Phonon-Driven Ultrafast Exciton Dissociation at Donor-Acceptor Polymer Heterojunctions

A quantum-dynamical analysis of exciton dissociation at polymer heterojunctions is presented, using a hierarchical electron-phonon model parametrized for three electronic states and 28 vibrational modes. Two representative interfacial configurations are considered, both of which exhibit an ultrafast exciton decay. The efficiency of the process depends critically on the presence of intermediate bridge states, and on the dynamical interplay of high- vs low-frequency phonon modes. The ultrafast, highly nonequilibrium dynamics is consistent with time-resolved spectroscopic observations.

Figure 1

Schematic illustration of (a) the eclipsed vs staggered configurations of the TFB:F8BT donor-acceptor heterojunction under study and (b) the photoexcitation process and potential crossings, involving the XT (green or gray), CT (mauve or dark gray), and IS (ochre or light gray) states. The minima and Franck-Condon (vertical) energies correspond to the computed adiabatic (relaxed) and vertical excitation energies, respectively, for the E configuration.

Figure 2

Projections of the coupled diabatic XT, CT, and IS PESs (E configuration) onto the XT-CT branching plane. The white and black circles indicate the conical intersection and Franck-Condon geometry, respectively.

Figure 3

Time-evolving state populations for (a) the full 3-state 28-mode wave packet propagation for the E configuration, and (b) a complementary 2-state 28-mode calculation for the E configuration that was restricted to the XT-CT subspace; (c) and (d) are analogous calculations for the S configuration.

Figure 4

Time-evolving XT population for the truncated HEP Hamiltonian ${\mathbf{H}}^{(n)}$ of Eq. (4), for $n=0$ (6 modes), $n=1$ (12 modes), and $n=2$ (18 modes), as compared with the full-dimensional 28-mode result, for both the E (left-hand side) and S (right-hand side) configurations [22].