Heteroatom‐Tailored Benzo[1,2,4]triazine Core: A Promising Building Block for High‐Efficiency Non‐Fullerene Acceptors ^†

Comprehensive Summary The rapid development of non‐fullerene acceptors (NFAs), particularly Y6 and its derivatives, has propelled organic photovoltaics (OPVs) to power conversion efficiencies (PCEs) exceeding 20%. This achievement stems from the rational design of core structures that regulate energy levels, optical absorption, and molecular packing. However, the potential of the 1,2,4‐triazine motif remains underexplored despite its unique electronic features. Here, we introduce the benzo[1,2,4]triazine (BTAZI) core as a promising building block for NFAs. Density functional theory calculations reveal that BTAZI possesses lower‐lying energy levels than the benzothiadiazole (BT) unit in Y6, arising from its reduced electron density. Through heteroatom substitution (S, O, and Cl), we finely tune the σ‐inductive and p‐π conjugative effects, yielding three BTAZI‐based acceptors: BTAZI‐IC‐SMe, BTAZI‐IC‐OMe, and BTAZI‐IC‐Cl. Among them, BTAZI‐IC‐SMe achieves an optimal balance between molecular orbital alignment and absorption profile with the donor polymer D18, affording a PCE of 18.14%, surpassing the others. This study highlights the benzo[1,2,4]triazine framework as a new core unit for efficient NFAs and offers valuable insights into the molecular design of high‐performance and stable OPVs.