Highly conductive ZnO:Al films are often used as a transparent conductive oxide (TCO) in photovoltaic devices. The high optical gap, high conductivity, high stability in hydrogen plasma, and the relative abundance of the composing elements make ZnO:Al ideal for application in mass production of silicon thin-film technology. The long-term electrical stability is an important issue. To investigate degradation processes, accelerated aging in a damp-heat was carried out in both H2O and D2O environments. A decrease in conductivity is observed over a period of 1000 hour, in particular for surface etched samples. We show that the sample degradation is caused by grain boundary modifications and suggest a barrier model to explain the observed degradation. The deuterium isotope was used as a marker for secondary ion mass spectrometry (SIMS) measurements to distinguish between hydrogen incorporated during or just after deposition and deuterium incorporated during the damp-heat treatment. SIMS measurements show a linear build-up of deuterium in samples treated up to 300 hours, as well as a D-H replacement mechanism. This suggests weak bonds exist between ZnO and H2O, and may explain the partial reversibility of damp-heat degradation observed by others.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 2774-2778