The conditions for the preparation of high quality and light stable a‐Ge:H material were investigated. By ‘‘hard’’ deposition, i.e., by strong ion bombardment during the rf‐plasma deposition, it is possible to obtain a material with a photoconductivity of &eegr;&mgr;&tgr;=10−6cm2/V (at 950nm). This value does not change after 2500 hours AM1 illumination. Also a stress test under keV‐electron irradiation shows the material to be stable. By small angle X‐ray scattering this stable materials was characterized as free from small voids. Larger voids are present in both ‘‘hard’’ and ‘‘soft’’ deposited a‐Ge:H material and therefore seem not to be mainly responsible for light degradation. The high photoconductivity and the good stability are a precondition for the use of a‐Ge:H in tandem solar cells.