Typical organic photovoltaic materials show high Urbach energies (ca. 25-50 meV), which is considerably higher than those of their inorganic counterparts and limits further improvement in the device efficiency of organic solar cells (OSCs). In this study, we introduce a facile method of selenium substitution to reduce the Urbach energy of organic photovoltaic materials to 20.4 meV (Y6Se), which is the lowest value reported for high-performance organic photovoltaic materials and very close to those (ca. 15 meV) of typical inorganic/hybrid semiconductors, such as crystalline silicon, gallium nitride, and lead-halide perovskite. Next, OSCs based on Y6Se showed 17.7% efficiency, which is among the best results for OSCs and the record efficiency of as-cast single junction OSCs to date.