Heteroatom Engineering of Nonfused Ring Electron Acceptors: Design Strategy for Optoelectronic Enhancement.

Nonfused ring electron acceptors (NFREAs) represent a pivotal subclass of nonfullerene acceptors, attracting growing attention in recent years due to their synthetic accessibility and low cost. Organic solar cells (OSCs) incorporating NFREAs have achieved impressive power conversion efficiencies (PCEs) of up to 19%. However, the development of high-performance NFREAs remains constrained, and the structure-property relationships governing their optoelectronic behavior have not yet been fully clarified. Herein, we report the systematic design and modeling of six novel NFREAs, derived from the state-of-the-art NFREA 2BTH-2F-C2 (with an OSC PCE exceeding 19%) via heteroatom substitutions on the terminal benzene rings and core thieno[3,2-b]thiophene unit. Using reliable density functional theory (DFT) and time-dependent DFT (TDDFT) calculations, we investigated the ground- and excited-state properties of these derivatives. Computational results demonstrate that the modified NFREAs outperform the parent 2BTH-2F-C2 in multiple key parameters, including molecular planarity, optical gap, and exciton binding energy (E_b). Notably, oxygen substitution in the core unit induces a series of favorable changes: reduced E_b (-0.115 eV), enhanced average electrostatic potential (+0.71 kcal/mol), upshifted LUMO energy (+0.029 eV), and a narrowed HOMO-LUMO gap (-0.671 eV). These findings provide critical guidelines for the rational design of high-performance NFREAs.