  Among silicon-based solar cells, heterojunction cells hold the world
efficiency record. However, their market acceptance is hindered by an initial
0.5\% per year degradation of their open circuit voltage which doubles the
overall cell degradation rate. Here, we study the performance degradation of
crystalline-Si/amorphous-Si:H heterojunction stacks. First, we experimentally
measure the interface defect density over a year, the primary driver of the
degradation. Second, we develop SolDeg, a multiscale, hierarchical simulator to
analyze this degradation by combining Machine Learning, Molecular Dynamics,
Density Functional Theory, and Nudged Elastic Band methods with analytical
modeling. We discover that the chemical potential for mobile hydrogen develops
a gradient, forcing the hydrogen to drift from the interface, leaving behind
recombination-active defects. We find quantitative correspondence between the
calculated and experimentally determined defect generation dynamics. Finally,
we propose a reversed Si-density gradient architecture for the amorphous-Si:H
layer that promises to reduce the initial open circuit voltage degradation from
0.5\% per year to 0.1\% per year.
