September 14, 2016

Studying Polaron Pair Formation with Ehrenfest Dynamics

A detailed understanding, with atomistic resolution, of the formation and dynamics of quasiparticles in semiconducting organic polymers is necessary in order to improve their performances. In this context a theoretical-computational approach based on the non-adiabatic excited state Ehrenfest dynamics is appropriate to describe both the formation and the evolution of such quasiparticles. In particular, in this work we have studied the formation of a polaron pair in a thiophene oligomer composed of seven monomers.

The formation and dynamics of the polaron pair is observed through the trend of the total dipole moment temporal evolution characterized by a peculiar decay and periods of dipole plateaus underlining that a charge trapping takes place during the dynamics. We have shown that the dynamics of the polaron pair is correlated to the nuclear dynamics, in particular to the inter-ring torsion motions causing changes in the rings coplanarity, and allowing to the charge to be blocked along the chain. The time resolved vibrational analysis based on the Wavelet transform allowed to catch the red shifts of the intra-ring C-C stretching motions, taking place because of the polaron pair formation, and to show that the dipole moment temporal evolution is also correlated to this motion.

Through this work we strongly underline that this technique is a precious instrument to give a molecular insight on complex non-equilibrium phenomena and the information obtained in this way can be used to improve the performances of such materials and to help in the design of more efficient ones. This work is published in the Journal of Physical Chemistry A and can be found here.