A low-bandgap small molecule, 7,7’-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b’]dithiophene-2,6-diyl)bis(6-fluoro-4-(5’-hexyl-[2,2’-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole), (p-DTS(FBTTh₂)₂) has been incorporated into polymer solar cells based on blend of poly(3-hexylthiophene) (P3HT) and a fullerene derivative (PCBM) to improve the light-harvesting efficiency in the near-infrared (near-IR) range. With addition of a low concentration of p-DTS(FBTTh₂)₂, the short-circuit current density (J_SC) increased significantly and hence the power conversion efficiency (PCE) was improved by more than 20% relative to that of the control device of P3HT/PCBM binary blend. This is due to the improvements in the external quantum efficiency (EQE) not only at the absorption band of p-DTS(FBTTh₂)₂ in the near-IR range but also at the P3HT absorption band in the visible range. Furthermore, the photoluminescence (PL) quenching efficiency of P3HT was much higher in the ternary blends than in the binary blend. These findings indicate that P3HT excitons can be dissociated into free charge carriers more efficiently in the presence of p-DTS(FBTTh₂)₂.