A First-Principles Study of 2D Bi-Based BiOClBr, BiOClI, and BiOBrI Monolayers with Ultralow Lattice Thermal Conductivities for Thermoelectric Application

Thermoelectric materials are important in waste heat utilization and clean energy development due to environmental and energy issues. In advanced thermoelectric applications, two-dimensional nanomaterials offer substantial advantages due to their exceptional bending stiffness and natural flexibility. The first-principles study has been used to predict the electronic structures and thermoelectric transport characteristics of BiOClBr, BiOClI, and BiOBrI monolayers. The results indicate that the three monolayers are semiconductors with an indirect band gap and high stability. The Seebeck coefficient and electrical conductivity are efficiently enhanced by the valence band valley and high carrier mobility. Moreover, a small phonon group velocity, short phonon relaxation time, large Grüneisen parameter, and phonon scattering phase space are responsible for the low lattice thermal conductivities of 1.66 (BiOClBr), 0.98 (BiOClI), and 1.51 W m–1 K–1 (BiOBrI) at 300 K. Under n-type doping, BiOClBr and BiOClI monolayers have optimal ZT values of 2.30 and 4.35 at 300 K and up to 4.78 and 7.83 at 700 K, respectively. For the BiOBrI monolayer with p-type doping, the optimal ZT values are 5.98 at 300 K and 10.01 at 700 K, which outperform some previously published Bi-based thermoelectric materials, suggesting that the three monolayers are promising Bi-based candidate thermoelectric materials.