Quantum Dot Light-emitting Diodes Research Articles

Development of Uniform Polydimethylsiloxane Arrays through Inkjet Printing

The inkjet printing method is a promising method to deposit polymer and functional nanoparticles at the microscale. It can be applied in the fabrication of multicolor polymer light emitting diodes (polyLEDs), polymer base electronics, multicolor color conversion layers, and quantum dot light emitting diodes (QLEDs). One of the main challenges is to print high-resolution polymer dots from dilute polymer solution. In addition, the quality of printed multicolor polyLEDs, QLEDs and multicolor color conversion layers is currently limited by non-uniformity of the printed dots. In this paper, polydimethylsiloxane (PDMS) is selected as the functional polymer, due to its high transparency, good reflective index value, inflammable and flexible properties. The optimal ink to form a uniform PDMS dot array is presented in this paper. Both the solvent and PDMS were tuned to form the uniform PDMS dot array. The uniform PDMS dot array was printed with a diameter of around 50 µm, and the array of closely spaced green quantum dots (QDs) mixed with PDMS ink was also printed on the substrate uniformly. While the green QD-PDMS film was printed at a resolution of 1693 dpi, the uniformity was evaluated using the photoluminescence (PL) spectrum and color coordinate value.

Bright and spectrally stable pure-red CsPb(Br/I)3 quantum dot LEDs realized by synchronous device structure and ligand engineering

All-inorganic CsPbBrxI3−x perovskite quantum dots (PQDs) show great potential for pure-red light-emitting diodes (LEDs). However, spectral instability caused by ion migration at high electric fields and lower luminance limit its practical applications. Here, the ITO/ZnO/PEI/CsPbBrxI3−x PQDs/N, N, N-tris (4-(9-carbazolyl) phenyl) amine (TCTA) (3 nm)/1-bis [4-[N, N-di(4-tolyl)amino]-phenyl]-cyclohexane (TAPC)/MoO3/Au device structure is first designed to reduce the overall operating voltage of the device by enhancing hole injection and transport, effectively avoiding ion migration under high electric fields, and then by post-treating CsPbBrxI3−x PQDs with n-octylphosphonic acid: K to remove surface defects and increase the activation energy of halogen migration, further suppressing ion migration while improving the optical properties of the material. Ultimately, the color-stable pure-red perovskite LED achieved a high luminance of 13730 cd m−2 at voltages as low as 4.9 V and a maximum EQE of 16.7%.

Multi-Color Light-Emitting Diodes

Multi-color light-emitting diodes (LEDs) with various advantages of color tunability, self-luminescence, wide viewing angles, high color contrast, low power consumption, and flexibility provide a wide range of applications including full-color display, augmented reality/virtual reality technology, and wearable healthcare systems. In this review, we introduce three main types of multi-color LEDs: the organic LED, colloidal quantum dots (CQDs) LED, and CQD–organic hybrid LED. Various strategies for realizing multi-color LEDs are discussed including red, green, and blue sub-pixel side-by-side arrangement; vertically stacked LED unit configuration; and stacked emitter layers in a single LED. Finally, according to their status and challenges, we present an outlook of multi-color devices. We hope this review can inspire researchers and make a contribution to the further improvement of multi-color LED technology.

Efficient Quantum-Dot Light-Emitting Diodes Based on Solvent-Annealed SnO2 Electron-Transport Layers

Due to the sensitivity of ZnO nanoparticles to water and oxygen, the hybrid quantum-dot light-emitting devices (QLEDs) based on ZnO electron-transport layers (ETLs) suffer from shelf stability and uncontrollable positive aging problems. Tin oxide (SnO2) is considered as an ideal alternative to ZnO as the ETL in QLEDs to improve their shelf stability. Currently, the SnO2-based device still suffers from poor efficiency. Herein solvent vapor annealing (SVA) is employed to reduce the roughness of SnO2 nanoparticle films, which suppresses the leakage current and enhances the device performance. Compared with the device with pristine SnO2, the maximum brightness and current efficiency (10.8 cd/A) of QLEDs based on solvent vapor annealed SnO2 are enhanced by 25.3% and 36.6%, respectively, being comparable to the ZnO device (11.0 cd/A). Moreover, SVA processing also has a positive effect on the device shelf stability, which is attributed to the dense interface between SnO2 and quantum dots due to the reduced surface roughness of SnO2 films.

Performance Improvement of Quantum Dot Light-Emitting Diodes Using a ZnMgO Electron Transport Layer with a Core/Shell Structure.

Highly efficient and all-solution processed quantum dot light-emitting diodes (QLEDs) with high performance are demonstrated by employing ZnMgO nanoparticles (NPs) with core/shell structure used as an electron transport layer (ETL). Mg-doping in ZnO NPs exhibits a different electronic structure and degree of electron mobility. A key processing step for synthesizing ZnMgO NPs with core/shell structure is adding Mg in the solution in addition to the remaining Mg and Zn ions after the core formation process. This enhanced Mg content in the shell layer compared with that of the core X-ray photoelectron spectroscopy showed a higher number of oxygen vacancies for the ZnMgO core/shell structure, thereby enhancing the charge balance in the emitting layer and improving device efficiency. The QLED incorporating the as synthesized ZnMgO NP core/shell A exhibited a maximum luminance of 55,137.3 cd/m2, maximum current efficiency of 58.0 cd/A and power efficiency of 23.3 lm/W. The maximum current efficiency and power efficiency of the QLED with ZnMgO NP core/shell A improved by as much as 156.3% and 113.8%, respectively, compared to the QLED with a Zn0.9Mg0.1O NP ETL, thus demonstrating the benefits of ZnMgO NPs with the specified core/shell structure.

Stable Quantum Dot Light-Emitting Diodes Employing TiO2 Nanoparticles as a Mixed Emission Layer

Colloidal quantum dots (QDs) have attractive optical and electrical characteristics for various applications in the display industry. They have superb properties including color tunability, which is achieved by controlling the particle size and solution processes used for quantum dot light-emitting diodes (QLEDs). Here we demonstrate high efficiency QLEDs that consist of a mixed layer of TiO2 nanoparticles (NPs) and QDs. ZnO is the key material responsible for the promising results of the electroluminescence devices, which exhibited well-matched energy levels and robustness. In this report, Li-doped TiO2 NPs were synthesized under ambient conditions as an alternative electron transport layer (ETL) for QLEDs. A stable mixture of TiO2 NPs and QDs was prepared in chlorobenzene and then applied to QLEDs without a conventional ETL. QLEDs with such a simplified structure produce a maximum luminance of 123,311 cd/m<sup>2</sup> with a current efficiency of 40.5 cd/A. These results are comparable to those of conventional QLEDs. Additionally, the predicted T50 at 100 cd/m<sup>2</sup> was 1,420 h, based on the T50 at 1,600 cd/m<sup>2</sup>. These clearly indicate not only the promising results of the TiO2 NPs as an inorganic ETL, but also the remarkable performance of the simplified device with less layers. The reduction in fabrication steps using this solution-based process is also advantageous for next-generation display technology.

Highly stable quantum dot light-emitting diodes with improved interface contacting via violet irradiation

The relatively low stability of quantum dot light-emitting diodes (QLEDs) caused by unbalanced carrier injection has not been completely solved. Among various influencing factors, interface contacting is one of the important factors affecting the carrier injection equilibrium. Herein, we proposed a method to improve the carrier injection equilibrium for QLEDs. We investigated and explored the effects of violet irradiation on the efficiency and operating stability of MoO3-based inverted QLEDs. Specifically, violet irradiation on the interface of MoO3/Mg:Ag is carefully analysed. Further research shows that the interfacial reaction between Mg:Ag anode and MoO3 hole injection layer accounts for such improvement. During violet irradiation, the Mg:Ag quickly reacts with the oxygen from MoO3, leading to the formation of Mo-O-Mg:Ag interlayer. The hole injection is effectively enhanced, resulting from the reduction of the electrons extraction barrier at MoO3/Mg:Ag interface and the improvement of the interface contacting. By optimizing the violet irradiation dosage and violet wavelength, the maximum current efficiency (CE) is increased from 20.2 cd/A to 31.1 cd/A. Moreover, devices exhibit a highly stable operation lifetime of 44161 h (T95@1000 nits), which is much longer than non-violet irradiation QLEDs and meet the stringent industrial requirements for display applications.

Carboxylic‐Free Synthesis of InP Quantum Dots for Highly Efficient and Bright Electroluminescent Device

AbstractIn the synthesis of InP‐based quantum dots (QDs), the environmentally friendly carboxylic‐free phosphor precursor tris(dimethylamino)phosphine ((DMA)3P) has been frequently used to replace tris (trimethylsilyl)phosphine (TMS)3P, in order to prevent the InP cores from oxidation and avoid the dangerous etching with hydrofluoric acid. However, the InP‐based QDs synthesized with (DMA)3P still perform poorly in quantum dot light‐emitting diodes (QLEDs) due to oxidation by other carboxylic sources. In this work, based on the synthesis with (DMA)3P, ZnCl2 precursor instead of zinc carboxylate is used for shell growing, and oleylamine is used as solvent instead of oleic acid, so that there is no oxidizing agent such as carboxylic groups during the whole synthesis process. As a result, highly efficient InP/ZnSe/ZnSeS/ZnS QDs with gradient varying shells are obtained, whose full width at half‐maximum (≈41 nm) and photoluminescence quantum yield (≈96%) are narrowest and highest among the reported ones, respectively. The QLEDs based on InP QDs synthesized with (DMA)3P achieve breakthrough efficiency of 12.39%. Especially, high efficiency and high brightness are achieved at the same time. This work opens a new avenue in the synthesis of high‐quality and environmental friendly InP‐based QDs.

Balancing Charge Injection via a Tailor-Made Electron-Transporting Material for High Performance Blue Perovskite QLEDs

One of the great challenges in perovskite quantum dot light-emitting diodes (Pe-QLEDs) is the unbalanced charge injection that significantly hinders the device performance and stability. Herein, we tailor-made a high mobility electron-transporting material (ETM), named B2, to balance the carrier injection in blue Pe-QLEDs. B2 with a tailored asymmetric anthracenyl structure exhibits a promising electron mobility of 2.7 × 10–4 cm2·V–1·s–1, which is almost 20 times higher than the commonly used ETM-TPBi (1.1 × 10–5 cm2·V–1·s–1). Subsequently, sky blue (490 nm) Pe-QLED with B2 as the ETM presented a remarkably high external quantum efficiency (EQE) of 13.17% and a low turn-on voltage of 2.2 V, which is much better than that of the TPBi-based device (EQE of 8.31% and Vturn-on of 3.2 V). In addition, B2 also demonstrated a universal application in green and deep blue Pe-QLEDs. This work provides an important guidance to rational design of high electron mobility ETMs for high-performance LEDs.

Does interfacial exciton quenching exist in high-performance quantum dot light-emitting diodes?

In quantum dot light-emitting diodes (QLEDs), even seemingly with interfacial exciton quenching between quantum dots (QDs) and the electron transport layer (ETL) limiting the device efficiency, the internal quantum efficiency of such QLEDs approaches 100%. Therefore, it is a puzzle that QLEDs exhibit high performance although they suffer from interfacial exciton quenching. In this work, we solve this puzzle by identifying the cause of the interfacial exciton quenching. By analyzing the optical characteristics of pristine and encapsulated QD-ETL films, the interfacial exciton quenching in the pristine QD-ETL film is attributed to O2-induced charge transfer. We further investigate the charge transfer mechanism and its effect on the performance of QLEDs. Finally, we show the photodegradation of the pristine QD-ETL film under UV irradiation. Our work bridges interfacial exciton quenching and high performance in hybrid QLEDs and highlights the significance of encapsulation in QLEDs.

Defect passivation and electron band energy regulation of a ZnO electron transport layer through synergetic bifunctional surface engineering for efficient quantum dot light-emitting diodes.

Zinc oxide nanoparticles (ZnO NPs) have been actively pursued as the most effective electron transport layer for quantum-dot light-emitting diodes (QLEDs) in light of their unique optical and electronic properties and low-temperature processing. However, the high electron mobility and smooth energy level alignment at QDs/ZnO/cathode interfaces cause electron over-injection, which aggravates non-radiative Auger recombination. Meanwhile, the abundant defects hydroxyl group (-OH) and oxygen vacancies (OV) in ZnO NPs act as trap states inducing exciton quenching, which synergistically reduces the effective radiation recombination for degrading the device performance. Here, we develop a bifunctional surface engineering strategy to synthesize ZnO NPs with low defect density and high environmental stability by using ethylenediaminetetraacetic acid dipotassium salt (EDTAK) as an additive. The additive effectively passivates surface defects in ZnO NPs and induces chemical doping simultaneously. Bifunctional engineering alleviates electron excess injection by elevating the conduction band level of ZnO to promote charge balance. As a result, state-of-the-art blue QLEDs with an EQE of 16.31% and a T50@100 cd m-2 of 1685 h are achieved, providing a novel and effective strategy to fabricate blue QLEDs with high efficiency and a long operating lifetime.

Improving the performance of quantum dot light-emitting diodes by tailoring QD emitters.

As the emitters of quantum dots (QDs) light-emitting diodes (QLEDs), QDs, which are responsible for the charge injection, charge transportation, and especially exciton recombination, play a significant role in QLEDs. With the crucial advances made in QDs, such as the advancement of synthetic methods and the understanding of luminescence mechanisms, QLEDs also demonstrate a dramatic improvement. Until now, efficiencies of 30.9%, 28.7% and 21.9% have been achieved in red, green and blue devices, respectively. However, in QLEDs, some issues are still to be solved, such as the imbalance of charge injection and exciton quenching processes (defect-assisted recombination, Auger recombination, energy transfer and exciton dissociation under a high electric field). In this review, we will provide an overview of recent advances in the study and understanding of the working mechanism of QLEDs and the exciton quenching mechanism of QDs in devices. Particular emphasis is placed on improving charge injection and suppressing exciton quenching. An in-depth understanding of this progress may help develop guidelines to direct QLED development.

Elaborating the interplay between the detecting unit and emitting unit in infrared quantum dot up-conversion photodetectors.

The quantum dot up-conversion device combines an infrared photodetector (PD) and a visible quantum-dot light-emitting diode (QLED) to directly convert infrared targets to visible images. However, large efficiency loss is usually induced by the integration of the detecting unit and the emitting unit. One of the important reasons is the performances of the PD and QLED units restraining each other. We regulated the equilibrium between infrared absorption and visible emission by changing the thicknesses of infrared active layers in up-conversion devices. A good balance could be achieved between the absorption of 980 nm incident light and the out-coupling of the 634 nm emission when the active layer thickness is 140 nm, leading to the best performance of the up-conversion device. As more photogenerated carriers are produced with the increase of infrared illumination intensity, the external quantum efficiency (EQE) of the QLED unit in the up-conversion device remains little changed. This suggests the limited amount of photogenerated holes in the PD unit does not limit the EQE of the QLED unit. However, a PD unit with a high ratio of photogenerated holes trapped near the interconnection decreased the EQE in the QLED unit. This work provides new insights into the interplay between the PD and QLED units in up-conversion devices, which is crucial for their further improvements.

Hole-transport-layer-free CdSe/ZnS core/shell red quantum-dot light-emitting diodes sensitized by TADF polymers

Energy transfer and charge transport in the thermally activated delayed fluorescence polymer doped quantum-dots were investigated. A high EQE of 18.1% was demonstrated for the first time in the quantum-dot red LEDs without any hole transport layers.

Spiro-based hole-transporting materials utilized in green perovskite quantum dot light-emitting diodes with high luminance

Pe-QLEDs based on two spiro-cored HTMs achieved high brightness of 45 056 and 46 382 cd m−2, respectively.

Performance enhancement by sol-gel processed Ni-doped ZnO layer in InP-based quantum dot light-emitting diodes

Performance enhancement by sol-gel processed Ni-doped ZnO layer in InP-based quantum dot light-emitting diodes

Efficient and bright green InP quantum dot light-emitting diodes enabled by a self-assembled dipole interface monolayer.

The interfacial state between the hole transport layer (HTL) and quantum dots (QDs) plays a crucial role in the optoelectronic performance of light-emitting diodes. Herein, we reported an efficient and bright green indium phosphide (InP) QD-based light-emitting diode (LED) by introducing a self-assembled monolayer of 4-bromo-2-fluorothiophenol (SAM-BFTP) molecule to improve interfacial charge transport in LED devices. The molecular dipole layer at the interface of the QD layer and HTL not only reduces the energy barrier of holes injected into QDs through vacuum energy level shift but also inhibits the fluorescence quenching of QDs caused by the HTL. Moreover, copper ions doped into phosphomolybdic acid (Cu:PMA) is selected as the hole injection layer (HIL) into the device system based on the SAM-BFTP molecule, and as a result, a green InP QD LED (QLED) with a maximum external quantum efficiency (EQE) of 8.46% and a luminance of 18 356 cd m-2 was realized. This work can inform and underpin the future development of InP-based QLEDs with concurrent high efficiency and brightness.

Sn-doped ZnO for efficient and stable quantum dot light-emitting diodes via a microchannel synthesis strategy.

ZnO nanocrystals (NCs) are widely employed as an electron transport layer (ETL) in quantum-dot light-emitting diodes (QLEDs). However, the excessive electron mobility, abundant surface defects and poor reproducibility of ZnO NC synthesis are currently the primary restrictive factors influencing the development of QLEDs. In this study, we developed Sn(IV)-doped ZnO NCs as the ETL for constructing highly efficient and long lifetime QLEDs. The introduction of Sn can reduce the surface hydroxyl oxygen defects and alter the electron transport properties of NCs, and thus is beneficial for improving the efficiency of hole-electron recombination in the emitting layer. Meanwhile, a microchannel (MC) reactor is utilized to finely control the synthesis of Zn0.96Sn0.04O NCs, enabling us to achieve uniform size distribution and consistent production reproducibility. Using the Sn(IV)-doped ZnO NCs as the ETL has led to a remarkable enhancement of external quantum efficiency (EQE) for the fabricated red QLED, from 9.2% of the ZnO only device to 15.5% of the Zn0.96Sn0.04O device. Furthermore, the T70 (@1000 cd m-2) of the Zn0.96Sn0.04O device reached 78 h, which is 1.77-fold higher than that of the ZnO only device (44 h). The present work provides an alternative ETL for efficient and stable QLEDs.

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

Correction for ‘Solution-processable Li-doped transition metal oxide hole-injection layer for highly efficient quantum-dot light-emitting diodes’ by Jae Seung Shin et al., J. Mater. Chem. C, 2022, 10, 5590–5597, https://doi.org/10.1039/D1TC06117H.

Ligand modification enhanced quantum dot LEDs: principles and methods

Many academics are exploring using quantum dots (QDs) to make better LED devices due to their narrow emission band, low reaction temperature, low self-absorption effect, and high photoluminescence quantum yields (PLQYs).