Quantum Dot Light-emitting Diodes Research Articles

Efficient and Stable Quantum‐Dot Light‐Emitting Diodes with Trilayer PIN Architecture

AbstractAlthough the performance of quantum dot light‐emitting diodes (QLEDs) has been greatly improved in recent years, the multilayer device structure has become increasingly complex, limiting the practical application of QLEDs. Here, a novel trilayer PIN QLED with only three functional layers, which are Spiro‐OMeTAD:TFB bulk‐heterojunction (BHJ) hole transport layer (HTL), quantum‐dot emitting layer and ZnMgO electron transport layer is demonstrated. Due to the enhanced hole injection capability and suppressed electron leakage of Spiro‐OMeTAD:TFB BHJ HTL, the trilayer PIN QLED can show an excellent external quantum efficiency (EQE) of 25.1% and an impressive brightness of 299300 cd m−2 at only 8 V, which are significantly higher than those of conventional QLED. Moreover, the device stability is also remarkably improved due to the mitigation of hole accumulation and removal of unstable PEDOT:PSS. By using liquid alloy EGaIn as cathode, a fully solution‐processed vacuum‐free trilayer PIN QLED with a higher EQE of 27.3% can be further realized. The developed trilayer PIN QLEDs, with better performance and fewer functional layers, can promote the commercialization of QLED technology.

Recent Advances in Light Outcoupling from Quantum-Dot Light-Emitting Diodes

Recent Advances in Light Outcoupling from Quantum-Dot Light-Emitting Diodes

Inkjet printing of mixed layers comprising multinary semiconductor quantum dots and charge transport materials for light‐emitting diode displays

AbstractQuantum dots (QDs) are important luminescent structures with applications in wide‐color‐gamut displays requiring exceptional color reproducibility. Multinary semiconductor QDs are expected to serve as eco‐friendly materials to replace conventional QDs owing to the narrow spectral widths and tunable bandgaps of these QDs. However, the application of multinary QDs, which tend to exhibit defect‐related emissions, to QD light‐emitting diode (QLED) displays will require electroluminescence to be obtained from QLEDs incorporating inkjet‐printed emitting layers. The present work examines QLEDs exhibiting vibrant color emissions based on blue‐emitting Zn–Se–Te QDs, green‐emitting Ag–In–Ga–S QDs, and red‐emitting Ag–Cu–In–Ga–S QDs. Each such QLED contains QD emitting layers comprising a mixture of charge transport materials. The spectra obtained from these RGB QLEDs fabricated by spin‐coating show very high color purity. Passive matrix QLED displays incorporating these QDs are also fabricated by inkjet printing to demonstrate the high color purity that can be obtained from multinary QDs in displays. In conjunction with passive matrix driving, these displays produce clear moving images with vibrant electroluminescence originating from the multinary QDs. The present results indicate that these QDs have significant potential for utilization in wide‐color‐gamut displays.

Atomic Layer Deposition for Stable InP-Based On-Chip Quantum Dot microLEDs: Hybrid Quantum Dot Pockets.

Recent advances in synthesis techniques yield InP-based QDs with optical properties comparable to those of benchmark Cd-based QDs, making InP-based QDs viable alternatives to toxic Cd-based QDs for applications such as quantum dot LEDs (QLEDs). However, QLEDs typically suffer from a loss of luminescence over time due to exposure of the QDs to ambient air. To avoid this, state-of-the-art hybrid barrier layers are explored consisting of alternating organic/inorganic layers. In this study, InP-based QD thin films and InP-based QDs embedded in Kraton polymers are encapsulated with a thin metal oxide barrier layer by atomic layer deposition (ALD). Specifically, Al2O3, TiO2, and ZnO thin films are deposited using trimethylaluminum (TMA), tetrakis(dimethylamino)titanium (TDMAT), and diethylzinc (DEZ), with H2O as the reactant. In situ photoluminescence (PL) is used to evaluate the optical response of the InP-based QDs during the ALD coating. The results show that ALD on pristine QD thin films causes degradation of luminescence, while this is not observed for polymer-embedded QDs. The long-term stability of the (ALD-coated) samples is investigated by accelerated degradation in a humidity chamber at a high temperature. Using a single Al2O3 ALD thin film as a capping layer for polymer-embedded QDs, greater stability of the QD-PL over a period of at least 300 h is found compared to pristine QD samples. A similar study is performed with InP-based QDs embedded in UV-patterned polymer (thiol-ene) structures, the so-called QD pockets, envisioned for use in on-chip quantum dot microLEDs. These QD pockets are purposefully designed for pick-and-place operations to reduce the complexity of the on-chip quantum dot microLED manufacturing process. The PL stability was significantly improved after incorporating Al2O3 ALD thin films, with these hybrid QD pockets showing no clear signs of degradation after 140 h. The combination of polymer embedding and ALD with the merits and scalability of the QD pocket structure is demonstrated to be an effective approach to improving the long-term QD stability and shows promise for the development of stable, InP-based on-chip quantum dot microLEDs.

Studies of Quantum Dots in Light-Emitting Diodes

Quantum dots are regarded as a revolutionary material especially in display and lighting applications due to their excellent optical properties. Their high quantum efficiency, high color purity, low-cost solution processability, and tunable emission wavelength, facilitate the realization of their potential for display applications. Therefore, researchers have made a lot of effort in the development and characterization of quantum dot light-emitting diodes (QD-LEDs) to achieve high color purity and brightness in light-emitting devices. Meanwhile, with the aim of commercialization, inefficiency, instability, and cost seem to be inevitable challenges that need to be faced. Progression and challenges of using quantum dots in light-emitting diodes are two aspects that are concentrated on in this review. It also affords methods to tackle these challenges. Lastly, the potential applications of QD-LEDs in displays and lighting are investigated. Their potential to revolutionize various industries are also highlighted. By overcoming current limitations and capitalizing on emerging opportunities, QD-LEDs technology will have further applications. This work will help promote in-depth research on QD-LEDs.

Bioinspired Three-Mode Photosensitive Synaptic LED for Optical Information Processing.

Inspired by human sense organs, AI is advancing toward multimodal perception, with display technology evolving into intelligent human-computer interaction tools. However, hardware networks with multimodal responses connected by different devices bring problems such as delayed information transfer and inefficiency. Thus, an innovative three-mode photosensitive synaptic LED (PSSL) is first proposed by adding a photosensitive layer indacenodithiophene-benzothiadiazole (IDTBT) to the quantum-dot light-emitting diode (QLED), switched by changing the bias voltage. The self-powered PSSL has a photoresponse range from 310 nm to 808 nm (ultraviolet-near-infrared, UV-NIR). The device exhibits a bipolar response under red and UV light at 1 V. When the voltage reaches the turn-on voltage, the PSSL device turns into a neuromorphic LED, exhibiting conductivity enhancement under red-light irradiation and suppression under UV-light irradiation. As a result, the PSSLs are expected to be applied in the field of optical encryption communication and in neuromorphic display.

Inverted Red Quantum Dot Light-Emitting Diodes with ZnO Nanoparticles Synthesized Using Zinc Acetate Dihydrate and Potassium Hydroxide in Open and Closed Systems

We developed inverted red quantum dot light-emitting diodes (QLEDs) with ZnO nanoparticles synthesized in open and closed systems. Wurtzite-structured ZnO nanoparticles were synthesized using potassium hydroxide and zinc acetate dihydrate at various temperatures in the open and closed systems. The particle size increases with increasing synthesis temperature. The ZnO nanoparticles synthesized at 50, 60, and 70 °C in the closed system have an average particle size of 3.2, 4.0, and 5.4 nm, respectively. The particle size is larger in the open system compared to the closed system as the methanol solvent evaporates during the synthesis process. The surface defect-induced emission in ZnO nanoparticles shifts to a longer wavelength and the emission intensity decreases as the synthesis temperature increases. The inverted red QLEDs were fabricated with a synthesized ZnO nanoparticle electron transport layer. The driving voltage of the inverted QLEDs decreases as the synthesis temperature increases. The current efficiency is higher in the inverted red QLEDs with the ZnO nanoparticles synthesized in the closed system compared to the devices with the nanoparticles synthesized in the open system. The device with the ZnO nanoparticles synthesized at 60 °C in the closed system exhibits the maximum current efficiency of 5.8 cd/A.

Full-angle light out-coupling enhancement of quantum dot light-emitting diodes by Mie-scattering micro-lens arrays

High efficiency, high brightness, and long-life quantum dot light-emitting diodes (QLEDs) are crucial for realizing integrated display and lighting applications. However, about 80% of the light is confined inside the device with substrate mode, waveguide mode, and plasma mode, which greatly weakens the brightness, efficiency and lifetime of the device. Here, a quasi-periodic concave template with large area was fabricated through the spontaneous condensation of droplets on the substrate surface. Based on the quasi-periodic template, SiO2 micro-lens arrays (SiO2-MLAs) Mie scattering composite structure was fabricated by imprinting on a SiO2-nanosphere thin film, which significantly improved light out-coupling at full angles with optimized quasi-Lambertian luminescence characteristics. In comparison to the planar QLED with state-of-the-art, the external quantum efficiency (EQE) demonstrated a qualitative improvement (>20%). Accordingly, the EQE, luminance (L), T50 lifetime (reduce to half brightness) of the green QLEDs with SiO2-MLAs structure have been optimized by 22%, 28%, 31%, and up to 24.21%, 381962.6 cd/m2, 111335 h, respectively. This strategy provides valuable insights into mass-producing and utilizing SiO2-MLA Mie scattering composite structures to boost QLED performance in high-efficiency display and lighting applications.

High-Performance, High-Resolution Quantum Dot Light-Emitting Diodes with Self-Assembly Single-Molecular Interface Modification.

With the development of near-eye displays, the demands for display resolution and performance are increasing. Quantum dot performance is virtually independent of pixel size, making it an efficient way to display ultrahigh resolution. However, the low efficiency of high-resolution quantum dot devices has been an urgent technical bottleneck to be solved. Here, we constructed a dense single-molecule modification layer and a leakage current blocking layer for high-resolution devices using self-assembly, thereby realizing ultrahigh-resolution, high-efficiency, and stable high-resolution quantum dot light-emitting diodes (QLEDs). The peak external quantum efficiencies of the red devices are 24.68% (8759 PPI) and 19.54% (26075 PPI), respectively, with an exceptional long lifetime (T95@1000 nit) up to 4871 h. In addition, we explored the feasibility of this modification strategy on non-Cd-based quantum dots. In conclusion, our strategy effectively improves the performance of high-resolution devices and provides a superior approach for realizing near-eye display applications.

Quantifying Efficiency Roll-Off Factors in Quantum-Dot Light-Emitting Diodes.

The application of quantum-dot light-emitting diodes (QLEDs) is hindered by efficiency roll-off at high current densities. Factors contributing to this roll-off include Auger recombination, electric field-induced quenching, Joule heating, and electron leakage into the hole transport layer. However, a method to quantitatively attribute the contribution of each factor to roll-off has not yet been available, leaving the primary cause of roll-off unidentified. This work addresses this gap using electrically pumped transient absorption spectroscopy, which measures the accumulated electrons and electric field in quantum dots (QDs). This study also introduces a method to quantify electron leakage in QLEDs using this spectroscopic technique. Based on the spectroscopic experimental results, the contribution of each factor to roll-off is quantified. A green QLED with a peak external quantum efficiency (EQE) of 26.8% is studied as an example. The EQE declines to 20.5% at a current density of 354mA cm-2, where field-induced quenching accounts for 5% of the efficiency roll-off, and electron leakage contributes 95%. Contributions from Auger recombination and heat-induced quenching are negligible. This work demonstrates strong correlations between roll-off and electron leakage amplitude using statistical data obtained in multiple QLEDs, confirming that electron leakage is the primary factor in EQE roll-off.

Hole-Induced Degradation of ZnMgO and Insights into the Causes of the Limited Electroluminescence Stability of Blue Quantum Dot Light-Emitting Devices

Hole-Induced Degradation of ZnMgO and Insights into the Causes of the Limited Electroluminescence Stability of Blue Quantum Dot Light-Emitting Devices

High Efficiency Ultra-Narrow Emission Quantum Dot Light-Emitting Diodes Enabled by Microcavity.

A wide-color-gamut display enableby a narrow emission linewidth facilitates a visually immersive experience akin to the real world. Quantum dot light-emitting diodes (QLEDs) with excellent color purity and high efficiency hold great promise as future candidates for high-definition displays. However, most devices typically exhibit emission linewidths exceeding 20nm, and lack a universal strategy for further enhancing the color purity. In this study, a planar microcavity structure for realizing ultra-narrow emissions is developed by incorporating a distributed Bragg reflector into normal electroluminescent devices. By leveraging the strong optical resonance effect derived from this microcavity structure, red QLEDs are successfully fabricated with an extraordinary full width at half maximum of 11nm in the normal direction, beyond the BT.2020 color coordinates. The fabricated red-microcavity QLEDs exhibit a considerable enhancement in the external quantum efficiency, which increases from 28.2% to 35.6%, together with an extended operating lifetime. The strategy adopted herein will serve as an effective reference for achieving ultra-narrow emission and high-efficiency QLEDs.

PEDOT:PSS-Free Quantum-Dot Light-Emitting Diode with Enhanced Efficiency and Stability.

Although high-performance quantum-dot light-emitting diodes (QLEDs) have been achieved, their stability is still limited due to the use of unstable PEDOT:PSS as the hole injection layer (HIL). Here, we developed a PEDOT:PSS-free QLED by using a binary PTAA:F4-TCNQ HIL. Because the PTAA, with a highest occupied molecular orbital (HOMO) level of ∼5.20 eV, can facilitate hole injection from ITO to the hole transport layer, and the F4-TCNQ can act as the electron acceptor dopant to improve the hole density and hole mobility of PTAA, the PTAA:F4-TCNQ HIL can exhibit excellent hole injection capability. As a result, the PEDOT:PSS-free QLED can exhibit a high EQE of 24.19% and an impressive brightness of 367,200 cd/m2, which are significantly higher than those of conventional QLEDs. Moreover, due to the improvement of device performance and the removal of PEDOT:PSS, the PEDOT:PSS-free QLED can also exhibit a high T95 operational lifetime of 4206 h at 1000 cd/m2 and an excellent T80 shelf lifetime of 207.41 h at 136400 cd/m2, which are about 1.6- and 3.3-fold those of conventional QLEDs, respectively. We believe that the demonstrated PEDOT:PSS-free QLED, with higher performance and stability, will promote the practical application of QLEDs in displays.

Optical Applications of CuInSe2 Colloidal Quantum Dots.

The distinctive chemical, physical, electrical, and optical properties of semiconductor quantum dots (QDs) make them a highly fascinating nanomaterial that has been extensively studied. The CuInSe2 (CIS) QDs demonstrates great potential as a nontoxic alternative to CdSe and PbSe QDs for realizing high-performance solution-processed semiconductor devices. The CIS QDs show strong light absorption and bright emission across the visible and infrared spectrum and have been designed to exhibit optical gain. The special characteristics of these properties are of great significance in the fields of solar energy conversion, display, and electronic devices. Here, we present a comprehensive overview of the potential applications of colloidal CIS QDs in various fields, with a particular focus on solar energy conversion (such as QD solar cells, QD-sensitized solar cells, and QD luminescence solar concentrators), solar-to-hydrogen production (such as photocatalytic and photoelectrochemical H2 production), and QD electronics (such as QD transistors, QD light-emitting diodes, and QD photodetectors). Furthermore, we offer our insights into the current challenges and future opportunities associated with CIS QDs for further research.

Elucidating the Impact of Electron Accumulation in Quantum-Dot Light-Emitting Diodes.

Quantum-dot (QD) light-emitting diodes (QLEDs) are promising candidates for future display technology. An imbalance in the injection of electrons and holes into QLEDs leads to the accumulation of excess charges, predominantly electrons, in the QDs. The precise effects of these accumulated electrons have not yet been fully quantified. This study examines how electron accumulation affects QLED efficiency by operating multiple QLEDs at the same voltage and analyzing the correlation between device efficiency and the number of accumulated electrons, as measured by using electrically pumped transient absorption technology. We analyzed 186 QLED devices made with QDs of different colors and quantum yields. Our results show that when QLEDs utilize QDs with a quantum yield of 95%, electron accumulation indeed reduces device efficiency. However, in QLEDs using QDs with a quantum yield below 70%, a higher density of accumulated electrons enhances the device efficiency.

Optimizing QLED Performance and Stability via the Surface Modification of PEDOT:PSS Experimental Insights and DFT Calculations.

The presence of the acidic and weak ionic conductor polystyrenesulfonate (PSS) in poly(3,4-ethylenedioxythiophene:PSS (PEDOT:PSS) leads to degradation and limits the charge transfer within quantum dot light-emitting diodes (QLEDs). Two-step solvent treatment resulted in a 40% reduction of PSS, which could be attributed to ethylene glycol (EG) attenuating the ionic interactions between PSS and PEDOT via interacting with PSS through hydrogen bonding. Methanol dissolved the predominant PSS and EG from the surface. The redshift of the peak representing the symmetrical vibration of Cα═Cβ in the Raman spectrum confirmed the conformation of benzoid structure to quinoid structure after the surface treatment. This conformation was attributed to the extension of the conjugation length and the reduction of the energy barrier within the PEDOT chain. This resulted in the improved conductivity and charge hopping of the PEDOT:PSS, which was also proven using density functional theory (DFT) calculations. Reducing the insulating and acidic PSS improved the electroluminescence performance and extended the operational lifetime of the QLEDs. The tris(dimethylamino)phosphine-based InP QLEDs exhibited an external quantum efficiency (EQE) of 6.4%, that value is comparable to those of tris(trimethylsilyl)phosphine-based QLEDs, and operational lifetime (T50) of 125.6 h.

Direct Photopatterning of Colloidal Quantum Dots with Electronically Optimized Diazirine Cross-Linkers.

Colloidal quantum dots (QDs) with a wide color gamut and high luminescent efficiency are promising for next-generation electronic and photonic devices. However, precise and scalable patterning of QDs without degrading their properties and their integration into commercially relevant devices, such as digitally addressable QD light-emitting diode (QLED) displays, remain challenging. Here, we develop electronically optimized diazirine-based cross-linkers for nondestructive, direct photopatterning of QDs and, ultimately, building the active-matrix QLED displays. The key to the cross-linker design is the introduction of electron-donating substituents that permit the formation of ground-state singlet carbenes for air-stable and benign QD photopatterning. Under ambient conditions, these cross-linkers enable the patterning of heavy metal-free QDs at a resolution of over 13,000 pixels per inch using commercial i-line photolithography. The patterned QD layers fully preserved their optical and optoelectronic properties. Pixelated electroluminescent devices with patterned InP/ZnSe/ZnS QD layers show a peak external quantum efficiency of 15.3% and a maximum luminance of about 40,000 cd m-2, outperforming those made by existing QD patterning approaches. We further show the seamless integration of patterned QLEDs with thin-film transistor circuits and the fabrication of dual-color active-matrix displays. These results underscore the importance of designing photochemistry for QD patterning, and promise the implementation of direct photopatterning methods in manufacturing commercial QLED displays and other integrated QD device platforms.

Interfacial Modification of ZnO/ZnMgO Bilayer for Efficient and Stable InP Quantum Dot Light-Emitting Diodes via Ultraviolet Ozone Treatment.

The development of efficient charge transport layers is crucial for realizing high-performance and stable quantum dot light-emitting diodes (QD-LEDs). The use of a ZnO/ZnMgO bilayer as an electron transporting layer (ETL) has garnered considerable attention. This configuration leverages the high electron mobility of ZnO and the favorable surface state of ZnMgO. Furthermore, the versatility of this configuration extends to its wide range of thickness tunability, rendering it suitable for the construction of thick devices for top-emitting structures with microcavities. However, despite the promising attributes of this bilayer configuration, the impact of the ZnO/ZnMgO bilayer ETL interface on QD-LEDs performance remains largely unexplored. Thus, this study investigated the effect of ultraviolet ozone (UVO) treatment on the stabilization of the ZnO/ZnMgO interface. UVO treatment was found to significantly enhance luminance uniformity across the QD-LEDs emission area while improving operational stability by over 4-fold. Comprehensive analyses employing X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy confirmed that UVO treatment significantly reduced the defect states of the hydroxyl groups and removed the insulating native ethanolamine ligands, thereby facilitating improved and uniform electron transport. Moreover, the effectiveness of UVO treatment in enhancing electron transport was supported by impedance analyses. Therefore, this paper presents an effective approach for enhancing the interface of a highly potent ZnO/ZnMgO bilayer ETL, which can ultimately improve the luminance uniformity and stability of QD-LEDs.

Transient Leakage Electroluminescence of Quantum-Dot Light-Emitting Diodes.

Parasitic emission or leakage emission caused by electron leakage to a hole transport layer in quantum-dot light-emitting diodes (QLEDs) critically impacts device efficiency and operational stability. The buildup dynamics of such emission channels, however, was insufficiently researched. Herein, we investigate transient electroluminescence dynamics of leakage emission in red/green/blue (R/G/B) QLEDs and reveal notable contrast for R and G. In RQLEDs, leakage emission exhibits delayed turning-on than primary emission, which is attributed to much slower filling up from lower-energy electron states and the initial quenching by nonradiative recombination with trapped holes. For GQLEDs, leakage emission turns on much more concurrently, and emission preferably starts from higher energy states. For R/G QLEDs, under varied offset voltage modulation, the current efficiency of primary emission is invert-correlated to leakage emission, reconfirming leakage emission as a loss indicator.

Synaptic Feature of Quantum Dot Light-Emitting Diodes for Visualization of Learning Process.

Brain-inspired electronics with synaptic functions hold significant promise for advancing artificial intelligent applications. In this study, we demonstrate the synaptic feature of quantum-dot light-emitting diodes (QLEDs), which can convert electrical pulses into synapse-like light signals (the brightness gradually increases as the electrical pulses are prolonged). These features are analogous to learning and forgetting in biological synapses. The enhancement of brightness can be attributed to the reduction of charge transfer from the quantum dots to ZnO electron transport layer and resistive switching effect. With an integrated complementary metal-oxide-semiconductor (CMOS) drive, arrayed synaptic QLEDs can simulate the visualization of brain-like learning processes, which can reduce the noise toward high image recognition rate (>95.0%) by deep neural networks. Our findings introduce a novel brain-inspired optoelectronic approach with potential applications in optical neuromorphic systems.