Irene Burghardt

Professor Irene Burghardt, Goethe-Universitat. (http://www.theochem.uni-frankfurt.de/) Elementary processes in functional organic polymer materials: Does quantum coherence matter?  Greater Boston Area Theoretical Chemistry Lecture 

Abstract:  As suggested by recent experimental studies, quantum coherence may be a driving force behind the ultrafast elementary processes in organic photovoltaics, very similarly to previous observations for biological light-harvesting systems. In this lecture, we will examine the intricate interplay of delocalized electronic excitations (i.e., electron-hole quasi-particle states, or excitons), electron-phonon coupling, and static and dynamic disorder, in order to assess the role of coherent transfer phenomena. The first part of the lecture will set the stage by introducing a vibronic coupling Hamiltonian in a generalized electron-hole representation, along with ab initio based parametrization strategies that include realistic phonon spectral densities. Quantum dynamical studies based upon this Hamiltonian are carried out using the Multi-Configuration Time-Dependent Hartree (MCTDH) method and non-Markovian reduced dynamics approaches. In the second part of the lecture, will specifically address (i) the dynamics of exciton migration across a torsional defect that locally breaks the pi-conjugation in oligo-(p-phenylene vinylene) and oligothiophene fragments, and (ii) ultrafast exciton dissociation in typical donor-acceptor complexes representing models of polymer-fullerene heterojunctions. Rapid free carrier generation from the primary interfacial charge transfer (CT) state is shown to be feasible, due to an effective lowering of the Coulomb barrier as a result of charge delocalization, along with the vibronically hot nature of the primary CT state. Against this background, a perspective will be given on the connection between ultrafast charge separation and high internal quantum efficiencies.