Oligothiophene-Based Photovoltaic Materials for Organic Solar Cells: Exciton Properties by the CNDOL Fockian Approach

In this study, we apply the CNDOL/2SS approximate Fockian with the configuration interaction of singles (CIS) method to explore the excitonic properties of oligothiophene-based materials for organic solar cells. Our calculations of the excited states and charge density distributions of isolated chromophores and a donor-acceptor pair align closely with experimental data. The methodology used is reliable and useful for addressing complex donor-acceptor systems and their eventual design. The prediction of exciton binding energy using the Coulomb and exchange (E^CE) term of the CIS energy transitions, combined with charge density difference maps to visualize the electronic structure of excitons, aids in distinguishing charge transfer states between multiple transitions present in the molecular aggregates representing the donor-acceptor pair. Our results indicate that the donor-acceptor blend Tz6T:eC9-4F exhibits strong low-energy light absorption and a state alignment that enables barrier-free charge transport. The sandwich-type arrangement of this pair reveals a charge transfer (CT) state characterized by low exciton binding energy (low E^CE term), highlighting its potential for optimizing organic solar cell performance. In contrast, less-ordered arrangements of the donor-acceptor pair show CT states at higher energies, which may compete with other deactivation processes and reduce the efficiency. This study provides a cost-effective approach to predicting and interpreting the feasibility of charge transfer in molecular aggregate designs for solar cells.