The molecular aggregation of nonfullerene acceptors (NFA) can significantly affect the light absorption, charge generation, and power conversion efficiency (PCE) of organic solar cells (OSCs). In this work, we demonstrate the regulation of J-aggregation of COi8DFIC NFA toward near- infrared absorption via solvent additives 1,8-diiodooctane (DIO), diphenyl ether (DPE), and 1-chloronaphthalene (CN). Molecular dynamics simulations reveal preferential interaction of DIO with the alkyl side chains of COi8DFIC, endowing side-chains with the flexibility to adjust conformations to promote the formation of "A-to-D" type J-aggregation among the COi8DFIC backbone, resulting in a significant red-shift of absorbance toward the near-infrared region. The enhanced J-aggregation via pi-pi stacking, evidenced by grazing-incidence wide-angle X-ray scattering, constructs three-dimensional charge transport channels at the molecular level to facilitate charge transport. The presence of 0.5 vol % DIO molecules, which is most effective among all three additives, boosts the maximum achievable PCE of CF cast PTB7-Th:COi8DFIC OSCs from 8.5% to 12.9%. Our results provide a new concept to enhance the efficiency of OSCs via dedicated control of molecular aggregations of nonfullerene acceptors.