Green Quantum Dot Light-emitting Diodes Research Articles

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

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

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

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

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.

Ultrabright and stable top-emitting quantum-dot light-emitting diodes with negligible angular color shift

Top emission can enhance luminance, color purity, and panel-manufacturing compatibility for emissive displays. Still, top-emitting quantum-dot light-emitting diodes (QLEDs) suffer from poor stability, low light outcoupling, and non-negligible viewing-angle dependence because, for QLEDs with non-red emission, the electrically optimum device structure is incompatible with single-mode optical microcavity. Here, we demonstrate that by improving the way of determining reflection penetration depths and creating refractive-index-lowering processes, the issues faced by green QLEDs can be overcome. This leads to advanced device performance, including a luminance exceeding 1.6 million nits, a current efficiency of 204.2 cd A−1, and a T95 operational lifetime of 15,600 hours at 1000 nits. Meanwhile, our design does not compromise light outcoupling as it offers an external quantum efficiency of 29.2% without implementing light extraction methods. Lastly, an angular color shift of Δu’v’ = 0.0052 from 0° to 60° is achieved by narrowing the emission linewidth of quantum dots.

Dewetting-assisted inkjet printing and optimized solvent mixture of Cd-free InP quantum dots for high-resolution pixel-pattern

Direct-printed quantum dot (QD) patterns utilizing inkjet method are a next-generation alternative to achieve high-resolution quantum dot light-emitting diodes (QLED). However, research on high-resolution pixel patterns is still limited due to issues such as thickness uniformity. In this study, we explored the feasibility of direct printing QLEDs to attain high-resolution pixel-patterned displays. Here, we successfully fabricated arrays of green QLED devices with a resolution of 200 pixels per inch, employing a precise 1 pL fine printer nozzle and optimizing ink formulation to achieve superior surface roughness and uniformity at the desired thickness. The inkjet-printed QD layers demonstrated a surface roughness of 1.9 nm and a flatness ratio of 0.2, comparable to those achieved through conventional spin-coating processes. This investigation offers the potential for extending the methodology to the fabrication of cost-effective, high-resolution full-color QLED displays.

Boosting the Efficiency of High-Resolution Quantum Dot Light-Emitting Devices Based on Localized Surface Plasmon Resonance.

With pixel miniaturization, the performance of high-resolution quantum dot light-emitting diodes (QLEDs) usually degrades. Considering the dimension of ultrasmall pixels, herein, a barrier architecture based on localized surface plasmon resonance (LSPR) that promotes the radiative recombination of neighboring quantum dots is rationally designed to improve the device performance. Au nanoparticles (NPs) are embedded in an insulating polymer to form a honeycomb-patterned barrier layer via the nanoimprint process. Each pixel fabricated in the void area (average diameter of 1.5 μm) of the barrier layer is surrounded by a number of LSPR-NPs to enhance the luminescence. The resultant green QLEDs with a resolution of 9027 pixels per inch show a maximum external quantum efficiency of 11.1%, a 42.8% enhancement compared to the control device. Additionally, the lifetime of high-resolution QLEDs is obviously improved by the LSPR effect.

Stable and efficient pure blue quantum-dot LEDs enabled by inserting an anti-oxidation layer

The efficiency and stability of red and green quantum-dot light-emitting diodes have already met the requirements for commercialization in displays. However, the poor stability of the blue ones, particularly pure blue color, is hindering the commercialization of full-color quantum-dot light-emitting diode technology. Severe hole accumulation at the blue quantum-dot/hole-transport layer interface makes the hole-transport layer prone to oxidation, limiting the device operational lifetime. Here, we propose inserting an anti-oxidation layer (poly(p-phenylene benzobisoxazole)) between this interface to take in some holes from the hole-transport layer, which mitigates the oxidation-induced device degradation, enabling a T50 (time for the luminance decreasing by 50%) of more than 41,000 h with an initial brightness of 100 cd m−2 in pure blue devices. Meanwhile, the inserted transition layer facilitates hole injection and helps reduce electron leakage, leading to a peak external quantum efficiency of 23%.

Rationally designed synthesis of bright Cu–Ga–Zn–Se-based nanocrystals for efficient green quantum-dot light-emitting diodes

Cd-free QLEDs with tunable emission from 603 to 524 nm were constructed by regulating the non-stoichiometric Cu : Ga molar ratio, and the green QLEDs showed high performance with an EQE of 5.8% and a high brightness of 7016 cd m−2.

Low-Temperature Cross-Linkable Hole Transport Materials for Solution-Processed Quantum Dot and Organic Light-Emitting Diodes with High Efficiency and Color Purity.

Cross-linkable hole transport materials (HTMs) are ideal for improving the performance of solution-processed quantum dot light-emitting diodes (QLEDs) and phosphorescent light-emitting diodes (OLEDs). However, previously developed cross-linkable HTMs possessed poor hole transport properties, high cross-linking temperatures, and long curing times. To achieve efficient cross-linkable HTMs with high mobility, low cross-linking temperature, and short curing time, we designed and synthesized a series of low-temperature cross-linkable HTMs comprising dibenzofuran (DBF) and 4-divinyltriphenylamine (TPA) segments for highly efficient solution-processed QLEDs and OLEDs. The introduction of divinyl-functionalized TPA in various positions of the DBF core remarkably affected their chemical, physical, and electrochemical properties. In particular, cross-linked 4-(dibenzo[b,d]furan-3-yl)-N,N-bis(4-vinylphenyl)aniline (3-CDTPA) exhibited a deep highest occupied molecular orbital energy level (5.50 eV), high hole mobility (2.44 × 10-4 cm2 V-1 s-1), low cross-linking temperature (150 °C), and short curing time (30 min). Furthermore, a green QLED with 3-CDTPA as the hole transport layer (HTL) exhibited a notable maximum external quantum efficiency (EQEmax) of 18.59% with a remarkable maximum current efficiency (CEmax) of 78.48 cd A-1. In addition, solution-processed green OLEDs with 3-CDTPA showed excellent device performance with an EQEmax of 15.61%, a CEmax of 52.51 cd A-1, and outstanding CIE(x, y) color coordinates of (0.29, 0.61). This is one of the highest reported EQEs and CEs with high color purity for green solution-processed QLEDs and OLEDs using a divinyl-functionalized cross-linked HTM as the HTL. We believe that this study provides a new strategy for designing and synthesizing practical cross-linakable HTMs with enhanced performance for highly efficient solution-processed QLEDs and OLEDs.

Improving Perovskite Green Quantum Dot Light-Emitting Diode Performance by Hole Interface Buffer Layers.

Perovskite quantum dot light-emitting diodes (QLEDs) are potential candidates for next-generation displays due to their high color purity and wide color gamut. Due to the strong electron-accepting ability of poly[bis(4-phenyl) (2,4,6-trimethylphenyl) amine] (PTAA), quantum dot (QD) films are prone to be charged, which leads to the imbalance of charge injection and the increase of nonradiative recombination, ultimately affecting the performance of the QLEDs. Here, we compared and studied two polymers, poly(methyl methacrylate) (PMMA) and poly(vinyl pyrrolidone) (PVP), as the hole interface buffer layers of QD films, which effectively reduced the defect density, suppressed nonradiative recombination, and greatly improved the efficiency and stability of QLEDs. The devices with PMMA achieved a maximum external quantum efficiency of 20.71%.

Bridging Chloride Anions Enables Efficient and Stable InP Green Quantum‐Dot Light‐Emitting Diodes

AbstractZnMgO nanoparticles (NPs) are commonly used as the electron transport layer (ETL) in indium phosphide (InP) based quantum dots light‐emitting diodes (QLEDs). It has been experimentally found that the inherent oxygen vacancy defects in ZnMgO can be passivated by halogen additives. However, an in‐depth understanding of how the halogen additives in ZnMgO affect the quantum dots (QDs) films is currently missing. Here, the study reports on efficient and stable indium phosphide (InP) green QLEDs by effectively bridging QDs and ETL using chloride (Cl) ions. As bi‐functional anchoring additives, Cl ions not only passivate the oxygen vacancy defects of ZnMgO NPs for suppressing the exciton quenching at the QDs/ZnMgO interfaces, but also facilitate the hole transport of QDs due to part replacement of insulated oleic acid ligands on the surfaces of InP QDs with Cl anions for more balanced charge injection in the devices. Consequently, the optimized green InP QLED achieves a peak external quantum efficiency (EQE) of 13.8% and an operational lifetime of 5944 h, which, to the best of current knowledge, represents the best overall performance among the reported green InP QLEDs.

Identifying the dominant carrier of CdSe-based blue quantum dot light-emitting diode

Unlike red and green quantum dot light-emitting diodes (QLEDs), it is not clear whether a blue QLED is an electron-dominated device. In this work, we identify that electron is over-injected in blue QLEDs by impedance spectroscopy. By analyzing the capacitance–voltage characteristics of the single-carrier devices, we find that the built-in voltage of the electron-only device is smaller than that of the hole-only device. Therefore, electron injection is more efficient than hole injection in blue QLEDs. To support our arguments, we employ a red QD as a fluorescent sensor to spatially investigate the exciton recombination zone of a blue QLED. Consequently, we observe that the exciton recombination zone of the blue QLED is close to the hole transport layer, and it shifts toward an electron transport layer with the increase in the applied bias. Our work provides a practical method for identifying the excess carrier in blue QLEDs, and it could be applied to other types of QLEDs.

Stable ZnS Electron Transport Layer for High-Performance Inverted Cadmium-Free Quantum Dot Light-Emitting Diodes

We report high-efficiency and long-lifetime inverted green cadmium-free (InP-based) quantum dot light-emitting diodes (QLEDs) using a stable ZnO/ZnS cascaded electron transport layer (ETL). We have successfully developed a strategy to spin-coat stable ZnS ETLs with a relatively higher conduction band minimum (CBM) and lower electron mobility than that of ZnO, which leads to balanced carrier injection and an improved device lifetime. Analysis shows that by using the ZnO/ZnS cascaded ETL, electron injection is reduced, resulting in an improved charge balance in the QD layer and suppressed exciton quenching, which preserves the emission properties of QDs. Optimized devices with ZnO/ZnS cascaded ETLs show a maximum external quantum efficiency of 10.8% and a maximum current efficiency of 37.5 cd/A; these efficiency values are an almost 2.2-fold improvement compared to those of reference devices without ZnS. The QLED devices also showed a remarkably long lifetime (LT70) of 265 h at an initial luminance of 1000 cd/m2. The predicted half-lifetime (LT50) at 100 cd/m2 is 60,255 h, which, to our knowledge, is currently the longest lifetime yet reported for InP-based green QLEDs.

Efficient green quantum dot light-emitting diodes enabled by high-quality alloyed gradient CdSeS/CdS/ZnS core/shell quantum dots

Efficient green quantum dot light-emitting diodes enabled by high-quality alloyed gradient CdSeS/CdS/ZnS core/shell quantum dots

Efficient InP Green Quantum‐Dot Light‐Emitting Diodes Based on Organic Electron Transport Layer

AbstractZnMgO thin film is commonly used as an electron transport layer (ETL) in quantum‐dot light‐emitting diodes (QLEDs); however, it often induces the problems of interface exciton quenching and electron over‐injection in the devices. Herein, an organic molecule 2,4,6‐tris(3‐(diphenylphosphoryl)phenyl)‐1,3,5‐triazine (PO‐T2T) is investigated as a ETL material for InP green QLEDs. Due to the high injection barrier and moderate electron mobility, the PO‐T2T can prevent electron over‐injection and accumulation in the InP QLEDs. Besides, with the organic ETL, the interfacial exciton quenching is effectively suppressed. By depositing the PO‐T2T using a solution‐assisted evaporation method, efficient InP green QLEDs are achieved, which exhibit a maximum external quantum efficiency of 15.0% and a luminance of 10 010 cd m−2, corresponding to 211% and 237% enhancements, respectively, compared to 7.1% and 4207 cd m−2 of the devices with ZnMgO ETL.

Highly Efficient Double‐Side‐Emitting Electroluminescent Quantum Dot Thin Film with Transparent MoO3/Ag:Cu/MoO3 Electrode Prepared by Thermal Co‐Evaporation

AbstractSee‐through double‐side‐emitting electroluminescent quantum‐dot light‐emitting‐diodes (QLEDs) with highly transparent tri‐layer MoO3/Ag:Cu/MoO3 electrode is demonstrated in this study. The tri‐layer MoO3/Ag: Cu/MoO3 electrode is prepared by a thermal co‐evaporation process, in which MoO3 is utilized as both a seed layer and a capping layer. This tri‐layer structure improves the transmittance as high as ≈88% and ≈86% for pure Ag and Ag:Cu at 550 nm, respectively. It is also found that minor Cu doping significantly improves the thermal stability and sheet resistance of Ag film. With the see‐through tri‐layer film as a device cathode, transparent double‐side‐emitting green QLED exhibits remarkable high brightness of 225, 500 cd m−2, the current efficiency of 58.68 cd A−1, the external quantum yield of 16.70%, and transmittance of 77% at 550 nm without any additional anti‐reflection coating.

Trap state-assisted electron injection in blue quantum dot light-emitting diode

We report trap state-assisted electron injection in a blue quantum dot light-emitting diode (QLED) in this work. By replacing an electron transport layer and a quantum dot emission layer, we identify trap states are indeed on blue quantum dots. We also analyze the equivalent circuit model and the density of trap state distribution by impedance spectroscopy. Furthermore, the trap states induce charge transfer in the blue QLED and lower the device efficiency, suggesting the competition between electron injection and trapping in a working device. Our work shows a distinct electron injection mechanism in blue QLEDs that has not been shown in red and green QLEDs.

Charge carrier analysis via impedance spectroscopy and the achievement of high performance in CdSe/ZnS:di-[4-(N,N-di-p-tolyl-amino)-phenyl]cyclohexane hybrid quantum dot light-emitting diodes

Despite the importance of charge carrier injection balance for achieving high performance with quantum dot light-emitting diodes (QLEDs), there have been comparatively few relevant studies. Thus, we extensively analyzed charge carrier behaviors of QLEDs using an impedance spectroscopy (IS) method and a QLED-equivalent circuit. This yielded both the capacitive and resistive values of each of the emission layer (EML) and the charge transport layer (CTL), revealing the relationships between these values and device performance. Using this analysis method, we examined the effects of the combination of hybrid quantum dot (QD) EMLs and CTLs on QLED performance. The CdSe/ZnS QD and di-[4-(N,N-di-p-tolyl-amino)-phenyl]cyclohexane blended hybrid QD EML, in combination with the di-[4-(N,N-di-p-tolyl-amino)-phenyl]cyclohexane hole transport layer, produced electron–hole balance in EMLs and resulted in a remarkable improvement in device performance. The maximum current efficiency of a green QLED using this combination was 43.7 cd/A, which was approximately two-fold greater than the maximum current efficiency of a QLED with a conventional CdSe/ZnS QD EML and much better than previously reported values for green QLEDs with CdSe/ZnS-based EMLs. In summary, we have greatly improved QLED performance via charge balance optimization. Herein, we present the methods for this improvement, along with an analysis of carrier behavior. • A CdSe/ZnS and TAPC hybrid layer improved light emission efficiency 1.9 times. • Hybrid emission layer and TAPC hole transport layer combination was the most efficient. • Energy band levels of the hybrid light emission layer could be controlled. • Capacitance and resistance of each layer were extracted by Impedance spectroscopy. • The device operation mechanism was well explained by the impedance spectroscopy.

P‐90: Boosting the Efficiency of Cd‐Free Blue Quantum Dot Light‐Emitting Diodes via Charge Transport Layer Optimization

Quantum dot light‐emitting diodes (QLEDs) show great potential in the display and solid‐state lighting fields. However, the more mature cadmium‐based quantum dots and perovskite nanocrystals contain elements such as cadmium and lead, which pose a greater threat to the environment and human health and limit their larger‐scale applications. Therefore, Cd‐free quantum dots and its electroluminescent devices show greater potential. At present, the research of red and green QLEDs is progressing rapidly, while Cd‐free blue QLEDs are less reported, and the performance of the device is obviously lagging behind. Based on the above considerations, we optimized the thickness of the electron transport layer (ETL) and selected a more suitable hole transport materials (HTM) to make the carrier injection and transport more balanced, and finally obtained Cd‐free blue QLEDs with a high EQE up to 6.18%.

Pure-colored red, green, and blue quantum dot light-emitting diodes using emitting layers composed of cadmium-free quantum dots and organic electron-transporting materials

This study focuses on the use of highly saturated and efficient red, green, and blue (RGB) cadmium (Cd)-free quantum dot light-emitting diodes (QD-LEDs). RGB QD-LEDs were fabricated using emitting layers (EMLs) comprising Cd-free QDs (red and green InP-based QDs and blue Zn–SeTe QDs) and organic electron-transporting materials (ETMs). The green QD-LED with high color purity was realized by adjusting the QD concentration in the EMLs and suppressing the defect-associated emission in the QD-LEDs. Narrow emissions of the full-width at half-maximum of 40, 34, and 23 nm were realized in the RGB QD-LEDs, respectively. Wide-area coverage of 80% of the available color reproduction area specified in Recommendation ITU-R BT.2020 was achieved using the developed RGB QD-LEDs without applying any color filter and cavity structure. Furthermore, the addition of organic ETMs into the EMLs improved the efficiency of all RGB QD-LEDs.