Tuning the properties of coumarin-based small molecule donors via strong acceptor substitution for advanced donor/PCBM organic solar cells

• Validated optoelectronic properties of coumarin-based donor (MR) via DFT methods. • Donor molecules with 1.51–1.72 eV gap designed for high-efficiency OSCs. • Maximum absorption red-shifted in designed molecules compared to MR. • Designed molecules show low electron reorganization energy vs MR, enhancing conductivity. • Power conversion efficiency of designed molecules is predicted to exceed 9 %. In this study, we designed and evaluated high-efficiency coumarin-based small-molecule donors for organic solar cells (OSCs) using density functional theory (DFT) calculations to address the challenge of improving charge transfer and photovoltaic performance. Building on previous work, we designed a series of four new molecules (M1-M4) by replacing the phenyl unit in the reference structure 2-((2E,4E)-1-(7-(diethylamino)-2-oxo-2H-chromen-3-yl)-5-(4-(dimethylamino)phenyl)penta-2,4-dien-1-ylidene) malononitrile (MR) with different strong electron-accepting groups, providing a systematic approach to enhance donor-acceptor interactions. The designed molecules exhibit significantly reduced HOMO-LUMO energy gaps (1.51–1.72 eV) compared to MR (1.87 eV), red-shifted maximum absorption (553–614 nm versus 518 nm), higher dipole moments (>10 D), and lower electron reorganization energies, indicating improved charge transport properties. The predicted power conversion efficiencies (PCE) of donor/PCBM heterojunctions increase from 5% for MR/PCBM to over 9% with the newly designed molecules, underscoring a marked improvement in photovoltaic performance. These findings provide new insights into the structure-property relationships governing optical absorption, intramolecular charge transfer, and charge mobility, establishing design rules for high-performance small-molecule donors. This study introduces a novel, systematic molecular design strategy that can guide future development of tunable, efficient donor materials, offering immediately relevant knowledge for the wider research community focused on next-generation OSCs.