Analyzing Electronic Excitations and Exciton Binding Energies in Y6 Films

Abstract

The Y6 molecule is one of the most promising non‐fullerene acceptors. Based on first‐principles calculations, this paper analyzes how the separation of an electronic excitation into electron and hole densities is influenced by the interaction between different Y6 molecules. By calculating the optical and the fundamental gaps of ensembles of Y6 molecules that realistically represent a film, their corresponding exciton binding energies are obtained. The combination of range separation, optimal tuning, and dielectric screening endows the presented density functional theory calculations with predictive power. The calculations reveal that the electronic excitations, characterized as electron–hole pairs via their natural transition orbitals, spread across multiple Y6 molecules. A distinct decrease in the exciton binding energy is correlated to a notable charge separation, and the exciton binding energy saturates in ensembles of six to seven Y6 molecules at ≈0.25 electronvolt (eV). These findings contribute to explaining the efficient charge separation in films of Y6. They also give a guideline for the number of molecules that theoretical models should take into account when they aim at a realistic description of charge separation.