Alkyl Chain Engineering of End Groups in Benzoselenadiazole‐Based Non‐Fullerene Small Molecule Organic Solar Cells: A First‐Principles Perspective

The development of high‐performance small‐molecule donors (SMDs) and their integration with suitable acceptor materials are critical for enhancing the efficiency of organic solar cells (OSCs). However, the limited molecular skeleton of SMDs often results in poor intermolecular interactions and insufficient pre‐aggregation properties in solution. To address this, we have optimized the alkyl chains of SMDs, leveraging their influence on intermolecular packing, thin‐film morphology, and charge transport. In this study, seven different donor molecules have been designed by selecting thiophene as the electron donor and benzoselenadiazole (BSz) and rhodanine as electron acceptors. Alkyl chains of varying lengths have been attached to the N‐atom of rhodanine to prevent hydrogen bonding and improve solubility in organic solvents. The resulting molecules exhibit nonplanar 3D structures with dihedral angles of 0 ∘ –30 ∘ , reducing self‐aggregation and stabilizing the active layer morphology. Molecular electrostatic potential surface and transition density matrix analyses manifest substantial charge separation in the studied donor molecules, implying efficient charge transport upon excitation. In addition, analysis of reduced density gradient plots confirms that attractive forces prevail over repulsive forces within the molecules, ensuring structural stability. Further, these molecules demonstrate broad absorption in the visible region due to π–π * transitions and intramolecular charge transfer, with promising photovoltaic performance (fill‐factor, = 0.92 and power conversion efficiency = 25.27%). These key findings highlight the potential of the newly designed donor molecules for advancing OSC technologies.