Ultralow thermal conductivity and thermoelectric performance of two-dimensional KCuX (X=S, Se) materials for energy harvesting

Two - dimensional (2D) materials have attracted considerable attention recently due to their outstanding electronic, optical, magnetic, and transport properties which can be widely used in electronic, optoelectronics, spintronic, sensing, energy storage (e.g. batteries and supercapacitors) and energy conversion devices (e.g., thermoelectric, and solar cells) [1,2]. The 2D materials possess excellent electrical and mechanical properties, which are suitable to fabricate high-performance composite thermoelectric materials as well [3]. In this work, we studied thermoelectric properties of KCuS and KCuSe monolayers which belong to a 2D ternary quintuple layer family named PQR (P = K, Na, and Rb), (Q =Cu, Au, and Ag), and (R = S, Se, and Te). Using First-principles based density functional theory (DFT) calculations and Boltzmann transport equation (BTE), we have calculated the thermoelectric (TE) properties of KCuX (X = S, Se). The result indicates that KCuS and KCuSe possess a direct band gap of 0.193 eV and 0.211 eV respectively. The obtained ultralow value of lattice thermal conductivity of 0.015 Wm⁻¹K⁻¹ for KCuS, and 0.006 Wm⁻¹K⁻¹ for KCuSe indicates towards large value of the figure of merit (ZT). We have estimated the values of various TE parameters like the Seebeck coefficient, electronic transport properties, power factor, and ZT at various temperatures. These combination of TE properties indicates towards potential use of KCuX (X=S, Se) family of materials for energy harvesting applications.