Top Junction Research Articles

Photoelectrochemical, all-soluble iron redox-flow battery for the direct conversion of solar energy

A photoelectrochemical redox-flow battery (RFB) employing an all-soluble, aqueous coordination chemistry of the element iron is developed. The system is based on the ferro/ferricyanide redox couple as posolyte, the iron-triethanolamine (TEOA) complex as negolyte and a Ge/GaAs/GaInP triple-junction solar cell (TJSC) as power source. Combining a discharging flow-cell with the photoelectrochemical unit, we enable an operando monitoring of degradation phenomena at the working electrode similar to a rotating ring-disc electrode (RRDE). In this manner, an irreversible deactivation of an unprotected TJSC is observed after 15 h of operation. In a post-mortem SEM/EDX analysis, this performance loss is identified to result from a dissolution of the top junctions of the photoabsorber. Mitigating this corrosion is achieved by introducing a 150 nm thick protection layer of TiO2 via reactive sputtering. In this manner, a stable performance for more than 120 h is obtained. In view of the fact that the iron species selected for this study are affordable, abundant, water-based and non-toxic our device is a prototype of a green photoelectrochemical RFB at laboratory scale.

Vertically Processed GaInP/InP Tandem-Junction Nanowire Solar Cells.

We present vertically processed photovoltaic devices based on GaInP/InP tandem-junction III-V nanowires (NWs), contacting approximately 3 million NWs in parallel for each device. The GaInP and InP subcells as well as the connecting Esaki tunnel diode are all realized within the same NW. By processing GaInP/InP tandem-junction NW solar cells with varying compositions of the top junction GaInP material, we investigate the impact of the GaInP composition on the device performance. External quantum efficiency (EQE) measurements on devices with varying GaInP composition provide insights into the performance of the respective subcells, revealing that the GaInP subcell is current-limiting for all devices. I-V measurements under AM1.5G illumination confirm voltage addition of the subcells, resulting in an open-circuit voltage of up to 1.91 V. However, the short-circuit current density is low, ranging between 0.24 and 3.44 mA/cm2, which leads to a resulting solar conversion efficiency of up to 3.60%. Our work shows a path forward toward high-efficiency NW photovoltaics and identifies critical issues that need improvement.

An Image Detection–Memory–Recognition Artificial Visual Unit Based on Dual‐Gate Phototransistors

Optoelectronic synapses integrating sensing–memory–processing functions have great advantages in neuromorphic computing for visual information processing and complex learning, recognition, and memory in an energy‐efficient manner. Herein, a light‐induced bidirectional response is demonstrated in the proposed WSe2/MoS2 junction field‐effect transistor (JFET) with an extra Ge back gate. The WSe2/MoS2 JFET exhibits high responsivity and detectivity owing to effective modulation by the top junction and back dielectric gates. The unique bidirectional photoresponse and interfacial state storage properties of the device result in significant synaptic excitatory effects under visible light stimulation and remarkable synaptic inhibitory effects under infrared illumination. Optical storage with visible light and optical erasure with infrared light can be achieved in the device based on its synaptic behavior. The conductance changes under different types of illumination can be used to mimic the weight update in a neural network for image recognition. A high accuracy exceeding 98% was achieved in handwritten digit recognition. This visible and infrared dual‐band phototransistor, which integrates image perception, memory, and recognition functionalities, provides a promising solution for self‐driving, surveillance, computer vision, and biomedical imaging applications.

Study of the mechanisms of the phonon bottleneck effect in CdSe/CdS core/shell quantum dots and nanoplatelets and their application in hot carrier multi-junction solar cells.

The hot carrier multi-junction solar cell (HCMJSC) is one of the promising advanced conceptual solar cells with theoretical efficiency greater than 65%, consisting of a thin top junction with a wide bandgap and a thicker junction at the bottom with a medium bandgap for absorption of high and low energy photons. The wide bandgap CdSe/CdS low-dimensional systems (e.g. quantum dots, QDs and nanoplatelets, NPLs) widely used in optoelectrical devices are expected to be an appropriate candidate for the top junction. However, the mechanisms underlying the carrier relaxation rate reduction (or phonon bottleneck effect, PBE) for HCMJSC in these material systems are not well understood so far. In this work, the carrier relaxation mechanisms in CdSe/CdS core/shell QDs and NPLs are quantitatively analyzed by calculating the thermalization coefficient (Qth) through steady state photoluminescence (SSPL) and picosecond-time resolved photoluminescence (ps-TRPL). A significantly extended carrier relaxation time of more than 20 ns was observed in the TRPL of QDs. This could be attributed to both the Auger reheating (AR) at the initial fast decay stage and acoustic phonon folding at the slow decay stage. For SSPL, the Qth value of QDs is much lower due to a 1 order of magnitude higher AR rate. A strong coupling may exist between AR and Qth with a high probability of PBE, where a lower Qth gives a higher AR rate. The AR may dominate carrier thermalization if the PBE level is high. Meanwhile, other mechanisms like acoustic phonon folding will also affect the carrier relaxation if the PBE is at a much lower level.

Determination of individual I(V) characteristics of each sub-cell of a triple junction device

Very high conversion efficiency is reached with triple junction solar devices integrated in concentrator photovoltaic (CPV) modules. However, reduction of the active area for micro-CPV applications increases the perimeter/area ratio, enhancing losses linked to the edges. It is therefore important to characterize the perimeter influence on the final conversion efficiency. For this purpose, I(V) characterization under dark and/or light could be used as a test of the sidewalls influence. We have designed an experiment to perform I(V) curves using the light of three lasers with adjustable powers at 405, 785, and 980 nm, preferentially absorbed by the top, middle or bottom junction of the device, respectively. This experiment was applied to commercial devices made from a stack of GaInP/GaAs/Ge. In parallel we have developed a numerical calculation modeling the device to reproduce the behaviors observed during I(V) experiments. Junction parameters and influence of leakage resistances are deduced from the fit of experimental results with the numerical calculation. The I(V) experiment as well as the numerical calculation are presented in details. It is also underlined that, combining both experiment and calculation, the I(V) characteristic of each junction as if it was isolated can be determined.

Improved GaInP/GaAs/GaInAs inverted metamorphic triple-junction solar cells by reduction of Zn diffusion in the top subcell

The growth of heavily doped tunnel junctions in inverted metamorphic multijunction solar cells induces a strong diffusion of Zn via a point-defects-assisted mechanism. The redistribution of Zn can compensate the n-type doping in the emitter of the GaInP top junction, degrading severely the conductivity of the whole solar cell and its conversion efficiency. This work evaluates different epitaxial growth strategies to achieve control on the Zn profile of an inverted metamorphic triple-junction structure, including: the reduction of the doping concentration in the tunnel junction to minimize the injection of point defects that trigger the diffusion mechanism; the use of different barrier layers to keep the injected point defects away from active layers and, finally, the minimization of Zn concentration in the AlGaInP back-surface-field layer of the GaInP subcell. This last approach enables a high-conductivity multijunction solar cell device without redesigning the tunnel junction as well as a high electronic quality in the GaInP subcell, which shows a collection efficiency higher than 93% and an open-circuit-voltage offset of 410 mV at 1 sun irradiance. The characterization of final triple-junction devices, including quantum efficiency, electroluminescence, and light current-density-voltage curves at different irradiances, demonstrates a successful integration of all the subcell and tunnel junction components. This way, final solar cells with peak efficiencies exceeding 40% at 500 suns are demonstrated, despite using doping levels in the AlGaInP:Zn back-surface-field of the GaInP subcell and using non-optimized antireflective coatings.

Switchable edge-line coupler based on parity time-reversal duality symmetry

A compact broadband Edge-Line Coupler (ELC) based on Parity Time-reversal Duality (PTD) symmetry has been conceived, designed, constructed and measured. The coupler connects four PTD bifilar edge lines (BELs), recently introduced by the authors. The PTD-BELs are constituted by a parallel plate waveguide whose walls are formed by a junction between Perfect Electric Conductor (PEC) and Perfect Magnetic Conductor (PMC) boundary conditions. Reversing the axis orthogonal to the plates interchanges the position of PEC and PMC. Such a waveguide supports unimodal transverse electromagnetic (TEM) propagation, extremely confined along the top and bottom junction edges; its propagation is protected against backscattering from any discontinuity that preserves the PTD symmetry. The ELC presented here is constituted by a 4-port junction in which each port is intrinsically matched due to the PTD symmetry, strongly coupled with a second port, strongly decoupled with a third port, and weakly coupled with a fourth port. The ELC is designed by using a mushroom metasurface for the PMC portion of the device; the connection is based on a switch circuit which imposes open and short conditions on the two opposite sides of the structure. Switching simultaneously the open and short circuits reroutes the signal in a different port, while maintaining the same level of coupling with the other ports. A static prototype has been built and its measurements have confirmed the matching performance and the good directionality of the coupler in a broadband frequency range between 24 and 30 GHz.

Comparison of the time resolution of unirradiated and irradiated LGADs and 3D sensors

Collider experiments as the upcoming Phase II-LHC or the future circular collider (FCC) will increase the demands on detectors used for tracking. In the FCC, sensors will not only face fluences of up to 1×1017neq/cm2, but also high pile-up scenarios. Therefore, sensors that not only have a good spatial resolution and a very high radiation hardness, but also an excellent time resolution of 5ps will be required. Currently, Low Gain Avalanche Diodes (LGADs), which have an additional gain layer to achieve fast signals through charge multiplication, are one of the main candidates when it comes to timing, reaching a resolution of below 30ps. However, their radiation hardness is not sufficient for future colliders. A second important candidate for timing are 3D sensors, which are known to be extremely radiation hard. In 3D sensors, the electrodes are etched into the sensor from the top (junction columns) and from the back (ohmic columns), resulting in short drift distances, low depletion voltages, a high electric field and therefore, fast signals. In this study, the time resolution of both LGADs and 3D sensors was investigated with MIP-like signals generated by a beta-source, as well as measurements using a laser with an infrared wavelength. It was found that LGADs and 3D sensors have compatible time resolutions. Transient current technique (TCT) timing measurements allow a position-resolved study of the time resolution. This is especially interesting for the 3D sensors, where the more complex electric field structure influences the time resolution strongly. This can be observed in the position dependent time resolution measurements. Additionally, the timing performance of 3D sensors before and after irradiation with reactor neutrons will be discussed.

FEM Modelling of Weld Damage in Continuous Cold Rolling of MIG/MAG Butt-Welded Stainless Steel Strips

Weld line fracture of butt-welded strips in stainless steel continuous rolling has been studied by numerical simulation using ForgeNxT®. The simulation plan includes weld line geometry as well as weld metal constitutive model parameters. The damage criterion used is the non-dimensional Latham & Cockroft function near the singular points of the weld line. Results are confronted to qualitative observations of fracture frequency and fracture initiation loci on the rolling line. Not surprisingly, the protrusion height of the weld line with respect to the strip top surface is found to be a major factor of risk. A second one pertains to the nature of the alloys used: due to the mushroom-like weld line cross-section geometry, damage at the top surface junction of the base metal and the weld metal becomes critical when the weld metal is harder than the base metal. Finally, on the rolling line investigated, prior to rolling properly speaking, the strip goes through a scale-breaking unit followed by acid pickling to eliminate oxides; the multiple, reverse plastic bending applied there contributes significantly to ductile damage, completing the explanation of why the fracture initiation locus is always found to be the same on the rolling line.

Hot-carrier multi-junction solar cells: A synergistic approach

Conventional single-junction solar cells have a theoretical efficiency limit around 33%, and multi-junction solar cells (MJSCs) are currently the only technology to overcome this limit. The demonstration of hot-carrier solar cells (HCSCs), another high-efficiency approach that relies on harvesting the kinetic energy of the photo-generated carriers, has so far been hindered due to the difficulty of mitigating carriers' thermalization. In this letter, we highlight the synergies of these two concepts by introducing the hot-carrier multi-junction solar cell (HCMJSC), a MJSC with a thin hot-carrier top junction. Using a detailed balance model, we compare the efficiency of different devices as a function of three parameters: the bandgap of the top and bottom junctions, the top cell thickness, and an effective thermalization coefficient, which encapsulates information on both thermalization and light trapping. Besides allowing for a much broader range of material combinations than MJSCs, we show that HCMJSCs can reach efficiencies higher than MJSCs with a larger thermalization coefficient than HCSCs. As such, HCMJSCs could provide a preferred route toward the development of hot-carrier-based high efficiency devices.

Perpendicular magnetic anisotropy and its voltage control in MgO/CoFeB/Mo/CoFeB/MgO junctions

We study the perpendicular magnetic anisotropy (PMA) in the MgO/CoFeB (CFB)/MgO junctions with an angstrom-thick Mo spacer layer separating the CFB layer. Perpendicularly magnetized CFB/Mo/CFB films are achieved for a wide range of CFB thicknesses, and a large PMA energy density of mJ m−2 is demonstrated by tuning the thickness ratio of the two CFB layers as well as the thickness of the Mo spacer layer. The PMA in the MgO/CFB/Mo/CFB/MgO is controlled by a voltage applied across the junction, and a sign inversion in the voltage-controlled magnetic anisotropy effect is clearly observed between the ‘top free’ and ‘bottom free’ magnetic tunnel junctions, in which the CFB/Mo/CFB layers are fabricated on top and bottom of an MgO barrier layer, respectively. Nanostructural analyses reveal the difference in the morphology of the top free and bottom free magnetic tunnel junctions and also suggest that the flatness of CFB/MgO interface is rather important for improving the efficiency of the voltage-controlled magnetic anisotropy.

Engineering Surface Architectures for Improved Durability in III–V Photocathodes

GaInP2 has shown promise as the wide bandgap top junction in tandem absorber photoelectrochemical (PEC) water splitting devices. Among previously reported dual-junction PEC devices with a GaInP2 top cell, those with the highest performance incorporate an AlInP2 window layer (WL) to reduce surface recombination and a thin GaInP2 capping layer (CL) to protect the WL from corrosion in electrolytes. However, the stability of these III-V systems is limited, and durability continues to be a major challenge broadly in the field of PEC water splitting. This work provides a systematic investigation into the durability of GaInP2 systems, examining the impacts of the window layer and capping layer among single junction pn-GaInP2 photocathodes coated with an MoS2 catalytic and protective layer. The photocathode with both a CL and WL demonstrates the highest PEC performance and longest lifetime, producing a significant current for >125 h. In situ optical imaging and post-test characterization illustrate the progression of macroscopic degradation and chemical state. The surface architecture combining an MoS2 catalyst, CL, and WL can be translated to dual-junction PEC devices with GaInP2 or other III-V top junctions to enable more efficient and stable PEC systems.

Failure Mode Determination of American Petroleum Institute 12F Shop-Welded Tanks With a New Roof-to-Shell Junction Detail

Abstract The American Petroleum Institute (API) 12F is the specification for vertical, aboveground shop-welded storage tanks published by the API. The nominal capacity for the twelve tank designs given in the current 13th edition of API 12F ranges from 90 bbl. (14.3 m3) to 1000 bbl. (159 m3). The minimum required component thickness and design pressure levels are also provided in the latest edition. This study is a part of a series research project sponsored by API that dedicates to ensure the safe operation of API 12 series storage tanks. In this study, the twelve API 12F tank designs presented in the latest edition are studied. The elastic stress analysis was conducted following the procedures presented in the ASME Boiler and Pressure Vessel Code 2019, Section VIII, Division 2 (ASME VIII-2). The stress levels at the top, bottom, and cleanout junctions subject to the design pressures are determined through finite element analysis (FEA). The bottom uplift subjected to design pressures are obtained, and the yielding pressure at the roof-shell and shell-bottom junctions are also determined. The specific gravity of the stored liquid is raised from 1.0 to 1.2 in this study. A new roof-shell attachment detail is proposed, and a 0.01 in. (0.254 mm) gap between the bottom shell course and the bottom plate is modeled to simulate the actual construction details. In addition, the flat-top rectangular cleanout presented in the current edition of API 12F is modeled. Moreover, various thickness combinations are considered to further investigate the effect due to different component thicknesses.

Simulation of double-junction III-phosphides/silicon solar cells

The characteristics of two-junction solar cells were calculated in this work, in which phosphides of group III are used as the top junction, and Si as the bottom junction. The following ternary phosphide compounds Ga0.52In0.48P (1.85 eV), GaPN0.02 (1.9 eV), and GaPN0.04 (1.7 eV) were considered as the active material of the top junction. The use of GaPN0.04 allowed one to reach the current matching with the bottom Si junction and an efficiency of 30% was achieved. In addition, the influence of the layer thickness, the lifetime of minority charge carriers and the electronic affinity on the efficiency of solar energy conversion were considered.

Junction Field‐Effect Transistors Based on PdSe2/MoS2 Heterostructures for Photodetectors Showing High Responsivity and Detectivity

Abstract2D materials have shown great promise for next‐generation high‐performance photodetectors. However, the performance of photodetectors based on 2D materials is generally limited by the tradeoff between photoresponsivity and photodetectivity. Here, a novel junction field‐effect transistor (JFET) photodetector consisting of a PdSe2 gate and MoS2 channel is constructed to realize high responsivity and high detectivity through effective modulation of top junction gate and back gate. The JFET exhibits high carrier mobility of 213 cm2 V−1 s−1. What is more, the high responsivity of 6 × 102 A W−1, as well as the high detectivity of 1011 Jones, are achieved simultaneously through the dual‐gate modulation. The high performance is attributed to the modulation of the depletion region by the dual‐gate, which can effectively suppress the dark current and enhance the photocurrent, thereby realizing high detectivity and responsivity. The JFET photodetector provides a new approach to realize photodetectors with high responsivity and detectivity.

Low-temperature dopant activation using nanosecond ultra-violet laser annealing for monolithic 3D integration

Nanosecond ultra-violet (UV) laser annealing was introduced to overcome the thermal degradation of the bottom layer device during top layer device junction formation in monolithic 3D integration. A simulation was performed to predict the thermal effects and the temperature distribution of the top and bottom layers during laser annealing. The dopant distribution, surface morphology, and crystallinity were characterized after laser annealing. The electrical properties were identified from the sheet resistance and current-voltage characteristics. A single crystal silicon phase was observed after applying top-hat shaped single pulse UV laser with the optimal fluence of 1.4 J/cm2. With a much lower thermal budget, the laser-annealed junction achieved an abrupt doping profile and low sheet resistance and low leakage current that were comparable to rapid thermal annealing or solid-phase epitaxial regrowth.

Fatigue Evaluation of API 12F Shop-Welded Tanks With a New Roof-to-Shell Junction Detail

Abstract The American Petroleum Institute (API) provides a series of standards and specifications on storage tanks, in which the API 12F specification provides 12 tank designs that can be fabricated in the shop and transported to the field. The nominal capacity of the 12 API 12F tank designs ranges from 90 bbl (14.3 m3) to 1000 bbl (158.99 m3). The minimum required thickness and operational pressure levels that each tank case can sustain are given in Table 1 of the current 13th edition of API 12F (API, 2019, “API 12F Specification for Shop-Welded Tanks for Storage of Production Liquids,” 13th ed., API Publishing Services, Washington, DC, Standard No. API 12F). The objective of this study is to estimate the fatigue life of API 12F tanks under normal operation pressure cycles following the procedure presented in ASME VIII-2. The stored liquid product specific gravity is assumed to be 1.2 when the liquid height is half of the tank height, while the specific gravity is assumed to be 0.7 when the stored liquid height is 18 in. (460 mm). Meanwhile, a new roof–shell attachment detail is proposed in this study, the new rectangular cleanout junction detail presented in the 13th edition of API 12F is modeled, and various component thickness combinations are considered to investigate the effect of component thickness on fatigue life. The roof–shell joint (top junction) and shell–bottom plate (bottom junction) are studied by axisymmetric models under axisymmetric idealization as they are away from the cleanout junction, while the cleanout junction is studied by applying a submodeling technique. Stress classification is performed at each location of interest to obtain the stress components to calculate the stress range within each loading cycle that is needed to perform fatigue evaluation. The results and discussion about fatigue evaluation of API 12F tanks are presented in this report.

Wide spectral coverage (0.7–2.2 eV) lattice‐matched multijunction solar cells based on AlGaInP, AlGaAs and GaInNAsSb materials

AbstractWe report on the progress in developing lattice‐matched GaAs‐based solar cells with focus on developing AlGaInP, AlGaAs, and GaInNAsSb materials, aiming at achieving a wide spectral coverage, that is, 0.7–2.2 eV. To this end, we first benchmark the performance of an upright four‐junction GaInP/GaAs/GaInNAsSb/GaInNAsSb solar cells grown by molecular beam epitaxy on p‐GaAs substrates with bandgaps of 1.88, 1.42, 1.17, and 0.93 eV, respectively. The four‐junction cell exhibited an efficiency of ~39% at 560‐sun illumination while showing good electrical performance even up to 1000 suns. As a first step to further improve the efficiency toward 50% level, we demonstrate AlGaInP (>2 eV) and GaInNAsSb (<0.8 eV) subcells. We prove that AlGaInP cells with 0.1 Al composition would exhibit current‐matching condition when being incorporated in a five‐junction architecture together with two GaInNAsSb bottom and AlGaAs top junctions. Furthermore, current matching required for a six‐junction tandem architecture is achieved for an Al composition of 0.26. Overall, the results open a practical path toward fabrication of lattice‐matched solar cells with more than four junctions.

Time-Gated and Multi-Junction SPADs in Standard 65 nm CMOS Technology

SPADs (Single-Photon Avalanche Diodes) are important detectors for a wide range of applications including positron emission tomography, Raman spectroscopy, light detection and ranging, and quantum key distribution. For some applications, custom image sensor technologies are used, but at a higher cost and lower performance imagers when compared to implementation in a standard planar CMOS technology. In this paper, we explore time-gating and multi-junction techniques to improve the SPAD's performance in smaller standard planar CMOS processes to take advantage of their potential for monolithic integration with other advanced, mixed-signal circuitry for simple, low-cost, high-performance imaging solutions. A passively quenched, unbuffered, triple-junction SPAD structure was designed in a standard 65 nm CMOS process from TSMC. The characterization of the SPAD junctions in this process is the first in literature and proves useful for SPAD designers aiming for advanced CMOS technology nodes. The time-gated (TG) pixel design used the top shallow junction. The potential for improved photon detection efficiency and wavelength distinction through a multi-junction design was investigated. Our testing demonstrated that the proposed implementation of the triple-junction SPAD in this technology node is not suitable for wavelength distinction. The TG design achieved a fill-factor of 28.6%, and at an excess voltage of 300 mV, it achieved a peak photon detection efficiency of ~2.1% at 440 nm, <; 1% afterpulsing probability for hold-off times >22 ns, and <; 200 ps timing jitter.

A Dual-Bit Spin-Based Analog-to-Digital Converter With Double-Check Estimation

This letter presents a three-bit magnetic analog-to-digital converter (MADC) based on the spin Hall effect, which exploits the spin current flowing at both sides of a heavy metal (HM) layer. The shape of the HM can be adjusted to produce multiple switching thresholds. After the input signal is injected, the magnetization of both free layers in the top and bottom magnetic tunnel junctions (MTJs) can be switched simultaneously. The converted results could be quantified by the states of the corresponding MTJs. In order to improve the accuracy and reliability of quantization, a multilayer structure with a double-check estimation method inspired by or gate operation is designed. This dual-bit MADC can reach up to 667 MHz sampling rate with only 0.17 pJ energy consumption, which is well suited for nonvolatile computing based on magnetic devices.