LeTID mitigation via an adapted firing process in p-type PERC cells from gallium-doped Czochralski silicon

Abstract

In this work, we analyse the light and elevated temperature induced degradation (LeTID) behaviour of gallium (Ga)-doped Czochralski (Cz) passivated emitter and rear cells (PERC). We show that these cells, based on industrial precursors, are prone to LeTID, degrading on average 8.3%rel in efficiency. The degradation and regeneration is slower than in boron (B)-doped PERC cells leading to a greater loss of energy over a 20 year lifespan of a solar module on a German rooftop, since, according to our experiments, regeneration does not start during that time frame. We show that the method of mitigating LeTID by slightly altering the temperature profile during fast firing, previously introduced on B-doped PERC cells, also works on Ga-doped PERC cells, reducing the maximum degradation to between 1%rel and 4%rel. This process does only slightly reduce the initial efficiency of the cell by up to 0.8%rel. Lastly, we show that increasing illumination and temperature from 0.15 suns eq. to 2 suns eq. 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 while achieving qualitatively and quantitatively similar results to those obtained in a standard test.