Microstructure Control of Polymer Electronic and Optical Properties: Polyfluorene Conformation and Orientation for Optimized Cavity Exciton-Photon Coupling and Light Emission
Donal Bradley b
a Physical Science and Engineering Division, KAUST, Thuwal St, Dhahran, Saudi Arabia
b Department of Physics, University of Oxford, Clarendon Laboratory, Oxford OX1 3PU, U.K.
Proceedings of International Conference on Emerging Light Emitting Materials (EMLEM22)
Aspects of Emergent Light Emitters:
Limasol, Cyprus, 2022 October 3rd - 5th
Invited Speaker, Donal Bradley, presentation 060
Publication date: 15th July 2022

The many conformations of glassy conjugated polymer chains define the characteristic ensemble broadening of their optical and electrical density of states functions, with adjustments in microstructure typically leading to relatively modest changes in distribution peak energy and width. This allows little prospect for the practical use of conformation control since the resulting changes in properties are insufficiently stark. However, in the circumstance that the distribution of conformational energies is not simply Gaussian but rather includes a distinct and readily generated low-energy subspecies it becomes possible to use conformation as a vector for function control. This is the situation for ß-phase formation in poly(9,9-dioctylfluorene) (PFO) and related materials, where the resulting ß-phase segments contribute a clearly resolved, red shifted peak that superposes on the co-existing glassy phase ensemble. In essence, this situation parallels the opportunities afforded by configuration control using photoisomer pairs such as the cis and trans forms of azobenzene molecules and represents a novel molecular approach to metamaterials creation. ß-phase formation modulates refractive index, emission colour, charge carrier mobility and several aspects of photophysics which, combined with spatial patterning, then allows the fabrication of novel structures and the enhancement of device performance.

Optical environment, namely whether the polymer film emits into free-space or is confined within an optical structure that defines the available modes, also provides a means to tune optoelectronic properties. Of particular interest is the situation in which the polymer exciton dipole moment and cavity confinement are large enough that hybridisation occurs to form exciton-polariton states. Two main regimes arise, strong- and ultrastrong-coupling (with less-explored variants deep-strong and very-strong coupling) and these regimes offer novel application possibilities in relation to emission characteristics. Additional control of the polymer chain conformation or its alignment within such a microcavity then allows subtle control over the polariton physics, yielding desirable emission characteristics in terms of colour saturation and angular dispersion and generating record coupling strengths for which new physics can be explored.

Specific examples will be presented in this talk for PFO and fluorene-based copolymers, including poly((9,9-dioctylfluorene)x-co-(phenylenediamine)y), poly(9,9-dioctylfluorene-co-benzothiadiazole (F8BT) and poly(9,9-dihexylfluorene-co-bithiophene (F6T2), also cases where thermal control can be used to tune conformation (e.g.poly[4-(octyloxy)-9,9-diphenylfluorene]-co-[5-(octyloxy)-9,9-diphenylfluorene] (PODPF)).

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