Absorption cross-sections of hole polarons in glassy and beta-phase polyfluorene

Abstract

Absorption induced by electrochemically injected holes is studied in poly-9,9-dioctylfluorene (PFO) films. Injected charges form positive polarons which are delocalised over four fluorene units in the glassy phase and about seven fluorene units in its beta-phase. Polaron absorption cross-sections at the 640 nm peak are similar to the published values of chemically reduced oligofluorenes in solution. The absorption cross-section of polaron in the beta-phase at 470 nm is about eight times smaller than the stimulated emission cross-section derived from published data. This indicates that beta-phase-rich PFO is an attractive candidate for a lightemitting layer in double-heterostructure organic laser diodes.