Exploring the fluorination effect mechanism on charge transport in organic solar cells

The fluorination in organic solar cells (OSCs) impacts the photoelectric properties of conjugated polymers. Here, two benzo [1,2-c:4,5-c'] dithiophene-4,8-dione (T1) based donor polymers with different numbers of fluorine substitutions (F1 and F2) have been investigated. Through density functional theory (DFT), optimized structures, frontier molecular orbitals (FMOs), energy gaps, electron affinity energies (EAs) and ionization potentials (IPs) of the fluorinated polymers were studied. Simultaneously, one used time-dependent density functional theory (TD-DFT) to simulate the absorption spectra of F1 and F2 polymer molecules. The impact mechanism of F atoms for the above parameters was explained in depth. Then, through constructing the molecular dimer model, the hole mobility based on Marcus' theory was calculated for two systems, suggesting that the hole mobility of F1 (with two fluorine atoms introduced in the specific para position of the benzene unit) was greater than that of F2. Finally, the ideal photovoltaic parameters of donor–acceptor compounds were theoretically investigated, and the reasons for performance differences were analyzed from the micro level. The research helps understand the influence of specific fluorine atom substitution on the structure, spectrum, electrochemical parameters, charge transfer and photovoltaic properties, and provides a theoretical guideline for the molecular fluorination modification strategy of excellent OSCs.