NOVELTY - Copolymer comprises a first unit containing substituted 9H-carbazole compounds (I), a second unit containing substituted 9H-fluorene compounds (II) and a third unit containing substituted 4-(tetrahydro-thiophen-2-yl)-7-thiophen-2-yl-benzo(1,2,5)thiadiazole compounds (III).    USE - The copolymer is useful in an organic solar cell (claimed).    ADVANTAGE - The copolymer: provides organic solar cell that exhibits increased open voltage and optical efficiency, improved cycle life characteristics, high charge mobility, excellent electrical properties and high photovoltaic conversion efficiency; and can be utilized in a pure or impure form in solar cell.    DETAILED DESCRIPTION - Copolymer comprises a first unit containing substituted 9H-carbazole compounds of formula (I), a second unit containing substituted 9H-fluorene compounds of formula (II) and a third unit containing substituted 4-(tetrahydro-thiophen-2-yl)-7-thiophen-2-yl-benzo(1,2,5)thiadiazole compounds of formula (III).    b = 0-4; either    R1a-R1d, R1-R7 = alkyl, cycloalkyl, alkoxy, aryloxy, alkylthioxy, arylthioxy, alkylsulfoxy, arylsulfoxy, alkenyl, silyl, B, alkylamine, aralkylamine, arylamine, heteroaryl amine, aryl, fluorenyl, carbazole or heterocyclyl containing at least one N, O or S (all optionally substituted), H, D, halo, nitrile, nitro, imide, amide or OH; or    two of R1a-R1d, R1-R7 = condensed ring; and    a = 0-3.    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 electrodes and including a photoactive layer, where the organic material layers comprise the copolymer; and    (2) fabricating the organic solar cell, comprising: preparing a substrate; forming the first electrode on a region of rear sides of the substrate; forming the organic material layer comprising the copolymer on the first electrode; and forming the second electrode on the organic material layer.