Theoretical study of phenyl-substituted indacenodithiophene copolymers for high performance organic photovoltaics

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The theoretical estimation of energy levels and energy gaps of conjugated polymers for organic photovoltaics (OPVs) represents in principle a useful tool for the prescreening of new donor systems as a suitable pair for the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). In this study, ten tetraphenyl-substituted indacenodithiophene (IDT) copolymers (eight in the form of donor-acceptor), whose energy gaps vary in the range of 1.48-2.11 eV have been selected and their highest occupied molecular orbitals (HOMOs), lowest unoccupied molecular orbitals (LUMOs), and gap energies have been calculated by applying density functional theory (DFT) and/or time-dependent density functional theory (TD-DFT) methods. In spite of the examined molecular structure variety, nice correlations (theoretical models) between experimental and theoretical electronic parameters were found. It is shown that the theoretical band gap estimated by the TD-DFT using dimer model compounds and DFT using tetramer model compounds provide in good agreement the optical band gap of these polymers. Finally, the optimum theoretical limits of the LUMO offset between the fullerene and the IDT tetramer model compounds, for which high performance OPVs (efficiency > 6%) are obtained, is presented for the first time.