Morphology-dependent energy transfer within polyfluorene thin films

We have performed a detailed study of the photoluminescence from thin films of blue-light-emitting poly(9,9-dioctylfluorene) containing different fractions of planarized (β-phase) chains within the glassy polymer film. By choosing solvents with a range of polarities and boiling points we were able to cast films with reliable control of the relative amounts of β-phase chains present. We analyzed the emission spectra in terms of Franck-Condon progressions and found that, at low temperatures (8 K), the luminescence can be modeled accurately by considering two distinct contributions from the two phases present in the film. The Huang-Rhys parameter for the β phase is shown to be approximately half the value obtained for the glassy phase, in agreement with a more delocalized exciton in the β phase. Time-resolved photoluminescence measurements on a film containing roughly 25% of β phase reveal a fast transfer of excitations from the glassy to the β phase, indicating that the two phases are well intermixed. Assuming the transfer dynamics to be governed by dipole-dipole coupling, we obtain a Förster radius of $8.2\pm 0.6\mathrm{nm},$ significantly larger than the radius typically found for excitation transfer within the glassy phase. These results are consistent with the large spectral overlap between the emission of the glassy phase and the absorption of the β phase and explain why the latter dominates the emission even from films containing only a small fraction of β-phase chains.