Density functional theory study of electronic band structures of fluorene-based conjugated polymers

Ling, Lin (2007) Density functional theory study of electronic band structures of fluorene-based conjugated polymers. Masters thesis, Memorial University of Newfoundland.

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Abstract

Alternating triphenylamine-fluorene, TPAFn (n=2,3), and fluorene-oxadiazole copolymers OxFn (n=2,3) are important components of novel high-efficiency multilayer polymeric blue light-emitting diodes [1]. In this work we investigate their electronic structure properties using a computational approach. For comparison and calibration purposes we also perform the calculations for poly(acetylene) (PA). The band gaps (E gap’s) and band widths (E width’s) are studied with the Hartree-Fock (HF) and density functional theory (DFT) approaches. The polymers are treated as one-dimensional (1D) infinite conjugated chains with periodic boundary condition. We consider a number of DFT exchange correlation functionals: B3LYP, O3LYP, OB95, PBEPBE, PBE1PBE and TPSSTPSS and employ the 6-31G* basis set and 32 k points in most of our calculations. We compare our results with experimental values whenever possible. The results show that HF method overestimates the band gap of trans-PA by 2.0 eV while the DFT theory underestimates it by 0.20 eV or more. In all DFT calculations with various functionals, B3LYP, O3LYP, OB95, PBEPBE, PBE1PBE and TPSSTPSS, all band gaps are direct and the best agreement with experiment is obtained with the B3LYP and O3LYP functionals for the fluorene-based polymers. The variation of bond length alternations (BLA's) and dipole moments as related to band gaps are discussed.

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