Optical gain characteristics of β-phase poly(9,9-dioctylfluorene)**Based on a presentation at the 2006 International Conference on Synthetic Metals, Dublin,Ireland.

Abstract

We report an investigation of the effect of morphology on thegain properties of poly(9,9-dioctylfluorene) (PFO). The PFOβ-phase morphology has previously been reported to be detrimental to lasing threshold, aconclusion at odds, however, with pump–probe measurements on PFO/poly(methylmethacrylate)blend films that show enhanced stimulated emission characteristics forβ-phase chains. In order to understand these conflicting indications, we have undertaken a detailedstudy of the gain properties for spin-coated PFO films, including samples in which the fraction ofβ-phase chains is deliberately enhanced by post-deposition exposure totoluene vapour. We find that the amplified spontaneous emission (ASE)threshold (390 nm pump, 10 ns pulses, 10 Hz repetition rate) is of order80 nJ/pulse, independent of the presence of a significantβ-phase component. Surface emitting distributed feedback lasers comprising polymer-coatedsecond-order gratings etched into silica substrates are also insensitive to theβ-phase morphology: lasing threshold energies are equivalent so long as the structuresare tuned to the correct peak gain wavelength for each film morphology. Thisoccurs at the 0–1 vibronic peak in the corresponding photoluminescence emissionspectra, namely 465 nm for films with and 450 nm for films without a significantβ-phase component. We can thus conclude that whilst the introduction ofβ-phase chains leads to new lasing wavelengths (some 15 nm red shifted from those for films withoutβ-phase chains) it is not obviously detrimental to lasing performance. An additional effectdoes occur, however, when the pump beam energy is increased by one to two ordersof magnitude above the ASE threshold energy: the ASE peak position for theβ-phase films then migrates from 465 to 450 nm. This phenomenonis irreversible and appears to be the result of exciton quenching onβ-phase chains due to the photo-oxidative formation of fluorenone moieties.