Environmental stability of transparent and conducting ITO thin films coated on flexible FEP and Kapton® substrates for spacecraft applications

• Published in: Solar energy materials and solar cells 2018-03, Vol.176, p.134-141
• Main Authors: Sibin, K.P., Mary Esther, A. Carmel, Shashikala, H.D., Dey, Arjun, Sridhara, N., Sharma, Anand Kumar, Barshilia, Harish C.
• Format: Article
• Language: English
• Subjects: Adhesion; Coating; Defluorination; Direct current; Electrical resistivity; Emission analysis; Emittance; Environmental testing; Etching; FEP; Field emission microscopy; Fluorinated ethylene propylenes; ITO thin film; Kapton; Kapton (trademark); Magnetron sputtering; Optical properties; Photoelectron spectroscopy; Plasma etching; Polyimide resins; Polymer films; Polymers; Relative humidity; Scanning electron microscopy; Space environment studies; Spacecraft; Sputtering; Stability; Studies; Substrates; Thermal cycling; Thin films; Vacuum
• ISSN: 0927-0248; 1879-3398
• DOI: 10.1016/j.solmat.2017.11.024

Acquiring good adhesion of ITO thin films on polymer substrate is a major concern, especially for space related applications. Delamination of ITO coating on these polymers can seriously damage the spacecraft. This paper presents the development of highly transparent and conducting ITO thin films on as-received and surface treated fluorinated ethylene propylene (FEP) and Kapton® substrates by reactive direct current magnetron sputtering. Stability of the ITO coating on FEP and Kapton® substrates was studied in simulated space environments. Environmental tests such as: relative humidity, thermal cycling and thermo vacuum were performed. Thermo-optical properties and sheet resistance of ITO coated FEP and Kapton® substrates were studied before and after environmental tests. Optimized ITO coating with thickness of ~ 15nm on FEP and Kapton® substrates showed sheet resistance in the range of 2–4kΩ/sq. with high average transmittance and high IR emittance. Adhesion of ITO coating on FEP substrate was improved by Ar plasma etching. X-ray photoelectron spectroscopy and field emission scanning electron microscopic studies of etched FEP substrate showed defluorination and high roughness of the etched surface which helped for better adhesion of ITO coating. We demonstrated that ITO coated plasma etched FEP substrate showed no change in the sheet resistance and thermo-optical properties. Moreover, ITO coated etched FEP substrate showed good environmental stability than ITO coated untreated FEP substrates. •ITO thin films were sputter deposited on flexible FEP, Kapton® and aluminized Kapton® substrates.•Thermo-optical and electrical properties of ITO coated FEP, Kapton® and aluminized Kapton® substrates were studied.•Stability of ITO coated flexible substrates in simulated space environments was investigated.•Detailed studies of plasma etched FEP substrate and ITO coating on plasma etched FEP substrate were carried out.