An ab initio method on large sized molecular aggregate system: Predicting absorption spectra of crystalline organic semiconducting films

Theoretical description of electronic excited states of molecular aggregates at an {\em ab initio} level is computationally demanding. To reduce the computational cost, we propose a model Hamiltonian (MH) approach that approximates the electronically excited state wavefunction of the molecular aggregate. We benchmark our approach on a thiophene hexamer, as well as calculate the absorption spectra of several crystalline non-fullerene acceptors (NFAs), including Y6 and ITIC, which are known for their high power conversion efficiency in organic solar cells. The method qualitatively predicts the experimentally measured spectral shape, which can be further linked to the molecular arrangement in the unit cell.