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.

Thick inorganic shell endows colloidal nanocrystals (NCs) with enhanced photochemical stability and suppression of photoluminescence intermittency (also known as blinking). However, the progress of using thick-shell heterostructure NCs in applications has been limited, due to low photoluminescence quantum yield (PL QY  60%) at room temperature. Here, we demonstrate thick-shell NCs with CdS/CdSe/CdS seed/spherical quantum well/shell (SQW) geometry that exhibit near-unity PL QY at room temperature and suppression of blinking. In SQW NCs, the lattice mismatch is diminished between the emissive CdSe layer and the surrounding CdS layers as a result of coherent strain, which suppresses the formation of misfit defects and consequently permits ~ 100% PL QY for SQW NCs with thick CdS shell (≥ 5 nm). High PL QY of thick-shell SQW NCs are preserved even in concentrated dispersion and in film under thermal stress, which makes them promising candidates for applications in solid-state lightings and luminescent solar concentrators.

Advance in wet chemistry enables the sophisticated design of nanocrystal quantum dots (QDs) and allows unprecedented color purity and brightness, promising their useful applications in a variety of light-emitting applications. A representative example is core/shell heterostructures, in which charge carriers are effectively decoupled from structural artifacts to generate photons efficiently. Despite the development of widely accepted synthetic protocols for Cd- or Pb-based QDs, the progress in heterostructuring environmentally benign QDs has been lagging behind, and so is the practical use of these QDs. Herein, we present a design principle for InP/ZnSe x S1–x heterostructured QDs. A principal design approach is the growth of uniformly thick inorganic shell consisting of a ZnSe x S1–x inner shell and a ZnS outermost shell that effectively confines electrons from spreading inward of QDs. Comprehensive studies across synthesis, spectroscopic analysis, and calculation uncover that the presence of Se near the InP emissive core enables a uniform shell growth to an extended thickness and the S-rich exterior shell ensures the decoupling of the electron wave function from the surface trap states. Engineering composition profile across multiple shells enables us to realize InP/thick-shell QDs meeting the requirements of light-emitting applications such as high photoluminescence quantum yield, narrow spectral bandwidth, and enhanced photochemical robustness. We capitalize on bright, robust, and color-pure InP/ZnSe x S1–x /ZnS QDs with a range of emission wavelength covering from cyan to red regions by exemplifying their use in the primary-color light-emitting diodes (peak external quantum efficiency of 3.78 and 3.92% for green- and red-emitting ones, respectively).

Digital microfluidics is a popular tool for lab-on-a-chip applications and is typically implemented in one of two formats: single-plate ͑"open"͒ devices or two-plate ͑"closed"͒ devices. Single-plate devices have some advantages relative to the more common two-plate format such as faster mixing, the capacity to move larger volumes on a given footprint, and easier access to droplets for handling or optical detection. In contrast with the two-plate format, in which ground potential is generally supplied via a top electrode, in the single-plate format, many different geometries of ground wires/ electrodes have been used. Until the present study, there has been no metric to determine which of these geometries is best suited for droplet actuation. Here, we present a combination of numerical simulations and experimental tests to compare six different single-plate designs. We applied finite element analysis, using the commercially available COMSOL software package to calculate the electrodynamic actuation forces in each of the different designs and used the results to optimize device design. Forces predicted by the electrodynamic model were in agreement with forces predicted using electromechanical models. More importantly, results were verified experimentally using a unique technique that permits indirect estimation of actuation forces on digital microfluidic devices. This work illustrates the promise of using numerical modeling to enhance the design and performance of digital microfluidic devices.

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