Fabrication Of Light-emitting Diodes Research Articles

Stability improvement of photoluminescent QLEDs based on Mn-doped all-inorganic metal halide perovskite quantum dots with silica shell

Perovskite quantum dots (PQDs) suffer from the long-time stability owing to their strong ionic structure. Wrapping them with silica or doping them with ions have been used to improve their structural stabilities. Here, transitional metal doping and surface engineering are employed to synthesize silica-coated Mn:CsPbCl 3 QDs for the fabrication of photoluminescent QD-based light-emitting diodes (QLEDs). The as synthesized QDs have dual emission, one is the intrinsic emission of the perovskite host, and the other is the Mn-related emission. The as synthesized Mn-doped CsPbCl 3 exhibit improved stability of luminous spectra compared to those of the pure ones. We also integrate the silica-coated Mn:CsPbCl 3 QDs as color converters with a purple chip. The luminous spectra of the photoluminescent QD-based light-emitting diodes (QLEDs) can be adjusted by controlling the QD concentration. Twenty-day observation and measurement show that devices based on the silica-coated Mn:CsPbCl 3 exhibit enhanced performance stabilities, including a maximum CRI fluctuation of 2.3, a maximum CCT fluctuation of 7 K and a maximum LER fluctuation of 5.2 lm/W opt . These results indicate that manganese doping and silica shell are helpful for the stability improvement of perovskite QDs, and the potential applications of silica-coated Mn:CsPbCl 3 QDs in lighting fields. • We report the synthesis of Mn-doped CsPbCl 3 QDs and silica-coated Mn:CsPbCl 3 QDs. • Light-emitting diodes based on silica-coated Mn:CsPbCl 3 QDs are fabricated. • The as fabricated devices exhibit improved performance stabilities.

Effect of Bis-diazirine-Mediated Photo-Crosslinking on Polyvinylcarbazole and Solution-Processed Polymer LEDs

The problems with fabrication of solution-processed organic light-emitting diodes (OLEDs) stem largely from the defects associated with sequential deposition of the layers, solvent-induced surface erosion, and layer mixing. Herein, we demonstrate that a photopolymerizable bis-diazirine molecule can easily convert soluble polymers into cross-linked insoluble materials, alleviating the problems associated with inter-layer mixing. Upon 5–20 min irradiation with long wavelength/low power UV (1.8 mW/cm2) bis-diazirines results in the formation of carbenes that can react via carbon–hydrogen bond insertion with polymers or small molecules yielding cross-linked networks. The effectiveness of the bisdiazirine-mediated photo-cross-linking has been studied by investigating the surface morphology of the hole-transporting material polyvinylcarbazole (PVK), which exhibited 45% decrease in surface roughness after 15 min of the UV irradiation. The effect of this cross-linking procedure on device performance was studied in OLEDs with the configuration of indium tin oxide/PEDOT:PSS/PVK:(0–10%) cross-linker/PFO:2% F8BT/TPBI/CsF/Al. After the photolysis of the diazirines, the experimental devices exhibited a 42% enhancement in the maximum external quantum efficiency (EQEmax), from 1.2 to 1.7%, and a maximum luminous efficiency improvement from 3.9 to 5.4 cd/A. Overall, this observation suggests that 3-trifluoromethyl(aryl)diazirine-based cross-linking processes is a promising method for fabrication of polymer LEDs.

Near-unity blue-orange dual-emitting Mn-doped perovskite nanocrystals with metal alloying for efficient white light-emitting diodes

The tunable dual-color emitting Mn2+ doped CsPbCl3-xBrx nanocrystals (NCs) with near-unity photoluminescence quantum yield (PL QY) were synthesized through post-treatment of metal bromide at room temperature for fabrication of efficient warm white light-emitting diodes (WLEDs). Especially, the CdBr2 treated blue-orange emitting Mn doped NCs with various Mn/Pb molar feed ratios exhibit higher PL QY of 97% and longer Mn2+ PL lifetime of 0.9ms. It is surprisingly found that the X-ray diffraction peak at 31.9° is almost not changed with increasing Br composition, meaning formation of metal alloying due to incorporation of amount of divalent cation in NCs. The strong and stable Mn2+ PL at temperature ranging from 80 K to 360K are revealed and the temperature-dependent energy transfer efficiencies in Mn2+ doped CsPbCl1.5Br1.5 NCs are obtained. The enhancement mechanism of Mn2+ PL QY was attributed to improved energy transfer from exciton to Mn2+ d-d transition and suppressed defect state density after post-treatment. The efficient warm WLEDs with color rendering index of 90 and luminous efficacy of 92lm/W at 10mA were fabricated by combining blue-orange dual-emitting Mn2+ doped CsPbCl3-xBrx@SiO2 and green emissive CsPbBr3@SiO2 NCs with violet GaN chips.

Demonstration of epitaxial growth of strain-relaxed GaN films on graphene/SiC substrates for long wavelength light-emitting diodes

Strain modulation is crucial for heteroepitaxy such as GaN on foreign substrates. Here, the epitaxy of strain-relaxed GaN films on graphene/SiC substrates by metal-organic chemical vapor deposition is demonstrated. Graphene was directly prepared on SiC substrates by thermal decomposition. Its pre-treatment with nitrogen-plasma can introduce C–N dangling bonds, which provides nucleation sites for subsequent epitaxial growth. The scanning transmission electron microscopy measurements confirm that part of graphene surface was etched by nitrogen-plasma. We study the growth behavior on different areas of graphene surface after pre-treatment, and propose a growth model to explain the epitaxial growth mechanism of GaN films on graphene. Significantly, graphene is found to be effective to reduce the biaxial stress in GaN films and the strain relaxation improves indium-atom incorporation in InGaN/GaN multiple quantum wells (MQWs) active region, which results in the obvious red-shift of light-emitting wavelength of InGaN/GaN MQWs. This work opens up a new way for the fabrication of GaN-based long wavelength light-emitting diodes.

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

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

Electrospun Supramolecular Hybrid Microfibers from Conjugated Polymers: Color Transformation and Conductivity Evolution

A type of novel electrospun supramolecular hybrid microfibers comprising poly(9-(4-(octyloxy)-phenyl)-2,7-fluoren-9-ol) (PPFOH) and poly(N-vinylcarbazole) (PVK) are successfully prepared for intriguing multi-color emission properties. The supramolecular tunable PPFOH aggregation in PVK matrix endows the complex with a smart energy transfer behavior to obtain the multi-color emissions. In stark contrast to PVK fibers, the emission color of PPFOH/PVK fibers with an efficient dispersion of PPFOH fluorophores at a proper dope ratio can be tuned in a wide spectrum of blue (0.1%), sky blue (0.5%), nearly white (1%), cyan (2%), green (5%) and yellow (10%). Besides, conductive behaviors of the microfiber were demonstrated in accompany with the increment of the doping ratio of PPFOH to PVK. Successful fabrication of polymer light-emitting diode (PLED) based on the blended electrospun fiber provided a further evidence of its excellent electrical property for potential applications in optoelectronic devices.

Simple Synthesis of Red Iridium(III) Complexes with Sulfur-Contained Four-Membered Ancillary Ligands for OLEDs

Phosphorescent iridium(III) complexes have been widely researched for the fabrication of efficient organic light-emitting diodes (OLEDs). In this work, three red Ir(III) complexes named Ir-1, Ir-2, and Ir-3, with Ir-S-C-S four-membered framework rings, were synthesized efficiently at room temperature within 5 min using sulfur-containing ancillary ligands with electron-donating groups of 9,10-dihydro-9,9-dimethylacridine, phenoxazine, and phenothiazine, respectively. Due to the same main ligand of 4-(4-(trifluoromethyl)phenyl)quinazoline, all Ir(III) complexes showed similar photoluminescence emissions at 622, 619, and 622 nm with phosphorescence quantum yields of 35.4%, 50.4%, and 52.8%, respectively. OLEDs employing these complexes as emitters with the structure of ITO (indium tin oxide)/HAT-CN (dipyra-zino[2,3-f,2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile, 5 nm)/TAPC (4,4′-cyclohexylidenebis[N,N-bis-(4-methylphenyl)aniline], 40 nm)/TCTA (4,4″,4″-tris(carbazol-9-yl)triphenylamine, 10 nm)/Ir(III) complex (10 wt%): 2,6DCzPPy (2,6-bis-(3-(carbazol-9-yl)phenyl)pyridine, 10 nm)/TmPyPB (1,3,5-tri(mpyrid-3-yl-phenyl)benzene, 50 nm)/LiF (1 nm)/Al (100 nm) achieved good performance. In particular, the device based on complex Ir-3 with the phenothiazine unit showed the best performance with a maximum brightness of 22,480 cd m−2, a maximum current efficiency of 23.71 cd A−1, and a maximum external quantum efficiency of 18.1%. The research results suggest the Ir(III) complexes with a four-membered ring Ir-S-C-S backbone provide ideas for the rapid preparation of Ir(III) complexes for OLEDs.

Nonlinear Optical Properties of CdSe and CdTe Core-Shell Quantum Dots and Their Applications

The strong nonlinear optical behavior of low-dimensional materials, such as quantum dots and core-shell quantum dots, has been a topic of intense research in recent years. As quantum dots have tunable emission via changes in their sizes, they are potentially useful in photo-electronics, photovoltaic nonlinear optics, light-emitting diode fabrication, and laser protections. Variation among core and shell shape and size, along with the chemical composition of quantum dots, define their enhanced nonlinear optical properties. Some specific nonlinear optical properties, such as nonlinear refraction, optical limiting, saturable absorption, reverse saturable absorption of CdTe and CdSe quantum dots (QDs), as well as core-shell QDs and their applications, were assessed in this paper.

Photoluminescence properties of certain trivalent RE ions (Dy/Sm/Eu) activated NaSrBi2(PO4)3 phosphors and cool to warm white color tuning in Dy3+-Sm3+ and Dy3+-Eu3+ codoped NaSrBi2(PO4)3 phosphors for indoor lighting applications

Abstract The host NaSrBi2(PO4)3 (abbreviated as NSBP) and the rare earth ions (RE3+) Dy3+/Sm3+/Eu3+ doped and codoped NSBP phosphors were prepared by a simple solid state reaction method. The phase identification and the morphological nature were studied by Powder XRD and HRSEM characterization tools respectively. The band gap of the host and RE3+ ions (Dy3+/Sm3+/Eu3+) doped phosphors were deduced from the diffused reflectance (DR) spectral data. The PLE spectra, the PL spectra and the energy transfer (ET) mechanism of Dy3+/Sm3+/Eu3+ doped and Dy3+-Sm3+, Dy3+-Eu3+ codoped NSBP phosphors were studied. The lifetime of the Dy3+/Sm3+/Eu3+ ions in NSBP was in the order of millisecond. The temperature dependent photoluminescence (TDPL) of the optimized phosphors were recorded from 23 °C to 210 °C, showed good thermal stability at 150 °C. The PL emission from the NSBP-xDy3+ phosphors found very closer to the ideal white region. The Sm3+ and Eu3+ doped NSBP phosphors exhibited excellent color coordinates and color purity when compared to the commercial phosphors. Combining the PL emission of the optimized NSBP-0.09Dy3+ phosphor with Sm3+ and Eu3+ ions separately, warm white emission was obtained when excited in the NUV region. Thus the prepared phosphors NSBP-0.09Dy3+,uSm3+ and NSBP-0.09Dy3+,vEu3+ might have promising applications in NUV excitable warm white light emitting diodes fabrication.

Fine regulation of linker and donor moieties to construct benzimidazole-based blue emitters for high-efficient organic light-emitting diodes

The investigation and development of novel blue emitters are still necessary for the fabrication of high-efficient organic light-emitting diodes (OLEDs). Benzimidazole and its derivatives have been attracting extensive attention due to the special characterizations to achieve high device efficiency. However, the systematic elucidation of the relationship between structure and properties is rarely reported. In this paper, four benzimidazole derivatives with virus linker and donor units were designed and synthesized, named PI-AN-3-Cz, PI-NP-3-Cz, PI-AN-TPA, and PI-NP-TPA. Their absorption, photoluminescence, electrochemistry, and thermology were systematically studied, and the results were found to be potential candidates for emitters of OLEDs. Interestingly, the fabricated OLEDs with PI-AN-3-Cz as emitter showed the best performance with the efficiencies of 13.75 cd/A, 5.87 lm/W, and 8.55%. Furthermore, it can be observed that these linkers/donors are changed from anthracene to naphthalene/carbazole to triphenylamine, the corresponding emission moved from sky to violet blue with commission international De L’ Eclairage (CIE) of (0.151, 0.055). The findings give a guidance for the preparation of adjustable emission and high-efficient benzimidazole derivatives.

Early defect identification for micro light‐emitting diode displays via photoluminescent and cathodoluminescent imaging

AbstractUltrahigh‐resolution micro light‐emitting diode (LED) displays are emerging as a viable technology for self‐emissive displays. Several of the critical issues facing micro LED displays with millions of pixels are fidelity, process control, and defect analysis during LED fabrication and transfer. Here, we investigate two non‐destructive test methods, photoluminescent and cathodoluminescent imaging, and compare them with electroluminescent images to verify LED fidelity and evaluate these methods as potential tools for defect analysis. We show that utilizing cathodoluminescent imaging as an analysis tool provides a rich data set that can identify and categorize common defects during micro LED display fabrication that correspond to electroluminescence. Photoluminescent imaging, however, is not an effective method for fidelity analysis but does provide information on dry‐etching uniformity.

Ultrashort Pulse Laser Lift-Off Processing of InGaN/GaN Light-Emitting Diode Chips

Gallium nitride (GaN) film delamination is an important process during the fabrication of GaN light-emitting diodes (LEDs) and laser diodes. Here, we utilize 520 nm femtosecond laser pulses, exploiting nonlinear absorption rather than single-photon absorption such as in conventional laser lift-off (LLO) employing excimer or Q-switched laser sources. The focus of this study is to investigate the influence of laser scanning speed and integrated fluence corresponding to laser energy per area during the LLO processing of GaN LED chips and their resulting structural properties. Because both the sapphire substrate and InGaN/GaN heterostructures are fully transparent to the emission of the laser system, a key question is related to the impact of laser pulses on the quality of a thin film structure. Therefore, several characterization methods (i.e., scanning electron microscopy, atomic force microscopy, X-ray diffraction, Raman spectroscopy, and electroluminescence spectroscopy) were employed to understand the material modifications made by femtosecond LLO (fs-LLO). We demonstrated that by adjusting the laser scanning speed, smooth GaN surfaces and good crystal quality could be obtained regardless of the existing delamination of metal contact, which then slightly downgraded the LED performance. Here, the integrated fluence level was set in the range of 2.6–4.4 J/cm2 to enable the fs-LLO process. Moreover, two mitigation strategies were developed and proven to improve the optoelectrical characteristics of the lifted-off LEDs (i.e., modification of the processing step related to the metal creation and reduction of laser energy).

On-Chip Integration of III-Nitride Flip-Chip Light-Emitting Diodes With Photodetectors

The fabrication of GaN light-emitting diodes (LEDs) with on-chip photodetectors (PDs) based on flip-chip configuration is reported. The exposed sapphire plays a key role not only in light extraction but also light guiding in the lateral direction. The same InGaN/GaN quantum well structure responsible for light emission in the LED and photodetection in the PD is characterized. The light-detecting mechanism of on-chip PDs at varying positions are also studied and verified by simulation. Compared with the design of PD adjacent to the LED, the PD formed in the center of the LED has less influence on the uniformity of the LED emission, and exhibits one-third increase in photocurrent. The flip-chip packaging device also enables a yellow phosphor film to be directly integrated onto the sapphire surface to generate white light emission. The on-chip PD is capable of detecting the variation of LED intensity without blocking the light output emission, which makes it highly valuable for real-time detection of the intensity of LED sources in various lighting applications.

Thermally cross-linkable spirobifluorene-core-based hole transport layer with high solvent-resistivity for solution processible OLEDs

Abstract The fabrication of organic light-emitting diodes (OLEDs) composed of multi-layered structure through a solution-based process suffers from the dissolution of the preformed lower parts during the coating of upper layers. To prevent this problem during the solution process, a promising approach of introducing a cross-linkable layer with a high solvent-resistivity has been proposed. Herein, thermally cross-linkable spirobifluorene-core-incorporated hole transport layers (HTLs) with a cross-linking temperature of 180 °C are designed for solution-processible OLEDs composed of multi-layered structures. The enhanced morphology stability and solvent-resistive property of the synthesized HTLs are evaluated through a rinsing-test with an organic solvent used for the emitting layer. Considering the charge transport property of HTLs in OLEDs, the appropriate energy level and triplet energy values of the synthesized HTLs promote the efficient cascade hole migration. Consequently, the OLED composed of thermally cross-linkable HTL shows higher quantum efficiency (QE) of 16.5% and lower operation voltage of 5.1 V at 1000 cd/m2, compared to that composed of a commercialized polymer, poly (9-vinylcarbazole) (PVK). Thus, in this study, the feasibility of the potential application of thermally cross-linkable HTL with the spirobifluorene-core-units as charge transport layers for solution-based optoelectronic devices composed of multi-layered structures has been verified.

The solution-processed fabrication of perovskite light-emitting diodes for low-cost and commercial applications

The solution-processed fabrication of perovskite light-emitting diodes (PeLEDs) is highly topical. Here, we summarize the recent advances, discuss the challenges and propose the outlooks for their further development.

Investigations on optical properties of Eu0.5 Sm0.5 (TTA)3 tppo hybrid organic complexes molecularly doped in PMMA and PS matrices.

An attempt has been made to synthesize orange-red light-emitting rare earth-doped polymer matrices Eu0.5 Sm0.5 (TTA)3 tppo (Eu=europium, Sm=samarium, TTA=thenoyltrifluoroacetone, tppo=triphenylphosphine oxide) hybrid organic complex by solution technique. Blended thin films were made by molecularly doping the complex in polymethylmethacrylate (PMMA) and polystyrene (PS) polymers at different weight percentages (5%, 10%). These films were solvated in basic and acidic media to explore the effect of solvent on its luminescence properties. UV-visible absorption spectra of these solvated films portray two peaks corresponding to π→π* and n→π* optical transitions in the range 240-275nm and 370-390nm in basic medium. Energy band gap of these thin films in basic medium was found between 3.16eV to 3.20eV and 3.12eV to 3.22eV in acidic medium. Photoluminescence spectra of all films in dichloromethane showed an intense peak at 614nm, whereas in formic acid, the same were found at 475nm, which fell in the orange-red and blue region of the visible spectrum, respectively. CIE coordinates of these solvated films in various solvents and at different weight percentages revealed tunable orange-red to blue light emission with change in polarity of the solvent. Therefore the synthesized complexes blended in the polymers can be shaped into flexible films for fabrication of organic light-emitting diodes (OLEDs) with consistent results, proving their prospect as colour tunable light emissive materials for OLED devices, lasers, displays, and solid-state lighting.

Novel yellow solid-state fluorescent-emitting carbon dots with high quantum yield for white light-emitting diodes

Fluorescence quenching of carbon dots (CDs) occurs in their aggregated state ascribed to direct π-π interactions or excessive resonance energy transfer (RET). Thus, CDs have been severely restricted for applications requiring phosphors that emit in the solid state, such as the fabrication of white light-emitting diodes (WLEDs). In this report, novel CDs with bright solid-state fluorescence (SSF) were synthesized by simple microwave-assisted synthesis method, using 1,4,7,10-tetraazacyclododecane (cyclen) and citric acid as precursors. Under 365nm UV light, these CDs emit bright yellow SSF, indicating they successfully overcome the aggregation-induced fluorescence quenching (ACQ) effect. When the excitation wavelength (λex) is fixed at 450nm, the emission peak of the CDs is centered at 546nm with the Commission Internationale de l'Eclairage chromaticity (CIE) coordinates of (0.43, 0.55), which means that they can be combined with a blue-emitting chip in order to fabricate WLEDs. More importantly, the absolute quantum yield (QY) of these CDs powder reached 48% at λex of 450nm, which was much higher than many previously reported SSF-emitting CDs and indicating their high light conversion ability in solid-state. Thanks to the excellent optical property of these CDs powder, they were successfully used in the preparation of high-performance WLEDs. This study not only enriches SSF-emitting CD-based nanomaterials with good prospects for application, but also provides valuable reference for subsequent research on the synthesis of solid-state fluorescent CDs.

Review of advanced techniques for multi‐gigabit visible light communication

Visible light communication (VLC) is a promising candidate for future indoor wireless communication. Light emitting diodes (LEDs) are popular choices as transmitters for VLC, since they are energy efficient and have the ability to provide illumination and data transmission simultaneously. VLC is attractive for reasons such as security, the ability to use a licence free spectrum, and broad bandwidth. Previous review articles on VLC mainly focused on VLC network algorithms, implementation challenges, system design, applications, and physical layer technologies. Unlike the existing reviews, this study presents state-of-the-art VLC systems for high-speed data communication in the gigabit range and the techniques to achieve such high data rates. In addition, concepts such as LED modelling and fabrication process, recent commercial advancements of VLC products, and hybrid/heterogeneous networks to achieve high data rate are highlighted in this study. For this purpose, some key technologies of VLC systems, including channel equalisation from both transmitter and receiver sides, higher-order modulation techniques, wavelength division multiplexing, multiple input multiple output technology, LED modelling, and advanced fabrication processes are discussed. This study also covers recent advancements in commercialisation of VLC technology, and recent progress made by various research groups.

Accurate Efficiency Measurements of Organic Light‐Emitting Diodes via Angle‐Resolved Spectroscopy

AbstractThe accurate characterization of thin‐film light emitting diodes (LEDs)—including organic light emitting diodes (OLEDs), perovskite LEDs, and quantum dot LEDs—is crucial to the understanding of the factors that influence their efficiency and thus to the fabrication of LEDs with improved performance and stability. In addition, detailed information about the angular characteristics of LED emission is useful to assess the suitability of individual architectures, e.g., for display applications. Here, the implementation of a goniometer‐based measurement system and corresponding protocol are described that allow to accurately determine the current–voltage–luminance characteristics, external quantum efficiency, and luminous efficacy of OLEDs and other emerging thin‐film LEDs. The system allows recording of angle‐resolved electroluminescence spectra and accurate efficiency measurements for devices with both Lambertian and non‐Lambertian emission characteristics. A detailed description of the setup and a protocol for assembling and aligning the required hardware are provided. Drawings of all custom parts and the open‐source Python software required to perform the measurement and to analyze the data are included.

Design and Assembly of a Thin-Film-Based Micro Pump for a Micro-slot Die

In printed electronics, coating is a key process used not only in the fabrication of solar cell and organic light-emitting diodes but also in display devices. For this purpose, coating devices are designed with several unique characteristics. In particular, slot die coaters and inkjet coaters are advantageous in terms of high-quality morphology and a small dispensing area, respectively. This study proposes a slot die with a narrow-width coating as a fusion device derived from a slot die coater and an inkjet printer. This device is termed a micro-slot die and comprises a micro pump and a narrow-width slot. The device was designed and assembled with appropriate consideration of both dynamic and fluid motions. Consequently, the fabricated device achieves a high-quality coating suitable for application in the display industry. The proposed device is expected to be used in laboratory-scale research applications that require a minimal coating of ink.