Solution processed tungsten oxide interfacial layer for efficient hole-injection in quantum dot light-emitting diodes

Nanosecond-pulsed electroluminescence from high current-driven quantum-dot light-emitting diodes

Ultrashort optical emission, with pulse duration ranging from nanoseconds to femtoseconds, is usually obtained from lasers. In this work, we achieve nanosecond-pulsed electroluminescence (EL) from a solution-processed fast-response quantum dot light-emitting diode (QLED). By modeling the QLED with a resistor-capacitor equivalent circuit and analyzing the transient current of the circuit, the dynamic of the carrier injection and transport process that fundamentally affects the transient EL of QLED is revealed, which helps to guide the optimization of fast-response QLED. Driven by a high current source, the optimized QLED can output stable and repeatable ultrashort EL with a pulse duration of 20 nanoseconds, a repetition rate of 50 kilohertz, and a high radiant exitance of 5.4 watts per square centimeter. Enabled by the nanosecond-pulsed EL, the developed QLED can be directly used as an instantaneous excitation source for time-resolved fluorescence spectroscopy. Meanwhile, its use as an exposure flash for high-speed imaging is also demonstrated.

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.

Recent Progress of Quantum Dots Light-Emitting Diodes: Materials, Device Structures, and Display Applications

Colloidal quantum dots (QDs), as a class of 0D semiconductor materials, have generated widespread interest due to their adjustable band gap, exceptional color purity, near-unity quantum yield, and solution-processability. With decades of dedicated research, the potential applications of quantum dots have garnered significant recognition in both the academic and industrial communities. Furthermore, the related quantum dot light-emitting diodes (QLEDs) stand out as one of the most promising contenders for the next-generation display technologies. Although QD-based color conversion films are applied to improve the color gamut of existing display technologies, the broader application of QLED devices remains in its nascent stages, facing many challenges on the path to commercialization. This review encapsulates the historical discovery and subsequent research advancements in QD materials and their synthesis methods. Additionally, the working mechanisms and architectural design of QLED prototype devices are discussed. Furthermore, the review surveys the latest advancements of QLED devices within the display industry. The narrative concludes with an examination of the challenges and perspectives of QLED technology in the foreseeable future.

Color Revolution: Prospects and Challenges of Quantum-Dot Light-Emitting Diode Display Technologies

Light-emitting diodes (LEDs) based on colloidal quantum-dots (QDs) such as CdSe, InP, and ZnSeTe feature a unique advantage of narrow emission linewidth of ≈20 nm, which can produce highly accurate colors, making them a highly promising technology for the realization of displays with Rec. 2020 color gamut. With the rapid development in the past decades, the performances of red and green QLEDs have been remarkably improved, and their efficiency and lifetime can almost meet industrial requirements. However, the industrialization of QLED displays still faces many challenges; for example, (1) the device mechanisms including the charge injection/transport/leakage, exciton quenching, and device degradation are still unclear, which fundamentally limit QLED performance improvement; (2) the blue performances including the efficiency, chromaticity, and stability are relatively low, which are still far from the requirements of practical applications; (3) the color patterning processes including the ink-jet printing, transfer printing, and photolithography are still immature, which restrict the manufacturing of high resolution full-color QLED displays. Here, the recent advancements attempting to address the above challenges of QLED displays are specifically reviewed. After a brief overview of QLED development history, device structure/principle, and performances, the main focus is to investigate the recent discoveries on device mechanisms with an emphasis on device degradation. Then recent progress is introduced in blue QLEDs and color patterning. Finally, the opportunities, challenges, solutions, and future research directions of QLED displays are summarized.

Highly Efficient All-Solution-Processed Quantum Dot Light-Emitting Diodes Using MoOx Nanoparticle Hole Injection Layer

This paper presents a study that aims to enhance the performance of quantum dot light-emitting didoes (QLEDs) by employing a solution-processed molybdenum oxide (MoOx) nanoparticle (NP) as a hole injection layer (HIL). The study investigates the impact of varying the concentrations of the MoOx NP layer on device characteristics and delves into the underlying mechanisms that contribute to the observed enhancements. Experimental techniques such as an X-ray diffraction and field-emission transmission electron microscopy were employed to confirm the formation of MoOx NPs during the synthesis process. Ultraviolet photoelectron spectroscopy was employed to analyze the electron structure of the QLEDs. Remarkable enhancements in device performance were achieved for the QLED by employing an 8 mg/mL concentration of MoOx nanoparticles. This configuration attains a maximum luminance of 69,240.7 cd/cm2, a maximum current efficiency of 56.0 cd/A, and a maximum external quantum efficiency (EQE) of 13.2%. The obtained results signify notable progress in comparison to those for QLED without HIL, and studies that utilize the widely used poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) HIL. They exhibit a remarkable enhancements of 59.5% and 26.4% in maximum current efficiency, respectively, as well as significant improvements of 42.7% and 20.0% in maximum EQE, respectively. This study opens up new possibilities for the selection of HIL and the fabrication of solution-processed QLEDs, contributing to the potential commercialization of these devices in the future.

Utilizing VO2 as a Hole Injection Layer for Efficient Charge Injection in Quantum Dot Light-Emitting Diodes Enables High Device Performance

Quantum dot light-emitting diodes (QLEDs) are promising devices for display applications. Polyethylenedioxythiophene:polystyrene sulfonate (PEDOT:PSS) is a common hole injection layer (HIL) material in optoelectronic devices because of its high conductivity and high work function. Nevertheless, PEDOT:PSS-based QLEDs have a high energy barrier for hole injection, which results in low device efficiency. Therefore, a new strategy is needed to improve the device efficiency. Herein, we have demonstrated a bilayer-HIL using VO₂ and a PEDOT:PSS-based QLED that exhibits an 18% external quantum efficiency (EQE), 78 cd/A current efficiency (CE), and 25,771 cd/m² maximum luminance. In contrast, the PEDOT:PSS-based QLED exhibits an EQE of 13%, CE of 54 cd/A, and maximum luminance of 14,817 cd/m². An increase in EQE was attributed to a reduction in the energy barrier between indium tin oxide (ITO) and PEDOT:PSS, caused by the insertion of a VO₂ HIL. Therefore, our results could demonstrate that using a bilayer-HIL is effective in increasing the EQE in QLEDs.

Efficient Tandem Quantum-Dot LEDs Enabled by An Inorganic Semiconductor-Metal-Dielectric Interconnecting Layer Stack

Light-emitting diodes (LEDs) in a tandem configuration offer a strategy to realize high-performance, multicolor devices. Until now, though, the efficiency of tandem colloidal quantum dot LEDs (QLEDs) has been limited due to unpassivated interfaces and solvent damage originating from the materials processing requirements of interconnecting layers (ICLs). Here an ICL is reported consisting of a semiconductor-metal-dielectric stack that provides facile fabrication, materials stability, and good optoelectronic coupling. It is investigated experimentally how the ICL enables charge balance, suppresses current leakage, and prevents solvent damage to the underlying layers. As a result record efficiencies are reported for double-junction tandem QLEDs, whose emission wavelengths cover from blue to red light; i.e., external quantum efficiencies (EQEs) of 40% (average 37+/-2%) for red, 49% (average 45+/-2%) for yellow, 50% (average 46+/-2%) for green, and 24% (average 21+/-2%) for blue are achieved.

Utilization of Nanoporous Nickel Oxide as the Hole Injection Layer for Quantum Dot Light-Emitting Diodes

Nickel oxide (NiOx) has been extensively investigated as the hole injection layer (HIL) for many optoelectronic devices because of its excellent hole mobility, high environmental stability, and low-cost fabrication. In this research, a NiOx thin film and nanoporous layers (NPLs) have been utilized as the HIL for the fabrication of quantum dot light-emitting diodes (QLEDs). The obtained NiOx NPLs have spongelike nanostructures that possess a larger surface area to enhance carrier injection and to lower the turn-on voltage as compared with the NiOx thin film. The energy levels of NiOx were slightly downshifted by incorporating the nanoporous structure. The amount of Ni2O3 species is higher than that of NiO in the NiOx NPL, confirming its good hole transport ability. The best QLED was achieved with a 30 nm thick NiOx NPL, exhibiting a maximum brightness of 68 646 cd m-2, a current efficiency of 7.60 cd A-1, and a low turn-on voltage of 3.4 V. More balanced carrier transport from the NiOx NPL and ZnO NPs/polyethylenimine ethoxylated (PEIE) is responsible for the improved device performance.

Highly Efficient and Super Stable Full-Color Quantum Dots Light-Emitting Diodes with Solution-Processed All-Inorganic Charge Transport Layers

High performance and super stable all-inorganic full-color quantum dot light-emitting diodes (QLEDs) are constructed by adopting solution-processed Mg-doped NiO_x (Mg-NiO_x ) nanoparticles as hole transport layer (HTL) and Al-doped ZnO (AZO) as electron transport layer (ETL). Mg-NiO_x nanoparticles possess the advantages of low-temperature solution processability, intrinsic stability, and controllable electronic properties. UV-ozone (UVO) treatment is applied to the Mg-NiO_x film to modulate its surface composition. By carefully controlling the UVO treating time, favorable energy levels can be achieved to minimize the energy barrier for hole injection. At the cathode side, Al-doping can reduce the conductivity of ZnO ETL and decrease the interface charge transfer, effectively, thus leading to more balanced charge injection and consequent high luminance and efficiency. The maximum luminance and EQE can reach as high as 38 444 cd m^-2 and 5.09% for R-QLEDs, 177 825 cd m^-2 and 10.1% for G-QLEDs, and 3103 cd m^-2 and 2.19% for B-QLEDs. The luminance values are the highest ever reported for all-inorganic QLEDs. Furthermore, the all-inorganic devices show much better resistance to water and oxygen existing in air. The results show that the ion-doped NiO_x and AZO nanoparticles would facilitate the design and development of highly efficient and super stable QLEDs.

Promoted Hole Transport Capability by Improving Lateral Current Spreading for High-Efficiency Quantum Dot Light-Emitting Diodes

Carrier imbalance resulting from stronger electron injection from ZnO into quantum-dot (QD) emissive layer than hole injection is one critical issue that constrains the performance of QDs-based light-emitting diodes (QLEDs). This study reports highly efficient inverted QLEDs enabled by periodic insertion of MoO3 into (4,4'-bis(N-carbazolyl)-1,1'-biphenyl) (CBP) hole transport layer (HTL). The periodic ultrathin MoO3/CBP-stacked HTL results in improved lateral current spreading for the QLEDs, which significantly relieves the crowding of holes and thus enhances hole transport capability across the CBP in QLEDs. Comprehensive analysis on the photoelectric properties of devices shows that the optimal thickness for MoO3 interlayer inserted in CBP is only ≈1 nm. The resulting devices with periodic two insertion layers of MoO3 into CBP exhibit better performance compared with the CBP-only ones, such that the peak current efficiency is 88.7 cd A-1 corresponding to the external quantum efficiency of 20.6%. Furthermore, the resulting QLEDs show an operational lifetime almost 2.5 times longer compared to CBP-only devices.

Tailoring hole injection of sol–gel processed WOx and its doping in PEDOT:PSS for efficient ultraviolet organic light-emitting diodes

We present an easily processed WO_x solution and its doping in PEDOT:PSS for tailoring hole injection and assembling efficient ultraviolet OLEDs.

Efficient and Stable Inverted Quantum Dot Light-Emitting Diodes Enabled by An Inorganic Copper-Doped Tungsten Phosphate Hole-Injection Layer

Inorganic interfacial buffer layers have widely been employed for efficient and long lifetime optoelectronic devices due to their high carrier mobility and excellent chemical/thermal stability. In this paper, we developed a solution-processed inorganic tungsten phosphate (TPA) as hole injection layer (HIL) in inverted quantum dot light-emitting diodes (QLEDs) achieving a high external quantum efficiency (EQE) of up to ∼20%. Further, the copper ions are doped into tungsten phosphate (Cu:TPA) which leads to an enhancement in hole injection due to increased hole mobility and conductivity of TPA as well as decreased hole injection barrier, enabling better charge balance in QLEDs and lower turn-on voltage from 5 to 2.5 V. Compared with the devices using conventional organic poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) HIL, the half-lifetime of Cu:TPA-based devices is over 3000 h at an initial brightness of 100 cd m^-2, almost 5-fold operating lifetime enhancement.

Stability of Quantum Dots, Quantum Dot Films, and Quantum Dot Light-Emitting Diodes for Display Applications

Quantum dots (QDs) are being highlighted in display applications for their excellent optical properties, including tunable bandgaps, narrow emission bandwidth, and high efficiency. However, issues with their stability must be overcome to achieve the next level of development. QDs are utilized in display applications for their photoluminescence (PL) and electroluminescence. The PL characteristics of QDs are applied to display or lighting applications in the form of color-conversion QD films, and the electroluminescence of QDs is utilized in quantum dot light-emitting diodes (QLEDs). Studies on the stability of QDs and QD devices in display applications are reviewed herein. QDs can be degraded by oxygen, water, thermal heating, and UV exposure. Various approaches have been developed to protect QDs from degradation by controlling the composition of their shells and ligands. Phosphorescent QDs have been protected by bulky ligands, physical incorporation in polymer matrices, and covalent bonding with polymer matrices. The stability of electroluminescent QLEDs can be enhanced by using inorganic charge transport layers and by improving charge balance. As understanding of the degradation mechanisms of QDs increases and more stable QDs and display devices are developed, QDs are expected to play critical roles in advanced display applications.

Interfacial electronic structure between a W-doped In2O3 transparent electrode and a V2O5 hole injection layer for inorganic quantum-dot light-emitting diodes

The interfacial electronic structure between a W-doped In₂O₃ (IWO) transparent electrode and a V₂O₅ hole injection layer (HIL) has been investigated using ultraviolet photoelectron spectroscopy for high-performance and inorganic quantum-dot light-emitting diodes (QLEDs). Based on the interfacial electronic structure measurements, we found gap states in a V₂O₅ HIL at 1.0 eV below the Fermi level. Holes can be efficiently injected from the IWO electrode into poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(4-sec-butylphenyl)diphenylamine)] (TFB) through the gap states of V₂O₅, which was confirmed by the hole injection characteristics of a hole-only device. Therefore, conventional normal-structured QLEDs were fabricated on a glass substrate with the IWO transparent electrode and V₂O₅ HIL. The maximum luminance of the device was measured as 9443.5 cd m⁻². Our result suggests that the IWO electrode and V₂O₅ HIL are a good combination for developing high-performance and inorganic QLEDs.

Interfacial electronic structure between W-doped In₂O₃ and V₂O₅ has been investigated, and we found gap states that can provide an efficient hole carrier injection pathway.

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.

A low-temperature-annealed and UV-ozone-enhanced combustion derived nickel oxide hole injection layer for flexible quantum dot light-emitting diodes

Sol-gel derived nickel oxide (NiOx) has been extensively investigated as the hole injection layer (HIL) for many optoelectronic devices because of its advantages of high environmental stability and low cost fabrication. Conventional sol-gel synthesis of NiOx requires high annealing temperature to convert precursors into crystal lattices, which limits its application in flexible devices. To address this issue, a low-temperature (150 °C) combustion method is used to synthesize NiOx. Besides, UV-ozone treatment is further performed to improve the electrical properties of low-temperature-grown NiOx, which leads to the formation of nickel oxyhydroxide (NiO(OH)) surface dipoles and Ni vacancies and thus modifies the energy structure and increases the conductivity of NiOx. Moreover, the formation of surface NiO(OH) induces a vacuum level shift and thus reduces the hole injection barrier. Owing to the enhanced hole injection, solution-processed green QLEDs with optimized UV-ozone treated NiOx HILs exhibit maximum current efficiencies of 45.8 cd A-1 and external quantum efficiencies of 10.9%, which outperform those of the devices with poly(3,4-ethylene dioxythiophene) : poly(4-styrenesulfonate) (PEDOT:PSS) HILs. Meanwhile, these devices also show better long-term stability with a 3.2-fold longer half-life time than that of the PEDOT:PSS-based devices. The demonstrated low-temperature-annealed and UV-ozone enhanced NiOx HILs would enable the realization of flexible QLEDs with high brightness, efficiency and stability.

Simultaneous Improvement of Efficiency and Lifetime of Quantum Dot Light-Emitting Diodes with a Bilayer Hole Injection Layer Consisting of PEDOT:PSS and Solution-Processed WO3

Even though chemically stable metal oxides (MOs), as substitutes for poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), have been successfully adopted for improving device stability in solution-processed quantum dot light-emitting diodes (QLEDs), the efficiencies of QLEDs are at a relatively low level. In this work, a novel architecture of QLEDs has been introduced, in which inorganic/organic bilayer hole injection layers (HILs) were delicately designed by inserting an amorphous WO₃ interlayer between PEDOT:PSS and the indium tin oxide anode. As a result, the efficiency and operational lifetime of QLEDs were improved simultaneously. The results show that the novel architecture QLEDs relative to conventional PEDOT:PSS-based QLEDs have an enhanced external quantum efficiency by a factor of 50%, increasing from 8.31 to 12.47%, meanwhile exhibit a relatively long operational lifetime (12 551 h) and high maximum brightness (>40 000 cd m^-2) resulting from a better pathway for hole injection with staircase energy-level alignment of the HILs and reduction of surface roughness. Our results demonstrate that the novel architecture QLEDs using bilayer MO/PEDOT:PSS HILs can achieve long operational lifetime without sacrificing efficiency.

Influence of Shell Thickness on the Performance of NiO-Based All-Inorganic Quantum Dot Light-Emitting Diodes

The effect of shell thickness on the performance of all-inorganic quantum dot light-emitting diodes (QLEDs) is explored by employing a series of green quantum dots (QDs) (Zn _xCd_{1- x}Se/ZnS core/shell QDs with different ZnS shell thicknesses) as the emitters. ZnO nanoparticles and sol-gel NiO are employed as the electron and hole transport materials, respectively. Time-resolved and steady-state photoluminescence results indicate that positive charging processes might occur for the QDs deposited on NiO, which results in emission quenching of QDs and poor device performance. The thick shell outside the core in QDs not only largely suppresses the QD emission quenching but also effectively preserves the excitons in QDs from dissociation of electron-hole pairs when they are subjected to an electric field. The peak efficiency of 4.2 cd/A and maximum luminance of 4205 cd/m² are achieved for the device based on QDs with the thickest shells (∼4.2 nm). We anticipate that these results will spur progress toward the design and realization of efficient all-inorganic QLEDs as a platform for the QD-based full-colored displays.

A New Facile Synthesis of Tungsten Oxide from Tungsten Disulfide: Structure Dependent Supercapacitor and Negative Differential Resistance Properties

Tungsten oxide (WO₃ ) is an emerging 2D nanomaterial possessing unique physicochemical properties extending a wide spectrum of novel applications which are limited due to lack of efficient synthesis of high-quality WO₃ . Here, a facile new synthetic method of forming WO₃ from tungsten sulfide, WS₂ is reported. Spectroscopic, microscopic, and X-ray studies indicate formation of flower like aggregated nanosized WO₃ plates of highly crystalline cubic phase via intermediate orthorhombic tungstite, WO_3. H₂ O phase. The charge storage ability of WO₃ is extremely high (508 F g^-1 at current density of 1 A g^-1 ) at negative potential range compared to tungstite (194 F g^-1 at 1 A g^-1 ). Moreover, high (97%) capacity retention after 1000 cycles and capacitive charge storage nature of WO₃ electrode suggest its supremacy as a negative electrode of supercapacitors. The asymmetric supercapacitor, based on the WO₃ as a negative electrode and mildly reduced graphene oxide as a positive electrode, manifests high energy density of 218.3 mWhm^-2 at power density 1750 mWm^-2 , and exceptionally high power density, 17 500 mW m^-2 , with energy density of 121.5 mWh m^-2 . Furthermore, the negative differential resistance (NDR) property of both WO₃ and WO₃ .H₂ O are reported for the first time and NDR is explained with density of state approach.

Synthesis of WOn -WX2 (n=2.7, 2.9; X=S, Se) Heterostructures for Highly Efficient Green Quantum Dot Light-Emitting Diodes

Preparation of two-dimensional (2D) heterostructures is important not only fundamentally, but also technologically for applications in electronics and optoelectronics. Herein, we report a facile colloidal method for the synthesis of WO_n -WX₂ (n=2.7, 2.9; X=S, Se) heterostructures by sulfurization or selenization of WO_n nanomaterials. The WO_n -WX₂ heterostructures are composed of WO_2.9 nanoparticles (NPs) or WO_2.7 nanowires (NWs) grown together with single- or few-layer WX₂ nanosheets (NSs). As a proof-of-concept application, the WO_n -WX₂ heterostructures are used as the anode interfacial buffer layer for green quantum dot light-emitting diodes (QLEDs). The QLED prepared with WO_2.9 NP-WSe₂ NS heterostructures achieves external quantum efficiency (EQE) of 8.53 %. To our knowledge, this is the highest efficiency in the reported green QLEDs using inorganic materials as the hole injection layer.

Inkjet-Printed Quantum Dot Light-Emitting Diodes with an Air-Stable Hole Transport Material

High-efficiency quantum dot light-emitting diodes (QLEDs) were fabricated using inkjet printing with a novel cross-linkable hole transport material N,N'-(9,9'-spirobi[fluorene]-2,7-diylbis[4,1-phenylene])bis(N-phenyl-4'-vinyl-[1,1'-biphenyl]-4-amine) (SDTF). The cross-linked SDTF film has excellent solvent resistance, high thermal stability, and the highest occupied molecular orbital (HOMO) level of -5.54 eV. The inkjet-printed SDTF film is very smooth and uniform, with roughness as low as 0.37 nm, which is comparable with that of the spin-coated film (0.28 nm). The SDTF films stayed stable without any pinhole or grain even after 2 months in air. All-solution-processed QLEDs were fabricated; the maximum external quantum efficiency of 5.54% was achieved with the inkjet-printed SDTF in air, which is comparable to that of the spin-coated SDTF in a glove box (5.33%). Electrical stabilities of both spin-coated and inkjet-printed SDTF at the device level were also investigated and both showed a similar lifetime. The study demonstrated that SDTF is very promising as a printable hole transport material for making QLEDs using inkjet printing.

Polyethylenimine Ethoxylated-Mediated All-Solution-Processed High-Performance Flexible Inverted Quantum Dot-Light-Emitting Device

We report on an all-solution-processed fabrication of highly efficient green quantum dot-light-emitting diodes (QLEDs) with an inverted architecture, where an interfacial polymeric surface modifier of polyethylenimine ethoxylated (PEIE) is inserted between a quantum dot (QD) emitting layer (EML) and a hole transport layer (HTL), and a MoO_x hole injection layer is solution deposited on top of the HTL. Among the inverted QLEDs with varied PEIE thicknesses, the device with an optimal PEIE thickness of 15.5 nm shows record maximum efficiency values of 65.3 cd/A in current efficiency and 15.6% in external quantum efficiency (EQE). All-solution-processed fabrication of inverted QLED is further implemented on a flexible platform by developing a high-performing transparent conducting composite film of ZnO nanoparticles-overcoated on Ag nanowires. The resulting flexible inverted device possesses 35.1 cd/A in current efficiency and 8.4% in EQE, which are also the highest efficiency values ever reported in flexible QLEDs.

Solution-processed inorganic copper(i) thiocyanate as a hole injection layer for high-performance quantum dot-based light-emitting diodes

The introduction of CuSCN as the hole injection material significantly improved the turn-on voltage of quantum dot-based LEDs.

Quantum dot light-emitting diodes using a graphene oxide/PEDOT:PSS bilayer as hole injection layer

Adoption of graphene oxide/PEDOT:PSS as a HIL layer dramatically improves the electro-optical performance of QLED devices.

Wafer-scale single-domain-like graphene by defect-selective atomic layer deposition of hexagonal ZnO

Large-area graphene films produced by means of chemical vapor deposition (CVD) are polycrystalline and thus contain numerous grain boundaries that can greatly degrade their performance and produce inhomogeneous properties. A better grain boundary engineering in CVD graphene is essential to realize the full potential of graphene in large-scale applications. Here, we report a defect-selective atomic layer deposition (ALD) for stitching grain boundaries of CVD graphene with ZnO so as to increase the connectivity between grains. In the present ALD process, ZnO with a hexagonal wurtzite structure was selectively grown mainly on the defect-rich grain boundaries to produce ZnO-stitched CVD graphene with well-connected grains. For the CVD graphene film after ZnO stitching, the inter-grain mobility is notably improved with only a little change in the free carrier density. We also demonstrate how ZnO-stitched CVD graphene can be successfully integrated into wafer-scale arrays of top-gated field-effect transistors on 4-inch Si and polymer substrates, revealing remarkable device-to-device uniformity.

Inverted Quantum-Dot Light Emitting Diode Using Solution Processed p-Type WOx Doped PEDOT:PSS and Li Doped ZnO Charge Generation Layer

Quantum dots (QDs) are a promising material for emissive display with low-cost manufacturing and excellent color purity. In this study, we report colloidal quantum-dot light emitting diodes (QLEDs) with an inverted architecture with a solution processed charge generation layer (CGL) of p-type polymer (tungsten oxide doped poly(ethylenedioxythiophene)/polystyrenesulfonate,

PEDOT

PSS:WOx) and n-type metal oxide (lithium doped zinc oxide, LZO). The effective charge generation in solution processed p-n junction was confirmed by capacitance-voltage (C-V) and current density-electric field characteristics. It is also demonstrated that the performances of CGL based QLEDs are very similar when various substrates with different work functions are used.

Patterning fluorescent quantum dot nanocomposites by reactive inkjet printing

Fluorescent quantum dot nanocomposites, including polymer and photonic crystal quantum dots, have been fabricated by reactive inkjet printing. This reactive inkjet printing method has the potential to be broadened to fabrication of other functional nanomaterials, which will find promising applications in optoelectronic devices.

Highly flexible, electrically driven, top-emitting, quantum dot light-emitting stickers

Flexible information displays are key elements in future optoelectronic devices. Quantum dot light-emitting diodes (QLEDs) with advantages in color quality, stability, and cost-effectiveness are emerging as a candidate for single-material, full color light sources. Despite the recent advances in QLED technology, making high-performance flexible QLEDs still remains a big challenge due to limited choices of proper materials and device architectures as well as poor mechanical stability. Here, we show highly efficient, large-area QLED tapes emitting in red, green, and blue (RGB) colors with top-emitting design and polyimide tapes as flexible substrates. The brightness and quantum efficiency are 20,000 cd/m(2) and 4.03%, respectively, the highest values reported for flexible QLEDs. Besides the excellent electroluminescence performance, these QLED films are highly flexible and mechanically robust to use as electrically driven light-emitting stickers by placing on or removing from any curved surface, facilitating versatile LED applications. Our QLED tapes present a step toward practical quantum dot based platforms for high-performance flexible displays and solid-state lighting.