Electroabsorption study of ordered polyfluorene thin films: Origin of oscillatory structure near the bottom of the continuum state

Abstract

The excited-state structure of ordered ($\ensuremath{\beta}$-phase) poly(9,9-dioctylfluorene) (F8) thin films has been investigated by electroabsorption (EA) spectroscopy. The EA spectra of a number of $\ensuremath{\pi}$ conjugated polymers, including disordered (glassy) F8, are well explained by a theoretical essential-state model, which consists of three characteristic excited states and the ground state [Y. Kawabe et al., Phys. Rev. B 44, 6530 (1991)]. However, it is found that the essential-state model fails to explain the EA spectrum of $\ensuremath{\beta}$-phase F8 because of the appearance of an oscillatory EA signal near the bottom of the continuum state. From the applied electric-field dependence and the directional dependence of the field and polarization of the light, it has been revealed that the oscillation results from several intrachain excited states, some of which are not observed in glassy F8 because of considerable inhomogeneous broadening. To reproduce the EA spectrum of $\ensuremath{\beta}$-phase F8 by a sum-over-states calculation, at least seven excited states, in addition to the ground state, have to be taken into account.