Microcrack Formation in Silicon Solar Cells during Cold Temperatures

Abstract

A single brief exposure of a photovoltaic (PV) module or coupon to cold temperatures down to -40°C, the lower limit in IEC photovoltaic testing standards, significantly degrades the fracture strength of silicon solar cells. To understand the mechanism behind the fracture strength degradation, we built a finite element model of a single cell encapsulated coupon and reduced the temperature isothermally from 25°C to -40°C and from 150°C to -40°C. Our modeling results confirm that the regions next to the interconnect wires see high stresses. The silicon wafer bends around the top wire towards the glass; whereas, the entire coupon curves in the opposite direction. The first-principle stress in the entire silicon wafer was found to be compressive, mostly in plane in the direction perpendicular to the wires, yet unable to cause a failure due to the much larger compressive strength of silicon. On the other hand, out-of-plane shear stresses on each side of the ribbons were observed to exceed considerably the shear strength of silicon, most likely causing the formation of microcracks. These microcracks that form during cooling can later propagate into full cracks at relatively low front side loads that place the cells into tensile stress. We also investigated whether the silicon cell may be buckling due to high compressive stresses due to the backsheet and encapsulant shrinkage. Index Terms— Cooling, Failure analysis, Numerical models, Photovoltaic cells, Solar Panels, Surface cracks, Thermal analysis, Thermal expansion, Thermal stresses.