Morphology dependence of the triplet excited state formation and absorption in polyfluorene

Poly(9,9-dioctylfluorene) (PFO) is widely used as a highly efficient blue-emitting polymer for light-emitting diode applications. Films of PFO are known to contain a planar $\ensuremath{\beta}$-phase or a disordered glassy phase that affects the emission characteristics. We have studied the morphology dependence of the photoinduced absorption in such PFO films and find that the ${T}_{1}\text{\penalty1000-\hskip0pt}{T}_{\mathrm{n}}$ absorption signal differs between the two phases in energy, linewidth, and intensity. The lower intensity we observe in the $\ensuremath{\beta}$-phase as compared to the glassy phase is evidence for a lower ${T}_{1}$ population in the former. We attribute this to firstly, reduced intersystem crossing and secondly, increased charge generation due to both more extended conjugation along the chains and higher interchain interactions in the planar, well-ordered $\ensuremath{\beta}$-phase. The reduced energy and smaller linewidth of the triplet absorption in the $\ensuremath{\beta}$-phase as compared to the glassy phase are indicative of a more delocalized ${T}_{\mathrm{n}}$ state and a higher degree of order in the $\ensuremath{\beta}$-phase, respectively.