Conductivity Mechanism of Shingling Adhesives
Electrically conductive adhesives (ECAs) typically consist of resins, hardeners, catalysts, functional additives such as adhesion promoter or conductivity promoter, and electrically conductive particles. The most widely used electrically conductive particles in shingling adhesives are silver flakes. In addition to providing thermal and electrically conductivities, conductive particles also influence adhesive’s rheological properties which affects workability and flow behavior.
Researchers have proposed two mechanisms on the electrical conductivity of ECAs. One is physical contact between conductive particles and the other is tunneling. Before cure, silver flakes are non-conductive, covered with lubricants to prevent them from agglomeration. During cure, volume shrinkage forces conductive particles to make contact with each other and conductive pathways are formed.
Chart 1：Traditional PV module
Chart 2:shingling module
Tunneling is a coherent process in which electrons move from one lattice site to another, maintaining a definite phase relationship between the amplitudes corresponding to finding an electron at different lattice sites. The probability of a tunneling event is an exponential in the area under the energy barrier between sites. So the thinner the insulating resin layer is between two conductive particles, the easier a conductive pathway can be formed.
Electrical conductivity of an ECA is governed by percolation theory, wherein the necessary fillers that host electrons transfer, via physical contact or tunneling, must reach some critical volume fraction to accommodate probable conductive pathways that would be large enough to be considered isotropic. Resins also affect electrical conductivity. Higher cure shrinkage resins such as acrylates or epoxies often yield adhesives with much higher electrical conductivity than lower cure shrinkage resins such as silicones.
Acrylate and epoxy systems are widely used in shingling adhesives. Because these systems exhibit higher electrical conductivity and higher adhesion strength, shingling module manufacturers can use less adhesives and reduce cost.
After years of product development, Yongoo has successfully developed shingling adhesives based on epoxy and acrylate chemistry, for both printing and jetting processes. These products demonstrate high electrical conductivity, high adhesion strength and high reliability.
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