# Solution-Based Synthesis and Processing of Metal Chalcogenides for Thermoelectric Applications
## 1 Introduction  
Given the current global energy crisis and environmental issues, the exploitation of green and sustainable resources, such as wind, solar, geothermal, and biomass resources, for energy generation is being extensively developed all over the world. In this context, thermoelectric materials that can directly convert waste heat into electricity have attracted tremendous research interest over the past decades [1; 2; 3; 4]. The thermoelectric conversion efficiency can be conveniently evaluated by the figure of merit ZT = S\({}^{2}\)\(\sigma\)T/\(\kappa\) (where S is the Seebeck coefficient, \(\sigma\) is electrical conductivity, T is absolute temperature, and \(\kappa\) is thermal conductivity). For easy comparison of the electrical and thermal transport properties, the thermal conductivity (\(\kappa\)) and power factor (S\({}^{2}\)\(\sigma\)) can be used to judge the performance of different thermoelectric materials [5]. Currently, the research in this field is mainly focused on (1) developing novel environmentally benign and naturally abundant thermoelectric materials, (2) improving thermoelectric performance of existing materials through nanostructure engineering, and (3) developing highly efficient modules or devices for practical thermoelectric energy conversion. Figure 1 summarizes the figure of merit values of current state-of-the-art thermoelectric materials with high conversion efficiency, which are mainly based on metal chalcogenide semiconductors, such as Bi\({}_{2}\)Te\({}_{3}\), Cu\({}_{2}\)Se, SnSe, and PbTe, etc. [4]. The outstanding ZT values of the metal chalcogenides are mainly ascribed to their relatively weaker covalent bonding strength and higher atomic weight compared to other thermoelectric materials, which lead to low thermal conductivity [6]. The electronic structures and formulations of the chalcogenide materials can be readily engineered through nanotechnologyand chemical doping. Moreover, the functional properties of metal chalcogenides can be easily tuned by p-type or n-type doping. Importantly, most of the metal chalcogenide thermoelectric materials are nontoxic, cheap, and Earth-abundant, holding great promise for large-scale production and manipulation. The practical application of high-performance thermoelectric materials largely relies on the development of scalable synthetic approaches with controllable size, composition, structure, and morphology. The conventional solid-state or gas-phase methodologies are often considered unsatisfactory due to consuming large amounts of energy, harsh reaction conditions, and/or low production rates. Comparatively, solution-based synthesis has been considered to be a powerful and industry-friendly approach for obtaining various metal chalcogenide materials. Currently, there are a number of review papers on metal chalcogenide materials for thermoelectric, photovoltaic, energy storage, electrocatalytic, and photocatalytic applications, some of which involve a specific method for preparation of these materials [6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17]. However, there is no comprehensive summary on solution-based synthesis and processing of metal chalcogenide materials for thermoelectric applications. Herein, we will provide an overview on the solution-based synthesis of nanostructured or thin-film metal chalcogenide materials as well as their thermoelectric applications reported recently. We mainly focus on the environmentally benign such materials with high ZT values, such as Bi\({}_{2}\)Te\({}_{3}\), Cu\({}_{2}\)Se, and SnSe. PbTe is also a high-performance thermoelectric material; however, the environmental concern due to its inherent toxicity limits its large-scale production and application. Other binary and multinary metal chalcogenide materials and their composites are not covered in this paper because of the overwhelming research in this field. Following that, we also introduce the state-of-the-art ink-based processing methods for fabrication of thermoelectric devices based on metal chalcogenides.