Thiophene-furan oligomers: beyond-DFT study of electronic and optical properties

Thiophene oligomers are an important class of organic materials for photovoltaic applications, owing to their unique optoelectronic properties. Recently it has been suggested that incorporation of furan units to the thiophene chains, maintaining the chain structure, namely thienylfuran linear oligomers, can bring improvements to the final material. In this work, we present a theoretical study of thiophene, furan and thienylfuran short chains, up to 4 units. Structural and electronic properties were obtained using Hartree-Fock (HF) and density functional theory (DFT) calculations plus beyond mean-field methodologies, specifically second-order M & oslash;ller-Plesset perturbation theory on HF (HF-MP2) and many-body perturbation theory by the G0W0 approximation on DFT (G0W0@DFT). The optical properties were calculated on top of G0W0@DFT data using the Bethe-Salpeter equation. We investigate properties from the monomers T and F to tetramers with different sequencing of units TT, TF or FF, always bonded through the usual carbon atoms. As well known for the uniform oligothiophene chains, also here for any TT sequencing we find a torsion angle of 150 degrees, while all other sequencing of units result planar, which can be relevant for film producing. Also we find that the first optical transitions for the oligomers reaches a promising threshold of eV at the tetramer length, combined with ionization potentials around similar to 6.5-7 eV compatible with usual semiconductor/substrate band alignments, which confirms the relevance of these organic compounds for photovoltaic applications.