A theoretical study of the effect of end-group and center backbone modifications on the optoelectronic properties of Y6-based asymmetric LL3 non-fullerene

Abstract

Y6-based non-fullerene acceptor (NFA) has garnered significant attention because of its unique A-DA'D-A molecular structure. However, the impact of asymmetric modification—a key strategy to enhance NFAs—on their photovoltaic properties is still not well understood. In this study, we optimized the high-performance asymmetric LL3, a Y6-based NFA characterized by its distinctive 3D end-group structure, by employing end-group and skeleton modification techniques. We designed six new asymmetric NFA candidates by expanding thiophene rings within the skeleton's core, incorporating π-bridges, and substituting chlorinated benzene rings or 3D segments at the end-groups with thiophene. Using density functional theory (DFT) and time-dependent DFT (TD-DFT), we calculated various molecular properties of these NFAs, such as molecular planarity, dipole moments, frontier molecular orbitals, electrostatic potential (ESP), electron-hole distributions, UV–Visible absorption spectra, singlet-triplet energy difference (ΔE_ST), exciton binding energy (E_b), and the open circuit voltages of organic solar cells based on these NFAs. Our results show that five of the new NFAs outperform the prototype LL3, with LL3-T-L standing out due to its red-shifted absorption peak, highest light absorption intensity, lower ΔE_ST and E_b, and enhanced ESP, indicating its potential as a high-performance NFA. These findings provide theoretical guidance for future experimental synthesis and device optimization.