Theoretical study on manipulating core structure and energy level of non-fused ring electron acceptors in organic...

Non-fused ring electron acceptors (NFREAs) have recently received a lot of interest as a possible alternative to fused-ring acceptors in bulk heterojunction organic solar cells (OSCs) because of their synthetic simplicity and structural tunability. Nevertheless, there is still a lack of systematic knowledge regarding how interfacial charge dynamics and overall device performance are determined by molecular engineering of the core structure of NFREA. The Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were used to describe the structure-property performance relationships of tetrathiophene-based NFREAs (4T-3, 4T-BOE, 4TThC-ICF, o-4TBC-2F, 4T-OEH, and LW-out-2F) and their donor/acceptor (D/A) interfaces with PBDB-T. We demonstrate that improved π-conjugation and electron-donating substitutions efficiently raise the highest occupied molecular orbital energy levels, narrow the bandgap, cause significant bathochromic shifts in optical absorption, and decrease nonradiative energy loss (E_loss) by altering the side chain of central backbone. Improved interfacial charge-transfer efficiency is also indicated by favorable changes in the free energies of charge separation (ΔG_CS) and charge recombination (ΔG_CR). The calculated optoelectronic parameters, including electron mobility (μ), E_loss, and charge-transfer energetics, are found to be comparable to or better than those of the benchmark Y6 acceptor, highlighting the potential of the designed systems for high-performance OSCs. Our findings show that side-chain engineering is essential for controlling interfacial electrical properties and electrostatic potential distribution, which directly affects NFREAs' photovoltaic efficiency. This work offers design guidelines and quantitative theoretical insights for optimizing NFREAs for high-efficiency OSC applications.