Iron related solar cell instability: Imaging analysis and impact on cell performance

• A method for imaging interstitial iron in silicon solar cells is presented.
• Iron may have a significant impact on stability of current–voltage parameters.
• Simulated and experimental cell parameter instabilities match well.
• Instructions for stable and reproducible cell measurements are given.

Iron is an omnipresent and efficiency-limiting impurity in p-type silicon solar cells. A very useful technique to detect the interstitial iron concentration in boron doped silicon is based on carrier lifetime measurements at two metastable states of iron, namely interstitial and bound to boron. We present a further development of this technique which images the iron concentration not only on wafer level but also on finished solar cells and in this way enables the most direct access to the impact of interstitial iron on cell performance. With the help of this method we reveal that remaining iron in solar cells may be responsible for significantly unstable current–voltage characteristic under illumination. Recommendations for stable cell measurements even for cells containing a significant concentration of iron are given.