NOVELTY - A copolymer (P1) comprising a first substituted 9H-carbazole unit (greater than 0 to 45) mole% based on the total moles of monomers constituting the copolymer, greater than or equal to 1 unit different from the first unit, and having substituent which increases solubility bound to the side chain of the copolymer; is new.    USE - For preparing organic solar cell (claimed).    ADVANTAGE - The copolymer is capable of providing organic semiconductor material for solar cell with excellent electrical properties due to high hole mobility and high photovoltaic conversion efficiency resulting from increased solubility attributable to specific substituent introduced into side chain. In 3,6-carbazole-containing copolymer, nitrogen atom is conjugated to main chain to stabilize holes and increase mobility of holes and electrons, so that hole mobility and electron mobility in organic solar cell comprising copolymer can be balanced. Thus, provides organic semiconductor material, which can exhibit excellent electrical properties. Conventional copolymers disadvantageously have low solubility in organic solvents, and thus phase separation occurs when conventional copolymer is mixed with electron acceptor to make solution. In addition, due to low solubility of conventional copolymer, process for dissolving copolymer is time consuming, and organic layer film formed using copolymer is non-uniform. The introduction of alkyl, sulfide, alkoxy or hydroxyl group into side chain of copolymer can increase solubility of copolymer. When copolymer having increased solubility is used, it can be highly miscible with electron acceptor, so that contact interface with electron acceptor can be increased. In addition, time required for dissolving copolymer can be reduced and organic layer film formed using copolymer can become uniform. Specifically, if alkyl, sulfide or alkoxy group has less than or equal to 40 carbon atoms it can increase solubility of copolymer without reducing mobility of electrons and holes and efficiency of organic solar cell comprising copolymer. The copolymer (P1) has solubility of 100 mg/ml in 1,2-dichlorobenzene (DCB) solvent.    DETAILED DESCRIPTION - A copolymer (P1) comprising a first substituted 9H-carbazole unit of formula (i) (greater than 0 to 45) mole% based on the total moles of monomers constituting the copolymer, greater than or equal to 1 unit different from the first unit, and having substituent which increases solubility bound to the side chain of the copolymer; is new.    o and p=0-3;    R'a, R'b and R1=T1;or    adjacent two R'a+R'b=condensed ring;    T1=(cyclo)alkyl, alkoxy, aryloxy, alkylthioxy, arylthioxy, alkylsulfoxy, arylsulfoxy, alkenyl, silyl, boron, alkylamine, aralkylamine, (hetero)arylamine, aryl, fluorenyl, carbazole, heterocyclyl containing greater than or equal to 1 heteroatom selected from N, O and S (all optionally substituted), H, deuterium, halo, nitrile, nitro, imide, amide, or hydroxyl.    INDEPENDENT CLAIMS are included for the following:    (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 electrode and the second electrode and including a photoactive layer, where at least one organic material layer(s) comprises the copolymer (P1); and    (2) method for fabricating organic solar cell, involving: 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 comprising the copolymer (P1); and forming a second electrode on the organic material layer.