Visible Light-Induced Oxidation of Poly(3-Hexylthiophene-2,5-Diyl) Thin Films on ZnO Surfaces Under Humid Conditions : Study of Light Wavelength Dependence

Abstract

The oxidation behavior of poly(3-hexylthiophene-2,5-diyl) (P3HT) thin films (~60 nm thickness)/ZnO irradiated with three different wavelengths of visible light (blue, green, and red) in a humid atmosphere were studied using X-ray photoelectron spectroscopy (XPS) and UV-vis spectroscopy. The formation of sulfoxide states upon visible light irradiation was observed in the S 2p core-level XPS spectra, and more importantly, this oxidation became pronounced with decreasing wavelength of incident light. Photo-oxidation of P3HT films also resulted in a reduction in optical absorption. In contrast to the XPS results, changes in the UV-vis absorption spectrum were rather insensitive to the wavelength of incident visible light. The wavelength dependency of the photo-oxidation of P3HT films seen in the XPS spectra is attributed to the more pronounced photo-induced oxidation of locally disordered thiophene rings on the surfaces of P3HT films under irradiation with shorter-wavelength visible light. The population of local-disordered sites that increases the optical transition gap compared to that of the well-ordered bulk P3HT film decreases from the top surface to the interior of P3HT films due to stronger interchain interactions in the interior portion of the films. Therefore, changes in the optical absorbance seen in the UV-vis absorption spectra of the entire P3HT film upon photo-induced oxidation are less sensitive to the wavelength of incident light in the visible regime.