Microscopic Investigation of the Excitons' Intermolecular Energy Migration in the β phase of Poly(9-9-Dioctyl Fluorene) by Confocal Laser Spectroscopy

We investigated, by confocal laser spectroscopy, the role of the microscopic morphology on the fluorescence spectra of poly(9,9-dioctylfluorene) (PF8) thin films self-doped by the PF8 β phase. We demonstrate the existence, on the micron scale, of different regions in the films, characterized by locally different fluorescence spectra. We show that the microscopic morphology irregularities lead to locally nonuniform β-phase density resulting in the switching on or off of the intermolecular energy migration within the β-phase excited-state distribution. This effect causes considerable local variation of the fluorescence spectra consisting of a progressive red shift (up to 26 meV) and a line width narrowing from about 70 (similar to the absorption one) down to 52 meV.