A rational molecular design approach has enabled organic solar cells (OSCs) to achieve a power conversion efficiency (PCE) of 19.9% (19.5% certified), approaching 20%. The study introduces o-BTP-eC9, an isomer of BTP-eC9 with modified chlorine positions, which exhibits a shallower LUMO level, higher dielectric constant, and weaker crystallinity. This modification reduces energy loss by 41 meV and enhances the PCE of the PM6:o-BTP-eC9 device to 18.7%. When integrated into a ternary blend with PM6:BTP-eC9, o-BTP-eC9 improves the nano-scale morphology, charge transport, and suppresses voltage loss, leading to a record PCE of 19.9% (19.5% certified). The molecule shows excellent miscibility, crystallinity, and energy level compatibility with BTP-eC9, enabling enhanced operational stability. Theoretical calculations guided the design of o-BTP-eC9, which optimizes the energy levels and reduces non-radiative recombination. The ternary device also demonstrates improved charge transport, reduced recombination, and efficient charge collection, contributing to higher PCE and fill factor. The study highlights the importance of molecular design in achieving high-performance OSCs with reduced energy loss and enhanced stability.A rational molecular design approach has enabled organic solar cells (OSCs) to achieve a power conversion efficiency (PCE) of 19.9% (19.5% certified), approaching 20%. The study introduces o-BTP-eC9, an isomer of BTP-eC9 with modified chlorine positions, which exhibits a shallower LUMO level, higher dielectric constant, and weaker crystallinity. This modification reduces energy loss by 41 meV and enhances the PCE of the PM6:o-BTP-eC9 device to 18.7%. When integrated into a ternary blend with PM6:BTP-eC9, o-BTP-eC9 improves the nano-scale morphology, charge transport, and suppresses voltage loss, leading to a record PCE of 19.9% (19.5% certified). The molecule shows excellent miscibility, crystallinity, and energy level compatibility with BTP-eC9, enabling enhanced operational stability. Theoretical calculations guided the design of o-BTP-eC9, which optimizes the energy levels and reduces non-radiative recombination. The ternary device also demonstrates improved charge transport, reduced recombination, and efficient charge collection, contributing to higher PCE and fill factor. The study highlights the importance of molecular design in achieving high-performance OSCs with reduced energy loss and enhanced stability.