Significance With the rapid development of society, the traditional fossil energy is increasingly exhausted, and the problems of energy shortage and environmental pollution are becoming more and more serious. Human demand for clean and efficient energy is becoming more and more urgent. Vigorously developing new energy has become the core issue of nowadays society. Now, a variety of new and sustainable energy sources are emerging, such as wind energy, solar energy, biomass energy, and water energy. Thermal power generation is one of the hot new energy sources. The thermoelectric conversion efficiency is directly proportional to the thermoelectric merit of thermoelectric materials. Tin selenide (SnSe) has a thermoelectric merit value of 2.6 (923K), which is one of the materials with the highest thermoelectric conversion efficiency and is widely used in many fields. However, the preparation method needs to be improved, and the thermoelectric properties of media and low temperature properties are not excellent. Therefore, the research progress of SnSe is reviewed in order to find a way to improve its preparation process and further enhance its thermoelectric properties.

Progress This review paper first introduces the basic structure and characteristics of SnSe. Then, the current research progress is reviewed from three aspects. The first is the preparation process. There are many ways to prepare different SnSe, such as single crystal, polycrystalline, and thin film. The main preparation methods of single crystal SnSe are Bridgman method and temperature gradient method ( Fig. 6), with strict crystal growth conditions and high production cost. The preparation methods of polycrystalline ( Fig. 7) mainly include spark plasma sintering, hot pressing, hydrothermal method, solvothermal method, and heat injection method, and the solution method can obtain relatively higher thermoelectric properties. The preparation methods of thin films ( Fig. 10) are mainly atmospheric pressure chemical vapor deposition, atomic layer deposition, thermal vapor deposition, pulsed laser deposition, molecular beam epitaxy, and magnetron sputtering. Chemical vapor deposition is commonly used. Molecular beam epitaxy can achieve accurate control of epitaxial layer at atomic scale, but the growth process is complex and the cost is high. The thermoelectric properties of SnSe are closely related to its structure and doping state. The thermoelectric properties of SnSe with different doping states ( Table 1) and different preparation methods ( Fig.12) are summarized. At present, the thermoelectric properties of undoped p-type single crystal SnSe and Ag doped p-type polycrystalline SnSe are the best. In addition, SnSe also possesses excellent photoelectric (thermoelectric) properties. The optical absorption band and absorption capacity can be controlled by the number of layers and band gap of SnSe. The photoelectric (thermoelectric) properties of SnSe in bulk and thin film states are introduced, and the applications of SnSe in photothermoelectric devices are discussed. The last aspect is potential applications of SnSe, which can be divided into photovoltaic devices, sodium-ion and lithium-ion batteries, flexible devices, topological crystalline insulators, and phase change memory.

Conclusion and Prospect In summary, there are still some problems in the research of SnSe: 1) the preparation conditions of SnSe are strict and the cost is high; 2) the thermoelectric properties of polycrystalline SnSe are still far lower than that of single crystal and need to be further optimized; 3) compared with traditional thermoelectric materials, the thermoelectric properties of SnSe in low temperature region are not ideal; 4) in terms of the new generation of flexible wearable devices, there are few research reports on SnSe. It should try to combine SnSe thin films with more flexible materials to make new self-powered devices. The following approaches are expected to improve the properties of SnSe.

1) Appropriate element doping. For example, doping SnSe single crystal with Na or Ag and optimizing carrier concentration can reduce the peak value of pyroelectric merit (Z_T) to medium temperature range; hole doping can optimize Fermi level, and enhance Seebeck coefficient and power factor, so as to improve thermoelectric performance.

2) Other materials with similar crystal structures (such as black phosphorus,GeSe and SnS) are used to composite with SnSe, which is expected to further reduce the optimal temperature of the device.

3) An appropriate preparation method is selected. For example, the solution method can effectively reduce the temperature range of peak Z_T and improve the average Z_T value. In the synthesis process, defects such as vacancy, crystal size, and type can be effectively controlled by adjusting the kinetic conditions (such as solvent, temperature, time, and catalyst), so as to improve the thermoelectric properties. The orientation and defects of the materials can be better controlled and the thermoelectric properties can be further improved by combining the preparation methods with complementary advantages.

4) In recent years, more and more new thermoelectric performance optimization technologies, such as magnetic interaction, introducing texture, adjusting bonding properties, and enhancing anharmonic bonding, have been paid attention to. The above methods can be used to optimize the thermoelectric performance of SnSe.