Low exciton binding energies from computational predictions: Towards the next generation of organic photovoltaics

Organic solar cells have the potential to become one of the key renewable energy technologies in future. This thesis addresses the problem of their low efficiency compared to the commonly used silicon solar cells. Currently, the best performing organic solar cell contains a blend composed of a donor material that conducts positive charges (holes) and an acceptor material that conducts negative charges (electrons).
The applied materials are typically characterised by a low dielectric constant. A low dielectric constant is responsible for the formation of bound electron-hole pairs, i.e., local excitons, on the donor or acceptor molecules after light absorption. These local excitons can split by electron or hole transfer between the donor and acceptor molecules. However, this step may lead to the generation of charge-transfer excitons in which the electron and hole are still bound. Two binding energies can be defined for these two types of excitons: the local exciton binding energy and the charge-transfer exciton binding energy. Lowering these binding energies is expected to improve the efficiency of organic solar cells.
Based on predictions using quantum chemical computational methods, several strategies are proposed to lower these binding energies in order to improve charge separation by increasing the dielectric constant of the applied donor and acceptor materials: including permanent dipoles in the side-chains of donor and acceptor molecules, and introducing functional chemical groups at certain positions in the molecules. Application of these promising strategies to the next generation of organic solar cells is expected to improve their efficiency.