Quantum Dot Light-emitting Diodes Research Articles

Time-Resolved Mechanism of Positive Aging in InP Quantum-Dot Light-Emitting Diodes.

Positive aging has been reported to be effective for enhancing electroluminescence characteristics of quantum dot (QD) based optoelectrical devices. This study investigated the intricate mechanisms underlying the positive aging effect in quantum-dot light-emitting diodes (QLEDs) influenced by encapsulation with ultraviolet-curable resin. A 120-h analysis assessed the impact of the resin on the electron transport layer and emission layer, utilizing a strategically positioned perfluorinated ionomer (PFI) interlayer. The PFI layer effectively delayed the Al2O3 formation at the zinc magnesium oxide (ZMO)/Al interface and further reduced the interactions within the QD/ZMO interface, thereby curtailing exciton quenching at the interfaces. The time-sequential effect of positive aging demonstrated that resin encapsulation effectively passivates the ZMO surfaces after 12 h. The positive aging facilitated the reaction between aluminum and oxygen from ZMO, contributing to Al2O3 formation within 48 h of aging. Furthermore, positive aging passivated the defect states of the QD surface and the QD/ZMO interface, reducing exciton quenching at the QD or QD/ZMO interface. The enhanced electron injection and reduced exciton quenching resulted in aged InP QLEDs, exhibiting an external quantum efficiency of 12.04%. This is a significant increase from the 3.16% observed in the control device. Finally, a sequential mechanism of positive aging in InP QLEDs was devised, providing new insights into the time-related operation of aging agents. This study elucidates an advanced time-resolved mechanism of positive aging, thereby offering valuable insights into the intricate dynamics of excitons within the domain of QLED physics.

Doping bilayer hole-transport polymer strategy stabilizing solution-processed green quantum-dot light-emitting diodes.

Quantum-dot light-emitting diodes (QLEDs) are solution-processed electroluminescence devices with great potential as energy-saving, large-area, and low-cost display and lighting technologies. Ideally, the organic hole-transport layers (HTLs) in QLEDs should simultaneously deliver efficient hole injection and transport, effective electron blocking, and robust electrochemical stability. However, it is still challenging for a single HTL to fulfill all these stringent criteria. Here, we demonstrate a general design of doping-bilayer polymer-HTL architecture for stabilizing high-efficiency QLEDs. We show that the bilayer HTLs combining the electrochemical-stable polymer and the electron-blocking polymer unexpectedly increase the hole injection barrier. We mitigated the problem by p-doping of the underlying sublayer of the bilayer HTLs. Consequently, green QLEDs with an unprecedented maximum luminance of 1,340,000 cd m-2 and a record-long operational lifetime (T95 lifetime at an initial luminance of 1000 cd m-2 is 17,700 hours) were achieved. The universality of the strategy is examined in various polymer-HTL systems, providing a general route toward high-performance solution-processed QLEDs.

Probing the Operation of Quantum-Dot Light-Emitting Diodes Using Electrically Pumped Transient Absorption Spectroscopy.

The quantum-dot light-emitting diode (QLED) is a new generation light emission source that holds great promise for display and lighting applications. Understanding the dynamics of electrons and holes in QLEDs during their operation is crucial for future QLED optimization, but a time-resolved technology capable of characterizing electrons is still lacking. To tackle this challenge, we develop a unique electrically pumped transient absorption (E-TA) spectroscopy to probe the density of electrons in the QD layer with a nanosecond time resolution. The E-TA result provides a comprehensive understanding of the electron dynamics in QLEDs by quantifying the electron injection time after external voltage on, electron release time after external voltage off, and equilibrated electron density (Ne) in the QD layer during device operation. By combining E-TA technology with time-resolved electroluminescence and transient current measurements, we present a comprehensive overview of the dynamics of both electrons and holes in a QLED during operation.

Narrow‐Bandwidth I–III–VI Semiconductor Nanocrystals: Synthesis, Luminescence and Applications in Quantum‐Dot Light‐Emitting Diodes

AbstractI‐III‐VI semiconductor nanocrystals (NCs) have emerged as promising candidates in quantum‐dot light‐emitting diodes (QLEDs) due to their environmental‐benign nature and capability for large‐scale tunable emission as well as straightforward synthesis. However, the photoluminescence (PL) emission of I–III–VI type NCs, as reported in numerous studies, exhibits a broader full width at half maximum (FWHM), adversely affecting their color purity. This review delineates the advancements in the development of narrow‐bandwidth I–III–VI NCs, focusing on their synthesis strategies, luminescence mechanisms, and applications in QLEDs. It concludes with a discussion on the challenges confronting narrow‐bandwidth I–III–VI‐based QLEDs and outlines potential strategies for improving device performance.

On the Electroluminescence of InP Quantum Dots with Varied ZnSe/ZnS Shell Thickness

Quantum dots (QDs) have emerged as a highly promising material for future display applications, offering advantages such as tunable emission wavelengths, narrow linewidths, and the ability to cover a broad color gamut. Traditionally, cadmium selenide (CdSe) QDs have dominated the electroluminescence (EL) QD light-emitting Diode (QLED) landscape. However, the presence of toxic metal cadmium in CdSe QDs has raised environmental and safety concerns. In recent years, indium phosphide (InP) materials, free from heavy metals, have become a focal point in EL QLED research. For InP EL QLED, the shell structure of InP/ZnSe/ZnS core/shell/shell QDs plays a crucial role in influencing carrier behavior and device performance. In this study, we prepare InP QDs with three different shell thickness, including ZnSeS shell with thickness of more than 6 nm (InP/ZnSe/ZnS QD size > 15 nm), and explore their EL properties. The research aims to investigate the potential mechanisms governing the interplay between shell thickness and carrier recombination, contributing to the optimization of InP QLEDs. Figure 1

(Invited) Toward Ultrathin Flexible Quantum Dot Light-Emitting Diodes and Photodetectors for the Wearable Electronics

Flexible optoelectronic devices, encompassing light-emitting diodes (LEDs) and photodetectors, capable of seamlessly transforming their shapes, represent a prominent trend in next-generation electronics. Quantum dots, widely employed as emitters and light absorption layers, are chosen for their high luminous performance, size-dependent color tunability, and excellent light absorption properties. Moreover, their solution processability and ultrathin form factor make them particularly appealing for the development of ultrathin wearable optoelectronic devices.Here, we introduce ultrathin flexible quantum dot light-emitting diodes and photodetectors designed for wearable electronics. Utilizing intaglio transfer printing, we can produce high-definition red-green-blue quantum dot LEDs, suitable for integration into skin-attachable displays. Moreover, our ultrathin, self-powered, heavy-metal-free photodetectors, based on copper indium selenide nanoparticles, showcase outstanding optical performance (specific detectivity: 2.10 × 1012 Jones at zero-bias) and find application in wearable photoplethysmography (PPG) sensors.

(Digital Presentation) Cooperative Quantum Dynamics of Excitons in Colloidal Quantum Dots for Novel Optoelectronic Applications

Colloidal semiconductor quantum dots (QDs) are excellent materials for combining photonics and nanoscience. They have been extensively studied to understand the photophysics of quantum confined excitons and to develop efficient optoelectronic devices such as QD solar cells and light-emitting diodes. The photophysical processes of QD devices are governed by a large number of QDs constructing the devices. Therefore, cooperative quantum dynamics of QDs, i.e., synchronous responses of QD ensembles, have a great potential for boosting optoelectronic device performances.Before the use of cooperative quantum dynamics in QD devices, the understanding of quantum dynamics of excitons, i.e., coherent properties of multiexcitons, is required to draw out quantum cooperativity from QD ensembles. In order to elucidate coherent dynamics of multiexcitons during resonant laser-pulse excitation, we performed a quantum interference spectroscopy of QD responses. We found that multiexcitons generate the high-frequency coherent oscillations with integer multiples of the exciton resonance frequency, which is called harmonic quantum coherence, and that the coherent responses govern the microscopic processes in the resonant multiphoton absorption [1-3]. Furthermore, it was demonstrated that the coherent responses were detectable as photocurrent signals in thin films of QD ensembles [4]. These developments build up an essential base of quantum cooperativities emerging in optoelectronics.Here, we report recent investigations of cooperative quantum dynamics in QDs. In optoelectronic devices, electronic couplings between QDs are expected to generate a new type of quantum cooperativities. To generate electronic coupling, we fabricated closely packed PbS QDs by using a ligand exchange method. We found that nonlinear photocurrent signals are strongly enhanced in the QD solids. The comparison analysis of different ligands clarified that the enhancement ratio increases monotonically with the decrease of the inter-QD distance [5]. This means that the signal enhancement originates from the electronic coupling and cooperative behavior of QDs. These findings show that the cooperative quantum dynamics provide a new route to advanced optoelectronic technology such as nonlinear amplifiers.Part of this work was supported by JSPS KAKENHI (JP19H05465, JP22H01990, and JP23K17877), JST PRESTO (JPMJPR23H3), and JST CREST (JPMJCR21B4).

(Invited) Electronic Trap Characterization in QD Devices Using Impedance Analysis Techniques

Colloidal quantum dots (QDs) are semiconductor nanocrystals with dimensions in the range of tens of nanometers. They exhibit unique optical and electronic properties, such as a size-dependent band gap and high absorption cross-section. These properties make them attractive for optoelectronic applications such as photovoltaics, light-emitting diodes (LEDs), and photodetectors. However, QD optoelectronics pose significant challenges due to trap states, which mainly originate from the surface states of nanocrystals. Therefore, it is necessary to analyze the trap states of QDs to understand their electronic properties and optimize their performance in optoelectronic applications. With this understanding, we want to correlate the cause of the trap states with the chemical defect to design better passivation strategies.In this discussion, we will explore the principles of electronic trap analysis in colloidal quantum dots based on impedance analysis techniques as a complementary method to spectroscopy. We will provide examples of electronic trap analysis in PbS QD photovoltaics and InP QD LEDs to demonstrate the effectiveness of this approach. We can electrically probe the QD layers in the device using an impedance analyzer to analyze the energy level and density of trap states. By combining chemical analysis and a theoretical approach with this technique, we attempted to correlate the trap states and chemical defects.

(Invited) Synthesis of Ultra-Bright Colloidal Silicon Quantum Dots (PLQY~80%) and Highly Efficient LEDs (EQE>10%)

Silicon is the second most abundant element in the Earth’s crust and could be considered an environmentally benign material. Silicon has been widely used in electronic devices and solar cells, and it has dominated the semiconductor research and development. However, the photoluminescence (PL) of the bulk material shows an invisible NIR wavelength (λ ~ 1100 nm). In addition, the PL quantum yield (PLQY) of bulk crystal Si is ~0.01% owing to the indirect transition nature. In contrast, silicon quantum dots show PL in the entire visible spectral region [1]. Furthermore, colloidal silicon quantum dots demonstrate PLQYs of 60% [2], and thereby they give highly impact as environmentally benign and heavy-metal-free quantum dots (i.e., In, Cd, and Pb free) for displays, lighting, and biomedical imaging via solution processing. Indeed, silicon quantum dot LEDs have recently been produced from a natural substance, by upcycling discarded rice husks [3].Herein, red PL with a PLQY of 60–80% and LEDs with an external quantum efficiency of >10% were obtained at 1/3600th of the cost of existing methods. This was achieved by using trichlorosilane and an organic solvent to prepare a hydrogen silsesquioxane (HSQ) polymer, and by optimizing the LED optoelectric properties. Based on many silicon quantum dots synthesized in our study, each of them enabled us to quantitative the relationships HSQ polymer structure, PLQY, and LED efficiency, exhibiting the critical parameters that governed their performances. Moreover, the simple and cost-effective methods proposed here could prove invaluable for the synthesis and characterization of silicon quantum dots in the near future.

Highly efficient printed quantum dot light-emitting diodes through ultrahigh-definition double-layer transfer printing

Highly efficient printed quantum dot light-emitting diodes through ultrahigh-definition double-layer transfer printing

Solution-Processed Thick Hole-Transport Layer for Reliable Quantum-Dot Light-Emitting Diodes Based on an Alternatingly Doped Structure.

The operating lifetime of quantum-dot light-emitting diodes (QLED) is a bottleneck for commercial display applications. To enhance the operational stability of QLEDs, we developed a robust solution-processed highly conductive hole-transport-layer (HTL) structure, which enables a thick HTL structure to mitigate the electric field. An alternating doping strategy, which involves multiple alternating stacks of N4,N4'-di(naphthalen-1-yl)-N4,N4'-bis(4-vinylphenyl)biphenyl-4,4'-diamine and phosphomolybdic acid layers, could provide significantly improved conductivity; more specifically, the 90 nm-thick alternatingly doped HTL exhibited higher conductivity than the 45 nm-thick undoped HTL. Therefore, when applied to a QLED, the increase in the thickness of the alternatingly doped HTL increased device reliability. As a result, the lifetime of the QLED with a thick, alternatingly doped HTL was 48-fold higher than that of the QLED with a thin undoped HTL. This alternating doping strategy provides a new paradigm for increasing the stability of solution-based optoelectronic devices in addition to QLEDs.

Stable and efficient all-inorganic quantum dot light-emitting diodes based on a double-layer electron transport layer

Abstract The most advanced quantum light-emitting diodes (QLEDs) have achieved almost unity external quantum efficiency (EQE) due to organic materials for high transport efficiency, but organic materials are less stable and can lead to irreversible degradation of the devices. The use of stable inorganic hole-transporting layers (HTLs), such as NiOx, becomes a promising alternative. ZnO is a common electron-transporting material in QLEDs, which possesses high mobility and tunable conductivity properties, but ZnO-based QLEDs devices undergo loss of efficiency due to exciton bursting, poor shelf stability, and pronounced positive aging effects. SnO2, as a common electron-transporting material, has better stability than ZnO. In this work, the SnO2/ZnO dual electron transport layer strategy is adopted, which can reduce the non-radiative burst and aging behavior, and at the same time, the electron transport is regulated by adjusting the film thickness. Inorganic QLEDs with high external quantum efficiency (9.5%) as well as high operational stability have been fully realized. This strategy provides effective ideas for future high-performance display devices.

TCO-free quantum dot light-emitting diodes based on PEDOT:PSS electrode treated with mild acid

Poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT: PSS) has emerged as a promising candidate for high-cost transparent conductive oxide (TCO) electrodes. However, achieving high conductivity in PEDOT:PSS typically requires treatment with strong acids, which presents inherent risks. In this study, we propose an alternative approach utilizing a mild methane sulfonic acid (MSA) treatment for PEDOT:PSS as a transparent conductive electrode (TCE). Scanning electron microscopy (SEM) and atomic force microscopy (AFM) analyses unveil heightened phase separation between PEDOT and PSS by MSA treatment, accompanied by structural modifications in PEDOT. Moreover, the interfacial morphology between the MSA-treated electrode and the hole injection layer underwent significant changes. It is precisely this improved interface morphology, combined with enhanced conductivity, that significantly improves the efficiency of the obtained TCO-free blue quantum dot light-emitting diode (QLED) by 300 % compared to the reference device. Finally, leveraging the high performance of the TCO-free blue QLEDs, we fabricate the above device on a yellow-emissive phosphors-in-glass substrate, enabling a white light-emitting LED (WLED) device. This experiment not only elucidates the structural changes in PEDOT:PSS under mild acid treatment but also introduces an innovative strategy for developing high-performance PEDOT:PSS electrodes.

One-Pot Synthesis of Color-Tunable Narrow-Bandwidth Ag-In-Ga-Zn-S Semiconductor Nanocrystals for Quantum-Dot Light-Emitting Diodes.

I-III-VI type semiconductor nanocrystals (NCs) have attracted considerable attention due to their environmental friendly nature and large-scale tunable emission. Herein, we report the successful synthesis of full-spectrum (470 to 614 nm) Ag-In-Ga-Zn-S (AIGZS) NCs by precisely regulating the In/Ga ratios using a facile one-pot method. Intriguingly, the photoluminescence (PL) peak width exhibits a continuous narrowing trend with extended reaction time, ultimately reaching a full width at half-maximum (fwhm) of 34 nm for green AIGZS NCs. Furthermore, the exciton relaxation dynamics of AIGZS NCs were systematically investigated using time-resolved photoluminescence and femtosecond transient absorption spectroscopy. Remarkably, we successfully fabricated blue, green, and red quantum-dot light-emitting diodes (QLEDs), forecasting the potential of AIGZS NCs with high color purity for applications in full-spectrum QLEDs.

Optimization the potential of solution process fluorine passivated zinc oxide electron transport layer for stable InP-quantum dot light emitting diodes

The pervasive use of zinc oxide (ZnO) as an electron transport layer in quantum dot (QD) electroluminescent devices is constrained due to its chemical instability with the QD layer and the formation of interface quenching sites. The effect of fluorine passivation of sol-gel processed ZnO in QD light-emitting devices (QLEDs) is investigated in depth. An examination of the interaction between the ZnO surface and fluorine species revealed that the passivation of oxygen vacancies and the formation of stable hydrogen bonds with hydroxyl groups on ZnO surface have a significant influence on the stability and efficiency of the device. Such exceptional functions of fluorine have been found to effectively capture defects at the interface between ZnO and the emissive layer, therefore mitigating the interface quenching sites. The initial fluorination device demonstrated a significant improvement in external quantum efficiency (EQE) from 5.72 % to 20.07 %, and half of the device lifetime (LT50 at an initial luminance of 1500 cd m−2) was 286 h. Further passivating the remaining active oxygen on the ZnO surface can extend the stability of the device to 542 h with an EQE of 15.2 %, which is among the longest lifetime reported so far for green InP-QLEDs. Our report offers the possibility of utilizing straightforward and highly effective fluorination by spin-coating technique to attain long-lasting InP-QLED devices with remarkable performance.

Recent Progress on Blue Quantum Dot Light-Emitting Diodes from Materials to Device Engineering

Recent Progress on Blue Quantum Dot Light-Emitting Diodes from Materials to Device Engineering

Impact of Alternating-Current Operation on All-Inorganic Quantum Dot Light-Emitting Diodes.

In colloidal quantum dot light-emitting diodes (QD-LEDs), replacing organic hole transport layers (HTLs) with their inorganic counterparts is expected to yield distinct advantages due to their inherent material robustness. However, despite the promising characteristics of all-inorganic QD-LEDs, some challenges persist in achieving stable operation; for example, the electron overflow toward the inorganic HTL and charge accumulation within working devices return a temporal inconsistency in device characteristics. To address these challenges, we propose an operational approach that employs an alternating-current (AC) in all-inorganic QD-LEDs. We carry out comprehensive studies on the optoelectrical characteristics of all-inorganic QD-LEDs under direct-current (DC) or AC operation and demonstrate that AC operation can facilitate efficient charge carrier recombination within the QD emissive layer, leading to improved device efficiency and temporally invariant optoelectronic characteristics. Leveraging the intrinsic material robustness of inorganic charge transport layers (CTLs), our current study suggests a promising pathway toward enhancing the performance and stability of QD-LEDs, particularly for futuristic display applications.

Controllable Synthesis of Cadmium-Free Blue-Emitting Cu-Ga-Zn-S-Based Nanocrystals for Solution-Processed Quantum-Dot Light-Emitting Diodes.

The utility of semiconductor nanocrystals (NCs) in light-emitting diodes (LED) has shown great potential in the field of display, whereas the challenge remains in developing efficient and stable cadmium-free blue-emitting LED devices due to the poor photophysical properties of blue-emitting NCs. Herein, we develop a controllable synthesis of Cu-Ga-Zn-S (CGZS) semiconductor NCs that show blue light emission with a relative photoluminescence quantum yield exceeding 90%. Furthermore, we have successfully fabricated a solution-processed quantum-dot LED (QLED) using CGZS NCs, achieving a notable maximum external quantum efficiency (EQE) of 1.00% at a luminance of 100 cd/m2. Our work lays a foundational framework for advancing cadmium-free blue-emitting QLEDs and facilitates the development of quantum dot electroluminescent panchromatic displays.

Electroluminescent white quantum dot light-emitting diodes based on high pressure synthesis of graphitic C3N4 semiconductor

Graphitic carbon nitride (g-C3N4) has shown promising potentials for quantum dot light-emitting diodes (QLEDs), which might be an effective alternative of the mostly used Cd(Se,S)/Zn(S,Se) emitting layers and can achieve both low toxicity and high stability, meeting the goal of sustainable development. Herein, we for the first report a white electroluminescent QLED device with blue and orange dual-color emissions, which were attributed to the σ*-LP, π*- π, and π*-LP transitions, respectively, from a single-compound g-C3N4 quantum dots (QDs) by synthesizing the precursor at 0.8 Mpa high-pressure N2. By selectively exciting the δ and π bonds with X-ray, the luminescence mechanism was approved with the synchrotron radiation techniques of X-ray absorption near edge structure (XANES) and the X-ray excited optical luminescence (XEOL).The chromaticity coordinate changes from cayan color space with CIE (0.2034, 0.1939) to white color space with CIE (0.3009, 0.2697) as pressure increases from 0.06 to 0.8 Mpa, with the increased overlap of the δ* and π* orbital upon the contraction of crystal lattice. The breakthrough achieved in this work will promisingly promote the development of future white light sources as thin as paper for human home and office lighting by using direct current-driven electroluminescence of QDs, instead of photoluminescence of phosphors.

Transparent CdSe/ZnS quantum-dot light-emitting diodes with WOx/Ag/WOx transparent electrodes achieving higher efficiency than opaque quantum-dot light-emitting diodes

High-performance transparent quantum-dot light-emitting diodes (TQLEDs) are achieved through fine-tuning the top dielectric/metal/dielectric (DMD) anode structure. The transparent DMD electrodes are utilized as both the bottom cathode and top anode of TQLEDs. Employing WOx/Ag/WOx DMD anodes serves a dual purpose of transparency and hole injection, thereby streamlining the TQLED design. Investigation into the effects of the thicknesses of WOx and Ag layers on the device characteristics reveals an optimal configuration of 10-nm WOx/27-nm Ag/40-nm WOx for the DMD anode. The resulting TQLED exhibits a remarkable device light transmittance of 47 % at 530 nm. With maximum bottom and top emission current efficiencies of 34.0 and 9.42 cd/A, respectively, the total emission obtained by summing the bottom and top emissions reaches the maximum current efficiency of 41.8 cd/A, surpassing that of conventional opaque quantum-dot light-emitting diodes. This advancement underscores the successful fabrication of TQLEDs boasting higher efficiency alongside substantial light transmittance.