Tight-binding model predicts exciton energetics and structure for photovoltaic molecules

Conjugated molecules and polymers are being 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 develop a tight-binding model to describe excitons on homo-oligomers, alternating co-oligomers, and a non-fullerene acceptor - IDTBR. We parameterize our model using density functional theory (DFT) energies of neutral, anion, cation, and excited states of constituent moieties. A symmetric molecule like IDTBR has two ends where an exciton can sit; but the product wavefunction approximation for the exciton breaks symmetry. So, we introduce a tight-binding model with full correlation between electron and hole, which allows the exciton to coherently explore both ends of the molecule. Our approach predicts optical singlet excitation energies for oligomers of varying length as well as IDTBR in good agreement with time-dependent DFT and spectroscopic results.

Tight binding models accurately predict band structures for copolymer semiconductors

Conjugated polymers possess a wide range of desirable properties including accessible band gaps, plasticity, tunability, mechanical flexibility and synthetic versatility, making them attractive for use as active materials in organic photovoltaics (OPVs). In particular, push-pull copolymers, consisting of alternating electron-rich and electron-deficient moieties, offer broad optical absorption, tunable band gaps, and increased charge transfer between monomer units. However, the large number of possible monomer combinations to explore means screening OPV copolymers by first-principles quantum calculations is computationally intensive. If copolymer band structures could be rapidly computed from homopolymer data, potential materials could be screened more efficiently. In this work, we construct tight binding models of copolymer band structures with parameters determined by density functional theory (DFT) calculations on homopolymers. We use these models to predict copolymer valence and conduction bands, which compare well to direct DFT calculations of copolymer band structures.