Implication of side-chain fluorination on electronic properties, ordering structures, and photovoltaic performance in asymmetric-indenothiophene-based semiconducting polymers

Side-chain fluorination of conjugated polymers has been proved to be a highly effective approach for optimizing optical and electrical properties of the relative polymers. However, current studies on the fluorination are all based on symmetric molecular structures. In this work, two new D-A type photovoltaic polymers, namely PITph-DTffBT and PITphf-DTffBT, based on asymmetric indenothiophene (IT) donor units with alkoxyphenyl or fluoroalkoxyphenyl substituents were designed and synthesized. Effects of the fluorine substitution in the asymmetric IT donor units on the electronic structure, ordering structure, photovoltaic properties, and charge generation and recombination dynamics were investigated. It is found that side-chain fluorination in the asymmetric donor units of the D-A polymers endowed the relative polymer with a deeper HOMO level, higher and more balanced charge mobilizes, increased charge dissociation efficiency and reduced bimolecular recombination. As a result, the bulk heterojunction solar cell based on the blend film of PITphf-DTffBT and PC71BM demonstrated an efficiency of 7.03%, whereas the cell efficiency based on PITph-DTffBT was only 5.68%. These results indicate that our design strategy by introducing a fluoroalkoxyphenyl unit as side chain to fabricate asymmetric IT-based polymer is efficient in improving the photovoltaic performance. We believe that the results provide new insights into the design of high-performance semiconducting photovoltaic polymers.