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a) Schematic energy level diagram of multilayer QLED structure. b) Current–density versus bias for hole‐only devices based on 40 nm thick Zn1−xCdxSe/ZnSe/ZnS core/shell QDs with OA or EHT surface ligands. c) Current density and luminance versus bias for devices based on Zn1−xCdxSe/ZnSe/ZnS core/shell QDs with OA or EHT surface ligands.
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In the study of hybrid quantum dot light‐emitting diodes (QLEDs), even for state‐of‐the‐art achievement, there still exists a long‐standing charge balance problem, i.e., sufficient electron injection versus inefficient hole injection due to the large valence band offset of quantum dots (QDs) with respect to the adjacent carrier transport layer. Her...
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... [8,10,12,[15][16][17][18][19][20][21] Furthermore, the effects of the QD properties, such as surface ligands, core/shell structure, shell thickness, etc., on the QLED device performance have been investigated in many publications. [22,23] However, limited researches on the relation between the QD EML properties (i. e., surface morphology, film thickness, and QD particle distribution of the QD layer) and device performance are available in the literature. ...
Blue light‐emitting CdSe@ZnS/ZnS quantum dot (QD) nanoparticles (NPs) were synthesized and their photophysical properties in both solution and film phases were investigated. The morphological properties of films prepared by different coating methods i. e. single layer coating from low to high concentrations of QD solutions and layer‐by‐layer (multilayer) coating within constant low QD solution concentration, were also examined in detail. Varying the concentration (1–10 mg/mL) and the number of layers (from 1–16) did not essentially affect the photophysical properties of QD films, although it resulted in a direct increment in QD film thickness. The concentration and layer‐dependent films were used as an emissive layer (EML) in QD light‐emitting diodes (QLEDs). Although the “6 mg/ml⁻¹ Layer” QD EML‐based device exhibited relatively high device efficiency compared to the “1 mg/ml⁻¹⁰ Layers” based one at working voltage region, it had ~2‐fold higher efficiency roll‐off at high voltage region. The performance differences for both devices with the same QD EML thickness were attributed to the morphological variations for the QD layer in terms of surface roughness, void density, aggregates/clusters, and trap sites that were directly related to the charge injection balance and Auger recombination.
... Isotropic spherical quantum dots (QDs) are the earliest discovered and most studied NCs, and their applications in the field of display have been widely studied. However, since the dipole moments in spherical QDs are randomly oriented, the theoretical maximum out-coupling efficiency (~20%) significantly limits the external quantum efficiency (EQE) of QLEDs [17,18]. On the other hand, the use of anisotropic nanocrystals including nanorods (NRs) and nanoplatelets (NPLs) as the light-emitting layer in QLEDs is expected to break the out-coupling efficiency limit and further improve the EQE of QLEDs [19]. ...
... Modeling and computational methods based on density functional theory have been used to guide the synthesis of core-shell structures [25,26]. Multi-shell structures with alloy transition shells have been shown to be effective in mitigating carrier delocalization and improving radiative recombinations [18,27,28]. In recent years, such core-shell structures have been increasingly explored in NRs [29,30]. ...
Semiconductor nanorods (NRs) have great potential in optoelectronic devices for their unique linearly polarized luminescence which can break the external quantum efficiency limit of light-emitting diodes (LEDs) based on spherical quantum dots. Significant progress has been made for developing red, green, and blue light-emitting NRs. However, the synthesis of NRs emitting in the deep red region, which can be used for accurate red LED displays and promoting plant growth, is currently less explored. Here, we report the synthesis of deep red CdSeTe/CdZnS/ZnS dot-in-rod core/shell NRs via a seeded growth method, where the doping of Te in the CdSe core can extend the NR emission to the deep red region. The rod-shaped CdZnS shell is grown over CdSeTe seeds. By growing a ZnS passivation shell, the CdSeTe/CdZnS/ZnS NRs exhibit a photoluminescence emission peak at 670 nm, a full width at a half maximum of 61 nm and a photoluminescence quantum yield of 45%. The development of deep red NRs can greatly extend the applications of anisotropic nanocrystals.
... Thus, it is essential to improve the device structure and materials to increase the outcoupling efficiency. Some recent studies have shown that QD-LEDs and PeLEDs can achieve an EQE above 25% without using any out-coupling enhancement method [20][21][22][23][24][25] . ...
Using a transfer printing technique, we imprint a layer of a designated near-infrared fluorescent dye BTP-eC9 onto a thin layer of Pt(II) complex, both of which are capable of self-assembly. Before integration, the Pt(II) complex layer gives intense deep-red phosphorescence maximized at ~740 nm, while the BTP-eC9 layer shows fluorescence at > 900 nm. Organic light emitting diodes fabricated under the imprinted bilayer architecture harvest most of Pt(II) complex phosphorescence, which undergoes triplet-to-singlet energy transfer to the BTP-eC9 dye, resulting in high-intensity hyperfluorescence at > 900 nm. As a result, devices achieve 925 nm emission with external quantum efficiencies of 2.24% (1.94 ± 0.18%) and maximum radiance of 39.97 W sr⁻¹ m⁻². Comprehensive morphology, spectroscopy and device analyses support the mechanism of interfacial energy transfer, which also is proved successful for BTPV-eC9 dye (1022 nm), making bright and far-reaching the prospective of hyperfluorescent OLEDs in the near-infrared region.
... Indeed, the quality of QD materials in actual devices is also crucial to the performance of QLEDs [28][29][30]. Binary or ternary type-I core/shell QDs are widely used for the preparation of QLED devices due to their superior optical properties [31][32][33][34]. The inevitably unbalanced charge injection and complex operating mechanism in QLEDs propose more rigorous requirements for the design of QDs. ...
Quantum dot (QD) light-emitting diodes (QLEDs) are promising for next-generation lighting and displays. Considering the optimization design of both the QD and device structure is expected to improve the QLED's performance significantly but has rarely been reported. Here, we use the thick-shell QDs combined with a dual-hole transport layer device structure to construct a high-efficiency QLED. The optimized thick-shell QDs with CdS/CdSe/CdS/ZnS seed/spherical quantum well/shell/shell geometry exhibit a high photoluminescence quantum yield of 96% at a shell thickness of 5.9 nm. The intermediate emissive CdSe layer with coherent strain ensures defect-free growth of the thick CdS and ZnS outer shells. Based on the orthogonal solvents assisted Poly-TPD&PVK dual-hole transport layer device architecture, the champion QLED achieved a maximum external quantum efficiency of 22.5% and a maximum luminance of 259955 cd m⁻², which are 1.6 and 3.7 times that of thin-shell QDs based devices with single hole transport layer, respectively. Our study provides a feasible idea for further improving the performance of QLED devices.
... For the Zn 1−x Mg x O (x = 0.1, 0.15, and 0.25) solutions, a specific amount of 0.55 M tetramethylammonium hydroxide (TMAH) was dissolved in ethanol as solution a. Separately, 0.1 M zinc acetate dihydrate and in materials and device structures, achieving external quantum efficiencies (EQE) of over 30% [6][7][8]. Compared to traditional light-emitting diodes, QLEDs usually feature a p-i-n structure, where ZnO nanoparticles (NPs) are widely acknowledged as one of the most frequently utilized electron transport layers (ETL) [9][10][11]. ...
Colloidal quantum-dot light-emitting diodes (QLEDs) are rapidly gaining recognition as formidable contenders in the realm of next-generation lighting and display devices. Notwithstanding, the journey to commercialization of QLED devices encounters a roadblock in the form of inadequate flexibility exhibited by the electron transport layer, which is typically made of ZnO nanoparticles. The hindrance stems from the substantial specific surface area, existence of surface defect states, and the fixed ZnO bandgap. To surmount these obstacles, we delved into the potential of integrating Mg element as a dopant in ZnO, aiming to enhance the surface chemistry, electrical characteristics, and film morphology of colloidal ZnO nanoparticles. The results clearly indicated that Mg doping played a critical role in diminishing surface defects in ZnO, while simultaneously reducing the density of oxygen vacancies, thereby regulating its electron mobility. Through modulation of the Mg doping concentration, the bandgap width of ZnO can be fine-tuned, leading to the creation of a more suitable electron transport layer. The inverted QLED devices based on Zn1−xMgxO electron transport layers exhibited remarkable advancements, with a peak external quantum efficiency and current efficiency of 6.7% and 29 cd A⁻¹, respectively. These values surpassed those of reference devices by 35 and 28%, underscoring the efficacy of Zn1−xMgxO as a viable approach for enhancing the efficiency of QLED devices.
... A single-exponential fluorescence lifetime implies that only one principal excited state is involved in the fluorescence process, leading to a more efficient energy conversion. [29][30][31] Multi-exponential fluorescence decay may encompass multiple non-radiative recombination processes and quenching mechanisms, potentially diminishing luminescence intensity. In summary, the above results indicate that our luminescence process involves a simple, direct, and efficient electronic transition process. ...
Self‐trapped exciton (STE) luminescence, typically associated with structural deformation of excited states, has attracted significant attention in metal halide materials recently. However, the mechanism of multiexciton STE emissions in certain metal halide crystals remains largely unexplored. This study investigates dual luminescence emissions in HCOO ⁻ doped Cs 3 Cu 2 I 5 single crystals using transient and steady‐state spectroscopy. The dual emissions are attributed to intrinsic STE luminescence originating from the host lattice and extrinsic STE luminescence induced by external dopants, respectively, each of which can be triggered independently at distinct energy levels. Theoretical calculations reveal that multiexciton emission originates from structural distortion of the host and dopant STEs within the 0D lattice in their respective excited states. By meticulously tuning the excitation wavelength and selectively exciting different STEs, the dynamic alteration of color change in Cs 3 Cu 2 I 5 :HCOO ⁻ crystals is demonstrated. Ultimately, owing to an extraordinarily high photoluminescence quantum yield (99.01%) and a diminished degree of self‐absorption in Cs 3 Cu 2 I 5 :HCOO ⁻ crystals, they exhibit remarkable X‐ray scintillation characteristics with light yield being improved by 5.4 times as compared to that of pristine Cs 3 Cu 2 I 5 crystals, opening up exciting avenues for achieving low‐dose X‐ray detection and imaging.
... [10] The red, green, and blue QLED devices based on CdE QDs have achieved an EQE of over 20%. [11][12][13] Among them, the efficiency of the red QLED device has exceeded 30%, [14] which is comparable to OLED. However, due to environmental and health hazards, Cd has been banned in numerous consumer electronics. ...
The “Nobel Prize in Chemistry 2023” is awarded to Moungi G. Bawendi, Louis E. Brus, and Alexey I. Yekimov for discovering and synthesizing Quantum Dots (QDs). Colloidal QDs possess fascinating size‐, morphological‐, composition‐, and assembly‐tunable electronic and optical properties, which makes them star materials for various optoelectronic applications, especially as luminescent materials for next‐generation wide color gamut ultra‐high‐definition displays. Perovskite QDs (PQDs) have gained widespread attention in recent years. In less than ten years, research on perovskite‐related materials and devices has basically been perfected in terms of quantum yield and external quantum efficiency (EQE). However, on the eve of its industrial application, some key technical indicators and technical processes need to be met and resolved. The development and transformation of QD materials and then focuses on the progress of luminescence linewidth and EQE of the PQD light‐emitting diode. Finally, several application avenues are reviewed for PQDs, and some challenges and opportunities in the field are proposed.
... [1,2] Such promising properties of QDs can materialize in the form of practical devices only if they accomplish reasonable photoluminescence quantum yield (PL QY) and stability, as indicated by many prior works. [1,[3][4][5][6][7][8] For this purpose, colloidal QDs have been constructed as the core/shell heterostructures; heteroepitaxy of wide band gap shell on cores allows for passivating defect states acting as non-radiative centers, improving PL QY. Moreover, spatial and compositional tuning of the shell region has opened a door for engineering carrier recombination dynamics, such as the following: controlled oscillator strength in quasi-type-II [9] or type-II structures; controlled Auger recombination through graded confinement potential; [9][10][11][12][13][14][15] reduced resonant energy transfer between dots in condensed films by increased shell thickness; [16,17] suppressed PL intermittency. ...
... Moreover, spatial and compositional tuning of the shell region has opened a door for engineering carrier recombination dynamics, such as the following: controlled oscillator strength in quasi-type-II [9] or type-II structures; controlled Auger recombination through graded confinement potential; [9][10][11][12][13][14][15] reduced resonant energy transfer between dots in condensed films by increased shell thickness; [16,17] suppressed PL intermittency. [18,19] Such scientific findings allowed for manipulating carrier dynamics in QDs for developing more efficient, stable, and high optical output devices [3,6,14,20] or laser diodes, [21][22][23][24] which requires the management of exciton species in optically active QD layers for preventing efficiency loss or lowering optical gain threshold. ...
... Therefore, the threshold shell thickness triggering the defect formation can be extended (e.g., recent high efficiency giant QDs for QD-based light-emitting diodes). [3,6,14,63] For these multi-or alloyed shell-based QDs, PL QY less than unity and its reduction with increasing shell thickness must be looked at the inclusion of atomic vacancies during shell growth. As long as such defects are left during shell growth, increasing shell thickness provides number of trap states lowering the PL QY of QDs, as observed in typical QD synthesis. ...
Photoluminescence quantum yield (PL QY) of colloidal quantum dots (QDs) can be improved by growing a shell, but it is rather limited if the shell thickness exceeds a threshold. Lattice mismatch between the core and shell is known to determine this critical shell thickness, securing QDs from defect formation through strain release. However, it cannot explain the recently reported high efficiency QDs with giant shells. Based on CdSe/ZnSe thick shell QDs, this study aims to identify the culprit limiting PL QY. In the shell growth process, the gradual reduction in PL QY is accompanied by a low‐energy tail emission, but the additional compressive strain by the outmost shell eliminates such abnormalities. It is revealed that the zinc vacancy in the shell provides shallow hole trap states. The computational study successfully explains the hole‐accepting zinc vacancy states above the CdSe 1S hh state, raised by compressive strain along radial direction. Additional hydrostatic compressive strain lifts the 1S hh state for this strained heterostructure to minimize the energetic gap with the zinc vacancy states. The finding suggests that critical shell thickness can be limited by atomic vacancy incorporated during shell growth, not by formation of misfit dislocation caused by strain release.
... Although the internal quantum efficiencies of QLEDs have reached near unity, the peak EQEs are limited to ≈20%, [8][9][10][11][12] and this is mainly due to the low light extraction efficiency. [13][14][15][16][17][18][19][20][21][22] Several approaches have been developed to enhance the outof-plane emission and increase the light extraction efficiency, including patterned emitting layer, structured substrate, high refractive index substrate, and the use of anisotropic nanocrystals. [23][24][25] The emergence and development of anisotropic nanorods (NRs) can potentially break the limit of the out-coupling factor (about 0.2) of QLEDs based on spherical QDs owing to their linearly polarized emission along the long axis of the NRs. ...
The external quantum efficiency (EQE) in light‐emitting diodes (LEDs) based on isotropic quantum dots has approached the theoretical limit of close to 20%. Anisotropic nanorods can break this limit by taking advantage of their directional emission. However, the progress towards higher EQE by using CdSe/CdS nanorods (NRs) faces several challenges, primarily involving the low quantum yield and unbalanced charge injection in devices. Herein, the seeded growth method is modified and anisotropic nanorods are obtained with photoluminescence quantum yield up to 98% by coating a gradient alloyed CdZnSe shell around conventional spherical CdSe seeds. This intermediate alloyed CdZnSe shell combined with a subsequent rod‐shaped CdZnS/ZnS shell can effectively suppress the electron delocalization in the typical CdSe/CdS nanorods due to their small conduction bandgap offset. Additionally, this alloyed shell can reduce the hole‐injection barrier and create a larger barrier for electron injection, both effects promoting a balanced injection of electrons and holes in LEDs. Hence, LEDs are reached with high brightness (160341 cd m⁻²) and high efficiency (EQE = 22%, current efficiency = 23.19 cd A⁻¹), which are the highest values to date for nanorod LEDs.
... Quantum dots (QDs) have emerged as a distinctive and advanced category of emissive materials, exhibiting exceptional material properties, such as high color saturation, emission wavelengths, a narrow spectral line width, compatibility with solution-based processing, and robust stability in various environmental conditions [1][2][3][4][5][6]. These inherent characteristics have significantly facilitated the development of QD-based devices, particularly in quantum dot light-emitting diodes (QLEDs), which exhibit unique and captivating optoelectronic characteristics [7][8][9][10][11]. Ongoing research in this field has greatly improved QLED performance. ...
Many quantum dot light-emitting diodes (QLEDs) utilize ZnO nanoparticles (NPs) as an electron injection layer (EIL). However, the use of the ZnO NP EIL material often results in a charge imbalance within the quantum dot (QD) emitting layer (EML) and exciton quenching at the interface of the QD EML and ZnO NP EIL. To overcome these challenges, we introduced an arginine (Arg) interlayer (IL) onto the ZnO NP EIL. The Arg IL elevated the work function of ZnO NPs, thereby suppressing electron injection into the QD, leading to an improved charge balance within the QDs. Additionally, the inherent insulating nature of the Arg IL prevented direct contact between QDs and ZnO NPs, reducing exciton quenching and consequently improving device efficiency. An inverted QLED (IQLED) utilizing a 20 nm-thick Arg IL on the ZnO NP EIL exhibited a 2.22-fold increase in current efficiency and a 2.28-fold increase in external quantum efficiency (EQE) compared to an IQLED without an IL. Likewise, the IQLED with a 20 nm-thick Arg IL on the ZnO NP EIL demonstrated a 1.34-fold improvement in current efficiency and a 1.36-fold increase in EQE compared to the IQLED with a 5 nm-thick polyethylenimine IL on ZnO NPs.
