The light‐induced effect of a‐Si films and solar cells fabricated by the super chamber (separated UHV reaction chamber) was systematically studied. It was confirmed that the reduction of impurities reduces the light‐induced effect in the range of 1018 cm−3 for oxygen. Furthermore, in the low impurity region, we found that other factors also have an influence on the light‐induced effect. In the low impurity region, the light‐induced degradations in a‐Si films and a‐Si solar cells were found to be reduced by reducing the SiH2 bond density. These results are effective for both the prevention of the light‐induced effect and the improvement of the conversion efficiency. As for cell structure, by using an n‐SiN/p‐SiC structure at the n/p junction in tandem solar cells, we obtained stable photovoltaic characteristics after light exposure for several hundred hours.