Tuning the frontier molecular orbitals of benzothiophene derivatives to suppress energy loss and improve organic solar cell efficiency: A DFT study

In the present work, a series of eighteen new donor-acceptor ( D-π-A ) organic molecules were designed using density functional theory (DFT) based on a benzothieno[3,2- b ]benzothiophene ( BT ) core. The design successfully achieved significantly improved optoelectronic properties over the reference molecule. Key results include a reduced energy gap as low as 1.73 eV ( 3A ), a lowered LUMO energy down to −3.37 eV ( 3A ) for better alignment with the PC₆₁BM acceptor (LUMO = −3.50 eV), and a red-shifted absorption maximum up to 442.14 nm ( 2F ). This yielded a higher theoretical open-circuit voltage, with the best-performing molecules ( 1D, 1E, 1F ) achieving a V OC of 1.76 eV. The highest estimated power conversion efficiency reached 37.35% for compound 1D . These results are explained by a comprehensive analysis of the molecular properties. The BT π-bridge provided excellent planarity and intramolecular charge transport (ICT) characteristics. The strategic incorporation of electron-withdrawing groups (NO₂, COOH, CN) at one terminal effectively lowered the LUMO energy, while donor groups (amine, methyl, ethyl) at the other end tuned the HOMO, optimizing the energy level alignment with PC₆₁BM. Computational analyses, including frontier molecular orbital (FMO) studies, global reactivity descriptors, electrostatic potential (ESP) mapping and transition density matrix (TDM), confirmed enhanced intramolecular charge transfer and chemical reactivity. Further investigation through parameters such as light harvesting efficiency (LHE up to 0.96 for 3A ), reorganization energy, and transition density matrix (TDM) supported superior charge transport and excitation dynamics. Overall, all newly designed chromophores demonstrated favorable optoelectronic behavior, with the ethyl-substituted scheme ( 3A-3F ) showing significant properties. Our results offer a strong computational methodology for creating D-π-A materials with great performance for next-generation organic solar cells ( OSCs ).