Two-dimensional (2D) materials with lower lattice thermal conductivity and high figure of merit are useful for applications in thermoelectric (TE) devices.In this work,the thermoelectric properties of monolayer Cu ₂X (X=S,Se) have been systematically researched through first-principles and Boltzmann transport theory.We have verified the dynamic stability of monolayer Cu ₂X (X=S,Se) through elastic constants and phonon dispersion.The results show that monolayer Cu ₂X (X=S,Se) together with small lattice constants,resulting in lower phonon vibration modes.Phonon transport calculations confirm that monolayer Cu ₂Se has lower lattice thermal conductivity (1.93W/mK) than Cu ₂S (3.25W/mK) at room temperature,which is due to its small Debye temperature and stronger anharmonicity.Moreover,the heavier atomic mass of Se atoms effectively reduces the phonon frequency,resulting in a ultra narrow phonon band gap (0.08THz) and lower lattice thermal conductivity for monolayer Cu ₂Se.The band degeneracy effect at the VBM of monolayer Cu ₂X (X=S,Se) significantly increases its carrier effective mass,resulting in higher Seebeck coefficient and lower conductivity under p-type doping.The electric transport calculation at room temperature shows that the conductivity of monolayer Cu ₂S (Cu ₂Se) under n-type doping about 10 ¹¹cm ^-2is 2.8×10 ⁴S/m (4.5×10 ⁴S/m),obviously superior to their conductivity about 2.6×10 ²S/m (1.6×10 ³S/m) under p-type doping.At the optimum doping concentration for monolayer Cu ₂S (Cu ₂Se),the n-type power factor is 16.5mW/mK ²(25.9mW/mK ²),which is far higher than p-type doping 1.1mW/mK ²(6.6mW/mK ²).Through the above results,the excellent figure of merit of monolayer Cu ₂S and Cu ₂Se under optimal n-type doping at 700 K can approach 1.85 and 2.82,which are higher than 0.38 and 1.7 under optimal p-type doping.The excellent thermoelectric properties of monolayer Cu ₂S and Cu ₂Se are comparable to those of many promising thermoelectric materials reported recently.Especially,the figure of merit of monolayer Cu ₂Se is larger than the well-known high-efficient thermoelectric monolayer SnSe (2.32).Therefore,monolayer Cu ₂X (X=S,Se) are potential thermoelectric material with excellent performance and good application prospects.Such results provide theoretical basis for the follow-up experiments exploring the practical applications of 2D thermoelectric semi-conductors materials and offer an in-depth insight into the effect of phonon thermal transport on improvement of TE transport properties.