Oxidation Behaviors of Poly(3‐hexylthiophene‐2,5‐diyl) on Indium Tin Oxide Surfaces without and with Additional TiO2 Thin Films#

Abstract

Poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) layers spin‐coated on bare and TiO2 ‐coated indium tin oxide (ITO) surfaces with two different TiO2 film thicknesses were exposed to visible light and humid atmosphere, and the oxidation behaviors these layers were studied using X‐ray photoelectron spectroscopy. S atoms in P3HT were oxidized to sulfoxide without ring opening of P3HT under our experimental conditions. When the mean thickness of TiO2 on ITO was ~19 nm, oxidation of the S atoms on this substrate was more pronounced than on bare ITO and ITO covered with thinner TiO2 films (mean thickness: ~7 nm). Studies using photoluminescence (PL) suggest that electron–hole pairs created in P3HT on bare ITO can be transferred into the ITO and most likely recombine efficiently in ITO by electron–electron scattering. Relatively thin TiO2 films on ITO do not show much change in PL with respect to the case of bare ITO; however, when a thicker TiO2 film is deposited between ITO and P3HT, charge transfer from optically excited P3HT to ITO seems to be suppressed. Therefore, the probability for charge transfer from optically excited P3HT to O2 and H2O, forming strong oxidizing agents such as and OH radicals, can be increased in the presence of a thicker TiO2 film on ITO.