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).

The charge injection imbalance into the quantum dot (QD) emissive layer of QD-based light-emitting diodes (QD-LEDs) is an unresolved issue that is detrimental to the efficiency and operation stability of devices. Herein, an integrated approach to harmonize the charge injection rates for bright and stable QD-LEDs is proposed. Specifically, the electronic characteristics of the hole transport layer (HTL) is delicately designed in order to facilitate the hole injection from the HTL into QDs and confine the electron overflow toward the HTL. The well-defined exciton recombination zone by the engineered QDs and HTL results in high performance with a peak luminance exceeding 410 000 cd/m 2 , suppressed efficiency roll-off characteristics (ΔEQE < 5% between 200 and 200 000 cd/m 2 ), and prolonged operational stability. The electric and optoelectronic analyses reveal the charge carrier injection mechanism at the interface between the HTL and QDs and provides the design principle of QD heterostructures and charge transport layers for high-performance QD-LEDs.

Wide interest in quantum dot (QD) light‐emitting diodes (QLEDs) for potential application to display devices and light sources has led to their rapid advancement in device performance. Despite such progress, detailed operation mechanisms of QLEDs, which are necessary for the fundamental understanding and further improvements, have been still uncertain because of the intricate interaction between charges and excitons in electrical operation. In this work, the transient electroluminescence (TREL) signals of dichromatic QLEDs which are purposely designed to consist of two different color‐emitting QD layers are analyzed. As a result, not only can the charge injection and exciton recombination processes be visualized but the electron mobility of the QD layer can also be estimated. Furthermore, the effects of Förster resonant energy transfer between two QDs and exciton quenching near the QD layer are quantitatively measured in QLED operation. The authors believe that their results based on TREL analyses will contribute to the understanding and development of high‐performance QLEDs.

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Quantum dot light‐emitting diodes (QLEDs) are considered promising candidates for several optoelectronic applications; however, they are plagued by the over‐injection of electrons compared to holes, which limits device efficiency. Studies have attempted to reuse the leaked electrons and transfer recombination energies via inserting an exciton‐harvesting layer (EHL) between the emissive layer (EML) and hole transport layer (HTL). This study conducts a detailed analysis of the energy transfer mechanisms to obtain better insights into improving the device performance. First, by analyzing the electroluminescence (EL) spectra and exciton dynamics, the effect of EHLs comprising phosphorescence (PH) or thermally activated delayed fluorescence (TADF) blue dopant is compared. Through parallel incorporation of those EHLs on QLEDs and organic LEDs, the minimal contribution of the PH‐EHL to energy transfer in QLEDs is confirmed, whereas the TADF‐EHL has a significant contribution. Second, highly efficient top‐emission green QLEDs with the TADF‐EHL are achieved. They exhibit a maximum luminance (L) and current efficiency (CE) of 40700 cd m−2 and 68.0 cd A−1, respectively, which are the highest among the reported values for green‐emitting InP QLEDs. The proposed approaches are expected to provide aid in the realization of highly efficient QLEDs from the analysis to the device optimization stage.