Efficient Quantum Dot Light-emitting Diodes Research Articles

Highly efficient green InP-based quantum dot light-emitting diodes regulated by inner alloyed shell component

InP-based quantum dot light-emitting diodes (QLEDs), as less toxic than Cd-free and Pb-free optoelectronic devices, have become the most promising benign alternatives for the next generation lighting and display. However, the development of green-emitting InP-based QLEDs still remains a great challenge to the environmental preparation of InP quantum dots (QDs) and superior device performance. Herein, we reported the highly efficient green-emitting InP-based QLEDs regulated by the inner alloyed shell components. Based on the environmental phosphorus tris(dimethylamino)phosphine ((DMA)3P), we obtained highly efficient InP-based QDs with the narrowest full width at half maximum (~35 nm) and highest quantum yield (~97%) by inserting the gradient inner shell layer ZnSexS1−x without further post-treatment. More importantly, we concretely discussed the effect and physical mechanism of ZnSexS1–x layer on the performance of QDs and QLEDs through the characterization of structure, luminescence, femtosecond transient absorption, and ultraviolet photoelectron spectroscopy. We demonstrated that the insert inner alloyed shell ZnSexS1−x provided bifunctionality, which diminished the interface defects upon balancing the lattice mismatch and tailored the energy levels of InP-based QDs which could promote the balanced carrier injection. The resulting QLEDs applying the InP/ZnSe0.7S0.3/ZnS QDs as an emitter layer exhibited a maximum external quantum efficiency of 15.2% with the electroluminescence peak of 532 nm, which was almost the highest record of InP-based pure green-emitting QLEDs. These results demonstrated the applicability and processability of inner shell component engineering in the preparation of high-quality InP-based QLEDs.

Highly efficient and eco-friendly InP-based quantum dot light-emitting diodes with a synergetic combination of a liquid metal cathode and size-controlled ZnO nanoparticles

Colloidal quantum dot light-emitting diodes (QLEDs) are the most promising candidates for next-generation displays due to wide color gamut, high contrast ratio, and narrow bandwidth emission. However, high-efficiency QLEDs based on toxic cadmium-based quantum dots (QDs) are strictly regulated by environmental law, and therefore environmentally friendly fabrication methods using non-cadmium-based QDs are indispensable for industrial applications. In this study, an eco-friendly and scalable fabrication strategy based on a synergetic combination of colloidal InP QDs, a eutectic gallium-indium liquid metal, and size-controlled ZnO nanoparticles was developed to produce highly efficient, environmentally friendly, solution-processable QLEDs. The cathode electrode of a gallium alloy liquid metal was applied using a simple screen-printing method, and the InP/GaP/ZnS green QDs with an excellent photoluminescence quantum yield of 85% were facilely prepared and incorporated into the device through spin coating process, ensuring that high device efficiency of InP-based QLED was achieved through uniform electroluminescence of active area. In addition, a size-controlled synthetic method for ZnO nanoparticles as the electron transfer layer was developed to improve the quantum efficiency of InP QDs, and consequently the optoelectronic performance of the resulting device was higher than that of conventional device in terms of current density, luminance, and external quantum efficiency.

Highly Efficient Red Quantum Dot Light-Emitting Diodes by Balancing Charge Injection and Transport.

Quantum dot light-emitting diodes (QLEDs) have promising commercial value and application prospects in the fields of displays and lighting. However, a charge-transfer imbalance always exists in the devices. In this work, the high-efficiency red QLEDs were obtained via employing the mixtures of poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4'-(N-(4-butylphenyl) (TFB) and 4,4'-bis(carbazole-9-yl)-1,1'-biphenyl (CBP) as hole-transport layers (HTLs) by solution processing. The optimized mixing concentration of CBP is 20 wt %. The corresponding red QLED exhibited a maximum luminance of 963 433 cd m-2, a maximum current efficiency of 38.7 cd A-1, an external quantum efficiency of 30.0%, a central wavelength of 628 nm with a narrow full width at half-maximum (fwhm) of 24 nm, and a 5-fold T50 lifetime enhancement at an extremely high luminance of 200 000 cd m-2. The characteristics of carrier-only devices with QD emissive layers (QD EMLs) and impedance characteristics of QLEDs demonstrate that these advances are chiefly ascribed to the more balanced charge transport and efficient hole-electron recombination in EML. We anticipate that our results could offer a low-cost and simple solution-processed method for preparing high-performance QLEDs.

Alleviating Electron Over-Injection for Efficient Cadmium-Free Quantum Dot Light-Emitting Diodes toward Deep-Blue Emission

Alleviating Electron Over-Injection for Efficient Cadmium-Free Quantum Dot Light-Emitting Diodes toward Deep-Blue Emission

Highly efficient blue quantum-dot light-emitting diodes based on a mixed composite of a carbazole donor and a triazine acceptor as the hole transport layer.

Solution-processed thermally activated delayed fluorescence (TADF) exciplexes were employed as the hole transport layer (HTL) of blue quantum dot (QD) light-emitting diodes (QLEDs) by blending polymer donors of poly(N-vinylcarbazole) (PVK) with small molecular acceptors of 2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine (T2T). As a result, the PVK:T2T HTL can harvest holes and electrons leaking from the QD active layer to form exciplex excitons and then this harvested exciton energy can be effectively transferred to the adjacent QD emitters through the Förster resonance energy-transfer process. Furthermore, the TADF exciplexes can enhance the hole mobility of the HTL due to the charge transfer process from the PVK donor to the T2T acceptor under an external electric field. The maximum current efficiency (CE) and external quantum efficiency (EQE) of the fabricated blue ZnCdS/ZnS core/shell QLEDs increase from 4.14 cd A-1 and 7.33% for the PVK HTL to 7.73 cd A-1 and 13.66% for the PVK:(5 wt%)T2T HTL, respectively. Our results demonstrate that the TADF exciplex HTL would be a facile strategy to design high-performance blue QLEDs.

Solution-processable Li-doped transition metal oxide hole-injection layer for highly efficient quantum-dot light-emitting diodes

Vanadium oxide (V2O5) doped with Li (5, 10, and 15 mol%) was synthesized using an easy and low-cost solution process method and was used as a hole-injection layer (HIL) for quantum-dot light-emitting diodes (QLEDs).

Efficient quantum dot light-emitting diodes with ultra-homogeneous and highly ordered quantum dot monolayer

Regarding conventional quantum dot light-emitting diodes (QLEDs) fabricated by using the spin-coating (SC) technique, voids and interstitial spaces are inevitable due to unordered quantum dots (QDs) stacking, generating device leakage current under an external bias. In the present study, we fabricated an ultra-homogeneous and highly ordered QD monolayer by adopting the Langmuir-Blodgett (LB) technique. The QD monolayer was transferred as a emissive layer with a horizontal lifting (HL) method to a red QLED, which exhibited high performance with an external quantum efficiency (EQE) of 19.0% and lifetime (T95@100 cd m−2) of 13,324 h. When compared with the SC-based device, the EQE and lifetime were improved by 15% and 183% due to the compact and ordered QD monolayer that lowered the leakage current. Moreover, white QLEDs with stacked QD monolayers could be obtained at a low voltage of 4 V because LB technique is an organic-solvent-free approach avoiding interlayer mixing and controlling the QD layer thickness precisely. In addition, we successfully fabricated an ultra-homogeneous large-area QD monolayer on a rectangular substrate with a size of 9 cm × 5 cm, indicating the promising size scalability of the LB-HL strategy.

A Study on the ZnO Thin Film Deposited by RF Sputtering Method as an Electron Transport Layer in Quantum Dot Light-Emitting Diodes

We report a highly efficient quantum dot light emitting diode (QLEDs) with a radio frequency (RF) sputtered ZnO thin film as an electron transport layer (ETL) instead of the conventional ZnO nanoparticles (NPs) by solution process. ZnO NPs have been used as a key material to improve the performance of QLEDs, but the charge imbalance in ZnO NPs resulting from fast electron injection, and their limited uniformity are significant disadvantages. In this study, ZnO layers were deposited by RF sputtering with various O2 partial pressures. All of the ZnO films showed preferential growth along the (002) direction, smooth morphology, and good optical transmittance. To test their feasibility for QLEDs, we fabricated devices with RF sputtered ZnO layers as an ETL, which has the inverted structure of ITO/RF sputtered ZnO/QDs/CBP/MoO3/Al. The optical/electrical characteristics of two devices, comprised of RF sputtered ZnO and ZnO NPs, were compared with each other. QLEDs with the sputtered ZnO ETL achieved a current efficiency of 11.32 cd/A, which was higher than the 8.23 cd/A of the QLEDs with ZnO NPs ETL. Next, to find the optimum ZnO thin film for highly efficient QLEDs, deposition conditions with various O2 partial pressures were tested, and device performance was investigated. The maximum current efficiency was 13.33 cd/A when the ratio of Ar/O2 was 4:3. Additional oxygen gas reduced the O vacancies in the ZnO thin film, which resulted in a decrease in electrical conductivity, thereby improving charge balance in the emission layer of the QLEDs. As a result, we provide a way to control the ZnO ETL properties and to improve device performance by controlling O2 partial pressure.

A Study on the ZnO Thin Film Deposited by RF Sputtering Method as an Electron Transport Layer in Quantum Dot Light-Emitting Diodes

We report a highly efficient quantum dot light emitting diode (QLEDs) with a radio frequency (RF) sputtered ZnO thin film as an electron transport layer (ETL) instead of the conventional ZnO nanoparticles (NPs) by solution process. ZnO NPs have been used as a key material to improve the performance of QLEDs, but the charge imbalance in ZnO NPs resulting from fast electron injection, and their limited uniformity are significant disadvantages. In this study, ZnO layers were deposited by RF sputtering with various O2 partial pressures. All of the ZnO films showed preferential growth along the (002) direction, smooth morphology, and good optical transmittance. To test their feasibility for QLEDs, we fabricated devices with RF sputtered ZnO layers as an ETL, which has the inverted structure of ITO/RF sputtered ZnO/QDs/CBP/MoO3/Al. The optical/electrical characteristics of two devices, comprised of RF sputtered ZnO and ZnO NPs, were compared with each other. QLEDs with the sputtered ZnO ETL achieved a current efficiency of 11.32 cd/A, which was higher than the 8.23 cd/A of the QLEDs with ZnO NPs ETL. Next, to find the optimum ZnO thin film for highly efficient QLEDs, deposition conditions with various O2 partial pressures were tested, and device performance was investigated. The maximum current efficiency was 13.33 cd/A when the ratio of Ar/O2 was 4:3. Additional oxygen gas reduced the O vacancies in the ZnO thin film, which resulted in a decrease in electrical conductivity, thereby improving charge balance in the emission layer of the QLEDs. As a result, we provide a way to control the ZnO ETL properties and to improve device performance by controlling O2 partial pressure.

A Study on the Stability of TiO2 Nanoparticles as an Electron Transport Layer in Quantum Dot Light-Emitting Diodes

We report highly efficient and robust quantum dot light-emitting diodes (QLEDs) with Li-doped TiO2 nanoparticles (NPs) as an electron transport layer (ETL). As core materials, ZnO-based inorganic NPs can enhance the performance of QLEDs due to their suitable energy level and solution processability. However, their fast electron mobility and instability in organic solvents are two main obstacles to practical display applications. The colloidal stability of TiO2 NPs in ethanol was confirmed after three day-storage, while ZnO NPs showed severe agglomeration. Inverted structure QLEDs using 3% Li-doped TiO2 NP were successfully fabricated and their optical/electrical properties were investigated. With 3% Li-doped TiO2 NPs, the charge balance in the emitting layer of the QLEDs was improved, which resulted in a maximum luminance of 159,840 cd/m<sup>2</sup> and external quantum efficiency (EQE) of 9.12%. These results were comparable to the performance of QLEDs with commonly used ZnO NPs. Moreover, the QLEDs with the Li-doped TiO2 NPs showed more stable characteristics than those with ZnO NPs after 7 days in ambient conditions. The EQE of the QLEDs with Li-doped TiO2 NPs was reduced by only 4.9%. These results indicate that Li-doped TiO2 NPs show great promise for use as a solution based inorganic ETL for QLEDs.

The Effect of Cation Doped ZnO Electron Transport Layer for Pure Blue Emitting Zntese/ZnSe/ZnS Quantum Dot Light Emitting Diodes

Colloidal quantum dot light emitting diodes (QLED) has considered as next generation display technology due to their accurate color information, high brightness, with low process cost, and potential flexible substrates. Red and green QLED achieved nearly the efficiency of theoretical limitation by optimizing quantum dot materials and device architecture with the tremendous efforts of many fellow researchers [1,2]. However, the authentic blue color QLED with high performance and high stability is yet remained as a obstacle for the commercialization. Herein, we report the fabrication of ZnTeSe based blue QLED device with multi shell quantum dots with very high quantum yield over >90% at 443nm with narrow FWHM of 15 nm. The peak wavelength of quantum dot could be tuned with the different Te/Se ratio of the core and the thickness of intermediate ZnSe shell. With the combination of zinc magnesium oxide (ZMO), which is the most common electron transport layer (ETL) for blue QLED, and our highly luminescent blue quantum dot, bright blue QLEDs with the max luminance of 700 cd/m2 in the inverted structure are demonstrated. Moreover, to obtain the better-performed and stable electron transport layer, different cations such as lithium, nickel, aluminum, cerium, erbium and indium-doped zinc oxides were successfully synthesized and studied. we have analyzed the band gap of ZnO can be fine tuned by doping of different cations and the effect of different zinc oxide in the device will be tested in the further study. According to the recent researches of the device stability of blue QLED [3,4], inferior stability of blue QLED is originated from the interface of ETL and quantum dots. I believe that the study of various ETL materials can give us a key to solve the poor lifetime problem of blue QLED.Reference1. Li, Y., Hou, X., Dai, X., Yao, Z., Lv, L., Jin, Y., & Peng, X. (2019). Stoichiometry-Controlled InP-Based Quantum Dots: Synthesis, Photoluminescence, and Electroluminescence. Journal of the American Chemical Society, 141(16), 6448–6452.2. Won, Y.-H., Cho, O., Kim, T., Chung, D.-Y., Kim, T., Chung, H., ... Jang, E. (2019). Highly efficient and stable InP/ZnSe/ZnS quantum dot light-emitting diodes. Nature, 575(7784), 634–638.3. Chen, S., Cao, W., Liu, T., Tsang, S.-W., Yang, Y., Yan, X., & Qian, L. (2019). On the degradation mechanisms of quantum-dot light-emitting diodes. Nature Communications, 10(1), 765.4. Kim, T., Kim, K.-H., Kim, S., Choi, S.-M., Jang, H., Seo, H.-K., ... Jang, E. (2020). Efficient and stable blue quantum dot light-emitting diode. Nature, 586(7829), 385–389.

Sol-gel processed tungsten trioxide nanocrystals layer for efficient hole-injection in quantum dot light-emitting diodes

In this work, we report highly bright and efficient quantum dot light-emitting diodes (QLEDs) using tungsten trioxide (WO3) nanocrystals (NCs) thin film as hole-injection layers (HIL). The WO3 thin film is prepared by a simple sol-gel method using W powder and H2O2 as precursors, stirring vigorously at room temperature, followed by calcinations at 420 °C for 1 h. The WO3 thin film is compact and smooth, enabling the formation of HIL in green QLED. The luminance, current efficiency and operational lifetime of the QLED is 18560 cd m−2, 11.8 cd A−1 and 11844 h, respectively.

Effect of Air Exposure of ZnMgO Nanoparticle Electron Transport Layer on Efficiency of Quantum-Dot Light-Emitting Diodes.

We demonstrate the effect of air exposure on optical and electrical properties of ZnMgO nanoparticles (NPs) typically exploited as an electron transport layer in Cd-based quantum-dot light-emitting diodes (QLEDs). We analyze the roles of air components in modifying the electrical properties of ZnMgO NPs, which reveals that H2O enables the reduction of hole leakage while O2 alters the character of charge transport due to its ability to trap electrons. As a result, the charge balance in the QDs layer is improved, which is confirmed by voltage-dependent measurements of photoluminescence quantum yield. The maximum external quantum efficiency is improved over 2-fold and reaches the value of 9.5% at a luminance of 104 cd/m2. In addition, we investigate the problem of electron leakage into the hole transport layer and show that trap-mediated electron transport in the ZnMgO layer caused by adsorbed O2 ensures a higher leakage threshold. This work also provides an insight into the possible disadvantages of device contact with air as well as problems and challenges that might occur during open-air fabrication of QLEDs.

Stable and efficient QLEDs with crystallographic TiO2 as the electron transportation layer and improved carrier transportation by chlorination

In order to obtain improvement in efficiency and stability of quantum dot light-emitting diodes (QLEDs), it is essential to seek carrier transport layer with higher carrier transport efficiency and better air stability. In terms of the high-performance carrier transport layer, solution-processed inorganic metal oxides show promise for efficient QLEDs because of their suitable energy band, high carrier mobility, especially their outstanding air stability. In this study, we employed high-quality oleic acid-capped TiO2 nanocrystals as the electron transportation layer to fabricate air-stable inverted QLEDs. These TiO2 nanocrystals have narrow size distribution, outstanding crystalline property, especially excellent air stability. To improve the interfacial carrier transportation performance of oleic acid-capped TiO2 nanocrystals, we introduce a thionyl chloride (SOCl2) post-ligand exchange process that replaces organic long-chain ligands with single inorganic ligands efficiently. The stability of oleic acid-capped TiO2-based QLEDs were evaluated by comparing with the traditional ZnO-based QLEDs. As a result, oleic acid-capped TiO2 nanoparticles show excellent long-term air stability. This study offers an alternative strategy to deal with the stability issue of optoelectronic devices.

The influence of H2O and O2 on the optoelectronic properties of inverted quantum-dot light-emitting diodes

The influence of H2O and O2 on the performances of Mg-doped zinc oxide (ZnMgO) and ZnMgO-based inverted quantum-dot light-emitting diodes (QLEDs) are studied. With the involvement of H2O from ambience, ZnMgO exhibits a high conductivity, whereas the resultant QLEDs show a low efficiency. The efficiency of QLEDs can be enhanced by annealing ZnMgO in H2O-free glovebox; however, the uniformity and the current of the devices are degraded due to the presence of O2, which adsorbs on the surface of ZnMgO and captures the free electrons of ZnMgO. By exposing the devices with ultraviolet (UV) irradiation, the adsorbed O2 can be released, consequently leading to the increase of driving current. Our work discloses the influence of the annealing ambience on the conductivity of ZnMgO, and reveals the interaction of H2O/O2 and UV with the ZnMgO and its effect on the performance of the resultant inverted QLEDs, which could help the community to better understand the mechanisms of ZnMgO-based QLEDs.

Efficient quantum-dot light-emitting diodes featuring the interfacial carrier relaxation and exciton recycling

Tremendous achievements of colloidal quantum-dot (QD) light-emitting diodes (QLEDs) in both efficiency and lifetime have been witnessed in the past decade. However, multiple interfacial losses such as defect-induced exciton quenching and multicarrier Auger recombination can severely suppress the device performance of QLEDs. Here, we present the efficient QLEDs by adopting the new liquid-solid-solution processed magnesium-doped zinc oxide nanocrystals (ZMO-LSS) as an electron-transport layer. A magic carrier relaxation dynamics is demonstrated at the QD emitter/ZMO-LSS interface for abnormal exciton recycling and enhanced radiative recombination, which arises from the subtle intragap band coupling of surface trap states as directly clarified by the prolonged electroluminescence decays of devices. Red-emitting QLEDs on rigid glass achieve a maximum external quantum efficiency (EQE) of 22.3% with virtual droop-free over a wide range of brightness from 10,000 to 200,000 cd m−2. By further combining a silver nanowires-based composited electrode on a plastic substrate, a substantial boost in EQE up to 24.0% is realized for flexible devices. The present results provide an in-depth study on interfacial recombination and convey a clear picture of constructing ZMO-LSS for efficient QLEDs and related optoelectronic devices.

Heterogeneous post-passivation of inorganic cesium lead halide perovskite quantum dots for efficient electroluminescent devices

A FABr-based heterogeneous post-passivation process, in which the excess FABr can be easily removed by filtration, has been demonstrated for efficient blue, green and red perovskite quantum dot light-emitting diodes.

Enhance the Light Extraction Efficiency of QLED with Surface Micro-Nanostructure

Quantum dot light-emitting diodes (QLEDs) are usually a flat multilayer structure. The luminous efficiency of QLEDs is limited because the light extraction is constrained owing to the substrate mode at the air/substrate interface, the waveguide mode at the conductive glass/organic interface, and the surface plasmon polaritons (SPPs) along the metal/organic interface. To improve the light extraction for the luminous efficiency of QLEDs, light transmission through the QLEDs coupled with three different surface micro-nanostructures is simulated by using a ray-tracing method. Furthermore, QLEDs (red, green, and blue emission) coupled with micro-nanostructures are also fabricated to investigate the influence of the micro-nanostructures on the luminous efficiency. Our simulation results show that more light escapes from the QLEDs coupled with surface micro-nanostructures than the control devices without micro-nanostructures. Our experimental results are consistent with the simulation results. By coupling with surface micro-nanostructures, the luminous efficiencies of the devices exhibited great improvement, and the EQE of R/G/B trichromatic reached 17.3%, 2.81%, and 10%, respectively, which are about 1~2 times higher than those of the control devices without any surface structure. Our results should make a contribution to building a strategy for improving the luminous efficiency of QLEDs.

Highly Efficient Quantum Dot Light-emitting Diode Based on Properly Charge Balanced and Suppressed Interfacial Exciton Quenching Process

Proceedings of the International Display Workshops Volume 27 (IDW '20),Highly Efficient Quantum Dot Light-emitting Diode Based on Properly Charge Balanced and Suppressed Interfacial Exciton Quenching Process

Efficient InP-based Quantum Dot Light-emitting Diodes Using an Emitting Layer Combined with Organic Electron-transporting Materials

Efficient InP-based Quantum Dot Light-emitting Diodes Using an Emitting Layer Combined with Organic Electron-transporting Materials