Optimizing Conductive Adhesives for High-Efficiency PV Cells

An electrically conductive adhesive for photovoltaic modules that combines low silver content with high conductivity, adhesion, and reliability. The adhesive comprises a resin matrix of epoxy and urethane acrylates, reactive diluents, and silver flake and glass particles. The composition achieves optimal properties through a balanced formulation of oligomers, monomers, and fillers, and can be cured thermally. The adhesive is suitable for shingled and ribbon-attached photovoltaic modules, providing stable electrical contacts and reliable performance under thermal cycling and humidity conditions.

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The widely used adhesive joining technique suffers from the drawback of being unable to be dismantled to examine for degradation. To counteract this weakness, several structural health monitoring (SHM) methods have been proposed to reveal the joint integrity status. Among these, doping the adhesive with carbon nanotubes to make the joint conductive and monitoring its electrical resistance change is a promising candidate as it is of relatively low cost and easy to implement. In this work, resistance change to monitor fatigue debonding of composite single-lap adhesive joints has been attempted. The debonded area, recorded with a liquid penetrant technique, related linearly to the fatigue life expended. However, it correlates with the resistance change in two different trends. Scanning electron microscopy on the fracture surface reveals that the two trends are associated with distinct failure micromechanisms. Implications of these observations on the practical use of the resistance change for SHM are discussed.

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A method for bonding interconnection elements to photovoltaic cells that reduces adhesive consumption and improves reliability. The method involves successively depositing a first electrically conductive adhesive film on each interconnection element, then placing the elements on the photovoltaic cell with the adhesive film in contact. The second adhesive film is deposited simultaneously with the first, eliminating the need for additional adhesive layers.

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Abstract Our previous work highlighted how microscopic structural effects influence the sheet and contact resistance of electrically conductive adhesives (ECAs). Herein, we delve further by investigating how the contact and bulk resistivity of several ECAs that are based on the same formulation, but with different filler content, are correlated with the filler content. Additionally, two different filler geometries high and low surface area (HSA and LSA) fillers are combined in different ratios to maintain a similar viscosity and therefore processability. Hence, contact and bulk resistivities are also correlated with the different geometry ratios of these two fillers. As expected, it was found that the contact and bulk resistivities decreased when the filler content was increased. However, the magnitude of the decrease was found to depend strongly on the filler geometry ratio. At extreme filler geometry ratios, when the bulk is either mostly loaded with HSAfillers or mostly with LSAfillers, the impact of changes in the filler content on the bulk and contact resistivities is mark... Read More

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This work deals with shingle solar modules, in particular the interconnection of shingle solar cells by using an electrically conductive adhesive (ECA), as well as by simply overlapping the same type of shingle solar cells but omitting the ECA. For both types of interconnection, the resistance due to interconnection Rint and the corresponding cell-to-module (CTM) power loss PMPP in the case of a full-size shingle module are quantified, finding Rint 0.26 m and PMPP 0.4 W for the ECA-based interconnection, and Rint 0.86 m and PMPP 1.2 W for the ECA-free interconnection. The work mainly focuses on the methodology used to derive these numbers from small-format modules containing shingle strings of different lengths. An extensive error discussion is given, as well as a discussion on the advantages over a different approach to determine Rint.

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This article reports on the 11th Workshop on Metallization and Interconnection for Crystalline Silicon Solar Cells, which took place in May 2023 in Neuchtel, Switzerland. An important observation at the workshop was that, while screen printing is still dominating metallization, alternative pastes with increasing Cu content are starting to be implemented in the industry in response to the need to decrease cost and improve sustainability. Compared to previous workshops, interconnection topics were more prevalent in this edition because of the need for interconnection solutions that work at lower temperatures, as is needed for emerging solar cell technologies. Electrically conductive adhesives (ECA) have gained in importance in this respect. Understanding of reliability aspects and insight into proper characterization of ECAs are increasing, yet the challenge is here is to substantially further reduce cost and silver content.

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Abstract Anisotropic conductive adhesives (ACAs) are the preferred interconnection technology for applications that employ large dies, flexible substrates, and ultra finepitch interconnects. Conventional ACAs require relatively high bonding pressures and temperatures, and ultra finepitch applications challenge the tradeoff between low interconnect resistance and risk of short circuits. This study introduces an ACAlike interconnection technology that addresses these limitations, allowing for lowpressure, lowtemperature assembly processes with enhanced particle control at the interconnects. Conductive particles are deposited onto a patterned carrier and subsequently transferred to electrical pads using either nonconductive film or Ag sintering. The Ag sintering process is performed at a low temperature (140 C) and low bonding pressure (1 N for a 10 10 mm 2 chip). The capability of controlling the position and number of conductive particles within individual interconnects is demonstrated. This presents possibilities for achieving ultrafine pitch interconnects with negligible ... Read More

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In this study, we aimed to develop and characterize Transparent Conductive Adhesive (TCA) materials for mechanically stacked tandem solar cell applications. The research objectives were to enhance the transparency, conductivity, and adhesive strength of TCAs to improve the efficiency and practicality of tandem solar cells. The TCA formulation comprises a combination of polymers and Ag-coated particles, with flexible poly-methyl methacrylate (PMMA) micro-spheres serving as conductive particles and SYLGARD 184 silicon elastomer poly-di-methyl-siloxane (PDMS), mixed in a 10:1 ratio of base to curing agent, serving as the transparent adhesive. We conducted systematic experiments with Ag-particle coverage areas ranging from 0.34 % to 21.61 % at temperatures of 80 C and 100 C, under 500 mb pressure. The optimal conductivity was achieved at 2 wt% Ag-particle coverage, measuring 5.26 10 S/m, with over 93 % transparency. These key results indicate the significant potential of the optimized TCA in enhancing the performance of mechanically stacked tandem solar cells, thus contributing to t... Read More

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The reduction of the series resistance in multijunction solar cells is of high importance for attaining peak efficiencies in concentrator photovoltaics. This study showcases strategies to reduce the sheet resistance of the uppermost subcell of a direct wafer bonded fourjunction devices, since it contributes significantly to the series resistance. Therefore, electron mobilities in ntype AlGaInP, lattice matched to GaAs, are investigated across bandgap energies between 1.9 and 2.1 eV and various doping concentrations. The sheet resistances for AlGaInP rear heterojunction cells are determined for the integration in a two terminal fourjunction solar cell. The rear heterojunction cell architecture effectively addresses the sheet resistance optimization because it features a thick ntype doped absorption layer. The sheet resistance of our current world record quadruplejunction solar cell with 47.6% efficiency under the 665fold concentrated AM1.5d spectrum is 550 sq 1 . Herein, it is shown that optimizing the nabsorption layer in the 1.90 eV GaInP top cell can reduce the sheet res... Read More

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A method for preparing photovoltaic cell strings and modules that reduces manufacturing costs by eliminating soldering and using adhesive films to connect cells. The method involves routing main grid lines to the back of cells, bonding them to an adhesive film, and then applying additional adhesive to secure the grid lines. This approach enables the use of thin silicon wafers and eliminates the need for soldering, reducing material costs and enabling the production of high-efficiency photovoltaic modules.

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Materials with high conductivity and low cost are urgently required for electronic products, particularly copper conductive adhesives. Developing reliable copper conductive adhesives (ECAs) remains a great challenge due to their low conductivity and easy oxidation. In this article, copper microflakes with surface passivation by formate ions and thiols were used to create an antioxidant copper adhesive. We demonstrate that the surface modification has no impact on the electrical conductivities, even in salt spray, acid, alkaline, high temperature, and humidity conditions. After being cured at 250 C for 30 min, the produced Cu ECAs displayed an ultralow resistivity of 65.12 cm1. Excellent stability enables its resistance to maintain a low level after 1000 h of aging at 200 C and 85%RH 85 C. This study establishes the enormous potential of Cu ECAs with antioxidants for low-cost microelectronics packaging.

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A conductive material for photovoltaic modules that enables accurate electrical connections between solar cells without the need for wide grid lines or soldering pads. The material features a conductive wire with a stepped or bent shape that passes through openings in an adhesive film, allowing for precise electrical connections between cells while minimizing material usage. The conductive material is prepared by fixing the wire to the film, heating the adhesive, and then melting the wire to secure it in place. The material is used to connect solar cells in series in a photovoltaic module, eliminating the need for traditional welding methods.

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In this work, the behavior of an electrically conductive adhesive (ECAs) was selected and studied, using shingled technology is a development in the connection of solar cells by means of superimposition with electrically conductive adhesives, thus obtaining higher powers than the previous welding process, also offers several advantages compared to standard photovoltaic modules, improving: efficiency, reduction of microcracks, better use of the surface, lower processing temperature, higher energy yield, and better aesthetics. The objective of this study was to analyze electrically conductive adhesives for use in the manufacture of photovoltaic modules to propose a technological development to obtain greater efficiency in energy capture. Polycrystalline cells and monocrystalline cells were studied. The photovoltaic module manufacturing set was made using the conventional fixing method to join solar cells, based on the union by welding with Sn-Cu connection tapes. Loctite Ablestik ICP 8282 adhesive was selected for use in the manufacture of the overlay prototype and the substitution of ... Read More

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This study presents a method to analyze the electrical resistance of planar contacts. The method can determine whether the contact resistance of the joint exhibits linear or non-linear behavior. By analyzing the current distribution over a planar contact, it can be determined whether an area-based contact resistance is justified or if other parameters define the contact resistance. Additionally, a quantitative evaluation of the factors that affect the measurement accuracy, including the positioning, the measurement equipment used, and the influence of the current injection on the sense pin was conducted. Based on these findings, the electrical contact resistance and the mechanical ultimate tensile force of a silver-filled epoxy-based adhesive are analyzed and discussed. The layer thickness and the lap joint length were varied. Overall, the investigated adhesive shows a low contact resistance and high mechanical strength of the same magnitude as that of well-established joining techniques, such as welding, press connections, and soldering. In addition to evaluating the mechanical and ... Read More

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For the industrialization of large area perovskite silicon tandem solar cells a low-resistance and low-cost contact metallization needs to be established. On the one hand, curing or even firing temperatures as applied for screen-printing metallization of Si single junction cells are too high for perovskites. On the other hand, screen-printing pastes with very low curing temperatures have a reduced specific conductivity. Thus, they require wide Ag consuming contacts for a sufficiently low resistance. We present a concept which comprises screen-printing the front contacts onto an isolating layer located on the uppermost layer of the bottom cell before the perovskite top cell is applied. This allows higher curing temperatures and thus lower finger resistivities than in a low curing temperature screen-printing process on a perovskite solar cell. Furthermore, this concept avoids mechanical pressure on the soft perovskite material during cell interconnection. We expect that the latter aspect is beneficial to prevent shunting issues. The concept is shown for perovskite single junction solar... Read More

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Conductive adhesive joints were climatically aged with temperature pulses in the range of -40 to 115 C. The joints were made of two types of electrically conductive adhesives with isotropic electrical conductivity. The insulating matrix of the adhesives was bis-phenol formaldehyde epoxy resin, and the conductive filler was silver flakes. One adhesive was two-component and the other one-component. Adhesives were applied to the printed circuit board by stencil printing, and then zero-value resistors were mounted by adhesive mounting. Thus, adhesion bonds were formed. The joints were first exposed for 15 minutes in the hot chamber of the thermal pulse testing device, then moved within 8 seconds to the cold chamber and exposed again for 15 minutes. This cycle was repeated two hundred times and five hundred times. The connection resistance was measured using the four-point method. Exposure to temperature pulses was found to cause small changes only in junction resistance.

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The temperature dependence of the resistance of adhesive conductive joints and the thermal coefficient of longitudinal expansion of conductive adhesives are parameters that can significantly affect the properties of this type of conductive joining. The dependence of these parameters on temperature was investigated. Adhesive mounting zero-value resistors to the test PCB prepared adhesion joints. The adhesive was applied by stencil printing. The resistance of the joints was measured by the four-point method and the thermal expansion of used adhesives was measured by the thermomechanical analysis technique. The junction resistance was found to increase approximately linearly with temperature, but the joints were found to harden when measured. It was also found that the coefficient of thermal longitudinal expansion varies depending on the adhesive and also on the sample when all samples are made from the same adhesive. The reason is the not completely reproducible adhesive application technology for this measurement.

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With fast developments in the environmental-friendly industry, electrically conductive adhesive (ECA) composites with high electrical conductivity, mechanical strength, and electrochemical migration resistance as well as low cost are highly desirable for emerging power electronics applications. In this study, a category of ECAs, namely, size-controllable sonochemical low-melting-point-alloy (LMPA) particle-incorporated transient liquid-phase ECAs (TLP-ECAs), with excellent electrical conductivity, considerable mechanical strength, and high electrochemical migration resistance has been successfully demonstrated. Experimental results showed that the bulk resistivity of TLP-ECA decreased to a minimum value of 2.37 104 -cm with the remaining filler content at a low level (the total content of metallic fillers, i.e., 70 wt % in TLP-ECAs), while the mechanical shear strength increased considerably (by 55.4%) compared with pure silver ECA with the same filler content, demonstrating that addition of sonochemical LMPA particles into ECA can simultaneously and considerably improve the elec... Read More

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The objective of this paper is to add to the fundamental understanding of the conduction mechanisms and mechanical behavior of electrically conductive adhesives and their applications. Consequently, the research work to be presented involves two parts: (a) analysis of mechanical behavior, and (b) analysis of electrical behavior of conductive adhesives. Electrical and mechanical behaviors of both bulk conductive adhesives and single lap joints bonded with a conductive adhesive are studied. For bulk conductive adhesives, a model is established to predict the adhesive conductivity. The effect of film thickness on the conduction behavior of conductive adhesives is also presented, and it is shown that in thin flim adhesives the resistivity values in planar directions are much larger than those encountered in 3-D measurements for the same adhesive material. The thermal mismatch stress induced at the interface between the adhesive and the substrate is studied in closed form and by finite element analysis, and the mechanical properties of the filled adhesive composite are obtained for this p... Read More

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The contact resistivity of interfaces in solar module interconnects has a direct impact on the series resistance of the entire module and fill factor. Thus, this impacts the performance of entire photovoltaic systems. Accurate measurement of the contact resistance is a key component of optimizing the performance of such interconnects. However, this is difficult for electrically conductive adhesive (ECA) based interconnects. This work shows that transmission line method (TLM) test structures based on ECA display nonnegligible inhomogeneities leading to inaccuracies when determining the contact resistivity. Seven methods based on two models the front and endcontact TLM models (or a combination of both) were investigated for four commercially available ECAs used in solar modules and their impacts on the extrapolation of the contact resistivity were quantified. It was determined that even when macroscale inhomogeneities (e.g., variations in the thickness of the ECA) are not present, microscopic structural effects influence the sheet and contact resistance. In particular, variation... Read More

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