Copolymer and organic solar cell comprising same

A copolymer comprising:

• a unit represented by the following Chemical Formula 1 or 2:
• in Chemical Formulas 1 and 2,
• l is a mol fraction and a real number of more than 0 and less than 1,
• m is a mol fraction and a real number of more than 0 and less than 1,
• a sum of 1 and m is 1,
• n is an integer of 1 to 10,000,
• A is unit A including a substituted or unsubstituted 5-membered or 6-membered monocyclic nitrogen-containing hetero-cyclic group,
• B and B′ are the same as or different from each other, and are each independently unit B including a substituted or unsubstituted monocyclic or polycyclic hetero-cyclic group,
• C is unit C including a substituted or unsubstituted monocyclic or polycyclic aryl group or a substituted or unsubstituted monocyclic or polycyclic hetero-cyclic group, and
• at least one of unit B and unit C includes a substituted or unsubstituted polycyclic aryl group or a substituted or unsubstituted polycyclic hetero-cyclic group.