Effect of static external electric field on bulk and interfaces in organic solar cell systems: A density-functional-theory-based study.

In this work, a detailed comparison of optical and electronic properties in bulk and interfaces of well-known organic semiconductor systems in presence of an external electric field is reported. We have used density functional theory (DFT) to model organic solar cell systems. The study promotes a deeper understanding of the connection between the chemical structures and the optical and electronic properties in the well-known organic solar cell systems based on thiophene and fullerene polymers. We have performed a vibration-mode analysis by simulating Raman spectra in presence of external electric fields. Time-dependent DFT has been used to investigate the effect of an external electric field on excited state properties. The charge-transfer rate controlled by the external electric field has been quantitatively extracted using the simulated excited state dipole moment, Gibbs free energy, and Marcus theory. Our results provide a detailed characterization of the effect of the external electric field on the neat polymers (bulk) and on the donor-acceptor heterojunctions (interfaces) in organic solar cell systems. This theoretical approach not only helps to understand the effect of an external field on bulk and interfaces in organic semiconductors, but it also supports the design of novel devices.