Community Research and Development Information Service - CORDIS


SOLARX Report Summary

Project ID: 307315
Funded under: FP7-IDEAS-ERC
Country: Finland

Final Report Summary - SOLARX (Riddle of light induced degradation in silicon photovoltaics)

Most of the solar cells that are in the market today are made of crystalline silicon (>90% market share). In general, the crystalline silicon solar panels are well-known to be long-term stable as the manufacturers provide 30-year (and even longer) guarantee for the panels. However, during the first 24-hours of sun-light exposure on these panels, the solar cells degrade severely, and even higher than 10% efficiency losses have been reported. This phenomenon is known as light-induced degradation (LID). Even though the LID is an old problem, there is still no clear understanding of its origin nor cheap solution to prevent it.

During the past few years, LID has gained further interest especially in industry, since the impact of LID increases along with the material and technology development. In SOLARX project, we proposed a patented solution to prevent the harmful LID by a relatively simple method using a charged insulator on top of the wafers. During the project we demonstrated experimentally that this method works in commercial solar grade silicon. In addition to the charging of the surface, we also discovered an alternative method to mitigate the harmful LID that is even better suited for the industrial environment. The invention is related to the surface engineering of antireflection layers and in the project we showed that it fully eliminates the LID in the multicrystalline solar cells. The results were verified at the production line of European PV manufacturer.

Solarx project also aimed at gaining a deep understanding of the origin and mechanism of the LID, therefore, a strong emphasis was placed on various advanced characterization methods as well as modelling the kinetics of degradation and recombination characteristics of the responsible defects. Our results show that the root cause for the LID present in the samples that we investigated (both silicon wafers as well as industrial solar cells) was due to nanoscale copper precipitates. As a result, a new universal model was created that is able to predict the amount of degradation when the material parameters are given as input. The modelling and characterization efforts also resulted in a new method to measure small amount of copper contamination in silicon that can find use also outside photovoltaic field, e.g. in semiconductor industry.

One of the latest findings in the project was related to the sources of Cu contamination in the industrial process line. While the original hypotheses that there were significant amount of Cu impurities already in the silicon feedstock and in the chemicals used in the processing lines turned out correct, the most surprising result was that a significant Cu source was present in the final process step, i.e. the contact firing furnace. During the cell processing it was possible to drive the copper contamination to the surfaces of the solar cells making them less detrimental for the degradation, but the final contact firing step caused copper to diffuse back to the silicon substrate. However, we were able to show that by properly tuning the firing processing parameters, it was possible to avoid Cu from backdiffusing to the silicon bulk. It required significant changes to the contact firing process as compared to the current industry standard.

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