Organofunctional Silane Modification of Aluminum-Doped Zinc Oxide Surfaces as a Route to Stabilization

Aluminum-doped zinc oxide (AZO) is a low-temperature processed transparent conductive oxide (TCO) made of earth abundant elements; its applications are currently limited by instability to heat, moisture, and acidic conditions. We demonstrate that the application of an organofunctional silane modifier mitigates AZO degradation and explore the interplay between performance and material composition and morphology. Specifically, we evaluate degradation of bare AZO and APTES (3-aminopropyltriethoxysilane)-modified AZO in response to damp heat (DH, 85 °C, 85% relative humidity) exposure over 1000 h and then demonstrate how surface modification impacts changes in electrical and optical properties and chemical composition in one of the most thorough studies to date. Hall measurements show that the resistivity of AZO increases due to a decrease in electron mobility, with no commensurate change in carrier concentration. APTES decelerates this electrical degradation, without affecting AZO optical properties. Percent transmission and yellowness index of an ensemble of bare and modified AZO are stable upon DH exposure, but haze increases slightly for a discrete sample of modified AZO. Atomic force microscopy (AFM) and optical profilometer (OP) measurements do not show evidence of pitting or delamination after 1000 h DH exposure but indicate a slight increase in surface roughness on both the nanometer and micrometer length scales. X-ray photoelectron spectroscopy data (XPS) reveal that the surface composition of bare and silanized AZO is stable over this time frame; oxygen vacancies, as measured by XPS, are also stable with DH exposure, which, together with transmission and Hall measurements, indicate stable carrier concentrations. However, after 1500 h of DH exposure, only bare AZO shows signs of catastrophic destruction. Comparison of the data presented herein to previous reports indicates that the initial AZO composition and microstructure dictate the degradation profile. This work demonstrates that surface modification slows the bulk degradation of AZO and provides insight into how the material can be more widely used as a transparent electrode in the next generation of optoelectronic devices.