Effect of annealing on the electronic structure of poly(3-hexylthiophene) thin film

Abstract

The electronic structure and film structure of poly(3-hexylthiophene) (P3HT) have been studied by X-ray diffraction (XRD) measurements, ultraviolet-visible (UV-vis) absorption spectroscopy, near-edge X-ray absorption fine structure (NEXAFS) measurements, ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). As reported in previous works, XRD results show that the crystallinity of the film with regioregular P3HT is significantly improved by annealing at 170 °C. The effects of annealing on the electronic structure strongly depend on the substrate and the degree of regioregularity of the P3HT polymer backbone. Even in the case of the regiorandom sample, annealing considerably changes the vacuum level energy, which is the result of changes in the conformation of the hexyl groups at the free surface of the film. The π- and π*-band onsets uniformly shift downward by the annealing resulting in an increased hole-injection barrier at the electrode interface. The effects of annealing on the electronic structure of regioregular samples are more complex and depend on multiple factors. It is necessary to take into account variations in the π- and π*-band widths and the polarization energy to determine the effects of annealing. The former is associated with the conformation of the backbones of the polymer chains, and the latter is associated with the packing density of the conjugated polymer planes. The combination of these variations determines the effects of annealing on the electronic structure of the regioregular film. This is a possible reason for the strong dependence of the effects of annealing on the surface roughness of the substrate, since substrate roughness has a considerable effect on the morphology and crystallinity of regioregular P3HT films.