On the trail of a cause for solar cell LID

Light-induced degradation (LID) has long been a problem for the PV industry, slowly eroding solar module performance, adding up to significant performance loss over years in the field.

Plenty of solutions have been suggested. Additional treatments during cell production have shown to be effective in mitigating LID. More recently, cell makers have begun to switch boron for gallium as a p-type doping material, since boron is known to be active in the LID mechanism. The switch to n-type technologies, which several manufacturers are currently in the early stages of making, should also mean modules are less susceptible to LID.

These solutions, however, rely on relieving the symptoms of LID rather than fully understanding the cause. And research is ongoing to fully understand the mechanisms at work within the cell that cause and cut them out without the need for additional processing in cell manufacturing.

Scientists led by the National Renewable Energy Laboratory in the U.S. and the Colorado School of Mines sought to contribute to this research, applying the latest microscope imaging techniques to boron-doped silicon cells. Using a technique called electron paramagnetic resonance, the group was able to identify a distinct defect signature that occurred as the cell performance was degraded by light. This signature disappeared when the group applied the widely used treatment for LID adopted by cell manufacturers.

The work is described in full in the paper Atomic structure of light-induced efficiency-degrading defects in boron-doped Czochralski silicon solar cells, published in Energy and Environmental Science. The group further identified another signature affected by light exposure, which did not match up with the number of defects in the material – suggesting other atomic changes may be occurring that don’t directly contribute to LID.

The group says the techniques it has developed to study LID could be applied to other degradation mechanisms affecting silicon cells, and could even work with cadmium telluride, perovskites, and other cell materials.