Central‐Core Cycloalkane Modified Nonfullerene Acceptors Reduce Energy Loss for 20.2% Efficiency in Organic Solar Cells

In recent years, power conversion efficiency (PCE) has exceeded 21% in organic solar cells (OSCs), thanks to the ongoing innovation of molecular design and device engineering. Among them, central‐core engineering is a key strategy for optimizing nonfullerene acceptors (NFAs) to reduce energy loss and enhance charge transport in OSCs. In this work, two novel quinoxaline (Qx)‐series NFAs, Qx‐CHX and Qx‐CP, are designed and synthesized by incorporating a rigid, nonaromatic cycloalkane (cyclohexane and cyclopentane) into the central core, respectively. The theoretical calculations and experiments indicate that, due to the smaller steric hindrance, a more planar molecular backbone, and better donor–acceptor miscibility, the D18:Qx‐CP‐based devices exhibit superior crystallinity and optimal phase separation, delivering a champion PCE of 18.75% with a low energy loss of 0.48 eV. In comparison, the Qx‐CHX‐based devices achieve a higher open‐circuit voltage ( V _OC ) of 0.905 V and a PCE of 18.02%. Notably, using Qx‐CP as a guest acceptor in the D18:BTP‐eC9 system further boosts the PCE to 20.25% with significantly enhanced short‐circuit current ( J _SC ) and fill factor (FF). This work highlights central‐core modification as an effective approach for high‐performance OSCs.