Regensburg 2004 – scientific programme

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SYOH: Organic and Hybrid Systems for Future Electronics

SYOH 2: Spectroscopy

SYOH 2.3: Talk

Thursday, March 11, 2004, 15:15–15:30, H37

Morphology-dependent energy transfer within polyfluorene thin films — •Anna Köhler, Amena L.T. Khan, Paiboon Sreearunothai, Laura M. Herz, and Michael J. Banach — Cavendish Laboratory, University of Cambridge, Cambridge CB3 OHE, U.K.

Polyflourenes are a class of blue luminescent organic semiconductors with photophysical properties that vary strongly with the morphology of the film. In Poly (9,9-dioctylfluorene) (PFO), distinct phases of molecular order have been identified such as the disordered glassy phase and crystalline alpha and beta-phases where the PFO chains are considered to be planar. We present a detailed study of the interrelationship between morphology and photophysics in thin films of PFO. Small amounts of the crystalline beta-phase dominate the emission characteristics. We demonstrate that the formation of the two phases during spin-coating can be controlled reliably through the choice of the boiling point and solubility parameter of the solvent. A Franck-Condon analysis shows that the planar chains are characterized by a low Huang-Rhys parameter, consistent with well delocalised excited states, while the non-planar chains of the glassy phase show high values for the Huang-Rhys parameter. The close intermixing of the two phases and the large spectral overlap of emission from the glassy phase and absorption from the b-phase leads to efficient transfer on a sub-picosecond timescale with a large Förster radius of about 8 nm. As a result, the steady state emission spectra show a significant fraction of emission from the beta-phase if only a few percent of the chains are planarized.