# Solution-Based Synthesis and Processing of Metal Chalcogenides for Thermoelectric Applications
## 3 Ink-Based Processing Method for Thermoelectric Device Fabrication
These inks allowed for the fabrication of varied geometry thermoelectric materials that gave rise to high thermoelectric performances, as evidenced by their ZT values of 0.6 for n-type and 0.9 for p-type materials. \begin{table}
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Figure 10: (**a**) Photograph showing viscoelastic all-inorganic TE ink. (**b**) Illustration of an extrusion-based 3D-printing process. (**c**) Optical microscopy image (top) and a photograph (bottom) of the 3D-printed TE materials. (**d**) Scheme of the structural changes of TE particles depending on the content of Sb\({}_{2}\)Te\({}_{3}\) ChaM binders in the Bi\({}_{2}\)Te\({}_{3}\) ink. (**e**) Photographs of the fabricated half-ring-based conformal TEG. The inset shows the 3D-printed n-type and p-type half rings. Readapted with permission from Macmillan Publishers Ltd. [123]. Recently, [PERSON] et al. initially reported the inkjet printing of phase-pure, single-crystalline Bi\({}_{2}\)Te\({}_{3}\) nanowires without adding any additives (Figure 11) [125]. The Bi\({}_{2}\)Te\({}_{3}\) nanowires with an average diameter of 30 nm were synthesized by a chemical batch process, and then directly dispersed into deionized water by sonication to form the printable ink. Bi\({}_{2}\)Te\({}_{3}\) films of different thickness (0.4-1.6 \(\upmu\)m) were printed onto glass substrates and then subjected to post-annealing to improve the thermoelectric performance. The printed networks exhibited a Seebeck coefficient up to 140 \(\upmu\)V K\({}^{-1}\), which is comparable to that of Bi\({}_{2}\)Te\({}_{3}\) nanowires that are hot-pressed into pellet form. High power factor (163 \(\upmu\)W m\({}^{-1}\) K\({}^{-2}\)) and thermoelectric efficiency (ZT \(\approx\) 0.26) at room temperature are achieved with these printed films, which are promising for the scalable roll-to-roll manufacturing of printed thin-film thermoelectric devices. In 2017, [PERSON] et al. first reported a truly soluble thermoelectric ink, prepared at room temperature by dissolving Cu\({}_{2}\)Se powder in a mixed solvent of ethylenediamine and ethanedithiol, which can be directly coated onto various substrates via a low-cost and scalable spin-coating process (Figure 12) [117]. Owing to the high structural perfection, the Cu\({}_{2}\)Se thin film derived from the truly soluble ink material exhibits a considerably outstanding power factor of 0.62 mW m\({}^{-1}\)K\({}^{-2}\) at 684 K on a rigid Al\({}_{2}\)O\({}_{3}\) substrate and 0.46 mW m\({}^{-1}\)K\({}^{-2}\) at 664 K on flexible polyimide film, which is greater than the values obtained from other solution-processed Cu\({}_{2}\)Se thin films (\(<\)0.1 mW m\({}^{-1}\)K\({}^{-2}\)) and amongst the highest values reported for flexible thermoelectric films (\(\approx\)0.5 mW m\({}^{-1}\)K\({}^{-2}\)). In addition, the prepared thin film shows excellent stability after 1000 bending cycles, showing great promise for integration into various flexible electronics. Figure 11: (**a**) Schematic diagram of the Bi\({}_{2}\)Te\({}_{3}\) nanowire (BTNW) ink formulation, inkjet printing of the resultant ink, and post-annealing processes. (**b**) Power factor (PF) of the printed films of different thickness (i.e., 25, 50, 75, and 100 printer passes) annealed in a forming (red) and a nitrogen (blue) gas ambient. Inset is the SEM image of the annealed nanowires in a nitrogen gas. Readapted with permission from WILEY-VCH [125]. DI: deionized.