Thermomechanical Property Measurement of Flexible Transparent Conductive-Film Substrates Based on Whole Folding Test by Reflective- Type Common-Path Liquid-Crystal Modulating Interferometry

To solve the temperature problem in the manufacturing process of flexible transparent conductive-film substrates, this study proposes reflective-type common-path liquid-crystal modulating interferometry with an innovative reflective optical heating mechanism that provides different tensile forces and temperatures to measure the thermal stress of flexible polyethylene terephthalate (PET) substrates with various indium tin oxide (ITO) thicknesses, as well as mono-layer and bilayer graphene/PET substrates. In addition, the durability of the flexible conductive substrates before and after 11000-cycle whole folding tests (WFTs) with a folding radius of 5 mm were analyzed using an automatic sliding-folding test platform. Consequently, the average thermal stress change rates of ITO/PET samples with ITO thicknesses of 80, 160, and 230 nm are 0.93%, 1.19%, and 1.85%, respectively. The average thermal stress change rates of the monolayer and bilayer graphene/PET samples are 0.67% and 2.38%, respectively. The resistance values of the samples and the resistance change rates of ITO/PET were measured with 80-, 160-, and 230-nm ITO were 1.57%, 2.44%, and 8.03%, respectively, and indicated that the rate increased as the film thickness increased. Moreover, confocal micro-Raman spectroscopy was used to measure the mechanical properties of monolayer and bilayer graphene/PET samples before and after WFT, indicating that the bilayer graphene/PET is inferior to the monolayer with respect to thermal stress. From the results obtained in this study, suggestions for improving the manufacturing process parameters of flexible transparent conductive-film substrates can be quickly provided using the proposed measurement system.