We use the density functional theory (B3LYP/6-31G(d)) to characterize band gap, photovoltaic properties and structural parameters of a series of conjugated organic oligomers. The chemical structure of these oligomers contains N-substituted carbazole, thiophene and phenylene rings, together with different moieties (e.g. ethylenedioxythiophene, EDOT). The theoretical calculations were performed by using Gaussian 09 program supported by GaussView 5.0. interface. The results reveal that the geometrical parameters (dihedral angles, bond lengths) of all oligomers possess nearly planar conformations. Moreover, the optoelectronic properties (HOMO, LUMO, Egap) were determined from the fully optimized structures by using B3LYP/6-31G(d) level. The absorption data (λmax, Etr, OS) of these systems were obtained by TD-B3LYP/6-31G(d) method. The studied oligomers Oligo-i (with i = 1-6) have low band-gaps with appropriate energy levels of HOMO and LUMO which are desired in solar cell applications, especially the oligomers with acceptor moieties in their chain. We conclude that these systems are excellent candidates for donor-acceptor heterojunctions used in organic solar cells.
