Polaron Energetics of Organic Semiconductors Analyzed by Broken-Symmetry ab-initio Molecular Dynamics

Molecular doping is crucial for enhancing the electric conductivity of organic semiconductors (OSCs). However, the insufficient mechanistic understanding of molecular doping hinders the optimization toward higher doping efficiency, and therefore calls for a thorough theoretical description of the polaron behavior. It is generally accepted that the integer charge transfer (ICT) between OSCs and molecular dopants, rather than electronic hybridization, plays a fundamental role in effective doping. The process of ICT is difficult to describe using spin-restricted DFT or computationally expensive multireference methods. Here we show that broken-symmetry (BS) DFT calculations can provide a good compromise by predicting ICT with manageable computational cost. By using BS-DFT we find that OSC:dopant complexes of increased size can lead to multiple ICT configurations that lead to delocalized polarons. Coulomb binding of the polaron is significantly reduced compared with individual bi-molecular pairs, facilitating an easier charge separation. Furthermore, we perform ab-initio molecular dynamics to study the polaron dynamics in OSC aggregates and the role of electron-phonon coupling therein. We find that the interchain polaron hopping is determined by the polaron energy level of the single polymer geometry. From our results, we deduce that the combined effects of polymer lattice dynamics and stabilization of the polaron by the OSC ordering crucially influence the tendency of polaron hopping.