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

The effect of the optical gap (E-opt) 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 E-opt. The independence of the activation energy from E-opt suggests that the variation of E-opt 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 E-opt cells exhibits greater temperature dependence after prolonged light soaking. This tendency was analytically attributed to the E-opt dependence of the light-induced defect creation process and the E-opt independence of the thermally induced annealing process. (C) 1998 American Institute of Physics. [S0021-8979(98)03519-1]