Chain Length Dependence in Diketopyrrolopyrrole/Thiophene Oligomers

Diketopyrrolopyrrole-based polymers have shown good performance in polymer solar cells. Monodisperse short oligomers, monomer up to tetramer, of one of these polymers, pBBTDPP1, have been prepared and the chain length dependence of their optical and electrochemical properties has been investigated. The optical and electrochemical data obtained in solution lead to the conclusion that conjugation in this system is very limited. The most probable reason for this is a twist in the backbone, caused by steric hindrance of the dodecyl side chains with the sulfur atoms. In thin films, the chain length dependence of the optical properties is much stronger, due to planarization of the oligomer chains upon aggregation.