Device Performance Of Light-emitting Diodes Research Articles

Rapid preparation of highly transparent paper with high built-in haze by an ion exchange approach

Highly transparent paper with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical-oxidized wood fibers (TOWFs) is a nascent sustainable photonic material for optoelectronics. However, its relatively low built-in haze and poor production efficiency are two remaining challenges. In this work, a facile and efficient ion exchange approach is proposed to address above obstacles by converting Na+ on their surface into Ca2+, thus enabling the rapid fabrication of highly transparent (90.0%) paper with high built-in haze (93.5%). Moreover, the relationship between Ca2+ concentration and the optical properties and production efficiency of transparent paper is explored. When the Ca2+ concentration increases from 0 to 0.8 mmol/g, the haze rises from 68.6% to 93.5% while maintaining a 90.0% transparency, and the filtration time decreases from 62.5 to 17.5 min. Finally, two application cases of this transparent paper for increasing the device performance of light-emitting diode (LED) and perovskite solar cell are demonstrated. This work proposes a facile and efficient method to simultaneously enhance the build-in haze and production efficiency of highly transparent paper with TOWFs and may bring us a step further towards preparing transparent and hazy paper on an industrial scale.

Quasi‐2D Perovskites: Controlling Spatial Crystallization Uniformity and Phase Orientation of Quasi‐2D Perovskite‐Based Light‐Emitting Diodes Using Lewis Bases (Adv. Mater. Interfaces 2/2020)

The stability of intermediate complexes formed by the addition of Lewis bases is a key factor in modulating orientation of quasi-2D perovskites as well as enhancing spatial uniformity of crystallization. Random orientation benefits the device performance of light-emitting diodes. More details can be found in article number 1901860 by Yaeeun Han, Inchan Hwang, and co-workers.

Performance of light-emitting diode device in detecting occlusal caries in the primary molars.

This in vitro study aimed to compare the performance of a light-emitting diode (LED) device (Midwest Caries I.D.: MID), International Caries Detection and Assessment System (ICDAS) visual criteria, and fluorescence-based devices (DIAGNOdent: LF; DIAGNOdent pen: LFpen; and Quantitative Light-induced Fluorescence: QLF) in detecting occlusal caries in the primary molars. Eighty-eight primary molars with sound occlusal surfaces or carious lesions at different stages were assessed twice, with a 1-week interval in between, by one examiner using all three methods. Subsequently, the teeth were sectioned and lesion depth was verified using stereomicroscopy as a gold standard. Sensitivity, specificity, and accuracy were calculated at D1 (all carious lesions-enamel and dentin) and D3 (dentin lesions) thresholds. Correlation with histological analysis was evaluated using Spearman's rank correlation coefficients (rho). Weighted Kappa and intraclass-correlation (ICC) coefficients were calculated to assess intra-examiner reproducibility. At D1 threshold, ICDAS and LFpen showed higher sensitivity than the other methods, whereas ICDAS, LF, and QLF showed higher specificity (p < 0.05), and MID showed lower accuracy. At D3 threshold, ICDAS, LFpen, and QLF showed higher sensitivity than MID, whereas ICDAS, LF, and MID showed higher specificity (p < 0.05). All methods, except MID, showed statistically similar accuracy values (p < 0.05). Correlations with histopathological analysis varied from 0.15 (MID) to 0.57 (ICDAS). Intra-examiner reproducibility varied from 0.30 (MID) to 0.92 (ICDAS, LF, and QLF). The MID device exhibited a poor performance in detecting occlusal carious lesions in the primary molars, and ICDAS visual criteria exhibited greater accuracy than LF, LFpen, and QLF devices.

Low cost inorganic white light emitting diode based on submicron ZnO rod arrays and electrodeposited Cu2O thin film

We present here the fabrication and characterization of low cost inorganic white light emitting diode (LED) based on submicron ZnO rod arrays and Cu2O thin film. Firstly, n-ZnO rod arrays were grown on FTO substrate by hydrothermal method at 150 °C. Then, p-Cu2O thin film was electrodeposited on top of ZnO arrays from alkaline cupric lactate solution at 60 °C. X-ray diffraction and scanning electron microscopy were employed to investigate the structural and the morphology of the prepared samples as well as the diode device. The results indicate the formation of hexagonal structure of ZnO arrays and cubic structure of polycrystalline Cu2O thin film. The performance of LED device was investigated employing I-V and electroluminescence measurements. The heterojunction device showed good rectification behavior and a broad emission covering visible spectrum for all applied voltage which indicates the white electroluminescence of the heterostructure device. Therefore, the inorganic ZnO arrays/Cu2O thin film is a promising heterojuniction for low cost and large-scale production of white LED.

Fabrication of Phosphor-Free III-Nitride Nanowire Light-Emitting Diodes on Metal Substrates for Flexible Photonics

In this paper, we report our study on high-performance III-nitride nanowire light-emitting diodes (LEDs) on copper (Cu) substrates via the substrate-transfer process. Nanowire LED structures were first grown on silicon-on-insulator (SOI) substrates by molecular beam epitaxy. Subsequently, the SOI substrate was removed by combining dry- and wet-etching processes. Compared to conventional nanowire LEDs on Si, the nanowire LEDs on Cu exhibit several advantages, including more efficient thermal management and enhanced light-extraction efficiency (LEE) because of the usage of metal reflectors and highly thermally conductive metal substrates. The LED on Cu, therefore, has stronger photoluminescence, electroluminescence intensities, and better current–voltage characteristics compared to the conventional nanowire LED on Si. Our simulation results further confirm the improved device performance of LEDs on Cu, compared to LEDs on Si. The LEE of the nanowire LED on Cu is nine times higher than that of the LED on Si at the same nanowire radius of 60 nm and spacing of 130 nm. Moreover, by engineering the device-active region, we achieved high-brightness phosphor-free LEDs on Cu with highly stable white-light emission and high color-rendering index of ∼95, showing their promising applications in general lighting, flexible displays, and wearable applications.

InGaN-based visible light-emitting diodes on ScAlMgO4(0001) substrates

High-quality InGaN-based visible light-emitting diodes (LEDs) are demonstrated on ScAlMgO4 (SCAM) (0001) substrates. GaN grown on SCAM by metal–organic vapor phase epitaxy is nearly strain-free with an in-plane compressive strain of −1.26 × 10−3, which is much smaller than that in conventional GaN/sapphire owing to the smaller thermal expansion mismatch between GaN and SCAM. We fabricate InGaN/GaN quantum well LEDs on GaN/SCAM templates, and observe bright blue electroluminescence at ∼470 nm wavelength. The device performances of LEDs on SCAM are comparable to those of LEDs on sapphire. Our achievements indicate that highly efficient InGaN-based light emitters are possible on SCAM substrates.

Electrochemical removal of hydrogen atoms in Mg-doped GaN epitaxial layers

Hydrogen atoms inside of an Mg-doped GaN epitaxial layer were effectively removed by the electrochemical potentiostatic activation (EPA) method. The role of hydrogen was investigated in terms of the device performance of light-emitting diodes (LEDs). The effect of the main process parameters for EPA such as solution type, voltage, and time was studied and optimized for application to LED fabrication. In optimized conditions, the light output of 385-nm LEDs was improved by about 26% at 30 mA, which was caused by the reduction of the hydrogen concentration by ∼35%. Further removal of hydrogen seems to be involved in the breaking of Ga-H bonds that passivate the nitrogen vacancies. An EPA process with high voltage breaks not only Mg-H bonds that generate hole carriers but also Ga-H bonds that generate electron carriers, thus causing compensation that impedes the practical increase of hole concentration, regardless of the drastic removal of hydrogen atoms. A decrease in hydrogen concentration affects the current-voltage characteristics, reducing the reverse current by about one order and altering the forward current behavior in the low voltage region.

A special issue on Light-Emitting Diodes

As an emerging and energy efficient lighting technology, light-emitting diode (LED) based solid state lighting is expected to significantly reduce global energy consumption and green-house emission for its extraordinary characteristics compared with traditional light sources, including high luminous efficiency, small size, potentially high device and system reliability, long lifetime, etc. Since the LEDs are unable to be applied before being packaged, the packaging technologies are critically needed in terms of materials selection, process development, co-design with multi-physics consideration, integrated consideration of chip and packaging design. Packaging is therefore essential in enhancing the desired long-term reliability and realizing the full potentials of optical performance of LED devices, which still needs us to conduct more fundamental research. Over the decades, many novel packaging technologies have been proposed and some of them have been commercially applied, yet some of them are still under development. To further improve the optical performance and reliability of the LED packages from the view of manufacturing, cost and application, the efforts are made in the whole LED packaging processes from the chip level to the application level. Any progress of LED packaging technologies will benefit the manufacturers, researchers, and of course the final users. Therefore, it is our intention to bring the research community’s attention to these particular research areas. In this “Special Issue on Light-Emitting Diodes”, 1 review article and 5 research articles focusing on relevant subjects by internationally active groups in the field are specially presented. More specifically, Prof. Lee of Hong Kong University of Science and Technology reviewed the advanced LED wafer level packaging technologies and covered some key packaging processes. Prof. Kim from Korea Institute of Ceramic Engineering and Technology suggested a new design and material processing technology to realize price and performance advantageous LED packaging solution. Prof. Rao from University of Electronic Science and Technology of China proposed a self-adaptive phosphor coating technology to further improve the quality of phosphor coating layer on the LED chips. Prof. Jang from Yeungnam University demonstrated the high-performance and current crowding-free InGaN/GaN LEDs using an electrically-reverse-connected Schottky diode and an Mgdelta doped layer. Prof. Luo from Huazhong University of Science and Technology studied the phosphor coating process by calculating the optical constants of phosphor layer and by investigating the effects of phosphor surface geometry on the optical performance of LEDs, respectively. Overall, articles in this special issue exhibit the good achievements of LED, particularly in the interests about the LED packaging technologies for better optical performance, better manufacturing processes and better applications. We hope they are helpful to the LED industry and researchers. Finally, we would like to give our acknowledgements to authors to present their work and to reviewers for their valuable comments.

Temperature Measurement of Visible Light-Emitting Diodes Using Nematic Liquid Crystal Thermography With Laser Illumination

In this letter, we present a new configuration with laser illumination to measure the temperature of visible light-emitting diode (LED) chips using nematic liquid crystals. This method is applied to measuring the junction temperature of multiquantum well (MQW) LEDs in InGaN-GaN-sapphire structure. A color filter is inserted in the optical path to attenuate the overwhelming LED light. A high-power laser beam is used as the sensing light to enhance the contrast of the thermal image on LED chips. This technique is nondestructive and can be performed in real-time during device operation. One objective is to investigate the effect of the junction temperature on the electrical and optical performance of the LED devices. For the LEDs measured, the conversion efficiency decreases by 67% when the junction temperature rises from 22/spl deg/C to 107/spl deg/C. The new measurement configuration is a valuable tool to study the thermal performance of GaN-based LED devices and subsequently to investigate the degradation on electrical and optical performance due to junction temperature increase.

Red light-emitting diodes based on PFO-BSeD

Device performance of light-emitting diodes from the first selenium-containing conjugated polymer - PFO-BSeD was optimized by both control of comonomer ratios and modification of electrode/polymer interfaces. By modifying both anode/polymer interface and cathode/polymer interface, an emission centered at around 600nm and luminous efficiency over 1 cd/A was demonstrated at a low operating voltage. The result indicates that PFO-BSeD can be a promising candidate for red polymer light-emitting diode (PLED).

Device Performance of Light-Emitting Diode with Poly(phenylenesulfidephenyleneamine) as a Promotor of Hole Injection

Charge carrier injection into and electroluminescence from single-layer, bilayer, and trilayer light-emitting diodes have been studied under cw and pulsed operation. The active materials were poly(...