Number Of Subcells Research Articles

Three-periodic 1D photonic crystals based on dielectric oxides SiO2, Al2O3, TiO2, ZrO2: an emerging class of structures with wide possibilities for applied photonics

One-dimensional three-periodic photonic-crystal (PC) structures based on dielectric nonmagnetic materials (SiO2, Al2O3, TiO2, ZrO2) that form [(ab)N(cd)M]K supercells including “internal” subcells of the (ab)N and (cd)M types are considered. The classification of this class of structures makes it possible to distinguish three large groups of subcells that differ in optical contrast values (low-, medium-, and high-contrast). Using the transfer matrix method, frequency-angle spectra and energy characteristics of the PC structures are studied and optimal combinations of layers (and contrast) are found, enabling obtaining controllable photonic band gaps (PBGs). A generalization of the obtained results shows that in the transparent region, the optical and energy properties of structures with high and medium optical contrast are of the greatest practical interest. It is found that the group of medium-contrast structures has a greater flexibility in the formation and rearrangement of the PBG due to a practically equivalent effect of the optical properties of both cells on the transmission spectrum. An analysis of the spatial-angular distribution of the transverse component of the Umov-Poynting vector and variation in the number of subcells (internal periods) shows that the structure of this group can provide a controlled radiation pattern (at a telecommunication wavelength of 1.55 μm) on the lateral surface of the PC. The findings may give a competitive advantage in the development of polarization-sensitive couplers, highly sensitive angle sensors for fiber-optic systems, and optical filters operating in the infrared range.

A precise method for the spectral adjustment of LED and multi‐light source solar simulators

AbstractSolar simulators based on light‐emitting diodes (LEDs) usually consist of many spectrally different LEDs, which in sum produce a sun‐like spectrum. On the one hand, this results in the advantage of a high spectral tunability of these systems and on the other hand, however, also in the challenge of a high number of parameters which have to be set for the adjustment of a suitable simulator spectrum. Multijunction solar cells consisting of series‐connected subcells are very sensitive to spectral irradiance conditions, which are affecting the current and the fill factor of the device. A precise adjustment of the simulator spectrum based on the spectral responsivity of the subcells is therefore essential for accurate multijunction measurements. Therefore, the number of spectrally different light sources used should be at least as high as the number of subcells in the device under test. However, for the measurement of multijunction devices, the much higher number of spectrally different light sources in common LED solar simulators results in a plethora of different simulator spectra, potentially suitable for the measurement. Furthermore, the nonlinear intensity characteristics of the utilized LEDs as well as the distance‐dependent illumination uniformity of such solar simulators add complexity when aiming for a precise spectral adjustment. To tackle these challenges, a new spectral adjustment procedure which is based on a least square's solution algorithm and the definition of appropriate boundary conditions for the calculation of suitable simulator settings is introduced in this publication. Focusing on measurements carried out under constant illumination makes the presented method especially applicable for perovskite‐on‐silicon multijunction devices. Therefore, an adapted method for the determination of the solar simulator's spectral properties, considering thermal influences which are particularly relevant when carrying out continuous illumination measurements, is introduced in this work. The presented method is verified applying it on a Wavelabs SINUS 220 LED solar simulator by performing a measurement comparison on a multijunction solar cell with Fraunhofer ISE CalLab's well‐established multilight source solar simulator.

Effectiveness of multi-junction cells in near-field thermophotovoltaic devices considering additional losses

Abstract Thermophotovoltaic (TPV) energy converters hold substantial potential in converting thermal radiation from high-temperature emitters into electrical energy through photovoltaic (PV) cells, offering applications ranging from solar energy harvesting to waste heat recovery. Near-field TPV (NF-TPV) devices, focused on enhancing power output density (POD), exhibit unique potential by harnessing photon tunneling. However, this potential can be mitigated by additional losses arising from high photocurrent densities and corresponding scalability issues. This study comprehensively investigates the effectiveness of multi-junction-based NF-TPV devices, accounting for additional losses. We propose two approximative expressions to quantify the impact of additional losses and characterize current density-voltage curves. Verification against rigorously optimized results establishes a criterion for effective performance. Our method provides precise POD estimations even for devices with 10 or more subcells, facilitating performance analysis across parameters like vacuum gap distance, cell width, emitter temperature, and the number of subcells compared to far-field counterparts. This research outlines a roadmap for the scalable design of NF-TPV devices, emphasizing the role of multi-junction PV cells. The analytical framework we developed will provide vital insights for future high-performance TPV devices.

Origins of the short circuit current of a current mismatched multijunction photovoltaic cell considering subcell reverse breakdown.

In the photovoltaic community, short circuit current (Isc) of a current mismatched multijunction photovoltaic (MJPV) cell was usually thought to be limited by the lowest subcell photocurrent (Imin). However, under certain conditions for multijunction solar cells, Isc≠Imin was observed by researchers, while this effect has not been studied in multijunction laser power converters (MJLPCs). In this work, we provide an in-depth analysis of the formation mechanisms for the Isc of the MJPV cell by measuring I-V curves of the GaAs and InGaAs LPCs with different number of subcells and simulating the I-V curves with the reverse breakdown of each subcell considered. It is found that Isc of an N-junction PV cell can be theoretically equal to any current value within a range from a current lower than Imin to the maximum subcell photocurrent, which is up to the number of subcell current steps in the forward biased I-V curve. An MJPV cell with a constant Imin will demonstrate a higher Isc if it has more subcells, smaller subcell reverse breakdown voltage and smaller series resistance. As a result, Isc tends to be limited by the photocurrent of a subcell closer to the middle cell and is less sensitive to the optical wavelength than Imin. This should be another possible reason why the measured EQE of a multijunction LPC exhibits a wider spectrum width than the calculated Imin-based EQE, whereas this was usually attributed to the luminescent coupling effect merely.

Numerical and experimental analysis of the stiffness and band-gap properties of shell structures with periodically variable cross sections

This paper describes one-dimensional periodic shell structures that have variable cross sections, a new type of periodic shell structures made from photopolymer. This paper will discuss the stiffness of periodic sub-cells that have variable cross sections and the band gaps of Bragg scattering shell structures based on numerical analysis and a series of experiments. This paper uses the Bloch theorem and lumped-mass method to create a band gap model for periodic shell structures. In this paper, an equivalent stiffness model for sub-cells is also created based on the principle of superposition and validated by experiments. Numerical studies and experiments are conducted to examine the effects of geometrical parameters, number of sub-cells, and stiffness of sub-cells on band gaps of one-dimensional periodic shell structures and to test the effectiveness of the models. The findings in this paper prove that by varying the stiffness of sub-cells under a fixed lattice constant, band gaps of one-dimensional periodic shell structures can be decreased. The findings also confirmed that the initial band gap of one-dimensional periodic shell structures can be lowered by increasing the number of sub-cells in a period. Unlike other types of Bragg scattering periodic structures, one-dimensional periodic shell structures allow their longitudinal band gaps to be adjusted under a fixed lattice constant. Those findings serve as a theoretical foundation for the application of Bragg scattering periodic shell structures in low-frequency vibration.

Modeling of superelastic auxetic structures of Ti–Zr base alloy

The superelastic properties of a novel shape memory Ti–24Zr–10Nb–2Sn alloy were evaluated in two different auxetic geometrics embedded in an airfoil by the nonlinear finite element software Abaqus. In the first part, a constitutive model was developed to determine the phenomenology and mechanical response of shape memory alloy in a superelastic state. In the second part, a numerical model of re-entrant and chiral auxetic structures was developed. The beam modeling was selected due to its efficiency in contrast to the shell and 3D solid elements. The chiral structure (simple AAS and complex AAC) is accommodated within the airfoil providing flexibility and ability to withstand loads. A larger displacement with limited strain of the airfoil was observed in both conditions. Numerical results of auxetic structural elements were consistent with experimental results. This study explores the superelastic capability of novel shape memory alloys embedded with auxetic core lattice as an alternative for soft morphing airfoil. • Finite element modeling of auxetic structures were developed to simulate superelastic capacity of Ti–24Zr alloy. • Two-dimensional model of the Eppler 420 airfoil structure was developed and evaluated for external loading conditions. • A constitutive model was developed to determine the mechanical response of shape memory alloy in a superelastic state. • The maximum stress is proportional to the number of sub-cells in auxetic structures. • A larger displacement with limited strain of the airfoil was observed in simple AAS and complex AAC chiral structures.

Effective properties of 0–3, 1–3, and 2–2 composites based on unified unit-cell micromechanics model

This paper presents a micromechanics method, dubbed unified unit-cell model, to predict the effective elastic constants of 0–3, 1–3, and 2–2 composites. Considering periodic composites, a representative volume element can be identified as a unit cell which is further uniquely divided into only four subcells. This novel configuration of a unit cell achieves concurrently modeling of 0–3, 1–3 and 2–2 composites by employing the minimum number of subcells, in comparison with existing unit cell-based micromechanics models. Effective properties are formulated by the use of concentration-factor tensors that are determined via the micromechanical relations based on the continuity conditions among the unit cell and its subcells. The presented formulation retains explicit predictions for the effective properties. Numerical results for particulate, fibrous, and laminated composites are presented to demonstrate the comparability of the presented model with the Mori-Tanaka model, and the models are compared in light of existing experimental data of 0–3 composites.

Linking a phase field model for polymer crystallization to full-field micromechanical simulations of semi-crystalline polymers

An enhanced phase field model recently proposed by the authors enables to model polymer crystallization and to predict spherulite growth and representative semi-crystalline micro-structures in 2D or 3D. After solidification, the value of the phase field variable in each subcell (2D pixel or 3D voxel) indicates whether the material is amorphous or crystalline. In the present paper, full-field micromechanical simulations are conducted on the microstructures generated by the enhanced phase field model in order to predict the effective mechanical properties. A Fast Fourier Transform (FFT) method with periodic boundary conditions is used. Care is taken to obtain representative volume elements (RVEs) by computing the number of subcells in each spherulite and the number of spherulite nucleations in each RVE. Numerical FFT predictions of elastic properties for a range of crystallinity ratios are validated against experimental data and compared to simpler composite inclusion models. Finite element simulations of thermal shrinkage are also shown. The approach was numerically implemented in a research version of the Digimat software.

Strain Dependent Performance Analysis of InGaN Multi-junction Solar Cell

This study reports the strain-dependent efficiency of the InGaN-based multi-junction solar cell (MJSC) for the first time. The route of strain in MJSC is identified to be the results of dissimilar lattice constants between layers of sub-cell grown epitaxially with bandgap stepping. Utilising multi-layered strain model, the state of strain and its magnitude is evaluated for three types of MJSC structures referred to as MJSC-1, MJSC-2, and MJSC-3. It is found that the MJSC position-dependent strain is strongly dependent on the sub-cell thickness as well as on the number of sub-cells. Employing the MJSC position-dependent strain in combination with deformation potentials, strain-induced energy bandgap is calculated when imposed under tensile strain condition. Finally, the strain-dependent efficiencies of different MJSC structures are estimated and obtained to be lower with that of reported with strain effects over sighted. The loss of efficiency is identified to be due to the open circuit voltage which decreases under tensile strain condition. Among the MJSC structures studied here, MJSC-3 with 7-layers is less efficient and its efficiency decreases up to 3.01% when strain effect is taken into consideration.

Automated simulation of voxel-based microstructures based on enhanced finite cell approach

A new and efficient method is proposed for the decomposition of finite elements into finite subcells, which are used to obtain an integration scheme allowing to analyse complex microstructure morphologies in regular finite element discretizations. Since the geometry data of reconstructed microstructures are often given as voxel data, it is reasonable to exploit the special properties of the given data when constructing the subcells, i.e. the perpendicularly cornered shape of the constituent interfaces at the microscale. Thus, in order to obtain a more efficient integration scheme, the proposed method aims to construct a significantly reduced number of subcells by aggregating as much voxels as possible to larger cuboids. The resulting methods are analysed and compared with the conventional Octree algorithm. Eventually, the proposed optimal decomposition method is used for a virtual tension test on a reconstructed three-dimensional microstructure of a dual-phase steel, which is afterwards compared to real experimental data.

RETRACTED ARTICLE: Investigation of rotated local Gabor features in face recognition using fusion techniques

One of the major challenges related to face recognition is the effective face representation. In this paper, a new rotation algorithm is proposed to make the slanting face to upright position. Gabor features have been identified to be effective for face recognition. To investigate the prospective of Gabor phase and its combination with magnitude for face recognition, Rotated Local Gabor XOR patterns (RLGXP), which encodes the Rotated Gabor phase by using the local XOR pattern (LXP) operator is first proposed, which increases the effectiveness of face representation. Then, Cell-based Fishers linear discriminant (CFLD) to reduce the dimensionality of the proposed descriptor is introduced. Finally, by using CFLD, local patterns of Rotated Gabor magnitude and phase for face recognition are combined. This proposed approach is evaluated on MIT, GTAVE, PIE, CMU and Home databases. Moreover, to enhance the effectiveness of face representation through feature design (RLGXP), the Gabor filter parameters like mask size, scale and orientation are very important. The effectiveness of the rotating local Gabor model + CFLD method was verified for upslopes and uprights of different data sets. The analysis of face feature representation using the RLGXP + CFLD method basically depends on the phase range, the appropriate choice of the number of sub-cells per pattern map and size of neasret region. Here, the sensitivity analysis used for the above three parameters is best selected and executed. This paper also investigates the effect of different Gabor filter parameters on face recognition accuracy. Parameter Selection Investigation result proves that Gabor wavelets comprising 5 scales and 8 orientations have been chosen to extract Facial Feature Points (FFP). The final result shows that Rotated Local Gabor Features provides the best face recognition accuracy.

Role of fiber arrangement in the thermal expanding behavior of unidirectional metal matrix composites

The influence of fiber arrangement on thermal expanding behavior of unidirectional fiber-reinforced metal matrix composites (MMCs) is evaluated using a unit cell micromechanical approach. Both regular and random fiber arrangements are considered in the MMC simulation by extending the number of sub-cells of the representative volume element (RVE). Moreover, the unit cell micromechanical model is used to investigate the role of fiber cross-section on the coefficients of thermal expansion (CTEs) of unidirectional MMCs. The validity of the model is verified by comparing the predictions and those available experimental measurements. The effective axial CTEs are not dependent on the MMC microstructure with different types of fiber arrays and cross-sections. However, the results show that besides the volume fraction, shape and the arrangement type of fibers plays a significant role in the effective transverse CTEs of MMCs. The discrepancy between the transverse predictions of the model with a random fiber arrangement and those of the model with a regular fiber arrangement increases with the rise of difference between the constituent material properties. Also, the effects of different statistical distributions of the random fiber arrangement, the number of RVE sub-cells and fiber orientation on the MMC thermal expanding response are examined.

Fine-Tuning of Multijunction Solar Cells: An In-Depth Evaluation

Multijunction (MJ) solar cells are currently seen as the most promising technology toward achieving incomparable solar to electricity conversion efficiency, largely surpassing the best conventional single-junction solar cells. It was recently shown that futuristic cell architectures encompassing increasing number of subcells will likely show increased sensitivity to the spectral distribution of sunlight, giving rise to dramatic variations in the energy output of MJ-based Photovoltaic (PV) systems from one location to the other. Here, we investigate the extent to which MJ cells fine-tuned to the spectral distribution of any particular site are likely to outperform conventionally designed solar cells (tailored to AM1.5 reference solar spectrum). We first tackle the question of the extent to which fine-tuning may improve the performance of MJ cells showing deviations in the values of the main atmospheric parameters, relative to the reference solar spectrum. We then evaluate the potential of fine-tuning in improving the energy output of MJ cell-based PV systems for several locations selected for being representative of the broad diversity of atmospheric and climatic conditions. We demonstrate that the improvement in the energy output one can expect from fine-tuned PV cells strongly depends on how the mean annual spectrum deviates from the reference AM1.5 solar spectrum, with values ranging from $\approx$ 0 to more than 30%, depending on the location and the cell architecture considered. Implications for future generations of MJ solar cells are finally discussed.

Improved efficiency of GaSb solar cells using an Al0.50Ga0.50As0.04Sb0.96 window layer

The increasing number of subcells in multi-junction structures imposes the use of narrow bandgaps for a full harvesting of the solar spectrum. In this context, gallium antimonide (GaSb) and its lattice-matched alloys offer a great potential. To date, GaSb-based multi-junction solar cells exhibit experimental results below theoretical expectations, due to a limiting GaSb subcell. In this paper, a new design of GaSb cells comprising an Al0.50Ga0.50As0.04Sb0.96 window layer is studied. With this design, an excellent conversion efficiency of 7.2% is achieved under 1 sun (AM1.5G) illumination. This performance is attributed to a significant improvement in short-circuit current.

Resource Allocation and Performance Analysis of Cellular-Assisted OFDMA Device-to-Device Communications

Resource allocation of cellular-assisted device-to-device (D2D) communication is very challenging when frequency reuse is considered among multiple D2D pairs within a cell, as intense inter D2D interference is difficult to tackle and generally causes extremely large signaling overhead for channel state information (CSI) acquisition. In this paper, a novel resource allocation framework for cellular-assisted D2D communication is developed with low signaling overhead while maintaining high system capacity. By utilizing the spatial dispersion property of the D2D pairs, a geography-based sub-cell division strategy is proposed to divide the cell into multiple sub-cells and the D2D pairs within one sub-cell are formed into one group. Then, sub-cell resource allocation is performed independently among the sub-cells without the need of any prior knowledge of inter D2D interference. Under the proposed resource allocation framework, a tractable approximation for the inter D2D interference modeling is obtained and a computationally efficient expression for the average ergodic sum capacity of the cell is derived. The expression further allows us to obtain the optimal number of sub-cells, which is an important parameter for maximizing the average ergodic sum capacity of the cell. It is shown that with small CSI feedback, the system capacity can be improved significantly by adopting the proposed resource allocation framework, especially in dense D2D deployed systems.

Multijunction photovoltaic converters for information and power transmission systems

Characteristics of heterostructure PV converters under ultra-high high power frequency modulated laser beams have been modeled and investigated. Dynamic properties of GaAs based multijunction cells have been studied, and estimations of efficiencies for such devices were carried out. It has been established that high efficiency values for conversion of modulated radiation can be obtained only at a minor depth of modulation (±3−3.5 % of the average power). And for converters comprising 3-6-junction, limitations for the acceptable modulation depth are stricter. The achievable bandwidth exceeds 300 MHz at modulation “up” with respect to the maximum power point, and at modulation “down” is more than 500 MHz in the range of laser power of 30-45 dBm. The number of subcells in the multijunction structure does not affect the limiting indicators of fast operation and efficiency, but shifts the point of their achievement towards the higher laser radiation powers. It has been shown that the influence of the photocurrent mismatch among photoactive subcells results in deterioration of both efficiency and fast operation parameters. So, at the mismatch of 5 % the bandwidth appears to be lower by about 6 % compared to single-junction converters operating under modulated laser radiation, and the efficiency − by 1.5 % correspondingly.

Series spreading resistance in single- and multi-junction concentrator solar cells

The series resistance of concentrator solar cells is determined by the processes of current spreading under the contact grid. The influence of the number of subcells in multi-junction solar cells on these processes is considered in the paper. Using the tube model of current spreading, it is shown that with the increase in the number of subcells, the current distribution of tubes becomes more uniform. This affects the shape of the IV-curve of the spreading resistance in such a way that the magnitude of the resistive losses at the maximum power point decreases.

In-plane crashworthiness of bio-inspired hierarchical honeycombs

Biological tissues like bone, wood, and sponge possess hierarchical cellular topologies, which are lightweight and feature an excellent energy absorption capability. Here we present a system of bio-inspired hierarchical honeycomb structures based on hexagonal, Kagome, and triangular tessellations. The hierarchical designs and a reference regular honeycomb configuration are subjected to simulated in-plane impact using the nonlinear finite element code LS-DYNA. The numerical simulation results show that the triangular hierarchical honeycomb provides the best performance compared to the other two hierarchical honeycombs, and features more than twice the energy absorbed by the regular honeycomb under similar loading conditions. We also propose a parametric study correlating the microstructure parameters (hierarchical length ratio r and the number of sub cells N) to the energy absorption capacity of these hierarchical honeycombs. The triangular hierarchical honeycomb with N = 2 and r = 1/8 shows the highest energy absorption capacity among all the investigated cases, and this configuration could be employed as a benchmark for the design of future safety protective systems.

Discrete‐level memristive circuits for HTM‐based spatiotemporal data classification system

The authors propose a discrete-level memristive memory design for analogue data processing in hardware implementations of hierarchical temporal memory (HTM). In this study, memristors were set to ternary and quaternary states in a sub-cell by application of different write voltage levels through a resistive network configuration. Simulations of the proposed circuit show that the highest number of discrete output levels of the memory was achieved using quaternary logic. However overall, using the same number of sub-cells and ternary logic exhibits the lowest relative error rate. For data classification purposes, the proposed discrete-level memristive cells are incorporated into the TM of HTM architecture, and its hardware circuit is presented for pattern recognition. They report improved results of face recognition using AR, ORL and UFI databases, and TIMIT database for speech recognition. These results are compared with the earlier design of HTM having only the spatial pooler (SP). Accuracy of the HTM architecture incorporating both SP and TM with discrete-level memristive cells for face recognition increased from 76.5 to 83.5% for AR database and speech recognition accuracy is improved from 73.3 to 93.3%.

Too Many Junctions? A Case Study of Multijunction Thin‐Film Silicon Solar Cells

AbstractThe benefit of two‐terminal multijunction solar cells in regard to the number of junctions (subcells) is critically evaluated. The optical and electrical losses inherent in the construction of multijunction cells are analyzed using information from thin‐film silicon photovoltaics as a representative case. Although the multijunction approach generally reduces the thermalization and nonabsorption losses, several types of losses rise with the number of subcells. Optical reflection and parasitic absorption are slightly increased by adding supporting layers and interfaces. The output voltages decline because of the tunnel recombination junctions, and more importantly of the illumination filtered and reduced by the top subcell(s). The loss mechanisms consume the potential gains in efficiency of multijunction cells. For thin‐film silicon, the triple‐junction is confirmed to be the best performing structure. More generally, only when each component subcell shows a high ratio between the output voltage and the bandgap of the absorber material, a multijunction cell with a large number of subcells can be beneficial. Finally, the high voltage and low current density of multijunction cells with a large number of subcells make them difficult to optimize and manufacture, vulnerable to any changes in the solar spectrum, and thus less practical for the ordinary terrestrial applications.