Recent progress in “2D phonon-3D charge” thermoelectric transport

<p indent="0mm">Thermoelectric materials can directly convert thermal and electrical energy to each other and have a huge potential in power generation and precise temperature control. High-performance thermoelectric materials pursue to achieve “phonon glass-electron crystal” transport, which requires both low thermal and high electrical transport properties. However, the strong coupling between the phonon and the electron in materials poses a significant challenge in realizing the “phonon glass-electron crystal” transport. Developing the thermoelectric performance of intrinsically low thermal conductivity materials, such as layered structure materials, is an effective strategy to promote the “phonon-electron” decoupling. Tin selenide (SnSe) is an intrinsically low thermal conductivity layered material. The low lattice thermal conductivity along the in-plane direction endows the basis of the excellent thermoelectric performance for p-type SnSe crystals. Layered structure materials possess much lower lattice thermal conductivity along the out-of-plane direction than in-plane generally. If the carrier mobility of SnSe and other intrinsically low thermal conductivity layered structure materials along the out-of-plane direction can be increased, the ultra-low thermal conductivity of layered structural materials along this direction can be utilized to form “two-dimensional (2D) phonon and three-dimensional (3D) charge” thermoelectric transport, which can be further realized “phonon glass-electron crystal” transport. Anion-doped n-type SnSe crystals possess overlapping interlayer charge density resulting in a large enhancement of carrier mobility along the out-of-plane direction, and thus show “2D phonon-3D charge” thermoelectric transport and have attracted huge attention in the thermoelectric field due to their excellent performance. Therefore, more materials with “2D phonon-3D charge” thermoelectric transport are being discovered and show exciting potential for thermoelectric performance. Seeking materials with “2D phonon-3D charge” thermoelectric transport among intrinsically low thermal conductivity materials is gradually becoming one of the criteria for the development of new high-performance thermoelectric materials. In this review, we first introduced the mechanism of “2D phonon-3D charge” thermoelectric transport. The intrinsically low thermal conductivity along the out-of-plane direction of the layered material forms the “2D phonon” transport, which is the basis for obtaining excellent thermoelectric performance. The overlapping interlayer charge density can facilitate electron transport along the out-of-plane direction, and thus forms the “3D charge” transport which is the key to obtaining excellent thermoelectric performance. We also introduced the advantages of “2D phonon-3D charge” thermoelectric transport with lattice thermal conductivity, carrier mobility, and the Seebeck coefficient. The overlapping interlayer charge density leads not only to high carrier mobility along the out-of-plane direction, but also to the higher scattering factor than the in-plane direction and thus to the larger Seebeck coefficients along the out-of-plane direction than in-plane. Finally, we summarized the research progress of materials with “2D phonon-3D charge” thermoelectric transport, and gave an outlook on how to utilize and enhance “2D phonon-3D charge” thermoelectric transport to realize high thermoelectric performance, as well as the challenges for the future development of thermoelectric materials with “2D phonon-3D charge” thermoelectric transport. This review can help researchers comprehensively understand the “2D phonon-3D charge” thermoelectric transport and also contribute to the development of new high-performance thermoelectric materials.