Recent progress in n-type SnSe crystals thermoelectric materials

Thermoelectric energy conversion technology can directly convert thermal and electrical energy to each other based on thermoelectric devices, and has a huge potential in power generation and thermoelectric cooling. The energy conversion efficiency of thermoelectric devices is determined by the average dimensionless figure of merit (ZT(ave)) of the thermoelectric material in a certain temperature range. Manufacturing high-performance thermoelectric devices requires both high-performance p- and n-type thermoelectric materials with matching properties. Tin selenide (SnSe) is an intrinsic p-type wide-bandgap thermoelectric material. The anisotropic layered structure and unique phase transition of SnSe endow both p- and n-type SnSe crystals with excellent thermoelectric properties. Thus, SnSe crystals have attracted much attention in the thermoelectric field and are considered as one of the potential next-generation commercial thermoelectric materials. Currently, p-type SnSe crystals achieve a ZT(ave) of 2.2 between 300-773 K and a ZT of 1.5 at 300 K. However, n-type SnSe crystals can only achieve a ZT(ave) of 1.7 between 300-773 K and a ZT of 0.5 at 300 K. Apparently, the thermoelectric performance of n-type SnSe crystals is still dissatisfactory compared to p-type SnSe crystals, especially for near room temperature. Optimizing the thermoelectric properties of n-type SnSe crystals has become the priority for the realization of all SnSe-based thermoelectric devices. Therefore, it is indispensable to summarize the research progress in n-type SnSe crystals and to develop a comprehensive understanding of their thermoelectric transport properties. In this review, we first summarized the recent progress in n-type SnSe crystals thermoelectric materials and analyzed the similarities and differences in the thermoelectric transport properties of n-type SnSe crystals doped with different donor impurities (bismuth, chlorine, bromine, and iodine). In general, halogen-doped SnSe crystals possess "two-dimensional phonon and three-dimensional charge" thermoelectric transport and show higher performance along the out-of-plane direction. Chlorine-doped SnSe crystals have a performance advantage in the near room and medium temperature regions resulting from the higher carrier mobility, and bromine-doped SnSe crystals show superior performance at high temperatures due to their relatively larger carrier concentration. Differently, bismuth-doped SnSe crystals show higher performance along the in-plane direction than the out-of-plane direction, and possess the highest thermoelectric performance along the in-plane direction compared to halogen-doped SnSe crystals due to their largest carrier concentration at high temperatures. Secondly, we compared the differences between the electrical properties of n-type SnSe crystals along the in-plane and out-of-plane directions due to the enhanced anisotropic carrier scattering factor induced by alloying lead selenide. Alloying lead selenide increases the scatter factor along the out-of-plane direction but decreases the scatter factor along the in-plane direction, which enlarges the Seebeck coefficient difference between the out-of-plane direction and the in-plane direction. Finally, we described the effect of SnSe phase transition on its thermal conductivity. The effect of the phase transition of SnSe on thermal conductivity mainly comes from the critical phase transition from Pnma to Cmcm phase; the continuous phase transition has little effect on thermal conductivity. We also discussed the upcoming research priorities and challenges for n-type SnSe crystals thermoelectric materials. This review describes the progress in n-type SnSe crystal thermoelectric materials, which helps researchers gain a comprehensive understanding of the thermoelectric transport of n-type SnSe crystals, and points out the direction for the future development of n-type SnSe crystals.