Effects of Fluorination on Exciton Binding Energy and Charge Transport of π‑Conjugated Donor Polymers and the ITIC Molecular Acceptor: A Theoretical Study

The intelligent addition of fluorine atoms in the chemical structure of conjugated polymers has been a popular approach to improve the efficiency of organic photovoltaic (OPV) devices. Recently, this strategy has been extended to nonfullerene acceptor (A) molecules in the best-performing bulk heterojunction (BHJ) devices. Yet, many details involved in the role of fluorination to enhance the photovoltaic response of organic semiconductors are still unclear. Here, we theoretically investigate the changes in the key properties of the representative fluorinated oligomers of polymers commonly used as donors (D) in BHJ-based OPVs. We then extend our analysis to consider the fluorination of ITIC, a very promising nonfullerene acceptor. We focus on the variation of the exciton binding energy (E_b) with the fluorination of an oligomer (molecule). Our calculations indicate that the fluorine substitution tends to lower the exciton binding energy which can enhance charge generation after light absorption. Considering the complexes of two oligomers (molecules), we also investigate the effects of fluorination on charge transport. We found that the intermolecular binding energy is considerably higher for the oligomers (molecules) with fluorine atoms. The increased electronic coupling tends to induce a better packing along the π–π direction which can explain the differences observed in the morphology of thin solid films. The calculation of the hole mobility for the oligomers (and electronic coupling for the acceptor molecules) showed higher values with fluorination. Our results are consistent with the space charge-limited current measurements performed in fluorinated conjugated materials and highlight the main reasons behind the better performance of fluorinated BHJ devices.