Field and Accelerated Aging of Cracked Solar Cells

Cracks can form in silicon solar cells in an otherwise intact photovoltaic module due to mechanical stresses such as rough handling or hail. The immediate impact on power due to these cracks can be readily measured, but it is also known from accelerated testing that the cracks can worsen over time. However, it is not clear how to predict the extent of future field degradation due to cracked cells, which requires a calibrated accelerated test. We describe progress toward such a test. In particular, we report on the outdoor aging of modules with cracked cells for nearly two years. We find that modules with cracked cells degraded in the field an average of 0.5% absolute more than uncracked modules over a period of 21 months. We also characterize the modules with multitemperature electroluminescence and find that the degradation is associated with cell fragments that become electrically isolated. We compare the weathering outdoors with the two types of accelerated tests: thermal cycling and a novel accelerated test, dynamic mechanical acceleration (DMX). DMX can apply thousands of pressure cycles at a frequency of approximately 10 Hz and pressures <200 Pa, which are relevant to the wind-driven pressure cycles experienced by modules outdoors. We find that the thermal cycles designed to reproduce the cumulative temperature change from the field overestimate field degradation and can excite noncell-crack degradation. DMX results were promising, reproducing degradation levels similar to those observed outdoors over 21 months with a test that can be performed in less than an hour.