A copolymer for organic solar cells represented by the following Chemical Formula 2: wherein, in Chemical Formula 2, n is an integer of up to 10,000; R1 and R2 are an ester group substituted with an alkyl group, or R1 and R2 are a carbonyl group substituted with an alkyl group; R3 and R4 are the same as or different from each other, and each independently hydrogen; or a halogen electron withdrawing group; A' is a substituted or unsubstituted monocyclic or multicyclic aryl group; or a substituted or unsubstituted monocyclic or multicyclic heterocyclic group including one or more of N, O, S and Se atoms; and A includes one, two or more of the following chemical formulae: wherein, in the structures, X1 to X6 are the same as or different from each other, and each independently CRR', NR, O, SiRR', PR, S, GeRR', Se or Te; Y1 and Y2 are the same as or different from each other, and each independently CR", N, SiR", P or GeR"; and R', R", R"', R10 to R13 are the same as or different from each other, and each independently hydrogen; a halogen group; a nitro group; a cyano group; a carboxyl group; a hydroxyl group; a substituted or unsubstituted carbonyl group; a substituted or unsubstituted alkyl group; a substituted or unsubstituted allyl group; a substituted or unsubstituted alkoxy group; a substituted or unsubstituted cycloalkyl group; a substituted or unsubstituted alkenyl group; a substituted or unsubstituted ester group; a substituted or unsubstituted amide group; a substituted or unsubstituted ether group; a substituted or unsubstituted sulfonyl group; a substituted or unsubstituted sulfoxy group; a substituted or unsubstituted arylalkyl group; a substituted or unsubstituted aryl group; or a substituted or unsubstituted heterocyclic group including one or more of N, O, S and Se atoms. n is an integer of up to 10,000; R1 and R2 are an ester group substituted with an alkyl group, or R1 and R2 are a carbonyl group substituted with an alkyl group; R3 and R4 are the same as or different from each other, and each independently hydrogen; or a halogen electron withdrawing group; A' is a substituted or unsubstituted monocyclic or multicyclic aryl group; or a substituted or unsubstituted monocyclic or multicyclic heterocyclic group including one or more of N, O, S and Se atoms; and A includes one, two or more of the following chemical formulae: X1 to X6 are the same as or different from each other, and each independently CRR', NR, O, SiRR', PR, S, GeRR', Se or Te; Y1 and Y2 are the same as or different from each other, and each independently CR", N, SiR", P or GeR"; and R', R", R"', R10 to R13 are the same as or different from each other, and each independently hydrogen; a halogen group; a nitro group; a cyano group; a carboxyl group; a hydroxyl group; a substituted or unsubstituted carbonyl group; a substituted or unsubstituted alkyl group; a substituted or unsubstituted allyl group; a substituted or unsubstituted alkoxy group; a substituted or unsubstituted cycloalkyl group; a substituted or unsubstituted alkenyl group; a substituted or unsubstituted ester group; a substituted or unsubstituted amide group; a substituted or unsubstituted ether group; a substituted or unsubstituted sulfonyl group; a substituted or unsubstituted sulfoxy group; a substituted or unsubstituted arylalkyl group; a substituted or unsubstituted aryl group; or a substituted or unsubstituted heterocyclic group including one or more of N, O, S and Se atoms.