Metallization Of Silicon Solar Cells Research Articles

Insight into the Contact Mechanism of Ag/Al-Si Interface for the Front-Side Metallization of TOPCon Silicon Solar Cells.

For N-type tunnel-oxide-passivated-contact silicon solar cells, optimal Ag/Al-Si contact interface is crucial to improve the efficiency. However, the specific roles of Ag and Al at the interface have not been clearly elucidated. Hence, this work delves into the sintering process of Ag/Al paste and examines the impact of the Ag/Al-Si interface structure on contact quality. By incorporating TeO2 into PbO-based Ag/Al paste, the Ag/Al-Si interface structure can be modulated. It can be found that TeO2 accelerates the sintering of Ag powder and increases Ag colloids within glass layer, while it simultaneously impedes the diffusion of molten Al. It leads to a reduced Al content near the Ag/Al-Si interface and a shorter diffusion distance of Al into Si. Notably, it can be demonstrated that the diffusion of Al in Si layer is more effective to reduce the contact resistance than the precipitation of Ag colloids. Therefore, the PbO-based Ag/Al paste, which favors Al diffusion, leads to solar cells with lower contact resistance and series resistance, higher fill factor, and superior photoelectric conversion efficiency. In brief, this work is significant for optimizing metallization of silicon solar cells and other semiconductor devices.

Screen printable copper pastes for silicon solar cells

The current work demonstrates the successful metallization of a PERC silicon solar cell with screen-printable copper (Cu) paste that is sintered at elevated temperature in air atmosphere. The existing state of the art in Silicon (Si) solar cell metallization is silver (Ag) paste; Cu cost is 1/100th the cost of Ag and has a comparable conductivity. However, Cu undergoes rapid oxidation at elevated temperatures and the high diffusion of Cu into Si restricts its usage in the metallization of silicon solar cells. In this paper, a Cu paste containing a proprietary mixture of antioxidant additives and diffusion inhibitors was used to make front gridlines on PERC cells. The Cu-printed cells were fired in an IR belt furnace with a peak temperature of 576 °C in air atmosphere. The solar cells discussed here reached an efficiency of 19%, which is 88% of the efficiency measured for a commercial Ag-printed cell. The working devices were further characterized using STEM, EDX, ToF-SIMS and long-term suns-Voc studies demonstrating an absence of Cu diffusion. Furthermore, thermally stressing the Cu-printed solar cells yielded a reduction of less than 1.9% of the initial pseudo fill factor (pFF) after 1000 h at 200 °C. The shunt resistance of cells monitored over 3 years remained within 2.5% of its initial value, which demonstrates the effectiveness of the Cu diffusion barrier.

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.

Printing technologies for silicon solar cell metallization: A comprehensive review

AbstractThis paper presents a comprehensive overview on printing technologies for metallization of solar cells. Throughout the last 30 years, flatbed screen printing has established itself as the predominant metallization process for the mass production of silicon solar cells. For this reason, we will provide a detailed review on its history, its evolution over time, and how the continuous efforts of the scientific and industrial community for further improvements revolutionized the entire PV industry. Furthermore, we will guide the reader through the physics on silicon solar cell metallization, the fundamentals on contact formation, and what type of challenges and requirements these topics create for printing technologies. The main topic of this review addresses the flatbed screen‐printing process mechanics, its different process sequences, corresponding screen technology, and the very important impact of paste rheology on the printing result. Finally, we will compare alternative upcoming printing approaches to flatbed screen printing and discuss how they might solve upcoming challenges in metallization in terms of increasing the throughput rates at an ever‐decreasing grid line width and reduced silver consumption.

Copper Screen Printed and Fired for Metallization of Silicon Solar Cells

Silicon solar cells almost exclusively include a front side metallization using silver that is deposited by screen printing and fired in belt furnaces. In 2020, the industry was consuming approximately 10% of the global supply of silver, and recent calls to dramatically increase photovoltaic production will add to the module price pressures from this precious metal. The ideal replacement for silver is copper; however, oxidation during firing and diffusion of copper into silicon has kept the industry from adopting copper. In this talk we will present results from a copper paste that was formulated to be screen printed and fired in a belt furnace without the use of an inert gas. The data presented will include the oxidation of the copper after the firing and the SiN etching performance of the paste. LIV data for the cells will be presented along with data describing the durability of the cell in an accelerated test.

Progress with screen printed metallization of silicon solar cells - Towards 20 μm line width and 20 mg silver laydown for PERC front side contacts

Within this work, we evaluate and compare different high-end screens for the fine line front side metallization of passivated emitter and rear cell (PERC) solar cells. Three types of high-end screens are considered including (i) a fine mesh screen configuration with a mesh count MC = 520 wires/inch and standard mesh angle of φ = 22.5°, (ii) a knotless screen configuration with a mesh angle of φ = 0° as well as (iii) a knotless screen configuration utilizing a rectangular mesh with different mesh count in warp and weft direction MC = 460/430 wires/inch. Using these advanced screen configurations, all PERC solar cells are metallized with a busbarless fine line front side grid at a nominal contact finger opening of 20 μm and 24 μm. All screen configurations obtained excellent results regarding finger geometry, silver laydown and electrical performance of the front side grid and the fabricated PERC solar cells. The most efficient group obtained a printed mean finger width of 21 μm with a silver laydown of only 19 mg and revealed a mean energy conversion efficiency of 22.7%. Furthermore, we investigate the geometric and electric results of the printed front side contacts in detail using advanced image analysis software and scanning electron microscopy and discuss the impact of the fine line front side metallization on the applicable cell interconnection concepts depending on the series resistance losses. • Evaluation of different high-precision screen configurations for fine line metallization of PERC solar cells. • Reduction of silver laydown to less than 20 mg on the front side while maintaining excellent I–V-results of the solar cells. • In-depth analysis of the geometric properties and electric results of the printed fine line front side contacts. • Discussion of the impact of different screen configurations on various aspects of solar cells.

Filament stretching during micro-extrusion of silver pastes enables an improved fine-line silicon solar cell metallization

The metallization of heterojunction solar cells requires a further reduction of silver consumption to lower production costs and save resources. This article presents how filament stretching of polymer-based low-temperature curing Ag pastes during micro-extrusion enables this reduction while at the same time offering a high production throughput potential. In a series of experiments the relationship between the printing velocity and the filament stretching, thus the reduction of Ag-electrode widths and Ag laydown is evaluated. Furthermore, an existing filament stretching model for the parallel dispensing process is advanced further and utilized to calculate the elongational viscosity. The stretching effect enables a reduction of the Ag-electrode width by down to Δwf = − 40%rel. depending on the nozzle diameter and paste type. The Ag laydown has been reduced from mAg,cal. = 0.84 mg per printed line to only mAg,cal. = 0.54 mg per printed Ag-electrode when 30 µm nozzle openings are used, demonstrating the promising potential of parallel dispensing technology for the metallization of silicon heterojunction solar cells.

Progress with Screen Printed Metallization of Silicon Solar Cells - Towards 20 Μm Line Width and 20 Mg Silver Laydown for Perc Front Side Contacts

Progress with Screen Printed Metallization of Silicon Solar Cells - Towards 20 Μm Line Width and 20 Mg Silver Laydown for Perc Front Side Contacts

Front side metallization of silicon solar cells – A high-speed video imaging analysis of the screen printing process

Aiming at higher cell efficiency and lower Ag consumption, research focuses on achieving smaller finger lines for silicon solar cell front side metallization. In-depth understanding of the printing process is mandatory to achieve uniform, defect free lines with high aspect ratio. An innovative high-speed video imaging setup allows for tracking of the mechanisms happening within ≈10–50 ms during the screen printing process. Two pastes differing significantly with respect to their rheological as well as wetting properties are used in this study. A paste spreading mechanism consisting of different consecutive process steps determines the final line shape. During nip contact the paste is squeezed between screen emulsion and substrate. Paste spreading further proceeds as long as the screen clings to the substrate, it occurs in a regular pattern and is most pronounced in the regions were mesh wires approach the emulsion surface. Then sudden retraction takes place during snap-off and the final line width is essentially fixed. Spreading is more pronounced for the paste exhibiting low viscosity at high shear rates and strong wetting of screen emulsion and substrate. The overall paste transfer is promoted by poor screen wetting and strong paste cohesion. These insights will stimulate further improvement of Ag paste formulation for solar cell metallization.

Studying Knotless Screen Patterns for Fine-line Screen Printing of Si-Solar Cells

Flatbed screen printing is the dominating process in industry for metallization of silicon solar cells. It offers high throughput rates, high flexibility of printing pattern, and an overall very cost-effective production compared with other printing technologies. However, the ongoing demand for an optimization of printed silver electrode shape creates an increasing challenge to paste and screen development. The recent trend of so called “knotless” screens with a screen angle of 0° offers an increased paste transfer, therefore, better electrode conductivity, which leads to an improved solar cell efficiency. The disadvantage of this screen architecture is a reduced screen lifetime and low production yield. This article presents a systematic simulation of screen pattern to investigate screen angles, which allow for an improved knotless screen architecture. Therefore, different types of wire crossings have been defined, simulated, and experimentally verified by microscope images. For example, when 14.036°, 18.435°, and 26.565° are used, knotless screen pattern openings emerge without showing any of the disadvantages of regular 0°-knotless screens. For this reason, statistical screen lifetime estimations are carried out by modeling the intersection lengths of all mesh wires and the emulsion edge across the screen opening. Furthermore, a detailed analysis on manufacturing tolerances is given, showing that the 26.565° screen angle offers the best compromise between challenges during manufacturing and potential performance in production of Si-solar cells.

Front side metallization of silicon solar cells by direct printing of molten metal

In this work, a new approach for the front side metallization of silicon solar cells is presented. Molten solder (Sn96Ag3Cu) is directly printed in a non-contact manner on solar cell precursors via StarJet technology. The StarJet technology features a pneumatically driven, heatable printhead with a reservoir of molten metal and a star-shaped silicon nozzle chip. Using this printhead, a jet of molten metal with 55 µm ± 5 µm diameter is generated and used to apply busbars as well as contact fingers on prefabricated electroplated seed layers. After deposition via StarJet, printed fingers have a minimum width of 70 µm and a mean aspect ratio of 0.94. The printed metallization is evaluated optically and electrically. Aluminum back surface field silicon solar cells with front side electroplated NiAg seed layers and StarJet metallization (busbars and fingers) show efficiencies of up to 18.1% after degradation. Solder is about 30–40 times cheaper than silver and therefore may allow cost-efficient solar cell metallization. The StarJet metallization on electroplated NiAg seed layers is fully functional and requires no additional post-processing steps. Only 6 mg of Ag per cell are consumed for the seed layer. As a proof-of-principle, a module is demonstrated, which consists of four solar cells that are metallized via StarJet.

High-throughput front and rear side metallization of silicon solar cells using rotary screen printing

Rotational screen printing (RSP) has recently attracted attention as a highly promising high-throughput alternative for the metallization of Silicon solar cells. Compared to other metallization approaches, RSP is already a well-developed printing method which has been used for various industrial applications during the last decades. The unique benefit of this technology is the ability to apply a thick film metallization combined with a high throughput. Within the present work, we will discuss the actual achievements and challenges of this approach. We show the results of Aluminium back surface solar cells with a RSP rear side metallization and a mean conversion efficiency of η = 19.4 % compared to reference solar cells with flatbed screen printed rear side metallization and a conversion efficiency of η = 19.3 %. We further investigate the properties of fine line cylinder screens used for RSP and conventional flatbed screen printing. Using RSP with a fine line cylinder screen, we printed contact fingers with a mean width down wf = 61 µm. Finally, we discuss the path to a further optimization of this highly promising approach.

Light Assisted Electrodeposition for Silicon Solar Cell Copper Metallization

Cu electroplating is of a great deal of interest for the metallization of silicon solar cells due to its comparable resistance with the current silver metallizaion, and its lower cost for both the material and the operation.[1,2] However, the notorious Cu diffusion into Si is of great concern to the devices performance degradation during the life cycle of the cells. Ni silicide and Ni metal layers have been recently reported as part of the integration scheme to enable the Cu metallizaion and greatly improved the solar cell reliability.[3,4] The formation of this Ni silicide and Ni stack structure is to provide a low-resistance contact between the silicon and metal grid, a good adhesion of metal grid, and a diffusion barrier for the Cu. Among them, the Ni silicide formation needs to be well controlled so that it is not too thick to shunt the solar cell. This paper will present the development of light assisted plating processes for the formation of nickel silicide and copper grids on silicon solar cells, including beaker scale, bench top tool, and pilot tool processing[5,6,7]. The solar cell substrates for plating comprise ~ 60 ohm/sq emitter formed by POCl diffusion, PECVD SiN passivation layer, and evaporated Al back surface field. Electrodeposition of Ni and copper was studied on 4 cm x 4 cm solar cells in small scale studies and 6 in x 6 in cells on tools. Figure 1 shows the absorption spectra of different electrolytes for deposition and the relation between the applied potential and the light intensity at different constant current densities. Nickel solution has a strong absorption at wavelength between 350 to 450 nm and between 600 to 800 nm. Two regions of operation were clearly observed with a threshold light intensity when a high intensity wide spectrum halogen light source is used for illumination during electroplating. At high illumination, when enough carriers were generated by the light, the plating occurs at the forward bias mode of the diode. As the light intensity decreases below a threshold value, the potential increases significantly and stays at a high value indicating the diode operates at reverse bias mode. The threshold light intensity is dependent on the illumination energy spectrum, solution absorbance, anode shadowing and applied current density. The learnings from beaker scale experiments were directly used to define the tool requirements and process specifications. REFERENCES D. A. Yates, US Patent 4,609,565 (1986).K. Holdermann, US patent 5,543,333 (1996).J Bartsch, A Mondon, K Bayer, C Schetter, M Horteis, and S W Glunz, J. Electrochem. Soc., 157, H942 (2010).J Bartsch, A Mondon, C Schetter, M Horteis, S W Glunz, IEEE 35th PVSC, Honolulu HI (2010).Q Huang, S S Papa Rao, K. Fisher, ECS J. Solid State Sci. Technol., 4, Q75 (2015).Q Huang, ECS J. Solid State Sci. Technol., 5, P51 (2016).Q Huang, K B Reuter, Y Zhu, V R Deline, ECS J. Solid State Sci. Technol., 5, Q24 (2016). Figure 1

134 - Early Mortality Due to Cardiovascular Disease for Patients with Myelodysplastic Syndromes (MDS) Living in Areas with Lower Cardiovascular Performance Outcomes

Rotational screen printing (RSP) has recently attracted attention as a highly promising high-throughput alternative for the metallization of Silicon solar cells. Compared to other metallization approaches, RSP is already a well-developed printing method which has been used for various industrial applications during the last decades. The unique benefit of this technology is the ability to apply a thick film metallization combined with a high throughput. Within the present work, we will discuss the actual achievements and challenges of this approach. We show the results of Aluminium back surface solar cells with a RSP rear side metallization and a mean conversion efficiency of η = 19.4 % compared to reference solar cells with flatbed screen printed rear side metallization and a conversion efficiency of η = 19.3 %. We further investigate the properties of fine line cylinder screens used for RSP and conventional flatbed screen printing. Using RSP with a fine line cylinder screen, we printed contact fingers with a mean width down wf = 61 µm. Finally, we discuss the path to a further optimization of this highly promising approach.

Impact of Wall Slip on Screen Printing of Front-Side Silver Pastes for Silicon Solar Cells

Printing of narrow electrodes with higher aspect ratio challenges the formulation of silver paste for front-side metallization of silicon solar cells. Here, we have investigated four pastes differing in solvent composition. The onset of slip, slip velocity on steel and on emulsion over mesh (EOM) have been determined using rotational rheometry. In both cases, v increases linearly with applied stress τ . Its absolute value, the slope v/τ , and the slip layer thickness $\delta _{{{slip}}}$ vary in a wide range, depending on paste composition. Silver deposit, residual silver on the screen, the geometry of the printed electrodes, and the connections between busbar and fingers have been examined using gravimetric methods and optical microscopy. The silver deposit systematically increases, and accordingly, the residual silver on the screen decreases with increasing slip velocity. The busbar/fingers connection area narrows, whereas height and cross-sectional area of the electrodes increase with slip velocity. However, electrode width seems to be related to the pastes’ yield stress. Mass deposit, shape of the busbar/finger connection, and electrode geometry are supposed to have an important impact on cell parameters, e.g., short-circuit current and fill factor. Accordingly, our results suggest that the slip behavior of the paste affects the overall cell efficiency.

Flexo-printed Busbarless Solar Cells for Multi-wire Interconnection

Flexographic printing is a high-throughput rotational relief printing method and represents a promising innovative approach for the front side metallization of silicon solar cells. The ability to realize narrow contact fingers with very low silver consumption makes this technology particularly interesting for busbarless solar cells with wire-interconnection like Meyer Burger's SmartWire Connection Technology (SWCT). This study intends to provide a comprehensive evaluation of the general feasibility for this approach. Fundamental printing tests are carried out to investigate important influence parameters like the flexible printing plate and the correlation between printing pressure and contact finger width. We found out that contact finger elements on elastomer printing plates down to an actual width of wf = 7 ± 2 μm can be realized using a high-resolution laser-engraving process. Furthermore, we show that printed contact finger width increases by approx. Δwf ≈ 10 μm for each Δploc = 0.05 MPa of pressure increase and thus has a strong impact on the contact grid. Busbarless Al BSF Cz-Si solar cells with flexo-printed front side metallization are fabricated and measured using Pasan GridTOUCH/SpotLIGHT-system. The best group of solar cells obtains a mean conversion efficiency of η = 19.0% and a maximum conversion efficiency of ηmax = 19.4%. A mini-module with the best two solar cells achieved a aperture conversion efficiency of η = 15.8%. Causes for cell-to-module losses are investigated and discussed in detail. In summary, we demonstrate the successful application of flexographic printing technology for the front side metallization of busbarless solar cells with multi-wire interconnection.

Post-plating Annealing of Light Induced Plated Copper Fingers: Implications for Reliable Metallization

Light-induced plating (LIP)of Ni/Cu presents a potentially lower-cost alternative to screen-printed Ag for silicon solar cell metallization. This paper presents results of self-annealing and post-plating rapid thermal processing (RTP) of plated Cu finger microstructure, texture and resistance. It is shown that the plated Cu conductors, if not thermally-annealed immediately after plating, self-anneal with time resulting in grain growth and increased (200) to (111) grain texture which occurs with increased tensile stress. Post-plating RTP annealing enables fast annealing and stable Cu fingers with a low ratio of (200) to (111) grain texture. These findings have important implications for plated finger adhesion and highlight the importance of annealing after plating for reliable Cu plated metallization.

Analysis of Al diffusion processes in TiN barrier layers for the application in silicon solar cell metallization

An evaporated Al layer is known as an excellent rear metallization for highly efficient solar cells, but suffers from incompatibility with a common solder process. To enable solar cell-interconnection and module integration, in this work the Al layer is complemented with a solder stack of TiN/Ti/Ag or TiN/NiV/Ag, in which the TiN layer acts as an Al diffusion barrier. X-ray photoelectron spectroscopy measurements prove that diffusion of Al through the stack and the formation of an Al2O3 layer on the stack's surface are responsible for a loss of solderability after a strong post-metallization anneal, which is often mandatory to improve contact resistance and passivation quality. An optimization of the reactive TiN sputter process results in a densification of the TiN layer, which improves its barrier quality against Al diffusion. However, measurements with X-ray diffraction and scanning electron microscopy show that small grains with vertical grain boundaries persist, which still offer fast diffusion paths. Therefore, the concept of stuffing is introduced. By incorporating oxygen into the grain boundaries of the sputtered TiN layer, Al diffusion is strongly reduced as confirmed by secondary ion mass spectroscopy profiles. A quantitative analysis reveals a one order of magnitude lower Al diffusion coefficient for stuffed TiN layers. This metallization system maintains its solderability even after strong post-metallization annealing at 425 °C for 15 min. This paper thus presents an industrially feasible, conventionally solderable, and long-term stable metallization scheme for highly efficient silicon solar cells.

Novel Ceramic Additives for Screen-Printable Silicon Solar Cell Metallization

The interfacial structure between front-side silver electrodes and n-type silicon emitters plays a very crucial role for the electrical and mechanical properties of silicon solar cells. Studies show that the residual glass layers at the Ag/Si interfaces will significantly increase the contact resistance, and this subsequently leads to a decrease in the overall efficiency of the silicon solar cells. In this work, silver-coated nano-sized non-glass frits using an electroless plating method were employed to improve the interfacial conductivity. Transfer length method was applied to evaluate the electrical performance of the samples made with different ceramic additives. For samples made with nano-sized silver-coated ceramic additives, the improvement of conductivity was found to be about 22% compared to additives with the same compositions with no surface treatment. The results indicate that the silver layer on the surface of ceramic additives provides a conducting channel within the residual insulating layer and therefore reduces overall electrical resistance.

Adhesion quality of evaporated aluminum layers on passivation layers for rear metallization of silicon solar cells

This paper characterizes the adhesion quality of evaporated aluminum rear metallization on rear passivation layers of silicon solar cells. A peel-test is introduced for adhesion rating and the adhesion quality between various Al and passivation layers on test structures is compared and discussed. The results show that the adhesion strongly varies with passivation type and evaporation parameters. Strong adherence is attributed to the formation of a strongly bonded, extended interphase layer. We derive factors that influence adhesion quality on our sample structures and find that an intimate contact between the layers and elevated temperatures are very important parameters. Additionally, a sufficiently high adhesion quality for an evaporated rear metallization of passivated emitter and rear cells (PERC) is achieved for all investigated passivation layers.