Understanding the impact of the cooling ramp of the fast-firing process on light- and elevated-temperature-induced degradation

In current silicon solar cell technologies, hydrogen is incorporated into the solar cell during the fast-firing process. It passivates defects at the surface and in the bulk, but also leads to light- and elevated-temperature-induced degradation (LeTID). Although it is known that the hydrogen content and the LeTID extent can be reduced by employing a slower cooling ramp during the fast-firing process, the exact mechanism behind this phenomenon remains unclear. This study aims at closing this gap by investigating the impact of cooling ramps with different temperature plateaus on hydrogen (complexes) in B-doped FZ-Si wafers. The fired wafers are analyzed with FT-IR spectroscopy, four-point-probe resistivity measurements, and LeTID tests via effective lifetime measurements. Our findings provide evidence that hydrogen not only diffuses into the silicon bulk but can also effuse out of it during the cooling ramp. A one-dimensional hydrogen model is built in Sentaurus TCAD to simulate the in- and out-diffusion of hydrogen and to compare it with the experimental results. The experimentally determined hydrogen concentrations align with our simulation, with the diffusion being dominated by the fast-diffusing neutral hydrogen H0. Moreover, we find stronger out-diffusion at higher temperatures, resulting in a lower total hydrogen concentration for slower cooling ramps. The extent of LeTID and surface-related degradation (SRD) are found to scale with the final total hydrogen concentration in the bulk. Therefore, modifying the cooling ramp can be an effective tool to optimize the hydrogen content and minimize the impact of degradation phenomena on silicon solar cells.