Optical gain characteristics of β-phase poly(9,9-dioctylfluorene)

We report an investigation of the effect of morphology on the gain properties of poly(9,9-dioctylfluorene) (PFO). The PFO β-phase morphology has previously been reported to be detrimental to lasing threshold, a conclusion at odds, however, with pump-probe measurements on PFO/poly(methylmethacrylate) blend films that show enhanced stimulated emission characteristics for β-phase chains. In order to understand these conflicting indications, we have undertaken a detailed study of the gain properties for spin-coated PFO films, including samples in which the fraction of β-phase chains is deliberately enhanced by post-deposition exposure to toluene vapour. We find that the amplified spontaneous emission (ASE) threshold (390 nm pump, 10 ns pulses, 10 Hz repetition rate) is of order 80 nJ/pulse, independent of the presence of a significant β-phase component. Surface emitting distributed feedback lasers comprising polymer-coated second-order gratings etched into silica substrates are also insensitive to the β-phase morphology: lasing threshold energies are equivalent so long as the structures are tuned to the correct peak gain wavelength for each film morphology. This occurs at the 0-1 vibronic peak in the corresponding photoluminescence emission spectra, namely 465 nm for films with and 450 nm for films without a significant β-phase component. We can thus conclude that whilst the introduction of β-phase chains leads to new lasing wavelengths (some 15 nm red shifted from those for films without β-phase chains) it is not obviously detrimental to lasing performance. An additional effect does occur, however, when the pump beam energy is increased by one to two orders of magnitude above the ASE threshold energy: the ASE peak position for the β-phase films then migrates from 465 to 450 nm. This phenomenon is irreversible and appears to be the result of exciton quenching on β-phase chains due to the photo-oxidative formation of fluorenone moieties. © IOP Publishing Ltd.