Electronic and optical excitations in crystalline conjugated polymers

Abstract

We calculate the electronic and optical excitations of crystalline polythiophene and polyphenylenevinylene, using the GW approximation for the electronic self-energy and including excitonic effects by solving the electron-hole Bethe-Salpeter equation. We compare with our earlier calculations on an isolated polythiophene chain and polymer chains embedded in a dielectric medium. Surprisingly, we find for the crystalline calculations optical gaps and exciton binding energies that are significantly smaller than present experimental values. We attribute the disagreement to the fact that the quantum-mechanical coherence between polymer chains, present in the calculations, is absent in most experimental situations. We discuss possible reasons for this absence. Our general conclusion is that the picture of a polymer chain in a dielectric medium is most appropriate in describing the present experimental data on electronic and optical excitations in conjugated polymers.

Equation-of-motion coupled-cluster study on exciton states of polyethylene with periodic boundary condition

“…Vracko et al applied RPA and TDA to the exciton energies of polyethylene. Rohlfing et al 40 and van der Horst et al 41 applied this method to the exciton states of conjugated polymers and showed that the excitation energies are well comparable with experiments. Kertész applied the intermediate exciton theory to the exciton states of polyacetylene and calculated the exciton energies from electron-hole pair Green's function from semiempirical Hartree-Fock wave function.…”

“…A standard method to calculate the exciton states is to use the configuration-interaction singles ͑CIS͒ method, random-phase approximation ͑RPA͒ method, or RPA with Tamm-Dancoff approximation ͑TDA͒. [37][38][39][40][41] In this approach, the Bethe-Salpeter equation for the two-particle Green's function is solved to obtain excitation energies. 31 Another approach to calculate exciton energies is Green's function method.…”