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Organofunctional Silane Modification of Aluminum-Doped Zinc Oxide Surfaces as a Route to Stabilization Rachael Matthews,† Emily Glasser,† Samuel C. Sprawls,‡ Roger H. French,‡,§,∥,⊥ Timothy J. Peshek,§ Emily Pentzer,*,†,∥ and Ina T. Martin*,‡,§ Department of Chemistry, ‡Department of Physics, §Department of Materials Science and Engineering, ∥Department of Macromolecular Science and Engineering, and ⊥Department of Biomedical Engineering, Case Western Reserve University, 10900 Euclid Ave., Cleveland, Ohio 44106, United States
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ABSTRACT: Aluminum-doped zinc oxide (AZO) is a lowtemperature 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 (3aminopropyltriethoxysilane)-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. KEYWORDS: surface modification, transparent conductive oxide, aluminum-doped zinc oxide, degradation, electron mobility
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INTRODUCTION Transparent conductive oxides (TCOs), a subset of metal oxides, are of immense societal value due to their high transparency and conductivity. TCOs are critical components of advanced optoelectronic and photonic devices including photovoltaics (PV), light-emitting diodes (LEDs), gas sensors, and smart windows.1−7 Whereas the most common TCOs used in these applications are indium tin oxide (ITO)8 and fluorinated tin oxide (FTO), the former is unattractive due to the scarcity and expense of In, and the latter is limited by high processing temperature requirements (350−450 °C).2,9 An alternative TCO, aluminum-doped zinc oxide (AZO), consists of abundant materials that can be processed at low temperature. AZO deposition methods and parameters impact film thickness, composition, and crystallinity and therefore also the initial optical and electrical properties of the material.9−18 The surface © 2017 American Chemical Society
of AZO is populated with hydroxyl groups (Figure 1, left) and can adsorb and desorb atmospheric oxygen, carbon dioxide, and water.11−13 Currently, compared to ITO and FTO, AZO is relatively unstable and degrades in the presence of water, limiting its widespread use.10 Numerous studies have focused on optimizing AZO performance by varying annealing temperature,11,19 sputtering conditions,19−21 and doping content.22,23 Although fewer studies have focused on the stability of AZO performance in response to external factors, damp heat (DH) is a stressor that has been shown to primarily affect AZO conductivity, but not its optical properties.10,24,25 Additionally, the thickness and Received: February 22, 2017 Accepted: May 1, 2017 Published: May 1, 2017 17620
DOI: 10.1021/acsami.7b02638 ACS Appl. Mater. Interfaces 2017, 9, 17620−17628
Research Article
ACS Applied Materials & Interfaces
Herein we evaluate the degradation of bare AZO and APTES-modified AZO (AZO/APTES) using the standard IEC 61646 and 61215 protocol of 1000 h exposure to damp heat (DH, 85 °C and 85% relative humidity). Select samples were further exposed to DH for a total of 1500 h. The electrical (Hall mobility, carrier concentration, resistivity) and optical (percent transmittance, yellowness index, % haze) properties and morphology/composition (surface roughness, chemical composition, binding environment) of the samples were analyzed. Significant findings include that the changes in resistivity and Hall mobility of AZO/APTES from 1000 h of DH exposure are mitigated compared to bare AZO and that carrier concentration does not significantly change for either system. Further and more severe differences in the stability of the electrical properties and morphology are observed when comparing bare and modified AZO after 1500 h of DH exposure. Taken together, these data support that electrical degradation of AZO is slowed when surface functionalities attributed to degradation are covalently passivated and help establish a foundation for characterizing, understanding, and ultimately preventing AZO degradation.
Figure 1. Overview of the work presented herein: functionalization of AZO on glass with 3-aminopropyltriethoxysilane (APTES) and exposure of both bare AZO and AZO/APTES to damp heat (DH), followed by characterization of the optical and electronic properties, composition, and morphology.
microstructure of AZO films, as dictated by deposition method and parameters, influence the degradation pathways. Tohsophon et al. reported that compact films with large grain sizes (≥40 nm) and film thicknesses of >700 nm have more uniform electrical properties than thinner films (
