An impurity intermediate band due to Pb doping induced promising thermoelectric performance of Ca5In2Sb6

Understanding the origin of the high thermoelectric figure of merit of Zintl-phase KCaBi

Herein, we have investigated the unexplored thermoelectric properties of Zintl-phase KCaBi using first-principles calculation and the solution of the Boltzmann transport equation. KCaBi shows intrinsically very low lattice thermal conductivities (κl) along the (x(y), and z)-directions of (1.78, 0.68) and (1.15, 0.43) W m-1 K-1 at 300 and 800 K, respectively. Along with the effect of very low κl, the high figure of merit (ZT) for p-type KCaBi results from the high Seebeck coefficients (S) and optimal electrical conductivities (σ), which originate from the high and steep total density of state (TDOS) at the valence band edge and the less dispersed multi-valley nature of the valence band edge in the band structure. On the other hand, large ZT for n-type KCaBi results from moderate S and high σ caused by the sloped TDOS at the conduction band edge and the highly dispersed nature of the conduction band edge in the band structure, and very low values of κl. The highest ZT of KCaBi that we obtained at 800 K along the (x(y), and z)-directions was (1.83, 0.80) for the p-type case at a hole concentration of 1021 cm-3 and (1.36, 1.22) for the n-type case at electron concentration 7 × 1018 cm-3. Our study demonstrates that both p-type and n-type KCaBi have the potential to be promising thermoelectric materials.

Chemical Composition Modulation Realizing Remarkable Improvement of Thermoelectric Performance in CuInTe2-Based Alloy

CuInTe2 (CIT) is one of the typical ternary chalcogenides known for its characteristic mixed polyanionic/polycationic site defects, making it a subject of continuous interest in the field of thermoelectrics. In this work, we propose a chemical composition modulation strategy for CIT by alloying GeTe and then introducing a copper deficiency (denoted by VCu). This strategy aims to unpin its Fermi level (Fr) and shift Fr into the valence band (VB) while simultaneously enabling coupling between the optical and acoustic phonon, thereby providing an extra phonon scattering path at low frequencies. The simultaneous composition regulations not only enhance the carrier concentration (nH) to 1019-1020 cm-3 but also significantly reduce the lattice thermal conductivity (κL) to ∼0.48 W m-1 K-1, thus effectively realizing electro-acoustic coordination in the present material. As a consequence, the thermoelectric (TE) performance is remarkably improved with the highest TE figure of merit (ZT) of 1.51 at ∼838 K. This value ranks at a higher level among CIT-based materials, which showcases the great significance of chemical composition modulation.

Interfacing Boron Monophosphide with Molybdenum Disulfide for an Ultrahigh Performance in Thermoelectrics, Two-Dimensional Excitonic Solar Cells, and Nanopiezotronics

A stable ultrathin 2D van der Waals (vdW) heterobilayer, based on the recently synthesized boron monophosphide (BP) and the widely studied molybdenum disulfide (MoS₂), has been systematically explored for the conversion of waste heat, solar energy, and nanomechanical energy into electricity. It shows a gigantic figure of merit (ZT) > 12 (4) for p (n)-type doping at 800 K, which is the highest ever reported till date. At room temperature (300 K), ZT reaches 1.1 (0.3) for p (n)-type doping, which is comparable to experimentally measured ZT = 1.1 on the PbTe-PbSnS₂ nanocomposite at 300 K, while it outweighs the Cu₂Se-CuInSe₂ nanocomposite (ZT = 2.6 at 850 K) and the theoretically calculated ZT = 7 at 600 K on silver halides. Lattice thermal conductivity (κ_l ≈ 49 W m^-1 K^-1) calculated at room temperature is lesser than those of black phosphorene (78 W m^-1 K^-1) and arsenene (61 W m^-1 K^-1). The nearly matched lattice constants in the commensurate lattices of the constituent monolayers help to preserve the direct band gap at the K point in the type II vdW heterobilayer of MoS₂/BP, where BP and MoS₂ serve as donor and acceptor materials, respectively. An ultrahigh carrier mobility of ∼20 × 10³ cm² V^-1 s^-1 is found, which exceeds those of previously reported transition metal dichalcogenide-based vdW heterostructures. The exciton binding energy (0.5 eV) is close to those of MoS₂ (0.54 eV) and C₃N₄ (0.33 eV) single layers. The calculated power conversion efficiency (PCE) in the monolayer MoS₂/BP heterobilayer exceeds 20%. It surpasses the efficiency in MoS₂/p-Si heterojunction solar cells (5.23%) and competes with the theoretically calculated ones, as listed in the manuscript. Furthermore, a high optical absorbance (∼10⁵ cm^-1) of visible light and a small conduction band offset (0.13 eV) make MoS₂/BP very promising in 2D excitonic solar cells. The out-of-plane piezoelectric strain coefficient, d₃₃ ≈ 3.16 pm/V, is found to be enhanced 4-fold (∼14.3 pm/V) upon applying 7% vertical compressive strain on the heterobilayer, which corresponds to ∼1 kbar of hydrostatic pressure. Such a high out-of-plane piezoelectric coefficient, which can tune top-gating effects in ultrathin 2D nanopiezotronics, is a relatively new finding. As BP has been synthesized recently, experimental realization of the multifunctional, versatile MoS₂/BP heterostructure would be highly feasible.

Hf/Sb co-doping induced a high thermoelectric performance of ZrNiSn: First-principles calculation

Previous experiments showed that Hf/Sb co-doping in ZrNiSn impressively improved the electrical conductivity (σ). To explore the physical reasons for this improvement, the electronic structures of HfxZr1-xNiSn1-ySby (x = 0, 0.25, 0.5; y = 0, 0.02) have been systematically investigated by using the first-principles method and semiclassical Boltzmann transport theory. 50% Hf doping at Zr site in ZrNiSn simultaneously increases the degeneracy and dispersion of energy bands near the conduction band edge, which are helpful to optimizing Seebeck coefficient and slightly improving σ. Furthermore, 2% Sb co-doping at Sn site in Hf0.5Zr0.5NiSn not only increases total density of states near the Fermi energy but also retains high mobility, and N v reaches eleven at the conduction band minimum, thereby inducing a large improvement in σ. Additionally, the Bader charge analysis shows the reason why Sb co-doping supplies more electrons. It is most likely derived from that Sb loses more electrons and Sb-Ni has a stronger hybridization than Sn-Ni. Moreover, we predict that the ZT of Hf0.5Zr0.5NiSn0.98Sb0.02 at 1000 K can reach 1.37 with the carrier concentration of 7.56 × 1018 cm-3, indicating that Hf/Sb co-doping may be an effective approach in optimizing thermoelectric properties of ZrNiSn alloy compounds.