The change in refractive index of poly(9,9-dioctylfluorene) due to the adoption of theβ-phase chain conformation

Abstract

The focus of this work is to study the refractive index in thin films of bluelight-emitting poly(9,9-dioctylfluorene) (PFO) that contain extended rigid chains(β-phase). Using a post-deposition exposure to toluene vapour,β-phase chains were induced in glassy PFO films that had been spin coated from a range ofsolution concentrations to provide thicknesses from 50–200 nm. With the aid of absorptionspectroscopy, a semi-empirical calculation of the change in refractive index due to the presence ofβ-phase chains is undertaken. Over the spectral region where optical gain occurs, the refractive indexof β-phase PFO is found to be appreciably larger than for as-spin-coated glassy PFO.Further development and explicit evaluation of the Kramers–Kronig relationslead to analytic and closed form expressions describing the change in indexΔn which are used to highlight the key contributions to index change. We find that the spectral responseof Δn is largely governed by the emergence of a characteristic absorption band associated with the presenceof β-phase chains. The changes in refractive index are experimentally studied by tracking thespectral shifts of optical resonances observed from films spin coated directly onto sub-wavelength1D grating structures. These measurements are supported and compared with rigorouscalculations of the spectra, which incorporate the full optical descriptions for glassy PFO andβ-phase PFO. The excellent agreement confirms the accuracy of our semi-empirical extractionapproach. Similar polymer-coated grating structures are examined under lasing conditionsand can also be used to support the expected spectral response and the decline ofΔn for longer wavelengths.