(559b) Low-Cost Composite Metallization to Reduce Cell-Crack-Induced Module Degradation

Microcracks in solar cells can eventually propagate through metal gridlines and busbars, leading to PV module power loss over time, and can be a root cause of hotspots – high-temperature areas within PV modules caused by Ohmic heating. Technical approaches to mitigate the impact of cell cracks include improved designs for cell shape, cell wiring, metallization patterns, and module construction. Multi-wire technology has also emerged as a possible solution to the cell-crack-induced module degradation. In this work, we focus on metal matrix composite (MMC) metallization that makes use of surface-functionalized multiwalled carbon nanotubes (MW-CNTs) embedded in commercially available silver paste. We have previously demonstrated that this composite metallization imparts unique properties to the metal gridlines and busbars: (1) increased fracture toughness and ductility, (2) electrical bridging of cracks ≥50 μm, and (3) “self-healing” of electrical continuity after repeated cycles of complete electrical failure under tensile strain and crack closure. Building on these enhanced electromechanical properties, we show that beginning-of-life performance of full-size Passivated Emitter and Rear Contact (PERC) solar cells integrated with MMC, statistically averaged over a large sampling pool (88 cells), matches that of PERC cells with standard metallization. Mini-module stress testing [e.g., temperature cycling (TC) from -40 to +85 °C and highly accelerated stress testing (HAST) involving damp heat] also shows a low degradation rate from the composite-enhanced modules, matching that of standard modules. More specifically, the MMC metallization shows metal corrosion comparable to the baseline metallization and power loss of only 0.6% after TC-200 and less than 4% after HAST-100. Overall, the inclusion of CNTs to conventional screen printable silver paste provides cell-crack-tolerance, while improving cell efficiency and providing comparable corrosion characteristics to the baseline. These properties open the opportunity to extend the PV module lifetime well beyond 25 years guaranteed under typical performance warranty.

This material is based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Solar Energy Technologies Office Award Number DE-EE0009013.