LAUSR

Abstract Three low band gap (below 1.6 eV) copolymers based on 2,5-bis(3-octylthiophen-2-yl)thieno[3,2-b]thiophene) donor unit and isoindigo acceptor units were designed and successfully synthesized by the direct arylation polycondensation method. The backbones of the copolymers were chosen to be similar while the alkyl side chain substituted on the isoindigo units were systematically tailored as 2-hexyldecyl in P1, 2-octyldecyl in P2 and 2-decyltetradecyl in P3. The thermal stabilities of the copolymers was found to be highly dependent on the lengths of alkyl side chains. The long alkyl side chains in P3 were found to be detrimental to its stability. P2 was found to have a higher thermal stability was found in due to the optimized side chains that reduced steric repulsion between the side chains in the donor and acceptor units. The thermogravimetric analysis also revealed that the backbone decomposition of P2 is at higher temperature than P3 and P1. The results confirmed the importance of alkyl side chain optimization for the stability of copolymers