Cathode interfacial materials (CIMs) stand as critical elemental in organic solar cells (OSCs), which can align energy levels, and foster ohmic contacts between the cathode and active layer of the OSCs. Nevertheless, the lagging advancement in CIMs has concurrently engendered the oversight of theoretical inquiries pertaining to the impact of molecular structure on their performance. Delving into this realm, we present two propeller-shaped isomers, 4,4',4''-(benzo[1,2-b:3,4-b':5,6-b'']trithiophene-2,5,8-triyl)tris(2-(3-(dimethylamino)propyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione) (3ONIN) and 6,6',6''-(benzo[1,2-b:3,4-b':5,6-b'']trithiophene-2,5,8-triyl)tris(2-(3-(dimethylamino)propyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione) (3PNIN), distinguished by their molecular planarity, as a promising foundation for crafting highly efficient OSCs. This study illuminates the superiority of 3PNIN with more plane structure, exemplified by its enhanced molar extinction coefficient, deeper lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels, intensified self-doping effect, heightened electron mobility, and elevated conductivity, in comparison to its counterpart, 3ONIN. As a result, 3PNIN and 3ONIN-treated OSC devices yield efficiencies of 17.73% and 16.82%, respectively. This finding serves as a compelling validation of the critical role played by molecular planarity in influencing CIM performance.