Large-area Emitters Research Articles

Influence of the AlN-sapphire template on the optical polarization and efficiency of AlGaN-based far-UVC micro LED arrays

Abstract Arrays of far-UVC micro LEDs based on AlGaN and emitting at 233-235 nm have been fabricated on different types of AlN-sapphire templates and the optical polarization, output power, and efficiencies have been studied in dependence of the template technology and the mesa diameter of the micro-pixels. While LEDs fabricated on MOVPE AlN-sapphire templates show dominant TM polarized emission with a degree of polarization of −0.2, LEDs on high temperature annealed AlN-sapphire templates show dominant TE polarized emission with a degree of polarization of 0.2–0.3. The output power and external quantum efficiency increases with decreasing diameter of the slanted and reflective micro LED mesa. Peak output powers of 18 mW at 200 mA and peak external quantum efficiencies of up to 2.7 % for mesa diameters of 1.5 µm on annealed templates were measured, corresponding to peak wall plug efficiencies of 1.7 %, while conventional LEDs with large mesa areas on the same template showed maximum EQEs of 1.1 %. The relative increase in output power by using the micro LED approach as compared to a conventional large area emitter is stronger for LEDs on MOVPE AlN templates than on annealed templates (about a factor of 3.7 vs. 2.3, respectively, at 50 mA) which is attributed to the polarization dependence of the light extraction.

The Improvement of Luminous Uniformity of Large-Area Organic Light-Emitting Diodes by Using Auxiliary Electrodes

The study developed a large emission area of flexible blue organic light-emitting diodes (BOLED) on a polyethylene terephthalate/ Indium tin oxide (PET/ITO) substrate using a polycyclic skeleton ν-DABNA Thermally Activated Delayed Fluorescence (TADF) material. Initially, a 1 × 1 cm2 blue OLED was fabricated to optimize the layer thickness. The blue OLED structure consisted of PET/ITO/HATCN/TAPC/UBH-21:ν-DABNA/TPBi/LiF/Al. However, as the emission area increased to 3.5 × 3.5 cm2, the current density decreased due to the resistance of PET/ITO, leading to luminance non-uniformity. To address this issue, auxiliary Au lines were added to the ITO anode to enhance current injection. Despite this, when the Au lines reached a thickness of 30 nm, average light emission was disrupted. To improve the luminescence characteristics of large-area PET/ITO OLEDs, a capping and planarization layer of PEDOT:PSS was applied. Grid uniformity revealed a significant increase in overall luminance uniformity from 74.1% to 87.4% with the addition of auxiliary Au lines. Further increases in grid line density slightly reduced uniformity but enhanced brightness, resulting in brighter, flexible, large-area blue OLED lighting panels.

Microstructured large-area photoconductive terahertz emitters driven at high average power.

Emitters based on photoconductive materials excited by ultrafast lasers are well-established and popular devices for THz generation. However, so far, these emitters - both photoconductive antennas and large area emitters - were mostly explored using driving lasers with moderate average powers (either fiber lasers with up to hundreds of milliwatts or Ti:Sapphire systems up to few watts). In this paper, we explore the use of high-power, MHz repetition rate Ytterbium (Yb) based oscillator for THz emission using a microstructured large-area photoconductive emitter, consist of semi-insulating GaAs with a 10 × 10 mm2 active area. As a driving source, we use a frequency-doubled home-built high average power ultrafast Yb-oscillator, delivering 22 W of average power, 115 fs pulses with 91 MHz repetition rate at a central wavelength of 516 nm. When applying 9 W of average power (after an optical chopper with a duty cycle of 50%) on the structure without optimized heatsinking, we obtain 65 µW THz average power, 4 THz bandwidth; furthermore, we safely apply up to 18 W of power on the structure without observing damage. We investigate the impact of excitation power, bias voltage, optical fluence, and their interplay on the emitter performance and explore in detail the sources of thermal load originating from electrical and optical power. Optical power is found to have a more critical impact on large area photoconductive emitter saturation than electrical power, thus optimized heatsinking will allow us to improve the conversion efficiency in the near future towards much higher emitter power. This work paves the way towards achieving hundreds of MHz or even GHz repetition rates, high-power THz sources based on photoconductive emitters, that are of great interest for example for future THz imaging applications.

Overcoming Challenges in OLED Technology for Lighting Solutions

In academic research, OLEDs have exhibited rapid evolution thanks to the development of innovative materials, new device architectures, and optimized fabrication methods, achieving high performance in recent years. The numerous advantages that increasingly distinguish them from traditional light sources, such as a large and customizable emission area, color tunability, flexibility, and transparency, have positioned them as a promising candidate for various applications in the lighting market, including the residential, automotive, industrial, and agricultural sectors. However, despite these promising attributes, the widespread industrial production of OLEDs encounters significant challenges. Key considerations center around efficiency and lifetime. In the present review, after introducing the theoretical basis of OLEDs and summarizing the main performance developments in the industrial field, three crucial aspects enabling OLEDs to establish a competitive advantage in terms of performance and versatility are critically discussed: the quality and stability of the emitted light, with a specific focus on white light and its tunability; the transparency of both electrodes for the development of fully transparent and integrable devices; and the uniformity of emission over a large area.

Achieving 9% EQE in light-emitting electrochemical cells via a TADF-sensitized fluorescence strategy.

Light-emitting electrochemical cells (LECs) are appealing for cost-effective, large-area emission applications; however, their luminescence efficiency is significantly limited by exciton annihilation caused by high concentration polarons. Here, we present thermally activated delayed fluorescence (TADF) sensitized fluorescence LECs (TSF-LECs) that achieve a record 9% EQE. The TADF sensitizers with rapid reverse intersystem crossing (RISC) rates can effectively convert triplet excitons to singlet excitons in LECs, thereby establishing a more efficient overall energy transfer pathway. Importantly, magneto-electroluminescence measurements indicate that the additional RISC route in TSF-LECs significantly suppresses the annihilation of triplet excitons and thus reduces exciton loss under high concentration polaron conditions. Compared to LECs without a sensitizer, TSF-LECs exhibit improved EQE and luminance, extended operational lifetimes, and suppressed efficiency roll-off. A flexible display prototype based on TSF-LECs was further fabricated, capable of stably displaying high-brightness preset patterns for extended periods. The exploration of the exciton dynamics in high concentration polaron environments offers valuable insights for future developments in high-efficiency LEC technology.

Improving the efficiency of an optical-to-terahertz converter using sapphire fibers

Objectives. The study aims to improve the efficiency of a large-area photoconductive terahertz (THz) emitter based on an optical-to-terahertz converter (OTC) having a radiating area of 0.3 × 0.3 mm2 for generating high-power THz radiation by using an array of close-packed profiled sapphire fibers having a diameter in the range of 100–300 μm as focusing optics.Methods. As a photoconductive substrate, we used a semi-infinite LT-GaAs layer (low-temperature grown GaAs; GaAs layer grown by molecular beam epitaxy at a low growth temperature). Additional Si3N4 and Al2O3 layers are intended for reducing leakage currents in the OTC and reducing the reflection of the laser pump pulse from the air/semiconductor interface (Fresnel losses), respectively, at a gap width of 10 μm. For forming the antenna electrodes and feed strips, the Ti/Au metal system was used. The simulation was carried out by the finite element method in the COMSOL Multiphysics environment.Results. The use of a profiled sapphire fiber whose diameter has been optimized with respect to the gap parameters to significantly increase the concentration of charge carriers in the immediate vicinity of the electrodes of an OTC is demonstrated. The integrated efficiency of a large-area photoconductive THz emitter was determined taking into account the microstrip topology of the array with a characteristic size of feed strips proportional to the gap width in the OTC and with the upper (masking) metal layer. The maximum localization of the electromagnetic field in close proximity to the edges of electrodes at the “fiber–semiconductor” interface is achieved with a profiled sapphire fiber diameter of 220 μm.Conclusions. By optimizing the diameter of the sapphire fiber, the possibility of improving the localization of incident electromagnetic waves in close proximity to the edges of the OTC electrodes by ~40 times compared to the case without fiber, as well as increasing the overall efficiency of a large-area emitter by up to ~7–10 times, was demonstrated.

Far-UVC emission of polarity-engineered AlGaN MQW using carbon nanotube-based cold cathode electron beam

Far-ultraviolet-C (UVC) light is an efficient and safe germicide because the wavelength band eradicates viruses but is harmless to human cells. In this study, electron-beam-pumped AlGaN emitters have been introduced as an alternative to far-UVC light emitting diodes which are limited by certain drawbacks, such as low conductivity and light absorption of the p-layer and side emission due to polarization at short wavelengths. We demonstrate a highly efficient far-UVC AlGaN emitter using a carbon nanotube-based cold cathode electron beam (C-beam). To maximize light emission, materials were grown via novel in situ lateral polarity engineering epitaxy. The C-beam enables the large-area emission of up to 528 mm2, and the maximum power conversion efficiency is 0.5% at 232 nm.

Study on explosive emission characteristics of the silicon carbide modified graphite cathodes with varied morphologies

Explosive emission cathodes are extensively used in high power microwave sources. Growing requirements are highlighted in the performance of the explosive emission cathode with the development of the high power microwave technology. In order to improve the emission properties of the graphite cathode, modifications using SiC particles or whiskers are carried out by the in situ chemical vapor reaction method. The experiments demonstrate that SiC whiskers accelerate the explosive emission turn-on speed of the graphite cathode for large field enhancement factors and improve the emission uniformity due to the surface flashover plasma generation mechanism. SiC particles increase the emission capability of the graphite cathode, possibly corresponding to the dielectric properties of SiC particles. These results suggest that the SiC whiskers modified graphite cathode is suitable for applications under low magnetic field and large emission area conditions, while the SiC particles modified graphite cathode has a brighter application prospect in the long-time stable operation.

Group IV THz large area emitter based on GeSn alloy.

THz photoconductive emitters based on III-V materials have demonstrated excellent THz radiation properties, enabling many unique applications. However, the incompatibility with the complementary-metal-oxide-semiconductor (CMOS) foundry fabrication process and the challenging growth condition hampers THz photoconductive emitters from large-scale production. To address this limitation, we proposed the GeSn alloy as the photoconductive material candidate through the CMOS-compatible epitaxy instrument. The GeSn photoconductor features a 518 cm2/V-s mobility and a 7187 cm-1 absorption coefficient at the wavelength of 1560 nm, resulting in sufficiently ultrafast photocurrent generation for THz radiation. As a result, the GeSn THz emitter provides over a bandwidth of 2 THz and a 40 dB signal-to-noise ratio, which shows its potential in realizing mass-producible, cost-effective THz integrated systems with CMOS technology.

Water stable and matrix addressable OLED fiber textiles for wearable displays with large emission area

Organic light-emitting diode (OLED) fibers with favorable electroluminescence properties and interconnectable pixel configurations have represented the potential for wearable electronic textile displays. Nevertheless, the current technology of OLED fiber-based textile displays still leaves to be desired due to several challenges, including limited emission area and lack of encapsulation systems. Here we present a fibrous OLED textile display that can attain a large emission area and long-term stability by implementing addressable networks comprised of integrated phosphorescence OLED fibers and by designing multilayer encapsulations. The integrated fiber configuration offers decoupled functional fiber surfaces for an interconnectable 1-dimensional OLED pixel array and a data-addressing conductor. Tailored triadic metal/ultrathin oxide/polymer multilayer enables not only the oxygen/water permeation inhibition but also the controllable conductive channels of dielectric antifuses. Together with reliable bending stability, the long-term operation of OLED textiles in water manifests the feasibility of the present device concept toward water-resistant full-emitting-area fibrous textile displays.

Effect of Ridge Width on the Lasing Characteristics of Triangular and Rectangular InAs/In0.53Ga0.47As Quantum Well Lasers

The lasing characteristics of InP-based InAs/In0.53Ga0.47As quantum well (QW) lasers with different ridge widths are investigated. Two groups of lasers are grown for comparison, one with active triangular QW regions and the other with rectangular QW regions. Their output powers, characteristic temperatures (T0), external differential quantum efficiencies (ηd) and junction temperatures (Tj) are analyzed and compared. The parameter of ridge width is found to play an important role in the performance of the lasers. In triangular QW lasers, by broadening the ridge width from 8 to 12 μm, output power and ηd of the lasers are decreased for the temperature range of 100–320 K due to heating effect. But by broadening the ridge width from 8 to 100 μm in rectangular QW lasers, output power has about 3.5 time increase at 100 K and ηd also has a little increase for temperatures from 100 to 180 K due to much larger emission area and much faster heat dissipation. Tj, the real temperature of the active region, is also found to have accelerated increase at high injection current and heat sink temperature. Besides, compared to the rectangular QW laser of the same ridge width, the improved thermal performance of triangular QW laser is also demonstrated.

Influencing Factors of Total Factor Energy Efficiency in Bohai Rim Region Based on DEA-Tobit Model

Bohai Rim region is an important economic development area and a large carbon emission area in China. It is of great significance to explore the total factor energy efficiency and its influencing factors for the low carbon transformation and high-quality development of the Bohai Rim region. Based on the total factor energy efficiency framework, the DDF-DEA model was used to calculate the total factor energy efficiency, and the internal and external differences of the total factor energy efficiency were further analyzed. The internal and external influencing factors were determined by ML index method and classical endogenous growth theory, and then the Tobit panel model was used to empirically analyze the action mechanism of all influencing factors of total factor energy efficiency in the Bohai Rim region. The results show that the pure technical efficiency, scale efficiency and technological progress among the internal influencing factors contribute to the improvement of energy efficiency in the Bohai Rim region. Industrial structure, industrial internal structure and ownership structure inhibit the improvement of energy efficiency. Energy consumption structure and energy endowment also have a negative impact on energy efficiency. Therefore, measures such as promoting technological progress, adjusting economic structure and optimizing energy structure will effectively improve total factor energy efficiency in the Bohai Rim region.

Structures and Materials in Stretchable Electroluminescent Devices.

Stretchable electroluminescent (EL) devices are obtained by partitioning a large emission area into areas specifically for stretching and light-emission (island-bridge structure). Buckled and textile structures are also shown effective to combine the conventional light emitting diode fabrication with elastic substrates for structure-enabled stretchable EL devices. Meanwhile, intrinsically stretchable EL devices which are characterized with uniform stretchability down to microscopic scale are relatively less developed but promise simpler device structure and higher impact resistance. The challenges in fabricating intrinsically stretchable EL devices with high and robust performance are in many facets, including stretchable conductors, emissive materials, and compatible processes. For the stretchable transparent electrode, ionically conductive gel, conductive polymer coating, and conductor network in surface of elastomer are all proven useful. The stretchable EL materials are currently limited to conjugated polymers, conjugated polymers with surfactants and ionic conductors added to boost stretchability, and phosphor particles embedded in elastomer matrices. These emissive materials operate under different mechanisms, require different electrode materials and fabrication processes, and the corresponding EL devices face distinctive challenges. This review aims to provide a basic understanding of the materials meeting both the mechanical and electronic requirements and important techniques to fabricate the stretchable EL devices.

Methods for measuring the local emission characteristics of CNT based multi-tip emitters

The work is aimed at obtaining microscopic emission characteristics of individual emission sites of a multi-tip field cathode or large-area emitter (LAFE) based on processing the current-voltage characteristics and emission glow patterns. Processing was carried out on a hardware-software complex for the study of field emission characteristics in real time. The calculation of the microscopic characteristics of the local emission sites — the field enhancement factor and emission area — was carried out by several different algorithms. A comparison of the results showed that the algorithms gave close values of the characteristics, which increases the reliability of the estimates made.

Organic light emitting diode for in vitro antimicrobial photodynamic therapy of Candida strains

Organic light emitting diodes (OLEDs) are very attractive light sources because they are large area emitters, and can in principle be deposited on flexible substrates. These features make them suitable for ambulatory photodynamic therapy (PDT). A few reports of in vitro or in vivo OLED based PDT studies for cancer or microbial inhibition are published but to our best knowledge, none against yeasts. Yeast infections are a significant health risk, especially in low income countries with limited medical facilities. In this work, OLED-based antimicrobial PDT (aPDT), using methylene blue (MB) as photosensitizer (PS), is studied to inactivate opportunistic yeast of four Candida strains of two species: Candida albicans and Candida tropicalis. Before aPDT experiments, fluconazole-resistance was evaluated for all strains, showing that both strains of C. tropicalis were resistant and both strains of C. albicans were sensitive to it. We found that 3 repetitive irradiations work better than a single dose while keeping the total fluence constant, and that this result applies whether or not the strains are resistant to fluconazole.

The effect of size distribution and degradation of carbon nanotubes on the Fowler–Nordheim plot behavior

The behavior of the Folwer-Nordheim (FN) plot is investigated for large-area emitters arrays composed of perfectly aligned single-walled carbon nanotubes (CNTs). The field emission currents are calculated based on the standard FN model using the Murphy-Good equation. The aspect ratio of the CNTs is varied such that the height of the CNT has predefined uniform or Gaussian distributions. Continuous degradation of the high-aspect-ratio CNTs is assumed to take place at predefined values of the scaled barrier field. The effects, on the FN plot behavior, of the size distribution and the scenario of the degradation of the emitter are determined and discussed. These factors are found to produce the two behaviors of the experimental FN plots, i.e. the upward and the downward bending behaviors. This work shows the possibility to expand the application of the standard FN model for the investigation of the field emission characteristics from nanoscaled emitters such as CNTs.

Unveiling the Nottingham Inversion Instability during the thermo-field emission from refractory metal micro-protrusions

The electron emission by micro-protrusions has been studied for over a century, but the complete explanation of the unstable behaviors and their origin remains an open issue. These systems often evolve towards vacuum breakdown, which makes experimental studies of instabilities very difficult. Modeling studies are therefore necessary. In our model, refractory metals have shown the most striking results for discontinuities or jumps recorded on the electron emitted current under high applied voltages. Herein, we provide evidence on the mechanisms responsible for the initiation of a thermal instability during the field emission from refractory metal micro-protrusions. A jump in the emission current at steady state is found beyond a threshold electric field, and it is correlated to a similar jump in temperature. These jumps are related to a transient runaway of the resistive heating that occurs after the Nottingham flux inversion. That causes the hottest region to move beneath the apex, and generates an emerging heat reflux towards the emitting surface. Two additional conditions are required to initiate the runaway. The emitter geometry must ensure a large emission area and the thermal conductivity must be high enough at high temperatures so that the heat reflux can significantly compete with the heat diffusion towards the thermostat. The whole phenomenon, that we propose to call the Nottingham Inversion Instability, can explain unexpected thermal failures and breakdowns observed with field emitters.

Solution processed fabrication of silver nanowire-MXene@PEDOT: PSS flexible transparent electrodes for flexible organic light-emitting diodes

Silver nanowire based flexible transparent electrodes (AgNW FTEs) are the most promising next-generation electrodes for flexible electronics. Novel AgNW-MXene@PEDOT:PSS (AgNW-MP) FTEs were fabricated by solution process, and flexible organic light emitting diodes (FOLED) were further demonstrated. Interactions between Ti3C2Tx MXene and PEDOT:PSS allow a high conductive quinoid state of PEDOT:PSS and AgNW-MP FTEs achieve a sheet resistance of 17 Ω/sq and transmittance of 97.6%. MP21 hybrids (mass ratio of 2:1) as nanowelding materials improve surface roughness (RMS roughness value of 8.2 nm) and enhance stability resisting 5000 cycles of bending or H2S gas corrosion. High efficiency FOLEDs with large emission area using commercial materials and structure were demonstrated, achieving a maximum external quantum efficiency of 25.9% and luminous efficiency over 21 cd/A due to better optoelectrical properties and higher work function. It suggests an important step toward producing commercial AgNW based hybrid FTEs for flexible OLEDs and other flexible electronics.

1.3‐µm buried‐heterostructure VCSELs with GaAs/AlGaAs metamorphic DBRs grown by MOCVD

The authors report buried-heterostructure long-wavelength VCSELs with GaAs/AlGaAs metamorphic DBRs, which has a potential for low thermal resistance and operation in the single transverse mode with a large emission area. In spite of their being upside-up and unmounted, they achieve CW operation at 17°C, and VCSELs with a 15-µm mesa exhibit single-mode operation.

Influence of the distribution of local field enhancement factors on the shape of the current-voltage characteristics of carbon-nanotube-based large-area emitters

Experimental current-voltage characteristics (IVCs) of carbon nanotube (CNT)-based large-area field emitters (LAFEs) were measured and analyzed using a “two-class model”, in which some emitters had a set of local field enhancement factors (FEFs) significantly larger than those of the other emitters. An online measurement of the experimental IVCs was carried out and, interestingly, showed the presence of strong nonlinearity in the form of “upward bends” in the corresponding orthodox Fowler-Nordheim (FN) plots. Our studies were conducted over a wide range of currents, and excellent agreement between our experimental results and the proposed theory confirmed the general validity of the two-class FEF model of orthodox field emission. The results presented here were also supported by a bimodal distribution of the brightness level observed by an analysis of the glow patterns, since the brightness was expected to depend on the site's emitted current. We believe this is an important step for LAFE characterization when strong nonlinearites are present in the FN plots, since an FEF similar to the classically defined FEF was recently proven to be applicable to CNTs in the context of full quantum ab initio calculations.