Exciton transfer in organic photovoltaic cells: A role of local and nonlocal electron-phonon interactions in a donor domain.

We theoretically investigate an exciton transfer process in a donor domain of organic photovoltaic cells focusing on the roles of local and nonlocal electron-phonon interactions. Our model consists of a three-level system described by the Holstein-Peierls Hamiltonian coupled to multiple heat baths for local and nonlocal molecular modes characterized by Brownian spectral distribution functions. We chose tetracene as a reference donor molecule, where the spectral distribution functions of the local and nonlocal modes are available. We then employ the reduced hierarchical equations of motion approach to simulate the dynamics of the system under the influence of the environment as a function of the electron-phonon coupling strength and temperature. We rigorously calculate the reduced density matrix elements to explain the time scale of dynamics under the influence of the dissipative local and nonlocal modes. The results indicate that the strong nonlocal electron-phonon interaction under high temperature conditions favors the exciton transfer process and enhances the efficiency of organic photovoltaic materials, while the lifetime of the exciton becomes shorter due to a low-frequency local mode.