The way to enhance thermoelectric properties of Bi$_{2}$O$_{2}$Q (Q$=$S and Se) system by introducing chalcogen mixture net.*

First principles density functional theory calculations were carried out for the series of bilayered semiconducting Bi$_{2}$O$_{2}$Q (Q$=$S, Se) and hypothetically constructed Bi2O2S0.5Se0.5 compounds in which chalogen square net of pure Bi$_{2}$O$_{2}$Q compound is replaced with having stripe type structure of chalcogen mixture net to explore their electronic structures, the change in their electronic structures under the chalcogen mixing, and the possibility for improving in their thermoelectric properties. By introducing chalcogen mixture net in pure Bi$_{2}$O$_{2}$Q, the band gap should be adjusted, and the indirect band gap in pure Bi$_{2}$O$_{2}$Q compound is changed to direct band gap. According to the analysis of calculated thermoelectric properties of Bi$_{2}$O$_{2}$Q and Bi$_{2}$O$_{2}$S$_{0.5}$Se$_{0.5}$ compounds, thermoelectric properties are strongly enhanced in Bi$_{2}$O$_{2}$S$_{0.5}$Se$_{0.5}$ compound. It seems to be due to the fact that the electrical conductivity is strongly enhanced by the decrease of dimensionality of its electronic structure and the broadening of sharpness of density of states near the Fermi level. Therefore, we believe that band modulation by introducing chalcogen mixture net in its pure compound Bi$_{2}$O$_{2}$Q compound gives a help to improve their thermoelectric properties.