Thickness-dependent linear and nonlinear optical properties of spin-coated iron (III) tetraphenylporphyrin chloride thin films

Abstract

This study investigated the thickness-dependent structural, linear, and nonlinear optical properties of spin-coated iron (III) tetraphenylporphyrin chloride (FeTPPCl) thin films. Thin films with thicknesses ranging from 90 to 480 nm were fabricated on quartz substrate via spin coating method, and then characterized using X-ray diffraction (XRD), scanning electron microscope (SEM), Raman spectroscopy, UV–Vis–NIR spectroscopy, and photoluminescence (PL) spectroscopy. The results revealed that film thickness significantly influenced crystallinity, surface morphology, and optical behavior. Thicker films exhibited significantly increased crystallite size (from 23.09 to 50.18 nm), enhanced crystallinity, and reduced structural defects, as evidenced by XRD analyses and SEM images. UV–Vis spectroscopy showed a redshift in absorption peaks and a systematic reduction in the optical bandgap with increasing thickness. Urbach energy (E_U) analysis indicated a decrease in disorder and localized states in thicker thin films, corroborating improved structural order. Photoluminescence (PL) studies revealed a thickness-dependent emission redshift and intensity increase, attributed to enhanced crystallinity and reduced surface traps. The absorbance coefficient (α) was employed to identify the nature of optical transitions in the films. The optical constants and dielectric properties exhibited notable thickness-dependent trends, consistent with the Wemple-DiDomenico and Sellmeier models. Furthermore, nonlinear optical properties, including third-order susceptibility $\left({\chi }^{\left(3\right)}\right)$, nonlinear refractive index $\left(n_2\right)\text{,}$ and two-photon absorption coefficient (βc), were modulated by film thickness, highlighting FeTPPCl's potential for nonlinear optical applications. These findings underscore the critical role of thickness control in tailoring the optoelectronic performance of solution-processed FeTPPCl thin films for advanced device applications.