Origin of the optical gap dependence of a-SiGe solar cell stability

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Abstract
The effect of the optical gap (Eopt) on the thermal recovery behaviors of a-SiGe solar cells was systematically investigated. It was found that the time constant and the activation energy required for the thermal annealing of metastable defects do not strongly depend on Eopt. The independence of the activation energy from Eopt suggests that the variation of Eopt causes little energy difference in the trap depth of hydrogen, which should diffuse to remove metastable defects during network relaxation by annealing. It was also shown that the degradation ratio for narrower Eopt cells exhibits greater temperature dependence after prolonged light soaking. This tendency was analytically attributed to the Eopt dependence of the light-induced defect creation process and the Eopt independence of the thermally induced annealing process.