Triple-junction Configuration Research Articles

Hydrogen-assisted intergranular fatigue crack initiation in metals: Role of grain boundaries and triple junctions

In this work, small-scale, low-cycle fatigue experiments on hydrogen charged nickel specimens are performed that highlight particular grain boundaries (GBs) and triple junctions as potential intergranular crack initiation sites. To understand the micromechanics and underlying physics, a dislocation density-based crystal plasticity model coupled with slip-rate based hydrogen transport model is developed. A fatigue indicator parameter (FIP) is also developed that models the crack initiation process by considering the contributions of accumulated plastic slip, GB normal stress, and local hydrogen concentration. Depending on the diffusivity, hydrogen binding energy, and misorientation, GBs are categorized as ‘special’ or ‘random’, and their role on hydrogen distribution is analysed using a model microstructure. Special GBs are ones with low diffusivity and high hydrogen binding energy whereas the random GBs have high diffusivity but low hydrogen binding energy. Complying with the experimental observations, the evolution of FIP with load cycles suggests certain triple junction configurations in the microstructure involving in the crack initiation process. For the case of uniform initial hydrogen concentration, special GBs are found to retain more hydrogen with load cycles primarily due to their low hydrogen diffusivity whereas the random GBs diffuse hydrogen out quickly to the bulk. The high hydrogen concentration and favourable stress state in the form of high hydrostatic stress spots generally found at triple junction of special/random GBs fulfil the necessary condition leading to an intergranular crack initiation.

Photovoltaic Partner Selection for High-Efficiency Photovoltaic-Electrolytic Water Splitting Systems: Brief Review and Perspective

Photovoltaic-electrolysis water splitting (PV-EWS) is the most promising approach for high solar-to-hydrogen (STH) efficiency. The present PV-EWS systems achieve the highest STH performance by using a III-V triple-junction configuration, which, however, involves a complex and expensive manufacture process. Therefore, in this work, we demonstrate a III–V double junction device that can be used as an alternative to the III–V triple-junction device for high STH conversion efficiency of the PV-EWS systems. We estimate the STH performance via coupling world-recorded multi-junction photovoltaic (PV) and our experimented cell configurations with an EWS system. The results show that the III–V double junction, owing to the good trade-off between the efficiency loss and compensation, exhibits a higher STH efficiency than the III–V triple-junction. Furthermore, strategies for improving the efficiency of the III–V double junction device for low-cost PV-EWS system are discussed.

Performance analysis of cesium formamidinium lead mixed halide based perovskite solar cell with MoOx as hole transport material via SCAPS-1D

Cesium formamidinium lead mixed halide (FA0.83Cs0.17PbI1.5Br1.5) perovskite solar cells (PSCs) have gained enormous attention being a top cell of perovskite-perovskite tandem cells and solar cells with triple-junction due to wide bandgap, prolonged diffusion length, and tremendous charge carrier mobility. But the performances of FA0.83Cs0.17PbI1.5Br1.5 based PSC are limited by the large valance band offset of conventional hole transport material (HTM). Molybdenum oxide (MoOx) can be an appropriate HTM in this regard as it exhibits a low valance band offset with FA0.83Cs0.17PbI1.5Br1.5 perovskite. In our work, we have simulated an n-i-p structured FA0.83Cs0.17PbI1.5Br1.5 based PSC using MoOx as HTM via SCAPS-1D. Investigating the impacts of perovskite layer thickness, defect density, hole transport layer thickness, back-contact work function, and interface defects, we find an encouraging efficiency of ∼20.1 %. The analysis reveals that FA0.83Cs0.17PbI1.5Br1.5 based solar cell using MoOx as HTM is compatible to be a benign top cell and may contribute to design a highly efficient perovskite-perovskite tandem or triple-junction configuration.

Diminished band discontinuity at the p/i interface of narrow-gap a-SiGe:H solar cell by hydrogenated amorphous silicon oxide buffer layer

We optimized hydrogenated amorphous silicon oxide (a-SiOx:H) as a buffer layer at the p/i interface of hydrogenated amorphous silicon germanium (a-SiGe:H) solar cells. This buffer layer was intended to effectively diminish the band discontinuity between the high-bandgap p-window layer and low-gap a-SiGe:H absorption layer, thereby enhancing the cell performance. The open-circuit voltage (Voc) increased by 3.7% as the [CO2]/[SiH4] gas ratio increased from 0 to 0.06. In addition, the short-circuit current density (Jsc) gradually increased with the gas ratio. Using the optimized a-SiOx:H buffer layer, a high performance of 9.6% was recorded for narrow-gap a-SiGe:H thin film solar cells. a-SiGe:H solar cells with optimized a-SiOx:H buffer layers, used as middle subcells, are expected to enhance the total Voc of triple-junction configuration solar cells.

The Geological Evolution of the Aden‐Owen‐Carlsberg Triple Junction (NW Indian Ocean) Since the Late Miocene

AbstractThe Aden‐Owen‐Carlsberg triple junction is the place where Arabia, Somalia, and India meet in the Arabian Sea (NW Indian Ocean). Here we present a new seismic data set crossing a key structure of the triple junction, namely, the Beautemps‐Beaupré pull‐apart basin at the southern end of the Owen Fracture Zone. The seismic data set is tied to Ocean Drilling Program Leg 117 Sites at the top of the Owen Ridge, which provides a detailed tectono‐stratigraphic framework since the Late Miocene. We show that the triple junction configuration has been disturbed by a major kinematic change at ~8 Ma and since then experienced a series of transient structural adjustments. A major structural episode is recorded at 2.4 Ma, expressed by the opening of the Beautemps‐Beaupré Basin and the uplift of its southern flank (the Beautemps‐Beaupré Ridge). This episode is coeval with the formation of the present‐day Owen Fracture Zone and must be considered as a part of a major structural reorganization of the entire India‐Arabia plate boundary up to the Makran subduction zone. This 2.4‐Myr‐old geological episode is unrelated to any significant kinematic change, leaving questions over its driving mechanism.

Highly Efficient Hybrid Polymer and Amorphous Silicon Multijunction Solar Cells with Effective Optical Management.

Highly efficient hybrid multijunction solar cells are constructed with a wide-bandgap amorphous silicon for the front subcell and a low-bandgap polymer for the back subcell. Power conversion efficiencies of 11.6% and 13.2% are achieved in tandem and triple-junction configurations, respectively. The high efficiencies are enabled by deploying effective optical management and by using photoactive materials with complementary absorption.

A generic concept to overcome bandgap limitations for designing highly efficient multi-junction photovoltaic cells.

The multi-junction concept is the most relevant approach to overcome the Shockley–Queisser limit for single-junction photovoltaic cells. The record efficiencies of several types of solar technologies are held by series-connected tandem configurations. However, the stringent current-matching criterion presents primarily a material challenge and permanently requires developing and processing novel semiconductors with desired bandgaps and thicknesses. Here we report a generic concept to alleviate this limitation. By integrating series- and parallel-interconnections into a triple-junction configuration, we find significantly relaxed material selection and current-matching constraints. To illustrate the versatile applicability of the proposed triple-junction concept, organic and organic-inorganic hybrid triple-junction solar cells are constructed by printing methods. High fill factors up to 68% without resistive losses are achieved for both organic and hybrid triple-junction devices. Series/parallel triple-junction cells with organic, as well as perovskite-based subcells may become a key technology to further advance the efficiency roadmap of the existing photovoltaic technologies.

Requirements for a GaAsBi 1 eV sub-cell in a GaAs-based multi-junction solar cell

Multi-junction solar cells achieve high efficiency by stacking sub-cells of different bandgaps (typically GaInP/GaAs/Ge) resulting in efficiencies in excess of 40%. The efficiency can be improved by introducing a 1 eV absorber into the stack, either replacing Ge in a triple-junction configuration or on top of Ge in a quad-junction configuration. GaAs0.94Bi0.06 yields a direct-gap at 1 eV with only 0.7% strain on GaAs and the feasibility of the material has been demonstrated from GaAsBi photodetector devices. The relatively high absorption coefficient of GaAsBi suggests sufficient current can be generated to match the sub-cell photocurrent from the other sub-cells of a standard multi-junction solar cell. However, minority carrier transport and background doping levels place constraints on both p/n and p-i-n diode configurations. In the possible case of short minority carrier diffusion lengths we recommend the use of a p-i-n diode, and predict the material parameters that are necessary to achieve high efficiencies in a GaInP/GaAs/GaAsBi/Ge quad-junction cell.

Variational Dynamics of Free Triple Junctions

We propose a level set framework for representing triple junctions either with or without free endpoints. For triple junctions without free endpoints, our method uses two level set functions to represent the three segments that constitute the structure. For free triple junctions, we extend our method using the free curve work of Schaeffer and Vese (J Math Imaging Vis, 1–17, 2013), Smereka (Phys D Nonlinear Phenom 138(3–4):282–301, 2000). For curves moving under length minimizing flows, it is well known that the endpoints either intersect perpendicularly to the boundary, do not intersect the boundary of the domain or the curve itself (free endpoints), or meet at triple junctions. Although many of these cases can be formulated within the level set framework, the case of free triple junctions does not appear in the literature. Therefore, the proposed free triple junction formulation completes the important curve structure representations within the level set framework. We derive an evolution equation for the dynamics of the triple junction under length and area minimizing flow. The resulting system of partial differential equations are both coupled and highly non-linear, so the system is solved numerically using the Sobolev preconditioned descent. Qualitative numerical experiments are presented on various triple junction and free triple junction configurations, as well as an example with a quadruple junction instability. Quantitative results show convergence of the preconditioned algorithm to the correct solutions.

Effects of phase fraction on phase morphology and triple junction configuration in anisotropic systems

It has been demonstrated that Herring’s anisotropic triple-junction equilibrium conditions do not predict a unique triple-junction configuration for junctions involving at least one anisotropic interface. In this paper, we minimize the total interfacial energy of a model two-dimensional anisotropic system to predict the preferred triple-junction configuration(s). Both the relative values of the interfacial energies and the phase fractions affect the energetics. Maps of the energetically preferred configuration(s) spanning a wide range of phase fraction and relative interfacial energies are presented. Parallel trends and key differences between the behaviors of the model anisotropic system and a (similarly configured) isotropic system are detailed. Lastly, we show how the approach used to predict equilibrium configurations (based on assumed interfacial energies) can be reversed to obtain relative interfacial energies for experimental systems where equilibrium has been reached but only geometrical parameters can be measured.

Equilibrium and stability of triple junctions in anisotropic systems

When a homophase or heterophase interface involves a crystalline solid, the interfacial energy is expected to depend upon the interface–plane orientation. In this paper, the equations governing equilibrium triple-junction configurations in anisotropic systems are reviewed. These equilibrium conditions were originally derived by considering the differential change in triple-junction interfacial energy associated with a differential change in triple-junction configuration and equating it to zero. However, the derived conditions do not distinguish between a triple-junction configuration that satisfies the equilibrium conditions by residing at a local energy minimum, a local energy maximum or at a saddle point. The present paper develops stability criteria for triple junctions with and without interfaces which have anisotropic energy, which can be used to determine whether triple-junction equilibrium conditions correspond to local energy minima (stable), maxima (unstable) or saddle points. For isotropic systems, there is a single solution to the triple-junction equilibrium conditions, and it is necessarily stable. For anisotropic systems, there are multiple solutions to the triple-junction equilibrium conditions, some of which may be stable, unstable or correspond to saddle points in triple-junction interfacial energy. Microstructure features and interface lengths/areas are expected to play a role in dictating the relative energies of stable configurations.

Electrical and Optical Modelling of Thin-Film Silicon Solar Cells

AbstractToday amorphous and microcrystalline silicon based solar cells use surface-textured substrates for enhancing the light absorption and buffer and graded layers in order to improve the overall performance of the cells. Tandem and triple-junction configurations are utilized to assure better use of the solar spectrum and, thus, achieve higher conversion efficiencies of the devices. Resulting structures of the solar cells are complex and computer modeling has become an essential tool for a detailed understanding and further optimization of their optical and electrical behavior.The performance limits of tandem and triple-junction silicon based solar cells are studied by simulations using the optical simulator SunShine developed at Ljubljana University and the opto-electrical simulator ASA developed at Delft University of Technology. First, both simulators were calibrated with realistic optical and electrical parameters. Then, they were used to study the required scattering properties, absorption in non-active layers, antireflective coatings, the crucial role of the wavelength selective intermediate reflector, and a careful current matching in order to indicate the way for achieving a high photocurrent, more than 15 mA/cm2 for a tandem a-Si:H/ìc-Si:H and 11 mA/cm2 for a triple-junction a-Si:H/a-SiGe:H/ìc-Si:H solar cells. By optimizing electrical properties of the layers and interfaces, for example using a p-doped a-SiC layer with a larger band gap (EG > 2 eV) and introducing buffer layers at p/i interfaces, the extraction of the charge carriers, the open-circuit voltage and the fill factor of the solar cells are improved. The potential for achieving the conversion efficiency over 15% for the a-Si:H/ìc-Si:H and 17 % for the triple-junction a-Si:H/a-SiGe:H/ìc-Si:H solar cells is demonstrated.

Higher Energy Solutions in the Theory of Phase Transitions: A Variational Approach

We establish the existence of a higher-energy solution to the vector Ginzburg–Landau equation with a triple-well potential on a bounded and smooth domain on the plane. This solution is obtained by a linking argument. In implementing this variational approach we make several considerations on the dynamics of the negative gradient flow. In particular, we use the Conley index to contruct a suitable one-dimensional invariant set. This solution has Morse index two in the nondegenerate case. We discuss its structure in connection with the so-called triple-junction configurations.

Relationship of the Central Indian Ridge segmentation with the evolution of the Rodrigues Triple Junction for the past 8 Myr

Located near 25°33′S, 70°00′E, the Rodrigues Triple Junction is the joining point of the intermediate‐spreading Southeast Indian and Central Indian Ridges with the ultraslow spreading Southwest Indian Ridge. Bathymetric data and magnetic anomalies are used to analyze the relationship between the evolution of the Central Indian Ridge segmentation and the evolution of the Rodrigues Triple Junction for the past 8 Myr. The Central Indian Ridge domain exhibits a complex morphotectonic pattern dominated by ridge‐normal and oblique bathymetric lows interpreted as the off‐axis traces of axial discontinuities. The short‐lived nontransform discontinuities as well as the segments that lengthen or shorten along the ridge axis reveal that the Central Indian Ridge segmentation is unstable near the Rodrigues Triple Junction. The combined study of the Central Indian Ridge and Southeast Indian Ridge domains shows that the triple junction evolves between two modes: a continuous mode where the Central Indian Ridge and Southeast Indian Ridge axes are joined and a discontinuous mode where the two ridge axes are offset. Owing to spreading asymmetry, and differences in axis direction or in lengthening rates of the Central Indian and Southeast Indian ridges, the continuous mode is unstable and evolves rapidly (<2 Myr) into a discontinuous mode. This last one is more stable and can evolve into a continuous mode only through the formation of a new Central Indian Ridge segment, which takes place facing the northern Southeast Indian Ridge segment. The evolution of the Rodrigues Triple Junction configuration and the evolution of the Central Indian Ridge segmentation are thus closely related.

Atomistic models of triple junctions and the origin of topological changes in microstructural evolution

ABSTRACTTriple junctions are crucial elements in microstructural evolution: for example, their mobility can be rate-limiting if lower than that of grain boundaries. However, very little is known about their atomic-level structure and properties. We studied the atomic structure of multiple-twin triple junctions in silicon, formed by the convergence of two {111} and one {221} symmetric-tilt grain boundaries. Molecular dynamics simulations with the Stillinger-Weber potential and constant-traction border conditions were performed on several triple junction configurations, obtained by different combinations of the three grain boundaries. All the configurations have a positive excess line energy, a measurable volume contraction and display regions of opposite, tensile and compressive, residual stress. Moreover, we tried to elucidate the role of triple junctions as being the seeds of theonlymicroscopic events that can lead to topological changes in the microstructure. Such events, usually dubbedT1andT2in mesoscopic models, correspond to grain switching (in the Ashby-Verrall sense) and grain-disappearance events, respectively. We present preliminary results for the atomic-scale modelling of both classes of topological events and discuss the connection between atomistic and mesoscopic modelling of microstructural evolution.

The calibration method for the mumford-shah functional

In this Note we adapt the calibration method to functionals of Mumford-Shah type, and provide a criterion (Theorem 1) to verify that a given function is energy minimizing. Among other applications, we use this criterion to show that certain triple-junction configurations are minimizing (Example 3)

Amorphous Silicon Alloy Materials and Solar Cells Near the Threshold of Microcrystallinity

AbstractOne of the most effective techniques used to obtain high quality amorphous silicon alloys is the use of hydrogen dilution during film growth. The resultant material exhibits a more ordered microstructure and gives rise to high efficiency solar cells. As the hydrogen dilution increases, however, a threshold is reached, beyond which microcrystallites begin to form rapidly. In this paper, we review some of the interesting features associated with the thin film materials obtained from various hydrogen dilutions. They include the observation of linear-like objects in the TEM micrograph, a shift of the principal Si TO band in the Raman spectrum, a sharp, low temperature peak in the H2 evolution spectrum, a shift of the wagging mode in the IR spectrum, and a narrowing of the Si (111) peak in the X-ray diffraction pattern. These spectroscopic tools have allowed us to optimize deposition conditions to near the threshold of microcrystallinity and obtain desired high quality materials. Incorporation of the improved materials into device configuration has significantly enhanced the solar cell performance. Using a spectral-splitting, triple-junction configuration, the spectral response of a typical high efficiency device spans from below 350 nm to beyond 950 nm with a peak quantum efficiency exceeding 90%; the triple stack generates a photocurrent of 27 mA/cm2. This paper describes the effect of the improved materials on various solar cell structures, including a 13% active-area, stable triple-junction device.

Propagation of the Southwest Indian Ridge at the Rodrigues Triple Junction

The analysis of multibeam bathymetric data of the Southwest Indian Ridge(SWIR) domain between the triple junction traces from 68° E to theRodrigues Triple Junction (RTJ; 70° E) reveals the evolution of thisridge since magnetic anomaly 4 (8 Ma). Image processing has been used toshow that the horizontal component of strain due to a network of normal stepfaults increases dramatically between 69°30′ E and the RTJ. Thisarea close to the RTJ is characterized by a deep graben at the foot of thetriple junction trace on the African plate and by a narrow fault-boundedridge that joins an offset of the trace on the Antarctic plate. In thatarea, spreading is primarily amagmatic and dominated by tectonic extensionprocesses. To the west of 69°30′ E, some lobate bathymetricfeatures atop of a large topographic high suggest volcanic constructions.Between 68°10′ E and 69°25′ E the southern flank of theSWIR domain is wider than the northern one and is characterized by a series of 7 en echelon bathymetric highs similar in size,shape and orientation to the one centred at 69°30′E near the present-day triple junction. Their en echelon organization along the triple junction trace on the Antarctic plate and the typical lack of conjugated parts on the northern flank show that these bathymetric highs have been shifted to the south by successive northward relocalisations of the SWIR rifting zone. This evolution results in the asymmetric spreading of the SWIR in the survey area. The off-axis bathymetric highs connect to the offsets of the triple junction trace on the Antarctic plate when the Southeast Indian Ridges lightly lengthenstoward the northwest and the triple junction is relocated to the north. We propose that the SWIR lengthens toward the northeast with two propagation modes: 1) a continuous and progressive propagation with distributed deformation in preexisting crust of the Central Indian Ridge, 2) a discontinuous propagation with focusing of the deformation in a rift zone when the triple junction migrates rapidly to the north. The modes of propagation of the SWIR are related to different localisation and distribution of strain which are in turn controlled by changes of the triple junction configurations due to propagation, recession or a symmetric spreading on the Central and Southeast Indian Ridges.

Stress singularities at triple junctions with freely sliding grains

Stress singularities at grain triple junctions due to freely sliding grain boundaries in idealized two-dimensional single-phase polycrystals have been analyzed. The shear stress is assumed to be completely relaxed along the grain boundaries while the normal stress is allowed to be fully transmitted. The singularity exponent was found to be independent of the elastic constants of the grains, and for some triple junction configurations, super-singularities (i.e., stronger than the standard −0.5 as found at the tip of a crack in homogeneous material) were obtained. Interestingly, for most geometrical configurations, the solution structure indicates a fixed mode near the triple junction, irrespective of the far-field load combination.

On the stability of triple junctions and its relation to episodicity in spreading

A convenient representation of triple junctions that involve only ridges (R) and transform faults (F) is proposed: this representation combines in a simple way information from geographic and velocity spaces. The velocity triangle provides the budget of lithospheric surface change which directly results from interactions of the three plates. A discussion of the relative positions of the triple junction with respect to the velocity triangle demonstrates that, in general, there are several triple junction configurations that are compatible with a given triangle. This discussion stresses the importance of oblique and asymmetric spreading. Pairs of triple junction configurations that are of particular interest are RRR‐RRF and RRR‐FFR. When the triple junction lies outside of the triangle, at least one of the ridges will shorten, leading to yet another type of potentially unstable configuration. The present configurations of the Bouvet, Galapagos, and Indian Ocean triple junctions are reviewed. A method is proposed to reconstruct past configurations of the Indian Ocean junction at the time of anomalies 23 and 28. The main parameters that influence triple junction evolution are the lengths of transform faults, the availability of magma and related connectivity of magma chambers and the (also related) spreading velocities. This study confirms an earlier suggestion that activity at constructive plate boundaries occurs in two preferred modes: the effusive and tectonic modes, corresponding here to RRR and to RRF‐RFF configurations respectively. These modes apparently alternate in episodes of typically 1 Ma duration. The significance of this time constant and consequences for lithospheric mechanics are briefly discussed.