In organic solar cells (OSCs), it is an effective way to improve the power conversion efficiency (PCE) by adding a guest component with appropriate absorption and energy levels in the host system. Herein, a new nonfullerene acceptor (NFA) named TBF-2Cl was developed by the strategy of expanding the π conjugated core of 2,2’-(((4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b’]dithiophene-2,7-diyl)bis(methaneylylidene))bis(5,6-dichloro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IDT-4Cl) with two benzene rings. With increase of benzene units, TBF-2Cl exhibits higher lowest unoccupied molecular orbital (LUMO) level of −3.75 eV than that of one benzene unit based NFA IDT-4Cl and fluorene core based NFA F-2Cl, which facilitates enhancing the open-circuit voltage (V_oc) of ternary devices. Moreover, TBF-2Cl film shows a medium optical bandgap with the absorption range from 500–800 nm, being well complementary with the wide bandgap polymer donor D18 and narrow bandgap NFA CH-6F. Accordingly, a remarkable PCE of 18.92% with a high short-circuit current density (J_sc) of 27.40 mA·cm⁻², a fill factor (FF) of 0.749, especially an outstanding V_oc of 0.922 V was achieved for the optimal ternary device based on D18:TBF-2Cl:CH-6F, surpassing the binary counterpart (17.08%). The findings provide insight into the development of new guest acceptors for obtaining more efficient OSCs.