Front Grid Research Articles

The Solar Aspect System of the Hard X-ray Imager Onboard ASO-S

The ASO-S/HXI is a bi-grids modulating instrument for solar hard X-ray imaging, whose collimator contains 91 pairs of tungsten grids. Since the solar disk is invisible in hard X-rays, a Solar Aspect System (SAS) is required to provide the pointing of hard X-ray imager (HXI) for locating X-ray sources on the solar disk. In addition, the knowledge of the alignment and relative twist of the corresponding front–rear grid pairs is important for image reconstruction as well as locating flares. Therefore, the SAS system was designed to monitor the alignment status of HXI grids and to provide the pointing direction of the HXI collimator with two subsystems DM and SA during the whole life cycle of HXI. DM measures the centroids of the front frosted glasses and the solar disk. SA images the Sun and provides precise relative locations of the solar disk center. Both work in the visible light of 565–585 nm. With all the data together, we can solve with an inversion algorithm the alignment status of the front and rear grids, the relative twist, and the pointing direction. We tested and validated the SAS design with both the simulation model and ground coordinate measuring machine. Here we present the detailed system design, the testing results, the inversion algorithm, and the in-orbit status of the SAS. Currently, the SAS has realized the rotational measurement accuracy of about 4 arcsec, and a translational measurement accuracy of about 15 μm, and the SAS pointing data has been used in both imaging calibration for flare locations and imaging corrections for the platform drifting effect. The high-cadence precise measurement (better than 0.3 arcsec) of the pointing will allow the study of source locations at different energies and therefore help us to understand electron acceleration and transportation in flares.

High‐Throughput Indirect Gravure Printing Applied to Low‐Temperature Metallization of Silicon‐Based High‐Efficiency Solar Cells

Herein, this work is dedicated to a both exotic and promising printing technique in silicon (Si) photovoltaics (PV). Indirect gravure printing is investigated as a metallization technique focusing on low‐temperature applications, such as Si heterojunction (SHJ) and perovskite Si tandem solar cells. One advantage of this rotary printing technique is that its components are very durable. From graphical industry, it is known that gravure cylinders show almost no wearing during multiple printing. Additionally, rotary printing techniques offer great throughput. Therefore, indirect gravure printing can reduce costs significantly in PV production. In this work, short process cycle times of 0.9 s cell−1 for industrial wafers (edge length of 156.75 mm) are demonstrated. Using an appropriate production platform even enables cycle times of down to 0.5 s cell−1. Busbarless SHJ half cells with non‐optimized gravure‐printed front grids reach a mean photoconversion efficiency that is (1.7 ± 0.5) %abs lower compared to screen‐printed reference samples, however, cycle time is reduced. The metal contacts exhibit a mean shading width of (65 ± 16) μm and an average height of (5 ± 1) μm. Applied to SHJ solar cells’ rear, such metallization can replace screen‐printed contacts with 0.1%abs efficiency loss only, as device simulations reveal.

Highly conductive and transparent metal microfiber networks as front electrodes of flexible thin-film photovoltaics

Simultaneously enhancing the optical transmittance and electrical conductivity of transparent conductors (TCs) applicable in various optoelectronic devices is a long-standing challenge. Herein, we present an innovative approach for fabricating electroplated Ni and Cu microfiber networks (NiMF and CuMF) as highly conductive TCs to realize high efficiency and desired aesthetics in thin-film solar cells and modules. The metal microfibers (MFs) are fabricated using electrospun polyacrylonitrile nanofibers as the template. The large cross-sectional aspect ratio of the metal MF networks remarkably and concurrently improves their electrical conductivity and optical transmittance. Between the NiMF and CuMF TCs, the CuMF sample exhibits a superior figure of merit owing to its exceptionally low electrical resistivity. The metal MF TC is a promising alternative to conventional patterned grids used in flexible Cu(In,Ga)Se2 thin-film solar cells, because it effectively reduces the series resistance, which is advantageous for large-area cells. The CuMF can be successfully employed as a ribbon to maintain the solar cell performance in centimeter-scale cells connected in series. The outstanding performance of the metal MF TCs indicates their potential to eliminate complicated monolithic integration processes or front grids and ribbons in flexible thin-film solar cells and modules.

Experiment on evacuation behavior: Applying the sector grid model to a clear area

Researching personnel evacuation is crucial for preventing and reducing emergencies. In this study, the movement and retention laws relating to personnel in building bottlenecks are derived from research on safe evacuation from building bottlenecks and personnel evacuation tests. Accordingly, a fan-grid partitioning method is proposed for building bottlenecks. Because the personnel distribution in the area is fan-shaped, a dynamic grid-division method is proposed and applied to the safe evacuation simulation model. Additionally, a dislocation arrangement is utilized for the front and rear grids, and the experimental analysis reveals that the time step in the safety evacuation simulation. Moreover, a grid partition model is proposed, and the evacuation parameters of the evacuation area bottleneck are characterized via the human flow rate sequence. The Visual Basic language can be utilized in the safe evacuation simulation algorithm for building bottleneck, which has great significance in guiding research on the safe evacuation of personnel during fire hazards.

Contribution of the photoluminescence effect of the stain etched porous silicon in improvement of screen printed silicon solar cell performance

In this work, we investigate the potential use of porous silicon (PS) nanostructures stain etched into photovoltaic technology as one possible way to increase the silicon solar cell (SC) efficiency at low cost. The process is based on a combination of texturization and porous silicon formed by metal assisted etching which used the screen printed front grid contact. The photoluminescence (PL), the reflectivity spectra of the PS layers, spectral response and electrical measurements are presented and discussed. As a result, the short circuit current density was improved by more than 21%. We observed an increase in internal quantum efficiency (iQE) in the region where the PL of the PS layers appears and the minority carriers’ diffusion length is enhanced by more than 60 . We report the correlation between the converter PL intensity and iQE and we demonstrate that the photoluminescence properties and the increased photocurrent result in an improvement of the solar cell efficiency.

Effectiveness of the front and rear grids as a result of silicon solar cell metallization patterns: A study using Griddler simulator

The objective of this investigation is to evaluate the effect of metallization patterns on the front and rear grids on the efficiency of the silicon heterojunction (SHJ) cells. Griddler 2.5 PRO simulator was utilized to analyze the effects of various front grid metallization designs and geometries on open circuit voltage (VOC), short circuit density (JSC), fill factor (FF), and efficiency (ƞ) of silicon-based PV cells. Griddler 2.5 PRO is designed to assess different cell types, and gain a better understanding of the limiting factors that influence solar cell characteristics in laboratory and field settings. The finite-element method (FEM) is applied in solar cell modeling to approximate the cell plane as a network of resistors and diodes. It also features an interface for generating H-patterns and back metal grids. The simulations varied the number of busbars used on the front side metal grids of solar cells from 1 to 5 and the number of metal fingers used for grid pattern optimization from 80 to 130, with finger widths ranging from 10 to 60 µm. For optimization of efficiency and fill factor, various styles (straight, rectangular pad tapered, round pad, digital, two split, and three split) and shapes (straight, pointed, digital, apollo, and wine bottle) were explored. The front and rear contact resistances were kept constant during these simulations to obtain the best fill factor and efficiency. In this model, with 110 fingers (the finger sheet resistance of front and rear sides is kept at 3 mΩ/□) and four busbars, with two split style and straight shapes and a finger width of 25 µm, was designed to achieve maximum performance of the solar cell. Various recombination loss factors in the cells are also analyzed. The results indicate that this is one of the most efficient silicon solar cells modeled, with a fill factor and efficiency of 80 % and 19.55 %, respectively, which is noteworthy for a planar solar cell.

High aspect ratio triangular front contacts for solar cells fabricated by string‐printing

AbstractWe are presenting a novel method to fabricate high aspect ratio, triangular cross‐section solar cell front contacts, henceforth referred to as string‐printing. We optimized string‐printing to yield contacts with an aspect ratio larger than 1 and a light redirection efficiency or effective transparency of 67%, thereby mitigating most of the optical losses inherent to flat metallic front grids. In string‐printing, a string coated with silver paste approaches a substrate until contact is made. Withdrawing the string then leaves behind silver paste on the substrate. Here, we describe the fabrication method and show initial results including current density‐voltage curves of string‐printed silicon heterojunction solar cells, as well as the effective transparencies of the contacts. String‐printing is a scalable, low‐temperature process with high potential to boost commercial solar cell efficiency and lower the module price per Watt.

A Pareto Front grid guided multi-objective evolutionary algorithm

For multi-objective optimization problems with irregular Pareto Fronts, most widely used decomposition methods in MOEA/D (multi-objective evolutionary algorithms based on decomposition) have shown to be lack of the ability to balance the diversity and the convergence to track the true Pareto Fronts during the search. This research investigates the recently proposed grid-based decomposition methods, which reflect the inherent characteristics of the neighborhood structure in the solution to address the issues of diversity and convergence. The performance of the grid-based decomposition method, however, depends on the size of its grid segmentation, and its time complexity increases with the number of grids. In order to improve the computational efficiency, we propose a new concept of Pareto Front grid to guide the search in MOEA. In order to reduce the computing time, a new nadir point estimation strategy based on statistical analysis has been proposed to estimate the whole population. In addition, based on the idea of knee point, a novel grid-based knee point selection method is proposed in the environmental selection of the next generation. Finally, a grid-based decomposition multi-objective evolutionary algorithm with Pareto Front Grid (PFG-MOEA) is proposed. Extensive experimental analysis demonstrates the effectiveness of the proposed PFG-MOEA against state-of-the-art multi-objective evolution algorithms. As the extension of the CDG-MOEA (Constrained Decomposition approach with Grids MOEA) algorithm in the literature, PFG-MOEA can obtain better performance by consuming much less computing time.

Design and characterization of multijunction photovoltaic laser power converters for nonuniform irradiance light profiles

AbstractNonuniform irradiance profiles of lasers used in power‐by‐light systems deteriorate the efficiency of photovoltaic laser power converters. We analyze three approaches to deal with this efficiency loss: (1) to design the power converter front grid for the nonuniform light received from the power‐by‐light system; (2) to strengthen the peak current and decrease the series resistance of tunnel junctions; and (3) to homogenize the light spot impinging the power converter by simply distancing it from the laser fiber. In the context of these approaches, this paper tackles the optimization of multijunction photovoltaic laser power converters to achieve their maximum efficiency under nonuniform irradiance profiles, without using any optical element to homogenize them and, this way, reproducing realistic operating conditions. As a direct application of the design shown in this paper, we have increased the experimental efficiency of our GaAs devices by up to 3% absolute for a nominal laser wavelength of 857 nm.

FlexTrail Printing as Direct Metallization with Low Silver Consumption for Silicon Heterojunction Solar Cells: Evaluation of Solar Cell and Module Performance

FlexTrail printing has been invented and developed for (fine‐line) printing of various fluids, e.g., particle‐based metal‐containing fluids, etchants, and liquid‐phase pyrophoric media. Compared to other printing techniques, FlexTrail is highly independent of the fluids’ viscosity. Using this printing approach, feature sizes of 10 μm and below are reached. This work utilizes FlexTrail as a direct metallization method for printing of silver‐nanoparticle‐based front electrodes on busbarless silicon heterojunction (SHJ) solar cells. Thereby, only (9.4 ± 0.9) mg of silver is consumed for printing of a busbarless front grid, which exhibits 80 contact fingers of 156 mm in length. This means a silver reduction of more than 60% compared to screen printing. Solar cells with M2+ wafer size and FlexTrail‐printed front grids reach conversion efficiencies of up to (22.87 ± 0.01)%, which is similar to screen‐printed reference cells. To further demonstrate the practicability of FlexTrail metallization beyond cell level, a FlexTrail‐printed SHJ cell is further processed into a 200 mm × 200 mm‐sized one‐cell module applying SmartWire Connection Technology for interconnection. This module exhibits a maximum power of (5.0 ± 0.1) W, underlining the great potential of FlexTrail printing for the metallization of high‐power SHJ devices with significant silver reduction.

Comprehensive analysis of photon dynamics in thin-film GaAs solar cells with planar and textured rear mirrors

This study describes a comprehensive analysis of light trapping and photon recycling (PR) in thin-film GaAs solar cells, explicitly considering the effects of (non-perfect) light scattering at a textured rear mirror. We calculate the probabilities of escape, reabsorption in the absorber and parasitic absorption for internally generated photons, as well as the reflectance, absorptance and parasitic absorptance of externally incident photons. Combined with the internal luminescent efficiency to account for non-radiative recombination non-explicitly, but in a phenomenological manner, device performance metrics such as the short-circuit current density ( J s c ), open-circuit voltage ( V o c ) and its radiative limit, the external luminescent efficiency, the contribution of PR to the V o c and the power conversion efficiency ( P C E ) are derived. We analyze a typical thin-film GaAs solar cell architecture, consisting of an n-GaAs/p-InGaP heterojunction structure cladded by commonly employed front- and rear-side layer stacks. The probabilities of escape, reabsorption and parasitic absorption and the resulting performance metrics are presented as a function of absorber thickness, rear-mirror reflectivity, haze factor, internal luminescent efficiency, front grid coverage and band tailing. The simulations show that the implementation of a textured rear mirror reduces PR and its importance for device performance. In these textured cells, a high rear-mirror reflectivity and, thereby, efficient PR, is less crucial to achieve V o c values close to the radiative limit. The increased J s c in textured cells renders light trapping favorable for the P C E , regardless of absorber thickness and material quality and despite reduced PR. Furthermore, the impact of an absorbing front grid and lateral transport of internal luminescence on PR is estimated, emphasizing the importance of transparent (non-absorbing) contact layers, at the rear, but importantly, also at the front of the device. To conclude, P C E limits of 300-nm-thick GaAs solar cells are deduced in the context of an experimentally realized light-trapping scheme based on a randomly textured rear mirror. P C E s exceeding 25.5% are deemed realistic. • Photon recycling and light trapping are studied in textured thin-film GaAs cells • Ray-optical modeling yields probabilities of escape and (parasitic) absorption • Textured rear mirrors reduce photon recycling and its impact on cell performance • Light trapping improves efficiency for any absorber thickness and material quality • Efficiency limits of 300-nm GaAs cells are currently 25.5% and ultimately ¿28%

A Grid Fusion Lifting Surface and Its Flow Control Mechanism at High Angles of Attack

A grid fusion lifting surface suitable for large subsonic angles of attack was designed. The influence of the grid position, grid diversion angle, and grid number on the aerodynamic performance of the grid fusion lifting surface under subsonic conditions was studied by numerical simulations. Based on the summary and analysis of the grid in the hypersonic state, the CFD numerical simulation method, which solves unsteady Reynolds averaged Navier-Stokes equations, was used to complete the research of the flow mechanism of the grid fin in the subsonic state. At low speed and subsonic conditions, the results show that the negative front grid fusion lift surface has better aerodynamic characteristics with a high angle of attack. Under subsonic conditions, the stall angle of attack of the front grid fusion lifting surface with a diversion angle of 20° increases by 16°, and the maximum lift coefficient increases by 22.1% compared to that of the conventional flat wing. The number of grids has an insignificant effect on the aerodynamic performance of the grid fusion lifting surface at a large angle of attack.

Losses in bifacial PERC solar cell due to rear grid design and scope of improvement

Photovoltaic (PV) industries are continuously trying to improve the module performance with the adoption of newer cell technologies. Recently, bifacial passivated emitter rear contact (PERC) solar cell technology has entered the market to further improve the module performance through the absorption of the extra photons reflected from the background. This work is mainly focused on optimizing the rear side grid pattern of the bifacial PERC solar cells for improvement of the total output power gain. We have developed an analytical model based on the front and rear grid pattern of commercially available bifacial PERC solar cells to calculate the optical shading and electrical losses for both sides of the solar cells. It is perceived that the rear shading loss due to rear grid design of industrial bifacial PERC solar cells is very high (∼30%) as compared to the front side (∼5%) of the cells. The model calculation shows that if with the improved laser scribing technology the finger width of the rear grid pattern be drastically reduced from ∼ 200 μm (measured value) to 25 μm the optical shading loss may be minimized to 11–14% from 30% without compromising the effective busbar resistance.

GaAs thermophotovoltaic patterned dielectric back contact devices with improved sub-bandgap reflectance

We demonstrate GaAs thermophotovoltaic (TPV) devices with a patterned dielectric back contact (PDBC) architecture, featuring a dielectric spacer between the semiconductor and back metal contact over most of the back surface for high reflectance, and metal point contacts over a smaller area for electrical conduction. In the TPV application, high sub-bandgap reflectance is needed to reflect unused sub-bandgap photons to the thermal emitter to minimize energy losses in this portion of the thermal spectrum. We explore different PDBC fabrication processes with SU-8 and SiO 2 dielectric spacer layers to maximize sub-bandgap reflectance while minimizing series resistance to increase TPV conversion efficiency. We successfully demonstrate GaAs SU-8 PDBC TPV devices with 2200 °C blackbody-weighted sub-bandgap reflectance of 94.9% and 96.5% with and without a front metal grid, respectively. This is 0.7% and 2.3% (absolute) higher than the mean sub-bandgap reflectance of 94.2% for GaAs baseline TPV devices with 100% Au back contact with front metal grid. Lower sub-bandgap reflectance in TPV devices with front grids indicates the front grid induces light scattering leading to additional parasitic absorption in the TPV device. We also show that for higher contact coverage fractions, the PDBC reflectance cannot in general be treated by a linear interpolation using simple 1D transfer matrix method modeling and should be treated instead as a diffraction grating by solving Maxwell's equations in 3D. • We explore different patterned dielectric back contact (PDBC) fabrication processes with SU-8 and SiO 2 for TPV cells. • Sub-bandgap reflectance over 96% demonstrated using dielectric-metal point contact rear mirror structure, a gain of >1%. • PDBC structures should be modeled as a 3-dimensional diffraction grating by solving Maxwell's equations. • Front grids increase diffuse reflectance thus increasing absorption in the device.

Impact of multi-busbar front grid patterns on the performance of industrial type c-Si solar cell

Solar cell performance is highly dependent upon the front contact grid design for minimizing the power losses due to shading (optical loss) and for proper collection of the photo-generated charge carriers (electrical loss). In this paper, theoretical calculations (optimization) have been carried out for the total power losses (viz. optical and electrical) due to different front grid multi-busbar patterns similar to the ones currently used in industrial production. Busbar width and finger spacing, the two important design parameters of solar cell with standard busbar structure, are optimized for multi busbar systems. Role of interlinks between the fingers to reduce the power loss has also been studied. The effect of each optimized grid design on the component in the levelized cost of electricity (LCOE) due to Silver (Ag) requirement has been estimated for a 10 MW power system with a 25 years lifespan. The study of EM field distribution due to incident photon flux shows that increasing the number of busbars can generate carriers within the shaded areas under the busbars.

Greatly enhanced hole collection of MoO x with top sub-10 nm thick silver films for gridless and flexible crystalline silicon heterojunction solar cells.

Greatly enhanced hole collection of MoOx is demonstrated experimentally with a top sub-10 nm thick Ag film, allowing for an efficient dopant-free contacted crystalline silicon (c-Si) heterojunction solar cell without a front grid electrode. With the removal of shadows induced by the front grid electrode, the gridless solar cell with the MoOx/Ag hole-selective contact (HSC) shows an increment of ∼8% in its power conversion efficiency (PCE) due to the greatly improved short-circuit current density (Jsc) as well as the almost undiminished fill factor (FF) and open-circuit voltage (Voc), while the gridless solar cells with the conventional MoOx/ITO and pure MoOx HSCs exhibit ∼20% and ∼43% degradations in PCE due to the overwhelming decrease in their FF and Jsc, respectively. Through systematic characterizations and analyses, it is found that the ultrathin Ag film (more conductive than ITO) provides an additional channel for photogenerated holes to transport on MoOx, contributing to the great enhancement in the hole collection and the great suppression of the shunt loss in the gridless solar cells. A 50 μm thick gridless c-Si heterojunction solar cell with the MoOx/Ag HSC is 75% thinner but is 86% efficient compared to its 200 μm thick counterpart (while the 50 μm thick gridless solar cell with the MoOx/ITO HSC is much less efficient). It is over 82% efficient after being bent to a curvature radius as small as 4 mm, also showing superior mechanical flexibility to its counterpart with the MoOx/ITO HSC. Our MoOx/Ag double-layer HSC can be easily fabricated through thermal evaporation without breaking the vacuum, saving both the time and cost of the fabrication of the whole device. Therefore, this work provides a guide for the design of efficient HSCs for high-efficiency, low-cost, and flexible solar cells.

Automated Temporal Tracking of Coherently Evolving Density Fronts in Numerical Models

AbstractOceanic density fronts can evolve, be advected, or propagate as gravity currents. Frontal evolution studies require methods to temporally track evolving density fronts. We present an automated method to temporally track these fronts from numerical model solutions. First, at all time steps contiguous density fronts are detected using an edge detection algorithm. A front event, defined as a set of sequential-in-time fronts representing a single time-evolving front, is then identified. At time stepi, a front is compared to each front at time stepi+ 1 to determine if the two fronts are matched. Anifront grid point is trackable if the minimum distance to thei+ 1 front falls within a range. Theifront is forward matched to thei+ 1 front when a sufficient number of grid points are trackable and the front moves onshore. A front event is obtained by forward tracking a front for multiple time steps. Within an event, the times that a grid point can be tracked is its connectivity and a pruning algorithm using a connectivity cutoff is applied to extract only the coherently evolving components. This tracking method is applied to a realistic 3-month San Diego Bight model solution yielding 81 front events with duration ≥ 7 h, allowing analyses of front event properties including occurrence frequency and propagation velocity. Sensitivity tests for the method’s parameters support that this method can be straightforwardly adapted to track evolving fronts of many types in other regions from both models and observations.

Trapezoidal grid fingers to reduce shadowing loss and improve short circuit current

Traditional metallic contacts on solar cells can cover a substantial portion of the device area, resulting in shadowing of light that diminishes the amount of current collected. In this paper, we developed trapezoidal electron-beam evaporated Ti/Al front grid fingers on GaAs single-junction solar cells to decrease the optical shadowing and increase the photogenerated current. We observe a high fill factor of 87% at 1-Sun light intensity and we observe no degradation on the open-circuit voltage. The short-circuit current density is found to increase by 0.4 mA/cm2 when compared to previously demonstrated electroplated Ni/Au front grid fingers, resulting in an efficiency improvement of ~0.8% absolute.

High-accuracy twist measurement based on the spherical wave Talbot effect for a bi-grid modulation collimator.

The bi-grid modulation collimator is a significant way for imaging solar flares in hard x rays. It implements many subcollimators that consist of separated grid pairs (so-called front grid and rear grid) whose line orientations are parallel. However, when the twist of the front grid with respect to the other will be induced during testing of the bi-grid modulation collimator in the ground verification phase, the line orientation of the grid pairs are no longer parallel. Knowledge of the relative twist between the rear grid and the front grid is very helpful in improving the imaging quality of the bi-grid collimator. However, because of the wide spacing between grid pairs and the requirement of high measurement accuracy, it is a challenge to measure the relative twist. To meet this demand, a method based on the spherical wave Talbot effect is proposed. The Talbot images of the front grid and the rear grid are imaged on the same plane, respectively, through two proper spherical waves. The relative twist can be figured out through the angle between the stripes in the Talbot images of the front grid and the rear grid. In experiments, the measurement accuracy of the relative twist angle can reach 9 arcsec in the range of 370 arcsec. It demonstrates that this method can effectively measure the relative twist between the grid pairs with very high accuracy.

Assessing the Impact of Front Grid Metallization Pattern on the Performance of BSF Silicon Solar Cells

Assessing the Impact of Front Grid Metallization Pattern on the Performance of BSF Silicon Solar Cells