Optical studies of photoexcitations of poly(9,9-dioctyl fluorene)

We have studied the interplay between the morphology and photophysics of poly(9,9-dioctyl fluorene) (PFO) films. Spin-casting from solution produces a glassy film with a broad absorption band and a large Stokes shift due to exciton migration. Upon slowly warming a sample from 80 to 300 K, a fraction of the sample converts into a new phase with extended conjugation, which we refer to as the β-phase. Its absorption band exhibits a clear vibronic structure, and the fluorescence red-shifts by approximately 0.1 eV. The photoinduced absorption (PA) spectrum of the glassy phase is dominated by triplet excitons, whereas, both charged polarons and triplet excitons are seen in the PA spectrum of a sample containing both phases.