The thermoelectric properties of CdBr, CdI, and Janus Cd₂BrI monolayers with low lattice thermal conductivity

The investigation and development of high thermoelectric value materials has become a research hotspot in recent years. In this work, based on the density functional theory on the Perdew-Burke-Ernzerhof (GGA-PBE) level, the thermoelectric properties of transition metal halides CdBr, Janus Cd₂BrI, and CdI monolayers have been systematically investigated using Boltzmann transport theory. The calculation of the electronic band structure shows that these three materials have indirect band gap semiconductor properties. For carrier transport, the electron mobilities for CdBr, Janus Cd₂BrI, and CdI monolayers are found to be 74, 16, 21 cm² s^-1 V^-1 for p-type doping and 116, 102, 78 cm² s^-1 V^-1 for n-type doping. Regarding their phonon transport, the CdBr, Cd₂BrI, and CdI monolayers all have very low lattice thermal conductivity (4.78, 2.46, and 1.65 W m^-1 K^-1, respectively) that decreases with increasing temperature, which is favorable for obtaining large zT values. The electrical transport results show that the performance of p-type doping is better than that of n-type doping. At 300 K, the Seebeck coefficients of p-type doping for the CdBr, Cd₂BrI, and CdI monolayers are 217.72, 246.43, and 226.24 μV K^-1, respectively. In addition, we predict that the zT values of the CdBr, Cd₂BrI, and CdI monolayers are 0.62, 1.64, and 0.87 for p-type doping at 300 K respectively. The zT values increase with the increase of temperature. In particular, the Janus Cd₂BrI monolayer has a zT value of 3.03 at 600 K. These results suggest that all these materials can be good candidates for thermoelectric materials.