Dynamical electronic screening and exciton binding in the Bethe-Salpeter approach

The Bethe-Salpeter equation (BSE) is a theoretical spectroscopy technique that accurately describes optical absorption by considering the screened electron-hole Coulomb interaction. In practice, the frequency dependence of screening is oftentimes neglected to reduce computational cost. This static approximation is often valid due to small exciton-binding energies in bulk inorganic semiconductors, compared to plasmon frequencies. However, for materials with large exciton-binding energies, such as finite and low-dimensional systems, dynamical screening can become important. To explore this, we incorporated dynamical electronic screening into the BSE approach and quantify its impact from first-principles calculations. We study the optical absorption of linear oligoacene crystals and show that the exciton binding energy is on the order of 1 eV. Furthermore, we compute corrections due to dynamic screening, and show that these can be of comparable size, i.e. an order of magnitude larger than in inorganic semiconductors. We also show that including this effect significantly improves agreement of exciton binding energies with experimental results.