A complete understanding of the reliability for ZnO-based transparent conducting oxides is essential for actual applications in photovoltaic devices or displays requiring long-term stability. The stability and degradation mechanisms under a controlled damp-heat environment (humid and hot atmosphere) for sputter-deposited aluminum-doped zinc oxide (ZnO:Al) thin films were quantitatively studied. The continuous degradations of carrier concentration and mobility with the Fermi-level shift were observed, and this behavior was resolved by separating the changes in the carrier-transport characteristics of the intragrain and grain boundary. By correlating the temperature dependence of electrical characteristics with x-ray photoelectron spectroscopy, the degradation is well explained by the increase of chemisorbed OH- in the grain boundaries. (C) 2013 Elsevier B.V. All rights reserved.