  Heterojunction Si solar cells exhibit notable performance degradation. We
developed the SolDeg platform to model this degradation as electronic defects
getting generated by thermal activation across energy barriers over time.
First, molecular dynamics simulations were performed to create a-Si/c-Si
stacks, using a machine-learning-based Gaussian approximation potential.
Second, we created shocked clusters by a cluster blaster. Third, the shocked
clusters were analyzed to identify which of them supported electronic defects.
Fourth, the distribution of energy barriers that control the generation of
these electronic defects was determined. Fifth, an accelerated Monte Carlo
method was developed to simulate the thermally activated time dependent defect
generation across the barriers. Our main conclusions are as follows. (1) The
degradation of a-Si/c-Si stacks via defect generation is controlled by a broad
distribution of energy barriers. (2) We developed the SolDeg platform to track
the microscopic dynamics of defect generation across this wide barrier
distribution, and determined the time dependent defect density $N(t)$ from
femtoseconds to gigaseconds, over 24 orders of magnitude in time. (3) We have
shown that a stretched exponential analytical form can successfully describe
the defect generation $N(t)$. (4) We found that in relative terms
$V_\mathrm{oc}$ degrades at a rate of 0.2%/year over the first year, slowing
with advancing time. (5) We developed the Time Correspondence Curve to
calibrate and validate the accelerated testing of solar cells. We found a
compellingly simple scaling relationship between accelerated and normal times
$t_\mathrm{accelerated} \propto t_\mathrm{normal}^{0.85}$. (6) We ourselves
carried out experimental studies of defect generation in a-Si:H/c-Si stacks. We
found a relatively high degradation rate at early times, that slowed
considerably at longer time scales.
