Showing papers on "Diode published in 2018"

Interpretation and evolution of open-circuit voltage, recombination, ideality factor and subgap defect states during reversible light-soaking and irreversible degradation of perovskite solar cells

Abstract: Metal halide perovskite absorber materials are about to emerge as a high-efficiency photovoltaic technology. At the same time, they are suitable for high-throughput manufacturing characterized by a low energy input and abundant low-cost materials. However, a further optimization of their efficiency, stability and reliability demands a more detailed optoelectronic characterization and understanding of losses including their evolution with time. In this work, we analyze perovskite solar cells with different architectures (planar, mesoporous, HTL-free), employing temperature dependent measurements (current–voltage, light intensity, electroluminescence) of the ideality factor to identify dominating recombination processes that limit the open-circuit voltage (Voc). We find that in thoroughly-optimized, high-Voc (≈1.2 V) devices recombination prevails through defects in the perovskite. On the other hand, irreversible degradation at elevated temperature is caused by the introduction of broad tail states originating from an external source (e.g. metal electrode). Light-soaking is another effect decreasing performance, though reversibly. Based on FTPS measurements, this degradation is attributed to the generation of surface defects becoming a new source of non-radiative recombination. We conclude that improving long-term stability needs to focus on adjacent layers, whereas a further optimization of efficiency of top-performing devices requires understanding of the defect physics of the nanocrystalline perovskite absorber. Finally, our work provides guidelines for the design of further dedicated studies to correctly interpret the diode ideality factor and decrease recombination losses.

High-efficiency perovskite–polymer bulk heterostructure light-emitting diodes

Abstract: Perovskite-based optoelectronic devices are gaining much attention owing to their remarkable performance and low processing cost, particularly for solar cells. However, for perovskite light-emitting diodes, non-radiative charge recombination has limited the electroluminescence efficiency. Here we demonstrate perovskite–polymer bulk heterostructure light-emitting diodes exhibiting external quantum efficiencies of up to 20.1% (at current densities of 0.1–1 mA cm−2). The light-emitting diode emissive layer comprises quasi-two-dimensional and three-dimensional (2D/3D) perovskites and an insulating polymer. Photogenerated excitations migrate from quasi-2D to lower-energy sites within 1 ps, followed by radiative bimolecular recombination in the 3D regions. From near-unity external photoluminescence quantum efficiencies and transient kinetics of the emissive layer with and without charge-transport contacts, we find non-radiative recombination pathways to be effectively eliminated, consistent with optical models giving near 100% internal quantum efficiencies. Although the device brightness and stability (T50 = 46 h in air at peak external quantum efficiency) require further improvement, our results indicate the significant potential of perovskite-based photon sources.

High-efficiency electroluminescence and amplified spontaneous emission from a thermally activated delayed fluorescent near-infrared emitter

Abstract: Near-infrared organic light-emitting diodes and semiconductor lasers could benefit a variety of applications including night-vision displays, sensors and information-secured displays. Organic dyes can generate electroluminescence efficiently at visible wavelengths, but organic light-emitting diodes are still underperforming in the near-infrared region. Here, we report thermally activated delayed fluorescent organic light-emitting diodes that operate at near-infrared wavelengths with a maximum external quantum efficiency of nearly 10% using a boron difluoride curcuminoid derivative. As well as an effective upconversion from triplet to singlet excited states due to the non-adiabatic coupling effect, this donor–acceptor–donor compound also exhibits efficient amplified spontaneous emission. By controlling the polarity of the active medium, the maximum emission wavelength of the electroluminescence spectrum can be tuned from 700 to 780 nm. This study represents an important advance in near-infrared organic light-emitting diodes and the design of alternative molecular architectures for photonic applications based on thermally activated delayed fluorescence. Near-infrared organic light-emitting diodes that operate in the 700 nm region with an external quantum efficiency of almost 10% are reported.

Minimising efficiency roll-off in high-brightness perovskite light-emitting diodes.

Abstract: Efficiency roll-off is a major issue for most types of light-emitting diodes (LEDs), and its origins remain controversial. Here we present investigations of the efficiency roll-off in perovskite LEDs based on two-dimensional layered perovskites. By simultaneously measuring electroluminescence and photoluminescence on a working device, supported by transient photoluminescence decay measurements, we conclude that the efficiency roll-off in perovskite LEDs is mainly due to luminescence quenching which is likely caused by non-radiative Auger recombination. This detrimental effect can be suppressed by increasing the width of quantum wells, which can be easily realized in the layered perovskites by tuning the ratio of large and small organic cations in the precursor solution. This approach leads to the realization of a perovskite LED with a record external quantum efficiency of 12.7%, and the efficiency remains to be high, at approximately 10%, under a high current density of 500 mA cm−2. Large drop in efficiency at high brightness has been holding back the development of various light-emitting diodes including halide perovskite. Here Zou et al. achieve high quantum efficiency of 10% under a high current density of 500 mA cm−2 in perovskite-based diodes by reducing luminescence quenching.

Tunneling Diode Based on WSe2 /SnS2 Heterostructure Incorporating High Detectivity and Responsivity.

Abstract: van der Waals (vdW) heterostructures based on atomically thin 2D materials have led to a new era in next-generation optoelectronics due to their tailored energy band alignments and ultrathin morphological features, especially in photodetectors. However, these photodetectors often show an inevitable compromise between photodetectivity and photoresponsivity with one high and the other low. Herein, a highly sensitive WSe2 /SnS2 photodiode is constructed on BN thin film by exfoliating each material and manually stacking them. The WSe2 /SnS2 vdW heterostructure shows ultralow dark currents resulting from the depletion region at the junction and high direct tunneling current when illuminated, which is confirmed by the energy band structures and electrical characteristics fitted with direct tunneling. Thus, the distinctive WSe2 /SnS2 vdW heterostructure exhibits both ultrahigh photodetectivity of 1.29 × 1013 Jones (Iph /Idark ratio of ≈106 ) and photoresponsivity of 244 A W-1 at a reverse bias under the illumination of 550 nm light (3.77 mW cm-2 ).

Diode fibres for fabric-based optical communications

Abstract: Semiconductor diodes are basic building blocks of modern computation, communications and sensing1. As such, incorporating them into textile-grade fibres can increase fabric capabilities and functions2, to encompass, for example, fabric-based communications or physiological monitoring. However, processing challenges have so far precluded the realization of semiconducting diodes of high quality in thermally drawn fibres. Here we demonstrate a scalable thermal drawing process of electrically connected diode fibres. We begin by constructing a macroscopic preform that hosts discrete diodes internal to the structure alongside hollow channels through which conducting copper or tungsten wires are fed. As the preform is heated and drawn into a fibre, the conducting wires approach the diodes until they make electrical contact, resulting in hundreds of diodes connected in parallel inside a single fibre. Two types of in-fibre device are realized: light-emitting and photodetecting p–i–n diodes. An inter-device spacing smaller than 20 centimetres is achieved, as well as light collimation and focusing by a lens designed in the fibre cladding. Diode fibres maintain performance throughout ten machine-wash cycles, indicating the relevance of this approach to apparel applications. To demonstrate the utility of this approach, a three-megahertz bi-directional optical communication link is established between two fabrics containing receiver–emitter fibres. Finally, heart-rate measurements with the diodes indicate their potential for implementation in all-fabric physiological-status monitoring systems. Our approach provides a path to realizing ever more sophisticated functions in fibres, presenting the prospect of a fibre ‘Moore's law’ analogue through the increase of device density and function in thermally drawn textile-ready fibres.

Reducing Architecture Limitations for Efficient Blue Perovskite Light-Emitting Diodes.

Abstract: Light-emitting diodes utilizing perovskite nanocrystals have generated strong interest in the past several years, with green and red devices showing high efficiencies. Blue devices, however, have lagged significantly behind. Here, it is shown that the device architecture plays a key role in this lag and that NiOx , a transport layer in one of the highest efficiency devices to date, causes a significant reduction in perovskite luminescence lifetime. An alternate transport layer structure which maintains robust nanocrystal emission is proposed. Devices with this architecture show external quantum efficiencies of 0.50% at 469 nm, seven times higher than state-of-the-art devices at that wavelength. Finally, it is demonstrated that this architecture enables efficient devices across the entire blue-green portion of the spectrum. The improvements demonstrated here open the door to efficient blue perovskite light-emitting diodes.

Red Organic Light-Emitting Diode with External Quantum Efficiency beyond 20% Based on a Novel Thermally Activated Delayed Fluorescence Emitter.

Abstract: A novel thermally activated delayed fluorescence (TADF) emitter 12,15-di(10H-phenoxazin-10-yl)dibenzo[a,c]dipyrido[3,2-h:2',3'-j]phenazine (DPXZ-BPPZ) is developed for a highly efficient red organic light-emitting diode (OLED). With rigid and planar constituent groups and evident steric hindrance between electron-donor (D) and electron-acceptor (A) segments, DPXZ-BPPZ realizes extremely high rigidity to suppress the internal conversion process. Meanwhile, the highly twisted structure between D and A segments will also lead to an extremely small singlet-triplet energy split to DPXZ-BPPZ. Therefore, DPXZ-BPPZ successfully realizes an efficient fluorescent radiation transition and reverse intersystem crossing process, and possesses an extremely high photoluminescence quantum efficiency of 97.1 ± 1.1% under oxygen-free conditions. The OLED based on DPXZ-BPPZ shows red emission with a peak at 612 nm and a Commission Internationale de L'Eclairage (CIE) coordinate of (0.60, 0.40), and it achieves high maximum forward-viewing efficiencies of 20.1 ± 0.2% (external quantum efficiency), 30.2 ± 0.6 cd A-1 (current efficiency), and 30.9 ± 1.3 lm W-1 (power efficiency). The prepared OLED has the best performance among the reported red TADF OLEDs. These results prove that DPXZ-BPPZ is an ideal candidate for red TADF emitters, and the designing approach is valuable for highly efficient red TADF emitters.

High-performance, multifunctional devices based on asymmetric van der Waals heterostructures

Abstract: Two-dimensional materials are of interest for the development of electronic devices due to their useful properties and compatibility with silicon-based technology. Van der Waals heterostructures, in which two-dimensional materials are stacked on top of each other, allow different materials and properties to be combined and for multifunctional devices to be created. Here we show that an asymmetric van der Waals heterostructure device, which is composed of graphene, hexagonal boron nitride, molybdenum disulfide and molybdenum ditelluride, can function as a high-performance diode, transistor, photodetector and programmable rectifier. Due to the asymmetric structure of the device, charge-carrier injection can be switched between tunnelling and thermal activation under negative and positive bias conditions, respectively. As a result, the device exhibits a high current on/off ratio of 6 × 108 and a rectifying ratio of ~108. The device can also function as a programmable rectifier with stable retention and continuously tunable memory states, as well as a high program/erase current ratio of ~109 and a rectification ratio of ~107. An asymmetric van der Waals heterostructure device, which is composed of graphene, hexagonal boron nitride, molybdenum disulfide and molybdenum ditelluride, can function as a high-performance diode, transistor, photodetector and programmable rectifier.

Universal strategy for Ohmic hole injection into organic semiconductors with high ionization energies.

Abstract: Barrier-free (Ohmic) contacts are a key requirement for efficient organic optoelectronic devices, such as organic light-emitting diodes, solar cells, and field-effect transistors. Here, we propose a simple and robust way of forming an Ohmic hole contact on organic semiconductors with a high ionization energy (IE). The injected hole current from high-work-function metal-oxide electrodes is improved by more than an order of magnitude by using an interlayer for which the sole requirement is that it has a higher IE than the organic semiconductor. Insertion of the interlayer results in electrostatic decoupling of the electrode from the semiconductor and realignment of the Fermi level with the IE of the organic semiconductor. The Ohmic-contact formation is illustrated for a number of material combinations and solves the problem of hole injection into organic semiconductors with a high IE of up to 6 eV. It is shown that Ohmic contacts for the injection of hole carriers into organic semiconductors with high ionization energy can be formed by adding ultrathin interlayers with higher ionization energy.

A Reconfigurable Broadband Polarization Converter Based on an Active Metasurface

Abstract: We propose an active metasurface whose functionalities can be dynamically switched among linear-to-linear, linear-to-elliptical, and linear-to-circular polarization conversions in a wideband. The active metasurface is composed of butterfly-shaped unit cells embedded with voltage-controlled varactor diodes. By controlling the bias voltage of the varactor diodes, the electromagnetic responses of the proposed metasurface can be tailored, leading to reconfigurable polarization conversions. The simulation results reveal that with no bias voltage, the proposed metasurface is able to reflect linear-polarization waves to cross-polarization waves in the frequency range from 3.9 to 7.9 GHz, with a polarization conversion ratio of over 80%; however, at the bias voltage of −19 V, the metasurface is tuned to be a circular polarization converter in a wideband from 4.9 to 8.2 GHz. Moreover, two equivalent circuits along the $x$ - and $y$ -directions are developed to elucidate the tunable mechanism. The experimental results are in a good agreement with the simulation results obtained from commercial software and from the equivalent circuit model.

Flexible active-matrix organic light-emitting diode display enabled by MoS2 thin-film transistor

Abstract: Atomically thin molybdenum disulfide (MoS2) has been extensively investigated in semiconductor electronics but has not been applied in a backplane circuitry of organic light-emitting diode (OLED) display. Its applicability as an active drive element is hampered by the large contact resistance at the metal/MoS2 interface, which hinders the transport of carriers at the dielectric surface, which in turn considerably deteriorates the mobility. Modified switching device architecture is proposed for efficiently exploiting the high-k dielectric Al2O3 layer, which, when integrated in an active matrix, can drive the ultrathin OLED display even in dynamic folding states. The proposed architecture exhibits 28 times increase in mobility compared to a normal back-gated thin-film transistor, and its potential as a wearable display attached to a human wrist is demonstrated.

Wide Spectral Photoresponse of Layered Platinum Diselenide-Based Photodiodes

Abstract: Platinum diselenide (PtSe2) is a group-10 transition metal dichalcogenide (TMD) that has unique electronic properties, in particular a semimetal-to-semiconductor transition when going from bulk to monolayer form. We report on vertical hybrid Schottky barrier diodes (SBDs) of two-dimensional (2D) PtSe2 thin films on crystalline n-type silicon. The diodes have been fabricated by transferring large-scale layered PtSe2 films, synthesized by thermally assisted conversion of predeposited Pt films at back-end-of-the-line CMOS compatible temperatures, onto SiO2/Si substrates. The diodes exhibit obvious rectifying behavior with a photoresponse under illumination. Spectral response analysis reveals a maximum responsivity of 490 mA/W at photon energies above the Si bandgap and relatively weak responsivity, in the range of 0.1–1.5 mA/W, at photon energies below the Si bandgap. In particular, the photoresponsivity of PtSe2 in infrared allows PtSe2 to be utilized as an absorber of infrared light with tunable sensitivit...

Hybrid perovskite light emitting diodes under intense electrical excitation

Abstract: Hybrid perovskite semiconductors represent a promising platform for color-tunable light emitting diodes (LEDs) and lasers; however, the behavior of these materials under the intense electrical excitation required for electrically-pumped lasing remains unexplored. Here, we investigate methylammonium lead iodide-based perovskite LEDs under short pulsed drive at current densities up to 620 A cm−2. At low current density (J 10 A cm−2), where EQE roll-off is dominated by a combination of Joule heating and charge imbalance yet shows no evidence of Auger loss, suggesting that operation at kA cm−2 current densities relevant for a laser diode should be within reach. Hybrid perovskite semiconductors are promising for wavelength-tunable laser diodes but their behavior under intense electrical excitation remains unexplored. Kim et al. investigate perovskite light emitting diodes at current densities nearing 1 kA cm−2 and suggest that a laser diode is within reach.

Light emission from a poly-silicon device with carrier injection engineering

Abstract: Development work was conducted on N+PN+PN+ Poly silicon light-emitting devices which are compatible with silicon-based integrated circuit technology. We discuss the emission characteristics of visible light by a monolithically integrated Poly-Si diode under reverse bias. With the structure of modified PN junctions, the carriers injection occurs through silicon slabs of only a few nanometer thick. The dominant role of non-radiative recombination at the N+ and P+ contacts is diminished by confining the injected carriers around the PN junction’s interface in which avalanche takes place. The current-dependent optical radiation presents a broad spectrum in the 400- to 900-nm range. Although the emission efficiency is low due to silicon’s indirect bandgap, it is advantageous to utilize these devices in all-silicon optoelectronic integrated circuits (OIC’s).

A New Method for Fitting Current–Voltage Curves of Planar Heterojunction Perovskite Solar Cells

Abstract: Herein we propose a new equivalent circuit including double heterojunctions in series to simulate the current–voltage characteristic of P–I–N planar structure perovskite solar cells. This new method can theoretically solve the dilemma of the parameter diode ideal factor being larger than 2 from an ideal single heterojunction equivalent circuit, which usually is in the range from 1 to 2. The diode ideal factor reflects PN junction quality, which influences the recombination at electron transport layer/perovskite and perovskite/hole transport layer interface. Based on the double PN junction equivalent circuit, we can also simulate the dark current–voltage curve for analyzing recombination current (Shockley–Read–Hall recombination) and diffusion current (including direct recombination), and thus carrier recombination and transportation characteristics. This new model offers an efficacious and simple method to investigate interfaces condition, film quality of perovskite absorbing layer and performance of transport layer, helping us further improve the device efficiency and analyze the working mechanism.

Control of oxygen vacancies in ZnO nanorods by annealing and their influence on ZnO/PEDOT:PSS diode behaviour

Abstract: ZnO is one of the most widely studied semiconductors due to its direct wide band gap and high exciton binding energy Due to its ease of synthesis, robustness and low cost, ZnO has been applied in a wide range of devices, including nanogenerators, solar cells, and photodetectors In this work, ZnO nanorods were synthesized in a single step using an aqueous method at temperatures below 100 °C The nanorods were annealed in oxygen and nitrogen and a p-type polymer poly(3,4-ethylenedioxythiophene)polystyrene sulfonate (PEDOT:PSS) was spray coated onto the top of ZnO nanorods to form a p–n junction The I–V characteristics of the device showed that the annealing atmosphere had a significant effect on the rectification ratio of the device Further analysis using Mott–Schottky, photoluminescence, and X-ray photoelectron spectroscopy (XPS) indicated that oxygen vacancy concentration correlated well with the free electron density in ZnO as well as the rectification ratio of the p–n junction devices Devices made with ZnO nanorods annealed in nitrogen had a better rectification ratio than oxygen, representing a simple method to improve p–n junction diode behaviour through tuning the defect properties of the nanorods via controlled annealing

Temperature-Dependent Characterization, Modeling, and Switching Speed-Limitation Analysis of Third-Generation 10-kV SiC MOSFET

Abstract: The temperature-dependent characteristics of the third-generation 10-kV/20-A SiC MOSFET including the static characteristics and switching performance are carried out in this paper. The steady-state characteristics, including saturation current, output characteristics, antiparallel diode, and parasitic capacitance, are tested. A double pulse test platform is constructed including a circuit breaker and gate drive with >10-kV insulation and also a hotplate under the device under test for temperature-dependent characterization during switching transients. The switching performance is tested under various load currents and gate resistances at a 7-kV dc-link voltage from 25 to 125 ˚C and compared with previous 10-kV MOSFETs. A simple behavioral model with its parameter extraction method is proposed to predict the temperature-dependent characteristics of the 10-kV SiC MOSFET. The switching speed limitations, including the reverse recovery of SiC MOSFET's body diode, overvoltage caused by stray inductance, crosstalk, heat sink, and electromagnetic interference to the control are discussed based on simulations and experimental results.

High performance, self-powered photodetectors based on a graphene/silicon Schottky junction diode

Abstract: Electron–hole pair separation and photocurrent conversion at two-dimensional (2D) and three-dimensional (3D) hybrid interfaces are important for achieving high performance, self-powered optoelectronic devices such as photodetectors. In this regard, herein, we designed and demonstrated a graphene/silicon (Gr/Si) (2D/3D) van der Waals (vdW) heterostructure for high-performance photodetectors, where graphene acts as an efficient carrier collector and Si as a photon absorption layer. The Gr/Si heterojunction exhibits superior Schottky diode characteristics with a barrier height of 0.76 eV and shows good performance as a self-powered detector, responding to 532 nm at zero bias. The self-powered photodetector functions under the mechanism of photovoltaic effect and exhibits responsivity as high as 510 mA W−1 with a photo switching ratio of 105 and a response time of 130 μs. The high-performance vdW heterostructure photodetector demonstrated herein is attributed to the Schottky barrier that effectively prolongs the lifetime of photo-excited carriers, resulting in fast separation and transport of photoexcited carriers. The self-powered photodetector with superior light harvesting and carrier transport behaviour is expected to open a window for the technological implementation of Si-based monolithic optoelectronic devices.

Control of Narrow-Band Emission in Phosphor Materials for Application in Light-Emitting Diodes

Abstract: Light-emitting diodes (LEDs) are widely used around the world. Scientists are attempting to develop LED devices that not only have high brightness but also have a high color rendering index (CRI). ...

1230 V β-Ga2O3 trench Schottky barrier diodes with an ultra-low leakage current of <1 μA/cm2

Abstract: β-Ga2O3 vertical trench Schottky barrier diodes (SBDs) are realized, demonstrating superior reverse blocking characteristics than the co-fabricated regular SBDs. Taking advantage of the reduced surface field effect offered by the trench metal-insulator-semiconductor structure, the reverse leakage current in the trench SBDs is significantly suppressed. The devices have a higher breakdown voltage of 1232 V without optimized field management techniques, while having a specific on-resistance (Ron,sp) of 15 mΩ cm2. An ultra-low leakage current density of <1 μA/cm2 is achieved before breakdown, the lowest among all reported Ga2O3 Schottky barrier diodes. Fast electron trapping and slow de-trapping near the Al2O3/Ga2O3 interface are observed by repeated C-V measurements, which show an interface state ledge and positive shifts of flat-band voltages with increasing voltage stress. By comparison between pulsed and DC measurements, the device self-heating effect and the trapping effect are uncoupled. It is found that the trapping effect at the trench sidewall affects the on-resistance of the trench SBDs, even under pulsed conditions. With reduced trapping effect and better field management technique, the trench SBDs could further harvest the promising material properties of β-Ga2O3.β-Ga2O3 vertical trench Schottky barrier diodes (SBDs) are realized, demonstrating superior reverse blocking characteristics than the co-fabricated regular SBDs. Taking advantage of the reduced surface field effect offered by the trench metal-insulator-semiconductor structure, the reverse leakage current in the trench SBDs is significantly suppressed. The devices have a higher breakdown voltage of 1232 V without optimized field management techniques, while having a specific on-resistance (Ron,sp) of 15 mΩ cm2. An ultra-low leakage current density of <1 μA/cm2 is achieved before breakdown, the lowest among all reported Ga2O3 Schottky barrier diodes. Fast electron trapping and slow de-trapping near the Al2O3/Ga2O3 interface are observed by repeated C-V measurements, which show an interface state ledge and positive shifts of flat-band voltages with increasing voltage stress. By comparison between pulsed and DC measurements, the device self-heating effect and the trapping effect are uncoupled. It is found tha...

Low-pressure CVD-grown β-Ga2O3 bevel-field-plated Schottky barrier diodes

Abstract: We report (010)-oriented β-Ga2O3 bevel-field-plated mesa Schottky barrier diodes grown by low-pressure chemical vapor deposition (LPCVD) using a solid Ga precursor and O2 and SiCl4 sources. Schottky diodes with good ideality and low reverse leakage were realized on the epitaxial material. Edge termination using beveled field plates yielded a breakdown voltage of −190 V, and maximum vertical electric fields of 4.2 MV/cm in the center and 5.9 MV/cm at the edge were estimated, with extrinsic R ON of 3.9 mΩcm2 and extracted intrinsic R ON of 0.023 mΩcm2. The reported results demonstrate the high quality of homoepitaxial LPCVD-grown β-Ga2O3 thin films for vertical power electronics applications, and show that this growth method is promising for future β-Ga2O3 technology.

A High Current Density Direct-Current Generator Based on a Moving van der Waals Schottky Diode.

Abstract: Traditionally, Schottky diodes are used statically in the electronic information industry while dynamic or moving Schottky diode-based applications are rarely explored. Herein, a novel Schottky diode named "moving Schottky diode generator" is designed, which can convert mechanical energy into electrical energy by means of lateral movement between the graphene/metal film and semiconductor. The mechanism is based on the built-in electric field separation of the diffusing carriers in moving Schottky diode. A current-density output up of 40.0 A m-2 is achieved through minimizing the contact distance between metal and semiconductor, which is 100-1000 times higher than former piezoelectric and triboelectric nanogenerators. The power density and power conversion efficiency of the heterostructure-based generator can reach 5.25 W m-2 and 20.8%, which can be further enhanced by Schottky junction interface design. Moreover, the graphene film/semiconductor moving Schottky diode-based generator behaves better flexibility and stability, which does not show obvious degradation after 10 000 times of running, indicating its great potential in the usage of portable energy source. This moving Schottky diode direct-current generator can light up a blue light-emitting diode and a flexible graphene wristband is demonstrated for wearable energy source.

Monolithically Integrated InGaN/GaN Light-emitting Diodes, Photodetectors and Waveguides on Si Substrate

Abstract: The characteristics of monolithically integrated light-emitting diodes (LEDs), photodetectors (PDs), and waveguides on a GaN-on-Si wafer are investigated. The InGaN/GaN multi-quantum wells, which are responsible for blue light emission in LEDs, are also used for photodetection in PDs. Despite the Stokes shift, a spectral overlap of ∼25 nm between the emission and absorption spectra provides the PDs with sufficient photosensitivity to signals from the emitter while remaining insensitive to ambient lighting. Optical interconnects in the form of linear or bent suspended waveguides bridging the LEDs and PDs are formed by selective detachment of etched GaN mesas from the Si substrate. Additionally, the PDs can be detached from the substrate and remounted on an elevated platform, owing to the flexibility of the thin-film waveguide. The 150 μm×150 μm LEDs and PDs exhibit rapid response on nanosecond time scales, which is attributed to fast radiative recombinations as well as minimized resistive-capacitive (RC) delays, enabling transmission of pseudorandom binary sequence (PRBS) data signals at rates of 250 Mb/s with an opening in the eye diagram. Together with multichannel transmission free of crosstalk, the ability of the planar and three-dimensional monolithic photonic systems to handle visible-light communication (VLC) applications is demonstrated.

Spectrum collapse, narrow linewidth, and Bogatov effect in diode lasers locked to high-Q optical microresonators

Abstract: We present a novel method allowing high-power single-frequency emission with sub-kHz linewidth from a compact multi-frequency diode laser locked to high-Q optical microresonator. Using high-Q MgF2microresonator and multi-frequency diode laser operating at 1535 nm with the output power of 100 mW and an emission spectrum consisting of approximately 50 lines with MHz linewidth, we observed a spectrum collapse to a single line or several lines with a sub-kHz linewidth and output power power of 50 mW. The Bogatov effect predicted more than 30 years ago was observed and studied in the spectrum of the locked laser. For analysis of the considered effect, original theoretical model taking into account self-injection locking effect, mode competition and Bogatov asymmetric mode interaction was developed and numerical modeling was performed. All numerical results are in a good agreement with our experimental data. Accurate analytical estimations for the parameters critical for the considered effect were obtained. The proposed method may be applied for different types of diode lasers operating in different spectral ranges.

Metal Halide Perovskites: Synthesis, Ion Migration, and Application in Field-Effect Transistors

Abstract: The past several years have witnessed tremendous developments of metal halide perovskite (MHP)-based optoelectronics. Particularly, the intensive research of MHP-based light-emitting diodes, photodetectors, and solar cells could probably reform the optoelectronic semiconductor industry. In comparison, in spite of the large intrinsic charge carrier mobility of MHPs, the development of MHP-based field-effect transistors (MHP-FETs) is relatively slow, which is essentially due to the gate-field screening effect induced by the ion migration and accumulation in MHP-FETs. This work mainly aims to summarize the recent important work on MHP-FETs and propose solutions in terms of the development bottleneck of perovskite-based transistors, in an attempt to boost the research of MHP transistors further. First, the advantages and potential applications of MHP-FETs are briefly introduced, which is followed by a detailed description of the MHP crystalline structure and various material fabrication techniques. Afterward, MHP-FETs are discussed, including transistors based on hybrid organic-inorganic perovskites, all-inorganic perovskites, and lead-free perovskites.

Accurate Transient Calorimetric Measurement of Soft-Switching Losses of 10-kV SiC mosfets and Diodes

Abstract: The characterization of soft-switching losses (SSL) of modern high-voltage SiC mosfet s is a difficult but necessary task in order to provide a sound basis for the accurate modeling of converter systems, such as medium-voltage-connected solid-state transformers, where soft-switching techniques are employed to achieve an improved converter efficiency. Switching losses (SL), in general, are typically measured with the well-known double pulse method. In the case of SSL measurements, however, this method is very sensitive to the limited accuracy of the measurement of the current and voltage transients, and thus is unsuitable for the characterization of fast-switching high-voltage mosfet s. This paper presents an accurate and reliable calorimetric method for the determination of SSL using the example of 10-kV SiC mosfet modules. Measured SSL curves are presented for different dc-link voltages and switched currents. Furthermore, a deeper analysis concerning the origin of SSL is performed. With the proposed measurement method, it can be experimentally proven that the largest share of the SSL arises from charging and discharging the output capacitance of the mosfet module and especially of the antiparallel junction barrier Schottky diode.

5.0 kV breakdown-voltage vertical GaN p–n junction diodes

Abstract: A high breakdown voltage of 5.0 kV has been achieved for the first time in vertical GaN p–n junction diodes by using our newly developed guard-ring structures. A resistance device was inserted between the main diode portion and the guard-ring portion in a ring-shaped p–n diode to generate a voltage drop over the resistance device by leakage current flowing through the guard-ring portion under negatively biased conditions before breakdown. The voltage at the outer mesa edge of the guard-ring portion, where the electric field intensity is highest and the destructive breakdown usually occurs, is decreased by the voltage drop, so the electric field concentration in the portion is reduced. By adopting this structure, the breakdown voltage (V B) is raised by about 200 V. Combined with a low measured on-resistance (R on) of 1.25 mΩ cm2, Baliga's figure of merit was as high as 20 GW/cm2.

Reconfigurable Linear-to-Linear Polarization Conversion Metasurface Based on PIN Diodes

Abstract: A reconfigurable linear-to-linear polarization conversion metasurface (Re-PCM) based on positive-intrinsic-negative (PIN) diodes is proposed, which can be reconfigured between the conversion mode and the reflection mode by switching the loaded PIN diodes. The unit cell of the Re-PCM consists of two slotted metal square rings and bias lines, which are all etched on a substrate backed by a metal ground. In the conversion mode, the Re-PCM reflects linearly polarized incident waves with 90° polarization rotation. The simulated results show that the polarization conversion ratio is more than 88% over 3.39–5.01 GHz for x -polarized and y -polarized normally incident waves. In the reflection mode, it exhibits total reflection characteristic like a metal plate. The magnitude of co-polarization reflection is over −1 dB from 3.83 to 4.74 GHz. The physical mechanism and equivalent circuit of the Re-PCM are analyzed. To validate the simulation, a prototype of the Re-PCM is fabricated and measured. Reasonable accordance between the simulated and measured results is obtained.