Film morphology and photophysics of polyfluorene

We have studied the interplay between photophysics and film morphology of poly(9,9-dioctyl)fluorene (PFO) using a variety of optical probes. Upon slowly warming a spin-cast PFO film from 80 to 300 K, a fraction of the sample is transformed into a different solid phase, the $\beta$ phase. Absorption and electroabsorption measurements show that the $\beta$ phase has more extended conjugation than the glassy phase. As a consequence, excited states of the $\beta$ phase are redshifted and have higher polarizability. The photoinduced absorption spectrum of a glassy PFO film is dominated by triplet excitons, whereas both polarons and triplet excitons are seen in a sample containing a fraction of the $\beta$ phase. The dependence of the photoinduced absorption and photocurrent upon the excitation wavelength shows that there is a clear link between polaron and triplet photogeneration.