TECHNOLOGY FOCUS - POLYMERS - Preparation (disclosed): No general method for preparation of the copolymer (P1) is given. Preferred Components: The copolymer (P1) has, in its side chain, greater than or equal to 1 group selected from 1-40C alkyl, 1-40C alkoxy, 1-40C sulfide (all optionally substituted), and hydroxyl. The unit different from the first unit includes ( greater than or equal to 1, preferably greater than or equal to 2) electron acceptor(s) (greater than 0 to 50) mole% based on the total moles of the monomers, and further greater than or equal to 1 electron donor. The copolymer is of formula -((-A'-B'-)l-(-C'-D'-)m-)n-. The electron acceptor is of formula (ii). The electron donor is of formula (iii). The copolymer further comprises a fourth unit of formula (iv), and a fifth unit of formula (v).l=greater than 0 to 1 mole fraction;m=0 to less than 1;l+m=1;n=1-10000;A'=group of formula (i);B', C' and D'=electron acceptor or electron donor;s and t=0-4;R8 to R13=1-40C alkyl, 1-40C alkoxy, 1-40C sulfide (all optionally substituted), H, or hydroxyl (preferably R10 and R11 are selected from 1-40C alkyl, 1-40C sulfide, or 1-40C alkoxy (all optionally substituted));oradjacent two of R8 to R13 together=condensed ring;R'10 and R'11=1-40C alkyl, 1-40C sulfide, 1-40C alkoxy (all optionally substituted), or hydroxyl;q and r=0-3;R6, R7 and R''=T1;oradjacent two R6+R7=condensed ring.Provided that: greater than or equal to 1 of B', C' and D' is an electron acceptor. In group of formula (ii), ( greater than or equal to 1, preferably greater than or equal to 2) of R8 to R13 is/are selected from 1-40C alkyl, 1-40C alkoxy, 1-40C sulfide (all optionally substituted), or hydroxyl. The copolymer comprises the first unit (i) (greater than 0 to 10) mole% based on the total moles of monomers constituting the copolymer, the second unit (ii) (greater than 0 to 50) mole% based on the total moles of the monomers, and the third unit (iii) (more than 0 to 40) mole%. The copolymer has an aryl and/or heterocyclyl group as its end group. The copolymer has a number-average molecular weight of 500-1000000 g/mol, and a molecular weight distribution of 1-100. Preferred Device: In the organic solar cell the organic material layer(s) are selected from an electron donor and an electron acceptor, where the electron donor is the copolymer (P1). The electron acceptor is selected from fullerene and its derivatives, carbon nanotubes and their derivatives, vasocuproin, semiconductor elements, and/or semiconductor compounds. The electron acceptor is C60 fullerene derivative or C70 fullerene derivative. The copolymer (P1) and the electron acceptor are mixed with each other at a weight ratio of 1:0.5-1:4. EXAMPLE - 3,6-Bis(4',4',5',5'-tetramethyl-1',3',2'-dioxaborolan-2'-yl)-N-9'-heptadecanylcarbazole (3.3 mg), 2,7-bis(4',4',5',5'-tetramethyl-1',3',2'-dioxaborolan-2'-yl)-N-9'-heptadecanylcarbazole (162 mg), 4,7-bis-(5'-bromo-(2,2')bithiophen-5-yl)-5,6-bis-octyloxy-benzo-(1,2,5)thiadiazole (180 mg), 20% aqueous solution of tetraethylammonium hydroxide (3 ml), (tetrakis(triphenylphosphine)palladium(0) (5 mg) and Aliquat 336 were added to toluene (3 ml) and stirred under reflux. After 72 hours, phenylboronic acid (0.05 g) was added to and reacted with reaction mixture for 3 hours, after which bromobenzene (0.12 g) was added and reacted with mixture for 4 hours. Then, mixture was cooled to room temperature and poured into methanol. The solid was filtered and Soxhlet-extracted with acetone, hexane and chloroform. The extract was precipitated in methanol, and solid was filtered. The copolymer was obtained in yield of 69%, had number-average molecular weight of 22200 g/mol, and weight-average molecular weight of 57900 g/mol.