An organic solar cell comprising: a first electrode; a second electrode disposed opposite to the first electrode; and one or more organic material layers disposed between the first electrode and the second electrode and including a photoactive layer, wherein the photoactive layer includes an electron donor and an electron acceptor, the electron donor includes a polymer including a first unit represented by the following Chemical Formula 1; a second unit represented by the following Chemical Formula 2; and a third unit represented by the following Chemical Formula 3, and the electron acceptor includes a non-fullerene-based compound: in Chemical Formulae 1 to 3, X1 to X5 are the same as or different from each other, and each independently selected from the group consisting of CRR', NR, O, SiRR', PR, S, GeRR', Se and Te; Y1 to Y4 are the same as or different from each other, and each independently selected from the group consisting of CR", N, SiR", P and GeR"; R, R', R", Q1 to Q4, R1 to R4, R10 and R11 are the same as or different from each other, and each independently hydrogen; deuterium; a halogen group; a hydroxyl group; a substituted or unsubstituted alkyl group; a substituted or unsubstituted cycloalkyl group; a substituted or unsubstituted alkoxy group; a substituted or unsubstituted aryloxy group; a substituted or unsubstituted alkenyl group; a substituted or unsubstituted amine group; a substituted or unsubstituted aryl group; or a substituted or unsubstituted heterocyclic group; R20 and R21 are the same as or different from each other, and each independently a substituted or unsubstituted alkoxy group; or a substituted or unsubstituted aryloxy group; a, b, d and e are each an integer of 0 to 3; when a, b, d or e is 2 or greater, structures in the two or more parentheses are the same as or different from each other; and A1 to A4 are the same as or different from each other, and each independently hydrogen, fluorine or chlorine, and at least one of A1 to A4 is fluorine or chlorine, and characterized in that the non-fullerene-based compound is represented by the following Chemical Formula A: in Chemical Formula A, Ra to Rf are the same as or different from each other, and each independently hydrogen; or a substituted or unsubstituted alkyl group; La to Ld are the same as or different from each other, and each independently a substituted or unsubstituted arylene group; or a substituted or unsubstituted divalent heterocyclic group; Ma and Mb are the same as or different from each other, and each independently hydrogen; a halogen group; or a substituted or unsubstituted alkyl group; p and q are the same as or different from each other, and each independently an integer of 0 to 2; and when p or q is 2, structures in the parentheses are the same as each other. a first electrode; a second electrode disposed opposite to the first electrode; and one or more organic material layers disposed between the first electrode and the second electrode and including a photoactive layer, wherein the photoactive layer includes an electron donor and an electron acceptor, the electron donor includes a polymer including a first unit represented by the following Chemical Formula 1; a second unit represented by the following Chemical Formula 2; and a third unit represented by the following Chemical Formula 3, and the electron acceptor includes a non-fullerene-based compound: in Chemical Formulae 1 to 3, X1 to X5 are the same as or different from each other, and each independently selected from the group consisting of CRR', NR, O, SiRR', PR, S, GeRR', Se and Te; Y1 to Y4 are the same as or different from each other, and each independently selected from the group consisting of CR", N, SiR", P and GeR"; R, R', R", Q1 to Q4, R1 to R4, R10 and R11 are the same as or different from each other, and each independently hydrogen; deuterium; a halogen group; a hydroxyl group; a substituted or unsubstituted alkyl group; a substituted or unsubstituted cycloalkyl group; a substituted or unsubstituted alkoxy group; a substituted or unsubstituted aryloxy group; a substituted or unsubstituted alkenyl group; a substituted or unsubstituted amine group; a substituted or unsubstituted aryl group; or a substituted or unsubstituted heterocyclic group; R20 and R21 are the same as or different from each other, and each independently a substituted or unsubstituted alkoxy group; or a substituted or unsubstituted aryloxy group; a, b, d and e are each an integer of 0 to 3; when a, b, d or e is 2 or greater, structures in the two or more parentheses are the same as or different from each other; and A1 to A4 are the same as or different from each other, and each independently hydrogen, fluorine or chlorine, and at least one of A1 to A4 is fluorine or chlorine, and characterized in that the non-fullerene-based compound is represented by the following Chemical Formula A: in Chemical Formula A, Ra to Rf are the same as or different from each other, and each independently hydrogen; or a substituted or unsubstituted alkyl group; La to Ld are the same as or different from each other, and each independently a substituted or unsubstituted arylene group; or a substituted or unsubstituted divalent heterocyclic group; Ma and Mb are the same as or different from each other, and each independently hydrogen; a halogen group; or a substituted or unsubstituted alkyl group; p and q are the same as or different from each other, and each independently an integer of 0 to 2; and when p or q is 2, structures in the parentheses are the same as each other.