The degradation behavior of pin solar cells from hydrogenated amorphous silicon (a‐Si:H) having uncontaminated i‐layers and that of solar cells in whih the i‐layer is intentionally contaminated with oxygen or water vapour, was studied using a voltage‐biased spectral response measurement method. The light‐soaked solar cells are characterized by a reduction of the transition voltage UT which separates the primary and the secondary photocurrent, by an enhancement of the secondary photocurrent, by a reduction of the μτ products of the charge carriers and by a kink of the collection efficiency curve in the blue part of the spectrum. A contamination of the i‐layer with impurities like O and H2O amplifies this degradation behavior of the cells. The kink is removable by a bias light as well as by a reverse bias voltage. The shape of the measured collection efficiency curves ar explained in detail by the electric field distribution in the i‐layer.