Stability of a-Si/Ge:H Device Material to Light and keV-Stress Exposure

The development of highly efficient and stable (a-Si:H/a-Ge:H) thin-film tandem solar cells presupposes a high quality and stable a-Ge:H material for the bottom cell. We have found conditions for manufacturing a high quality material which shows excellent stability to illumination with white light. The evaluation of the defect density with PDS did not reveal any increase in defects after illumination. Nor did the photo and dark conductivity change after 2500 hours AM1 light soaking. Furthermore, using keV electron irradiation, we demonstrated a significant increase of stability of a-Si/Ge:H alloys with rising germanium content. This behavior might be explained by the higher diffusion mobility of hydrogen in a-Ge:H compared to a-Si:H, so that a lower annealing temperature may result. In order to verify this assumption, the temperature-dependent dark conductivity sd of different doped and undoped a-Ge:H films was measured at different cooling rates. The results indicate a glass-like transition between a thermal equilibrium state above a certain freeze-in temperature T C * and a slowly relaxing state below that temperature. The values for T C * depend on the position of the Fermi level and agree well with the glass transition temperatures T G * defined by the diffusion coefficient DH of the hydrogen sub-lattice. For undoped a-Ge:H, T G * and T C * are about 50°C to 70°C lower than in a-Si:H. This will improve the annealing and stability behavior of a-Si/Ge tandem solar cells.