First-principle investigations with the thermoelectric study for monolayer of group III Nitrides (III = B, Al, Ga & In)

Thermoelectric (TE) material, as one of new energy materials, is regarded as one of the most important energy-saving materials, which can directly achieve the interconversion between heat and electricity. Presently, inorganic semi-conductors are considered to be the best thermoelectric materials. However, the development of new thermoelectric material has attracted great attention owing to the scarce resource and limited performance of inorganic thermoelectric materials. As the discovery and wide application of conducting polymers (CPs), organic thermoelectric materials have come into the sights of people over the past 30 years. The development of CPs as a promising thermoelectric material has gained great attention over the past decades. Various CPs have been developed and investigated on thermoelectric performance such as polyacetylene (PA), polyaniline (PANi), polypyrrole (PPy), polythiophene (PTh) and their derivatives since 1989. Among numerous CPs, poly(3,4-ethylenedioxythiophene) (PEDOT) shows many superior properties compared to others due to its excellent environmental stability, water solubility, easy processibility, and high electrical conductivity, which brings new strategy for studies of high performance organic thermoelectric materials. The conductive PEDOT combined with an insulating poly(styrenesulfonate) (PSS) can form a stable aqueous PEDOT:PSS with a good film-forming property and has been regarded as one of the most potential thermoelectric material. In 2008, the thermoelectric performance of PEDOT:PSS pellets were reported systematically for the first time. Although its thermoelectric figure-of-merit ( ZT ) was as low as 10 - 3, it was one of the highest values for CPs at that time. Soon afterwards, the thermoelectric performance of the free-standing PEDOT:PSS film achieved a ZT value as high as 10 - 2 in 2010. During these years, great attention focused on the derivatives of PTh, polyselenophene (PSh), PANi, and polycarbazole (PCz). Since 2010, PEDOT:PSS has come into sight of researchers in the world. The past few years witnessed great development of thermoelectric performance of conducting PEDOT:PSS ( ZT ~10 - 1). In recent ten years, the thermoelectric figure-of-merit ( ZT ) of PEDOT has been enhanced by three orders of magnitude from 10 - 4 to 10 - 1 as one of the most promising organic thermoelectric materials. Presently, the enhanced thermoelectric performance for PEDOT concern in the increased electrical conductivity via an easy method, especially for PEDOT:PSS. A large electrical conductivity of PEDOT:PSS thin film more than 3000 S cm - 1 has been achieved by adding an organic solvent or post-treatment with organic solvents, acid, and ionic liquids. Compared to inorganic thermoelectric materials, PEDOT:PSS can achieve a high electrical conductivity without an obviously decreasing Seebeck coefficient. A further improvement of thermoelectric performance for PEDOT:PSS has been devoted to the optimization of Seebeck coefficient via the pH value adjustment, the reduction of the oxidized level of conductive PEDOT, or composite with inorganic thermoelectric materials. A large thermoelectric power factor has become a reality. Although there are large gaps from actual industrial application for thermoelectric PEDOT ( ZT ˃1), yet most efforts focus on the high performance PEDOT. A large number of new techniques and methods have been developed to improve the thermoelectric performance of PEDOT. This review pays the attention to the advantages and characteristics, development history, and performance improvement of PEDOT as a promising organic thermoelectric CP from discovery to development. In order to achieve a high performance thermoelectric PEDOT, more attention should be paid to the development of low dimensional PEDOT crystal, control of oxidized level, extension of conjugated chain length of PEDOT, new preparation method and techniques, and the effects of crystal structure on electron transport properties as well as the flexible PEDOT thermoelectric devices in the future. Additionally, it is necessary to keep up with the development of n-type organic thermoelectric materials.

Thermoelectric materials with crystal-amorphicity duality induced by large atomic size mismatch

As two pivotal functional segments, triazine and porphyrin can be coupled to form a highly cross-linked conjugated polymer. Although the obtained conjugated polymers are almost insoluble in most so...

The geometry structures, vibrational, electronic, and thermoelectric properties of bilayer GeSe, bilayer SnSe, and van der Waals (vdW) heterostructure GeSe/SnSe are investigated by combining the first-principles calculations and semiclassical Boltzmann transport theory. The dynamical stability of the considered structures are discussed with phonon dispersion. The phonon spectra indicate that the bilayer SnSe is a dynamically unstable structure, while the bilayer GeSe and vdW heterostructure GeSe/SnSe are stable. Then, the electronic structures for the bilayer GeSe and vdW heterostructure GeSe/SnSe are calculated with HSE06 functional. The results of electronic structures show that the bilayer GeSe and vdW heterostructure GeSe/SnSe are indirect band gap semiconductors with band gaps of 1.23 eV and 1.07 eV, respectively. The thermoelectric properties, including electrical conductivity, thermal conductivity, Seebeck coefficient, power factor, and figure of merit (ZT) are calculated with semiclassical Boltzmann transport equations (BTE). The results show that the n-type bilayer GeSe is a promising thermoelectric material.

The present work reports systematic study of a series of calcium chalcogenides CaX (X = O, S, Se and Te) in the architecture of rock-salt and hexagonal monolayer phases. Using first principle investigation within density functional theory (DFT) framework, we have computed the equilibrium structure and phonon dispersion curves for the dynamic stability, which follows the calculation of electronic properties like electronic band structure and projected density of state for the considered chalcogenide series. Furthermore, the thermoelectric properties such as thermal and electrical conductivities, Seebeck coefficient (S) and figure of merit (ZT) of the considered compounds are computed using the semi-classical Boltzmann transport equations (BTE). The present work reports the monolayer calcium chalcogenides as potential candidate for thermoelectric applications.

Computational advances for energy conversion: Unleashing the potential of thermoelectric materials

Understanding the Temperature Dependence of the Seebeck Coefficient from First-Principles Band Structure Calculations for Organic Thermoelectric Materials

Thermoelectric (TE) materials with efficiencies comparable to Carnot efficiency are highly desirable for applications in devices that utilize the TE effect to convert heat into electricity. The efficiency of a TE material is measured by its figure of merit, which is influenced by the complex interplay between electronic and thermal transport parameters. Janus materials, with their large specific surface area, can enhance the movement of electrons and holes generated by heat towards the reaction interface. One example of a two-dimensional (2D) transition metal dichalcogenide used in this study is molybdenum-based materials. In this research, we investigated the electronic and thermoelectric properties of 2D Janus materials using first-principles electronic structure methods combined with semiclassical Boltzmann transport theory, calculating relaxation times using molybdenum-based materials and analyzing their impact on TE properties. We found that relaxation time calculations played a critical role in significantly altering transport phenomena, leading to a higher figure of merit and thus enhancing TE efficiency. This study is significant as it highlights the important structure-property relationship in determining TE efficiency, which could pave the way for the discovery of new TE materials with higher efficiencies. The results show that the thermoelectric properties of molybdenum-based materials indicate that MoSeO exhibits the highest maximum power factor (PF) across all temperatures. For p-type carriers in MoSeO, the PF reaches 32.568 mW/mK2 at 900 K, while for n-type carriers, the maximum PF reaches 3.419 mW/mK2 at the same temperature. These findings suggest that MoSeO has a high thermoelectric potential, particularly at high temperatures, making it a promising candidate for future thermoelectric applications.

Hybrid CaS/CaSe bilayer as a wide temperature range thermoelectric material

Disorder-induced Anderson-like localization for bidimensional thermoelectrics optimization

Black phosphorus is a promising thermoelectric (TE) material because of its high Seebeck coefficient and high electrical conductivity. In this work, the TE performance of bulk black phosphorus and single-layer phosphorene under uniaxial strain is studied using first-principles calculations and Boltzmann transport theory. The results show relatively excellent TE performance along the armchair direction for both black phosphorus and phosphorene in our study. However, high lattice thermal conductivity is the key adverse factor for further enhancing the TE performance of phosphorus. The ZT value can only reach up to 0.97 and 0.73 for n- and p-type black phosphorus at 700 K, respectively. Owing to quantum size effects, black phosphorene has lower lattice thermal conductivity than black phosphorus. At the same time, two-dimensional (2D) phosphorene exhibits increased electronic energy compared with bulk black phosphorus, resulting in a larger bandgap and reduced electrical conductivity due to the quantum confinement effect. Thus, the TE performance of n-type phosphorene can be partially improved, and the ZT value reaches up to 1.41 at 700 K. However, the ZT value decreases from 0.73 to 0.70 for p-type phosphorene compared with bulk phosphorus at 700 K. To further improve the TE performance of phosphorene, a tensile strain is applied along the armchair direction. Subsequent work indicates that uniaxial strain can further optimize phosphorene's TE properties by tuning hole relaxation time to improve electrical conductivity. Strikingly, the ZT values exceed 1.7 for both n- and p-type phosphorene under 4.5% tensile strain along the armchair direction at 700 K because of increased electrical conductivity and decreased lattice thermal conductivity.

Recent Advances in Molecular Design of Organic Thermoelectric Materials

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

High thermoelectric efficiency fluoride perovskite materials of AgMF3 (M = Zn, Cd)

: In the present work we explore the electronic, vibrational and thermoelectric properties of new Li based quaternary Heusler compound LiYNiSn that is recently proposed by Jiangang He et.al. [Chem. Mater. 30 (2018) 4978] which is based on the 18-electron rule. Here the theoretical calculations are performed within the approach of density functional theory and semi classical Boltzmann transport equations with the constant relaxation time approximation. The band gap of the proposed compound is 0.38 eV that is in agreement with the available results in literature. The Seebeck coefficient and the Figure of Merit (ZT) are calculated at the three different temperatures (300K, 600K and 700K) with respect to the chemical potential (μ). The maximum ZT recorded is 0.14 at 700 K temperature. The compound is reported first time as the thermoelectric material and can be beneficial in the experimental research of thermoelectrical materials.