Fatigue crack growth in Silicon solar cells and hysteretic behaviour of busbars

Photovoltaic modules are subject to cyclic deformation during their lifetime as a result of vibration, applied loads, and thermal effects. Vibration and applied loads induce cyclic bending on the modules, while operating temperature excursions during the day lead mostly to cyclic axial deformation. In both cases, the region between two solar cells is severely stressed. For cyclic bending, cracks can nucleate near the points where busbars are soldered onto Silicon and might propagate due to fatigue. For cyclic axial deformation, on the other hand, busbars are stressed above the elastic regime and may experience plasticity and hysteretic energy dissipation. The present study focuses on the experimental characterization of such material degradation phenomena related to the above two types of cyclic deformation. For the former, fatigue crack growth in Silicon and its evolution have been quantified by using two independent nondestructive monitoring techniques based on electroluminescence and thermal infrared imaging. For the latter, plasticity and degradation of the material response of busbars has been assessed in relation to different applied cyclic strain levels. The obtained results shed light onto the cyclic response of materials used in photovoltaics, and pinpoint features that should be taken into account in the development of refined standard qualification tests for photovoltaics including cyclic deformation.