Broken metal fingers in silicon wafer solar cells and PV modules

Photovoltaic (PV) modules undergo accelerated aging tests as part of the certification procedure in order to confirm that they are not susceptible to the most common failure mechanisms that would strongly reduce their lifetime in the field. Though not yet part of the standard test sequence, it is well known that silicon camera based electroluminescence (EL) imaging would be a good addition to the test sequence as it enables the identification of defects in PV modules such as micro-cracks and poor electrical contact that will limit the lifetime of the module in the field, but do not necessarily result in a power reduction of more than 5% that would have them fail the certification tests. After IEC tests, we observe two distinct types of dark areas in the EL images of silicon wafer based PV modules, irregularly shaped regions which are the result of cracks in the silicon wafers, and regular rectangular shaped areas which we postulate are due to broken front grid fingers. In order to identify the mechanism responsible for the dark rectangular regions in the EL images of silicon wafer based PV modules, we investigate a soldered solar cell which exhibits similar rectangular dark areas in its EL image. SEM microscopy reveals that the dark areas in this cell are due to broken fingers caused by contraction of the tin during the soldering process. We hypothesise that a similar mechanism is responsible for the dark rectangular areas seen in the EL images of silicon wafer based PV modules after the accelerated aging tests. (C) 2012 Elsevier B.V. All rights reserved.