Micro structural root cause analysis of potential induced degradation in c-Si solar cells
The Potential Induced Degradation (PID) of crystalline Si solar modules has attracted a strong interest in recent years as one of the most prominent failure modes observed in solar park installations. In recent publications the influence of elevated voltages applied to the modules has been studied in detail. Depending on the electrical interconnection scheme, material properties of the modules as well as environmental conditions, in particular cases the total breakdown of module power has been observed. However, until now a clear understanding of the underlying degradation mechanism and the physical failure mode is still missing. Based on PID experiments on mini modules we have reproduced the degradation mechanism under laboratory conditions (elevated voltage, increased temperature and humidity). The local electrical shunting of the degraded mini modules has been investigated by high resolution Lock-in Thermography (LIT). Samples from regions with different degrees of degradation have been prepared. The material properties have been investigated using Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and Scanning Electron Microscopy (SEM) with Electron Beam Induced Current (EBIC) measurements at cross sections. The shunted regions of the solar cells show an accumulation of alkali metals at the interface of the front side coatings of the solar cell. In the same regions dramatic changes of the p-n junction contrast can be detected by SEM/EBIC. Based on these data, we propose a simple model that may explain the PID effect in solar cells through induced negative charges at SiNx/Si interface.