Low Temperature Diffusivity for Aluminum and Silver in Amorphous Silicon

The authors present experimental results which determine the diffusivities for aluminum and silver in amorphous silicon using segments cut from test coupons and a fabricated module. The data extend those found in the literature to a temperature range of 50°C-300°C. This study has been motivated in part by the rapid degradation of a-Si module performance. This degradation is known to be attributable to the generation of light-induced effects, but it has also been noted that aluminum diffuses into the n⁺-i barrier and thereby decreases its contribution to the generated potential. The results agree with the relative order of magnitude of previously published work on aluminum over the temperature range of these data. The activation energy differs significantly. For the temperature range cited above, the aluminum diffusivity varies from 10^-19 to 10^-14 cm²/s and that for silver varies from 10^-18 to 10^-16 cm²/s. The activation energies are calculated to be E_a(Al) = 0.73 eV and E_a(Ag) = 0.33 eV. These values affect module life significantly.