Electromechanical Characterization of Crack-Tolerant, Carbon-Nanotube-Reinforced Composite Gridlines Using in Situ Scanning Electron Microscope Strain Test

Cell cracks, over time, lead to module degradation and reduced power generation. To minimize this degradation, we have previously demonstrated that silver paste reinforced with multiwalled carbon nanotubes, also known as metal matrix composites, can impart enhanced fracture toughness, electrical gap-bridging, and 'self-healing' to gridlines and busbars. In this work, we peer into mechanisms responsible for these enhanced material properties, using in situ scanning electron microscope strain test. We discover that micro void coalescence exists in printed and fired metal matrix composite gridlines. These voids can turn the crack propagation to take more tortuous paths than in gridlines from standard silver paste, increasing the composite's fracture toughness. The presence of carbon nanotubes also appears to cause the fractured surface to have pronounced intrusions/extrusions that help maintain the electrical continuity after fracture. In rare cases, we observe that fracture branching is another mechanism by which partially fractured macroscopic section of gridlines can electrically bridge large gaps (= 70 microns). The intrusions/extrusions on the fractured surface and fracture branching are also suspected to provide the 'self-healing' properties when the gap in the cracks narrows.