(274c) Tight-Binding Model Predicts Exciton Energetics and Structure on Conjugated Molecules Used in Organic Photovoltaics.

Conjugated molecules and polymers are designed as acceptor and donor materials for organic photovoltaic [OPV] cells. OPV performance depends on generation of free charge carriers through dissociation of excitons, which are electron-hole pairs created when a photon is absorbed. Here, we extend the tight-binding model to describe excitons on homo-oligomers, alternating co-oligomers, and a non-fullerene acceptor – IDTBR. We devise a novel approach to parametrize our model using DFT energies of neutral, anion, cation, and excited states of constituent moieties. For a molecule like IDTBR, with two ends where an exciton can sit, the product wavefunction exciton breaks symmetry. But with a tight-binding model, we can introduce full correlation between electron and hole, which allows the exciton to explore both ends of the molecule. We show that the tight-binding model predicts the singlet excitation (or optical gap) for oligomers of varying length in good agreement with time-dependent DFT and spectroscopic results.