Electronic band structure of alternating fluorene-oxadiazole conjugated copolymer - A 1D solid-state DFT study

Fluorene-oxadiazole alternating copolymers OxFn (n = 1, 2, 3) are important electron transport and blue light-emitting materials of the (multilayer) polymeric light-emitting diodes. In this work we investigate their electronic structure properties using the density functional theory (DFT) approach. For comparison and calibration purposes we also perform the calculations for the simpler system such as poly(acetylene) (PA). All polymers are treated as one-dimensional (1D) infinite conjugated chains and periodic boundary condition (PBC) is applied to the repeat units. We consider six DFT approximations (PBE1PBE, B3LYP, O3LYP, OB95, TPSSTPSS, and PBEPBE) in the study. The 6-31G∗ basis set and 32 k points are employed in most of our solid state calculations. We compare the band gap (Egap), band width (Ewidth) and highest occupied and lowest unoccupied crystal orbital (HOCO and LUCO) energy levels with experimental data whenever possible. In all DFT calculations for OxFn, the band gaps are direct and the best agreement with experiment is obtained with hybrid DFT functionals. Hybrid functionals also give a better agreement with experimental data for ionization potentials. For electron affinities, the gradient-corrected functionals give better performance.