Excitons and Polarons in Organic Materials

Excitons and polarons play a critical role in the electronic and optical properties of aggregates, thin films and crystals of π-conjugated molecules and polymers. In such systems neutral excitations (excitons) and charged excitations (polarons) are coupled strongly to nuclear degrees of freedom involving the symmetric vinyl-stretching mode (i.e. the aromatic-quinoidal vibration when conjugated rings are present). In this talk the photophysical properties of excitons and polarons in organic materials are compared and contrasted within the framework of the Holstein Hamiltonian, which treats electronic coupling and local electron-vibrational coupling on equal footing.[1] Applications are made to excitons  and positively-changed polarons (holes) in poly(3-hexylthiophene) films.[2, 3] It is shown how simple optical probes can be used to extract the exciton and polaron coherence lengths, quantities which are important for understanding energy and charge transport.

References:

[1]   R. Ghosh and F. C. Spano, Excitons and Polarons in Organic Materials, Acc. Chem. Res 53 (10), 2201-2211 (2020).

[2]   J. Clark, C. Silva, R. H. Friend and F. C. Spano, Role of intermolecular coupling in the photophysics of disordered organic semiconductors: Aggregate emission in regioregular polythiophene, Phys. Rev. Lett. 98 (20), 206406 (2007).

[3]   R. Ghosh, A. R. Chew, J. Onorato, V. Pakhnyuk, C. K. Luscombe, A. Salleo and F. C. Spano, Spectral Signatures and Spatial Coherence of Bound and Unbound Polarons in P3HT Films: Theory Versus Experiment, J. Phys. Chem. C 122(31), 18048-18060 (2018)