Photoactive polymers comprising first and second co-monomer repeat units, the first co-monomer repeat unit comprising a moiety selected from the group consisting of an alkylthieno[3,4-c]pyrrole-4,6-dione moiety and a 1,3-dithiophene-5-alkylthieno[3,4-c]pyrrole-4,6-dione moiety, and the second co-monomer repeat unit comprising a moiety selected from the group consisting of a 4,4'-dialkyl-dithieno[3,2-b:2'3'-d]silole moiety, an ethylene moiety, a thiophene moiety, an N-alkylcarbazole moiety, an N-(1-alkyl)dithieno[3,2-b:2'3'-d]pyrrole moiety and a 4,8-dialkyloxylbenzo[1,2-b:3,4-b]dithiophene moiety are described herein. These polymers are suitable for use in photovoltaic cells and field effect transistors.