Effect of fluorination on electronic properties of polythienothiophene-co-benzodithiophenes and their fullerene complexes.

Abstract

Fluorination of conjugated polymers is a popular way of designing new electron donors for the bulk heterojunction (BHJ) based organic solar cells (OSCs). However, not all fluorinated polymers observed experimentally enhance the power conversion efficiency of OSCs, and the fundamental understanding of the effect of fluorination is not yet fully uncovered. Herein, we report the effect of fluorine substitution on the electronic properties of polythienothiophene-co-benzodithiophenes as well as their complexes with fullerene, using density functional theory (DFT) and time-dependent DFT methods at the molecular level. Systematic computations of energy gaps (E(g)(opt) and E(g)(hl)), ionization potentials (IP), electron affinities (EA), molecular electrostatic potential (MEP) surfaces, and dipole moments (μ) are carried out for these systems. We found that the fluorination of the thienothiophene unit favors lower E(g)(opt), E(g)(hl), IP, and EA as well as stronger μ compared to the fluorination of the benzodithiophene unit, suggesting that efficient exciton dissociation and charge carriers formation may take place efficiently for the former case. These results support recent experimental findings that the performance of polythienothiophene-co-benzodithiophene-based organic solar cells enhances when thienothiophene unit is fluorinated. The present results highlight that more efficient conjugated polymers for OSC can be designed if the gap engineering is carried out by focusing on the low IP, low EA, and high dipole moment.