Optimizing thermoelectric performance of layered PbBi<sub>4</sub>Te<sub>7</sub> compound through Cu doping

<p indent="0mm">Global energy and environmental challenges are an urgently required on the development of efficient, green and clean energy materials due to the consumption of fossil fuels. Thermoelectric materials, which enable the direct conversion between heat and electricity, hold great promise in applications such as waste heat recovery, energy utilization and refrigeration. In this context, more and more researchers are focusing on exploring ternary high-performance thermoelectric materials. Among the newly discovered ternary chalcogenides (IV-VI)<italic>_n</italic>(V₂-VI₃)<italic>_m</italic> (IV = Ge, Sn, Pb; VI = Se, Te; V = Sb, Bi), PbBi₄Te₇ has attracted increasing attention due to its low lattice thermal conductivity and unique layered crystal structure. Previous studies have indicated that the intrinsic low lattice thermal conductivity of PbBi₄Te₇ is primarily attributed to strong phonon scattering at the interfaces with its complex layered stacking structure. However, this structure also significantly reduces carrier mobility and limits the enhancement of its thermoelectric properties, which is also an issue commonly encountered in most layered thermoelectric materials. In recent years, Cu doping has been proven to be an effective method for enhancing interlayer carrier transport and achieving high <italic>ZT</italic> value in layered thermoelectric materials such as Bi₂Te₃, Sb₂Te₃ and SnSe₂. Understanding the effects of Cu on the thermoelectric properties of layered PbBi₄Te₇ in detail is of great importance. In this study, Cu doping is adopted to synergistically regulate its electrical and thermal transport to improve the thermoelectric properties of PbBi₄Te₇. A series of PbBi₄Te₇-<italic>x</italic>%Cu (<italic>x</italic> = 0–1) compounds were prepared by the melting method. These compounds were comprehensively characterized using techniques such as X-ray diffraction (XRD) and Hall coefficient measurement systems, etc. The results show that PbBi₄Te₇ with unique layered structure exhibits anisotropic carrier and phonon transport behavior. Specifically, PbBi₄Te₇ demonstrates an extremely low lattice thermal conductivity of <sc>0.53 W m⁻¹ K⁻¹</sc> along the direction parallel to the applied pressure, as well as a high electrical conductivity of <sc>938 S cm⁻¹</sc> and a considerable Seebeck coefficient of –76 μV K⁻¹, indicating its potential as a promising thermoelectric material. Cu doping can strengthen the interlayer electronic transport, increasing the interlayer room-temperature carrier mobility from <sc>100 cm² V⁻¹ s⁻¹</sc> in PbBi₄Te₇ to <sc>160 cm² V⁻¹ s⁻¹</sc> in PbBi₄Te₇-0.1%Cu. This enhanced carrier mobility enables the maximum power factor of PbBi₄Te₇-0.1%Cu to reach <sc>11.3 μW cm⁻¹ K⁻².</sc> Additionally, Cu doping can enhance interlayer phonon scattering, resulting in a minimum lattice thermal conductivity of <sc>0.20 W m⁻¹ K⁻¹</sc> in PbBi₄Te₇-0.1%Cu. Ultimately, with the synergistic optimization of Cu doping on interlayer electronic and thermal transport properties, the maximum <italic>ZT</italic> (<italic>ZT</italic>_max) value of PbBi₄Te₇-0.1%Cu is increased to 0.59, and the near-room-temperature average <italic>ZT</italic> (<italic>ZT</italic>_ave) value <sc>(300–773 K)</sc> can reach 0.48. PbBi₄Te₇-0.1%Cu sample demonstrates significant advantages over other typical Pb-Bi-based ternary compounds, indicating their promising potential as thermoelectric materials near room temperature.