Quantum Dot Light-emitting Diode Devices Research Articles

Highly Luminescent CsPbBr3@Cs4PbBr6 Nanocrystals and Their Application in Electroluminescent Emitters.

Zero-dimensional perovskite nanocrystals (NCs) are becoming the most attractive material due to their excellent optical performance and better stability compared with high-dimensional perovskite. However, their application in electroluminescent (EL) emitters for high-quality displays is still limited. In this work, we successfully achieved CsPbBr3@Cs4PbBr6 NCs around 13.9 ± 0.2 nm by using the hot-injection method. Additional SnBr2 was mixed in the PbBr2 precursor to provide extra Br- ions and reduce the excessive amount of Pb2+ ions to promote the formation of CsPbBr3@Cs4PbBr6. Time resolution photoluminescence analysis indicated that the green emission of our CsPbBr3@Cs4PbBr6 NCs originated from the embedded CsPbBr3 NCs, which corresponds to our previous research. The Cs4PbBr6 crystals passivated the surface of CsPbBr3 NCs, resulting in the absence of trions for the high photoluminescence quantum yield. The as-synthesized CsPbBr3@Cs4PbBr6 NCs were used to fabricate quantum dot light-emitting diode (QLED) devices with the highest current efficiency of 4.89 cd/A. This is the best performance of the CsPbBr3@Cs4PbBr6-system QLED device, which reveals the great potential of CsPbBr3@Cs4PbBr6 NCs and will inspire further study of zero-dimensional perovskite composite NCs for EL emitters.

Highly Efficient Deep Blue Cd-Free Quantum Dot Light-Emitting Diodes by a p-Type Doped Emissive Layer.

Environmentally friendly ZnSe/ZnS core/shell quantum dots (QDs) as an alternative blue emission material to Cd-based QDs have shown great potential for use in next-generation displays. However, it remains still challenging to realize a high-efficiency quantum dot light-emitting diode (QLED) based on ZnSe/ZnS QDs due to their insufficient electrical characteristics, such as excessively high electron mobility (compared to the hole mobility) and the deep-lying valence band. In this work, the effects of QDs doped with hole transport materials (hybrid QDs) on the electrical characteristics of a QLED are investigated. These hybrid QDs show a p-type doping effect, which leads to a change in the density of the carriers. Specifically, the hybrid QDs can balance electrons and holes by suppressing the overflow of electrons and improving injection of holes, respectively. These electrical characteristics help to improve device performance. In detail, an external quantum efficiency (EQE) of 6.88% is achieved with the hybrid QDs. This is increased by 180% compared to a device with pure ZnSe/ZnS QDs (EQE of 2.46%). This record is the highest among deep-blue Cd-free QLED devices. These findings provide the importance of p-type doping effect in QD layers and guidance for the study of the electrical properties of QDs.

Impact of the resistive switching effects in ZnMgO electron transport layer on the aging characteristics of quantum dot light-emitting diodes

The phenomenon of positive aging has been frequently reported in quantum dot light-emitting diodes (QLEDs). However, the root cause for this phenomenon remains illusive. On the other hand, the commonly used electron transport material in QLEDs, ZnMgO, has been extensively studied as a resistive switching material. In this work, we found that the ZnMgO nano-particle film used in QLEDs showed a clear resistive switching effect. It is, thus, reasonable to relate the resistive switching mechanism of ZnMgO to the aging characteristics of QLED devices. We found that during the first stage of QLED aging, the efficiency of the QLED was improved due to the migration of off-lattice ions and formation of conductive filaments in the ZnMgO layer. Subsequently, as active oxygen ions migrated to the interface between quantum dots and ZnMgO, the barrier for electron transport increased due to the oxidation of quantum dots. At the same time, the conductive filaments were gradually fused due to the continuous external electric field. As a result, the performance of QLED devices continuously deteriorated.

65‐1: Invited Paper: Enhanced Efficiency of InP‐based Red and Green Quantum‐Dot Light‐emitting Diodes

Due to the inherent toxicity of cadmium selenide (CdSe)‐based quantum dots (QDs), Cd‐free alternatives are being widely investigated. Indium phosphide (InP) QDs have shown great potential as a replacement for CdSe QDs in display applications. However, the performance of InP‐based quantum dot light emitting diodes (QLEDs) is still far behind that of the CdSe‐based devices. In this study, we wanted to show the effects of different approaches to improve the performance of InP‐based QLED devices. We investigated the effect of magnesium (Mg) doping in ZnO nanoparticles, which is used as an n‐type electron transport layer, in balancing the charge transfer in InP‐based QLED devices. We found that an increasing Mg doping level can broaden the ZnO band gap, shift its energy levels, but most importantly, increase its resistivity; as a result, the electron current density is significantly reduced and the device efficiency is improved. We also investigated the effect of high‐photoluminescence quantum yield emitters and different QLED architectures on the device performance. Through optimizing QD structures and devices, red InP QLEDs with the current efficiencies reaching 11.6 cd/A and green InP QLEDs with the current efficiencies reaching 37.0 cd/A were fabricated.

P‐111: Red, Green, and Blue Quantum Rod Based Electroluminescent Light‐Emitting Diodes

Quantum dot light emitting diodes (QLEDs) emerges as a potential display technology due to its ultra‐saturated colors, small turn on voltage (Von) and high efficiency (ŋ). However, the internal quantum efficiency (IQE) of QLED devices nearly reach its ideal level, and leave light outcoupling as the only possible way to improve the external quantum efficiency (EQE). This might need a different shape of the core shell, device structure and re‐optimization of the device. In this paper, we synthesized CdSe with CdS/ZnS based red, blue and green quantum rods (QRs) and fabricated electroluminescent QRs light emitting diodes (QRLEDs), this is the first study to report for the blue QRLEDs. The solution processed zinc oxide (ZnO) nano‐particles (NP) are used as an electron transport layer. The maximum luminance and EQE values of the devices are 1288, 420 and 13 cd/m2 and 5.7% and 0.18% and 0.093 %, for the red, green and blue LEDs, respectively. This kind of new QRLEDs shows a great potential for the use of next generation high color gamut display.

65‐4: Control of Carrier Injection and Transport Behavior in QLEDs via Modulating Schottky Barrier

Carrier injection and transport behavior in quantum dot light‐emitting diodes (QLEDs) was modulated by tuning mobility and Schottky barrier of charge transport layers. Theoretical analysis indicated that not only those 2 parameters investigated but also other parameters including density of charge traps and their energy level must be considered to match charge balance in the QD layers. Experimental results demonstrated that doping strategy could modulate Schottky barrier and mobility of charge transport layers as well as density of charge traps and their energy level. With the improved charge balance, the efficiency of the QLED device was improved from 26 to 38 cd/A.

72‐3: Influence of Mobility Effect on Top‐Emission Red Quantum‐Dot Light‐Emitting Diode with Weak‐Cavity Structure

Optimization of mobility is critical to achieving carrier balance in quantum dot light‐emitting diodes (QLED) which in turn is essential for high performance devices. In this paper, our red top‐emitting QLED devices possess maximum current efficiency of ~25 cd/A. We find that with appropriate annealing temperature of quantum dots (QDs)/ZMO the charge transfer balance of the device can be improved. We also present a simulation results of carrier mobility V.S. device performance based on the software SETFOS from Fluxim AG. The carrier mobility simulation is agreed with the experimental results. We believe that an applicable annealing temperature is favorable to achieve carrier balance, highly efficient and color saturation devices for display applications.

Quantum dot light-emitting diodes with an Al-doped ZnO anode

A study of hybrid ZnCdSeS/ZnS quantum dot light-emitting diodes (QLEDs) device fabricated with indium tin oxide-free transparent electrodes is presented. Al-doped zinc oxide (AZO) prepared by magnetron sputtering is adopted in anode transparent electrodes for green QLEDs with different sputtering pressures. A Kelvin probe force microscopy measurement showed that AZO has a work function of approximately 5.0 eV. The AZO/poly(ethylene-dioxythiophene)/polystyrenesulfonate (PEDOT:PSS) interface can be adjusted by the sputtering pressures, which was confirmed by hole-only devices. AZO films with low surface roughness can form a good AZO/PEDOT:PSS interface, which can increase the holes’ injection, and result in an improved charge balance. The maximum current efficiency, luminance, and external quantum efficiency of the optimized QLED devices under a sputtering pressure of 1 mTorr can achieve values of 50.75 cd A−1, 102 500 cd m−2, and 12.94%, respectively.

High‐Resolution Inkjet Printing of Quantum Dot Light‐Emitting Microdiode Arrays

AbstractThe direct printing of microscale quantum dot light‐emitting diodes (QLEDs) is a cost‐effective alternative to the placement of pre‐formed LEDs. The quality of printed QLEDs currently is limited by nonuniformities in droplet formation, wetting, and drying during inkjet printing. Here, optimal ink formulation which can suppress nonuniformities at the pixel and array levels is demonstrated. A solvent mixture is used to tune the ejected droplet size, ensure wetting, and provoke Marangoni flows that prevent coffee stain rings. Arrays of green QLED devices are printed at a resolution of 500 pixels in.−1 with a maximum luminance of ≈3000 cd m−2 and a peak current efficiency of 2.8 cd A−1. The resulting array quality is sufficient to print displays at state‐of‐the‐art resolutions.

CdSe/ZnS Quantum-Dot Light-Emitting Diodes With Spiro-OMeTAD as Buffer Layer

We report a high-efficient green CdSe/ZnS quantum-dot light-emitting diode (QLED) using 2,2’,7,7’-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9’-Spirobifluorene (Spiro-OMeTAD) as an electron buffer layer. The high lowest unoccupied molecular orbital level of Spiro-OMeTAD suppresses the electron transfer and promotes the charge balance. The external quantum efficiency (EQE) of the green QLED device is increased by 1.93 times to 14.65%, and the brightness is increased by 1.37 times to 55 760 cd/m2. The results show that the Spiro-OMeTAD layer can effectively prevent excessive electron injection into QDs, and balance the combination of electron and hole carriers.

Investigation on Thermally Induced Efficiency Roll-Off: Toward Efficient and Ultrabright Quantum-Dot Light-Emitting Diodes.

Quantum-dot light-emitting diodes (QLEDs) with high brightness have potential application in lighting and display. The high brightness is realized at high current density (J). However, at high J, the efficiency drops significantly, thereby limiting the achievable brightness. This notorious phenomenon has been known as efficiency roll-off, which is likely caused by the Auger- and/or thermal-induced emission quenching. In this work, we show that the Joule heat generated during device operation significantly affects the roll-off characteristics of QLEDs. To realize ultrabright and efficient QLEDs, the thermal stability of QDs is improved by replacing the conventional oleic acid ligands with 1-dodecanethiol. By further using a substrate with high thermal conductivity, the Joule heat generated at high J is effectively dissipated. Because of the effective thermal management, thermal-induced emission quenching is significantly suppressed, and consequently, the QLEDs exhibit a high external quantum efficiency (EQE) of 16.6%, which is virtually droop-free over a wide range of brightness (e.g., EQE = 16.1% @ 105 cd/m2 and 140 mA/cm2). Moreover, due to the reduced efficiency roll-off and enhanced heat dissipation, the demonstrated QLEDs can be operated at a very high J up to 3885 mA/cm2, thus enabling the devices to exhibit a record-high brightness of 1.6 × 106 cd/m2 and a lumen density of 500 lm/cm2. Our work demonstrates the significance of thermal management for the development of droop-free and ultrabright QLED devices for a wide variety of applications including lighting, transparent display, projection display, outdoor digital signage, and phototherapy.

Enhanced Efficiency of InP-Based Red Quantum Dot Light-Emitting Diodes.

Due to the inherent toxicity of cadmium selenide (CdSe)-based quantum dots (QDs), Cd-free alternatives are being widely investigated. Indium phosphide (InP) QDs have shown great potential as a replacement for CdSe QDs in display applications. However, the performance of InP-based quantum dot light-emitting diodes (QLEDs) is still far behind that of the CdSe-based devices. In this study, we wanted to show the effects of different approaches to improving the performance of InP-based QLED devices. We investigated the effect of magnesium (Mg) doping in ZnO nanoparticles, which is used as an n-type electron transport layer, in balancing the charge transfer in InP-based QLED devices. We found that an increasing Mg doping level can broaden ZnO band gap, shift its energy levels, but most importantly, increase its resistivity; as a result, the electron current density is significantly reduced and the device efficiency is improved. We also investigated the effect of high-photoluminescence quantum yield emitters and different QLED architectures on the device performance. Through optimizing QD structures and devices, red InP QLEDs with the current efficiencies as high as 11.6 cd/A were fabricated.

Stepwise Bi-Layer Hole-Transport Interlayers With Deep Highest Occupied Molecular Orbital Level for Efficient Green Quantum Dot Light-Emitting Diodes

The imbalanced charge transport/injection in quantum dot (QD) light-emitting diodes (QLEDs) due to the existing excess electrons has greatly restricted the device performance. Here, we designed a stepwise bi-layer structured hole-transport layer (HTL) containing 4,4'-bis-(carbazole-9-yl)biphenyl (CBP)/1,3-bis(9H-pyrido[2,3-b]indol-9-yl) benzene (mCaP) with deep highest occupied molecular orbital (HOMO) level to realize more efficient hole injection into QD emissive layer for balancing charge in the QLED devices. The resulting green QLEDs show a maximum current efficiency (CE) of 41.2 cd A <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> corresponding to an external quantum efficiency (EQE) of 12.6%, which is a significant efficiency enhancement compared with the devices with only single-layer HTL. In addition, the device turn-on voltage is also reduced from 5 to 3 V, and meanwhile the operational lifetime is increased by more than twofold.

Realizing 17.0% external quantum efficiency in red quantum dot light-emitting diodes by pursuing the ideal inkjet-printed film and interface

Abstract Since the layer erosion often happens during the solution-processable optoelectronic devices, and thus the efficiency of inkjet-printed quantum dot light emitting diodes (QLEDs) lags behind those of spin-coated counterparts severely. Indeed, the pursuit of high-performance inkjet-printed QLEDs is fundamentally challenging. Herein, a binary QDs ink based on cyclohexylbenzene (CHB) and indane was developed with red CdZnSe/ZnS core/shell structure without using any other additives for inkjet printing. Three common hole transport materials (HTMs), poly (9-vinlycarbazole) (PVK), poly[(9,9'-dioctylfluorenyl-2,7- diyl)-co-(4,4'-(N-(4-sec-butyl))diphenylamine)] (TFB) and poly (N, N′-bis (4-butylphenyl-N, N′-bis (phenyl) benzidine) (Poly-TPD), were invoked as the underlying layer to print QDs emissive layer, and their surface wettability and solvent resistance were systematically investigated. Among them, PVK exhibited excellent surface wettability and solvent resistance, accordingly, the PVK-based printed red QLEDs exhibited the current efficiency of 28.8 cd A−1 and the maximum external quantum efficiency (EQE) over 17.0%, which was comparable to the spin-coated QLEDs (19.8%). These results demonstrate the fabrication of efficient inkjet-printed QLED devices, which will be meaningful for printed QLEDs displays.

All-solution-processed high-performance quantum dot light emitting devices employing an inorganic thiocyanate as hole injection layer

We report here the all-solution-processed, high-efficiency quantum dot light emitting diode (QLED) employing inorganic copper (I) thiocyanate (CuSCN) as hole injection layer. In comparison with the widely used injection material of poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS), the hole injection into the QD layer is significantly improved, allowing low turn-on voltage, high luminance and efficiency. By optimizing the multilayer structure and synergistically balancing the carrier injection, the resulting QLEDs exhibit high performance with the maximum current efficiency of 52.4 cd/A and external quantum efficiency of 12.0% for green device, 17.0 cd/A and 16.2% for red device. These results indicate that CuSCN is a reliable hole transport materials for low-cost, high-efficiency QLED devices.

Improving blue quantum dot light-emitting diodes by a lithium fluoride interfacial layer

Unlike green and red quantum dot light-emitting diodes (QLEDs), electron injection is not sufficient for blue QLEDs due to a higher potential barrier at the quantum dot (QD)/ZnO interface. In this work, a lithium fluoride (LiF) interlayer is inserted between ZnO and the quantum dot layer to improve the efficiency and stability of blue QLEDs. The LiF interfacial layer facilitates electron injection into QDs through the electron tunneling effect and suppresses the exciton quenching at the QD/ZnO interface. As a result, the blue QLED devices show the maximum external quantum efficiency and current efficiency of 9.8% and 7.9 cd A−1, respectively, which are 1.45 times and 1.39 times, respectively, higher than those of control devices. The operational lifetime of devices is also improved by two times. Our works indicate that interface engineering is an effective method for high efficiency and stable blue QLEDs.

Beyond OLED: Efficient Quantum Dot Light-Emitting Diodes for Display and Lighting Application.

The unique features of solution-processed quantum dots (QDs) including emission tunability in the visible range, high-quality saturated color and outstanding intrinsic stability in environment are highly desired in various application fields. Especially, for the preparation of wide color gamut displays, QDs with high photoluminescence quantum yield are deemed as the optimal fluorescent emitter that has been utilized in the backlight for liquid crystal display. Nevertheless, the commercialization of electrically driven self-emissive quantum dot light-emitting diode (QLED) display is the ultimate target due to its merits of high contrast, slim configuration and compatibility with flexible substrate. Through the great efforts devoted to material engineering and device configuration, astonishing progresses have been made in device performance, giving the QLED technology a great chance to compete with other counterparts for next-generation displays. In this review, we retrospect the development roadmap of QLED technology and introduce the essential principles in the QLED devices. Moreover, we discuss the key factors that affect the QLED efficiency and lifetime. Finally, the advances in device architectures and pixel patterning are also summarized.

An Experimental Method on the EMP Resistance of QLED Device

An experimental method on the electromagnetic pulse resistance of quantum dot light-emitting diode device is researched in this paper. If there is a strong electromagnetic pulse in the environment, the electromagnetic pulse can disturb the QLED device’s work or even destroy it through coupling into the cables of the device. The noises could overwhelm the interference wave if using irradiation experiment. So, the equivalent coupling energy is calculated, and the pulse voltage injection experiment method is adopt in this paper. The experiment results supply references to the engineering practice.

Electron transport phenomena at the interface of Al electrode and heavily doped degenerate ZnO nanoparticles in quantum dot light emitting diode

ZnO nanoparticles (NPs) of 4–5 nm, widely adopted as an electron transport layer (ETL) in quantum dot light emitting diodes (QD-LEDs), were synthesized using the solution-precipitation process. It is notable that synthesized ZnO NPs are highly degenerate intrinsic semiconductors and their donor concentration can be increased up to ND = 6.9 × 1021 cm–3 by annealing at 140 °C in air. An optical bandgap increase of as large as 0.16–0.33 eV by degeneracy is explained well by the Burstein–Moss shift. In order to investigate the influence of intrinsic defects of ZnO NP ETLs on the performance of QD-LED devices without a combined annealing temperature between ZnO NP ETLs and the emissive QD layer, pre-annealed ZnO NPs at 60 °C, 90 °C, 140 °C, and 180 °C were spin-coated on the annealed QD layer without further post-annealing. As the annealing temperature increases from 60 °C to 180 °C, the defect density related to oxygen vacancy (VO) in ZnO NPs is reduced from 34.4% to 17.8%, whereas the defect density of interstitial Zn (Zni) is increased. Increased Zni reduces the width (W) of the depletion region from 0.21 to 0.12 nm and lowers the Schottky barrier (ФB) between ZnO NPs and the Al electrode from 1.19 to 0.98 eV. We reveal for the first time that carrier conduction between ZnO NP ETLs and the Al electrode is largely affected by the concentration of Zni above the conduction band minimum, and effectively described by space charge limited current and trap charge limited current models.

Electrohydrodynamic jet-sprayed quantum dots for solution-processed light-emitting-diodes

Quantum dot light-emitting diodes (QD-LEDs) have attracted considerable interest for their wide-range of color tunability, high brightness, and good color purity due to their quantum confinement effect. However, challenges remain in achieving the necessary multilayer device structures using printing processes for display applications. Many printing technologies need to be employed to build multi-stacking patterns and devices. For the first time, QD emitting layers were printed by electrohydrodynamic (EHD) jet spray technology and applied to all solution-processed QD-LEDs except for the electrodes. The QD-LEDs have a structure of ITO/PEDOT:PSS/PVK/EHD-sprayed QDs/ZnO/Al. The sprayed QD patterns were controlled with various parameters to obtain uniform surface morphology. The optimized QD-LED device showed a luminance of 5154 cd/m2 and a current density of 2.18 cd/A, and EQE of 1.47%.