Impact of Nitrogen-to-Phosphorus Substitution on End Group of the Y6 Nonfullerene Molecule for Organic Photovoltaics: A Comprehensive Modeling Study.

The N-to-P substitution in nonfullerene acceptors (NFAs) is an intriguing strategy to address environmental and economic concerns associated with the cyano (-C≡N) group. In this study, we performed a comprehensive computational investigation of N-to-P substitution in the end groups of the Y6 molecule, a benchmark NFA for organic photovoltaics (OPV). Using density functional theory (DFT), time-dependent DFT (TD-DFT), and molecular dynamics, we analyzed the effects on the electronic structure and intermolecular interactions. Our results reveal significant changes in intramolecular properties depending on the P atom's position. The P≡C bond is longer than the N≡C bond, enhancing electronic delocalization, reducing both fundamental and optical gaps, and improving light absorption and exciton dissociation. However, P incorporation reduces the quadrupole moment, slightly weakening the intermolecular interactions and electronic coupling. Despite this, the electron transfer rate remains stable due to a compensation effect with intramolecular reorganization energy. Overall, our findings suggest that N-to-P substitution enhances the key optoelectronic properties of Y6, potentially benefiting OPV performance. This study provides valuable insights into the feasibility of this modification for organic electronics.