Abstract We present the study of high efficiency single and multi-junction solar cells, focusing on the stability against degradation under illumination. In both single and multijunction solar cells, the thickness of the a-Si:H absorber layer was varied over a wide range up to 790 nm. While single junction a-Si:H solar cells show reduced stability against prolonged light illumination with an increase in layer thickness, themultijunction solar cells are significantly more stable. In these cells the total thickness of the a-Si:H absorber layers (first and second sub-cells) can be significantly increased up to 790 nm while keeping the degradation level low (below 13%) after 1000 hrs of light soaking. The possible origins of an improved stability against degradation are addressed in the paper.