Hydrogen in Silicon Solar Cells: The Role of Diffusion

A model for hydrogen in silicon is presented, which accounts for both in-diffusion and out-diffusion from a passivation layer (e.g., SiNx), as well as the known hydrogen reactions within the silicon matrix. The model is used to simulate hydrogen diffusion and reactions during contact firing in a solar cell process, with a particular focus on variations in the cooling process, the sample thickness, and boron doping levels. The model reproduces the measured differences in hydrogen concentration due to these variations and thus helps to understand hydrogen-induced surface degradation and the dependencies of light and elevated temperature-induced degradation (LeTID) on the cooling process due to the close relation of LeTID and hydrogen. The same model and parameters are utilized to simulate the subsequent annealing of the fired samples at temperatures ranging from 160 to 290 degrees C. By successfully modeling the development of boron-hydrogen pairs during dark annealing processes across varying temperatures and doping levels, it is demonstrated that diffusion toward the Si/SiNx interface explains the observed decrease in resistivity and reductions in boron-hydrogen concentrations over extended dark annealing durations. Our simulations show the necessity of considering the depth-dependent hydrogen distributions after the firing process for analyzing the dark annealing.