Theoretical Insight into Fluorination on Low-Cost A-π-D-π-A Type Donor Materials for All-Small-Molecule Organic Photovoltaic Cells

Low-cost organic photovoltaic (OPV) devices have shown enormous potential in large-scale industrial applications. And it has attracted widespread attention in the past few decades. However, the photophysical characteristics of these budget-friendly materials haven't been explored much. Here, low-cost small materials, including small molecule 1 (asm1) with ortho-fluorinated side chain and small molecule 2 (asm2) with meta-fluorinated side chain were selected to probe the fluorination effect on the absorption spectra, electrochemical energy levels, electrostatic potential (ESP), etc. The results show that the molecules asm1 and asm2 have good planarity of the backbone. And the meta-fluorinated side chain of asm2 contributes more to the highest occupied molecular orbital and less to the lowest unoccupied molecular orbital than asm1. Moreover, differences in ESP are found between donor and acceptor materials. Furthermore, strong and broad light absorption in the visible region of these low-cost molecules is observed, resulting in a better short-circuit current density for the devices constructed by the donors asm1, asm2, and acceptor Y6. In addition, more charge transfer mechanisms are characterized for the asm1/Y6 system. The introduction of ortho-fluorination in the conjugated side chain of the molecule is a favorable approach, which will provide theoretical guidance for further molecular design experiments.