A study has been carried out on a-Si:H solar cell materials fabricated under a wide range of deposition conditions in different laboratories. The results on both thin films and corresponding Schottky barrier cell structures demonstrate that analysis and characterization based solely on the neutral dangling bonds are clearly inadequate. Contributions of charged defects to the properties of a-Si:H their effect on light-induced changes are identified together with the limitations of methods commonly used to characterize the solar cell properties and stability of a-Si:H materials. Self-consistent fitting of a wide range of results on films and Schottky barrier cell structures is obtained with a gap state distribution in which charged defects are included. (C) 2001 Published by Elsevier Science B.V. All rights reserved.