Tight-Binding Model Accurately Describes Frontier Orbitals of Conjugated Oligomer Acceptors for Organic Solar Cells

Conjugated organic small molecules are designed as acceptor materials for organic photovoltaic [OPV] cells. OPV optoelectronic properties, including light absorption, intramolecular and intermolecular charge transfer, depends on the properties of frontier molecular orbitals of these conjugated molecules. Recently, we have shown that tight-binding models can efficiently describe a broad range of optoelectronic properties for copolymers. Tight-binding parameters derived from density functional theory [DFT] calculations on constituent homopolymers reasonably predict copolymer valence and conduction bands. Here, we extend this approach to heterogeneous oligomers designed for non-polymeric acceptors including IDTBR, which gives high-efficiency OPV in combination with poly(3-hexylthiophene) [P3HT], a widely used donor. IDTBR consists of 7 aromatic moieties: an indacenodithiophene [IDT] core, flanked by benzothiadiazole and 3-ethylrhodadine on either side. We show that the tight-binding parameters for copolymers and dimers of the constituent moieties can be used to define the IDTBR tight-binding model, which reasonably predicts the energy and wavefunction of multiple frontier orbitals. As an example of the usefulness of the tight-binding approach, we calculate the absorption spectrum of IDTBR melt by averaging over possible conformational disorder, which would be challenging using DFT directly.