Basic technologies for solving problems in thermal and light-induced degradation have been developed. The tandem type a-SiC/a-Si solar cell with blocking layers exhibits excellent stability for both thermal and sun light conditions. An observable light-induced degradation is not seen in the cell performance after light exposure test of 2000 hours. Two instability modes, that is, thermal and light-induced degradation have been investigated. For thermal degradation, a blocking layer for preventing diffusion has been inserted between the back side metal electrode and a n-layer and another blocking layer has been introduced between the np tunnel junction. To prevent light-induced degradation, p-type a-SiC layer of the pin structure on the side of a glass substrate/SnO2 has been deposited at the temperature of 70°C. The highest efficiency is 9.0% at the present stage, but it is expected to be improved to more than 10%.