The Fermi-Level Dependence of Hydrogen Dynamics and Its Effects on Degradation of Silicon Heterojunction Solar Cells

We investigate the Fermi level dependence of hydrogen diffusivity in silicon. We connect the atomic description of the Si-H bond breaking via density functional theory and nudged elastic band method to the cell level degradation simulated with device modeling package SCAPS. Contrary to the common belief that hydrogen's charge state controls its mobility, we find that a complex charge rearrangement dynamics governs the diffusivity. We found that the doping-induced shift of the Fermi level modulated the bond-breaking barrier. This result is a compelling theory for the experimental trends of H diffusivity. The H diffusivity controls the passivation stability, critical for improving the longevity of silicon-based photovoltaic devices.