Design Proposals for Organic Materials Exhibiting a Low Exciton Binding Energy

Organic photovoltaics (OPV) have the potential for a low cost energy conversion from solar to electrical power. However, minimizing voltage losses, inherent to the currently used intermolecular donor acceptor system, represents the main challenge to make this technology suitable for commercial large area applications. One key parameter to overcome is the high exciton binding energy in organic materials. Based on density functional theory (DFT) simulations, it is shown how polar side chains may significantly increase the dielectric constant, which lowers the exciton binding energy and enables charge generation in organic materials without a donor acceptor system. Furthermore, based on time dependent (TD)-DFT calculations, we show another path for low exciton binding energy organic materials, by inducing a charge transfer state between two hydrogen bonded oligomers along the backbone. The charge transfer is achieved by fluorination of the acceptor oligomer investigated, leading to a wide-range intermolecular exciton, where the frontier orbitals are spatially separated from each other. Furthermore, we show that intramolecular charge transfer (CT) states and CT states between two pi-stacked molecules do not significantly influence the Coulomb interactions.