A conjugated polymer that is an electron donor, that is soluble without aggregation, that is solution-coatable and is dryable at a temperature below 70° C., that has an energy conversion efficiency of 7 % or more over an area of 5 cm2 or more, and that is composed of a repeating unit represented by Chemical Formula 1A below: where x is a real number from 0.1 to 0.2; and n is an integer from 1 to 1,000. The conjugated polymer forms a uniform thin film over a large area of, for example, an organic solar cell, without a heat treatment due to superior solubility and crystallinity at low temperature and, thus, allows fabrication of an organic solar cell with high efficiency at a low temperature.