Quantum Dot Light-emitting Diode Devices Research Articles

Well-type thick-shell quantum dots combined with double hole transport layers device structure assisted realization of high-performance quantum dot light-emitting diodes

Quantum dot (QD) light-emitting diodes (QLEDs) are promising for next-generation lighting and displays. Considering the optimization design of both the QD and device structure is expected to improve the QLED's performance significantly but has rarely been reported. Here, we use the thick-shell QDs combined with a dual-hole transport layer device structure to construct a high-efficiency QLED. The optimized thick-shell QDs with CdS/CdSe/CdS/ZnS seed/spherical quantum well/shell/shell geometry exhibit a high photoluminescence quantum yield of 96% at a shell thickness of 5.9 nm. The intermediate emissive CdSe layer with coherent strain ensures defect-free growth of the thick CdS and ZnS outer shells. Based on the orthogonal solvents assisted Poly-TPD&PVK dual-hole transport layer device architecture, the champion QLED achieved a maximum external quantum efficiency of 22.5% and a maximum luminance of 259955 cd m-2, which are 1.6 and 3.7 times that of thin-shell QDs based devices with single hole transport layer, respectively. Our study provides a feasible idea for further improving the performance of QLED devices.

Highly enhanced Quantum dot light-emitting diode performance by controlling energy resonance in inorganic insertion layers

Quantum-dot light-emitting diodes (QLED) have become a research trend in the field of new displays due to their low cost, wide color gamut, narrow bandwidth, and characteristics that enable production through the solution-gel method. However, the electrical performance of QLED is consistently constrained by energy losses and imbalanced charge carrier injection. This motivates our focus on exciton recombination and energy losses within the quantum-dot layer to enhance the electrical efficiency of QLED. In this work, we introduce a method using a CdZnS quantum dot (B-QD) interlayer to modulate energy transfer and charge carrier transport in QLED devices employing CdSe quantum dot (G-QD) as the emissive layer. By strategically incorporating a B-QD layer between the G-QD and HTL/ETL, we facilitate energy transfer due to the overlap between the excitation wavelength of B-QD and the absorption wavelength of G-QDs. This leads to enhanced energy injection in QLED devices, resulting in a high current efficiency of 39.54 Cd/A and a peak brightness of 522,272 cd/m2 for efficient QLED. The corresponding external quantum efficiency (EQE) is greatly improved from 5.62 % to 9.4 %. Our work provides a straightforward and effective approach to modulate exciton recombination and energy injection and further can be applicable to other photo-electronics devices.

Recent Developments in Quantum Dot Patterning Technology for Quantum Dot Display

Colloidal quantum dot (QDs) have emerged as a crucial building block for LEDs due to their size-tunable emission wavelength, narrow spectral line width, and high quantum efficiency. Tremendous efforts have been dedicated to improving the performance of quantum dot light-emitting diodes (QLEDs) in the past decade, primarily focusing on optimization of device architectures and synthetic procedures for high quality QDs. However, despite these efforts, the commercialization of QLEDs has yet to be realized due to the absence of suitable large-scale patterning technologies for high-resolution devices., This review will focus on the development trends associated with transfer printing, photolithography, and inkjet printing, and aims to provide a brief overview of the fabricated QLED devices. The advancement of various quantum dot patterning methods will lead to the development of not only QLED devices but also solar cells, quantum communication, and quantum computers.

High efficiency quantum dot light-emitting diodes with femtosecond laser post-treatment

Quantum dot light-emitting diodes (QLEDs), as an emerging display technology, have garnered widespread attention due to their excellent color rendering, high efficiency, and long lifespan. However, the inherent differences in the properties of charge transport layer materials inevitably lead to charge injection imbalances and low device performance. Herein, we developed a simple technique by using femtosecond laser scanning over the QLED devices. The results indicate that scanning with a femtosecond laser improves the conductivity of the hole transport layer and increases the external quantum efficiency of the QLED devices. Our work provides an effective route for realizing high performance QLED devices with efficient post-treatment.

The warming-up effects of quantum-dot light emitting diodes: A reversible stability issue related to shell traps.

The aging phenomenon is commonly observed in quantum-dot light emitting diodes (QLEDs), involving complex chemical or physical processes. Resolving the underlying mechanism of these aging issues is crucial to deliver reliable electroluminescent devices in future display applications. Here, we report a reversible positive aging phenomenon that the device brightness and efficiency significantly improve after device operation, but recover to initial states after long-time storage or mild heat treatment, which can be termed as warming-up effects. Steady and transient equivalent circuit analysis suggest that the radiative recombination current dramatically increases but electron leakage from the quantum dots (QDs) to hole transport layer becomes more accessible during the warming-up process. Further analysis discloses that the notable enhancement of device efficiency can be ascribed to the filling of shell traps in gradient alloyed QDs. This work reveals a distinct positive aging phenomenon featured with reversibility, and further guidelines would be provided to achieve stable QLED devices in real display applications.

Large-Emitting-Area Quantum Dot Light-Emitting Diodes Fabricated by an All-Solution Process.

Quantum dots (QDs) have attracted a lot of attention over the past decades due to their sharp emission spectrum and color, which can be tuned by changing just the particle size and chromophoric stability. All these advantages of QDs make quantum dot light-emitting diodes (QLEDs) promising candidates for display and light-source applications. This paper demonstrates a large-emitting-area QLED fabricated by a full-solution process. This QLED is composed of indium tin oxide (ITO) as the anode, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT: PSS) as the hole injection layer (HIL), and poly(N,N'-bis-4-butylphenyl-N,N'-bisphenyl)benzidine (poly-TPD) as the hole-transport layer (HTL). The light-emitting layer (EML) is composed of green CdSe/ZnS quantum dots. By applying the ZnO nanoparticles as the electron-injection/transport layer, QLED devices are prepared under a full-solution process. The large-emitting-area QLED exhibits a low turn-on voltage of around 2~3 V, and the International Commission on Illumination (CIE) 1931 coordinate value of the emission spectrum was (0.31, 0.66). The large emitting area and the unique QLED structure of the device make it possible to apply these features to inkjet printing quantum dot light sources and quantum dot display applications.

A full high‐definition quantum dot light‐emitting diode‐on‐silicon microdisplay

AbstractWe report a 0.39‐in. quantum dot light‐emitting diode (QLED) microdisplay with full high‐definition (FHD, 1920 × 1080) resolution by integrating a red top‐emitting QLED on a complementary metal–oxide–semiconductor (CMOS) backplane. By optimizing the microcavity structure and constructing a suitable energy‐level structure for the QLED devices, the performance of the large‐area (4.9 × 8.7 mm2) top‐emitting device with normal structure reached 13,936 cd/m2 of brightness at 5‐V bias with 13.3% external quantum efficiency (EQE). Notably, the optimal device showed a low turn‐on voltage of 1.7 V, which matched well the voltage output of the CMOS backplane. Our work demonstrates the great promise of QLED microdisplays for applications in head‐mounted augmented reality/virtual reality (AR/VR).

Revisiting Hole Injection in Quantum Dot Light‐Emitting Diodes

AbstractInjecting holes from the hole transport layer (HTL) into the quantum dot (QD) emitting layer in quantum dot light‐emitting diodes (QLEDs) is considered challenging due to the presence of a relatively high hole injection barrier at the HTL/QD interface. However, QLEDs with exceptional brightness and efficiency are achieved, prompting a reevaluation of the traditional hole injection mechanisms. This study examines the hole injection mechanism in QLEDs using a combination of experiments and simulations. The results demonstrate that the applied bias significantly reduces the barrier height between the highest occupied molecular orbital level of the HTL and the valence band (VB) of the QDs, facilitating hole injection. The bending of the lowest unoccupied molecular orbital energy level of the HTL at the HTL/QD interface confines electrons within the QD, effectively minimizing leakage current. Additionally, the triangle‐shaped potential barrier arising from the bending of the VB energy level of the QDs creates favorable conditions for hole–tunneling injection. Moreover, both simulations and experiments consistently demonstrate that the predominant pathway for hole injection from the HTL to the QDs in the QLED device involved thermally assisted tunneling. This study is important to understand the hole injection mechanism in QLEDs.

High-Resolution, Highly Transparent, and Efficient Quantum Dot Light-Emitting Diodes.

Aiming at next-generation displays, high-resolution quantum dot light-emitting diodes (QLEDs) with high efficiency and transparency are highly desired. However, there is limited study involving the improvements of QLED pixel resolution, efficiency, and transparency simultaneously, which undoubtedly restricts the practical applications of QLED for next-generation displays. Here, the strategy of electrostatic force-induced deposition (EF-ID) is proposed by introducing alternating polyethyleneimine (PEI) and fluorosilane patterns to synergistically improve the pixel accuracy and transmittance of QD patterns. More importantly, the leakage current induced by the void spaces between pixels that is usually reported for high-resolution QLEDs is greatly suppressed by substrate-assisted insulating fluorosilane patterns. Finally, high-performance QLEDs with high resolution ranging from 1104 to 3031 pixels per inch (PPI) and a high efficiency of 15.6% are achieved, among the best performances of high resolution QLEDs. Notably, the high resolution QD pixels greatly enhance the transmittance of the QD patterns, thus prompting an impressive transmittance of 90.7% for the transparent QLEDs (2116 PPI), which represents the highest transmittance of transparent QLED devices. Consequently, this work contributes an effective and general approach for high-resolution QLEDs with high efficiency and transparency.

Machine Learning Assisted Stability Analysis of Blue Quantum Dot Light-Emitting Diodes.

The operational stability of the blue quantum dot light-emitting diode (QLED) has been one of the most important obstacles to initialize its industrialization. In this work, we demonstrate a machine learning assisted methodology to illustrate the operational stability of blue QLEDs by analyzing the measurements of over 200 samples (824 QLED devices) including current density-voltage-luminance (J-V-L), impedance spectra (IS), and operational lifetime (T95@1000 cd/m2). The methodology is able to predict the operational lifetime of the QLED with a Pearson correlation coefficient of 0.70 with a convolutional neural network (CNN) model. By applying a classification decision tree analysis of 26 extracted features of J-V-L and IS curves, we illustrate the key features in determining the operational stability. Furthermore, we simulated the device operation using an equivalent circuit model to discuss the device degradation related operational mechanisms.

Highly Efficient ITO-Free Quantum-Dot Light Emitting Diodes via Solution-Processed PEDOT:PSS Semitransparent Electrode.

We present a study on the potential use of sulfuric acid-treated poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as a viable alternative to indium tin oxide (ITO) electrodes in quantum dot light-emitting diodes (QLEDs). ITO, despite its high conductivity and transparency, is known for its disadvantages of being brittle, fragile, and expensive. Furthermore, due to the high hole injection barrier of quantum dots, the need for electrodes with a higher work function is becoming more significant. In this report, we present solution-processed, sulfuric acid-treated PEDOT:PSS electrodes for highly efficient QLEDs. The high work function of the PEDOT:PSS electrodes improved the performance of the QLEDs by facilitating hole injection. We demonstrated the recrystallization and conductivity enhancement of PEDOT:PSS upon sulfuric acid treatment using X-ray photoelectron spectroscopy and Hall measurement. Ultraviolet photoelectron spectroscopy (UPS) analysis of QLEDs showed that sulfuric acid-treated PEDOT:PSS exhibited a higher work function than ITO. The maximum current efficiency and external quantum efficiency based on the PEDOT:PSS electrode QLEDs were measured as 46.53 cd/A and 11.01%, which were three times greater than ITO electrode QLEDs. These findings suggest that PEDOT:PSS can serve as a promising replacement for ITO electrodes in the development of ITO-free QLED devices.

Degradation phenomena of quantum dot light-emitting diodes induced by high electric field

Quantum dots possess exceptional optoelectronic properties, such as narrow bandwidth, controllable wavelength, and compatibility with solution-based processing. However, for efficient and stable operation in electroluminescence mode, several issues require resolution. Particularly, as device dimensions decrease, a higher electric field may be applied through next-generation quantum dot light-emitting diode (QLED) devices, which could further degrade the device. In this study, we conduct a systematic analysis of the degradation phenomena of a QLED device induced by a high electric field, using scanning probe microscopy (SPM) and transmission electron microscopy (TEM). We apply a local high electric field to the surface of a QLED device using an atomic force microscopy (AFM) tip, and we investigate changes in morphology and work function in the Kelvin probe force microscopy mode. After the SPM experiments, we perform TEM measurements on the same degraded sample area affected by the electric field of the AFM tip. The results indicate that a QLED device could be mechanically degraded by a high electric field, and work function changes significantly in degraded areas. In addition, the TEM measurements reveal that In ions migrate from the indium tin oxide (ITO) bottom electrode to the top of the QLED device. The ITO bottom electrode also deforms significantly, which could induce work function variation. The systematic approach adopted in this study can provide a suitable methodology for investigating the degradation phenomena of various optoelectronic devices.

Solution‐Processable Small‐Molecule Hole Transport Material for High‐Performance QLED via Manipulating Dipole Moments

AbstractSolution‐processed quantum‐dot light‐emitting diodes (QLEDs) have achieved huge successes in terms of both efficiency and stability. However, the polymeric hole transport materials (HTMs) utilized in QLEDs, to some degree, cause severe damages to performance consistency due to their intrinsic batch‐to‐batch problems. Thus, adopting small‐molecule HTMs with definitive chemical structures and molecular weight is a plausible way to solve this issue. In this work, a new solution‐processable small‐molecule (SM)‐HTM (FLCZ) is designed and synthesized, through elaborately modifying the widely used 4,4′‐bis(carbazole‐9‐yl)‐biphenyl via manipulating dipole moments, for solution‐processed QLEDs to replace polymeric HTMs. The FLCZ‐based QLED has achieved an external quantum efficiency (EQE) of 14.63% (18.74 lm W−1, 20.01 cd A−1), approaching that of polymer HTMs‐based QLEDs. In addition, the large surface energy of 45.29 mN m−1 is also observed on the film of FLCZ, which is highly desirable for inkjet‐printed (IJP) QLED devices, results in the IJP fabricated QLED with an EQE of 10.69%. To the best of knowledge, the results of this work represent the highest device performance of conventional solution‐processed QLED based on a single SM‐HTM.

Improved Current Efficiency of Quantum Dot Light‐Emitting Diodes by Utilizing ZnO Nanoparticles and an Organic Ionic Interlayer

AbstractThe solution‐processed QLED (quantum dot light‐emitting diode) has a great potential for low‐cost and large‐scale displays. However, the solution‐processed QLED has a limitation in the current efficiency due to the rough surface of the quantum dot layer causing shunt leakage. This paper reports solution‐processed highly current‐efficient QLED utilizing zinc oxide nanoparticles (ZnO NPs) and an organic ionic interlayer as an electron injection layer. The organic ionic materials create the permanent interface dipole by shifting the vacuum energy using tetrabutylammonium (TBA) cations and tetrafluoroborate (BF4) anions under the applied electric field. The dipole effectively reduces the electronic injection barrier to provide charge carrier balance of the holes and electrons in the emissive layer. As a result, the maximum current efficiency and power efficiency for the device structure of QLEDs ITO/PEDOT : PSS/PVK/CdSe@ZnS QDs/ZnO NPs/TBABF4+PEG/Al (23.9 cd A−1 and 12.9 lm W−1) enhanced significantly compared to the structure of QLEDs ITO/PEDOT : PSS/PVK/CdSe@ZnS QDs/ZnO NPs/PEI/Al (9.7 cd A−1 and 5.8 lm W−1) and the structure of QLEDs ITO/PEDOT : PSS/PVK/CdSe@ZnS QDs/ZnO NPs/Al (6.8 cd A−1 and 4.1 lm W−1). The improvement of the current efficiency can be analyzed under the Space Charge‐Limited Current regime. In addition, this paper examines the existence of parasitic resistances including series resistance and shunt resistance by modeling the QLED device as a single exponential diode model. Finally, we can evaluate the influence of the ionic interlayer on the entire QLED device in reducing the leakage current and the current‐voltage (I–V) characteristic of the fabricated QLED devices. We believe the presented high‐performance quantum dot light‐emitting diodes have a high potential and impact on lighting and display technologies.

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.

A dC/dV Measurement for Quantum-Dot Light-Emitting Diodes

We present dC/dV analysis based on the capacitance-voltage (C–V) measurement of quantum-dot light-emitting diodes (QLEDs), and find that some key device operating parameters (electrical and optical turn-on voltage, peak capacitance, maximum efficiency) can be directly related to the turning points and maximum/minimum of the dC/dV (versus voltage) curve. By the dC/dV study, the behaviors such as low turn-on voltage, simultaneous electrical and optical turn-on process, and carrier accumulation during the device aging can be well explained. Moreover, we perform the C–V and dC/dV measurement of aged devices, and confirm that our dC/dV analysis is correct for them. Thus, our dC/dV analysis method can be applied universally for QLED devices. It provides an in-depth understanding of carrier dynamics in QLEDs through simple C–V measurement.

Color-tunable quantum-dot light emitting diode and its integration with GaN-based blue LED for smart white-light emission

White-light emission with tunable color temperature have potential applications in the fields of smart lighting and backlight. In this study, we introduced an electron blocking layer (EBL) between red and green emission layers (EMLs) in a quantum-dot light-emitting diode (QLED). The emitting color of QLED can be tuned from red to green as the carrier distribution and recombination region are regulated by the applied voltage. In addition, the color tunable QLED device was integrated with a GaN-based blue light emitting diode (LED), achieving a hybrid QLED/LED device for white-light emission. Under the alternate-current (AC) driving method, the QLED was forwardly driven while the LED was reversely biased, and thus, both devices can be independently addressed by the polarity of the AC driving voltage. The results showed that the color temperature of white-light emission can be tuned from 3102K (warm white) to 12853K (cold white) and the highest color rendering index (CRI) of 88.6 can be also achieved, exhibiting great prospects in the application of lighting.

Efficient InP/ZnSe/ZnS quantum dot shelling and the effect of a bi-layered organic-inorganic electron-transport layer on the performance of quantum dot light-emitting diode devices

We report the fabrication of red-emitting multi-shell InP quantum dots (QDs) via a two-pot synthesis method and their application to high-efficiency QD light-emitting diodes (QD-LEDs). We synthesized highly efficient and monodisperse deep-red-emitting InP/ZnSe/ZnS QDs with a photoluminescence quantum efficiency of 90% and a full width at half maximum of 49 nm using an eco-friendly phosphine precursor, tris(dimethylamino)phosphine. Furthermore, conventional QD-LED device structures in which metal oxide nanoparticles are used as the electron-transport layer (ETL) suffer from charge imbalance problems related to the charges injected from the cathode and anode. Therefore, in this study, we explored a new approach for efficiently balancing the electrons and holes, which involved the insertion of an additional organic layer of 2,2′,2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) between the inorganic zinc magnesium oxide (ZnMgO) as ETL and LiF as electron injection layer. The bi-layered organic-inorganic ETL reduced the electron mobility and modulated charge balance, thereby enhancing the properties of the conventional QD-LED device. The QD-LED device with the bi-layered ETL was 1.35 times and 1.36 times higher than those of the QD-LED device employing a mono-layered ZnMgO ETL, respectively.

Solvent-mediated surface ligand exchange to enhance the performance of quantum-dot light-emitting diodes

Colloidal quantum dots (QDs) with core-shell structure and long-chain organic ligands are widely used as emitting-material layers (EMLs) for quantum-dot light-emitting diodes (QLEDs). However, the poor conductivity of long-chain organic ligands severely affects charge injection into QD-EMLs and charge transport among QDs, usually leading to poor device performance. To address the issue, we partially replaced the original long-chain oleic acid (OA) ligands capping on the QD surface with short-chain 1,2-ethanedithiol (EDT) ligands via solvent-mediated solid-state ligand exchange to fabricate high-performance QLEDs. The effects of solvent (acetonitrile and ethanol)-mediated EDT ligand exchange on the properties of QDs and device performance were comparatively investigated. Our results show that the ligand exchange is more sufficient when using ethanol instead of acetonitrile as the solvent for EDT, which is not only capable of improving charge transport among QDs but also capable of enhancing hole-electron injection balance in QLEDs. As a result, the QLED device based on QDs after EDT (in ethanol) ligand exchange exhibits a maximum luminescence of 190,200 cd/m 2 , a maximum current efficiency of 18.34 cd/A, a maximum power efficiency of 9.60 l m/W and a peak external quantum efficiency of 12.26%, appreciably outperforming those based on pristine QDs and QDs after EDT (in acetonitrile) ligand exchange. After aging, its maximum current efficiency, maximum power efficiency and peak external quantum efficiency can reach up to 35.91 cd/A, 23.30 l m/W and 24.00%, respectively. Our results highlight the importance of surface chemistry control via solvent-mediated ligand exchange to fabricate high-performance QLEDs, and suggest that ethanol may be a more suitable solvent for EDT to fulfill solid-state ligand exchange of core/shell QDs. • Solvent mediated 1,2-ethanedithiol (EDT) ligand exchange is reported to enhance the performance of QLEDs. • The ligand exchange is more sufficient when using ethanol instead of acetonitrile as the solvent for EDT. • Ethanol mediated ligand exchange can improve charge transport among QDs and enhance hole-electron injection balance in QLEDs. • Devices based on EDT/ethanol ligand-exchanged QDs surpass those based on pristine or EDT/acetonitrile ligand-exchanged QDs.

Top-Emitting Active-Matrix Quantum Dot Light-Emitting Diode Array with Optical Microcavity for Micro QLED Display.

An electroluminescent quantum-dot light-emitting diode (QLED) device and a micro QLED device array with a top-emitting structure were demonstrated in this study. The QLED device was fabricated in the normal structure of [ITO/Ag/ITO anode]/PEDOT:PSS/PVK/QDs/[ZnO nanoparticles]/Ag/MoO3, in which the semi-transparent MoO3-capped Ag cathode and the reflective ITO/metal/ITO (IMI) anode were designed to form an optical microcavity. Compared with conventional bottom-emitting QLED, the microcavity-based top-emitting QLED possessed enhanced optical properties, e.g., ~500% luminance, ~300% current efficiency, and a narrower bandwidth. A 1.49 inch micro QLED panel with 86,400 top-emitting QLED devices in two different sizes (17 × 78 μm2 and 74 × 40.5 μm2) on a low-temperature polysilicon (LTPS) backplane was also fabricated, demonstrating the top-emitting QLED with microcavity as a promising structure in future micro display applications.