LeTID in PERC Cells from Gallium-Doped Czochralski Silicon: Extent, Accelerated Testing and Mitigation via an Adapted Firing Process

In this work, we analyse the light and elevated temperature induced degradation (LeTID) behavior of passivated emitter and rear cells (PERC) based on gallium (Ga)-doped Czochralski (Cz) silicon. The investigated cells which we made using industrial precursors exhibit LeTID, degrading between 8%rel and 14%rel in efficiency under 0.15 suns illumination at 75°C. The degradation and regeneration processes are slower than in boron (B)-doped PERC cells leading to a greater yield reduction over a 20-year lifespan of a solar module since the regeneration process typically does not begin during that time frame. Besides our standard fast firing process, we apply firing processes with adapted temperature profiles during contact firing with a reduced cooling rate after reaching the peak temperature to these cells. As previously shown for B-doped cells, this approach to mitigate LeTID also works for Ga-doped PERC cells. With this adapted firing process, we reduce the maximum degradation to between 1%rel and 4%rel. At the same time, this process only slightly reduces the initial efficiency of the cells by up to 0.8%rel. We expect that the advantage of strongly reducing LeTID therefore far outweighs the disadvantage of a slightly lower initial efficiency over the lifetime of the module and, hence, increases the energy yield. Lastly, we show that altering the degradation conditions by increasing illumination and temperature from 0.15 suns to 2 suns and 75°C to 140°C, respectively, is an adequate way to accelerate LeTID testing on Ga-doped samples by more than two orders of magnitude. This allows to speed-up R&D cycles and the further development of research and technology to mitigate LeTID.