NOVELTY - Copolymer comprising a substituted 9H-carbazole-3,6-diyl unit (I) (as first unit) and at least one unit different from the first unit, where the first unit is present at greater than 0 to 45 mole% based on the total moles of monomers constituting the copolymer, is new.    USE - The copolymer is useful for fabricating an organic solar cell (claimed).    ADVANTAGE - The copolymer: provides organic solar cell with excellent electrical properties (due to the balance between hole mobility and electron mobility) and high photovoltaic conversion efficiency; provides organic solar cell with high charge mobility; has the first unit (I) which contributes to the stability of the copolymer; and has a 2,7-carbazole unit in addition to a 3,6-carbazole unit, which provides low band gap energy and is highly stable against chemical and thermal attacks, air and moisture compared to other electron donors, so that it improves the efficiency of the organic solar cell.    DETAILED DESCRIPTION - Copolymer comprising a substituted 9H-carbazole-3,6-diyl unit of formula (I) (as first unit) and at least one unit different from the first unit, where the first unit is present at greater than 0 to 45 mole% based on the total moles of monomers constituting the copolymer, is new.    u, p = 0-3; either    R1a, R1b = heterocyclyl (containing at least one heteroatom comprising N, O or S), alkyl, cycloalkyl, alkoxy, aryloxy, alkylthioxy, arylthioxy, alkylsulfoxy, arylsulfoxy, alkenyl, silyl, boron group, alkylamine, aralkylamine, arylamine, heteroarylamine, aryl, fluorenyl, carbazolyl (all optionally substituted), H, D, halo, nitrile, nitro, imido, amido, or OH; or    adjacent two of R1a and R1b = condensed ring; and    R1 = R1a.    INDEPENDENT CLAIMS are also included for:    (1) an organic solar cell comprising a first electrode, a second electrode opposite the first electrode, and at least one organic material layer (interposed between the first and second electrodes) including a photoactive layer, where at least one of the organic material layers comprise the copolymer; and    (2) fabricating an organic solar cell, comprising preparing a substrate, forming the first electrode on a region of rear sides of the substrate, forming an organic material layer (comprising the copolymer) on the first electrode, and forming the second electrode on the organic material layer.