LeTID Stability Tests on Gallium-Doped Industrial PERC Cells and Wafers

This study investigates the light and elevated temperature induced degradation (LeTID) stability of industrial gallium-doped PERC solar cells which now account for >90% of production. We compare wafers to PERC cells processed from neighbouring wafers. The wafers from a mass-market supplier were processed to cells as usual in the manufacturing lines at a major manufacturer. The seven investigated materials feature a resistivity variation of 0.4 to 1.0 Ωcm and lifetime samples were processed to create high hydrogen content (with PECVD SiN) or low hydrogen content (with ALD Al2O3), using the facilities of two research institutions. We test the stability of charge carrier lifetime and cell performance under a variety of illumination conditions and temperatures. Our experiments demonstrate that the material itself is prone to LeTID effects. However, experiments on the finished PERC cells from the same material stock indicate that the production process successfully suppresses LeTID. We observe only small degradation and sometimes even improvement of cell parameters under LeTID testing conditions. Even dedicated pre-annealing treatments to destabilize the material do not cause relevant subsequent degradation of the cells.