In this work, hydrogenated amorphous and polymorphous silicon (a-Si:H and pm-Si:H) solar cells were tested for the reversibility of their light-induced degradation by subjecting them to light-soaking and thermal annealing cycles. The tests were done under one sun at 40 degrees C. The pm-Si:H solar cells show an irreversible degradation at the initial stage of light-soaking (five hours), while having higher stabilized efficiency with respect to a-Si:H cells. To the best of our knowledge, such irreversible degradation of pm-Si:H solar cells has never been reported and cannot be explained by the conventional paradigm based on the Staebler-Wronski Effect. The results lead us to view the kinetics from a different perspective in which stimulated hydrogen motion-such as that which occurs in a-Si:H under extreme conditions-takes place in pm-Si:H under conditions close to typical device operation, and introduces structural changes that affect the solar cell parameters in an irreversible way. (c) 2012 Elsevier B.V. All rights reserved.