Intensive Static Quantum Chemistry and AIMD Theoretical Investigations of Molecular Stacking and Charge-Transfer Mechanisms in PM6:PY-X Organic Solar Cells

Abstract

Organic solar cells based on donor–acceptor heterojunctions rely on interfacial charge transfer (CT) for exciton dissociation and charge transport. This study combines DFT, TD-DFT, and AIMD simulations to investigate the electronic structure and CT properties of PM6 donor dimers with PY-X non-fullerene acceptors (X = CH₃, OCH₃, CN, COCH₃, NH₂, NHCH₃), using PM6:PY-CH₃ as the reference. Three substituted derivatives are highlighted: PY-CN and PY-COCH₃ exhibit strong CT character (∼54%–51%), low band gaps (∼2.0 eV), and deep LUMO levels favoring electron transfer, while PY-NHCH₃ offers a high open-circuit voltage (1.76 eV) and fill factor (92%) despite weak CT (∼6.6%). Notably, after AIMD simulations in explicit chloroform, the most optically active excited state of the PM6:PY-CN dimer exhibits a high CT character (∼56.56%), together with the smallest band gap indicating highly efficient exciton dissociation. AIMD simulations in chloroform reveal that solvent-induced structural relaxation enhances CT interactions and stabilizes the donor–acceptor interface. Overall, substituent effects, solvent interfacial interactions, and π–π stacking are key modulators of exciton dissociation and charge transport, offering design guidelines for high-performance, stable organic solar cells.