  In this paper, the structural and electronic properties of polythiophene and
polyprrrole-based systems have been investigated using first-principles
calculations both in periodic and oligomer forms. Of particular interest is the
band gap modulation through substitutions and bilayer formation. Specifically,
S has been substituted by Se and Te in polythiophene, leading to polyseleophene
and polytellurophene, respectively, and N has been substituted by P and As in
polypyrrole. The values obtained of the binding energy suggest that all the
systems studied can be realized experimentally. Stacking (bilayer formation) of
pure polythiophene, polypyrrole and their derivatives leads to linear
suppression of the band gap or HOMO-LUMO gap as a function of the stacking.
Mixed bilayers, including one formed from polythiophene on top of polypyrrole,
have also been considered. Overall, a wide range of band gaps can be achieved
through substitutions and stacking. Hybrid (B3LYP) calculations also suggest
the same trend in the band gap as PBE calculations. Trends in the binding
energy are similar for both periodic and molecular calculations. In addition,
the $\Gamma$-point phonon calculation are performed in order to check the
stability of selected systems.
