First Principles Investigation of Thermoelectric Materials

P C, Sreeparvathy and V, Kanchana (2019) First Principles Investigation of Thermoelectric Materials. PhD thesis, Indian institute of technology Hyderabad.

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Abstract

The rapidly increasing energy demand in each and every domain of human life highlights the necessity of clean and renewable energy sources. Among them, thermoelectric (TE) energy conversion technique stands out due to the global availability of heat energy in the form of waste heat and find wide span of applications ranging from wrist watch to space applications. TE materials can play a major role in both power generation and waste heat recovery and can convert the heat energy to electricity. The search for better TE materials is still demanding due to the lesser efficiency of the same. The present thesis focuses on the search of novel TE materials fetching applications for a wide temperature range within the frame work of Density Functional Theory (DFT). DFT is a prominent tool for predicting the electronic structure and the diverse physical properties of materials and plays a leading role in computational condensed matter theory. Here in this thesis, we have investigated the TE properties of few interesting family of compounds and also tried to improve the TE properties by the application of hydrostatic/uni-axial strain and further attempted to explore the advantages of few exotic materials such as Dirac/topological insulator for TE applications. In the first chapter, we present a detailed investigation of electronic structure and TE properties of Zn based pnictide semiconductors in the form of ZnXPn2 (X: Si, Ge, Sn; Pn: P, As, Sb). The entire compounds in this chapter belong to the well known chalcopyrite family and crystallize in tetragonal structure with space group I¯42d. The appropriate selection of exchange correlation functional is important and here the precise electronic structure of all the studied compounds are computed using Tran-Blaha modified Becke-Johnson (TB-mBJ) functional. The presence of a mixture of light and heavy bands in the band structure in general strengthen the TE properties, and the studied compounds shows this property. Computed TE properties unveils the p-type conduction to be favorable in ZnXP2 (X: Si, Ge, Sn) and n-type conduction in ZnGeP2 and ZnSiAs2. Mechanical stability of the material is one of the criteria for device applications, and here we have confirmed the mechanical stability using the computed elastic constants. The TE properties such as thermopower, electrical conductivity scaled by relaxation time (will be addressing as electrical conductivity) and power factor for all the compounds are investigated by combining DFT with semi-classical Boltzmann transport theory. The thermopower of the investigated compounds are found to be higher compared with the prototype chalcopyrite TE materials, together with comparable values for electrical conductivity. In addition, the investigated compounds possess high thermopower compared to other established TE materials, which motivates further research in these compounds. Overall the considered compounds are found to possess appreciable TE coefficients, and we have analyzed the effect of hydrostatic strain on two of the investigated compounds ZnGeSb2 and ZnSnSb2. Among these, ZnGeSb2 turned to be very promising TE material with huge power factor of the order of 3×1017 W m−1 K−2 s −1 due to the huge electrical conductivity around 8.5×1025 Ω −1 m−1 s −1 , observed in its massive Dirac state. The calculated electrical conductivity is higher than the well established Dirac materials, and is almost carrier concentration independent with similar behaviour for both ‘n’ and ‘p’ type carriers. Detailed analysis of the projected band structure reveals the ‘s’, ‘p’ band inversion around Γ high symmetry point in the tensile strained state of ZnGeSb2 and ZnSnSb2. The possibility of low value for thermal conductivity is also evident from the phonon dispersion plot of ZnGeSb2 and from Debye temperature. Application of systematic hydrostatic strain on ZnGeSb2 and ZnSnSb2 reveals the gradual phase change of ZnGeSb2 from a normal semiconducting state, through massive Dirac states, to a topological semi-metal, whereas ZnSnSb2 is transfered from normal semiconductor to metal together with a band inversion. The maximum power factor is observed in the massive Dirac states of ZnGeSb2 compared to all other compounds. In the next chapter, we have investigated the electronic structure, mechanical and TE properties of few natural bulk super lattice materials, and report the high thermopower values together with the probability of low thermal conductivity in the studied series. The study includes BaXFCh (X: Cu, Ag, Ch: S, Se, Te), LaXSO (X: Cu, Ag) and SrCuTeF , and crystallize in tetragonal structure with space group P4/nmm. The possibility of low thermal conductivity is predicted from the obtained elastic constants and few well established models such as Cahill’s model and Slack’s model. The huge difference in the band dispersion along the different crystallographic directions of the investigated compounds reveal the quasi two dimensional behavior of band structure in the valence band, and this is confirmed through effective mass calculations. The significant difference in effective mass along different crystallographic directions in valence band introduces an anisotropy in the transport properties. The properties along ‘a’ axis is found to be more favourable for hole doping. The magnitude of thermopower of these compounds are highly comparable with other established TE materials. In addition to these, the parameter A (S2σ/τT/ κe/τ ), which helps to decouple the relaxation time from our calculations is also calculated, and it reveals the potential TE properties of the considered compounds. Next chapter deals with a detailed electronic structure calculation, which reveals the strong topological insulating nature of a series of compounds CaSrX (X: Si, Ge, Sn, Pb), together with striking TE properties. The electronic structure of all the compounds are studied as a function of uni-axial strain and an emergence of Dirac semi-metallic state has been observed in CaSrX (X: Si, Ge, Sn, Pb), which is induced by uni-axial strain along ‘b’ axis. CaSrSi and CaSrGe evolved as normal semiconductor with uni-axial strain, and remaining compounds are found to preserve metallic states within the studied strain range. Since the investigated compounds preserve time reversal symmetry and inversion symmetry, the trivial and non-trivial topological phases are evaluated by band inversion and Z2 topological invariants. An unusual thermopower oscillations has been observed at these Dirac semi-metallic states. Further the TE properties at strong topological insulating state and normal insulating state are summarized, which reveals the potential TE properties of these materials. In the last chapter of the results, we deal with the electronic and TE properties of few transition metal dichalcogenides, which are less investigated. We have chosen pyrite structure OsX2 (X: S, Se, Te) and triclinic structure ReX2 (X: S, Se). First we report the electronic structure and TE properties of OsX2 (X: S, Se, Te), and find a giant value of thermopower of magnitude ranging from 600 µV K−1 to 800 µV K−1 for a wide temperature range of 100 K - 500 K for hole doping (at 1018cm−3 ), which is higher than the value found for well established TE materials. The optimized structural parameters are in good agreement with available experimental reports. The mechanical stability of OsX2 is confirmed from the computed elastic constants. The band gap of the investigated compounds is examined by several exchange correlation functionals, and TB-mBJ with modified parameters is found to be the best for OsX2. The heavy valence bands stimulates the thermopower value for hole doping and light conduction bands enhances the electrical conductivity values for electron doping, enabling both ‘n’ and ‘p’ type doping to be favourable for TE applications at higher concentrations (1020cm−3 ), which brings out the device application. Study on OsX2 unveils the possibility of TE applications for all the examined compounds for a wide temperature range (100 K to 500 K), and OsS2 specifically is a good alternate with the operating temperature ranging from 100 K - 900 K. Further, we have studied a highly versatile system ReS2, which transforms from a semiconductor to a two dimensional metal under uni-axial compressive strain along ‘a’ direction in both bulk and monolayer. The 2D nature is realised from highly flat Fermi surfaces and anisotropic transport properties. Moreover the layer independent electronic structure properties are revisited and TE properties of ReS2 in bulk, monolayer and bilayer forms reveals the competing TE coefficients in each form. The in-plane power-factor shows an enhancement over ‘c’-axis value as a function of strain, which is almost two orders of magnitude. In addition, strain induced tunable in-plane anisotropy of almost one order has been observed in both bulk and monolayer ReS2 (around 20%), which further open up the possibility of TE application as nanowires. Our analysis reveals a wide range of application for ReS2 in the field of thermoelectrics as bulk and thin films for a wide temperature range. The magnitude of TE coefficients are comparable with other well established transition metal dichalocogenides. Overall, the present thesis addresses the potential TE properties in few compounds for a wide temperature range, and in each family of compounds we have attempted to enhance the power factor by the application of hydrostatic/uni-axial strain. In the case of ZnGeSb2, we could enhance the power factor value to 3×1017 W m−1 K−2 s −1 , which is huge compared to normal power factor range. We believe that, the experimental realization of this potential TE material would be very fascinating and the understanding about thermal conductivity would further help in predicting the ZT. From our calculations, we could show that the super lattice structures possess low thermal conductivity, and if one can realize the potential TE application from experimental studies, it would be a great opening. Further, in the case of Ca based compounds, we could predict the coexistence of strong topological insulating nature and potential TE properties, and the exact benefit of strong topological nature with conducting surface states towards TE applications can be understood from the total electrical conductivity (from bulk and surface states), which is beyond the scope of present thesis, and we believe that this could be taken as future study. In the case of ReS2, the layer independent TE properties can lead to several applications in thin film TE, and further strain induced in-plane anisotropy also can be taken as future study, where one can understand the TE properties in nano wire ReS2. Altogether, the present thesis opens up new possibilities from the electronic structure point of view, but in reality for a TE device, the understanding of thermal conductivity, and figure of merit are very crucial. If one can utilize the predicted results, and calculate all the other vital parameters, we believe few of the compounds might turn out to be good TE materials.

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