Quantitative characterization of exciton from GW+Bethe-Salpeter calculation

We propose a method of classifying excitons into local-, Rydberg-, or charge transfer-type as a step toward enabling a data-driven material design of organic solar cells. The classification method is based on the first-principles many-body theory and improves over the conventional method based on state-by-state visualization of the one-electron wave functions. In our method, the exciton wave function is calculated within the level of the GW+Bethe-Salpeter equation, which is used to obtain two dimensionless parameters for the automatic classification. We construct criteria for exciton classification from experiences with a model molecule, dipeptide. Then we check the validity of our method using a model β-dipeptide which has a geometry and an excitation spectrum similar to the model dipeptide. In addition, we test the effectiveness of the method using porphyrin molecules, or P₁TA and P₂TA, for which the conventional method is hampered by the strong state hybridization associated with excitation. We find that our method works successfully for P₁TA, but the analysis of P₂TA is hindered by its centrosymmetry.