Inorganic Light-emitting Diodes Research Articles

Shape-deformable Micro-LEDs for advanced displays and healthcare

Recently, flexible/stretchable micro-scale light-emitting diodes (LEDs), with dimensions significantly smaller than conventional diodes used for illuminations, have emerged for promising applications in areas such as deformable displays, wearable devices for healthcare, etc . For such applications, these devices must have some unusual features that common inorganic LEDs do not intrinsically own, including conformability, biocompatibility, mechanical flexibility, etc . This Perspective focuses on summarizing the most recent progress in developing such flexible emitters based on inorganic semiconductors, followed by reviewing their potential applications. Finally, major challenges and future research directions of deformable micro-scale LEDs are presented.

Advantageous occupation of Cu2+ in the B site of double-perovskite Rb2BB'Br6 (B = K; B′ = Bi) framework for white-light-emission

Research on lead-free double perovskite has demonstrated its potential as a solution to the stability and toxicity issues associated with lead-based inorganic perovskite white light-emitting diodes (WLED). This current investigation was designed to fabricate, examine, and compare the photoluminescence (PL) characteristics of Cu2+-activated Rb2BB'Br6 (B = K and B′ = Bi) double-perovskite (DP) [A2BB'Br6] luminous materials. Bi3+ ions are substituted with Cu2+ ions at their B′ sites. These ions are distributed to the equilibrium valence state and demonstrate minimal variation in ion radius. The microstructure and phase purity of Rb2BB'Br6:Cu2+ DP were assessed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The Rietveld refinement was also used to identify the crystal structures, and the resulting samples all exhibit cubic crystal structures. The Fm3m space group in Rb2BB'Br6 DP frameworks is significantly impacted by the B sites. The PL spectra were used to thoroughly examine the associated mechanism of the Cu2+ → Bi3+ energy-transfer process. Cu2+ ions can be successfully doped in the Rb2KBiBr6 host with an optical band gap (Eg) of 2.74 eV (RKBBr: 0.025Cu2+). The Rb2KBiBr6:Cu2+ phosphor exhibits distinct emission bands from 450 to 650 nm upon UV excitation at 366 nm. The Cu2+→ Bi3+ energy transfer design allows for systematic luminescence tuning from blue to white due to spectral overlap between the Cu2+ emission and Bi3+ excitation spectra. It has been established that Cu2+ → Bi3+ energy transfer significantly increases the white emission intensity of Bi3+, with an associated colour temperature of 7892 K and an outstanding colour rendering index of 88.8. As a down-conversion luminous material in solid-state illumination for the upcoming generation of display devices, the discovered Rb2KBiBr6: 0.025Cu2+ phosphor exhibits remarkable promise.

High-Efficiency and Emission-Tunable Inorganic Blue Perovskite Light-Emitting Diodes Based on Vacuum Deposition.

The fabrication of perovskite light-emitting diodes (PeLEDs) with vacuum deposition shows great potential and commercial value in realizing large-area display panel manufacturing. However, the electroluminescence (EL) performance of vacuum-deposited PeLEDs still lags behind the counterparts fabricated by solution process, especially in the field of blue PeLEDs. Here, the fabrication of high-quality CsPbBr3- x Clx film through tri-source co-evaporation is reported to achieve high photoluminescence quantum yield (PLQY). Compared with the conventional traditional dual-source co-evaporation, the tri-source co-evaporation method allows for freely adjustable elemental ratios, enabling the introduction of the lattice-matched Cs4 Pb(Br/Cl)6 phase with the quantum-limited effect into the inorganic CsPb(Br/Cl)3 emitter. By adjusting the phase distribution, the surface defects of the emitter can be effectively reduced, leading to better blue emission and film quality. Further, the effects of Cs/Pb ratio and Br/Cl ratio on the PLQY and carrier recombination dynamics of perovskite films are investigated. By optimizing the deposition rate of each precursor source, spectrally stable blue PeLEDs are achieved with tunable emission ranging from 468 to 488nm. Particularly, the PeLEDs with an EL peak at 488nm show an external quantum efficiency (EQE) of 4.56%, which is the highest EQE value for mixed-halide PeLEDs fabricated by vacuum deposition.

Organic and inorganic light-emitting diodes for photodynamic therapy of cutaneous leishmaniasis

For effectively fighting worldwide infectious diseases such as cutaneous leishmaniasis, novel approaches are required. Photodynamic Therapy (PDT) is one such possibility. PDT involves applying a light-sensitive chemical (photosensitizer), which should be highly efficient, non-toxic, and work at longer light wavelengths. This photosensitizer needs to be activated by a light source that provides uniform emission over a large area, high intensity, easy to fabricate, compact, and low cost. In this work, we designed and built light sources based upon commercially available Organic Light Emitting Diode (OLED) and LED parts to experimentally validate the combination with methylene blue photosensitizer to kill Leishmania major and Crithidia fasciculata cells in vitro. Our results showed that suitable-sized OLEDs, as compact and uniform light sources, are very good candidates for photodynamic therapy and can be used to efficiently kill such kinetoplastids in vitro. Therefore, it has real potential to be used in wearable devices for ambulatory treatment of patients.

Fully implantable and battery-free wireless optoelectronic system for modulable cancer therapy and real-time monitoring

Photodynamic therapy (PDT) is attracting attention as a next-generation cancer treatment that can selectively destroy malignant tissues, exhibit fewer side effects, and lack pain during treatments. Implantable PDT systems have recently been developed to resolve the issues of bulky and expensive conventional PDT systems and to implement continuous and repetitive treatment. Existing implantable PDT systems, however, are not able to perform multiple functions simultaneously, such as modulating light intensity, measuring, and transmitting tumor-related data, resulting in the complexity of cancer treatment. Here, we introduce a flexible and fully implantable wireless optoelectronic system capable of continuous and effective cancer treatment by fusing PDT and hyperthermia and enabling tumor size monitoring in real-time. This system exploits micro inorganic light-emitting diodes (μ-LED) that emit light with a wavelength of 624 nm, designed not to affect surrounding normal tissues by utilizing a fully programmable light intensity of μ-LED and precisely monitoring the tumor size by Si phototransistor during a long-term implantation (2–3 weeks). The superiority of simultaneous cancer treatment and tumor size monitoring capabilities of our system operated by wireless power and data transmissions with a cell phone was confirmed through in vitro experiments, ray-tracing simulation results, and a tumor xenograft mouse model in vivo. This all-in-one single system for cancer treatment offers opportunities to not only enable effective treatment of tumors located deep in the tissue but also enable precise and continuous monitoring of tumor size in real-time.

Fluidic self-assembly for MicroLED displays by controlled viscosity.

Displays in which arrays of microscopic 'particles', or chiplets, of inorganic light-emitting diodes (LEDs) constitute the pixels, termed MicroLED displays, have received considerable attention1,2 because they can potentially outperform commercially available displays based on organic LEDs3,4 in terms of power consumption, colour saturation, brightness and stability and without image burn-in issues1,2,5-7. To manufacture these displays, LED chiplets must be epitaxially grown on separate wafers for maximum device performance and then transferred onto the display substrate. Given that the number of LEDs needed for transfer is tremendous-for example, more than 24 million chiplets smaller than 100 μm are required for a 50-inch, ultra-high-definition display-a technique capable of assembling tens of millions of individual LEDs at low cost and high throughput is needed to commercialize MicroLED displays. Here we demonstrate a MicroLED lighting panel consisting of more than 19,000 disk-shaped GaN chiplets, 45 μm in diameter and 5 μm in thickness, assembled in 60 s by a simple agitation-based, surface-tension-driven fluidic self-assembly (FSA) technique with a yield of 99.88%. The creation of this level of large-scale, high-yield FSA of sub-100-μm chiplets was considered a significant challenge because of the low inertia of the chiplets. Our key finding in overcoming this difficulty is that the addition of a small amount of poloxamer to the assembly solution increases its viscosity which, in turn, increases liquid-to-chiplet momentum transfer. Our results represent significant progress towards the ultimate goal of low-cost, high-throughput manufacture of full-colour MicroLED displays by FSA.

Technological Breakthroughs in Chip Fabrication, Transfer, and Color Conversion for High-Performance Micro-LED Displays.

The implementation of high-efficiency and high-resolution displays has been the focus of considerable research interest. Recently, micro light-emitting diodes (micro-LEDs), which are inorganic light-emitting diodes of size <100 µm2 , have emerged as a promising display technology owing to their superior features and advantages over other displays like liquid crystal displays and organic light-emitting diodes. Although many companies have introduced micro-LED displays since 2012, obstacles to mass production still exist. Three major challenges, i.e., low quantum efficiency, time-consuming transfer, and complex color conversion, have been overcome with technological breakthroughs to realize cost-effective micro-LED displays. In the review, methods for improving the degraded quantum efficiency of GaN-based micro-LEDs induced by the size effect are examined, including wet chemical treatment, passivation layer adoption, LED structure design, and growing LEDs in self-passivated structures. Novel transfer technologies, including pick-up transfer and self-assembly methods, for developing large-area micro-LED displays with high yield and reliability are discussed in depth. Quantum dots as color conversion materials for high color purity, and deposition methods such as electrohydrodynamic jet printing or contact printing on micro-LEDs are also addressed. This review presents current status and critical challenges of micro-LED technology and promising technical breakthroughs for commercialization of high-performance displays.

Vacuum-Deposited Inorganic Perovskite Light-Emitting Diodes with External Quantum Efficiency Exceeding 10% via Composition and Crystallinity Manipulation of Emission Layer under High Vacuum.

Although vacuum-deposited metal halide perovskite light-emitting diodes (PeLEDs) have great promise for use in large-area high-color-gamut displays, the efficiency of vacuum-sublimed PeLEDs currently lags that of solution-processed counterparts. In this study, highly efficient vacuum-deposited PeLEDs are prepared through a process of optimizing the stoichiometric ratio of the sublimed precursors under high vacuum and incorporating ultrathin under- and upper-layers for the perovskite emission layer (EML). In contrast to the situation in most vacuum-deposited organic light-emitting devices, the properties of these perovskite EMLs are highly influenced by the presence and nature of the upper- and presublimed materials, thereby allowing us to enhance the performance of the resulting devices. By eliminating Pb° formation and passivating defects in the perovskite EMLs, the PeLEDs achieve an outstanding external quantum efficiency (EQE) of 10.9% when applying a very smooth and flat geometry; it reaches an extraordinarily high value of 21.1% when integrating a light out-coupling structure, breaking through the 10% EQE milestone of vacuum-deposited PeLEDs.

Monolithic Multicolor Emissions of InGaN-Based Hybrid Light-Emitting Diodes Using CsPbBr3 Green Quantum Dots.

To address the increasing demand for multicolor light-emitting diodes (LEDs), a monolithic multicolor LED with a simple process and high reliability is desirable. In this study, organic-inorganic hybrid LEDs with violet and green wavelengths were fabricated by depositing CsPbBr3 perovskite green quantum dots (QDs) as the light-converting material on InGaN-based violet LEDs. As the injection current was increased, the total electroluminescence (EL) intensities of the hybrid LEDs increased, whereas the light-converted green emission efficiency of the CsPbBr3 QDs decreased. The maximum green-to-violet EL spectral intensity ratio of the hybrid LEDs with CsPbBr3 QDs was achieved with the injection current of <10 mA. Moreover, the EL spectral ratio of the green-to-violet emission decreased at an injection current of 100 mA. The light-conversion intensity of the CsPbBr3 QDs decreased linearly as the junction temperature of the hybrid LEDs was increased with increasing injection current, similar to the temperature-dependent photoluminescence degradation of CsPbBr3 QDs. In addition, the junction temperature of the hybrid LED was minimized by pulse injection to suppress the thermal degradation of QDs and increase the light conversion efficiency to green emission. Therefore, the overall emission spectrum color coordinates of the hybrid LEDs exhibited a red shift from violet to blue in the low-current region and a blue shift toward violet as the green emission of the QDs was decreased above 10 mA.

Super-Bright Green Perovskite Light-Emitting Diodes Using Ionic Liquid Additives.

Halide perovskites have potential for use in next-generation low-cost, high-efficiency, and highly color-pure light-emitting diodes (LED) that can be used in various applications, such as flat and flexible displays and solid-state lighting. However, they still lag behind other mature technologies, such as organic LEDs and inorganic LEDs, in terms of performance, particularly brightness. This lag is partly due to the insulating nature of the long-chain organic ligands used to control the perovskite-film morphology. Herein, a 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid (IL) is incorporated as a potential additive with CsPbBr3 perovskite precursors, which results in a super-bright green perovskite light emitting diode (PeLED) achieving a peak luminance of 3.28 × 105 cd m-2 only at a bias voltage of 6V, with a peak external quantum efficiency of 13.75%. This achievement is the outcome of multirole support from IL that simultaneously enables superior control over the perovskite-film morphology, passivates defects, modifies the band energy levels, and prevents ion migration. Hence, this work demonstrates IL as a novel alternative additive with the potential to outperform conventional long-chain ligands in high-performance PeLED device fabrication.

Integration Technology of Micro-LED for Next-Generation Display.

Inorganic micro light-emitting diodes (micro-LEDs) based on III-V compound semiconductors have been widely studied for self-emissive displays. From chips to applications, integration technology plays an indispensable role in micro-LED displays. For example, large-scale display relies on the integration of discrete device dies to achieve extended micro-LED array, and full color display requires integration of red, green, and blue micro-LED units on the same substrate. Moreover, the integration with transistors or complementary metal-oxide-semiconductor circuits are necessary to control and drive the micro-LED display system. In this review article, we summarized the 3 main integration technologies for micro-LED displays, which are called transfer integration, bonding integration, and growth integration. An overview of the characteristics of these 3 integration technologies is presented, while various strategies and challenges of integrated micro-LED display system are discussed.

Application of porous materials and structures for improving optical and thermal performance of inorganic and organic light-emitting diodes: A review

Light-emitting diodes (LEDs) are considered the most promising technology for future display and lighting applications due to their high efficiency, durability, long lifespan, and eco-friendliness. Inorganic and organic LEDs (LEDs and OLEDs) are the key components in current display and lighting applications. Porous materials are a fast-developing field of study that has unraveled and expanded an extensive array of novel properties and applications. The structural and morphological parameters of the hosted semiconductor can be controlled by modifying the pore size distribution, thickness, composition of the pore walls, geometry of the pore system, and its topology. Porous hosts provide the guest semiconductor with enhanced stability and versatility in terms of processing, which favors its integration in devices. This article reviews the current progress in the applications of porous materials and structures to improve the performance of LEDs and OLEDs. The use of the porous materials to enhance the optical performance of GaN-/InGaN-/ and AlGaN-based LEDs, porous silicon LEDs, OLEDs are presented. Furthermore, the applications of porous materials for the thermal management of LEDs are discussed.

Freestanding High-Resolution Quantum Dot Color Converters with Small Pixel Sizes

Designing the next generation of high-resolution displays requires high pixel density per area and small pixel sizes without compromising the optical quality. Quantum dots (QDs) have been demonstrated as a promising material system for down-conversion of blue emission as they provide pure colors on the wide color gamut. However, for high color-conversion efficiency, the required QD film thickness has not been compatible with small pixel sizes. In this work, we develop a new type of freestanding QD-based color converter for efficient optical down-conversion from inorganic blue light-emitting diodes (LEDs) in a color-by-blue configuration. CdSe/ZnS core-shell QDs in a UV-curable polymer matrix are encapsulated within cavities formed by patterning and bonding a pair of patterned quartz substrates. By controlling the required QD thickness and the pixel size independently, we demonstrate freestanding monochrome red and green converters with small pixel sizes down to 5 × 5 μm2 and a high resolution of >3600 ppi. The optical studies show that the QD film thickness required for efficient color conversion can be successfully realized even for the small pixel sizes. We further combine green and red pixels in a single converter to achieve white emission when combined with blue LED emission. The QD color converter design and processing are decoupled from the LED fabrication and can be easily scaled to wafer-size integration with arbitrary pixel sizes for QD-based RGB displays with ultrahigh resolution.

Understanding the Sensing Mechanism of ZnO Nanoparticles through Identifying Intrinsic Defects

Progress in crystal growth made ZnO applicable as the next generation of inorganic light-emitting diodes and lasers, which has attracted much attention. Our previous work is a proof that Zn vacancies are occupied by three protons to form VZnH3 complexes in high-quality ZnO single crystals. In this work, electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) studies on high-quality ZnO single crystals further confirm that three proton clusters or cations with three positive charges are the intrinsic defects and/or impurities related to doping in the host-lattice of high-quality ZnO single crystals. Unintentionally doped hydrogen (H) clusters might be relating to ZnO’s sensing capabilities. The sensor response of ZnO semiconductors is promoted by a decreasing size; however, the nature of the size effects has not been elucidated satisfactorily. In this work, broccoli-like ZnO nanoarrays, which can detect H2S gas in the parts per billion (ppb) region, has been synthesized under a hydrothermal method. High-resolution transmission electron microscopy (HRTEM) experimental results demonstrate that there are multiple polar faces exposed on the surfaces of ZnO nano crystals. The high catalytic active of oxygen species adsorbed on the multiple polar surfaces should respond to the high sensing performance and the high selectivity of ZnO toward reducing H2S gas. However, the enormous variations in electric current cannot be produced by releasing electrical charges from O– adsorbates into semiconductors. EPR results show that Zn+ interstitials have been produced after the reaction between H2S and O– adsorbates on ZnO. In contrast to the unintentionally doped hydrogen (H) clusters, our experimental results show that the Zn+ interstitials acting as shallow donors is what really offer the extra free-electron carriers.

Flexible GaN-based microscale light-emitting diodes with a batch transfer by wet etching.

Flexible inorganic GaN-based microscale light-emitting diodes (µLEDs) show potential applications in wearable electronics, biomedical engineering, and human-machine interfaces. However, developing cost-effective products remains a challenge for flexible GaN-based µLEDs. Here, a facile and stable method is proposed to fabricate flexible GaN-based µLEDs from silicon substrates in an array-scale manner by wet etching. Circular and square µLED arrays with a size and pitch of 500 µm were fabricated and then transferred to a flexible acrylic/copper substrate. The as-fabricated flexible µLEDs can maintain their structure intact while exhibiting a significant increase in external quantum efficiency. This Letter promotes the application of simple and low-cost flexible µLED devices, especially for virtual displays, wearables, and curvilinear displays.

Polycrystalline InGaO Thin-Film Transistors with Coplanar Structure Exhibiting Average Mobility of ≈78 cm2 V-1 s-1 and Excellent Stability for Replacing Current Poly-Si Thin-Film Transistors for Organic Light-Emitting Diode Displays.

Highly ordered polycrystalline indium gallium oxide (PC-IGO) film is obtained by the crystallization of room temperature sputtered amorphous IGO on a hot plate at 350°C for 1 h and then annealed for 1 h in an N2 O environment. A high-density PC-IGO of ≈7.15g cm-3 with reduced oxygen vacancy (≈14.83%) and hydroxyl (OH) related defects (≈10.96%) has been obtained by N2 O annealing. Self-aligned coplanar thin-film transistor (TFT) with the PC-IGO exhibits the average saturation mobility of 78.73 cm2 V-1 s-1 , threshold voltage of -1.07V, subthreshold swing of 0.147V dec-1 , and the on/off current ratio of over 108 . The TFTs show excellent stability under bias-temperature stress with a negligible threshold voltage shift (ΔVTH ) of + 0.1 and -0.1V for the positive and negative bias stresses, respectively. The TFTs exhibit very stable environmental stability when the TFTs are stored under high humidity (85%) and a high temperature (85°C) for 2 days. The ring oscillator and the gate driver mode of the PC-IGO TFTs exhibit the propagation delay of 7.44ns/stage with rising/falling times of less than 0.7 μs, respectively. Therefore, the PC-IGO TFTs are suitable for large area, high-resolution active-matrix organic, and inorganic light-emitting diodes displays.

Control of the horizontal dipole ratio and emission color of deep blue tetradentate Pt(II) complexes using aliphatic spacer groups

The dedication of researchers has gradually allowed organic light-emitting diodes (OLEDs) to replace liquid crystals and/or inorganic light-emitting diodes (LEDs) in displays, signage, and lighting. However, the blue-emitting materials of OLEDs exhibit low efficiency and short lifetimes. Tetradentate Pt(II) complexes have the potential to serve as the next generation of blue emitters because of their narrow emission spectra, high photoluminescence quantum yields (PLQYs), and rigid molecular structures. The horizontal dipole ratio (Θ) is one of the most important factors that determines OLED efficiency. Unfortunately, however, high-level horizontal orientation data concerning Pt(II) complexes are available for only non-doped crystalline Pt(II) complexes, which exhibit excimer emission. Herein, we report a new strategy by which to control the orientation of Pt(II) complexes doped into host materials. The use of various aliphatic groups in different positions changes the molecular orientation. Therefore, a Θ of 79% is achieved; this is the highest value thus far for doped Pt(II) complexes. Additionally, we fabricated deep blue OLEDs with CIE y values < 0.1 and external quantum efficiencies over 20% using the developed Pt(II) complexes. The results offer new guidance for how to increase the Θ of Pt(II) complexes.

Efficient Radiative Enhancement in Perovskite Light-Emitting Devices through Involving a Novel Sandwich Localized Surface Plasmon Structure.

In recent years, CsPbX3 (X = Cl, Br, I) perovskite quantum dots (QDs) have been considered as the most promising materials for light-emitting diodes (LEDs). However, the advances of CsPbX3 quantum dot-based light emitting diodes (QLEDs) still lagged behind inorganic III-V LEDs and other organic LEDs. Herein, a strategy to improve the performances of perovskite QLEDs is reported by utilizing the localized surface plasmon resonance (LSPR) effects of Au nanospheres (NSs). It is accomplished by introducing a Au NS layer into the electron transport layer of Ca2+ -CsPbBr3 QLEDs, where the diameter and spacing of Au NSs and the interaction distance between the Ca2+ -CsPbBr3 QD and Au NS layers are modulated, according to the theoretical simulation of Finite-difference time-domain. As a result, the photoluminescence quantum yield of Ca2+ -CsPbBr3 QD layer is improved from 31.5% to 73.3%. Finally, the luminance of Ca2+ -CsPbBr3 QLEDs is improved from 16824 to 63931cd m-2 and external quantum efficiency (EQE) is improved from 4.2% to 10.5%. The radiative transition rate can be remarkably modulated from 0.7 × 107 to 6.6 × 107 s-1 . The enhancement in luminance and EQE are the best values in the LSPR modified perovskite QLEDs and the strategy offered in this work fits with other LEDs and optoelectrical devices.

Self‐Assembled Nanowrinkle‐Network‐Structured Transparent Conductive Zinc Oxide for High‐Efficiency Inorganic Light‐Emitting Diodes (Adv. Mater. Interfaces 4/2022)

ZnO Nanowrinkle Network (NWN) Film in Conductive Electrodes A ZnO NWN film is used as a transparent conductive electrode to improve the light extraction efficiency of inorganic light-emitting diodes (LEDs), as reported in article number 2101460 by Gun-Woo Lee, Young-Chae Na, and Sung-Nam Lee. The cover depicts: forming NWN structure by evaporating a solvent from ZnO-sol spin-coated on LED wafer using thermal heating process, and a schematic diagram of light escape comparing the light extracted from NWN structure and smooth surface of the LED.

Stretchable Inorganic LED Displays with Double-Layer Modular Design for High Fill Factor.

The recent commercial success of flexible and foldable displays has resulted in growing interest in stretchable electronics which are considered to be the next generation of the optoelectronic technology. Stretchable display technologies are being intensively studied for versatile applications including wearable, attachable, and shape changeable electronics. In this paper, we present high fill factor, stretchable inorganic light-emitting diode (LED) displays fabricated by connecting mini-LEDs and stretchable interconnects in a double-layer modular design. The double-layer modular design enables an increased areal coverage of LEDs and stretchable interconnectors with both electrical and mechanical stability. The main features of the double-layer modular design, fabrication processes, and device characteristics for the high fill factor, stretchable inorganic LED display are discussed, with experimental and computational results. Demonstrations of a passive matrix LED display confirm the potential value of the multi-layer structured, stretchable electronics in a wide range of applications that need high fill factor with high stretchability.