in NRs versus NQDs, which is attributed primarily to a decrease in the electron-hole pair creation energy. These findings suggest that shape control is a promising approach for enhancing the CM process. Further, our work demonstrates the effectiveness of the SNSPD technique for the rapid screening of CM performance in infrared nanomaterials.

We conduct measurements of photocharging of PbSe and PbS nanocrystal quantum dots (NQDs) as a function of excitation energy (ℏω). We observe a rapid growth of the degree of photocharging with increasing ℏω, which indicates an important role of hot-carrier transfer in the photoionization process. The corresponding spectral dependence exhibits two thresholds that mark the onsets of weak and strong photocharging. Interestingly, both thresholds are linked to the NQD band gap energy (E(g)) and scale as ∼1.5E(g) and ∼3E(g), indicating that the onsets of photoionization are associated with specific nanocrystal states (tentatively, 1P and 2P, respectively) and are not significantly dependent on the energy of external acceptor sites. For all samples, the hot-electron transfer probability increases by nearly 2 orders of magnitude as photon energy increases from 1.5 to 3.5 eV, although at any given wavelength the photoionization probability shows significant sample-to-sample variations (∼10(-6) to 10(-3) for 1.5 eV and ∼10(-4) to 10(-1) for 3.5 eV). In addition to the effect of the NQD size, these variations are likely due to differences in the properties of the NQD surface and/or the number and identity of external acceptor trap sites. The charge-separated states produced by photoionization are characterized by extremely long lifetimes (20 to 85 s) that become longer with increasing NQD size.

We show that excitation of solutions of well-passivated PbSe semiconductor nanocrystals (NCs) with ultraviolet (3.1 eV) photons can produce long-lived charge-separated states in which the NC core is left with a nonzero net charge. Since this process is not observed for lower-energy (1.5 eV) excitation, we ascribe it to hot-carrier transfer to some trap site outside the NC. Photocharging leads to bleaching of steady-state absorption, partial quenching of emission, and additional fast time scales in carrier dynamics due to Auger decay of charged single- and multiexciton states. The degree of photocharging, f, saturates at a level that varies from 5 to 15% depending on the sample. The buildup of the population of charged NCs is extremely slow indicating very long, tens of seconds, lifetimes of these charge-separated states. Based on these time scales and the measured onset of saturation of f at excitation rates around 0.05-1 photon per NC per ms, we determine that the probability of charging following a photon absorption event is of the order of 10(-4) to 10(-3). The results of these studies have important implications for the understanding of photophysical properties of NCs, especially in the case of time-resolved measurements of carrier multiplication.