# Solution-Based Synthesis and Processing of Metal Chalcogenides for Thermoelectric Applications
## 3 Ink-Based Processing Method for Thermoelectric Device Fabrication  
The ink-based processing method is highly favorable as a cheaper method, including for large-scale device production. This method allows for easy assembly of thermoelectric device components, especially in the shaping and scaling of the thermoelectric legs. Ink-based processing can effectively reduce energy consumption for device fabrication and provide a high degree of design ability for thermoelectric generators (TEGs) [105]. Generally, thermoelectric inks are prepared from conductive polymers [106; 107; 108; 109; 110], hybrids of conductive polymers with inorganic or carbon nanofillers [111; 112; 113; 114; 115; 116], and inorganic thermoelectric materials [117; 118; 119; 120; 121; 122; 123; 124; 125]. Currently, most thermoelectric devices are developed from inorganic materials due to their intrinsically higher power factors and ZT values compared to conductive polymers and organic-inorganic hybrid thermoelectric materials. The conventional route for fabrication of thermoelectric devices with inorganic materials involves multiple costly and time-consuming steps such as ingot synthesis, dicing, electroplating, assembling, cleaning, and soldering (Figure 9a). Moreover, the shapes of thermoelectric materials generated by the conventional method are mainly cuboids, giving rise to exclusively planar TEG geometries, and thus narrowing the application scope of thermoelectric power generation technology [105]. In comparison, the ink-based process exhibits great simplicity for patterning and shaping of thermoelectric materials into 2D films or even 3D bulk forms by drop-casting, spin-coating, spray-coating, inkjet-printing, screen-printing, dispenser-printing, and 3D-printing processes (Figure 9b) [126]. Table 2 summarizes the thermoelectric properties of various ink-processed metal chalcogenides and output power density of the derived TEG devices. For thermoelectric inks of inorganic materials, several parameters such as concentration, dispersibility, and rheological properties should be optimized to make them suitable for the subsequent coating or printing process. Organic and polymer binders are always necessary to provide certain viscoelastic properties to the inks, which are eventually decomposed in the post-annealing step [118; 119; 120; 121; 124; 125]. For example, [PERSON] et al. prepared a slurry of Bi\({}_{2}\)Te\({}_{3}\) particles and epoxy for dispenser printing of n-type Bi\({}_{2}\)Te\({}_{3}\) thermoelectric generators [131]. [PERSON] et al. developed a composite paste consisting of Bi and Te powders, organic binder, glass powder, and solvent by a ball-milling process [124], which can be used for fabrication of a flexible glass fabric-based TEG via the screen-printing method [118]. Figure 9: Comparison of the fabrication process of a thermoelectric generator (TEG) by (**a**) conventional ingot-based and (**b**) ink-based processes. Readapted with permission from WILEY-VCH [105]. TE: Thermoelectric. One problem with the addition of organic binders is that undesirable binder residues are at times trapped between the inorganic grains after the post-sintering stage, leading to compromised electrical properties and ZT values of the final thermoelectric products. In recent reports, inorganic chalcogenidometallate (ChaM) ions were observed to act as surface ligands for nano- and microscale particles, stabilizing these particles in the solution through electrostatic interactions [132]. In 2018, [PERSON] and coworkers reported the extrusion-based 3D-printing of shape-conformable TE materials via full-inorganic Bi\({}_{2}\)Te\({}_{3}\)-based colloid inks using Sb\({}_{2}\)Te\({}_{3}\) ChaM ions as inorganic binders (Figure 10) [123]. The ChaM inorganic ions can act as ligands to provide a charged surface for the Bi\({}_{2}\)Te\({}_{3}\) particles, enhancing electrostatic interactions that contribute to the stability and viscoelasticity of the inks, and meanwhile facilitating 3D shape engineering and ensuring homogeneity of thermoelectric properties.