*3.2.1.2. Sol-gel chemistry*

In metal salt-based sol-gel chemistry, the precursor solution is synthesized by dissolving metal salt precursors in solvents with stabilizing agents and water. The resulting metal complexes undergo a hydrolysis reaction through the loss of a proton by one or more of the water molecules that surround the metal cations in the first solvation shell. As a consequence, the aquo ligand molecule, water, bonded to the metal cation is transformed into either a hydroxo ligand, OH− , or an oxo ligand, O2−. Subsequently, a condensation reaction occurs due to the oxolation generating an oxo-bridge, M-O-M, allowing the formation of a metal oxide skeleton framework with a dense microstructure.

Elimination of impurities such as hydroxides, anions, and carbon along with the formation of oxygen vacancies is critical for determining properties of AOS because they not only hinder the sol-gel oxide framework formation reaction but also interrupt the efficient transport of charge carriers [49]. To enable low-temperature processable products, the thermal decomposition of metal precursors should be completed at a low temperature, as evidenced by the use of metal nitrate precursors, the anion of which is almost completely decomposed even at 250°C, instead of other metal precursors (chloride, acetate) [50]. In addition, the chemical structure should be tailored toward a framework containing less hydroxide, which can be realized by doping an element that has a high electronegativity [51].

*3.2.2. Physical pathways*

*3.2.2.1. Microwave annealing (MW)*

*3.2.2.2. Deep-ultraviolet assisted annealing (DUV)*

*3.2.2.3. High-pressure annealing (HPA)*

high-bias stability [62].

the room temperature DUV-IGZO is as high as 14.0 cm<sup>2</sup> V−1 s−1.

The use of microwave-assisted annealing enabled quick fabrication of ZnO-TFT at 140°C with a *μ* of 1.7 cm<sup>2</sup> V−1 s−1, which is improved to three- to sixfolds in comparison with TFTs processed by conventional hot-plate annealing [56]. This enhanced TFT performance is attributed to the improved development of ZnO grains suitable for reducing the length of transport paths across grain boundaries and the shorter Zn-Zn distance enabling better overlap between the Zn s-orbitals, thus creating chemical structures appropriate for high-performance AOS.

Low-Temperature Solution-Processable Functional Oxide Materials for Printed Electronics

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Much efforts have also been focused on the treatment of sol-gel films by photochemical activation using deep-ultraviolet (DUV) irradiation [36, 57–61]. When the as-deposited film is exposed to DUV irradiation, high-energy DUV photons induce photochemical cleavage of alkoxide groups and activate metal and oxygen atoms to facilitate M-O-M network formation (**Figure 5**, step 1, condensation). Further irradiation induces a gradual removal of oxygen and carbon (and, thereby, near-complete condensation) and a transition to a film densification (**Figure 5**, step 2, densification). This DUV technique is applicable to numerous AOS. The *μ* of

HPA influences thermodynamics of solution-processed AOS films, facilitating denser film formation by reducing its thickness under a high pressure (2 MPa). In addition, high-oxygen pressure changes the Gibbs free energy of the system and strengthens the bonding between metal ions and oxygen. As a result, TFT processed at 220°C exhibited a *μ* of 1.7 cm<sup>2</sup> V−1 s−1 and

**Figure 5.** Schematic diagram for the condensation mechanism of metal-oxide precursors by DUV irradiation.

A "sol-gel on chip" hydrolysis approach from soluble metal alkoxide precursors was reported [52], which affords unprecedented high *μ* of ~10 cm<sup>2</sup> V−1 s−1, reproducible and stable turn-on voltage (~0 V) at maximum process temperature as low as 230°C. The approach uses the *insitu* hydrolysis and condensation of transition metal alkoxides when they are exposed to an aqueous environment by nucleophilic substitution, thus affording the M-O-M framework at low temperature. The process is applicable to a broad range of AOS that are of immediate interest in TFT applications.
