Characterization of transparent conductive ZnO and Ga-doped ZnO films on polyethylene naphthalate sheets aged for six years in ambient atmospheric

Abstract

Transparent conductive ZnO and Ga-doped (GZO) films that had been deposited on polyethylene naphthalate (PEN) sheet substrates were aged in ambient atmosphere conditions for six years. The films then evaluated to characterize the degradation in their sheet resistance over the substrate surface due to moisture permeation. The conductive properties of the ZnO and GZO films deposited on PEN substrates were more degraded compared to those for films deposited on glass substrates, exhibiting a nonuniform distribution of resistance. While ZnO and GZO films thinner than 100 nm were particularly sensitive to moisture infiltration, the increase in sheet resistance relative to the initial value became less obvious as thickness increased. This observation indicates that moisture permeation is limited to a certain depth from the surface. While GZO films thicker than 100 nm were free from signs of aging at Ga concentrations corresponding to standard transparent electrodes, regions with Ga concentrations more than 10% suffered from aging. This phenomenon has been observed in GZO films on glass substrates. The signs of aging were more serious for the ZnO films compared to the GZO films, as the oxygen vacancies in the ZnO films acted as reaction centers with H2O molecules. Furthermore, ZnO films thicker than 200 nm cracked because of misfit strain between the film and the PEN substrate, and cracks were widened and deeper for the thicker films. The cracked films experienced far greater deterioration than the films without cracks because moisture could easily enter the cracks and reach deep regions of the films.