Anisotropic optical constants of electroluminescent conjugated polymer thin films determined by variable-angle spectroscopic ellipsometry

Abstract

This article reports on in-plane and out-of-plane refractive index (n) and extinction coefficient (k) values measured using variable-angle spectroscopic ellipsometry for poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) thin films of different molecular weights (Mn=9−255 kg∕mol), both in the pristine and annealed states. The in-plane n and k values are generally larger than the out-of-plane values for all pristine films leading to a measurable optical anisotropy which becomes much stronger as F8BT molecular weight increases. This indicates that polymer chains lie preferentially in the plane of the substrate and this configuration is more energetically favorable for longer polymer chains. Upon annealing, a larger reduction in kout-of-plane than in kin-plane is measured leading to a further increase in optical anisotropy. A redistribution of oscillator strengths and a broadening toward lower energies in absorption spectra are also observed indicating significant restructuring of F8BT chains upon annealing.