Enhanced charge-carrier mobility inβ-phase polyfluorene

Abstract

Alkyl substituted polyfluorenes are promising candidates for use in organic display applications due to efficient, pure blue (polarized) luminescence, high charge carrier mobility, and good processabilty. Poly(9,${9}^{\ensuremath{'}}$-dioctylfluorene) (PFO) is an especially interesting polyfluorene derivative, because of its self-organization into distinct supramolecular structures at room temperature. In addition to the amorphous glassy phase, PFO exhibits a unique packing behavior, the so-called $\ensuremath{\beta}$-phase formation, which leads to a higher degree of organization. We show that the $\ensuremath{\beta}$-phase is an energetically favorable environment for charge carriers. We look into the migration of charges from glassy polyfluorene to areas where PFO exhibits $\ensuremath{\beta}$-phase organization and find that the charge carrier mobility is higher in $\ensuremath{\beta}$-phase polyfluorene than in glassy polyfluorene. Our results illustrate that the order on a supramolecular scale determines the conductive properties of conjugated polymers to a large extent. We conclude that the performance of devices based on polyfluorene can be significantly improved by the enhancement of the supramolecular order.