Excitons and Polarons in Organic Materials*

Excitons and polarons play a critical role in the electronic properties of organic molecular aggregates as well as conjugated polymers and aggregates thereof. For many conjugated organic molecules 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 conjugated organic materials are compared and contrasted within the framework of a theory based on the Holstein Hamiltonian, which treats local electron-vibrational coupling exactly. Applications are made to excitons in perylene diimide crystals [1] and dopant-induced positively-changed polarons (holes) in poly(3-hexylthiophene) films. [2,3 ] It is shown how simple optical probes like steady-state absorption and photoluminescence can be used to extract the exciton and polaron coherence lengths, quantities which are important for understanding energy and charge transport.



(1) Oleson, A.; Zhu, T.; Dunn, I. S.; Bialas, D.; Bai, Y.; Zhang, W. Q.; Dai, M. J.; Reichman, D. R.; Tempelaar, R.; Huang, L. B.et. al. Perylene Diimide-Based Hj- and Hj-Aggregates: The Prospect of Exciton Band Shape Engineering in Organic Materials. J. Phys. Chem. C 2019, 123, 20567-20578.
(2) Ghosh, R.; Chew, A. R.; Onorato, J.; Pakhnyuk, V.; Luscombe, C. K.; Salleo, A.; Spano, F. C. Spectral Signatures and Spatial Coherence of Bound and Unbound Polarons in P3ht Films: Theory Versus Experiment. J. Phys. Chem. C 2018, 122, 18048-18060.
(3) Chew, A. R.; Ghosh, R.; Pakhnyuk, V.; Onorato, J.; Davidson, E. C.; Segalman, R. A.; Luscombe, C. K.; Spano, F. C.; Salleo, A. Unraveling the Effect of Conformational and Electronic Disorder in the Charge Transport Processes of Semiconducting Polymers. Adv. Funct. Mater. 2018, 28, 1804142.