Effects of alloying chalcopyrite CuTlSe2 with Na on the electronic structure and thermoelectric coefficients: DFT investigation.

The optimized structural parameters, electronic structure, and thermoelectric coefficients of the chalcopyrite alloys Cu_1–xNa_xTlSe₂, with x = 0.00, 0.25, and 0.50 were studied through density functional theory calculations. The Wu–Cohen generalized gradient and the Tran–Blaha modified Becke–Johnson approximations have been employed to describe the exchange–correlation potential. Energy band structure analysis reveals that CuTlSe₂ is a semi-metal, while Cu_0.25Na_0.75TlSe₂ and Cu_0.50Na_0.50TlSe₂ alloys are semiconductors with gaps of approximately 0.17 eV and 0.35 eV, respectively. The total and partial densities of states were calculated and discussed. Examining the charge density, we point out the formation of the Na–Se ionic bond when Cu is replaced by the Na atom, which is responsible for the metal–semiconductor transition in the Cu_1–xNa_xTlSe₂ alloys. Moreover, variations of the Seebeck coefficient, electrical conductivity, electronic and lattice thermal conductivity, power factor, and figure of merit of the Cu_1–xNa_xTlSe₂ alloys with temperature and chemical potential were explored. The obtained results show that the value of the figure of merit increases when doping CuTlSe₂ with sodium to reach 0.46 and 0.87 for p-type Cu_0.75Na_0.25TlSe₂ and n-type Cu_0.50Na_0.50TlSe₂, respectively. [ABSTRACT FROM AUTHOR]