Effects of deposition conditions of TiO₂ film and substrate prestrain on film void geometries and optical properties in TiO₂/PET specimens

TiO₂/poly(ethylene terephthalate) (PET) specimens with various PET substrate prestrains are prepared to investigate the effect of the deposition conditions of the TiO₂ film on the geometries of TiO₂ film porosities and the effect of these void geometries on three optical parameters. The response surface methodology is used to express these three geometric parameters and three optical parameters as functions of the substrate prestrain and four deposition parameters. An increase in the O₂/Ar flow ratio increases the mean width of voids. The trends of the curved surface of optical transmittance with L∗ and L∗/W∗ are opposite to those of reflection. The highest transmittance is obtained with the largest L∗/W∗ and the smallest L∗. A nonzero flow rate of O₂ gas in the deposition process reduces the optical band gap and increases the TiO₂ film thickness generally, thus resulting in a reduction in transmittance and an increase in absorption. An increase in the prestrain of the PET substrate decreases the TiO₂ film thickness. Both the mean index of refraction (n)_mean and the mean extinction coefficient (k)_mean due to the provision of the O₂ gas are reduced. The n and k parameters for various wavelengths vary in a narrow range.