First-principles Study Of Electronic Structures And Thermoelectric Properties Of CuGaTe2

The energy crisis and environmental pollution are increasingly serious. How to properly solve the energy shortage and environmental pollution is the worldwide concern. Ones expect that new energy materials can convert industrial waste heat and automobile exhaust into the available energy. Since it not only effectively reduces environmental pollution, but also solves the energy shortage. Thermoelectric material is a kind of new energy material, which is able to convert the low quality heat into electricity, based on the thermoelectric effect. However, the current thermoelectric material is not widely used, due to the low conversion efficiency. Thus, the researchers are focus on seeking the new thermoelectric materials with high conversion efficiency and exploring the ways to improve their performance.Based on density functional theory and Boltzmann theory, we studied the electronic structures and thermoelectric properties of chalcopyrite semiconductor CuGaTe2. The research contents and main results are as follows:(1) We have investigated the electronic structures, elastic constants and thermoelectric properties of chalcopyrite semiconductors CuGaTe2and CuInTe2. The results displayed that MBJ-GGA can predict more accurate energy gap values than GGA. Through the study of thermoelectric properties, it was found that optimizing the carrier concentration can improve the thermoelectric performance. The results of the lattice thermal conductivities showed that in the temperature range of300-800K, the lattice thermal conductivities of CuGaTe2and CuInTe2were mainly from the phonon scattering. And the phonon scattering was dominated by Umklapp scattering.(2) We have studied the electronic structures and thermoelectric properties of CuGa1-xInxTe2solid solutions. The calculation of formation energies proved that the stable CuGa1-xInxTe2solid solutions can be formed under the certain temperature. The calculated equilibrium lattice constants showed that they almost linearly increased with the increase of In content. However, the results of calculated band structures indicated that the energy gap values of solid solutions almost linearly decreased with increasing In content. The estimated average speed of sound suggested that In dopant was favorable to reduce the thermal conductivity. Based on the calculated S2σ/τ as a function of carrier concentrations, we found that the optimal doping concentration is in the range from2.0×1026m-3to9.0×1026m-3. (3) The electronic structures and thermoelectric properties of Ag doped CuGaTe2have been researched. When Ag doped, the system converted direct band gap into indirect band gap semiconductor. What’s more, it was found that three bands at valence band edge would degenerate together, when the content of Ag dopant was0.074. Thus, we predicted that a small amount of Ag dopant (x=0.074) was more conducive to improve the seebeck coefficient. The Fermi level shifted downward to the valence band. Due to the hybridization between the states of valence band and Ag dopant, the density of states at Fermi level increased. The calculated results showed that doping of Ag not noly increased the carrier concentration, but also decreased the lattice thermal conductivity.(4) The electronic structures and thermoelectric properties of CuGaTe2under several biaxial plane strains have been explored. It was found that there was a combination of heavy and light bands at valence band edge and these three bands almost converged at-0.5%compressive strain, which would result in the large Seebeck coefficient. The optimal doping concentrations of strained and unstrained CuGaTe2were estimated based on the maximum of S2σ/τ. The peak value of S2σ/τ for CuGaTe2under-0.5%compressive strain at750K was about6.1×1011Wm-1K2s-1with a carrier concentration of7×1026m-3. By the calculated results, we obtained two ways to improve the thermoelectric performance of CuGaTe2. One was imposing the appropriate compressive strain on the compound, and the other was optimizing the carrier concentration.