Probing Side-Chain Engineering for Modulating Exciton Dynamics in Non‑fullerene Acceptors

The last 2 years have witnessed rapid progress in organic solar cells (OSCs), and precise modulation of frontier orbital energies without compromising light absorption is crucial for optimizing organic semiconductors. This study presents a predictive side-chain engineering approach for non-fullerene acceptors (NFAs) using quantum-chemical calculations to identify substituents that selectively shift the HOMO and LUMO levels while preserving the optical bandgap. Guided by structural and excited-state simulations, two non-fullerene acceptors, PDIEH and PDIIN, were designed, incorporating flexible 2-ethylhexyl and rigid indanyl groups, respectively. Electronic structure modeling revealed that flexible alkyl groups stabilize the electronic structure via inductive effects, whereas rigid aromatic groups introduce partial conjugative perturbations, resulting in distinct energy level shifts. Despite similar optical gaps (∼2.27 eV), these electronic modulations critically impact exciton dynamics as revealed by ultrafast transient absorption spectroscopy (UTAS). PDIEH exhibits deeper LUMO levels and prolonged charge-separated lifetimes, while PDIIN shows faster recombination. This study establishes a chemically intuitive framework where side-chain engineering enables predictive control of electronic properties and charge behavior in organic photovoltaics.