From molecular design to high performance: interpreting the combined effect of π-linkers and acceptor groups in benzotriazole-based non-fullerene acceptors.

The combined effect of π-linkers and terminal groups presents a powerful yet underexplored strategy for boosting the performance of benzotriazole (BZ)-based non-fullerene acceptors (NFAs). In this computational study, we employ density functional theory (DFT) and time-dependent DFT (TD-DFT) to design and evaluate the photovoltaic performance of five benzotriazole (BZ)-core-based non-fullerene small molecule acceptors (BD1-BD5). These molecules are engineered by modifying the terminal acceptor groups of a reference structure (BZE-31) and incorporating a methyl thieno[3,4-b]thiophene-2-carboxylate π-linker. The designed molecules exhibit a pronounced bathochromic shift in absorption (λ max = 690-939 nm in chloroform) and significantly reduced optical band gaps (1.66-2.24 eV) compared to the reference. In particular, BD4, functionalized with a 4-methylene-5-oxocyclopent-1-ene-1-sulfonyl chloride end-group, demonstrates the most promising optoelectronic profile. It achieves the narrowest band gap (1.66 eV), the highest red-shifted absorption (939 nm), the lowest exciton binding energy (0.31 eV), and an exceptional short-circuit current density (J sc ≈ 22 mA cm-2). This superior optoelectronic synergy transforms into an idealized theoretically estimated power conversion efficiency (PCE) of 23.48% for BD4. Concurrently, BD1 (with a malononitrile end-group) also shows superior performance, with a high predicted PCE. A comprehensive analysis of frontier molecular orbitals, density of states, transition density matrices, and global reactivity descriptors confirms effective intramolecular charge transfer and favorable charge separation in the designed systems. The estimated PCEs for the most promising candidates, BD4 and BD1, reach 23.48% and 22.61%, respectively. This work provides a detailed theoretical blueprint demonstrating that π-linker incorporation and strategic terminal group engineering work in a combined manner and are highly effective strategies for tailoring the optoelectronic properties of BZ-based NF-SMAs. The high-performance candidates (BD1 and BD4) offer valuable theoretical insights for the experimental synthesis of next-generation, high-efficiency organic photovoltaic materials.