A copolymer includes a first unit and at least two units different from the first unit. An organic solar cell includes a first electrode, a second electrode opposite the first electrode, and one or more organic material layers interposed between the first electrode and the second electrode and including a photoactive layer, and one or more of the organic material layers include the copolymer. A method for fabricating an organic solar cell includes preparing a substrate, forming a first electrode on a region of rear sides of the substrate, forming on the first electrode an organic material layer containing the copolymer, and forming a second electrode on the organic material layer.