Central core regulation by methoxy in quinoxaline-based non-fullerene acceptors for over 19% efficiency organic solar cells

The power conversion efficiency (PCE) of organic solar cells (OSCs) is intricately linked to the molecular design strategy of non-fullerene acceptors (NFAs). Researchers frequently modificated the end group and side chain in prior investigations. The central core of quinoxaline (Qx) possesses abundant modification sites, and the alkoxy groups exhibit high electron-accepting capability based on its meta-substitution. In this work, we designed and synthesized three small molecule acceptors (SMAs) with a phenyl substituted with an alkoxy group on their central core of quinoxaline, named Qx-B1, Qx-B2, and Qx-B3, respectively. The molecular backbones of these substances share similarities, yet the substitution points of methoxy groups are different, which significantly affect absorption, energy levels, electrostatic potentials, and molecular stacking of acceptors. As a result, the PM6:Qx-B2 device possessed excellent crystallinity and uniform morphology in the blend film, indicating outstanding charge transport and collection characteristics. This resulted in a high short-circuit current density (J_SC) of 25.97 mA cm⁻² and fill factor (FF) of 77.17 %, contributing to its champion PCE of 17.88 %. Furthermore, by using ternary and interface engineering strategies, when 15 wt% of BTP-eC9 was doped into the PM6:Qx-B2 two-component device as a guest acceptor, the 2PACz as the hole transparent layer, the device achieved a PCE of 19.25 %. This work investigates Qx-based asymmetric NFAs with three different methoxy substitution positions, suggesting a research direction for alkoxy substitution in the central core.