Excited State Properties of Organic Semiconductors: Breakdown of the Tamm-Dancoff Approximation

TL;DR

This paper investigates the limitations of the Tamm-Dancoff approximation in calculating optical properties of organic semiconductors, revealing significant deviations and improved accuracy when the approximation is broken down.

Contribution

It demonstrates that the Tamm-Dancoff approximation can fail for organic semiconductors, leading to more accurate exciton binding energy predictions.

Findings

TDA breaks down in organic semiconductors, affecting exciton energy calculations.

Breaking TDA increases exciton binding energy estimates by up to 44%.

Results improve agreement with experimental data.

Abstract

The solution of the Bethe-Salpeter equation within the framework of many-body perturbation theory has turned out to be a benchmark for ab-initio calculations of optical properties of semiconductors and insulators. Commonly, however, the coupling between the resonant and anti-resonant excitations is neglected which is referred to as the Tamm-Dancoff approximation (TDA). This is well justified in cases where the exciton binding energy is much smaller than the band gap. Here, we report on the optical properties of a representative series of organic semiconductors where we find the TDA to no longer hold. We observe an increase of the exciton binding energy of up to 44% thereby improving the agreement with experiment.